Phosphate-induced ORAI1 expression and store-operated Ca2+ entry in aortic smooth muscle cells

Compromised renal phosphate elimination in chronic kidney disease (CKD) leads to hyperphosphatemia, which in turn triggers osteo-/chondrogenic signaling in vascular smooth muscle cells (VSMCs) and vascular calcification. Osteo-/chondrogenic transdifferentiation of VSMCs leads to upregulation of the transcription factors MSX2, CBFA1, and SOX9 as well as tissue-nonspecific alkaline phosphatase (ALPL) which fosters calcification by degrading the calcification inhibitor pyrophosphate. Osteo-/chondrogenic signaling in VSMCs involves the serum- and glucocorticoid-inducible kinase SGK1. As shown in other cell types, SGK1 is a powerful stimulator of ORAI1, a Ca2+-channel accomplishing store-operated Ca2+-entry (SOCE). ORAI1 is stimulated following intracellular store depletion by the Ca2+ sensor STIM1. The present study explored whether phosphate regulates ORAI1 and/or STIM1 expression and, thus, SOCE in VSMCs. To this end, primary human aortic smooth muscle cells (HAoSMCs) were exposed to the phosphate donor beta-glycerophosphate. Transcript levels were estimated by qRT-PCR, protein abundance by western blotting, ALPL activity by colorimetry, calcification by alizarin red S staining, cytosolic Ca2+-concentration ([Ca2+](i)) by Fura-2-fluorescence, and SOCE from increase of [Ca2+](i) following re-addition of extracellular Ca2+ after store depletion with thapsigargin. As a result, beta-glycerophosphate treatment increased ORAI1 and STIM1 transcript levels and protein abundance as well as SOCE in HAoSMCs. Additional treatment with ORAI1 inhibitor MRS1845 or SGK1 inhibitor GSK650394 virtually disrupted the effects of beta-glycerophosphate on SOCE. Moreover, the beta-glycerophosphate-induced MSX2, CBFA1, SOX9, and ALPL mRNA expression and activity in HAoSMCs were suppressed in the presence of the ORAI1 inhibitor and upon ORAI1 silencing. In conclusion, enhanced phosphate upregulates ORAI1 and STIM1 expression and store-operated Ca2+-entry, which participate in the orchestration of osteo-/chondrogenic signaling of VSMCs. Key messages center dot In aortic SMC, phosphate donor ss-glycerophosphate upregulates Ca2+ channel ORAI1. center dot In aortic SMC, ss-glycerophosphate upregulates ORAI1-activator STIM1. center dot In aortic SMC, ss-glycerophosphate upregulates store-operated Ca2+-entry (SOCE). center dot The effect of ss-glycerophosphate on SOCE is disrupted by ORAI1 inhibitor MRS1845. center dot Stimulation of osteogenic signaling is disrupted by MRS1845 and ORAI1 silencing.