Modulation of DNA synthesis in cultured muscle cells by 1,25-dihydroxyvitamin D-3

Biphasic effects of 1,25-dihydrovitamin D-3 on DNA synthesis were shown in primary cultured (24 h) chick embryo myoblasts exposed to physiological concentrations of the hormone. The sterol stimulated [³H]thymidine incorporation into DNA in proliferating myoblasts, e.g., at early stages of culture prior to cell fusion or in high serum-treated cells. The opposite effects were observed during the subsequent stage of myoblast differentiation in low-serum media. The mitogenic effect of 1,25-dihydroxyvitamin D-3 was correlated with an increase in c-myc mRNA and a decrease in c-fos mRNA levels, whereas its inhibitory action on DNA synthesis was accompanied by increased myofibrillar and microsomal protein synthesis and an elevation of creatine kinase activity, the latter suggesting a stimulation of muscle cell differentiation by the sterol. These data are in agreement with the results of previous morphological studies. Treatment of myoblasts with the calcium ionophore X-537 A or the phorbol ester TPA caused only a transient stimulation of [³H]thymidine incorporation into DNA, which occurred earlier than the response elicited by 1,25-dihydroxyvitamin D-3, suggesting that changes in intracellular Ca²⁺ and kinase C activity are not major mediators of the hormone effects. A similar temporal profile of changes in calmodulin mRNA levels as that of [³H]thymidine incorporation into DNA was observed after treatment of myoblasts with the sterol, in accordance with the role of calmodulin in the regulation of cell proliferation. 1,25-dihydroxyvitamin D-3 may play a function in embryonic muscle growth and differentiation.