MECHANISM OF THE MITOGENIC EFFECT OF FLUORIDE ON OSTEOBLAST-LIKE CELLS - EVIDENCES FOR A G-PROTEIN-DEPENDENT TYROSINE PHOSPHORYLATION PROCESS

Recent results indicate that a fluoroalumino complex (AIFx) is probabl y the molecule responsible for the mitogenic effect of fluoride in MC3 T3-E1 osteoblast-like cells, Initial analysis suggested that a tyrosin e phosphorylation (tyr phos) process similar to that induced by thromb in and activation of the p42 MAP kinase (ERK 2) mediate this cellular response, In the present study, the signaling mechanism activated by A IFx was further investigated, The results indicated that AIFx dose-dep endently enhanced the tyr phos of the cell adhesion proteins FAK and p axillin, as well as of the adaptor molecules p46(shc), p52(shc), and p 66(shc) and their association with GRB2, Pretreatment of MC3T3-E1 cell s with cytochalasin D completely prevented FAK and paxillin tyr phos w ithout any alteration in the tyr phos of Shc proteins and activation o f ERK2 induced by AIFx, This observation suggests that in confluent MC 3T3-E1 cells, there is no link between the activation of FAK induced b y AIFx and the stimulation of ERK2, Pretreatment of the cells with per tussis toxin inhibited Shc phosphorylation, activation of ERK2, and ma rkedly reduced cell replication induced by AIFx, This toxin also signi ficantly reduced the stimulation of Pi transport activity induced by A IFx in these cells, Alteration in tyr phos induced by AIFx was not ass ociated with any detectable inhibition of tyrosine phosphatase activit y in MC3T3-E1 cell homogenates, suggesting that enhanced tyr phos indu ced by AIFx probably resulted from activation of a tyrosine kinase, In conclusion, the results of this study suggest that the mitogenic effe ct of fluoride in MC3T3-E1 osteoblast-like cells is mediated by the ac tivation of a pertussis toxin-sensitive Gi/o protein and suggest an im portant role for these heterotrimeric G proteins in controlling the gr owth and differentiation of bone-forming cells.