CYTOKINES AND GROWTH-FACTORS IN THE REGULATION OF BONE REMODELING

Osteoporosis and periodontal disease both represent examples of abnormal bone remodeling. As knowledge of the cellular and molecular events in the normal bone remodeling process has accumulated in the last decade, better understanding of the pathophysiology of bone loss associated with periodontal disease and with aging has occurred. This short review does not attempt to include all aspects of this topic but covers specific areas in which there have been recent advances. (1) Observations made in the last few years have indicated that a hierarchy of both receptor and nonreceptor tyrosine kinases may be involved in normal osteoclastic bone resorption and that certain members of these tyrosine kinase families may mediate cytokine effects. Studies in the op/op variant of murine osteopetrosis have shown that normal production of monocyte-macrophage colony-stimulating factor 1 (M-CSF, also called CSF-1) and activation of its receptor (the receptor tyrosine kinase c-fms) are required for normal osteoclast formation. (2) Studies in mice made deficient in nonreceptor tyrosine kinase by gene knockout have shown that expression of this nonreceptor tyrosine kinase is required for normal osteoclast action and ruffled border formation, although not for osteoclast formation. (3) Recent studies have shown that in addition to protaglandins of the E series, other arachidonic acid metabolites may be involved in normal and abnormal osteoclastic bone resorption. 5-Lipoxygenase metabolites, the leukotrienes, stimulate isolated osteoclasts to form resorption pits as well as cause osteoclastic bone resorption in organ cultures of neonatal mouse calvariae. These compounds, which are unstable in tissue culture media, are readily inhibitable by agents that inhibit 5-lipoxygenase enzymes. Since they accumulate in chronic inflammatory cells, they may be important in bone remodeling associated with periodontal disease. (4) Recently, a new growth regulatory factor for osteoclasts has been described. This factor, called the osteoclastpoietic factor (OPF), is likely akin to the colony-stimulating factors for the formed elements of the blood, since the osteoclast shares a common precursor with these cells. OPF enhances the differentiation of osteoclasts from committed precursors. Its role in normal and abnormal bone remodeling is still to be determined. (5) Recent studies using prolonged cultures of fetal rat calvarial cells have shown that members of the TGF-beta superfamily (TGF-beta and the BMPs) may work in concert to stimulate normal bone formation. In this in vitro model culture system, TGF-beta enhances cell proliferation but inhibits differentiated function. In contrast, BMP-2, a prototype for the BMP family, enhances differentiated function in these cells, including expression of osteocalcin, alkaline phosphatase, and the formation of bone nodules. For more comprehensive reviews of the role of cytokines in bone remodeling, the reader is referred to Mundy (1993), Manolagas and Jilka (1992), and Canalis et al. (1989).