Abstract

RationaleThe ubiquitin-proteasome system (UPS) and the autophagic-lysosomal pathway (ALP) are pivotal to proteostasis. Targeting these pathways is emerging as an attractive strategy for treating cancer. However, a significant proportion of patients who receive a proteasome inhibitor-containing regime for example, show cardiotoxicity. Moreover, UPS and ALP defects are implicated in the pathogenesis of a large subset of heart disease. Hence, a better understanding of the cross-talk between the two catabolic pathways should help advance cardiac pathophysiology and medicine. ObjectiveSystemic pharmacological proteasome inhibition (PSMI) was shown to increase p62/SQSTM1 expression and induce myocardial macroautophagy. The present study investigates whether cardiomyocyte-restricted PSMI activates myocardial ALP and, more importantly, how proteasome malfunction activates the ALP in the heart. Methods and ResultsMyocardial macroautophagy, transcription factor EB (TFEB) expression and activity, and p62 expression were markedly increased in mice with either cardiomyocyte-restricted ablation of Psmc1 (a 19S proteasome subunit gene) or pharmacological PSMI. In cultured cardiomyocytes, PSMI-induced increases in TFEB activation and p62 expression were blunted markedly by calcineurin inhibition (cyclosporine A) and by siRNA-mediated Molcn1 silence. PSMI induced remarkable increases in myocardial autophagic flux in wild type mice but not p62 null mice. In cultured wild type, but not p62-null, mouse cardiomyocytes, PSMI induced increases in LC3-II flux and in the lysosomal removal of ubiquitinated proteins. Myocardial TFEB activation by PSMI as reflected by TFEB nuclear localization and target gene expression was strikingly less in p62 null mice compared with wild type mice. Conclusions(1) The activation of cardiac macroautophagy by proteasomal malfunction is mediated by the Mocln1-calcineurin-TFEB-p62 pathway; (2) both Mocln1 and p62 form a feed-forward loop with TFEB during TFEB activation by proteasome malfunction; and (3) targeting the Mcoln1-calcineurin-TFEB-p62 pathway may provide new means to intervene cardiac ALP activation in a proteasome malfunction setting.