Cyclical and Alternating Infusions of Glucose and Intralipid in Rats Inhibit Insulin Gene Expression and Pdx-1 Binding in Islets

Abstract

OBJECTIVE—Prolonged exposure of isolated islets of Langerhans to elevated levels of fatty acids, in the presence of high glucose, impairs insulin gene expression via a transcriptional mechanism involving nuclear exclusion of pancreas-duodenum homeobox-1 (Pdx-1) and loss of MafA expression. Whether such a phenomenon also occurs in vivo is unknown. Our objective was therefore to ascertain whether chronic nutrient oversupply inhibits insulin gene expression in vivo. RESEARCH DESIGN AND METHODS—Wistar rats received alternating 4-h infusions of glucose and Intralipid for a total of 72 h. Control groups received alternating infusions of glucose and saline, saline and Intralipid, or saline only. Insulin and C-peptide secretion were measured under hyperglycemic clamps. Insulin secretion and gene expression were assessed in isolated islets, and β-cell mass was quantified by morphometric analysis. RESULTS—Neither C-peptide secretion nor insulin sensitivity was different among infusion regimens. Insulin content and insulin mRNA levels were lower in islets isolated from rats infused with glucose plus Intralipid. This was associated with reduced Pdx-1 binding to the endogenous insulin promoter, and an increased proportion of Pdx-1 localized in the cytoplasm versus the nucleus. In contrast, MafA mRNA and protein levels and β-cell mass and proliferation were unchanged. CONCLUSIONS—Cyclical and alternating infusions of glucose and Intralipid in normal rats inhibit insulin gene expression without affecting insulin secretion or β-cell mass. We conclude that fatty acid inhibition of insulin gene expression, in the presence of high glucose, is an early functional defect that may contribute to β-cell failure in type 2 diabetes.

Chronic Exposure to Excess Nutrients Left-shifts the Concentration Dependence of Glucose-stimulated Insulin Secretion in Pancreatic β-Cells

“…In contrast, it is known that LC fatty acids can deplete insulin stores by promoting secretion of insulin while simultaneously inhibiting proinsulin synthesis (26,27). Additionally, multiple reports have documented the ability of excess lipid to inhibit insulin gene transcription both in vitro (28) and in vivo (29). It has also been recognized that culture in excess nutrients enhances basal secretion, which has been difficult to reconcile with the reduced maximal secretion.…”

A free-choice high-fat high-sugar diet induces glucose intolerance and insulin unresponsiveness to a glucose load not explained by obesity

“…In addition, we cannot exclude the possibility that alterations in glucose metabolism because of a HFHS-choice diet involve the 'toxic' effect of the combined increased levels of glucose and fatty acids on the pancreas (the so-called 'glucolipotoxicity' theory). 29 However, as the glucose elevations in HFHS-choice diet rats are much less than glucose concentrations necessary (twofold) to affect b-cell function, 30 it seems unlikely that the effect of the HFHS-choice diet involves glucolipotoxicity.…”

Regulation of beta cell replication

“…For example, studies of alternating 4-hour glucose (50%) infusions over a 72-hour period showed a total lack of effect of glucose on beta cell mass and proliferation in Wistar rats (Hagman et al, 2008), whereas, as mentioned above, continuous glucose infusion (20%) increased beta cell mass significantly after 48-hour in Sprague-Dawley rat . These results suggest considerations regarding infusion protocols, animal strains, dose-dependency, must be taken into accounts when comparing data between studies, and also their interpretations.…”