Annexin A6-mediated tumor suppression requires inhibition of calcium influx and down-regulation of the calcium-activated RasGRF2

Membrane association of the tumor suppressor, annexin A6 (AnxA6), has been shown to regulate plasma membrane permeability to extracellular Ca2+, inhibit anchorage-independent tumor cell growth and paradoxically, promote tumor cell motility by mechanisms that remains poorly understood. Here, we identified RasGRF2, a Ca2+-activated Ras-specific guanine nucleotide exchange factor, as a major effector of AnxA6-elicited tumor-associated phenotypes in breast cancer cells. We demonstrate that reduced expression or loss of AnxA6 in breast cancer cells is associated with up-regulation of RasGRF2, increased Ras activity and consequently, early onset and rapid growth of xenograft tumors. Meanwhile, up-regulation of AnxA6 in AnxA6-low breast cancer cells is associated with a decrease in Ras activity and growth of xenograft tumors but on the contrary, an increase in Cdc42 activity and EGF-stimulated cell motility. Inhibition of Ca2+ influx into AnxA6-low breast cancer cells via Ni2+-sensitive or non-selective Ca2+ channels dose-dependently suppressed the expression of RasGRF2, induced the expression of AnxA6 and consequently, inhibited tumor cell proliferation. Finally, aberrant expression of RasGRF2 may be triggered by AnxA6-mediated or pharmacological inhibition of non-selective Ca2+ channels and occurs at least in part, by promoter methylation. These data for the first time identify RasGRF2 as a major effector of AnxA6-dependent breast tumor growth and motility and that regulated Ca2+ influx and/or RasGRF2 may be potential therapeutic targets for rapidly growing AnxA6-low breast carcinomas

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