Abstract 1590: Autophagic Processing Of Ubiquitinated Protein Aggregates During Unloading-induced Cardiac Atrophy
Background: Mechanical unloading of the diseased ventricle has emerged as an important treatment strategy in advanced heart failure. In rare instances, dramatic improvements in ventricular performance are seen, yet little is known regarding underlying mechanisms. Autophagy is an ancient mechanism of bulk protein degradation recently shown to be activated by a variety of forms of cardiac stress.
Methods and Results: To test for a role of autophagy in atrophic remodeling of the unloaded left ventricle (LV), murine wild-type (WT) and “autophagy reporter” (αMHC-GFP-LC3 transgenic) hearts underwent heterotopic transplantation into the abdomen of same-strain recipient mice. The donor hearts were thereby mechanically unloaded while the native recipient hearts served as controls. In WT hearts, cardiomyocyte cell surface area declined 27±2% at 7 days. Processing of LC3 (microtubule-associated light chain 3) increased in atrophying donor hearts, indicative of increased autophagosome formation. Further, significant increases in the number of GFP-LC3 puncta in “autophagy reporter” hearts were observed. Levels of the pro-autophagic protein Beclin 1 increased in donor hearts, further evidence of increased autophagic flux during atrophic remodeling. In atrophic donor hearts, we detected significant increases in polyubiquitinated proteins as well as in the abundance of the ER chaperone proteins GRP78/BIP and GRP94, indicative of an unfolded protein response. As autophagy degrades numerous cellular components including polyubiquitinated protein aggregates, we transplanted hearts haploin-sufficient for beclin 1, a gene required for early processing of autophagosomes (≈50% decrease in autophagy). In these studies, we detected robust increases in the abundance of polyubiquitin proteins in beclin 1± donor hearts relative to WT (62% vs 12%, p<0.05).
Conclusions: Autophagy is activated during cardiac atrophy and participates in processing of protein aggregates. Together, these findings implicate autophagic activity in the remodeling of the unloaded LV.
This research has received full or partial funding support from the American Heart Association, AHA South Central Affiliate (Arkansas, New Mexico, Oklahoma & Texas).