Obesity- or diabetes-related epicardial adipose tissue (EAT), a form of visceral fat, has been implicated in the development and progression of various heart diseases including hypertensive heart disease, ischemic cardiomyopathy, diabetic cardiomyopathy and so on [34, 35]. EAT can supply free fatty acids for myocardial energy production. However, substantially increased EAT results in its pathophysiology changes leading to secretion of deleterious factors to cardiac myocytes, including “bad” adipokines, pro-inflammatory factors, and oxidative factors [34, 35]. All of them create a suitable environment for the development of heart diseases [4–6]. Currently, adipocyte-derived exosomes have been suggested to mediate the impaired effects of EAT on cardiac structure and function through releasing specific miRNAs. In mice fed with high-fat diet, miR-130b-3p from dysfunctional adipocyte-derived small extracellular vesicles exacerbates myocardial ischemia/reperfusion injury . It is interesting to note that adipocyte-derived exosome functions as a mediator between adipocytes and insulin resistance [9, 37]. For example, adipocyte-derived exosomal miR-27a mediates obesity-triggered insulin resistance in skeletal muscle . In the present study, we demonstrated that hypertrophic adipocyte-derived exosome induced insulin resistance in NRVMs (Fig. 1). Furthermore, miR-802-5p enriched in hypertrophic adipocyte-derived exosome and negatively regulated insulin sensitivity (Fig. 1 and Figure S1). Thus, consistent with previous studies, our findings indicate that hypertrophic adipocyte-derived exosomal miR-802-5p caused cardiac insulin resistance.
MiR-802 possesses multiple function. It has been reported to regulate cancer development, alleviate lipopolysaccharide (LPS)-induced acute lung injury, and modulate the expression of human angiotensin II type I receptor [38–40]. In the term of metabolism, miR-802 impairs glucose metabolism and causes nephropathy in both obese mice and human [41, 42]. Thus, miR-802 is considered as a promising biomarker for obesity- or diabetic-related disorders . In the present study, our findings identify that hypertrophic adipocyte-derived exosomal miR-802-5p functions as a key modulator for EAT-induced cardiac insulin resistance.
HSP60 has diverse effects on heart, which dependent on its location. HSP60 can be released into the extracellular space including serum by various cell types . Extracellular HSP60, even at low concentration, causes cardiac myocyte apoptosis and necrosis . The higher mean plasma levels of HSP60 are closely associated with clinically manifest cardiovascular diseases in the patients with type 1 or type 2 diabetes . Additionally, the increased levels of anti-HSP60 antibody in the plasma is recognized as a risk factor for coronary heart disease and ischemic stroke [46, 47]. Hence, extracellular HSP60 is possibly dangerous to the cell function. Interestingly, the impacts of intracellular HSP60 on heart remains controversial. Transgenic HSP60 expression in the embryonic stage causes neonatal death in mice, accompanied with increased apoptosis and myocyte degeneration that possibly contributes to neonatal heart failure . In contrast, intracellular HSP60 is low expressed in diabetic heart . Decreased HSP60 inhibits insulin-like growth factor (IGF)-1 signaling pathway leading to the development of diabetic cardiomyopathy . Furthermore, abnormal distribution of HSP60 on the cell surface trigger cell apoptosis leading to heart failure . Loss of HSP60 in adult mouse hearts results in dilated cardiomyopathy, heart failure, and lethality . But overexpression of HSP60 in NRVMs protects cardiac cells from apoptotic cell death induced by stress stimuli like ischemia and ischemia/reoxygenation [52, 53]. In present study, our results demonstrated that HSP60 silence induced insulin resistance in NRVMs (Fig. 4). Given that cardiac insulin resistance is an importantly promotive factor for diabetic cardiomyopathy [1–3], our findings indicate that HSP60 deficiency is a risk factor contributing to the development of diabetic cardiomyopathy.
HSP60 is a highly conserved mitochondrial chaperone responsible for the protein folding, transport, trafficking, and quality control of mitochondrial proteostasis. Under stressful conditions, the abundance of HSP60 protein is compensatorily upregulated and increased HSP60 protects cells from oxidative stress, inflammation, and apoptosis. Therefore, loss of HSP60 will impaired mitochondrial function, which has been recognized as a primary abnormality contributing to the pathogenesis of cardiac insulin resistance and diabetic cardiomyopathy [2, 3]. In the present study, we found that HSP60 depletion significantly raised PERK phosphorylation and CHOP protein levels, increased intracellular ROS formation, and enhanced expression levels of protein carbonylation (Fig. 5a-d). These findings are consistent with previous study showing that knockdown of HSP60 in adult mouse hearts upregulates ROS production and increases CHOP mRNA levels at age of 9 weeks and 11 weeks , suggesting an impairment of mitochondrial function. It is well-known that excessive or prolonged UPR activation and ROS accumulation can trigger JNK/IRS-1 signaling pathway leading to insulin resistance [30, 31]. In the present study, the impacts of HSP60 knockdown on the UPR and ROS were accompanied with increased phosphorylation of JNK and IRS-1 S307, further confirming HSP60 knockdown-induced insulin resistance is dependent on its impairment on mitochondrial function.
Adiponectin, an adipokine produced by white adipose cells, has been proposed to treat obesity- or diabetes-related cardiomyopathy, at least partly through its insulin-sensitizing properties [29, 54–56]. In the present study, our results found that HSP60 depletion diminished the positive effects of adiponectin on insulin-stimulated Akt phosphorylation and glucose uptake (Figure S2), suggesting that HSP60 silencing resulted in adiponectin resistance. This inhibitory effects on adiponectin action may attribute to HSP60 knockdown-induced degradation of adiponectin receptor (Zhang D et al., Paper in press).
In the present study, like HSP60 knockdown, miR-802-5p mimic and hypertrophic adipocyte-derived exosome generated a similar promotion effect on the UPR and ROS (Fig. 6). Moreover, these changes were greatly abrogated by both inhibition of exosome uptake and deletion of miR-802-5p in adipocytes (Fig. 6 and S3). Therefore, hypertrophic adipocyte-derived exosome induced cardiac insulin resistance through exosomal miR-802-5p/HSP60 signaling pathway.