Rac1-NADPH oxidase signaling promotes CD36 activation under glucotoxic conditions in pancreatic beta cells

Abstract

We recently reported that cluster determinant 36 (CD36), a fatty acid transporter, plays a pivotal role in glucotoxicity-induced β-cell dysfunction. However, little is known about how glucotoxicity influences CD36 expression. Emerging evidence suggests that the small GTPase Rac1 is involved in the pathogenesis of beta cell dysfunction in type 2 diabetes (T2D). The primary objective of the current study was to determine the role of Rac1 in CD36 activation and its impact on β-cell dysfunction in diabetes mellitus. To address this question, we subjected INS-1 cells and human beta cells (1.1B4) to high glucose conditions (30 mM) in the presence or absence of Rac1 inhibition either by NSC23766 (Rac1 GTPase inhibitor) or small interfering RNA. High glucose exposure in INS-1 and human beta cells (1.1b4) resulted in the activation of Rac1 and induced cell apoptosis. Rac1 activation mediates NADPH oxidase (NOX) activation leading to elevated ROS production in both cells. Activation of the Rac1-NOX complex by high glucose levels enhanced CD36 expression in INS-1 and human 1.1b4 beta cell membrane fractions. The inhibition of Rac1 by NSC23766 inhibited NADPH oxidase activity and ROS generation induced by high glucose concentrations in INS-1 & human 1.1b4 beta cells. Inhibition of Rac1-NOX complex activation by NSC23766 significantly reduced CD36 expression in INS-1 and human 1.1b4 beta cell membrane fractions. In addition, Rac1 inhibition by NSC23766 significantly reduced high glucose-induced mitochondrial dysfunction. Furthermore, NADPH oxidase inhibition by VAS2870 also attenuated high glucose-induced ROS generation and cell apoptosis. These results suggest that Rac1-NADPH oxidase dependent CD36 expression contributes to high glucose-induced beta cell dysfunction and cell death.

Glucotoxicity promotes aberrant activation and mislocalization of Ras-related C3 botulinum toxin substrate 1 [Rac1] and metabolic dysfunction in pancreatic islet β-cells: reversal of such metabolic defects by metformin

“…A growing body of evidence implicates CD36, a fatty acid transport protein, in cell apoptosis under glucolipotoxic conditions [19, 28]. Data from our current studies have provided evidence to suggest that Rac1 activation is upstream to CD36 expression since EHT1864, a known inhibitor of Rac1 [31, 32], attenuated HG-induced CD36 expression in INS-1 832/13 cells.…”

“…Several recent studies have established novel roles for CD36, a membrane-associated fatty acid transporter protein, in the onset of β-cell dysfunction and demise under chronic hyperglycemic conditions [19, 28]. Therefore, we asked if HG conditions increase expression of CD36 in INS-1 832/13 cells, and if so, whether Rac1 activation represents an upstream signaling mechanism for HG-induced expression of CD36.…”

“…Considerable experimental evidence suggests that increases in intracellular generation of reactive oxygen species [ROS] triggers the metabolic dysfunction of the islet β-cell under the duress of glucotoxicity, lipotoxicity or exposure to proinflammatory cytokines or biologically-active sphingolipids, such as ceramide [11–19]. The phagocytic NADPH oxidase [Nox2] has been shown to be one of the contributors of ROS generation under these pathological conditions [20, 21].…”

“…Interestingly, Rac1 is an integral member of the Nox2 holoenzyme and, therefore, sustained activation of Rac1 under metabolic stress conditions is felt to be one of the signaling events necessary for the activation of Nox2. Using molecular biological [siRNA, antisense and dominant negative mutants] and pharmacological inhibitors of Rac1 [NSC23766, EHT1864, Ehop-016] and Nox2 [ gp91-ds-tat peptide, VAS2870], recent studies have implicated requisite roles of accelerated Rac1-Nox2 module in islet β-cell dysfunction in in vitro and in vivo models of impaired GSIS and diabetes [8, 13, 14, 19–21]. …”

Antioxidant defenses in diabetes mellitus: a clinical and molecular approach

“…Under excessive levels of glucose, Rac1 activation induced expression of a cell surface lipid transporter, the CD36, which triggers mitochondrial dysfunction, oxidative stress and apoptosis of beta cells [98]. In the same study, inhibition of Rac1 abrogated the deleterious effects of high glucose on pancreatic beta cells.…”