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Glucose-induced insulin secretion from hyperglycemic 90% pancreatectomized rats is markedly impaired, possibly because of loss of β cell differentiation. Association of these changes with β cell hypertrophy, increased mRNA levels of the transcription factor c-Myc, and their complete normalization by phlorizin treatment suggested a link between chronic hyperglycemia, increased c-Myc expression, and altered β cell function. In this study, we tested the effect of hyperglycemia on rat pancreatic islet c-Myc expression both in vivoand in vitro. Elevation of plasma glucose for 1–4 days (glucose infusion/clamp) was followed by parallel increases in islet mRNA levels (relative to TATA-binding protein) of c-Myc and two of its target genes, ornithine decarboxylase and lactate dehydrogenase A. Similar changes were observed in vitroupon stimulation of cultured islets or purified β cells with 20 and 30 mmol·liter−1glucose for 18 h. These effects of high glucose were reproduced by high potassium-induced depolarization or dibutyryl-cAMP and were inhibited by agents decreasing cytosolic Ca2+or cAMP concentrations. In conclusion, the expression of the early response gene c-Myc in rat pancreatic β cells is stimulated by high glucose in a Ca2+-dependent manner and by cAMP. c-Myc could therefore participate to the regulation of β cell growth, apoptosis, and differentiation under physiological or pathophysiological conditions.

Vascular endothelial growth factor (VEGF) and its two receptor tyrosine kinases, Flk-1/KDR and Flt-1, may play an important role in mediating the revascularization of transplanted pancreatic islets.

Differentiated pancreatic β cells are unique in their ability to secrete insulin in response to a rise in plasma glucose. We have proposed that the unique constellation of genes they express may be lost in diabetes due to the deleterious effect of chronic hyperglycemia. To test this hypothesis, Sprague-Dawley rats were submitted to a 85–95% pancreatectomy or sham pancreatectomy. One week later, the animals developed mild to severe chronic hyperglycemia that was stable for the next 3 weeks, without significant alteration of plasma nonesterified fatty acid levels. Expression of many genes important for glucose-induced insulin release decreased progressively with increasing hyperglycemia, in parallel with a reduction of several islet transcription factors involved in β cell development and differentiation. In contrast, genes barely expressed in sham islets (lactate dehydrogenase A and hexokinase I) were markedly increased, in parallel with an increase in the transcription factor c-Myc, a potent stimulator of cell growth. These abnormalities were accompanied by β cell hypertrophy. Changes in gene expression were fully developed 2 weeks after pancreatectomy. Correction of blood glucose by phlorizin for the next 2 weeks normalized islet gene expression and β cell volume without affecting plasma nonesterified fatty acid levels, strongly suggesting that hyperglycemia triggers these abnormalities. In conclusion, chronic hyperglycemia leads to β cell hypertrophy and loss of β cell differentiation that is correlated with changes in c-Myc and other key transcription factors. A similar change in β cell differentiation could contribute to the profound derangement of insulin secretion in human diabetes.

1The cytoplasmic Ca2+concentration ([Ca2+]i) was measured in single cells and cell clusters of different sizes prepared from mouse pancreatic islets.2During stimulation with 15 mM glucose, 20 % of isolated cells were inert, whereas 80 % showed [Ca2+]ioscillations of variable amplitude, duration and frequency. Spectral analysis identified a major frequency of 0.14 min−1and a less prominent one of 0.27 min−1.3In contrast, practically all clusters (2–50 cells) responded to glucose, and no inert cells were identified within the clusters. As compared to single cells, mean [Ca2+]iwas more elevated, [Ca2+]ioscillations were more regular and their major frequency was slightly higher (but reached a plateau at ≈0.25 min−1). In some cells and clusters, faster oscillations occurred on top of the slow ones, between them or randomly.4Image analysis revealed that the regular [Ca2+]ioscillations were well synchronized between all cells of the clusters. Even when the Ca2+response was irregular, slow and fast [Ca2+]ioscillations induced by glucose were also synchronous in all cells.5In contrast, [Ca2+]ioscillations resulting from mobilization of intracellular Ca2+by acetylcholine were restricted to certain cells only and were not synchronized.6Heptanol and 18α‐glycyrrhetinic acid, two agents widely used to block gap junctions, altered glucose‐induced Ca2+oscillations, but control experiments showed that they also exerted effects other than a selective uncoupling of the cells.7The results support theoretical models predicting an increased regularity of glucose‐dependent oscillatory events in clusters as compared to isolated islet cells, but contradict the proposal that the frequency of the oscillations increases with the number of coupled cells. Islet cell clusters function better as electrical than biochemical syncytia. This may explain the co‐ordination of [Ca2+]ioscillations driven by depolarization‐dependent Ca2+influx during glucose stimulation.

The potential roles of protein tyrosine kinases (TKs) and of phosphotyrosine phosphatases (PTPs) in pancreatic islet function are not known. In this study, we investigated whether vanadate, a potent PTP inhibitor, affects phosphoinositide (PI) metabolism by a TK-dependent pathway in isolated mouse islets. To avoid the confounding effects of changes in Ca2+ influx, all experiments were performed in the absence of Ca2+. In the presence of 15 mM glucose, vanadate, acetylcholine (ACh) or [Arg]vasopressin (AVP) strongly stimulated InsP production. Vanadate also increased PtdInsP levels in membranes. The TK inhibitor genistein (not its inactive analogues genistin and daidzein) significantly reduced vanadate effects, but was without effect in the absence of stimulation or in the presence of ACh or AVP. Islet proteins resolved by SDS/PAGE were analysed by immunoblotting with anti-phosphotyrosine antibody. Under control conditions, several phosphotyrosyl-proteins (PYPs) were present. Vanadate increased phosphotyrosine residues on several PYPs, notably two proteins of 145 and 85 kDa. This effect was prevented by genistein. p145 and p85 could correspond to phospholipase Cγ (PLCγ) and the regulatory subunit of PtdIns-3-kinase (PtdIns-3K) respectively. Both proteins are expressed in islets, as revealed by immunoblots with specific antibodies. Tungstate, another PTP inhibitor, reproduced vanadate effects, but inhibition of PtdIns-3K by wortmannin failed to affect vanadate-increased PtdInsP levels. Incubation of the islets in the presence of 10% (v/v) fetal calf serum instead of BSA increased InsP production and this effect was prevented by genistein. These results suggest that inhibition of PTP increases InsP production in mouse islets by a TK-dependent pathway. They also provide evidence for a potential role of TK and PTP in pancreatic B-cell function.

Pharmacological inhibition of AQP1 prevents H2O2transport and hypertrophy of cardiac myocytes.