January 2004, Issue 1

The Intricacies of Bone Growth

Jinkun (Jake) Chen, DDS, PhD, studies the molecular and cellular basis of periodontal regeneration, bone formation, and bone metastasis of malignant tumors. He came to Tufts University School of Dental Medicine in 2002 to serve as director of the division of oral biology in the Department of General Dentistry. He is also a member of the faculty of the Sackler School of Graduate Biomedical Sciences (Program in Cell, Molecular, and Developmental Biology). Chen's laboratory, located on the sixth floor of the Dental School, has seven staff members, including technicians, research associates, postdoctoral fellows and graduate students. The group conducts research in three areas.

In the area of gene expression and regulation in mineralized tissue, the Chen lab is focusing on the roles of bone sialoprotein (BSP) and the transcription factor Cbfa1 (core binding factor). Cbfa1 is a master gene in osteogenesis — when the gene is knocked out, there is no bone formation. BSP is the major noncollagenous protein in bone and other mineralized tissue, and BSP is expressed during de novo bone formation and the initial stage of mineralization. To study how BSP promotes bone formation in vivo, the lab transplants tissue from BSP-overexpressing mice into severe combined immune deficient (SCID) mouse recipients.

The Chen lab's second area of study is the role of BSP in the metastasis of malignant tumors into bone, a project conducted in collaboration with Toshiyuki Yoneda, DDS, PhD, of the University of Texas Health Sciences Center at San Antonio and Osaka University in Japan. "BSP is found in malignant breast and prostate cancer cells that tend to invade bone," says Chen, "so it's a good marker for these tumors, and it also might be involved in bone metastasis. We haven't done clinical studies on breast cancer, but it has been shown that the BSP expression level is correlated with the stages of breast cancer — it's related to a higher tendency for the tumor to metastasize into bone, and it's even related to the patient's prognosis and lymph node metastasis. So BSP expression is related to the aggressiveness of these cells." To invade a secondary site, tumor cells must get into the blood system and then out of it again in order to get to the secondary site. BSP is involved in penetration of the blood vessels.

The lab's third area of research is cell differentiation in bone formation and periodontal regeneration. Periodontal disease is caused by resorption of bone and is a major problem in America and around the world. In their investigations into periodontal regeneration, Chen's lab has been collaborating with David Kaplan's tissue engineering laboratory at the School of Engineering. Says Chen of Kaplan, "he has a very nice system in which he uses silk as a scaffolding for bone formation. He has many in vitro studies done. But these materials have not been tested in animal models. We are performing a variety of in vivo studies — doing mouse surgery, transplantation and subsequent analysis (x-ray examination, histology, histomorphometry, immunohistochemistry, in situ hybridizations) to detect gene expression and evaluate the amount of bone formed."

Tissue engineering involves three major components. Chen explains: "Number one is a scaffold, which provides a shell, a matrix for the cell to grow into, and provides the durability. Also, the matrix can carry the second component, a growth factor. Something like a BMP, a bone morphogenetic protein, which can promote bone formation. Transcription factors (such as Cbfa1) aren't actually growth factors but are in that category in that they can promote the cell to differentiate and promote bone mineralization. The third element is the most important element — the cell. That's the engine in tissue engineering. Without the cell there can be no tissue formed." Chen is currently investigating all three factors (scaffold, growth factor, cell). He has been using type 1 collagen for the scaffold, as well as biodegradable silk proteins. To promote cell growth and differentiation, he soaks with a mix of factors such as BSP, Cbfa1, and BMP. The cells are adult human stem cells and mouse bone marrow cells. In a preliminary study the Chen lab has been able to induce bone formation in vivo.

For more information, go to http://www.tufts.edu/sackler/cmdb/chen-lab.htm.