‘Classical’ genomic progesterone receptors appear relatively late in phylogenesis, i.e. it is only in birds and mammals that they are detectable. In the different species, they mediate manifold effects regarding the differentiation of target organ functions, mainly in the reproductive system. Surprisingly, we know little about the physiology, endocrinology, and pharmacology of progesterone and progestins in male gender or men respectively, despite the fact that, as to progesterone secretion and serum progesterone levels, there are no great quantitative differences between men and women (at least outside the luteal phase). In a prospective cohort study of 1026 men with and without cardiovascular disease, we were not able to demonstrate any age-dependent change in serum progesterone concentrations. Progesterone influences spermiogenesis, sperm capacitation/acrosome reaction and testosterone biosynthesis in the Leydig cells. Other progesterone effects in men include those on the central nervous system (CNS) (mainly mediated by 5α-reduced progesterone metabolites as so-called neurosteroids), including blocking of gonadotropin secretion, sleep improvement, and effects on tumors in the CNS (meningioma, fibroma), as well as effects on the immune system, cardiovascular system, kidney function, adipose tissue, behavior, and respiratory system. A progestin may stimulate weight gain and appetite in men as well as in women. The detection of progesterone receptor isoforms would have a highly diagnostic value in prostate pathology (benign prostatic hypertrophy and prostate cancer). The modulation of progesterone effects on typical male targets is connected with a great pharmacodynamic variability. The reason for this is that, in men, some important effects of progesterone are mediated non-genomically through different molecular biological modes of action. Therefore, the precise therapeutic manipulation of progesterone actions in the male requires completely new endocrine-pharmacological approaches.
Key words: MEN, PROGESTERONE, PROGESTINS, NON-GENOMIC ACTIONS
From a phylogenetic point of view, the action of the steroid hormone progesterone (4-pregnen-3,20-dione;progesterone) via its nuclear receptor (PR) is a relatively young acquisition in the animal kingdom . We find PRs only in birds and mammals in which the progesterone plays an important regulatory role in the oviduct, including oviposition, in the uterus including pregnancy, and in the mammary gland including lactation2,3.
Not surprisingly, the main interest of progesterone endocrinology remains focused on the female physiology. Therefore, the history of medical indications for progesterone preparations or progestins is a story of gynecological developments, like replacement therapy in oophorectomized women (1934, by Kaufmann), the treatment of oligo- and hypomenorrhea (1937, by von Kehrer), the treatment of anovulation (1938, by Clauberg), the treatment of menorrhagia (1953, by Kaufmann), female contraception with norethynodrel (1956, by Pmcus), the treatment of endometriosis (1969, by Kistner), and, since the 1970s, postmenopausal hormone replacement therapy with estrogens. Only in the field of hormonal male contraception, has a relatively broader experience been gained, with progestins being clinically administered to men. Antiandrogenically acting progestins are also indicated in some forms of sexual deviation and prostate cancer4,5.
Defined by most encyclopedias and textbooks as a female hormone, the importance of progesterone in the male endocrine system has remained in the shadow. Testicular and adrenal progesterone has been regarded as a physiologically unimportant by-product of steroidogenesis that is not converted to testosterone. But, in many conditions, including aging, the serum progesterone/androgen ratio increases. Only during the past few years has the role of progesterone as a modulator of the male endocrine system become more and more evident6.
The aim of this review is to summarize and critically discuss the rather scattered, often controversial results about progesterone and progesterone action in men.
Pregnenolone is the precursor of progesterone (catalyzed by 3β-hydroxysteroid dehydrogenase) and the main serum metabolites of progesterone are 17α-OH-P (catalyzed by 17-hydroxylase), desoxycorticosterone (catalyzed by 21-hydroxylase), and to the main urinary metabolite pregnanediol. Considering that 17α-OH-P gives 11-deoxycortisol (catalyzed by 21-hydroxylase), which in turn gives the essential cortisol (catalyzed by 11β-hydroxylase) as well as aiidrostenedionc and dehydroepiandrosterone (intermediate steroids in the biosynthesis of androgens and estrogens), the fundamental importance of progesterone for the maintenance of steroid hormone homeostasis independently of the known, directly sex- specific actions of progesterone is shown7. The importance of progesterone as a precursor of the so-called neurosteroids is discussed below. The surprising finding of Nadjafi-Triebsch and colleagues, namely that the serum progesterone levels rise in men after the oral administration of dehydrocpiandrosterone (DHEA) (unusual upstream metabolism), does not fit in with the general scheme of biosynthesis and the metabolism of progesterone. On the other hand, only two men were investigated.
3β-Hydroxysteroid dehydrogenase (3β-HSD) has been purified from steroidogenic organs (adrenal cortex, testes, ovaries) of different species, including humans, and has been characterized9- 11. This shows that, in men, progesterone is synthesized not only in the Leydig cells of testicles, but also in the adrenals, and is secreted from there into the circulatory system.
The reference range for progesterone levels in adult men is 0.13- 0.97 ng/ml12. Zumoff and colleagues reported a mean serum progesterone level of 0.18 0.03 ng/ml for men (n = 7) and of 0.21 0.05 ng/ml for young women in the fbllicular phase (n = 8). In contrast to this, Muneyyirci-Delale and colleagues14 measured 0.78 0.28 ng/ml for healthy men and 0.26 0.18 ng/ml for postmcnopausal women (Coat-a-Count RIA kit).
Data in the literature are contradictory regarding age-dependent changes in blood serum concentrations of progesterone. Collecting 252 saliva profiles from healthy children and adolescents (125 boys and 127 girls), Grschl and colleagues15 found that salivary progesterone was highest (p 60 years. In contrast to these findings, Blanger and colleagues17 found no age-related changes in progesterone concentrations in 2423 men (40-80 years old). This discrepancy caused us to reassess the relationship between serum progesterone levels and men’s ages. The data we show here originate from the cooperation of Jenapharm GmbH & Co. KG/Jena (Germany) with the LURIC study (Ludwigshafen Risk and Cardiovascular Healthy Study), a joint project of the Herzzentrum (Heart Center) in Ludwigshafen and the Universittskliniken (Teaching Hospitals) in Freiburg and Ulm, Germany18. We analyzed serum samples from 1015 men aged 20-90 years and serum samples from 330 postmenopausal women aged 50-90 years by a radioimmunoassay for progesterone. We found 1.21 0.41 SD nmol/l (0.38 0.13 ng/ml) for men and 1.24 1.18 SD nmol/l (0.38 0.37 ng/ml) for women, i.e. there were no differences between men and women. As shown in Figure 1, there were no age- dependent changes in serum progesterone levels in both men and postmenopausal women.
No details are given here of the main metabolite of progesterone, 17α-hydroxyprogestcrone, and its importance for the diagnosis of congenital adrenal hyperplasia (21-hydroxylase deficiency). In male patients with cytochrome P450C17 (steroid 17α-hydroxylase/ 17,20-lyase; EC 220.127.116.11) deficiency, the progesterone levels are clearly elevated19.
Figure 1 Serum progesterone levels and age (Luric-Jenapharm- study). There are no age-related changes. The mean serum concentrations are 1.21 0.41 SD nmol/l for men and 1.24 1.18 SD nmol/l for postmenopansal women
As with all steroid hormones, the classical paradigm of the progesterone action is that intracellular receptors bind to progesterone to modulate the gene expression within the nuclei of target cells. Two progesterone receptors (PRs), termed A and B, are derived from alternate promoters of only one gene located on chromosome 11 q22-23. PR-B contains 933 amino acids while PR-A is a truncated version lacking the initial 164 amino acids (the B- upstream segment). In vitro evidence reveals that the PR-A isoform is necessary to oppose the estrogen-induccd proliferation as well as the PR-B-dependent proliferation. In contrast to this, PR-A predominace is an early event in mamma carcinogenesis and is associated with poor clinical features20,21. In other words, the cellular ratio of PR-A : PR-B is likely to be an important determin\ant of the tissue-specific progesterone action22-24. On the other hand, studies with female knock-out mice showed that PR-A is both necessary and sufficient to elicit the progesterone-dependent reproductive responses necessary for female fertility, while the PR- B isoform is required to elicit normal proliferative and differentiative responses of the mammary gland to progesterone25.
In addition to the classical, genomic action via two nuclear PR isoforms, rapid progesterone effects incompatible with the model of nuclear receptors have been identified. The proposed mechanisms of non-genomic progesterone action are:
(1) Progesterone acts directly via subsets of the classical intracellular receptor bound to the membrane;
(2) Progesterone acts directly via one (or more) non-classical membrane-bound receptor;
(3) Progesterone may interact with a partner ligand via non-PR;
(4) Progesterone, at high concentrations, may get into the plasma membrane and affect membrane fluidity.
Examples of the membrane-dependent progesterone action in male gender are:
(1) Sperm capacitation/acrosome reaction (e.g. rapid increase in [Ca^sup 2+^]^sub i^)
(2) LH receptor expression and subsequent influence on testosterone biosynthesis in Ley dig cells;
(3) Increased classical PR concentrations in prostate (BPH as well as prostate cancer);
(4) Interactions with the GABA^sub A^ receptor complex in the CNS, including sedative and anesthetic actions;
(5) Progesterone-interactions in adipose tissue and kidney.
This list suggests that many progesterone effects in the male are rapid and therefore non-genomically mediated.
Progesterone-binding membrane proteins have been identified in liver26, sperm27,28, and lens epithelial cells29. Zhu and colleagues30 cloned, expressed, and characterized a membrane PR using fish oocytes. On the basis of the sequence of the membranous messenger PR, the authors then identified a whole family of mPR proteins from a number of different species, including frog, human, and mouse, some of which bound progesterone30,31
A variety of other rapid progesterone effects have been demonstrated; however, they occur at non-physiologically high steroid concentrations, rendering their relevance questionable. For example, progesterone at micromolar concentrations induces a dose- dependent relaxation of rat saphenous artery segments (precontracted with norpeinephrine). In a similar manner, progesterone dose- dependently decreases the contractile activity of murine jejunum (for review, see reference 28).
Finally, several polymorphisms have been identified in nuclear PRs; they include S344T, G393G, + 331G/A, Exon 4 V660L, Exon 5 H770H (C/T), and the PROGINS allele (Al insertion32). These polymorphisms will be useful markers in the genetic study of disorders affecting female endocrine systems, such as so-called progesterone resistance and breast, uterine, and ovarian cancers. However, nothing is known at present about the diagnostic and therapeutic relevance of functional polymorphisms of PRs in the male.
PROGESTERONE AND THE IMMUNE SYSTEM
Data from both human and animal studies clearly demonstrate that both 17β-estradiol and progesterone influence most components of innate as well as adaptive immunity. Evidence of these effects is found in the differences in immune responses between females and males. For example, women show more vigorous T and B cell responses and have higher circulating CD4 T cell numbers than men. The incidence of most autoimmune diseases, including multiple sclerosis, is higher in womenjand, in many women, the hormone changes associated with pregnancy lessen the severity of the disease33.
Various in vivo and in vitro studies have demonstrated that progesterone inhibits the functions of human macrophages and T lymphocytes within physiological concentrations, and thus it has been suggested that progesterone acts mainly as an immunosuppressant during prognancy. However, various immune cells have been shown to lack the classical PR and, hence, the mechanism of anti- inflammatory effects of progesterone still remains more or less unclear34. In this context, studying the so-called FK506-binding proteins could be an interesting approach to answer this question. The ability to bind immunosuppressive drugs such as cyclosporine and FK506 defines the immunophihn protein family, and the FK506-binding proteins form the FKBP subfamily of immunophilins. The large FK506- binding protein FKDP51 is a component of the PK complex and is transcriptionally regulated by the ‘pure’ progestin R5020 and attenuates progestin responsiveness in hormone-conditioned T-471) cells35,36.
Studies have shown that glucocorticoid receptors can bind progesterone with a high affinity. In vitro progesterone and cyproterone acetate have, in some models, antiglucocorticoid effects37,38. But, in contrast to this, it is interesting to note that Allolio and colleagues39 found that high-dose progesterone infusion in healthy men affects neither plasma ACTH levels, nor serum or saliva cortisol. Therefore, under these specific clinical- pharmacological conditions, an antiglucorticoid action of progesterone can be neglected.
In 132 human thymomas, immunoreactivity for PK-B was dominant (49%) compared with that for PR-A (4%). A significant positive correlation was detected between immunoreactivity for estrogen receptor (ER) α and PR-B. Therefore, the PR-B status in human thymoma may also reflect estrogenic actions in this immune competent tissue. The ERα immunoreactivity was positively correlated with a better clinical outcome and negatively con-elated with tumor size, clinical stage, WHO classification, and the Ki-67 labeling index40.
PROGESTERONE ACTIONS ON SPERM
Most of the available data about the role of progesterone in sperm function have been obtained in humans (reviewed in references 41-43). The process of capacitation renders the sperm capable of interacting with the oocyte and of engaging acrosome reaction. Progesterone facilitates human sperm capacitation44-46. Acrosome reaction is a process marked by the fusion of the outer acrosomal membrane with the plasma membrane. The physiological induccr of aerosome reaction in many species is the zona pellucida (ZP) glycoprotein, ZP3. Also, progesterone secreted by cumulus cells and contained in the fbllicular fluid induces acrosomc reaction in humans47,48.
Although early data suggested direct effects of progesterone on sperm49, it was not until 1989 that progesterone induction of intracellular free calcium ([Ca^sup 2+^]^sub i^) was found to result in phosphatidylinositol-4,5-bisphosphonatc hydrolysis in human sperm50. In 1990, Dlackmore and coworkers51 demonstrated that high concentrations of progesterone and 17-OH-P rapidly increased [Ca^sup 2+^]^sub I^ in both capacitated and non-capacitated human spermatids, actions which were shown to be blocked by a Ca^sup 2+^- channel antagonist. Since then, more than 100 studies have investigated, in detail, the action of progesterone on [Ca^sup 2+^]^sub I^ in sperm, in addition to the role of voltage-dependent calcium channels VDCC and sigmoidal dose sensitivity6,52,53. Besides the increasing intracellular calcium concentrations, progesterone has led to a stimulation of activity of phospholipases and tyrosine phosphorylation of sperm proteins (reviewed in reference 41).
Non-genomic progesterone effects have been better demonstrated at the sperm plasma membrane using bovine serum albumin-conjugated progesterone48,54. These progesterone-binding sites in sperm were further characterized and shown to share homology with the steroidbinding site of the genomic progesterone receptor; however, they lacked the long 120-kDa protein. After all, two surface receptors (of 54 and 57 kDa) with different affinity to progesterone (one in the nanomolar and the other in the micromolar range) have been identified in humans. The two proteins were detected after membrane preparation with antibodies directed to the hormone- binding region D of the genomic PR, but are not seen with antibodies to either the DNA-binding domain or the amino-termmal domains of the ‘classical’ progesterone receptor55. The availability of the membranous progesterone receptor on the sperm surface increases during the epididymal transit and after capacitation56. Using the hamster egg penetration test for the demonstration of stimulatory effects on the human sperm/oocyte fusion, Francavilla and colleagues57 found that the metabolite of progesterone, 17α-OH- progesterone, is nearly as active as progesterone itself. The addition of the synthetic progestin levonorgestrel in vitro to capacitated spermatozoa from fertile men is associated with a dose- dependently increased rate of acrosome reaction58. Both progesterone and the synthetic progestin norethisterone (NET) increased acrosome reaction in porcine spermatozoa, while the 5α-reduced metabolite of NET, 5α-NET, not only did not induce this reaction, but was able to block the effect of progesterone59. Therefore, 5α-NET has to be categorized as a ‘non-classical’ progesterone antagonist in this respect. Other data have demonstrated that these membrane actions of progesterone in sperm are neither mimicked nor blocked using ‘classical’ progesterone antagonists, such as RU486 (mifepristone)46.
Earlier clinical studies showed that sperm obtained from oligospermic semen had reduced responses to progesterone stimulation, suggesting that this membrane effect of progesterone can be crucial for sperm development and fertilizing capacity60,61. Other studies went even further, demonstrating that the absence of progesterone actions on sperm can be the sole reason for some cases of male infertility62.
USE OF PROGESTINS IN MALE CONTRACEPTION
Since follicle stimulating hormone (FSH) (through the Sertoli cell), luteinizing hormone (LH) (through the Leydig cell), and testosterone are required for normal spermatogenesis, t\he two gonadotropins need to be suppressed as strongly as possible for effective male hormonal contraception. Therefore, the exogenous testosterone administration is combined with gonadotropin releasing hormone (GnRH) antagonists or, preferably, with progestins. The gonadotropin suppression by progestins in the human male is mediated by genomic progesterone receptors, whereas the androgenicity of some progestins seems to contribute only minimally to gonadotropin inhibition63. Since excellent reviews on hormonal male contraception have been published recently; this indication for progestins in fertile men will not be treated in greater detail here64-69.
PROGESTERONE AND LEYDIG CELL FUNCTIONS
Generally, the steroidogenic capacity in aging Leydig cells is markedly reduced. This has been explained by an age-related reduction in the expression of a number of genes relevant to testosterone biosynthesis and genes involved on stress/free radical scavenging70-72. Moreover, expression of relaxin-like factor, which is present only in Leydig cells in testis, decreases in parallel with a reduction in the rate of testosterone production in aging testis73. In addition to a quantitative reduction in testosterone biosynthesis, there is a deflection of the direction of testicular steroidogenesis. An example is the increased secretion of progesterone by the aging rat Leydig cell due to lesion at the cytochrome P450 17-hydroxylase/C^sub 17-20^ step74,75. Classical nuclear PR. is absent in Leydig cells (Mukhopadhyay AK, unpublished data). In murine Leydig tumor cells (mLTC-1 cells), El-Hefhawy and Huhtaniemi76 could not demonstrate a classical PR and concluded that progesterone actions in these cells were mediated by a non- classical receptor. The authors found that binding of [^sup 3^H] progesterone to the mLTC-1-cells revealed a high (K^sub d^, ~ 9.3 nmol/l) and a low testosterone affinity (K^sub d^, ~ 284 nmol/l) component, and the binding displayed with specificity (progesterone > dehydroepiandrosterone > 17-OH-P). The binding was apparently different from that of the classical nuclear PR76. Also in MA-10 mouse Leydig tumor cells, using reverse transcription-polymerase chain analysis and immunoblotting experiments, the absence of the classical form of PR has been clearly demonstrated, although progesterone was found to have a direct stimulatory effect77 on the steroidogenic acute regulatory protein (StAR) and on VEGF production78 in these cells. From a variety of data, it appears plausible that progesterone acts on Leydig cells via a non- classical type of receptor, whose specific mode of action remains obscure as yet. The putative progesterone receptor in the Leydig cell is possibly located on the cell membrane, indicating a non- genomic or non-classical mode of progesterone action in this target cell as well.
In aging men, despite a significant elevation of mean serum LH concentrations80, the mean serum testosterone concentrations go down, indicating an increased LH insensitivity of Leydig cells or a reduction of the number of Leydig cells. Interestingly enough, incubation of Leydig cells with progesterone inhibits the expression of the promoter for the LH receptor gene75. In a detailed study, Pirke and co-workers81 reported that testosterone and its precursors decreased in the testicular tissue of old men. In contrast, progesterone and 17α-hydroxyprogesterone increased in relation to testosterone in the testicular tissue and in the spermatic vein of old men. Therefore, it is conceivable that a local increase in testicular progesterone concentration may have a detrimental effect on Leydig cell function75. A reduced expression of LH receptor may make the Leydig cells in aging testis refractory to LH action. Therefore, it would be logical to consider increasing testosterone secretion in older men with antiprogestins, which may ameliorate the action of increased progesterone in aging testes. This caused us to investigate the effects of progestins and antiprogestins on the testosterone secretion of isolated rat Leydig cells. It is well established that steroidogenesis in Leydig cells is regulated by LH/ human chorionic gonadotropin (hCG) via the second-messenger cAMP signal transduction pathway82. Therefore, we additionally used the intracellular cAMP levels as a marker of steroidogenesis. Initially, we observed a higher rate of progesterone production by isolated Leydig cells from aged Wistar rats, compared to young ones, with and without stimulation by 8 Ur-cAMP or hCG (Figure 2). This was compatible with the observations of Pirke and coworkers81 in aging human testis (see above). When cells were preincubated with the progestin antagonist mifepristone (RU 486), we found a dose-related increase in testosterone production (Table 1). After that, we investigated several other antiprogestins as well as progestins in this in vitro system. AU steroids had in common was that they showed ‘classical’, genomic agonistic or antagonistic activities. They had all been screened previously in the same pharmacological model, the so-called McPhail assay (transformation of the estrogen-primed rabbit endometrium). The findings regarding the influence on the steroid biosynthesis of Leydig cells were very heterogeneous, indicating that conventional screening and selection for ‘classical’ antiprogestins are not sufficient for specific manipulation of testosterone biosynthesis in Leydig cells. Sometimes, U-shaped dose- response curves were seen. All things considered, ‘classical’ screened progestins or antiprogestins have an insufficient influence on Leydig cell function. For the non-classical PR, there is probably a need for a new class of agonists or antagonists different from those which are known to act on the classical PR.
PROGESTERONE AND THE PROSTATE
It has been suggested that prostate cancer cells can survive in an androgen-deprived milieu by using estrogens for their own growth83. Since the PR is a widely accepted marker for a functional estrogen receptor pathway, the evidence of elevated PR concentrations in metastatic and androgen-insensitive adcnocarcinomas is considered as proof of a continuing steroid metabolism directed to estrogens. The observed increasing activity of 17β-hydroxysteroid dehydrogcnase type 7 (17HSD7) could lead to the increased intracellular production of 17β-estradiol during disease progression . In primary tumors, the PR was detectable in 36% of primary Gleason grade 3; 33% of primary Gleason grade 4, and in 58% of primary Gleason grade 5 tumors. None of the 41 primary tumors investigated revealed a significant PR expression in more than 50% of tumor cells. Conversely, moderate to strong receptor expression was observed in 60% of metastatic lesions. Irrespective of grades and stages, the presence of the PR was invariably associated with high steady-state levels of ERα mRNA85.
Kumar and colleagues86 found that cytosolic PR was detectable in all cases of benign prostatic hypertrophy (BPH) as well as in all cases of prostatic carcinoma. In contrast to this, three other groups observed a higher expression of PR in BPH than in prostate cancer87-89. This raises the question whether progesterone or PR may play an independent role in the etiology of BPH or prostate cancer. Chlormadinone acetate (CMA) and its derivatives cyproterone acetate and TZP-4238 (Osaterone) are steroidal progestins with strong antiandrogenic properties, which are sometimes used in the medical management of human BPH or prostatic carcinoma. The atrophic effects on human BPH have been reported by several authors. In the prostate cell, these progestim are reported to inhibit the cellular uptake of testosterone and the binding of androgen to its receptor, and to decrease the level of androgen receptors (ARs). The anti-androgenic mechanism has been evaluated biochemically, for example, on the basis of the AR content and the steroid 5a-reductase activity in the prostate. The reduction of volume of BPH may be due to shrinkage of both glandular and stromal compartments in the prostate tissue90- 95.
Figure 2 Progesterone secretion by isolated Leydig cells from young and old Wistar rats with or without stimulation by 8 Br-cAMP or hCG. Note the increased progesterone secretion from Leydig cells of aged rats
Table 1 The influence of different progestins administered postnatally on sexual behavior of male Wistar rats. Daily administration of 0.5 mg progestin from day 1 to day 14. Apomorphine (A) was given 10 min prior to mating period (0.1 mg/kg b.w. subcutaneously)
On the other hand, Tassinari and colleagues observed rapid progression of advanced ‘hormone-resistant’ prostate cancer during palliative treatment with progestins for cancer cachexia. This brings up the question of the sense of antiprogestin treatment of prostate disorders. In vitro, pretreatment with the antiprogestin mifepristone overcomes the resistance of prostate cancer cells to tumor necrosis factor α-related apoptosis-inducing ligand (TRAIL)97. Mifepristone is a potent antiandrogen with minimal androgen agonistic activity. Compared with other known antiandrogens, mifepristone is a very strong inducer of the interaction between androgen receptor and its co-repressors NCoR and SMRT, and, therefore, could be used as a selective receptor modulator. In view of the unique molecular, pharmacological profile of this antiprogestin, a phase II trial of mifepristone for treatment of progressive prostate cancer seems justified98.
The interrelations between progesterone, PK, and several tumors within the CNS have been of interest for a long time. Progesterone may be involved in the regulation of the growth and development of neurogenic tumors via PR, especially in the inhibition of tumor proliferation via PR-A. Whereas PR-A and PR-B were expressed in equal amounts in meningiomas, in astrocytic tumors and Schwannomas, PR-B was the predo\minant isoform compared with PR-A. Additionally, there was a statistically significant inverse correlation between PR- A and the proliferation index in meningiomas and astrocytic tumors99,100. ER expression is lost or reduced in non-malignant meningiomas, whereas loss of PR expression is an indicator of increased apoptosis and early recurrence101. A case report indicated that prolonged therapy with the progestin megestrol acetate could promote the growth of benign intracranial meningioma102. Therefore, it is not a surprise that Eid and colleagues in 2002 announced a phase III clinical study with the antiprogestin mifepristone (RU486). Unfortunately, at present, nothing is published about the outcome of this study.
McLaughlin and Jacks103 found that the majority (75%) of neuroflbromas express PR, whereas only a minority (5%) express ER. Within neurofibromas, PR was expressed by non-neoplastic tumor- associated cells and not by neoplastic Schwann cells. The authors hypothesize that progesterone may play an important role in neurofibroma growth and suggest that antiprogestins may be useful in the treatment of this tumor.
17β-Hydroxysteroid dehydrogenase type 2 (17HSD type 2) is a member of the short-chain dehydrogenases/reductases (SDR) enzyme family. Substrate specificity for the enzyme shows that it efficiently converts 17β-estradiol, testosterone and 5α- dihydrotestosterone into their corresponding inactive 17-ketoforms, thereby decreasing the influence of sex steroids on various target tissues and organs. On the other hand, it also converts 20α- hydroxyprogesterone to active progesterone and is expressed in the surface and fbveolar epithelium of normal gastric mucosa and in the duodenum. Gender did not have an effect on epithelial expression, but 17HSD type 2 mRNA expression decreased with increasing age. Chronic gastritis was associated with decreased expression. Regenerating epithelium close to ulcers and active gastritis showed up-regulation. Type I intestinal metaplasia also showed up- regulation, while type HI metaplasia and gastric cancer showed decreased expression . On the other hand, no literature about PR expression in different compartments of the human gastrointestinal tract is available to date. Using bovine samples, PR mRNA was not abundant in the stomach and guts. Interestingly, under these conditions, PR seems not to be estrogcn-dependent105. Wu and colleagues found, in 122 male patients with gastric adenocarcinoma, that the serum progesterone levels were significantly higher than in the male control group (0.26 0.26 vs. 0.14 0.11 ng/ml). Patients with presurgical serum progesterone levels > 0.26 ng/ml survived for significantly shorter periods than those with levels ≤ 0.26 ng/ ml.
PROGESTERONE AND THE CENTRAL NERVOUS SYSTEM
In the 1980s, Baulieu and co-workers107 demonstrated that some steroids, such as the precursor of progesterone, pregnenolone (PREG), DHEA and their sulfates, are present in higher concentrations in the brain than in blood and arc synthesized de novo in the ClNS (e.g. by astrocytes, oligodendrocytes, neurons)108. Such steroids are now universally referred to as ncurosteroids. They act as modulators of several neurotransmitter receptors (γ- aminobutyric acid^sub A^, N-methyl-D-aspartate, and δ1 receptors), either as stimulators or inhibitors, and are involved in learning and memory performance. The biosynthesis of the neurosteroids in glial cells starts with cholesterol, which is first converted to PREG, progesterone, 5α-pregnane-3,20-dione (5αDH-PROG or 5α-DHP) and then to 3α-hydroxy-5α- pregnane-20-one (3α,5α-TH-PROG) or 3α,5α-THP or allopregnanolone109,110. 5α-Reductase type II, one of the two 5α-reductase isoforms, is thought to be a key enzyme in the generation of neuroactive steroids in the brain, particularly allopregnanolone. The gene expression of 5α-reductase type II in the brain is transcriptionally regulated by progesterone111. It could be that estrogens induce directly the de novo synthesis of progesterone in astrocytes112. In adult rats, neuroactive derivatives of progesterone (i.e. dihydroprogesterone, allopregnanolone) exert direct effects on adult neurogenesis, strongly affecting both neuroblasts and astrocytes of the subependymal layer113.
Steroids can influence neuronal function through ‘classical’ binding to cognate intracellular receptors, which may act as transcription factors in the regulation of gene expression. Receptors for gonadal steroids have been identified in several brain areas: amygdala, hippocampus, cortex, basal forebrain, cerebellum, locus ceruleus, mid-brain rafe nuclei, glial cells, pituitary gland, hypothalamus, and central gray matter114. These intracellular steroid hormone receptors have often been considered to be activated solely by cognate hormone. However, during the past decade, numerous studies have shown that the receptors can be also activated by neurotransmitters and intracellular signaling systems, through a process that does not require hormone115.
Neuroactive steroids not only modify neuronal physiology, but also intervene in the control of glial cell function110,116. In addition, certain neuroactive steroids modulate ligand-gated ion channels via non-genomic mechanisms. Especially distinct 3α- reduced metabolites of progesterone are potent positive allosteric modulators of γ-aminobutyric acid type A (GABA^sub A^) receptors. However, progesterone itself is also an allosteric agonist of the GABA^sub A^ receptor117 and, in addition to this, it may act as a functional antagonist at the 5-hydroxytryptamine type 3 (5-HT^sub 3^) receptor, a ligand-gated ion channel, or certain glutamate receptors. There is evidence that neurosteroids interact allosterically with ligand-gated ion channels at the receptor membrane interface. On the other hand, 3α-reduced neuroactive steroids, too, may regulate gene expression via the PR after intracellular oxidation into 5α-pregnane steroids. Animal studies have shown that progesterone is converted rapidly into GABAergic neuroactive steroids in vivo. Progesterone reduces locomotor activity in a dose-dependent fashion in male Wistar rats. Moreover, progesterone and 3α-reduced neuroactive steroids produce a benzodiazepine-like sleep EEG profile in rats and humans. In extremely low concentrations, sulfated neurosteroids, such as PREG sulfate, can regulate learning and memory. Femtomolar doses of PREG sulfate infused into the ventricles of mice could enhance memory In a presynaptic mode of action, PREG sulfate also increases the spontaneous glutamate release via the activation of a presynaptic G^sub i/o^_coupled δ receptor and an elevation in intracellular Ca^sup 2+^ levels119. In men receiving androgen ablation therapy for prostate cancer, treatment with the progestin medroxyprogesterone acetate (MPA) may be an effective and well- tolerated option for the alleviation of hot flushes120.
Animal studies have shown neuroprotective effects for progesterone, which protects, for example, against necrotic damage and behavioral abnormalities caused by traumatic brain injury, e.g. by increasing the activity of antioxidative catalase or by modifying the microtubule-associated protein-2 content109,121-124. In this context, progesterone and allopregnanolone inhibited cell death and cognitive deficits, including recovery of select behaviors after a contusion of the rat pre-frontal cortex125,126. Progesterone- mediated neuroprotection has also been reported in peripheral nerve and spinal cord injury127. Furthermore, inhibition of 3α- hydroxysteroidoxidoreductase (3α-HSOR) by the progestin medroxyprogestcronc acetate resulted in enhanced synaptic and extrasynaptic GABA^sub A^ receptor-mediated inhibition of neurotransmission in the dentate gyrus but not in the CA1 region in the hippocampus, also indicating a regionally dependent manner of neurosteroid action128.
Recent observations have indicated that both the central nervous and the peripheral nervous system are able to synthesize neurosteroids. After cryolesion of the male mouse sciatic nerve, blocking the local synthesis or action of progesterone impairs remyelination of the regenerating axons, whereas administration of progesterone to the lesion site promotes the formation of new myelin sheaths110,129. Neuroactive steroids are able to reduce aging- associated morphological abnormalities of myelin and aging- associated myelin fiber loss in the sciatic nerve130. Two important proteins are expressed by myelin of peripheral nerves, the glycoprotein (Po; controlled by progesterone via PR) and the peripheral myelin protein 22 (PMP-22; controlled by allopregnanolone via GABA^sub A^ receptor127,131-133). Systemic progesterone administration resulted in a partial reversal of the age-associated decline in CNS remyelination following toxin-induced demyelination in male rats134. On the other hand, the Charcot-Marie-Tooth disease (CMT-1A) – the most common inherited neuropathy – is associated with overexpression of PMP-22. In a transgenic model for CMT-1A with male rats, daily administration of progesterone elevated the steady- state levels of PMP-22 and myelin protein zero (Mpz) in the sciatic nerve, resulting in enhanced Schwann cell pathology and a more progressive clinical neuropathy. In contrast, administration of the antiprogestin mifepristone reduced overexpression of PMP-22 and improved the CMT phenotype135.
Neuroactive steroids affect a broad spectrum of behavioral functions through their unique molecular properties and may represent a new treatment strategy for neuropsychiatric disorders. The background is that both the genomic and the non-genomic effects of progesterone and reduced progesterone metabolites in the brain may contribute to the pathophysiology of psychiatric disorders and the mechanisms \of action of antidepressants.
In major depression, there is disequilibrium of 3α-reduced neuroactive steroids, which can be corrected by treatment with antidepressant drugs. Neuroactive steroids may further be involved in the treatment of depression and anxiety with antidepressants in patients during ethanol withdrawal. A deregulation in concentrations of the neurosteroids allopregnanolone and 3α,5α- tetrahydrodeoxycortico-sterone (3α,5α-TH DOC) has been found in depressed patients. Indirect genomic (allopregnanolone) and non-genomic (allopregnanolone and DHEA) mechanisms are involved in the neurosteroidogenic pathophysiology of depression. The neurosteroid levels in depressive patients normalize following treatment with selective serotonin uptake inhibitors. Additionally, studies in patients with panic disorder suggest that neuroactive steroids may also play a role in modulating human anxiety136-138.
Furthermore, blood concentrations of progesterone are significantly lower in catamenial epilepsy patients compared to non- epileptic controls. Over 60 years ago, Selye in 1942(139) demonstrated that progesterone protected animals against pentylenetetrazol-induced seizures. Cinza and colleagues140 showed that the protective effect of progesterone against kainic excitotoxicity in vivo in rats is also mediated by the 5α%- reduced metabolites of progesterone. Chronic slow spike-and-wave discharges (SSWDs) induced by the cholesterol synthesis inhibitor AY9944 in Long Evans rats were exacerbated by the administration of both progesterone and allopregnanolone. This effect was not blocked by mifepristone141.
Cyclic natural progesterone administration may lessen the seizure frequency in women with catamenial seizure exacerbation. Under clinical conditions, the progesterone-efficacy can be diminished by the concomitant administration of the 5α-reductase inhibitor finasteride, indicating thnt 5α-reduced metabolites rather than progesterone itself are responsible for improved seizure control. In contrast to convulsive epilepsy, progesterone seems to aggravate absence seizures142,143. Interestingly, the antiprogestin mifepristone failed to affect the electroconvulsive threshold or the efficacy of antiepileptic drugs in maximal electroshock in mice144.
Although progesterone is relatively well tolerated, certain hormonal side-effects, such as disturbances of the mineral balance due to the metabolism of progesterone to desoxycorticosterone or (perhaps) breast tenderness, may occur. The short half-life makes it inconvenient to administer to men. Neurosteroid analogs that do not mimic progesterone’s genomic actions and have improved pharmacokinetic properties may overcome these drawbacks (for a review see references138,145,146).
PROGESTERONE AND SLEEP
Intramuscular injection of 200 mg progesterone produces mild sedative-like effects in men and women147. A single oral administration of 300 mg micronized progesterone at 21.30 induced a significant increase in the amount of non-rapid eye movement (non- REM) sleep in nine healthy male volunteers. The EEG spectral power during non-REM sleep showed a significant decrease in the slow wave frequency range (0.4-4.3 Hz), whereas the spectral power in the higher frequency range (> 15 Hz) tended to be elevated. Some of the observed changes in the sleeping pattern and sleep-EEG power spectra are similar to those induced by agonistic modulators of the GABAA receptor complex and appear to be mediated in part by the conversion of progesterone into its GABA-active, 5α-reduced metabolites148. The oral administration of progesterone at the same dosage and at the same time (300 mg in the evening) produced no consistent effects on attention performance. Thus, dosages of progesterone that are sufficient to modulate sleep in men are not likely to exert sedative hangover effects149. It seems that only progesterone including its 5α-reduced metabolites is involved in positive sleep regulation, whereas, in contrast to this, the synthetic progestin megestrol acetate reduces REM sleep150.
PROGESTERONE AND SEXUAL FUNCTION
Administering various progestins, including progesterone, to male rats postnatally (critical hypothalamic differentiation phase), we found that some progestins (progesterone, levonorgestrel and dienogest) were able to reduce mating activities. In the case of levonorgestrel and dienogest, the additional application of apomorphine 10 min prior to the 30-min mating periods caused only a marginal improvement of sexual activity, indicating a more peripheral effect of inhibition of mounting behavior by progesterone and selected progestins. These experimental-pharmacological results also substantiate what we already know from a variety of clinical observations that a progestin is not a progestin. For example, the progestin with distinct antiandrogenic action, chlormadinone acetate, was unexpectedly ineffective in our model, indicating that the influence of progestins on the maturation of the hypothalamus is independent of given antiandrogenic effects (Table 1).
The antiandrogenically acting progestins MPA and CPA are widely used in Europe and in the USA for the treatment of deviant behavior of male sex offenders. Given orally in a high dosage or intramuscularly as weekly injections of 200-600 mg, the two progestins have been reported to reduce a variety of paraphilias, including pedophilia, incest, sadism and rape4,5. Interestingly, testosterone and sexual experience increase the levels of plasma membrane binding sites for progesterone in the male rat brain151. In this context, the down-regulation of sex hormone receptors, including PR, in the aging rat penile crura is associated with erectile dysfunction152.
PROGESTERONE AND THE RESPIRATORY SYSTEM
Hasselbach recognized progesterone’s potential role in the regulation of ventilation already in 1912(153,154), when he reported hyperventilation in pregnant women. Moreover, he found that women also hyperventilate during the luteal phase of the menstrual cycle. Consequently, these cyclic breathing variations disappear in postmenopausal women155. A direct effect of progesterone is suggested here, because the concentrations of progesterone in the rat lung are much higher than those of the progesterone metabolites; the P/P metabolites ratio is 6 : 1(156). PR-A is the predominant progesterone receptor isofomi in the rat lung, in an A : B ratio of 2 : 1(157). The classical PR is also present in the mouse fetal lung tissue and reveals distinct developmental profiles, with the highest expression during the prenatal period158.
Logically, synthetic progestins (MPA and chlormadinone acetate, CMA) have also been used for respiratory stimulation in men, mainly within the management of chronic obstructive pulmonary disease (COPD). Clinical studies have reported some improvement in blood gas levels and in number or duration of apneic and hypopneic events (for a review, see reference 159). A recent publication underlines the usefulness of another progestin, megestrol acetate (MGA), for selected patients with COPD160. Also, the combination of the carbonic anhydrase inhibitor acetazolamide with cither CMA or MPA seems to be effective for the treatment of COPD161,162.
Here, we will discuss briefly the renal action of progesterone, the influence of progestins on kidney, the cardiovascular system, some effects of progesterone on the adipose tissue metabolism, and, finally, the clinical use of progestins for stimulating weight gain in men.
The kidney is one of the sites in the body expressing progesterone receptors, as reported in various studies. The incubation of rabbit proximal tubules with progesterone had no influence on the Ca^sup 2+^ or Na+ transport by brush border membrane vesicles. By contrast, the hormone significantly increased the Ca^sup 2+^ and decreased the Na+ uptake by the distal tubule luminal membranes. These effects were significant following 1 min of incubation. Finally, 10^sup -11^ mol/l progesterone also enhanced the Ca^sup 2+^ uptake by distal tubules-membranes through a direct non-genomic mechanism163. In the same context, the group of W. Oelkers164 found enhanced downstream metabolism of progesterone in human kidney. This may be the mechanism responsible for the protection of the mineralocorticoid receptor (MR) from the antimineralocorticoid action of progesterone, by which water balance is maintained.
As far as we know, the influence of progesterone or progestins on the hemostatic system in men has been described in only one publication. A single intramuscular 200-mg dose of the depot- progestin norethindrone enanthate (NET-EN) alone to seven healthy white men, aged 28-38 years, led to a significant suppression of serum free and total testosterone and of serum 17β-estradiol on day 14 post injectionem. There was a marked shift in hemostatic parameters with increasing levels of Factor XHc, fibrinogen, antithrombin, F1 + 2, and plasmin-α2-antiplasmin complex (PAP), whereas levels of Factors VIIc and VIIa decreased165. The intravenous infusion of progesterone dilated mesenteric, renal, and iliac circulations in pigs. This dilatative effect on the arteries was inhibited by N-nitro-L-argmine methylester (NAME), indicating the involvement of NO-dependent mechanisms166. Plasma membrane- bound PR in vascular endothelial cells may regulate the non-genomic actions of progesterone167,168. Additionally, progesterone at physiological concentrations inhibits the cell proliferation in cultures of aortic smooth muscle cells 6 in a dose-dependent manner.
Progesterone acts as an antiglucocorticoid in adipose tissue in vivo. When progesterone was given concomitantly, the glucocorticoid effects of dexamethasone on adipose tissue mass, lipolytic activity, and lipolysis were blocked37. The expression of each adrenergic receptor (AdR) subtype gene is distinctly reg\ulated by sex hormones (naturally besides norepincphrine) in brown adipocytes. Testosterone- treated cells had lower lipolytic activity and increased expression of antilipolytic receptors α2A-AdR. Both 17β-estradiol and progesterone decreased α2A-AdR expression and α^sub 2A^/ β^sub 3^-AdR protein ratio, but progesterone had a higher potency than 17β-estradiol, increasing β-AdR levels, mainly β^sub 3^-AdR expression, and enhancing lipolysis stimulated by norepinephrine . The uncoupling protein 1 (UCP1) is the main mediator of brown adipose tissue (BAT). Progesterone stimulated in vitro the norepinephrine-stimulated UCP1 mRNA expression at very low concentrations (10^sup -9^ mol/l). Surprisingly, the antiprogestin and antiglucorticoid mifepristone (RU486) acted in this model as a progesterone agonist, strengthening the progesterone activity171. This observation possibly indicates that the effects of progesterone on adipose tissue are non- genomicauy mediated.
The synthetic progestin, megestrol acetate (MGA), is used clinically to treat a reduction in appetite and weight loss in AIDS and cancer patients and in elderly people who arc underweight172- 174. However, the composition of the body mass gained with MGA in AIDS and cancer patients has been shown to be predominantly or entirely fat. This may be due to the reduction in serum testosterone associated with MGA ingestion. Lambert and colleagues175 performed a randomized, controlled, clinical trial with 30 older men (body mass index
Finally, there are some unclear relationships between serum progesterone and certain physiological or pathological conditions. For example, the serum Mg concentration in young healthy men was directly and significantly related to the progesterone level, and the Ca^sup 2+^ /Mg^sup 2+^ ratio was inversely related to the serum progesterone level14. At the end, serum progesterone concentrations were elevated significantly in HIV-positive men at different stages of their disease177 In this context, there are positive correlations between serum ACTH and progesterone levels178.
Is progesterone the forgotten hormone in men? To answer this question, we have three possibilities to explain the physiological role of this steroid in male gender:
(1) Progesterone is a physiologically unimportant by-product in steroidogenesis;
(2) The expression and the function of the progesterone receptors are only the result of the action of estrogens;
(3) Progesterone plays a specific physiological and pathophysiological role in men with smart possibilities for new therapeutic approaches.
Naturally, depending on the given tissue or cell type or state of scientific clearing up, all three opportunities are applicable. Despite the relative broad knowledge about the progesterone actions in the male (reviewed in this paper), the exact physiological ranking of progesterone in comparison with other steroids and the therapeutic value of progestins and antiprogestins in the male for gender-specific approaches remains more or less unclear.
The situation is furthermore complicated by the fact that the obviously important progesterone-dependent conditions in males are mediated either by the uncommon PR isoform A (e.g. lung) or by membraneous progesterone-effects (Table 2). Both targets are not typical of the hitherto performed screening for selecting progestins or antiprogestins. Therefore, the precise pharmacological manipulation of progesterone actions in the male requires completely new molecular biological approaches. But this investment could be valuable because it seems reasonable to identify new compounds for male contraception, stimulation of endogenous testosterone biosynthesis in aged Leydig cells, prostate cancer and/or BPH, meningioma/fibroma, chronic obstructive pulmonary disease, weight loss and – last but not least – specific diseases of the central nervous system.
Progesterone – the forgotten hormone in men? This title is not quite correct for a publication at the beginning of the 21st century. We have to wait for the future with new pharmacological and clinical results, hopefully.
Table 2 Expected genomic and non-genoniic actions of progesterone in the male
This paper was presented at the 4th World Congress on The Aging Male, Prague, February 26-29, 2004. We thank Doris Hbler, Ulrike Schumacher and Vladinir Patchev from Jenaphann GmbH & Co. KG for wonderful cooperation, scientific input and suggestions.
1. Thornton JW, Need E, Crews D. Resurrecting the ancestral steroid receptor: ancient origin of estrogen signalling. Science 2003;301:1714-7
2. Thornton JW. Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions. PNAS 2001;98:5671-6
3. Mc Lachlan JA. Environmental signalling: what embryos and evolution teach us about endocrine disrupting chemicals. Endocr Rev 2001 ;22:319-41
4. Kravitz HM, Haywood TW, Kelly J, Liles S, Cavanaugh JL. Medroxyprogesteronc and paraphues: do testosterone levels matter? Bull Am Acad Psychiatry Law 1996;24:73-83
5. Zumpe D, Clancy AN, Michael RP. Progesterone decreases mating and estradiol uptake in preoptic areas of male monkeys. Physiol Bcliav 2001;74:603-12
6. El-Hemawy T, Huhtaniemi IT. Progesterone and testicular function. Aging Male 1999;2:240-5
7. Shackleton C, Malunowicz E. Apparent pregnene hydroxylation deficiency (APDH): seeking the parentage of an orphan metabolome. Steroids 2003;68:707-17
8. Nadjafi-Triebsch C, Huell M, Burki D, Rohr UD. Progesterone increase under DHEA-substitution in males. Maturitas 2003;45:231-5
9. Naville 1), Keeney DS. Jenkin G, Murry DA, Head JR, Mason JI. Regulation of expression of male-specific rat liver microsomal 3 beta-hydroxysteroid dcliydrogenase. Mol Endocrinol 1991;5: 1090-100
10. Liang JH, Sankai T, Yoshida T, Cho F, Yoshikawa Y. Localization of testosterone and 3beta-hydroxysteroid dehydrogenase/ delta5-delta4-isomerase in cynomolgus monkey (Macaco fnsciciilaris) testes. J Mcd Piimalol 1998;27:10-4
11. Pelletier G, Li S, Luu-The V, Tremblay Y, Belanger A, Labrie F. Immunoelectron microscopic localization of three key steroidogenic enzymes (cytochrome P450(scc), 3 beta-hydroxysteroid dehydrogenase and cytochrome P450(c17)) in rat adrenal cortex and gonads. J Endocrinol 2001 ;171:373-83
12. Burtis CA, Ashwood RE, eds. Tietz Textbook of Clinical Chemistry. Philadelphia: WB Saunders Co, 1999
13. Zumoff B, Miller L, Levin J, et al. Follicular-phase serum progesterone levels of nonsmoking women do not differ from the levels of nonsmoking men. Steroids 1990;55:557-9
14. Muneyyirci-Delale O, Dalloul M, Nacharaju VL, Altura BM, Altura BT. Serum ionized magnesium and calcium and sex hormones in healthy young men: importance of serum progesterone level. Fertil Steril 1999;72:817-22
15. Grschl M, Rauh M, Drr H-J. Orcadian rhythm of salivary cortisol, 17α-hydroxy progesterone, and progesterone in healthy children. Clin Chem 2003;49:1688-91
16. Genazzani AR, Petraglia F, Bernardi F, et al. Circulating levels of allopregnanolone in humans: gender, age, and endocrine influences. J Clin Endocrinol Metab 1998;83:2099-103
17. Belanger A, Candas B, Dupont A, et al (Changes in serum concentrations of conjugated and unconjugated steroids in 40-80- year-old men. J Clin Endocrinol Metab 1994;79:1086-90
18. Winkelmann BR, Mrz W, Boehm BO, et al. on behalf of the LURIC study group. Rationale and design of the LURIC study – a resource for functional genomics, pharmacogenomics and long-term prognosis of cardiovascular disease. Pharmacogenomics 2001;2(Suppl 1):S1-73
19. Martin RM, Lin CJ, Costa EMF, et al P450c17 deficiency in Brazilian patients: biochemical diagnosis through progesterone levels confirmed by CYP17 genotyping. J Clin Endocrinol Metab 2003;88:5739-6
20. McGowan EM, Weinberger RP, Graham JD, et ai Cytoskeletal responsiveness to progestins is dependent on progesterone receptor A levels. J Moke Endocrinol 2003;31:241-53
21. McGowan EM, Saad S, Bendall LJ, Bradstock KF, Clarke CL. Effect of progesterone receptor A predominance on breast cancer cell migration into bone marrow fibroblasts. Breast Cane Res Treatm 2004;83:211-20
22. Arnett-Mansfield RL, deFazo A, Wain GV, et al. Relative expression of progesterone receptors A and 13 in endometrioid cancers of the endometrium. Cancer Res 2001 ;61:4576-82
23. Conneely OM, Jericevic BM. Progesterone regulation of reproductive function through functionally distinct progesterone receptor isoforms. Rev Endocr Metab Disorders 2002;3:201-9
24. Takimoto GS, Tung L, Abdel-Hafiz H, et al. Functional properdes of the N-terminal region of progesterone receptors and their mechanistic relationship to structure. J Steroid Biochem Molec BiW 2003:85:209-19
25. Conneely OM, Mulac-Jericevic B, Lydon JP. Progesterone- dependent regulation of female reproductive activity by two distinct progesterone receptor isoforms. Steroids 2003;68:771-8
26. Haseroth K, Christ M, Falkenstein E, Wehling M. Aldosterone- and progesterone-membrane-binding proteins: new concepts of nongenomic steroid action. Curr Protein Peptide Sci 2000;1: 385-401
27. Falkenstein E, Heck M, Gerdes D, et al. Specific progesterone binding to a membrane protei\n and related nongenomic effects on Ca^sup 2+^ -fluxes in sperm. Endocrinology 1999;140:5999-6002
28. Falkenstein E, Tillmann H-C, Christ M, Feuring M, Wehling M. Multiple actions of steroid hormones – a focus on rapid, nongenomic effects. Pharmacol Rev 2000;52:513-55
29. Samadi A, Carlson CG, Gueorguiev A, Cenedella RJ. Rapid, non- genomic actions of progesterone and estradiol on steady-state calcium and resting calcium influx in lens epithelial cells. Pfgers Arch-Eur J Physiol 2002;444:700-9
30. Zhu Y, Rice CD, Pang Y, Pace M, Thomas P. Cloning expression, and characterization of a membrane progestin receptor and evidence it is an intermediary in meiotic maturation of fish oocytes. PNAS 2003;100:2231-6
31. Zhu Y, Bond J, Thomas P. Identification, classification, and partial characterization of genes in humans and other vertebrates homologous to a fish membrane progestin receptor. PAMS 2003;100:2237- 42
32. De Vivo I, Hankinson SE, Colditz GA, Hunter DJ. A functional polymorphism in the progesterone receptor gene is associated with an increase in breast cancer risk. Cancer Res 2003;63:5236-8
33. Beagley KW, Gockel CM. Regulation of innate and adaptive immunity by the female sex hormones oestradiol and progesterone. FEMS Immunol Mg J Microbiol 2003;38:13-22
34. Suzuki T, Murry BA, Darnel AD, Sasano H. Progesterone metabolism in human leukemic monoblast U937 cells. Endocr J 2()02;49:539-46
35. Sinars CR, Cheung-Flynn J, Rimerman RA, Scammell JG, Smith DF, Clardy J. Structure of the large FK506-binding protein FKBP51, an Hsp90-binding protein and a component of steroid receptor complexes. PNAS 2003; 100: 868-73
36. Hubler TR, Denny WB, Valentine DL, Cheung-Flynn J, Smith DF, Scamniell JG. The FK506-binding immunophilin FKBP51 is transcriptionally regulated by progestin and attenuates progestin responsiveness. Endocrinology 2003;2380-7
37. Pedersen SB, Kristensen K, Richelsen B. Anti-glucocorticoid effects of progesterone in vivo on rat adipose tissue metabolism. Steroids 2003;68: 543-50
38. Honer C, Nam K, Fink C, et al. Glucocorticoid receptor antagonism by cyproterone acetate and RU 486. Moke Pharmacol 2003;63:1012-20
39. Allolio B, Oremus M, Reincke M, et al. High-dose progesterone infusion in healthy males: evidence against antiglucocorticoid activity of progesterone. Eur J Endocrinol 1995; 133:696-700
40. Ishibashi H, Suzuki T, Suzuki S, et al. Sex steroid hormone recptors in human thymoma. J Clin Endocrinol Metab 2003;88:2309-17
41. Baldi E, Luconi M, Bonaccorsi L, Forti G. Signal transduction pathways in human spermatozoa. J Reprod Immunol 2002;53:121-31
42. Kirkman-Brown JC, Punt EL, Barratt CL, Publicover SJ. Zona pellucida and progester-one-induced Ca2+ signaling and acrosome reaction in human spermatozoa. J Androl 2002; 23:306-15
43. Thrien I, Manjunath P. Effect of progesterone on bovine sperm capacitation and acrosome reaction. Biol Reprod 2003;69:1408-15
44. Foresta C, Rossato M, Mioni R, Zorzi M. Progesterone induces capacitation in human spermatozoa. Andrologia 1992;24:33-5
45. Uhler ML, Leung A, Chan SY, Wang C. Direct effects of progesterone and antiprogesterone in human sperm hyperactivated motility and acrosome reaction. Fertil Steril 1992;58:1191-8
46. Emiliozi C, Cordonier H, Guerin JF, Ciapa B, Benchaib M, Fenichel P. Effects of progesterone on human spermatozoa prepared for in-vitro fertilization. Int. J Androl 1996;19:39-47
47. Osman RA, Andria ML, Jones AD, Meizel S. Steroid induced exocytosis: the human sperm acrosome reaction. Biochem Biopliys Res Commun 1989; 160:828-33
48. Meizel S, Turner KG. Progesterone acts at the plasma membrane of human sperm. Mol Cell Endocrinol 1991;77:R1-5
49. Lindahl PE. Effects of some steroid hormones on head-to-head association in bovine spermatozoa. Exp Cell Res 1978;111:73-81
50. Thomas F, Meizel S. Phosphatidylinositol-4,5-bisphosphate hydrolysis in human sperm stimulated with follicular fluid or progesterone is dependent upon Ca2 + influx. Biochem J 1989; 264:539- 40
51. Blackmore PF, Beebe SJ, Danforth DK, Alexander N. Progesterone and 17alpha-hydroxy-progesterone. Novel stimulators of calcium influx in human sperm. J Biol Chem 1990;265: 1376-80
52. Gonzlez-Martinz MT, Bonilla-Hernandez M, Guzman-Grenfell AM. Stimulation of voltage-dependent calcium channels during capacitation and by progesterone in human sperm. Arch Bioclicm Biophys 2002;408:205-10
53. Harper CV, Kirkman-Brown JC, Barrrat CLR, Publicover SJ. Encoding of progesterone stimulus intensity by intracellular [Ca^sup 2+^] ([^sup Ca2+^]i) in human spermatozoa. Biochem J 2003;372:4()7- 17
54. Blackmore PF, Neulen J, Lattanzio F, Beebe SJ. Cell surface- binding sites for progesterone mediated calcium uptake in human sperm. J Biol diem 1991;266:18655-9
55. Luconi M, Bonaccorsi L, Maggi M, et al. Identification and characterization of functional nongenomic progesterone receptors on human sperm membrane. J Clin Endocrinol Metab 1998; 83:877-85
56. Pictrobon EO, De Los Angeles Monclus, Alberdi AJ, Pomes MW. Progesterone receptor availability in mouse spermatozoa during epididyinal transit and capacitation: Ligand blot detec