Search

Alteration of pancreatic [beta]-cell survival and Preproinsulin gene expression by prolonged hyperglycemia may result from increased c-MYC expression. However, it is unclear whether c-MYC effects on [beta]-cell function are compatible with its proposed role in glucotoxicity. We therefore tested the effects of short-term c-MYC activation on key [beta]-cell stimulus-secretion coupling events in islets isolated from mice expressing a tamoxifenswitchable form of c-MYC in [beta]-cells (MycER) and their wild-type littermates. Tamoxifen treatment of wild-type islets did not affect their cell survival, Preproinsulin gene expression, and glucose stimulussecretion coupling. In contrast, tamoxifen-mediated c-MYC activation for 2-3 days triggered cell apoptosis and decreased Preproinsulin gene expression in MycER islets. These effects were accompanied by mitochondrial membrane hyperpolarization at all glucose concentrations, a higher resting intracellular calcium concentration ([[[Ca.sup.2+].sub.i]], and lower glucose-induced [[[Ca.sup.2+]].sub.i] rise and islet insulin content, leading to a strong reduction of glucose-induced insulin secretion. Compared with these effects, 1-wk culture in 30 mmol/l glucose increased the islet sensitivity to glucose stimulation without reducing the maximal glucose effectiveness or the insulin content. In contrast, overnight exposure to a low [H.sub.2][O.sub.2] concentration increased the islet resting [[[Ca.sup.2+]].sub.i] and reduced the amplitude of the maximal glucose response as in tamoxifen-treated MycER islets. In conclusion, c-MYC activation rapidly stimulates apoptosis, reduces Preproinsulin gene expression and insulin content, and triggers functional alterations of [beta]-cells that are better mimicked by overnight exposure to a low [H.sub.2][O.sub.2] concentration than by prolonged culture in high glucose. [beta]-cell mass; apoptosis; cytosolic calcium concentration; insulin secretion; mitochondrial membrane potential

Stimulatory concentrations of glucose induce two patterns of cytosolic [Ca.sup.2+] concentration ([[[Ca.sup.2+]].sub.c]) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [[[Ca.sup.2+]].sub.c] and insulin release from single islets revealed that mixed [[[Ca.sup.2+]].sub.c] oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single [beta]-cell within an islet and of [[[Ca.sup.2+]].sub.c] in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most [beta]-cells of the islet. Each slow [[[Ca.sup.2+]].sub.c] increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum [Ca.sup.2+]-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [[[Ca.sup.2+]].sub.c] oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA[3.sup.+/+] [beta]-cells. We conclude that glucose-induced mixed [[[Ca.sup.2+]].sub.c] oscillations result from compound bursting in all [beta]-cells of the islet. Disruption of SERCA3 abolishes mixed [[[Ca.sup.2+]].sub.c] oscillations and augments [beta]-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the [beta]-cell membrane potential during glucose stimulation. electrical activity; insulin-secreting cell; thapsigargin

Adaptation of [Beta]-cell mass to substrate oversupply: enhanced function with normal gene expression. Am J Physiol Endocrinol Metab 280: E788-E796, 2001.--Although type 2 diabetes mellitus is associated with insulin resistance, many individuals compensate by increasing insulin secretion. Putative mechanisms underlying this compensation were assessed in the present study by use of 4-day glucose (GLC; 35% Glc, 2 ml/h) and lipid (LIH; 10% Intralipid + 20 U/ml heparin; 2 ml/h) infusions to rats. Within 2 days of beginning the infusion of either lipid or glucose, plasma glucose profiles were normalized (relative to saline-infused control rats; SAL; 0.45% 2 ml/h). During glucose infusion, plasma glucose was maintained in the normal range by an approximately twofold increase in plasma insulin and an ~80% increase in [Beta]-cell mass. During LIH infusion, glucose profiles were also maintained in the normal range. Plasma insulin responses during feeding were doubled, and [Beta]-cell mass increased 54%. For both groups, the increase in [Beta]-cell mass was associated with increased [Beta]-cell proliferation (98% increase during GLC and 125% increase during LIH). At the end of the 4-day infusions, no significant changes were observed in islet-specific gene transcription (i.e., the expression of islet hormone genes, glucose metabolism genes, and insulin transcription factors were unaffected). Two days after termination of the infusions, the glucose-stimulated plasma insulin response was increased ~67% in glucose-infused animals. No sustained effect on insulin secretory capacity was observed in the LIH animals. The increase in plasma insulin response after glucose infusion was achieved in the absence of any change in insulin clearance. We conclude that, in rats, an increase in insulin demand after an increase in glucose appearance or free fatty acid leads to an increase in [Beta]-cell mass, mediated in part by an increase in [Beta]-cell proliferation, and that these compensatory changes lead to increased insulin secretion, normal plasma glucose levels, and the maintenance of normal islet gene expression. insulin resistance; insulin secretion; [Beta]-cell; mitosis; maturity onset diabetes of the young

Stimulatory concentrations of glucose induce two patterns of cytosolic Ca2+ concentration ([Ca2+]c) oscillations in mouse islets: simple or mixed. In the mixed pattern, rapid oscillations are superimposed on slow ones. In the present study, we examined the role of the membrane potential in the mixed pattern and the impact of this pattern on insulin release. Simultaneous measurement of [Ca2+]c and insulin release from single islets revealed that mixed [Ca2+]c oscillations triggered synchronous oscillations of insulin secretion. Simultaneous recordings of membrane potential in a single β-cell within an islet and of [Ca2+]c in the whole islet demonstrated that the mixed pattern resulted from compound bursting (i.e., clusters of membrane potential oscillations separated by prolonged silent intervals) that was synchronized in most β-cells of the islet. Each slow [Ca2+]c increase during mixed oscillations was due to a progressive summation of rapid oscillations. Digital image analysis confirmed the good synchrony between subregions of an islet. By contrast, islets from sarco(endo)plasmic reticulum Ca2+-ATPase isoform 3 (SERCA3)-knockout mice did not display typical mixed [Ca2+]c oscillations in response to glucose. This results from a lack of progressive summation of rapid oscillations and from altered spontaneous electrical activity, i.e., lack of compound bursting, and membrane potential oscillations characterized by lower-frequency but larger-depolarization phases than observed in SERCA3+/+ [3-cells. We conclude that glucose-induced mixed [Ca2+]c oscillations result from compound bursting in all β-cells of the islet. Disruption of SERCA3 abolishes mixed [Ca2+]c oscillations and augments β-cell depolarization. This latter observation indicates that the endoplasmic reticulum participates in the control of the β-cell membrane potential during glucose stimulation. [ABSTRACT FROM AUTHOR]

In rat pancreatic islets, β-cell gene expression, survival, and subsequent acute glucose stimulation of insulin secretion (GSIS) are optimally preserved by prolonged culture at 10 mM glucose (G10) and markedly altered by culture at G5 or G30. Here, we tested whether pharmacological glucokinase (GK) activation prevents these alterations during culture or improves GSIS after culture. Rat pancreatic islets were cultured 1-7 days at G5, G10, or G30 with or without 3 μM of the GK activator Ro 28-0450 (Ro). After culture, β-cell apoptosis and islet gene mRNA levels were measured, and the acute glucose-induced increase in NAD(P)H autofluorescence, intracellular calcium concentration, and insulin secretion were tested in the absence or presence of Ro. Prolonged culture of rat islets at G5 or G30 instead of G10 triggered β-cell apoptosis and reduced their glucose responsiveness. Addition of Ro during culture differently affected β-cell survival and glucose responsiveness depending on the glucose concentration during culture: it was beneficial to β-cell survival and function at G5, detrimental at G10, and ineffective at G30. In contrast, acute GK activation with Ro increased the glucose sensitivity of islets cultured at G10 but failed at restoring β-cell glucose responsiveness after culture at G5 or G30. We conclude that pharmacological GK activation prevents the alteration of β-cell survival and function by long-term culture at G5 but mimics glucotoxicity when added to G10. The complex effects of glucose on the β-cell phenotype result from changes in glucose metabolism and not from an effect of glucose per se., (Copyright © 2015 the American Physiological Society.)