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 expre ssion of insulin-like growth factor I (IGF-I), IGF-I receptor level, a nd IGF binding proteins (IGFBPs) in murine clonal osteoblastic MC3T3-E 1 cells. 12-O-Tetradecanoylphorbol-l 3-acetate (TPA), an activator of protein kinase C, decreased the secretion of immunoreactive IGF-I into the medium, whereas dibutyryl cAMP (Bt(2)cAMP) augmented the secretio n. In contrast, TPA increased the level of type I IGF receptor on the cells. Furthermore, MC3T3-E1 cells produced and secreted at least thre e different IGFBPs with molecular masses of 24, 30, and 34kDa, and the 24-kDa IGFBP was predominant under normal conditions. However, TPA sp ecifically 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 immunorea ctive to anti-IGFBP-2 antiserum. The level of IGFBP-2 mRNA in the cell s was increased by TPA, indicating that the increase in IGFBP-2 secret ion results from the stimulation of IGFBP-2 production. In contrast, B t(2)cAMP affected neither IGF-I receptor number nor the IGFBP secretio n. These results indicate that the production of IGF-I and the express ion of IGF-I receptors and IGFBP-2 are up-regulated by the activation of adenylate cyclase and protein kinase C, respectively, in osteoblast ic MC3T3-E1 cells. (C) 1994 Wiley-Liss, Inc.