TMEM97 facilitates the activation of SOCE by downregulating the association of cholesterol to Orai1 in MDA-MB-231 cells

Abstract

The expression of TMEM97, a regulator of cholesterol transport, has been reported to be enhanced in some tumour cells. We have recently shown that TMEM97 is involved in the proliferation of the breast cancer cell line MDA-MB-231, probably through changes in store-operated calcium entry (SOCE). By using silencing and overexpression of TMEM97 in MDA-MB-231 cells (two manoeuvres that either reduce or increase the calcium influx, respectively), we show enhanced cholesterol uptake in these cells as compared to the non-tumoral breast cell line, MCF10A. The enhanced cholesterol uptake in MDA-MB-231 cells was inhibited by silencing TMEM97, while overexpression of this protein increased cholesterol uptake in MCF10A cells and, therefore, indicating that this protein plays a role in the enhanced cholesterol uptake in MDA-MB-231 cancer cell line. TMEM97 silencing and overexpression resulted in an increase and decrease in the association of cholesterol to the SOCE calcium channel Orai1, respectively. Interestingly, silencing of TMEM97 in MDA-MB-231 cells significantly reduced the co-localization of Orai1 with the SOCE regulatory protein STIM1. Finally, neither silencing nor overexpression of TMEM97 altered SOCE in MDA-MB-231 cells transfected with the cholesterol insensible mutant of Orai1(Y80E). Our results reveal a novel regulatory mechanism of SOCE that relies on TMEM97 activity that courses through the reduction of the cholesterol content in the plasma membrane, and subsequently, by impairing its interaction with Orai1.