ACTIVATION OF PROTEIN-KINASE-C INHIBITS CA-45 ACCUMULATION IN CULTURES OF OSTEOBLAST-LIKE CELLS - POSSIBLE INVOLVEMENT OF INSULIN-LIKE GROWTH FACTOR-I

In a previous report, we have demonstrated that exogenous insulin-like growth factor-I (IGF-1) stimulates Ca-45-accumulation into extracellular matrix in long-term cultures of osteoblast-like MC3T3-E1 cells and that Ca-45-accumulation occurs even in the cultures without exogenous IGF-I. In this study, effects of protein kinase C (PKC) on IGF-I secretion and Ca-45-accumulation into extracellular matrix were examined in 6-week cultured MC3T3-E1 cells. The MC3T3-E1 cells secreted IGF-I spontaneously. The PKC activator, 12-O-tetradecanoylphorbol-13-acetate (TPA) suppressed IGF-I secretion in a dose-dependent manner. 4alpha-Phorbol 12,13-didecanoate (4alpha-PDD), which is inactive for PKC, had little effect on the secretion. 1-Oleoyl-2-acetylglycerol, a specific activator for PKC, also suppressed the IGF-I secretion dose dependently. H-7, a PKC inhibitor, recovered the inhibitory effect of TPA. On the other hand, TPA inhibited the Ca-45-accumulation into extracellular matrix in cultures of these cells dose dependently, whereas 4alpha-PDD was ineffective in this capacity. The TPA-induced inhibition of Ca-45-accumulation was recovered almost to the control level by H-7. Exogenous IGF-I recovered the inhibitory effect of TPA on Ca-45-accumulation. In spite of the inhibitory effects of TPA as above, TPA had little effect on DNA synthesis in these cells. These results suggest that the activation of PKC inhibits calcification via suppression of IGF-I secretion in osteoblast-like cells.