Methods br Results br Discussion In
Discussion In this report, the therapeutic effect of a GCGR antagonistic antibody REMD2.59 was tested in 2 mechanistically divergent disease models of heart failure without confounding defects in global metabolism. Based on histological and functional analyses in both MI-injured and pressure-overloaded hearts, REMD2.59 treatment showed significant protection against cardiac hypertrophy and fibrosis remodeling with better preserved contractile function. These findings support a broadly applicable cardioprotective effect of GCGR inhibition against heart failure with different etiologies. As both these pathological stressors are imposed specifically and directly on heart rather in a systemic fashion, the observed cardioprotection of REMD2.59 is likely the result of its direct impact on GCGR signaling in cardiomyocytes rather than its impact on global glucose metabolic activities. This observation is consistent with the previous observations made in the cardiomyocyte-specific GCGR knockout mice, which have demonstrated the cardioprotective effect of GCGR antagonism against myocardial infarction in a receptor-dependent and cardiomyocyte cell-autonomous manner 29, 32. GLC and insulin are both pancreatic but counterbalancing hormones important to maintain systemic glucose regulation. GLC exerts its function via a peptide G protein–coupled receptor, GCGR. The canonical GCGR-mediated signaling involves classic G protein–coupled cAMP-dependent protein kinase A activation in hepatocytes, leading to induction of gluconeogenesis and glycogen catabolism, while inhibiting glycolysis 7, 35. In addition to its predominant Adefovir Dipivoxil in liver, GCGR is also expressed at modest to low levels in the kidney, heart, pancreas, and many other tissues 3, 8, 36. Although G protein–coupled canonical signaling for GLC is well established in hepatocytes, other mechanisms involving intracellular calcium regulation have also been reported in nonhepatocytes including cardiomyocytes 3, 10, 28, 36. In this report, we investigated GCGR inhibition in 2 mechanistically divergent disease models (i.e., myocardial infarction vs. pressure overload), the treatment resulted in similar cardioprotective effects against a broad spectrum of sequential pathological features in the failing heart, including cardiomyocyte hypertrophy, marker gene induction, interstitial fibrosis, and most importantly, cardiomyocyte contractile dysfunction. Apparently, GCGR antagonism is affecting cellular processes shared by different etiologies of cardiac pathology, including diabetes (32), ischemic injury, and mechanical overload. It is conceivable that abnormal GCGR activity may impact on cellular metabolism and energetic status via AMPK-dependent modulation in working heart as shown by Sharma et al. (32). However, our understanding to noncanonical signaling mechanism of GCGR is still very limited, and more studies are needed to illustrate the mechanistic basis of GCGR antagonism–mediated cardioprotection in response to different pathological stressors and energy homeostasis in failing hearts. It is important to note that when REMD2.59 was applied 2 weeks after the onset of pressure overload, GCGR antagonism no longer had any significant impact on cardiac hypertrophy, but still preserved the residual functions of the heart (Figures 6 and 7). This is consistent with previously reported observation that cardiomyocyte hypertrophy is established rather early in response to pressure overload while contractile dysfunction and fibrotic remodeling will continue to manifest (33). Our results highlight the potential limitation in the therapeutic window for heart failure. Nevertheless, REMD treatment can halt the further progression of heart failure and remodeling despite the limitation that GCGR antagonism may not be sufficient to reverse established cardiac hypertrophy and to fully restore contractile function. It is clear that more studies will be needed to fully establish the therapeutic efficacy of GCGR antagonism. Clinically relevant large animal models with established heart failure will be needed, and longer-term treatment and better outcome-based measurements (e.g., death and exercise tolerance) will be required.