Potent induction of chondrocytic differentiation of human adipose-derived adult stem cells by bone morphogenetic protein 6

A novel growth factor significantly improves the ability of specialized stem cells derived from human fat to be transformed into cartilage cells, according to Duke University Medical Center and Pratt School of Engineering researchers.

Such growth factors are crucial to the bioengineering of tissues for clinical use in humans, the researchers said, because cells would need to be grown quickly and in large numbers in order to be practical. For the current study, as well as for past experiments in this area, the Duke team isolated the specialized cells, known as human adipose-derived adult stem cells (hADAS), from fat obtained during liposuction procedures, and then exposed the cells to a cocktail of various growth factors in order to stimulate their transformation into cartilage cells.

The growth factor that the Duke team used in hADAS cells for the first time is called bone morphogenetic protein 6 (BMP-6), a naturally occurring protein that is involved in hardening, or ossifying, the soft ends of long bones that come into contact with cartilage.

The researchers found that BMP-6 significantly increased the production of two important biochemical markers of cartilage cell proliferation. Specifically, hADAS cells treated with BMP-6 increased by 205 times the expression of aggrecan, a component of articular cartilage, and they increased by 38 times the production of a type of collagen uniquely present in cartilage, compared with cells without BMP-6 in the cocktail.

"Our studies suggest that growing hADAS cells with BMP-6 could provide tissue that could be used to repair damaged cartilage," said Bradley Estes, a graduate student in Pratt's Department of Bioengineering and lead author of a paper published in the April 2006 issue of the journal Arthritis and Rheumatism. The team's research was supported by the National Institutes of Health.

"One potential approach would be to take cells from a patient and then treat and grow the cells outside the body to create a tissue that could be reimplanted into the joint," Estes said. "Another strategy would be to use genetic engineering techniques to insert the gene for the production of BMP-6 into hADAS cells and then inject these cells into the site of damage."

Cartilage damage is difficult to treat, the researchers said, because the tissue lacks a supply of blood, nerve and lymph and has limited capacity for repair. Current strategies for treating cartilage damage, such as microfracture surgery or cartilage transplants, have been largely disappointing, they said.

However, over the past five years, Duke researchers under the direction of Farshid Guilak, Ph.D., director of orthopedic research, have been investigating novel approaches to treating cartilage damage. In their experimental system, the team exposes hADAS cells to different cocktails of nutrients, vitamins and growth factors. This chemical reprogramming forces the stem cells to progress along different paths, whether to bone, cartilage or nerve cells.

In their latest experiments, the researchers added BMP-6 to the cocktail in which hADAS cells were grown in tiny spheres of a complex carbohydrate known as alginate. The three-dimensional scaffold provided by the alginate spheres promotes differentiation of treated hADAS cells into cartilage tissue.

Interestingly, the Duke team also found that hADAS cells comprise a distinct lineage of stem cells.

Other researchers have found that a certain type of stem cell, called mesenchymal stem cells, which come from bone marrow, also can be transformed into cartilage cells when exposed to the right cocktail of growth factors. But that is where their similarity with hADAS cells ends, according to the Duke team.

"While the treatment of mesenchymal stem cells with BMP-6 tends to stimulate the transformation into bone cells, the treatment of hADAS cells with BMP-6 stimulates cartilage cell growth, as well as the blockage of bone cell growth," Estes said. "This shows that the hADAS cells we use are very different from the mesenchymal stem cells. They may look alike, but they act quite differently."

Based on their current tests, the researchers are confident that hADAS cells already have demonstrated the potential to serve as a readily available source for creating new cells and tissues to treat cartilage damage. Moreover, the researchers said, evidence suggests that the addition of other growth factors and in differing combinations could generate an even more robust response in the cells that would increase their utility even more.

"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 could 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."

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