Background: Curcumin functions as a proteasome inhibitor. However, the molecular mechanisms behind this action need more detailed explanations. Purpose: This study aimed to investigate the inhibitory effect of curcumin on 20S proteasome activity and to elucidate its exact mechanism in triple-negative breast cancer (TNBC) MDA-MB-231 cells. Methods: Proteasomal peptidase activities were assayed using synthetic fluorogenic peptide substrates. Knockdown or overexpression of microRNA (miRNA or miR) or protein was used to investigate its functional effect on downstream cellular processes. BrdU (5-bromo-2'-deoxyuridine) assay was performed to identify cell proliferation. Western blot and quantitative real-time PCR(qRT-PCR) were carried out to determine protein abundance and miRNA expression, respectively. Correlations between protein expressions, miRNA levels, and proteasome activities were analyzed in TNBC tissues. Xenograft tumor model was performed to observe the in vivo effect of curcumin on 20S proteasome activity. Results: Curcumin significantly reduced PSMB5 protein levels, accompanied with a reduction in the chymotrypsin-like (CT-l) activity of proteasome 20S core. Loss of PSMB5 markedly inhibited the CT-1 activity of 20S proteasome. Furthermore, curcumin treatment significantly elevated miR-142-3p expression. PSMB5 was a direct target of miR-142-3p and its protein levels were negatively regulated by miR-142-3p. Moreover, histone acetyltransferase p300 suppressed miR-142-3p expression. Overexpression of p300 mitigated the promotive effect of curcumin on miR-142-3p expression. The correlations among p300 abundances, miR-142-3p levels, PSMB5 expressions, and the CT-1 activities of 20S proteasome were evidenced in TNBC tissues. In addition, loss of p300 and PSMB5 reduced cell proliferation. Inhibition of miR-142-3p significantly attenuated the inhibitory impact of curcumin on cell proliferation. These curcumin-induced changes on p300, miR-142-3p, PSMB5, and 20S proteasome activity were further confirmed in in vivo solid tumor model. Conclusion: These findings demonstrated that curcumin suppressed p300/miR-142-3p/PSMB5 axis leading to the inhibition of the CT-1 activity of 20S proteasome. These results provide a novel and alternative explanation for the inhibitory effect of curcumin on proteasome activity and also raised potential therapeutic targets for TNBC treatment.

Abstract Background : Epicardial adipose tissue (EAT) is implicated in insulin resistance, which has been recognized as a strongest predictor of the development of diabetic cardiomyopathy and subsequent heart failure. However, the underlying mechanism remains incompletely understood. Herein, we investigated the effect of hypertrophic adipocytes on cardiac insulin resistance. Methods : Palmitate was used to induce hypertrophic 3T3-L1 adipocytes. Exosomes were purified from normal control or hypertrophic 3T3-L1 adipocyte-associated conditioned medium. Exosome-exposed neonatal rat ventricular myocytes (NRVMs) were treated with insulin to investigate the effects of exosomes on insulin signaling. Insulin sensitivity was evaluated by measuring insulin-stimulated Akt phosphorylation and glucose uptake. SiRNA techniques were used to downregulate protein levels and its efficiency was evaluated by western blot.Results : Hypertrophic adipocyte-derived exosomes (h-Exo) induced insulin resistance in NRVMs. Furthermore, h-Exo high-expressed miR-802-5p. Insulin sensitivity of NRVMs was impaired by miR-802-5p mimic but improved by its inhibitor. TargetScan and luciferase reporter assays revealed that heat shock protein 60 (HSP60) was a direct target of miR-802-5p. Both h-Exo and miR-802-5p mimic could downregulate HSP60 protein levels. In addition, HSP60 silencing induced insulin resistance and mitigated the insulin-sensitizing effects of adiponectin. HSP60 depletion also significantly increased the expression levels of CHOP, a marker of the unfolded protein response (UPR), and enhanced oxidative stress, accompanied by the increased phosphorylation of JNK and IRS-1 Ser307. Inhibition of both miR-802-5p and endocytosis abolished the impacts of HSP60 knockdown on the UPR and oxidative stress. Conclusion : Hypertrophic adipocyte-derived exosomal miR-802-5p caused cardiac insulin resistance in NRVMs through downregulating HSP60. These findings provide a novel mechanism by which EAT impairs cardiac function.

Adiponectin, an adipokine produced and secreted by adipocytes, is involved in regulating the development and progression of insulin resistance, diabetes, and diabetic complications. Heat shock protein 60 (HSP60) is a molecular chaperone, most commonly presenting in mitochondria and participating in the maintenance of protein homeostasis. Accumulating studies have demonstrated that the elevated circulating HSP60 and the decreased intracellular HSP60 are closely associated with diabetic complications such as diabetic cardiomyopathy. However, the underlying mechanism remains poorly understood. In the present study, we reported that HSP60 interacted directly with adiponectin receptors. Its abundance was positively associated with adiponectin action. Furthermore, HSP60 depletion markedly mitigated the protective impacts of adiponectin on high glucose-induced oxidative stress and cell apoptosis in rat cardiac H9c2 cells. In addition, HSP60 knockdown significantly enhanced proteasome activity leading to the degradation of adiponectin receptor 1. Taken together, we showed for the first time that HSP60 interacted with adiponectin receptors and mediated adiponectin signaling through stabilizing adiponectin receptor. This in vitro study also provides an alternative explanation for mechanism by which adiponectin exerts its action.

Neuronal insulin resistance is implicated in neurodegenerative diseases. Icariin has been reported to improve insulin resistance in skeletal muscle cells and to restore impaired hypothalamic insulin signaling in the rats with chronic unpredictable mild stress. In addition, icariin can exert the neuroprotective effects in the mouse models of neurodegenerative diseases. However, the molecular mechanisms by which icariin affects neuronal insulin resistance are poorly understood. In the present study, amyloid-β (Aβ) was used to induce insulin resistance in human neuroblastoma SK-N-MC cells. Insulin sensitivity was evaluated by measuring insulin-stimulated Akt T308 phosphorylation and glucose uptake. We found that the phosphatase and tensin homologue deleted on chromosome 10 (PTEN) mediated Aβ-induced insulin resistance. Icariin treatment markedly reduced Aβ-enhanced PTEN protein levels, leading to an improvement in Aβ-induced insulin resistance. Accordingly, PTEN overexpression obviously abolished the protective effects of icariin on Aβ-induced insulin resistance. Furthermore, icariin activated proteasome activity. The proteasome inhibitor MG132 attenuated the effects of icariin on PTEN protein levels. Taken together, these results suggest that icariin protects SK-N-MC cells against Aβ-induced insulin resistance by activating the proteasome-dependent degradation of PTEN. These findings provide an experimental background for the identification of novel molecular targets of icariin, which may help in the development of alternative therapeutic approaches for neurodegenerative diseases.

Objective This study aimed to elucidate the mechanism by which hypertrophic adipocytes regulate insulin signaling in cardiac myocytes. Methods Palmitate was used to induce hypertrophic 3T3-L1 adipocytes. Exosomes were purified from normal control or hypertrophic 3T3-L1 adipocyte-associated conditioned medium. Exosome-exposed neonatal rat ventricular myocytes were stimulated with insulin to investigate the effects of exosomes on insulin signaling. Small interfering RNA techniques were used to downregulate protein levels, and their efficiency was evaluated by Western blot. Results Hypertrophic adipocyte-derived exosomes highly expressed miR-802-5p. Insulin sensitivity of neonatal rat ventricular myocytes was negatively regulated by miR-802-5p. TargetScan and luciferase reporter assays revealed that heat shock protein 60 (HSP60) was a direct target of miR-802-5p. HSP60 silencing was found to induce insulin resistance and to mitigate the insulin-sensitizing effects of adiponectin. In addition, HSP60 depletion significantly increased the expression levels of C/EBP-homologous protein and enhanced oxidative stress, accompanied by the increases in the phosphorylation of JNK and IRS-1 Ser307. Moreover, the effects of HSP60 knockdown on C/EBP-homologous protein and oxidative stress were abolished by the inhibition of either miR-802-5p or endocytosis. Conclusions Hypertrophic adipocyte-derived exosomal miR-802-5p caused cardiac insulin resistance through downregulating HSP60. These findings provide a novel mechanism by which epicardial adipose tissue impairs cardiac function.