Evidence for the participation of protein kinase C and 3′,5′-cyclic AMP-dependent protein kinase in the stimulation of muscle cell proliferation by 1,25-dihydroxy-vitamin D 3

Abstract

Treatment with 1,25-dihydroxy-vitamin D 3 (1,25(OH) 2D 3) (1–12 h, 10 −10 M) stimulates DNA synthesis in proliferating myoblasts, with an early response at 2–4 h of treatment followed by a maximal effect at 10 h. To investigate the mechanism involved in the mitogenic action of the hormone we studied the possible activation of intracellular messengers by 1,25(OH) 2D 3. The initial phase of stimulation of [ 3H]thymidine incorporation into DNA by the sterol was mimicked by the protein kinase C activator tetradecanoylphorbol acetate (TPA) in a manner which was dose dependent and specific as the inactive analog 4α-phorbol was without effect. Maximal responses to TPA (100 nM) were obtained at 4 h. Staurosporine, a protein kinase C inhibitor, blocked the effect of 1,25(OH) 2D 3 on myoblast proliferation at 4 h. In addition, a fast (1–5 min) elevation of diacylglycerol levels and membrane-associated protein kinase C activity was observed in response to 1,25(OH) 2D 3. The adenylate cyclase activator forskolin (20 μM) and dibutyryl-cAMP (50 μM) increased DNA synthesis reproducing the second 1,25(OH) 2D 3-dependent stimulatory phase at 10 h. Inhibitors of protein kinase A blocked the increase in muscle cell DNA synthesis induced by 1,25(OH) 2D 3 at 10 h. Significant increases in cyclic AMP levels were detected in myoblasts treated with the sterol for 1–10 h. The calcium channel antagonist nifedipine (5–10 μM) abolished both the effects of 4-h treatment with 1,25(OH) 2D 3 or TPA and 10-h treatment with 1,25(OH) 2D 3 or dibutyryl-cAMP. Similar to the calcium channel agonist Bay K8644, 1,25(OH) 2D 3 stimulated myoblast 45Ca uptake and its effects were blocked by nifedipine. Our results suggest that activation of calcium channels by phosphorylation via protein kinases C and A and is involved in the mitotic response of myoblasts to 1,25(OH) 2D 3.