Protein samples (7.5–10 μg protein/sample) were applied to SDS-PAGE using 4–15% TGX Stain-Free gels (Bio-Rad Laboratories, Hercules, CA). Both endogenous FA synthesis and FA uptake are considered causally important for increased lipid accumulation in β-cells ( 13, 14). Hence, together these findings indicate that insulin resistance and defective insulin secretion are likely to share common etiologies in terms of lipid accumulation. Moreover, insulin signaling in β-cells is essential not only for growth but also for proper regulation of the cellular function ( 10– 12). A number of in vitro studies have identified mechanisms involved in impaired insulin secretion by chronic fatty acid (FA) elevation ( 7, 8), including those affecting exocytosis ( 9). In fact, increased accumulation of lipid droplets is observed with increased BMI in human β-cells ( 6). In this condition, accumulated lipids, specifically triacylglycerol, cause cellular stress, dysfunction, and death of the β-cell. Like insulin target tissues, the insulin-producing β-cells have been shown to be damaged by excessive lipid accumulation, a concept known as β-cell lipotoxicity ( 5). These findings suggest that those who cannot adapt to the extra demand by increased insulin secretion are prone to T2D. Indeed, reduced first-phase insulin response can, at least in some individuals, be observed already before the development of T2D ( 4). Pancreatic β-cell dysfunction is central in the failure to adjust for the increased insulin resistance.
However, not all obese individuals develop T2D because pancreatic β-cells can adjust, to a certain extent, for an increasing demand of insulin. The intracellular lipid accumulation in ectopic tissues leads to impaired insulin signaling and promotes systemic insulin resistance ( 3). In obesity, lipid accumulation is common not only in adipose tissue but also in ectopic tissues such as the liver and skeletal muscle. Obesity is one of the strong risk factors for the development of T2D.
Insulin resistance and defective insulin secretion are the two major pathogenic factors of the disease, and both are strongly associated with lifestyle and genetic components ( 1, 2). Hyperglycemia caused by insufficient insulin action characterizes type 2 diabetes (T2D). Hence, CD36 could be a key molecule to limit β-cell function in T2D associated with obesity.
Our results demonstrate that β-cells from obese donors with T2D have dysfunctional exocytosis likely due to an abnormal lipid handling represented by differential CD36 expression. CD36 antibody treatment of the human β-cell line EndoC-βH1 increased IRS1 and exocytotic protein levels, improved granule docking, and enhanced insulin secretion. This was accompanied by reduced expression of the exocytotic proteins SNAP25, STXBP1, and VAMP2, likely because CD36 induced downregulation of the insulin receptor substrate (IRS) proteins, suppressed the insulin-signaling phosphatidylinositol 3-kinase/AKT pathway, and increased nuclear localization of the transcription factor FoxO1. Indeed, CD36 overexpression led to decreased insulin secretion, impaired exocytosis, and reduced granule docking. We tested the hypothesis that CD36 is a key molecule in the reduced insulin secretion capacity. Islets from obese donors with T2D had reduced insulin secretion, decreased β-cell exocytosis, and higher expression of fatty acid translocase CD36. In this study, we aimed to investigate the cause of differential insulin secretion capacity of pancreatic islets from donors with T2D and non-T2D (ND), especially obese donors (BMI ≥30 kg/m 2). Obesity is a risk factor for type 2 diabetes (T2D) however, not all obese individuals develop the disease.
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