We investigated the possible involvement of the melastatin family protein TRPM7 in Ca
2+-mediated proliferative control of human retinoblastoma (RB) cells. The growth of RB cell was facilitated by elevating the extracellular Ca
2+ concentration with a parallel increase in the magnitude of spontaneous Ca
2+ influx. Under nystatin-perforated voltage-clamp, RB cells exhibited an outward-rectifying, spontaneous cation current (I
spont) having Ca
2+/Mg
2+-inhibited but -permeating properties. Various cation channel blockers inhibiting I
spont (Gd
3+, La
3+, LOE908, 2-APB) suppressed the spontaneous Ca
2+ influx and decelerated the growth of RB cells with similar efficacies. Excision of the RB cell membrane (inside-out) into MgATP-free solution induced a 70pS single channel activity, which was effectively inhibited by millimolar concentrations of Mg
2+ or MgATP. RT-PCR and immunocytochemical experiments revealed the expression of TRPM7 mRNA and protein in RB cells, and heterologous expression of TRPM7 in HEK293 cells reproduced the key features of I
spont. In contrast, elimination of this protein from RB cells by siRNA silencing markedly reduced I
spont density and the magnitude of spontaneous Ca
2+ influx, which was paralleled by decreased TRPM7 immunoreactivity, decelerated cell proliferation, and retarded G
1/S cell cycle progression. These results suggest a significant regulatory role of TRPM7 for RB cell proliferation as a spontaneously activated Ca
2+ influx pathway.
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