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Stimulatory effect of a phorbol ester on expression of insulin‐like growth factor (IGF) binding protein‐2 and level of IGF‐I receptors in mouse osteoblastic MC3T3‐E1 cells

We examined the relationship between signal transduction and the expression of insulin‐like growth factor I (IGF‐I), IGF‐I receptor level, and IGF binding proteins (IGFBPs) in murine clonal osteoblastic MC3T3‐E1 cells. 12–O‐Tetradecanoylphorbol‐13‐acetate (TPA), an activator of protein kinase C, decreased the secretion of immunoreactive IGF‐I into the medium, whereas dibutyryl cAMP (Bt2cAMP) augmented the secretion In contrast, TPA increased the level of type IIGF receptor on the cells. Furthermore, MC3T3‐E1 cells produced and secreted at least three different IGFBPs with molecular masses of 24, 30, and 34 kDa, and the 24‐kDa IGFBP was predominant under normal conditions. However, TPA specifically increased the secretion of the 34‐kDa IGFBP. The N‐terminal amino acid sequence of the purified 34‐kDa IGFBP was nearly identical with that of rat IGFBP‐2. Furthermore, the 34‐kDa IGFBP was immunoreactive to anti‐IGFBP‐2 antiserum. The level of IGFBP‐2 mRNA in the cells was increased by TPA, indicating that the increase in IGFBP‐2 secretion results from the stimulation of IGFBP‐2 production. In contrast, Bt2cAMP affected neither IGF‐l receptor number nor the IGFBP secretion. These results indicate that the production of IGF‐l and the expression of IGF‐l receptors and IGFBP‐2 are up‐regulated by the activation of adenylate cyclase and protein kinase C, respectively, in osteoblastic MC3T3‐E1 cells. © 1994 Willey‐Liss, Inc.