Nov. 9, 2001 ANN ARBOR, Mich. - Oxygen may be good for you, but it's not so great for your stem cells -- according to a new study by scientists at the University of Michigan Medical School. Too much oxygen can kill stem cells, slow growth and even trigger an alternate developmental pathway that converts pre-muscle stem cells into fat cells.
The U-M study, published in the November 2001 issue of the Journal of Cellular Physiology, shows that gene expression patterns change significantly when stem cells are exposed to varying amounts of oxygen, and that these changes alter the basic biologic function of stem cells. In addition to its scientific importance, the U-M study could have important clinical implications for treatment of obesity and diabetes.
"The more primitive the stem cell, the more sensitive it is to oxygen," says Marie Csete, M.D., Ph.D., an assistant professor of cell and developmental biology and an associate professor of anesthesiology in the U-M Medical School, who directed the study.
"We found that skeletal muscle satellite cells grew faster, lived longer and developed into muscle cells more consistently when cultured with the amount of oxygen found in their natural environment." In their natural environment in the body, stem cells never are exposed to the amount of oxygen they encounter in the typical biomedical laboratory.
Csete and colleagues compared growth rates and developmental patterns of stem cell lines and skeletal muscle satellite cells grown in a laboratory atmosphere of 20 percent oxygen to cells grown with the 2 percent to 6 percent oxygen levels found inside the body. Csete grows stem cell cultures in a custom-designed facility, which she can adjust to create an atmosphere with specific amounts of oxygen and other gases.
"The big surprise was that satellite cells isolated from muscle fibers often converted spontaneously to fat precursor cells, also called adipocytes, when cultured with 20 percent oxygen, especially for long periods of time," said Csete.
Csete's study focused on adult stem cells called satellite cells from muscle tissue in adult mice. Unlike embryonic stem cells, which are capable of transforming into any cell in the body, adult stem cells are limited to becoming just a few cell types. The satellite cells in Csete's study normally develop into muscle cells. They provide a continuous source of new cells to replace those damaged during daily wear-and-tear. Under abnormal conditions, however, Csete discovered they can morph into fat cells instead.
Csete suspects the abnormal behavior of cells grown with lots of oxygen may mimic the reaction of aging cells exposed to free radicals and oxidative stress. "It seems plausible that some clinical conditions, such as aging and diabetes, which involve lost muscle mass and increased amounts of fat, may be related to satellite cell adipogenesis or fat cell development," says Csete.
The toxic effects of oxygen may not be limited to just one type of precursor stem cell. In related experiments, Csete obtained the same results with progenitor cells from the central and peripheral nervous systems of adult mice.
The U-M study is available online (see citation and web address at the end of this release). Results include:
* Over one-third of 3T3 stem cell lines cultured at 20 percent oxygen contained a protein called PPARgamma, which signals stem cells to become fat cells instead of muscle. Only 12 percent of 3T3 cells expressed the protein when grown with 6 percent oxygen.
* After one week in culture, 0.77 adipocytes were identified in each mouse muscle fiber cultured in 20 percent oxygen, while only 0.27 adipocytes per fiber were found in stem cell cultures grown with 6 percent oxygen.
* After 48 hours in culture, 70 percent of muscle fibers cultured in 6 percent oxygen were alive versus 31 percent of those grown in 20 percent oxygen. After 10 days, 30 percent were still alive in the low-oxygen facility, while only five percent were alive in the traditional laboratory oxygen environment.
* During the five years she has been studying the effects of oxygen and other gases on stem cells, Csete has encountered more than her share of skepticism. "It was difficult initially to get people to even consider the idea that oxygen matters, because scientists have been culturing cells the same way for decades.
"These studies suggest that the high levels of oxygen used in routine cultivation of stem cells can alter their efficiency," adds Csete. "Our results indicate that the conditions under which approved human embryonic stem cell lines were cultured may have limited their potential in important ways."
In her current research, Csete is focusing on questions related to oxidative stress and muscle-fat conversion in aging. "Our preliminary results suggest that these stem cells are active players in normal or pathological remodeling of muscle tissue throughout life," she says. "The developmental path they take is very responsive to the oxidative stress around them."
The U-M study was supported by the Defense Advanced Research Projects Administration (DARPA) of the Department of Defense. U-M collaborators included Nicole Slawny, graduate student; and research associates Yuewang Wei, Ph.D., and Sheryl Korsnes. Other members of the research team from the California Institute of Technology included Jean Walikonis, research scientist; Barbara Wold, Ph.D., professor of biology; and John C. Doyle, Ph.D., professor of control and dynamical systems.
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