Cardiac hypertrophy is a compensatory response to cardiac stress such as development, repetitive endurance exercise, pressure overload, volume overload, hypoxia, storage diseases, and inherited diseases. However, sustained cardiac hypertrophy usually progresses to maladaptive cardiac remodeling involving fibrosis, cardiomyocyte death, increased collagen synthesis, decreased pumping ability, arrhythmias, and aberrant gene expression, eventually leading to heart failure and even sudden cardiac death [1]. Therefore, pathological cardiac hypertrophy is considered as a critical therapeutic target for many heart diseases. Hitherto, hypertrophic signaling pathways, involving calcineurin/nuclear factor of activated T cell (NFAT) [2], AMP-activated protein kinase (AKT)/mammalian target of rapamycin (mTOR) [3], Ca2+/calmodulin-dependent protein kinase (CaMK)II4, cyclic guanosine monophosphate (cGMP)-protein kinase G (PKG) [5], mitogen-activated protein kinase (MAPK) [6], and non-coding RNAs [7], have been identified. Nevertheless, the fundamental mechanisms underlying pathological cardiac hypertrophy, especially the involvement of mitophagy program, are still poorly understood.
Mitophagy, a selective autophagic response, specifically the traffics of superfluous, aging or damaged mitochondria to lysosomes for degradation, serves as a crucial mechanism in mitochondrial quality control and cellular homeostasis. Depending on mitophagy, paternal mitochondria are eliminated from the fertilized eggs [8] and mitochondria are cleaned progressively during erythrocytes maturation [9]. Since mitochondria are the major energy production organelle in cardiomyocytes, mitophagy is particularly necessary to heart development and maintenance of cardiac homeostasis. Mitophagy defects contribute greatly to cardiac dysplasia and cardiac aging [10, 11]. Recent advance has revealed that mitophagy is implicated in the pathogenesis of heart diseases. It has been demonstrated that mitophagy is repressed in myocardial infarction and heart failure, and mitophagy reversion using genetical or pharmacological methods attenuates myocardial injury and remodeling, and improves cardiac function [12,13,14]. Diabetic cardiomyopathy is exacerbated by impaired mitophagy and prevented by activated mitophagy [15]. Conversely, other studies revealed that advanced heart failure and pathologic hypertrophy are accompanied by dramatically increased mitophagy. Inhibition of mitophagy rescues cardiac remodeling and heart failure [16, 17]. Given all above, what a role mitophagy plays in heart diseases, either positive or negative, and how it plays, are still enigmas.
PARKIN, also known as PARK2, is an E3 ubiquitin ligase and participates in multiple cellular processes and mitochondrial homeostasis through modulating post-translational modification of proteins in a ubiquitin–proteasome dependent pathway. By ubiquitinating receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and Cyclophilin-D (CYPD), PARKIN prevents both receptor-dependent and mitochondrial ways of necroptosis [18, 19]. Defects on mono-ubiquitination of VDAC1 by PARKIN promotes apoptosis by augmenting the mitochondrial calcium uptake [20]. Loss of Parkin impairs mitochondrial biogenesis and mitochondrial respiration, leading to cell death [21]. Another widely appreciated role of PARKIN is mediating mitophagy. Upon phosphorylation by mitochondrial outer membrane (MOM)-localized PTEN-induced putative kinase protein-1 (PINK1), PARKIN is recruited to MOM and extensively catalyzes poly-ubiquitination of a dozen OMM substrates among which modified mitofusion (MFN) 1/2, MIRO and VDAC1 have been demonstrated to recruit autophagy adaptor such as p62/sequestosome 1 (SQSTM1), NDP52, and optineurin (OPTN), and ultimately target mitochondria removal [22,23,24,25,26]. Affluent effectors downstream of PARKIN-mediated mitophagy have been identified. However, the upstream triggering mechanism remains to be elucidated fully.
Albeit Parkin was first identified in Parkinson’s disease (PD), emerging evidence indicates that Parkin is deeply implicated in cardiovascular system. During heart development, PARKIN evokes the switch of mitochondria from nascent to mature through mediating fetal mitochondria degradation. Cardiomyocyte-specific deletion of Parkin at birth results in perinatal cardiomyopathy and premature death [27]. PARKIN is required for melatonin-mediated inhibition of mitochondrial dysfunction and cardiac remodeling in diabetic cardiomyopathy [28]. Our previous work has demonstrated that PARKIN alleviates cardiac ischemia/reperfusion injury and improved cardiac function [19]. The Parkin-deficient mice exhibited aggravated cardiac injury and increased mortality in response to myocardial infarction [29]. PARKIN participates in cardiac physiological and pathological processes, but the underlying mechanism, especially referring to hypertrophic program, is largely unknown. Thus, we were interested in exploring the role of PARKIN in cardiac hypertrophy.
Forkhead box O3a (FOXO3a), a member of forkhead family of transcription factors, extensively regulates gene transcription depending on its 100-amino acid DNA binding domain. In a state of non-phosphorylation and deacetylation, FOXO3a regulates diverse cellular functions, including proliferation, differentiation, metabolism, cell death, and stress response [30,31,32,33]. FOXO3a is highly expressed in hearts and functions as a negative regulator in cardiac disorders. FOXO3a inhibits cardiac apoptotic and necrotic cell death, and maintains calcium homeostasis in response to myocardial infarction. Foxo3a transgenic mice exhibit reduced cell death and infarct size, and improved cardiac function [30, 31, 34]. Phosphorylated FOXO3a accumulates upon hypertrophic stimuli such as insulin, angiotensin II (Ang II), phenylephrine, and pressure overload, and enforced expression of FOXO3a inhibits cardiac hypertrophy [35, 36]. FOXO3a suppresses mitochondrial fission and apoptosis, and protects against doxorubicin-induced cardiotoxicity [32]. Recent advances have shown that FOXO3a is implicated in mitophagy signaling pathways. FOXO3a can upregulate the expression level of Bcl-2 E1B 19-KDa interacting protein 3 (BNIP3), an autophagy receptor partially mediating non-canonical mitophagy [17]. Loss of Sirt3 impairs autophagy and mitophagy accompanied by decreased deacetylation of FOXO3a and levels of PARKIN [37]. FOXO3a is seemingly co-localized with mitochondrial PINK1/PARKIN proteins activated by antioxidants [38]. However, the role of FOXO3a in mitophagy and whether FOXO3a targets mitophagy program in the pathogenesis of cardiac disorders is still poorly understood.
The present study aimed at exploring the role of PARKIN in cardiac hypertrophy. Mitophagy is impaired upon hypertrophic stimulation. PARKIN was found to regulate cardiac hypertrophy by modulating mitophagy process. Parkin transgenic mice exhibits rescued mitophagy, decreased hypertrophic responses, and improved cardiac function. In searching for the upstream regulators of PARKIN, we identified that FOXO3a transcriptionally activated PARKIN expression. FOXO3a participated in regulating mitophagy and cardiac hypertrophy through targeting PARKIN. Taken together, our results revealed a novel hypertrophic regulating model composed of FOXO3a, PARKIN and mitophagy program.