STIM2β is a Ca2+ signaling modulator for the regulation of mitotic clonal expansion and PPARG2 transcription in adipogenesis

Abstract

Intracellular Ca2+ is crucial in the regulation of adipocyte lipid metabolism and adipogenesis. In this study, we aimed to investigate the regulation mechanism of intracellular Ca2+ levels ([Ca2+](i)) during adipocyte differentiation. We found that the expression of stromal interaction molecule 2 beta (STIM2 beta), which is the inhibitor of store-operated Ca2+ entry (SOCE), is upregulated throughout the differentiation process. Evaluation of [Ca2+](i) in 3 T3-L1 and primary stromal vascular fraction (SVF) cells revealed that the basal Ca2+ level is downregulated after differentiation. Knockout (KO) of STIM2 beta in 3T3-L1 and primary SVF cells showed increased [Ca2+](i), indicating the involvement of STIM2 beta in the regulation of [Ca2+](i) during adipogenesis. We further evaluated the function of STIM2 beta-mediated [Ca2+](i) in early and terminal differentiation of adipogenesis. Analysis of cell proliferation rate during mitotic clonal expansion (MCE) in wild-type and STIM2 beta KO 3T3-L1 cell lines revealed that a larger population of KO cells underwent G1 arrest, suggesting that reduced [Ca2+](i) by STIM2 beta induces MCE. Additionally, ablation of STIM2 beta increased differentiation efficiency, with more lipid accumulation and rapid transcriptional activation of adipogenic genes, especially proliferator-activator receptor gamma 2 (PPARG2). We found that PPARG2 transcription is regulated by store-operated calcium entry (SOCE) downstream transcription factors, confirming that increased [Ca2+](i) by STIM2 beta ablation promotes PPARG2 transcription during adipogenesis. Additionally, STIM2 beta KO mice showed hypertrophic adipose tissue development. Our data suggest that STIM2 beta-mediated [Ca2+](i) plays a pivotal role in the regulation of mitotic clonal expansion and PPARG2 gene activation and provides evidence that MCE is not a prerequisite process for terminal differentiation during adipogenesis.