A hypothetical model to solve the controversy over the involvement of UCP2 in palmitate-induced β-cell dysfunction

Resveratrol protects against ethanol-induced impairment of insulin secretion in INS-1 cells through SIRT1-UCP2 axis

SIRT1 has been proposed to enhance insulin secretion in β-cell through repressing the expression of uncoupling protein2 (UCP2), but whether ethanol-induced β-cell dysfunction is mediated by the disrupted SIRT1-UCP2 axis remains unknown. This study was conducted to explore the underlying mechanisms by which ethanol resulted in β-cell dysfunction and the potential protective effects of resveratrol in this process. INS-1 cells (rat pancreatic β-cell line) were cultured with ethanol in the presence or absence of resveratrol (2.5, 12.5 μmol/L). The results showed that ethanol exposure reduced glucose-stimulated insulin secretion, ATP production and SIRT1 expression but increased UCP2 expression, while supplementation with resveratrol restored the function of INS-1 cell by upregulating SIRT1 and inhibiting UCP2. Moreover, the critical role of SIRT1-UCP2 axis was further supported by the results that SIRT1 activator SRT1720 reversed ethanol-induced impairment of glucose-stimulated insulin secretion by decreasing UCP2, while SIRT1 inhibitor Ex527 abolished the beneficial effects of resveratrol. Meanwhile, NAD⁺ booster nicotinamide mononucleotide also counteracted the deleterious effects of ethanol by increasing SIRT1, suggesting the regulation of SIRT1-UCP2 axis may be associated with cellular NAD⁺/NADH ratio. In conclusion, our observations imply that ethanol induces impaired insulin secretion from INS-1 cell through disrupting SIRT1-UCP2 axis, while resveratrol may reverse this process by augmenting SIRT1 and inhibiting UCP2.

Novel role of PKR in palmitate-induced Sirt1 inactivation and endothelial cell senescence

Endothelial cell senescence is regarded as a vital characteristic of cardiovascular diseases. Elevated palmitate (PA) is an independent risk factor of cardiovascular diseases, but its role in endothelial cell senescence is currently unknown. During the course of studying the prosenescent role of PA, we discovered a key role of dsRNA-dependent protein kinase [protein kinase R (PKR)] in endothelial senescence. Exposure of human umbilical vein endothelial cells (HUVECs) to PA-induced cell senescence is characterized by increased levels of senescence-associated β-galactose glucosidase activity, excessive production of reactive oxygen species production, impaired cellular proliferation, and G1 phase arrest. This phenomenon is associated with an increase of PKR autophosphorylation and decreased activity of sirtuin 1 (Sirt1), a pivotal antisenescent factor. PKR inactivation by PKR siRNA or its phosphorylation inhibitor 2-aminopurine significantly attenuated PA-induced HUVEC senescence by reversing Sirt1 activity and its downstream signaling. Moreover, to study the regulatory mechanism between PKR and Sirt1, we found that PKR promotes JNK activation to inhibit Sirt1 activity and that this effect could be reversed by the JNK inhibitor SP600125. These findings provide evidence that PKR mediates PA-induced HUVEC senescence by inhibiting Sirt1 signaling. Our study provides novel insights into the actions and mechanisms of PKR in endothelial senescence.

NEW & NOTEWORTHY This study first provides a novel observation that dsRNA-dependent protein kinase (PKR) mediates palmitate-induced sirtuin 1 inactivation and subsequent human umbilical vein endothelial cell senescence. Most importantly, these new findings will provide a potential therapeutic strategy to improve free fatty acid-induced endothelial senescence by targeting PKR in cardiovascular diseases.