Stress oxydatif et altérations phénotypiques des cellules β pancréatiques par les concentrations extrêmes de glucose : rôle du facteur de transcription c-MYC et de la glycation des protéines intracellulaires
(2011)
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- Supervisors
- Jonas, Jean-Christophe
- Abstract
- (en) As the main physiological stimulus of insulin secretion, glucose exerts beneficial effects on functional β-cell mass. However, chronic hyperglycemia or severe hypoglycemia are deleterious for β-cells. In vitro, β-cell function and survival are preserved during culture in the presence of 10 mM de glucose (G10) and strongly altered by a prolonged culture at low (5 mM glucose ; G5) or high glucose (30 mM glucose ; G30) concentrations. These alterations are associated with an increase in stress response gene expression (c-Myc, Heme oxygenase 1 (Hmox-1), Methallothionein (Mt1a)), suggesting a role of the pro-apoptotic transcription factor c-MYC and oxidative stress in these phenotypic alterations. In a first study, we investigated whether increased expression of the transcription factor c-MYC may contribute to the alterations of β-cell differentiation, survival and function induced by high glucose concentrations. Our results show that one week culture in G30 increased the islet sensitivity to glucose stimulation without affecting their maximal glucose responsiveness or their insulin content. In contrast, c-MYC activation induced β-cell apoptosis, decreased Preproinsulin gene expression and alterations of glucose stimulus-secretion coupling events that are better mimicked by 58 µM H2O2 than G30 (one week culture). This suggests that c-MYC does not contribute to the increase in islet glucose sensitivity but that it could be involved in the alterations of β-cell function provoked by oxidative stress. Non-enzymatic protein glycation, increased by chronic hyperglycemia, could play a role in high glucose-induced β-cell dysfunction by inducing oxidative stress and altering protein function. To address that question, we tested whether the deficiency in Fructosamine-3 kinase (FN3K) protein-repair enzyme increases the alterations of β-cell survival and function induced by G30 or 25 mM ribose. We have shown that Fn3k ablation increase fructoselysine level in islet cultured in G10 and to a lesser extent in those cultured in G30. However, this increase did not alter mouse β-cell function, islet cell survival and stress response gene expression (c-Myc, Mt1a, Hmox1) under control conditions and did not increase the sensitivity of mouse islets to the deleterious effects of a 1-3 weeks culture in G30 or ribose on β-cell function. On the contrary, Fn3k-/- islets cultured for 3 weeks in G30 displayed a lower induction of β-cell apoptosis and c-Myc gene expression. In contrast, FN3K deficiency further altered survival of islet cultured in ribose while reducing the stimulation of Mt1a gene expression. Chronic exposure of rat islets to low glucose concentrations (G5) markedly alters β-cell function while increasing β-cell apoptosis. The latter effect likely results from an increase in oxidative stress. However, it is still unclear whether oxidative stress also contributes to β-cell dysfunction after prolonged culture in low glucose. To investigate that question, we tested the effects of several antioxidants on the alterations of rat islet function and survival during one week culture in G5. First, we have shown that one week culture in G5 increases the oxydation state of the mitochondrial compartment, stress response gene expression and β-cell apoptosis. As expected, culture in G5 strongly reduced glucose-induced calcium rise, insulin content and Preproinsulin gene expression, leading to an almost complete suppression of insulin secretion. Despite beneficial effets on β-cell survival and oxidative stress, MnTBAP only sligthly improved the glucose-induced [Ca2+]I rise but was almost ineffective at restoring glucose-induced insulin secretion. These results suggest that prolonged culture in low glucose provoked islet oxidative stress that contributes largely to the increase in islet cell apoptosis and slightly to the strong reduction in β-cell glucose responsiveness. Alternatively, the moderate level of oxidative stress persisting in the presence of MnTBAP is sufficient to almost fully suppress β-cell function.
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Citations
Pascal, S. (2011). Stress oxydatif et altérations phénotypiques des cellules β pancréatiques par les concentrations extrêmes de glucose : rôle du facteur de transcription c-MYC et de la glycation des protéines intracellulaires. https://hdl.handle.net/2078.5/152464