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

Islets. Nov-Dec 2012;4(6):379-92. doi: 10.4161/isl.22720. Epub 2012 Nov 1.

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: ADRP; human islets; insulin; lipid droplets; mTORC1; nutrient overload; rapamycin; time lapse studies.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Blotting, Western
  • Diabetes Mellitus, Type 2 / metabolism
  • Fatty Acids, Nonesterified / administration & dosage*
  • Fatty Acids, Nonesterified / metabolism
  • Glucose / administration & dosage*
  • Glucose / metabolism
  • Humans
  • Immunohistochemistry
  • Insulin-Secreting Cells / drug effects
  • Insulin-Secreting Cells / metabolism*
  • Insulin-Secreting Cells / ultrastructure
  • Male
  • Mechanistic Target of Rapamycin Complex 1
  • Membrane Proteins / metabolism
  • Microscopy, Phase-Contrast
  • Multiprotein Complexes
  • Perilipin-2
  • Proteins / antagonists & inhibitors
  • Proteins / metabolism
  • Rats
  • Rats, Sprague-Dawley
  • Sirolimus / pharmacology
  • TOR Serine-Threonine Kinases
  • Triglycerides / metabolism

Substances

  • Fatty Acids, Nonesterified
  • Membrane Proteins
  • Multiprotein Complexes
  • PLIN2 protein, human
  • Perilipin-2
  • Plin2 protein, rat
  • Proteins
  • Triglycerides
  • TOR Serine-Threonine Kinases
  • Mechanistic Target of Rapamycin Complex 1
  • Glucose
  • Sirolimus