β-cell metabolic alterations under chronic nutrient overload in rat and human islets

Abstract

The aim of this study was to assess multifactorial β-cell responses to metabolic perturbations in primary rat and human islets. Treatment of dispersed rat islet cells with elevated glucose and free fatty acids (FFAs, oleate:palmitate = 1:1 v/v) resulted in increases in the size and the number of lipid droplets in β-cells in a time- and concentration-dependent manner. Glucose and FFAs synergistically stimulated the nutrient sensor mammalian target of rapamycin complex 1 (mTORC1). A potent mTORC1 inhibitor, rapamycin (25 nM), significantly reduced triglyceride accumulation in rat islets. Importantly, lipid droplets accumulated only in β-cells but not in α-cells in an mTORC1-dependent manner. Nutrient activation of mTORC1 upregulated the expression of adipose differentiation related protein (ADRP), known to stabilize lipid droplets. Rat islet size and new DNA synthesis also increased under nutrient overload. Insulin secretion into the culture medium increased steadily over a 4-day period without any significant difference between glucose (10 mM) alone and the combination of glucose (10 mM) and FFAs (240 μM). Insulin content and insulin biosynthesis, however, were significantly reduced under the combination of nutrients compared with glucose alone. Elevated nutrients also stimulated lipid droplet formation in human islets in an mTORC1-dependent manner. Unlike rat islets, however, human islets did not increase in size under nutrient overload despite a normal response to nutrients in releasing insulin. The different responses of islet cell growth under nutrient overload appear to impact insulin biosynthesis and storage differently in rat and human islets.

Keywords: :

• lipid droplets
• mTORC1
• ADRP
• rapamycin
• nutrient overload
• human islets
• time lapse studies
• insulin

Disclosure of Potential Conflicts of Interest

No potential conflicts of interest were disclosed.

Acknowledgments

This work was supported by NIH Grant 1R15DK094142 (G.K.), SIUE Internal Grants (G.K.), American Association of Colleges of Pharmacy (G.K.), NIH Grant DK06181 (M.L.M.), DK00618146S1 (M.L.M.),Washington University Diabetes Research and Training Center Morphology and BioImaging Core DK20579 (M.L.M.), ADA 7-08-RA-57 (A.S.G.), USDA, Agricultural Research Service 58-1950-7-707 (A.S.G.), NIH RO1-DK0822574 (A.S.G.),1RC2ES018781 (A.S.G.) and R24DK087669 (A.S.G.). All procedures performed on animals were approved by the SIUE Institutional Animal Care and Use Committee.