Endocrine Abstracts

JOINT1571

Introduction: We and others have linked intracellular lipid accumulation in pancreatic islets with beta-cell dysfunction. Here, we report the effects of advancing hepatic steatosis on islet lipid accumulation during hyperlipidemic conditions. Increase in circulatory free fatty acids (FFA) enhances the expression and secretion of FetuinA (FetA) from the liver into the circulation. FetA, in combination with FFAs, activates TLR4, which induces insulin resistance and islet inflammation. This study thus targets TLR4 using its inhibitor C34 to investigate the regulation of islet lipid accumulation and beta cell function during the progression of hepatic steatosis.

Methods: C57BL/6J male mice were divided into 6 groups and given a standard diet (SD) for 20 weeks or a high-fat diet (HFD) for 4, 8, 12, 16 and 20 weeks. Another experiment had three groups receiving SD, HFD, HFD+TLR4 inhibitor C34 (1 mg/kg/day, i.p.) for 12 weeks. To assess the effects of FetA, mice on 4-week HFD received FetA i.p. injection for 5 days. Pancreatic islets from treated mice were subjected to immunoblot, qPCR and Nile Red staining. Immunofluorescence and H&E staining were performed on liver and pancreatic sections. Serum FetA level was measured using ELISA. Beta-cell function was evaluated by in-vivo GSIS.

Results: Results showed progressive increase in intracellular lipid accumulation in the pancreatic islet, along with hepatic steatosis and serum FetA levels, with HFD treatment from 4-16 weeks. However, in the 20-week HFD group, significant islet mass reduction with damage in islet morphology was observed. Increase in vesicular steatosis was seen in the liver with prominent adipocyte infiltration in the 20-week HFD, as evident from H&E-stained liver sections. Confocal images revealed higher lipid accumulation in islets of FetA-injected 4-week HFD mice compared to HFD (4-week) alone (~2-fold), suggesting the involvement of FetA in aggravating lipid accumulation. A concomitant increase in TLR4 expression and reduction in insulin content in the islet was observed in the HFD+FetA mice. TLR4 inhibition by C34 administration in HFD mice significantly reduced islet lipid accumulation and lowered circulating FetA (~0.4-fold) and IL-1β levels. Both lipid uptake and de-novo lipogenesis markers CD36 and SREBP1 were reduced in the islet of C34-treated HFD mice. Inhibiting TLR4 improved in-vivo GSIS, insulin gene expression and Pdx1 levels in HFD islets.

Conclusion: Elevated serum FetA level with advancing hepatic steatosis poses a significant increase in islet lipid accumulation and consequent dysfunction of beta cells in obese mice and could be prevented by early inhibition of TLR4.