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Title: The role of diet in development and reversal of type 2 diabetes
Other Titles: De rol van dieet in ontwikkeling en remissie van type 2 diabetes
Authors: Vangoitsenhoven, Roman
Issue Date: 6-Jun-2016
Abstract: Type 2 diabetes is a chronic disease, characterized by insulin resistance and beta-cell dysfunction. It is thought to be a consequence of both genetic predisposition and environmental factors. Although sedentary lifestyle and Western diet are the main drivers of the rising prevalence of type 2 diabetes, environmental pollutants have been reported to play a role as well.
In the first part of this thesis, we evaluated the role of the cereulide toxin in beta cell failure and the onset of type 2 diabetes, as cereulide had been implicated in beta-cell toxicity and it had been found frequently in low concentrations in starchy dishes in Belgian restaurants. We confirmed the increased sensitivity to cereulide of different beta-cell models when compared to other mammalian cell lines. Rat and mouse beta-cell lines (INS-1E and MIN6) and freshly isolated murine pancreatic islets showed very high rates of apoptosis after exposure to 5 ng/ml cereulide for 24 h, whereas human hepatocellular carcinoma (HepG2) and renal fibroblast cells (COS-1) remained viable when exposed to the same concentration. Hoechst/propidium Iodide staining and electron microscopy evaluation revealed that the observed cell death was mainly due to apoptosis, which was further supported by increased caspase 3/7 activation, elevated cytochrome C release into the cytoplasm and upregulation of pro-apoptotic mRNA markers in MIN6 cells. Transmission electron microscopy showed that mitochondria of the MIN6 cells were swollen and disintegrated after exposure to 0.5 ng/ml cereulide. At this dose, reactive oxygen species culminated more than twofold, and basal respiration rate was reduced to half, as compared to unexposed MIN6 cells. Presumably, such high mitochondrial toxicity explains the reduced insulin secretion by MIN6 cells or whole mouse islets after cereulide exposure.Taken together, we show that cereulide causes apoptotic beta-cell death at low concentrations and impairs beta-cell function at even lower concentrations, with mitochondrial dysfunction underlying these defects.
In the second part of the thesis, we investigated the impact of Roux-en-Y Gastric Bypass (RYGB) on pancreatic islet mass and function in a mouse model. Diet-induced obese C57BL/6J mice underwent RYGB or sham surgery at the age of 22 weeks. Postoperatively, the mice consumed the same high fat diet (HFD) as before, and sham mice were either pair-fed (PFS) or weight-matched (WMS) to the RYGB mice and compared to age-matched normal chow fed LEAN control mice. PFS mice regained body weight quickly after sham surgery to become obese and glucose intolerant by the end of the 8 week postsurgical follow-up period. RYGB and WMS on the other hand, had a similar body weight and body fat percentage as LEAN mice. Total energy expenditure was higher in RYGB compared to WMS mice. In vitro glucose stimulated insulin secretion was not significantly altered in isolated islets after RYGB, compatible with the hypothesis that changes in the gastrointestinal tract are necessary for the RYGB-specific observed effects on beta-cell function. Histological examination showed similar islet numbers in LEAN, WMS, RYGB and PFS mice. However, islets of PFS mice were larger than islets of LEAN mice. Interestingly, the islet size in RYGB and WMS also tended to be larger than in LEAN mice. We speculate that the islet size increased during the high fat feeding that rendered the mice obese. Subsequently, islet partial involution occurs when weight is lost by either RYGB or caloric restriction. We did not detect differences in islet composition, in particular in terms of relative insulin content, between experimental groups. Ki67-staining showed very low proliferation of islet cells in all groups, as can be expected in 30 week old mice at steady state. Pathway analysis of the RYGB islet transcriptome suggested no difference in inflammation or endoplasmic reticulum stress in islets post-RYGB compared to WMS mice. However, several proliferation/differentiation markers were upregulated in islets following RYGB and most members of the Reg gene family were expressed in the pancreases of the RYGB but not the WMS mice. Overexpression of Reg2 and Reg3b in MIN6 cells line, did not affect the insulin secretion. As no difference in proliferation was noted in the histology samples, the relevance of the Reg gene upregulation in islets after RYGB remains to be determined and will be subject of further research. In this chapter, we concluded that islet histology, function and gene profile did not differ dramatically between the mice that underwent RYGB or lost similar weight by caloric restriction.
In the third part of the thesis we investigated the metabolic impact after different weight loss strategies. Twenty two weeks old diet-induced obese C57BL/6J mice underwent RYGB-HF or caloric restriction on either normal chow (RESTR-NC) or HFD (RESTR-HF), in order to attain a similar body weight as the LEAN control mice on normal chow at the end of the 8 weeks intervention period. Thirty weeks old HFD fed mice were used as age-matched OBESE controls. Oxygen consumption, heat production and total energy expenditure decreased with weight loss through adaptive thermogenesis and there was no significant difference between RESTR-NC and RESTR-HF mice. The epididymal white adipose tissue (eWAT) pads and individual adipocytes of the RESTR-HF mice were significantly larger than in RESTR-NC and RYGB-HF mice. The adipocytes in RESTR-NC mice were not enlarged anymore, and the increased density compared to NC-CO mice suggests that adipocyte size rather than number is affected by weight loss. In addition, mRNA markers of inflammation were at normal levels in RESTR-NC and RYGB-HF mice, but were increased in the eWAT of OBESE mice and RESTR-HF mice. Both histological examination and triglyceride (TG) content evaluation of hepatic tissues showed massive steatosis in all OBESE mice, but also signs of steatosis in RESTR-HF and RYGB-HF mice, while RESTR-NC and LEAN were free of fatty liver disease. The eWAT size and hepatic TG content was tightly correlated with the glucose and insulin tolerance tests. Actually, the intraperitoneal glucose tolerance tests revealed that the RESTR-HF mice were only slighty less glucose intolerant than the OBESE mice, despite a major weight loss. If the same weight loss was achieved on a normal chow diet on the other hand (RESTR-NC) or by RYGB the glucose tolerance was restored. Thus, we clearly show that metabolic improvement is not solely weight driven. We demonstrate that shifting to lower fat foods has a greater metabolic benefit than losing weight by restricting calories or surgery without modifications of the food content. Food with lower fat content causes less fat disposition and by consequence less glucose intolerance than weight loss achieved through restriction on fat.
The overall conclusion of the present thesis is that external pollutants can contribute to type 2 diabetes by direct detrimental effects on beta-cell survival and function and that the nature of weight loss strategies matters by its impact on fat disposition and insulin resistance, but has no direct effect on beta-cells.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Clinical and Experimental Endocrinology

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