CONTROL OF TGF-BETA RECEPTOR EXPRESSION IN BONE

Michael Centrella, Changhua Ji, Thomas L. McCarthy

Department of Surgery, Plastic Surgery Section, Yale University School of Medicine, 333 Cedar Street, PO Box 208041, New Haven, CT 06520-8041,

Received 12/1/97 Accepted 12/5/97

7. EFFECTS OF TGF-BETA ON BONE CELL ACTIVITY

In vivo, local administration of TGF-beta increases bone formation (65-67). However, the molecular mechanisms of TGF-beta activity in bone have principally been determined by studies with primary and continuous cultures of bone cells that express varying degrees of the osteoblast phenotype. TGF-beta modestly enhances replication of fibroblasts and undifferentiated periosteal cells, but it is a potent mitogen in osteoblast-enriched cultures from fetal rat bone. The stimulatory effect of TGF-beta on DNA synthesis decreases at high TGF-beta concentrations and in cell cultures derived from more mature organisms, and it inhibits replication by certain osteosarcoma derived cells that are thought to represent highly differentiated osteoblasts. The mitogenic effect of TGF-beta therefore appears to be focused on cells that are first prominent at an intermediate stage of bone development. In order to allow effective skeletal tissue formation, cell populations like these must need to re-emerge during bone remodeling or fracture repair (20,21,68).

TGF-beta also alters the expression of many genes related directly to osteoblast activity. Consistent with its activity in many tissues, TGF-beta induces the synthesis of several matrix proteins by bone cells. Type I collagen comprises 90% of the organic matrix of bone and it is an essential element in skeletal calcification and structure. TGF-beta enhances type I collagen synthesis and the apposition of a collagen-containing matrix in cultures of isolated bone cells and in bone fragments (69-72). Studies in osteoblast-enriched cultures show that part of this effect is transcriptional, part is related to type I procollagen mRNA stability, and part derives from an increase in collagen secretion and its subsequent re-deposition to the cell layer (71). However, in cells prepared from human bone obtained after hip replacement surgery, TGF-beta only transiently increases type I procollagen mRNA and polypeptide levels, suggesting differences that could derive from disease state, age, or perhaps species variations (72). TGF-beta also induces the expression of other bone matrix components such as osteopontin and osteonectin. In contrast, it reduces the expression of a late stage osteoblast-derived protein, osteocalcin, in several osteoblast-like or osteosarcoma-derived cell cultures. Like the effect that occurs with most mitogenic factors, short term exposure to TGF-beta reduces the activity of alkaline phosphatase, a protein thought to be involved in matrix calcification, in fetal bone cells, but enhances its expression by some osteosarcoma and mature human bone cells (20,72). Furthermore, TGF-beta rapidly re-organizes its own receptor profile on osteoblasts (73). Overall, the response patterns that occur in osteoblasts derived from fetal, neonatal, mature and osteoporotic bone, and from osteosarcomas predict distinct changes in sensitivity to TGF-beta at specific stages of osteoblast differentiation.

Variations in TGF-beta activity could perhaps in part be related to one or another isform of TGF-beta. Indeed, in osteoblast-enriched cell cultures, TGF-beta3 is 3-10 fold more potent than TGF-beta1 or TGF-beta2. However, all three TGF-betas bind to the same three TGF-beta receptors, although with slightly different affinities, and in the end cause the same qualitative effects on osteoblast activity (74,75). Therefore, unlike the patterns that are found with endothelial cell cultures (41), dissimilarities in TGF-beta activity on bone cells do not seem to result from differences in the association of certain TGF-beta isoforms with only specific combinations of TGF-beta receptors.