Involvement of TRPM7 in Cell Growth as a Spontaneously Activated Ca²⁺ Entry Pathway in Human Retinoblastoma Cells

Abstract

We investigated the possible involvement of the melastatin family protein TRPM7 in Ca²⁺-mediated proliferative control of human retinoblastoma (RB) cells. The growth of RB cell was facilitated by elevating the extracellular Ca²⁺ concentration with a parallel increase in the magnitude of spontaneous Ca²⁺ influx. Under nystatin-perforated voltage-clamp, RB cells exhibited an outward-rectifying, spontaneous cation current (I_spont) having Ca²⁺/Mg²⁺-inhibited but -permeating properties. Various cation channel blockers inhibiting I_spont (Gd³⁺, La³⁺, LOE908, 2-APB) suppressed the spontaneous Ca²⁺ 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 Mg²⁺ 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 Ca²⁺ influx, which was paralleled by decreased TRPM7 immunoreactivity, decelerated cell proliferation, and retarded G₁/S cell cycle progression. These results suggest a significant regulatory role of TRPM7 for RB cell proliferation as a spontaneously activated Ca²⁺ influx pathway.