A Novel Role for Lipoic Acid Synthase in Pancreatic β-Cell Function.

Padmalayam, Indira; Hamm, Rebecca; Saxena, Uday; Pillarisetti, Sivaram

Type 2 diabetes is characterized by impaired insulin secretion, attributable to progressive decline in pancreatic beta cell mass. Apoptosis of β-cells is the major contributor of the reduced β-cell mass. Therapeutic approaches designed to arrest apoptosis are a new development in the management of Type 2 diabetes since these approaches could potentially reverse the disease progression rather than just control hyperglycemia. Lipoic acid Synthase (LASY) is a mitochondrial enzyme that is responsible for endogenous synthesis of lipoic acid, a powerful antioxidant. We have previously shown that LASY plays a critical role in inflammation and insulin resistance associated with Type 2 diabetes. The purpose of this study was to determine the effect of LASY inducer, Compound 1 on β-cell function, and its effects on apoptosis of pancreatic cells. We first studied the effect of siRNA-based knock-down of LASY on apoptosis in a hamster pancreatic β-cell line, HIT-T15, by looking at expression of caspase 3 using real-time PCR. Knock-down of LASY resulted in 50% increase in caspase 3 mRNA levels over the levels obtained with scrambled control RNA. Treatment with Compound 1 restored LASY mRNA levels and decreased caspase 3 mRNA levels significantly, even below the normal levels obtained with the scrambled oligo. Treatment of pancreatic β-cells with Compound 1 also resulted in a >75% decrease in caspase 3/7 activity in a luminescence assay. Decrease in caspase activity correlated with a 4 fold increase in LASY gene expression. These experiments suggest a direct relationship between LASY induction and β-cell apoptosis in vitro. Next we looked at the expression of pancreatic and duodenal homeobox domain-1 (PDX-1), a critical player that is intimately involved with pancreatic β-cell development, function and insulin secretion. Deficiency of PDX-1 leads to decreased insulin secretion and results in the MODY (maturity-onset disease in the young) phenotype. Treatment of β-cells with Compound I resulted in a significant increase in PDX-1 expression, which correlated strongly with LASY gene expression. High concentrations of glucose decreased LASY gene expression and therefore PDX-1 expression. Treatment with Compound 1 was able to restore LASY and PDX-1 mRNA levels that were decreased by glucose. These experiments reveal a new role for LASY in pancreatic β-cell function. They also suggest that pharmacological induction of LASY could be an effective means of preventing β-cell apoptosis, thereby providing a novel approach to management of Type 2 diabetes.

LIPOIC acid; PANCREATIC beta cells; TYPE 2 diabetes; INSULIN; APOPTOSIS; GENE expression; BLOOD sugar

Diabetes, 2007, Vol 56, pA420

0012-1797

Academic Journal