Metformin prevents glucotoxicity by alleviating oxidative and ER stress–induced CD36 expression in pancreatic beta cells

Abstract

Aim/hypothesis

Cluster determinant 36 (CD36), a fatty acid transporter, was reported to have a pivotal role in glucotoxicity-induced beta cell dysfunction. However, little is known about how glucotoxicity influences CD36 expression, and it is unknown whether this action can be counteracted by metformin. In the present study, we showed that metformin counteracts glucotoxicity by alleviating oxidative and endoplasmic reticulum (ER) stress–induced CD36 expression.

Methods

We used primary rat islets as well as INS-1 cells for 72 h to 24 h with 30 mM glucose, respectively. Thapsigargin was used as strong ER stressor, and Sulfo-N-succinimidyl oleate (SSO) and RNA interference were chosen for CD36 inhibition. Free fatty acid uptake was measured by radioisotope tracing technique.

Results

Exposure of isolated rat islets to high glucose (HG) for 3 days decreased insulin and pancreatic duodenal homeobox1 (Pdx1) mRNA expression, with the suppression of glucose-stimulated insulin secretion (GSIS) along with elevation of reactive oxygen species (ROS) levels. Incubation with metformin restored insulin and Pdx1 mRNA expression with significant improvements in GSIS and decrease in ROS production. HG exposure in INS-1 cells increased free fatty acid uptake via induction of CD36 along with impaired insulin and Pdx1 mRNA expression. Moreover, thapsigargin also increased the induction of CD36 expression. Metformin blocked HG- and thapsigargin-induced CD36 expression. In addition, the simultaneous inhibition of intracellular ROS production by metformin or CD36 activation by SSO or CD36 siRNA significantly decreased the apoptotic response in HG-treated INS-1 cells.

Conclusion/interpretation

In conclusion, metformin conferred protection against HG-induced apoptosis of pancreatic beta cells, largely by interfering with ROS production, and inhibited the CD36-mediated free fatty acid influx. This report provides evidence that the inhibition of CD36 may have potential therapeutic effects against hyperglycemia-induced beta cell damage in diabetes.

Introduction

Hyperglycemia, an important pathologic characteristic of type 2 diabetes, primarily results from insulin resistance and beta cell dysfunction. The latter is essential for the development of diabetes because hyperglycemia does not develop until compensatory insulin secretion failure. Excessive nutrition and obesity increase the metabolic load in pancreatic beta cells, and this leads to pancreatic beta cell dysfunction and death (Han, 2016, Kim and Yoon, 2011, Poitout and Robertson, 2002, Poitout and Robertson, 2008, Robertson et al., 2004). Although, the molecular signals that trigger functional deterioration of the beta cell are unknown, several studies have provided evidence that the dysfunction occurs in the presence of elevated glucose levels in type 2 diabetes. Moreover, pancreatic islets are relatively vulnerable to oxidative stress due to a low capacity of antioxidant enzymes (Jung et al., 2014, Karunakaran and Park, 2013, Robertson et al., 2003). Thus, beta cell preservation is an essential component of type 2 diabetes management.

Metformin is currently recommended as first-line therapy for all newly diagnosed type 2 diabetes patients. Metformin regulates blood glucose levels primarily by decreasing hepatic glucose production and improving insulin sensitivity (Bailey and Turner, 1996, Prager and Schernthaner, 1983). Meanwhile, there have been controversies whether metformin directly has beneficial effects on beta cell function. In particular, metformin restores insulin secretion altered by chronic exposure to high glucose or high free fatty acid levels in vitro (Lupi et al., 1999, Patane et al., 2000) and ameliorates the deleterious effects of free fatty acid levels in pancreatic human islets (Lupi et al., 2002). Marchetti et al.(2004) showed that the beneficial effects of metformin on beta cell function may be mediated by the alleviation of oxidative stress. However, the exact mechanism of these effects has not been clarified. Recently, we reported that fatty acid translocase cluster determinant 36 (CD36), a membrane glycoprotein, influences the induction of glucotoxicity in pancreatic beta cells (Kim et al., 2012). Previous findings showed that CD36 is not only a fatty acid transporter but also related to a signaling cascade under metabolic stress conditions. Thus, CD36 would be an important therapeutic target in diabetic conditions (Karunakaran, Moon, Lee, & Won, 2015). In this study, we examined the effects of metformin on high glucose-induced CD36 expression and found that metformin decreased high glucose-induced CD36 expression via the inhibition of oxidative and endoplasmic reticulum(ER) stress signaling.