Evidence that fluoride-stimulated 3[H]-thymidine incorporation in embryonic chick calvarial cell cultures is dependent on the presence of a bone cell mitogen, sensitive to changes in the phosphate concentration, and modulated by systemic skeletal effectors

Abstract

In previous studies we have shown that clinically effective concentrations of fluoride (5 to 30 μmol/L) could also have direct effects in vitro on skeletal tissues to increase embryonic chick bone formation and bone cell proliferation ( 3[H]-thymidine incorporation into DNA). From these observations, we hypothesized that fluoride-stimulated bone formation might be mediated by a direct effect of fluoride to increase bone cell proliferation. The current studies were intended to investigate the mechanism of fluoride-stimulated 3[H]-thymidine incorporation, in chick calvarial cell cultures, by assessing mitogenic interactions between fluoride and inorganic phosphate, bone-derived growth factors, and systemic skeletal effectors. With respect to fluoride-phosphate interactions, the results of our studies indicate that the effect of fluoride was dependent on the phosphate concentration in the medium. Fluoride did not increase 3[H]-thymidine incorporation in BGJ b medium containing 1 mmol/L (total) phosphate; but, in 1.6 mmol/L phosphate medium, fluoride caused a dose-dependent increase in 3[H]-thymidine incorporation, between 1 and 20 μmol/L ( P<.001). The action of fluoride was also dependent on the presence of a bone cell mitogen. Fluoride increased 3[H]-thymidine incorporation when added to calvarial cell cultures in the cell-conditioned medium, but had no effect in unconditioned (ie, fresh) medium. The action of fluoride could be restored by adding an exogenous growth factor (ie, concentrated cell-conditioned medium, bone-derived growth factors, or a systemic bone cell mitogen) to the unconditioned culture medium, P<.05 for each effector. All of the systemic bone cell mitogens we tested (insulin, IGF-1, PTH, and calcitonin) increased the maximum response to fluoride ( P <.05), and, conversely, fluoride increased the maximum response to each of these effectors. The activations were noncompetitive. (1,25-dihydroxy-vitamin D decreased 3[H]-thymidine incorporation, with or without added fluoride.) Finally, our data showed that the activity of fluoride was dependent on the presence of mitogen-responsive osteoprogenitor cells. Using calvarial cell cultures enriched for differentiated osteoblasts and for osteoprogenitor cells, we found an inverse correlation between flouride-stimulated 3[H]-thymidine incorporation and alkaline phosphatase activity per milligram of cell protein ( r = −0.88, P < .05). Together, these data demonstrate that fluoride-stimulated 3[H]-thymidine incorporation can be modulated by systemic skeletal effectors, is dependent on the phosphate concentration in the medium, and requires both a bone cell mitogen and mitogen-responsive osteoprogenitor cells. With regard to mechanism, our data indicate that fluoride increases bone cell proliferation by enhancing the activity of bone cell mitogens.