Prevention of glucose toxicity in HIT-T15 cells and Zucker diabetic fatty rats by antioxidants

Abstract

Chronic exposure of pancreatic islets to supraphysiologic concentrations of glucose causes adverse alterations in ␤ cell function, a phenomenon termed glucose toxicity and one that may play a secondary pathogenic role in type 2 diabetes. However, no mechanism of action has been definitively identified for glucose toxicity in ␤ cells. To ascertain whether chronic oxidative stress might play a role, we chronically cultured the ␤ cell line, HIT-T15, in medium containing 11.1 mM glucose with and without the antioxidants, N-acetyl-L-cysteine (NAC) or aminoguanidine (AG). Addition of NAC or AG to the culture medium at least partially prevented decreases in insulin mRNA, insulin gene promoter activity, DNA binding of two important insulin promoter transcription factors (PDX-1͞STF-1 and RIPE-3b1 activator), insulin content, and glucose-induced insulin secretion. These findings suggested that one mechanism of glucose toxicity in the ␤ cell may be chronic exposure to reactive oxygen species, i.e., chronic oxidative stress. To ascertain the effects of these drugs on diabetes, NAC or AG was given to Zucker diabetic fatty rats, a laboratory model of type 2 diabetes, from 6 through 12 weeks of age. Both drugs prevented a rise in blood oxidative stress markers (8-hydroxy-2-deoxyguanosine and malondialdehyde ؉ 4-hydroxy-2-nonenal), and partially prevented hyperglycemia, glucose intolerance, defective insulin secretion as well as decrements in ␤ cell insulin content, insulin gene expression, and PDX-1 (STF-1) binding to the insulin gene promoter. We conclude that chronic oxidative stress may play a role in glucose toxicity, which in turn may worsen the severity of type 2 diabetes.

High glucose and hydrogen peroxide increase c-Myc and haeme-oxygenase 1 mRNA levels in rat pancreatic islets without activating NF?B

“…It is also compatible with earlier reports on the beneficial effect of antioxidants, including NAC, on beta cell glucose toxicity in models of type 2 diabetes [32,33,49,50]. It should be noted, however, that very high NAC or cysteine concentrations can be detrimental (pro-oxidant?)…”

Molecular mechanisms for hyperinsulinaemia induced by overproduction of selenium-dependent glutathione peroxidase-1 in mice

“…Infection of rat islets with adenovirus encoding human GPX1 gene protected against the ribose-induced loss of insulin mRNA, content and secretion. However, a sixfold increase in GPX1 activity and 72 h infection seemed to be insufficient to alter baseline levels of these three parameters [6,24].…”

“…Uncoupling protein 2 (UCP2) serves as a negative regulator of mitochondrial membrane potential (Δ= m ) [18], which positively correlates with glucosestimulated insulin secretion (GSIS) [19]. Levels and function of PDX1 and UCP2 are affected by intracellular ROS status [20,21], glucotoxicity [22] and antioxidants [6,23,24]. However, it is not known whether in vivo global overexpression of Gpx1 could overly diminish intracellular production of hydroperoxides and subsequently dysregulate production of PDX1 and UCP2 and their role in insulin synthesis and secretion.…”

“…Although post-transcriptional regulation of PDX1 by ROS or antioxidants has been postulated [20,[23][24][25], new mechanisms such as epigenetic modification [26] of Pdx1 have not been studied. The proximal region of Pdx1 promoter consists of an islet-specific expression consensus E-box motif, which predominantly binds the upstream transcription factor [27].…”

Chronic treatment with a glucokinase activator delays the onset of hyperglycaemia and preserves beta cell mass in the Zucker diabetic fatty rat

“…This genetic defect renders these animals especially vulnerable to lipotoxicity and 'lipoapoptosis' [28][29][30][31]. In the ZDF rat, lipid overload leads to inflammation and oxidative stress, beta cell dysfunction, and then beta cell loss beginning at 7-8 weeks of age.…”