Role of the unfolded protein response in pathological cardiac remodeling

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Project Summary Heart failure (HF) is a leading cause of morbidity and mortality worldwide. Hypertension is one of the most important risk factors of HF. Despite paramount interests and urgent needs, our understanding of the mechanisms of HF development remains limited. To accommodate the elevated demand of cardiac contractility under high blood pressure, the heart mounts an acute reaction through hypertrophic growth. This once adaptive response may decompensate and progress into HF. The overall goal of this renewal application is to delineate the role of the unfolded protein response (UPR) in HF under pressure overload. Cumulative evidence shows that the UPR is activated in the heart under pressure overload and in cultured cardiomyocytes by growth stimulation. During the previous funding period of this project, both gain- and loss-of- function studies have demonstrated that spliced XBP1 (sXBP1), a downstream transcriptional factor of the UPR, is critical for adaptive cardiac hypertrophy under hemodynamic stress, which is partly mediated by its target GFAT1. sXBP1 is produced by IRE1α, one of the three signaling transducers of the UPR, under protein-folding stress. In the course of the previous funding studies, a novel role of IRE1α in translation, not sXBP1-controlled transcription, was uncovered. This action, not mediated by sXBP1, may open a new research direction of IRE1α and the UPR in cardiac hypertrophy and HF. Preliminary results through an unbiased pulse SILAC assay demonstrate that UPR proteins are enriched under cell growth conditions. Importantly, only the IRE1α, not PERK or ATF6, branch of the UPR is required for protein synthesis and cell growth. This new action of IRE1α is independent of sXBP1 since sXBP1 restoration does not rescue growth defect from IRE1α silencing. Moreover, age-matched IRE1α conditional knockout (cKO) mice present more severe cardiomyopathy and HF compared to sXBP1 cKO animals. Further pilot data from a proteomic assay suggest that IRE1α directly binds components of the translation initiation complex. Based on these findings, a central hypothesis has been proposed: IRE1α exerts a new role in translational regulation, which is essential for the heart to mount an adaptive growth response and antagonize HF under pressure overload. Ribosome profiling has been conducted to identify translation-only targets of IRE1α. The role of IRE1α in translation initiation will be delineated by testing the assembly and functionality of the translation initiation complex. Next, the mechanism of targeting of IRE1α on 5’- and 3’-UTRs of its targets will be determined by deducing secondary mRNA structures and validation at the molecular level. Finally, effectors of IRE1α, particularly membrane proteins, will be evaluated for their contributions to cardiac hypertrophy and HF in vivo. Elucidation of the novel role of IRE1α and the UPR in cardiac remodeling and HF will advance our understanding of the pathophysiology of hypertensive heart disease and pave a way for novel, more effective therapeutic design.