X-box binding protein 1 regulates brain natriuretic peptide through a novel AP1/CRE-like element in cardiomyocytes

Abstract 

The unfolded protein response (UPR) is triggered to assist protein folding when endoplasmic reticulum (ER) function is impaired. Recent studies demonstrated that ER stress can also induce cell-specific genes. In this study, we examined whether X-box binding protein 1 (XBP1), a major UPR-linked transcriptional factor, regulates the expression of brain natriuretic peptide (BNP) in cardiomyocytes. In samples from failing human hearts, extensive splicing of XBP1 was observed along with increased expression of glucose-regulated protein of 78kDa (GRP78), a target of spliced XBP1 (sXBP1), suggesting that the UPR was induced in heart failure in humans. Interestingly, quantitative real-time PCR revealed a positive correlation between cardiac expression of GRP78 and BNP, leading us to test the hypothesis that sXBP1 regulates BNP as well as GRP78 in cardiomyocytes. A pharmacological ER stressor caused a dose-dependent increase in the expression of sXBP1 and BNP by cultured cardiomyocytes. Short interfering RNA targeting XBP1 suppressed the induction of BNP expression by a pharmacological ER stressor or norepinephrine, which was rescued by the adenovirus-mediated overexpression of sXBP1. The promoter assay with overexpression of sXBP1 or norepinephrine showed that the proximal AP1/CRE-like element in the promoter region of BNP was critical for transcriptional regulation of BNP by sXBP1. Direct binding of sXBP1 to this element was confirmed by the chromatin immunoprecipitation assay. These findings suggest that ER stress observed in failing hearts regulates cardiac BNP expression through a novel promoter region of the AP1/CRE-like element.

Abbreviations: BNP, brain natriuretic peptide, cDNA, complementary DNA, ER, endoplasmic reticulum, ERSE-I, -II, ER stress response elements I, II, GAPDH, glyceraldehyde-3-phosphate dehydrogenase, GRP78, glucose-regulated protein of 78kDa, IRE1α, inositol requiring kinase 1α, mUPRE, mammalian UPR element, NE, norepinephrine, RT-PCR, real-time polymerase chain reaction, siRNA, short interfering RNA, sXBP1, spliced X-box binding protein 1, TU, tunicamycin, uXBP1, unspliced X-box binding protein 1, UPR, unfolded protein response

Keywords: Endoplasmic reticulum, Unfolded protein response, XBP1, Brain natriuretic peptide, AP1/CRE-like element, Transcriptional regulation