Abstract 17132: IRE1α Protects Against Cardiac Fibrosis via Regulating Selective Transcript Degradation

Abstract

Background: Upon cardiac injury, cardiac fibroblasts become activated and differentiate intomyofibroblasts to maintain cardiac structure and function through secretion of stabilizingextracellular matrix (ECM) components. However, if this persists, it leads to stiffening of themyocardium and heart failure. The unfolded protein response (UPR) responds to an increaseddemand for folding of nascent secreted ECM proteins trafficked through the endoplasmicreticulum such as collagen. IRE1α, the most evolutionarily conserved arm of the UPR, actsadaptively both by enhancing protein folding of secreted proteins and reducing prolongedincreases in protein synthesis as occurs during pathologic cardiac fibrosis. Hypothesis: IRE1α protects against cardiac fibrosis via inhibiting pathologic sustained fibroblastactivation. Methods: Primary cardiac fibroblasts (CFBs) were cultured with transforming growth factor beta(TGF-β) to promote activation, in vitro . Small molecule targeting approaches were utilized tospecifically inhibit or activate IRE1α. IRE1α-floxed mice were injected with AAV9 to knockoutIRE1α specifically in myofibroblasts (IRE1mfbKO) and were subjected to transverse aorticconstriction (TAC) as a model of heart failure to stimulate fibroblast activation, in vivo . Results: While small molecule activation of IRE1α significantly blunted CFB activation inresponse to TGF-β, specific inhibition of IRE1α’s endonuclease domain enhanced CFBresponsiveness to TGF-β. Importantly, this effect occurred independent of IRE1α’s canonicalmechanism of activating the transcription factor, XBP1, implicating regulated IRE1α-dependentdecay (RIDD) as a novel anti-fibrotic protective mechanism. Transcriptomics of CFBs identifieda novel “RIDD-ome” of pro-fibrotic mRNA targets of IRE1α-mediated degradation. Finally, inresponse to TAC, IRE1mfbKO mice exhibited an exacerbated decrease in cardiac function andmarked increase interstitial fibrosis. Conclusions: IRE1α protects against pathologic differentiation of CFBs in response to TGF-β or TACvia regulating the degradation of a novel subset of pro-fibrotic transcripts. Thus, small moleculetargeting of IRE1α represents a novel approach to mitigating cardiac fibrosis.