DOWN-REGULATION OF GAP JUNCTIONAL INTERCELLULAR COMMUNICATION BETWEENOSTEOBLASTIC MC3T3-E1 CELLS BY BASIC FIBROBLAST GROWTH-FACTOR AND A PHORBOL ESTER (12-O-TETRADECANOYLPHORBOL-13-ACETATE)

To address the relation between osteoblast growth and cell-to-cell com munication, we examined the effects of basic fibroblast growth factor (bFGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA), both potent sti mulators of osteoblastic proliferation, on gap junctional intercellula r communication between osteoblastic MC3T3-E1 cells, The level of inte rcellular communication was estimated by a photobleaching method, TPA inhibited the degree of intercellular communication in two different t ime-dependent manners, The early (<1 h) inhibition by TPA was consiste nt with an increase in the phosphorylation of connexin 43 (Cx43), The later inhibition was caused by reduction in the total amount of Cx43 o n the plasma membrane, due to the decrease in the level of Cx43 transc ripts, These qualitative and quantitative modulations by TPA were inhi bited by a selective inhibitor of protein kinase C, GF109203X. bFGF al so attenuated the gap junctional intercellular communication, However, short exposure (<5 h) to bFGF did not affect the communication, The f act that the growth factor immediately stimulated the phosphorylation of Cx43 indicates that the phosphorylation site(s) affected by bFGF wa s not involved in the inhibition of communication. The decrease in the intercellular communication level was detected by the longer exposure (>8 h) to bFGF and paralleled the decline in the Cx-mRNA level, This inhibitory effect of bFGF was abolished by the addition of a tyrosine kinase inhibitor, herbimycin A, Thus, gap junctional intercellular com munication between osteoblasts was down-regulated by osteoblastic mito gens through different mechanisms of the modulation of Cx43.