April 21, 2014
Single Cell Type Found To Cause Most Invasive Bladder Cancers: Study
Lawrence LeBlond for redOrbit.com - Your Universe Online
Invasive bladder cancer (IBC), a malignant disease that currently affects more than 375,000 people worldwide, has been found to be caused by a single type of cell in the lining of the bladder, according to researchers with the Stanford University School of Medicine.
The researchers say this is the first study to pinpoint the normal cell type that can give rise to IBC. It is also the first study to show that most bladder cancers and their precancerous lesions arise from just one cell, which could also explain why many bladder cancers recur after therapy.
"We've learned that, at an intermediate stage during cancer progression, a single cancer stem cell and its progeny can quickly and completely replace the entire bladder lining," Philip Beachy, PhD, professor of biochemistry and of developmental biology, said in a statement. "All of these cells have already taken several steps along the path to becoming an aggressive tumor. Thus, even when invasive carcinomas are successfully removed through surgery, this corrupted lining remains in place and has a high probability of progression."
Beachy and colleagues found that while cancer stem cells and the precancerous lesions they form express an important signaling protein known as sonic hedgehog, the cells of subsequent invasive cancers invariably do not – a critical switch that appears vital for invasion and metastasis. This switch may explain certain confusing aspects of previous studies on the cellular origins of bladder cancer in humans. It also pinpoints a possible weak link in cancer progression that could be targeted by therapies.
"This could be a game changer in terms of therapeutic and diagnostic approaches," said Michael Hsieh, MD, PhD, assistant professor of urology and a co-author of the study. "Until now, it's not been clear whether bladder cancers arise as the result of cancerous mutations in many cells in the bladder lining as the result of ongoing exposure to toxins excreted in the urine, or if it's due instead to a defect in one cell or cell type. If we can better understand how bladder cancers begin and progress, we may be able to target the cancer stem cell, or to find molecular markers to enable earlier diagnosis and disease monitoring."
Bladder cancer is the fourth most common cancer in men and the ninth in women. There are two main types of bladder cancer: one that invades the muscle around the bladder and then metastasizes to other organs, and another that remains confined to the bladder lining. Unlike noninvasive cancers, most invasive bladder cancers are untreatable. Those that can be treated are expensive and difficult to cure, and with a high likelihood of recurrences, ongoing monitoring is required.
To determine what genes or cell types are at play in the formation of bladder cancer, the study team used a mouse model that closely mimicked what happens in humans. Usually, researchers rely on prior knowledge or guesses as to what genes are involved and often genetically alter cell types in animals to induce overexpression of a gene known to be involved in tumorigenesis or to block the expression of a gene that inhibits cancer development.
LINK TO SMOKING
Previous work by Beachy and his colleagues suggested that basal cells play a role in bladder cancer. However, the new study offered an unbiased approach.
Because smoking has been determined to be a big factor in the development of bladder cancer and cigarette smoke contains carcinogens known as nitrosamines, the team decided to put the chemical compound N-butyl-N-4-hydroxybutyl nitrosamine (BBN) in the drinking water of mice and watch the animals over a period of months.
BBN is a form of the nitrosamine chemical that is specifically activated in the bladder. After four months of the study, many of the mice had developed precancerous lesions in their bladders that had very closely resembled those in human patients. After six months, all of the animals had developed invasive bladder cancer.
Once they had the model, the team then conducted two experiments:
In the first experiment, Beachy and colleagues looked to see what would happen in mice exposed to BBN when the sonic hedgehog-expressing cells were marked with a distinctive fluorescent color.
In this experiment, the researchers found something surprising: after just a few months of BBN exposure, nearly the entire lining of the bladder was labeled with the fluorescent green marker indicating cells had arisen from the sonic hedgehog-expressing basal stem cells. When transplanted into other mice, those labeled cells were able to give rise to bladder cancers, but those cells not expressing the sonic hedgehog did not.
In the second experiment, the team used genetic techniques to selectively kill those same cells in animals prior to exposure with BBN. The team found no tumors had grown in these animals in which the stem cells had selectively been killed. However, the bladder architecture became severely compromised in the absence of stem cells to regenerate cells lost during the normal course of bladder function.
"So now we have two lines of evidence indicating that the bladder stem cells are solely responsible for tumorigenesis," Shin said. "When we mark the bladder stem cells, the tumors are also marked. When we remove, or ablate, the stem cells, no tumors arise after BBN treatment."
With these experiments under their belt, the team then tackled the question of whether bladder cancers arise as the result of genetic changes to one or more of these bladder stem cells. To determine this, they used a genetically engineered mouse with cells that fluoresce green, but which can be triggered to randomly fluoresce red, blue or yellow as well. This “rainbow mouse,” as it was called, allowed the team to more precisely determine the origin of groups of cells. If all cells in the tumor are red, for example, it is much more likely that they originated from a single cell.
"After four months of BBN treatment," Beachy said, "we'd most often see just one color dominating the entire epithelium. This clearly indicates that a single cell has taken over the lining of the entire bladder, elbowing out its neighbors in a way that's not been seen in other organs."
In subsequent studies, none of the cells in the most advanced, invasive carcinomas in the BBN-treated animals expressed sonic hedgehog, despite the fact that only sonic hedgehog-expressing cells are able to give rise to the earlier stages of bladder cancer. One obvious implication of the lack of sonic hedgehog expression in these cells is that the hedgehog pathway somehow inhibits steps required for tissue invasion or metastasis.
"We know that the hedgehog pathway is widely used throughout the animal kingdom to tightly regulate cellular and tissue differentiation," Hsieh said. "So its loss could make sense in this context because cancer is essentially a loss of normal regulation."
"One really important lesson from this study," Beachy said, "is the idea that, by the time you get to a full-blown tumor, the properties of the cells in that tumor may have changed quite significantly from the cell type that gives rise to the tumors. This can complicate understanding how human tumors arise, because even if you identify the tumor-propagating cells within a mature tumor, conclusions about the origins of a cancer based on properties of these cells may be inaccurate."
Their study is published in the April 20 issue of the journal Nature Cell Biology.