Sigmar1 regulates endoplasmic reticulum stress-induced C/EBP-homologous protein expression in cardiomyocytes.

Shafiul Alam,Christopher B Pattillo,Manikandan Panchatcharam,Chowdhury S Abdullah,Sumitra Miriyala,James Traylor,A Wayne Orr,Md Shenuarin Bhuiyan,Richa Aishwarya

doi:10.1042/bsr20170898

Abstract

C/EBP-homologous protein (CHOP) is a ubiquitously expressed stress-inducible transcription factor robustly induced by maladaptive endoplasmic reticulum (ER) stresses in a wide variety of cells. Here, we examined a novel function of Sigma 1 receptor (Sigmar1) in regulating CHOP expression under ER stress in cardiomyocytes. We also defined Sigmar1-dependent activation of the adaptive ER-stress pathway in regulating CHOP expression. We used adenovirus-mediated Sigmar1 overexpression as well as Sigmar1 knockdown by siRNA in neonatal rat ventricular cardiomyocytes (NRCs); to induce ER stress, cardiomyocytes were treated with tunicamycin. Sigmar1-siRNA knockdown significantly increased the expression of CHOP and significantly induced cellular toxicity by sustained activation of ER stress in cardiomyocytes. Sigmar1 overexpression decreased the expression of CHOP and significantly decreased cellular toxicity in cells. Using biochemical and immunocytochemical experiments, we also defined the specific ER-stress pathway associated with Sigmar1-dependent regulation of CHOP expression and cellular toxicity. We found that Sigmar1 overexpression significantly increased inositol requiring kinase 1α (IRE1α) phosphorylation and increased spliced X-box-binding proteins (XBP1s) expression as well as nuclear localization. In contrast, Sigmar1 knockdown significantly decreased IRE1α phosphorylation and decreased XBP1s expression as well as nuclear transport. Taken together, these results indicate that Sigmar1-dependent activation of IRE1α-XBP1s ER-stress response pathways are associated with inhibition of CHOP expression and suppression of cellular toxicity. Hence, Sigmar1 is an essential component of the adaptive ER-stress response pathways eliciting cellular protection in cardiomyocytes.

Highlights

Endoplasmic reticulum (ER) is called the protein folding ‘factory’ of the secretory pathway as approximately one-third of the cellular proteome is synthesized, modified, and folded in the ER [1,2]
The expression of CCAAT-enhancer binding proteins (C/EBP)-homologous protein (CHOP) suggested the induction of maladaptive ER-stress induced by tunicamycin treatment in cardiomyocytes
We found that sigma 1 receptor (Sigmar1) overexpression increased the expression of spliced X-box binding protein 1 (XBP1) following tunicamycin treatment in cardiomyocytes (Figure 8A)

Summary

Introduction

Endoplasmic reticulum (ER) is called the protein folding ‘factory’ of the secretory pathway as approximately one-third of the cellular proteome is synthesized, modified, and folded in the ER [1,2]. Molecular chaperones and enzymes assist in protein folding by interacting with them, and function as key determinants for their exit from the ER [2]. Misfolded proteins are dislocated across the ER membrane and degraded by the cytosolic c 2017 The Author(s).

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Results

Conclusion