Peroxisome Proliferator-Activated Receptor-α Regulates the Expression of Pancreatic/Duodenal Homeobox-1 in Rat Insulinoma (INS-1) Cells and Ameliorates Glucose-Induced Insulin Secretion Impaired by Palmitate

Abstract

Both peroxisome proliferator-activated receptor-alpha (PPARalpha) and pancreatic/duodenal homeobox-1 (PDX-1) have been reported to be associated with glucose-stimulated insulin secretion (GSIS), but the relationship between PPARalpha and PDX-1 is not yet fully understood. In the present study, we tested the hypothesis that PPARalpha regulates the expression of PDX-1 in beta-cells. Isolated pancreatic islets from Wistar rats and rat pancreatic insulinoma (INS-1) beta-cells were cultured in media supplemented with and without 0.2 or 0.4 mm palmitate, and treated with and without a PPARalpha agonist (fenofibrate) or PPARalpha antagonist (MK886). Results indicated that treatment with fenofibrate significantly enhanced PPARalpha mRNA and protein expression in cells cultured with elevated palmitate concentrations compared with cells that did not receive fenofibrate treatment. In turn, this enhanced expression led to an increase in PDX-1 mRNA and nuclear protein, as well as DNA binding activity of PDX-1 with the insulin promoter. Accordingly, the expression of the PDX-1 downstream targets, insulin and glucose transporter-2, increased, resulting in increased intracellular insulin content and GSIS. Treatment with MK886 inhibited expression of PPARalpha, blocking PPARalpha-regulated PDX-1 expression, and the downstream transcription events of PDX-1. EMSA revealed that nuclear protein might bind with the peroxisome proliferator response element sequence located in the PDX-1 promoter. Collectively, these results demonstrate a regulatory relationship between PPARalpha and PDX-1 in INS-1 cells. Furthermore, PPARalpha activation potentiates GSIS under elevated palmitate conditions possibly via up-regulation of PDX-1. Our findings have potential clinical implications for the use of PPARalpha agonists in the treatment of type 2 diabetes.

ChREBP Mediates Glucose Repression of Peroxisome Proliferator-activated Receptor α Expression in Pancreatic β-Cells

“…PPAR␣ is a regulator of fatty acid metabolism in pancreatic ␤-cells, and PPAR␣ has been reported to protect against lipidinduced ␤-cell dysfunction (9,(12)(13)(14). Studies performed in rodents suggest that reduced activity of PPAR␣ is implicated in nutrient-induced ␤-cell dysfunction.…”

A chemical chaperone 4-PBA ameliorates palmitate-induced inhibition of glucose-stimulated insulin secretion (GSIS)

“…Whereas acute application of FFAs enhanced GSIS by augmentation of K ATP -independent amplifying pathways [21], prolonged exposure of FFAs decreased GSIS [2,22]. Although the mechanism behind FFA-induced GSIS-inhibition has not yet been fully established, it is believed that the down-regulation of PDX-1 [23], depression of ATP/ADP ratio [24], activation of K ATP channel [25], and interference with insulin exocytosis [26] contribute toward FFA-induced GSIS-inhibition. In our studies, treatment of cells with 200 lM palmitate for 24 h significantly inhibited GSIS whereas 300 lM palmitate for 24 h almost completely suppressed the glucose-mediated stimulatory effect on insulin secretion, demonstrating that palmitate had an inhibitory effect on GSIS and that this effect was palmitate-dose-dependent.…”

A novel role for thyroid-stimulating hormone: Up-regulation of hepatic 3-hydroxy-3-methyl-glutaryl-coenzyme a reductase expression through the cyclic adenosine monophosphate/protein kinase A/cyclic adenosine monophosphate-responsive element binding protei

“…qPCR was performed by using the primers listed in Supporting Table 1, according to a method as described previously. 14 Western Blot Analysis and Immunofluorescence. -See Supporting Materials and Methods.…”