Publications

Abstract

Ca2+ plays a complex and often conflicting role in the differentiation of committed pre- adipocytes into mature, fat laden adipocytes. Stim1 is a single pass transmembrane protein that has recently found to control the influx of Ca2+ ions through specific plasma membrane Ca2+ channels. Stim1 is a sensor of endoplasmic reticulum Ca2+ store content and when these stores are discharged following activation of receptor-mediated IP3 production, Stim1 clusters in puncta on the endoplasmic reticulum membrane where it induces Ca2+ influx through plasma membrane store-operated Ca2+ channels. Ca2+ entering through store-operated Ca2+ channels plays an important role in sustaining the activity of Ca2+-dependent enzymes, including the serine/threonine phosphatase calcineurin. In this study, we investigated the role of Stim1 in the regulation of adipocyte differentiation. Western blotting revealed that Stim1 was expressed at low levels in 3T3-L1 preadipocytes and was upregulated 4 days following induction of differentiation. We found that overexpression of STIM1 in 3T3-L1 preadipocytes potently inhibited the ability of the cells to undergo differentiation as measured by Oil Red O staining, and this was accompanied by an increase in receptor and store-operated Ca2+ entry. Overexpression of STIM1 did not appear to disrupt endogenous proliferation or the process of mitotic clonal expansion and subsequent growth arrest. Rather, overexpression of STIM1 was associated with reduced expression of C/EBP alpha and adiponectin. siRNA-mediated knockdown of endogenous Stim1 had the opposite effect to that of STIM1 overexpression, and increased 3T3-L1 differentiation under sub-optimal stimulation conditions. This was accompanied by an increase in C/EBP alpha and adiponectin expression. Our results suggest that Stim1 is a negative regulator of 3T3-L1 differentiation. It is likely that its increased expression during 3T3-L1 differentiation may act, through its ability to modify the level of Ca2+ influx through store-operated channels, to balance the level of differentiation in these cells in vitro.