GLUCOCORTICOID REGULATION OF TRANSFORMING GROWTH FACTOR-BETA-1 ACTIVITY AND BINDING IN OSTEOBLAST-ENRICHED CULTURES FROM FETAL-RAT BONE

Transforming growth factor beta-(TGF-beta) enhances replication and bone matrix protein synthesis and associates with distinct binding sites in osteoblast-enriched cultures from fetal rat bone. In the organism high levels of or sustained exposure to glucocorticoids alters bone cell activity and decreases bone mass, effects that may be mediated in part by changes in local TGF-beta-actions in skeletal tissue. Preexposure of osteoblast-enriched cultures to 100 nM cortisol reduced the stimulatory effects of TGF-beta-1 on DNA and collagen synthesis by 40 to 50%. Binding studies showed that cortisol moderately enhanced total TGF-beta-1 binding, but chemical cross-linking and polyacrylamide gel electrophoretic analysis revealed an increase only within M(r) 250,000 (type III) TGF-beta-binding complexes, which are thought to represent extracellular TGF-beta storage sites. In contrast, a decrease in TGF-beta-1 binding was detected in M(r) 65,000 (type I) and 85,000 (type II) complexes, which have been implicated as signal-tranducing TGF-beta receptors. Our present studies therefore indicate that glucocorticoids can decrease the anabolic effects of TGF-beta-1 in bone, and these may occur in part by a redistribution of its binding toward extracellular matrix storage sites. Alterations of this sort could contribute to bone loss associated with glucocorticoid excess.