Oxygen key switch in transforming adult stem cells from fat into cartilage

In their ongoing research on turning adult stem cells isolated from fat into cartilage, Duke University Medical Center researchers have demonstrated that the level of oxygen present during the transformation process is a key switch in stimulating the stem cells to change.

Their findings were presented today (Feb. 2, 2003) at the annual meeting of the Orthopedic Research Society.

Using a biochemical cocktail of steroids and growth factors, the researchers have “retrained” specific adult stem cells that would normally form the structure of fat into another type of cell known as a chondrocyte, or cartilage cell. During this process, if the cells were grown in the presence of “room air,” which is about 20 percent oxygen, the stem cells tended to proliferate; however, if the level of oxygen was reduced to 5 percent, the stem cells transformed into chondrocytes.

This finding is important, the researchers say, because this low oxygen level more closely simulates the natural conditions of cartilage, a type of connective tissue that cushions many joints throughout the body. However, since it is a tissue type poorly supplied by blood vessels, nerves and the lymphatic system, cartilage has a very limited capacity for repair when damaged. For this reason, the Duke investigators are searching for a bioengineering approach to correct cartilage injury.

“Our findings suggest that oxygen is a key determinant between proliferation and differentiation, and that hypoxia, or low oxygen levels, is an important switch that tells cells to stop proliferating and start differentiating,’ said David Wang, a fourth-year medical student at Duke, who presented the results of the Duke research.

Farshid Guilak, Ph.D., director of orthopedic research and senior member of the Duke team, said that the combination of growth factors sets the adult stem cells on the right path, while controlling oxygen levels inspires the cells to more readily transform into chondrocytes. Without the growth factors, he said, changing oxygen levels has no effect on the cells.

“For us, the ultimate goal is the development of a bioreactor where we can very carefully control the physical and chemical environment of these cells as they transform,” Guilak said. “The results of these experiments which demonstrated the role of oxygen levels in the process represent another important step in achieving this goal.”

Two years ago at the Orthopedic Research Society meeting, the Duke team for the first time reported that cartilage cells can be created from fat removed during liposuction procedures. Not only were the researchers able to make cells change from one type into another, they grew the new chondrocytes in a three-dimensional matrix, a crucial advance for success in treating humans with cartilage damage.

In their latest experiments, the team used the materials collected from liposuction procedures performed on multiple human donors. These materials were then treated with enzymes and centrifuged until cells known as adipose-derived stromal cells remained. These isolated cells were infused into three-dimensional beads made up of a substance known as alginate, a complex carbohydrate that is often used as the basis of bioabsorbable dressings, and then treated with the biochemical cocktail.

Those cells grown in hypoxic conditions saw growth inhibited by as much as 44 percent, but saw as much as an 80 percent increase in chondrocyte differentiation.

“No one has looked at the role of hypoxia in the creation of chondrocytes, but it made sense since cartilage normally exists in an hypoxic environment,” Wang said. “While we know oxygen plays a role, we don’t know the mechanism. The next questions to answer are how the cells sense the level of oxygen around them and then turn that into a metabolic change.”

The researchers anticipate that the first patients to benefit from this research would be those who have suffered some sort of cartilage damage due to injury or trauma. Farther down the line, they foresee a time when entire joints ravaged by osteoarthritis can be relined with bioengineered cartilage.

“We don’t currently have a satisfactory remedy for people who suffer a cartilage-damaging injury,” Guilak said. “There is a real need for a new approach to treating these injuries. We envision being able to remove a little bit of fat, and then grow customized, three-dimensional pieces of cartilage that would then be surgically implanted in the joint. One of the beauties of this system is that since the cells are from the same patients, there are no worries of adverse immune responses or disease transmission.”

The Duke researchers have developed several animal protocols to test how this cartilage fares in a living system.

The research was supported by the National Institutes of Health; Artecel Sciences, Inc., Durham, N.C.; the North Carolina Biotechnology Center, Research Triangle Park, N.C.; and the Kenan Institute for Engineering, Technology, and Science at North Carolina State University, Raleigh, N.C.

Joining Wang and Guilak in the research were Beverley Fermor, Ph.D., from Duke, and Jeff Gimble, M.D., from Artecel Sciences.