Enhanced expression of hexokinase I in pancreatic islets induced by sucrose administration

The HDAC Inhibitor Butyrate Impairs β Cell Function and Activates the Disallowed Gene Hexokinase I

Histone deacetylase (HDAC) inhibitors such as butyrate have been reported to reduce diabetes risk and protect insulin-secreting pancreatic β cells in animal models. However, studies on insulin-secreting cells in vitro have found that butyrate treatment resulted in impaired or inappropriate insulin secretion. Our study explores the effects of butyrate on insulin secretion by BRIN BD-11 rat pancreatic β cells and examined effects on the expression of genes implicated in β cell function. Robust HDAC inhibition with 5 mM butyrate or trichostatin A for 24 h in β cells decreased basal insulin secretion and content, as well as insulin secretion in response to acute stimulation. Treatment with butyrate also increased expression of the disallowed gene hexokinase I, possibly explaining the impairment to insulin secretion, and of TXNIP, which may increase oxidative stress and β cell apoptosis. In contrast to robust HDAC inhibition (>70% after 24 h), low-dose and acute high-dose treatment with butyrate enhanced nutrient-stimulated insulin secretion. In conclusion, although protective effects of HDAC inhibition have been observed in vivo, potent HDAC inhibition impairs β cell function in vitro. The chronic low dose and acute high dose butyrate treatments may be more reflective of in vivo effects.

Impaired endocrine-metabolic homeostasis: underlying mechanism of its induction by unbalanced diet

To characterize the intrinsic mechanism by which sucrose induces β-cell dysfunction. Normal rats received for 3 weeks a standard diet supplemented with 10% sucrose in the drinking water (high sucrose (HS)) with/out an antioxidant agent (R/S α-lipoic acid). We measured plasma glucose, insulin, triglyceride, leptin, and lipid peroxidation levels; homeostasis model assessment (HOMA)-insulin resistance (HOMA-IR) and HOMA for β-cell function (HOMA-β) indexes were also determined. Insulin secretion, β-cell apoptosis, intracellular insulin and leptin mediators, and oxidative stress (OS) markers were also measured in islets isolated from each experimental group. HS rats had increased plasma triglyceride, insulin, leptin, and lipid peroxidation (OS marker) levels associated with an insulin-resistant state. Their islets developed an initial compensatory increase in glucose-induced insulin secretion and mRNA and protein levels of β-cell apoptotic markers. They also showed a significant decrease in mRNA and protein levels of insulin and leptin signaling pathway mediators. Uncoupling protein 2 (UCP2), peroxisome proliferator-activated receptor (PPAR)-α and -δ mRNA and protein levels were increased whereas mRNA levels of Sirtuin-1 (Sirt-1), glutathione peroxidase, and catalase were significantly lower in these animals. Development of all these endocrine-metabolic abnormalities was prevented by co-administration of R/S α-lipoic acid together with sucrose. OS may be actively involved in the mechanism by which unbalanced/unhealthy diet induces β-cell dysfunction. Since metabolic-endocrine dysfunctions recorded in HS rats resembled those measured in human pre-diabetes, knowledge of its molecular mechanism could help to develop appropriate strategies to prevent the progression of this metabolic state toward type 2 diabetes (T2D).

CNC-bZIP protein Nrf1-dependent regulation of glucose-stimulated insulin secretion

AIMS

The inability of pancreatic β-cells to secrete sufficient insulin in response to glucose stimulation is a major contributing factor to the development of type 2 diabetes (T2D). We investigated both the in vitro and in vivo effects of deficiency of nuclear factor-erythroid 2-related factor 1 (Nrf1) in β-cells on β-cell function and glucose homeostasis.

RESULTS

Silencing of Nrf1 in β-cells leads to a pre-T2D phenotype with disrupted glucose metabolism and impaired insulin secretion. Specifically, MIN6 β-cells with stable knockdown of Nrf1 (Nrf1-KD) and isolated islets from β-cell-specific Nrf1-knockout [Nrf1(b)-KO] mice displayed impaired glucose responsiveness, including elevated basal insulin release and decreased glucose-stimulated insulin secretion (GSIS). Nrf1(b)-KO mice exhibited severe fasting hyperinsulinemia, reduced GSIS, and glucose intolerance. Silencing of Nrf1 in MIN6 cells resulted in oxidative stress and altered glucose metabolism, with increases in both glucose uptake and aerobic glycolysis, which is associated with the elevated basal insulin release and reduced glucose responsiveness. The elevated glycolysis and reduced glucose responsiveness due to Nrf1 silencing likely result from altered expression of glucose metabolic enzymes, with induction of high-affinity hexokinase 1 and suppression of low-affinity glucokinase.

INNOVATION

Our study demonstrated a novel role of Nrf1 in regulating glucose metabolism and insulin secretion in β-cells and characterized Nrf1 as a key transcription factor that regulates the coupling of glycolysis and mitochondrial metabolism and GSIS.

CONCLUSION

Nrf1 plays critical roles in regulating glucose metabolism, mitochondrial function, and insulin secretion, suggesting that Nrf1 may be a novel target to improve the function of insulin-secreting β-cells.

Effect of dexamethasone on insulin secretion: examination of underlying mechanisms

OBJECTIVE

To study the mechanism of increased insulin secretion in response to short-term administration of dexamethasone.

METHODS

Male Wistar rats were injected intraperitoneally with dexamethasone (dexamethasone; 200 mcg/kg body weight per day) or saline for 3 consecutive days. Insulin secretion in response to glucose, ionomycin, and KCl was quantified in islets isolated from the animals, and the amount of glucokinase was measured by Western blot.

RESULTS

Dexamethasone-treated animals had 1.18-fold higher fasting blood glucose concentration and 6.5-fold increase in fasting serum insulin concentration compared with findings from animals injected with saline. Compared with islets isolated from control rats, islets from dexamethasone-treated rats secreted more insulin at 60 minutes in response to 5.5 mM glucose (416.4 vs 115.6 fmoles/10 islets, P = .011) and in response to 16.6 mM glucose (985.5 vs 520.6 fmoles/10 islets, P = .014); no change in insulin secretion was observed at 10 minutes. Insulin secretion from islets of dexamethasone-treated rats and control rats was not differentially augmented in response to either ionomycin or potassium chloride. Glucokinase expression was not altered by treatment with dexamethasone.

CONCLUSIONS

Augmentation of insulin secretion in response to glucose in the pancreatic islets from dexamethasone-treated rats is preserved in islets studied in vitro. The increase in glucose-stimulated insulin secretion appears to be mediated by steps upstream to β-cell membrane depolarization and the attended increase in intracellular calcium in the signaling pathway of insulin secretion.

Time-dependent mechanisms in beta-cell glucose sensing

The relation between plasma glucose and insulin release from pancreatic beta-cells is not stationary in the sense that a given glucose concentration leads to a specific rate of insulin secretion. A number of time-dependent mechanisms appear to exist that modify insulin release both on a short and a longer time scale. Typically, two phases are described. The first phase, lasting up to 10 min, is a pulse of insulin release in response to fast changes in glucose concentration. The second phase is a more steady increase of insulin release over minutes to hours, if the elevated glucose concentration is sustained. The paper describes the glucose sensing mechanism via the complex dynamics of the key enzyme glucokinase, which controls the first step in glucose metabolism: phosphorylation of glucose to glucose-6-phosphate. Three time-dependent phenomena (mechanisms) are described. The fastest, corresponding to the first phase, is a delayed negative feedback regulating the glucokinase activity. Due to the delay, a rapid glucose increase will cause a burst of activity in the glucose sensing system, before the glucokinase is down-regulated. The second mechanism corresponds to the translocation of glucokinase from an inactive to an active form. As the translocation is controlled by the product(s) of the glucokinase reaction rather than by the substrate glucose, this mechanism gives a positive, but saturable, feedback. Finally, the release of the insulin granules is assumed to be enhanced by previous glucose exposure, giving a so-called glucose memory to the beta-cells. The effect depends on the insulin release of the cells, and this mechanism constitutes a second positive, saturable feedback system. Taken together, the three phenomena describe most of the glucose sensing behaviour of the beta-cells. The results indicate that the insulin release is not a precise function of the plasma glucose concentration. It rather looks as if the beta-cells just increase the insulin production, until the plasma glucose has returned to normal. This type of integral control has the advantage that the precise glucose sensitivity of the beta-cells is not important for normal glucose homeostasis.