c-myb affects intracellular calcium handling in vascular smooth muscle cells

The protooncogene c-myb is responsible for elevating intracellular calcium concentration ([Ca2+]i) at the G1/S interface in vascular smooth muscle cells (VSMC). However, the molecular components of this pathway are undefined, and the biological effects of increased levels of divalent cation are unknown. We have demonstrated that growth-arrested c-myb-transfected VSMC, compared with wild type VSMC, exhibit a fourfold increased number of insulin-like growth factor I (IGF-I) receptors, increased amount of secreted IGF-I activity, and a twofold increased level of [Ca2+]. The c-myb transfected cells, compared with wild type cells, also possess a twofold increased rate of calcium influx and a twofold decreased rate of calcium efflux. The elevated calcium influx rate of transfected cells is decreased to that of wild type cells with IGF-I neutralizing antibody, whereas the decreased calcium efflux rate of transfected cells is increased to that of wild type cells with antisense c-myb oligonucleotides. Proliferating wild type VSMC exhibit an increased calcium influx rate in late G1, which is dependent on production of augmented amounts of IGF-I activity but not increased levels of IGF-I receptors. The wild type VSMC also show a decreased calcium efflux rate at the same point in the cell cycle, which is dependent on expression of c-myb. The treatment of wild type cells with antisense c-myb or IGF-I receptor oligonucleotides induces a late G1 block in cell proliferation, which can be overcome by exposure to the calcium ionophore, 4-bromo-A-27318, in amounts sufficient to raise [Ca2+]i to levels observed at the G1/S interface. We conclude that IGF-I/IGF-I receptors and c-myb are involved in control of [Ca2+]i at the G1/S interface by separately regulating the rates of calcium influx and efflux and that elevated levels of divalent cation are necessary for progression of VSMC into the S phase of the cell cycle.