Induction of specific proteins in cultured skeletal muscle cells by 1,25-dihydroxyvitamin D-3.

The presence in myoblasts of an intracellular receptor specific for 1,25-dihydroxyvitamin D-3 [1,25(OH)2D3) and 1,25(OH)2D3-dependent changes in myoblast Ca2+ transport and phospholipid metabolism which are suppressed by RNA and protein synthesis inhibitors have been shown. In agreement with these observations, incubation of chick embryo myoblasts, precultured for 24 h in a medium containing low levels of vitamin D-3 metabolites, with 1,25(OH)2D3 at conditions which induce maximum cell responses (10(-10) M, 24 h) markedly stimulated the incorporation of [3H]leucine into total cell proteins and this effect was abolished when sterol treatment was performed in the presence of cycloheximide or puromycin. To investigate whether 1,25(OH)2D3 selectively stimulates the de novo synthesis of muscle cell proteins, mixtures of myoblast proteins from control and sterol-treated cultures labelled with [14C]leucine and [3H]leucine, respectively, were separated by SDS-polyacrylamide gel electrophoresis and isoelectric focussing. Examination of 3H/14C ratios in gel fractions revealed that 1,25-(OH)2D3 stimulates the production of proteins of molecular masses (isoelectric points) of 9 kDa (4.1 and 8.5), 17 kDa (7.5), 30 kDa (7.2), 40 kDa (5.5), 55 kDa (4.5) and 100 kDa (8.6). Cell fractionation studies showed the following subcellular distribution: 9 kDa (85% cytosol, 15% microsomes); 17 and 100 kDa (100%, 1200 X g pellet); 30 kDa (65% cytosol, 35% mitochondria); 40 kDa (100% microsomes); 55 kDa (65% microsomes, 35% mitochondria). Marker enzyme data indicated that this distribution is not due to cross-contamination between fractions. Affinity chromatography of double-labelled myoblast proteins on an immobilized lectin showed that the 55 kDa protein contains carbohydrate. Labelling of myoblast proteins with 45CaCl2 after their separation on SDS-polyacrylamide gels showed in addition that the 1,25(OH)2D3-dependent proteins of 9, 17, 40 and 100 kDa are major Ca2+-binding components of the cells. Synthesis of these proteins may mediate the effects of the sterol on myoblast calcium metabolism.