FLUORIDE AT MITOGENIC CONCENTRATIONS INCREASES THE STEADY-STATE PHOSPHOTYROSYL PHOSPHORYLATION LEVEL OF CELLULAR PROTEINS IN HUMAN BONE-CELLS

This study was designed to test the hypothesis that treatment of human bone cells with mitogenic concentrations of fluoride would lead to an increase in the steady state level of tyrosyl phosphorylation of spec ific cellular proteins. With an immunoblot assay method, it was found that mitogenic concentrations of fluoride (i.e. 50-200 mu mol/L) induc ed a dose- and time-dependent increase in the level of tyrosyl phospho rylation of at least 13 cellular proteins in both normal human bone ce lls and human TE85 osteosarcoma cells. Time-course studies revealed th at a statistically significant increase in tyrosyl phosphorylation of these 13 cellular proteins in human bone cells was observed after 3-6 h of fluoride treatment and was sustained for up to 24 h. This time co urse was not compatible with a direct activation of tyrosyl kinases, a s epidermal growth factor, which activates tyrosyl kinase activity, in duced an immediate and acute response that was rapidly reversible with in 1 h. Although fluoride increased the steady state tyrosyl phosphory lation of the cellular proteins in human bone cells, the same micromol ar doses of fluoride had no effect on human skin fibroblasts, which ar e fluoride-nonresponsive cells. The effects of fluoride were rapidly r eversible in the absence of fluoride and could be acutely potentiated by pretreatment with epidermal growth factor. In summary, we have show n for the first time that mitogenic concentrations (i.e. 50-200 mu mol /L) of fluoride increased the steady state level of tyrosyl phosphoryl ation of at least 13 cellular proteins in human bone cells, and that t he increases were relatively slow in onset and sustained. In conclusio n, these findings are consistent with the hypothesis that the osteogen ic actions of fluoride are mediated at least in part by an inhibition of the activity of one or more fluoride-sensitive phosphotyrosyl prote in phosphatases in human bone cells.