Differential effects of overexpressed glucokinase and hexokinase I in isolated islets. Evidence for functional segregation of the high and low Km enzymes

Androgen-induced upregulation of CFTR in pancreatic β-cell contributes to hyperinsulinemia in PCOS model

PURPOSE

Polycystic ovarian syndrome (PCOS) is an endocrine-metabolic condition affecting 5-10% of reproductive-aged women and characterized by hyperandrogenism, insulin resistance (IR), and hyperinsulinemia. CFTR is known to be regulated by steroid hormones, and our previous study has demonstrated an essential role of CFTR in β-cell function. This study aims to investigate the contribution of androgen and CFTR to hypersecretion of insulin in PCOS and the underlying mechanism.

METHODS

We established a rat PCOS model by subcutaneously implanting silicon tubing containing Dihydrotestosterone (DHT). Glucose tolerance test with insulin levels was performed at 9 weeks after implantation. A rat β-cell line RINm5F, a mouse β-cell line β-TC-6, and mouse islets were treated with DHT, and with or without the androgen antagonist flutamide for CFTR and insulin secretion-related functional assays or mRNA/protein expression measurement. The effect of CFTR inhibitors on DHT-promoted membrane depolarization, glucose-stimulated intracellular Ca2+ oscillation and insulin secretion were examined by membrane potential imaging, calcium imaging and ELISA, respectively.

RESULTS

The DHT-induced PCOS model showed increased body weight, impaired glucose tolerance, and higher blood glucose and insulin levels after glucose stimulation. CFTR was upregulated in islets of PCOS model and DHT-treated cells, which was reversed by flutamide. The androgen receptor (AR) could bind to the CFTR promoter region, which was enhanced by DHT. Furthermore, DHT-induced membrane depolarization, enhanced glucose-stimulated Ca2+ oscillations and insulin secretion, which could be abolished by CFTR inhibitors.

CONCLUSIONS

Excessive androgen enhances glucose-stimulating insulin secretion through upregulation of CFTR, which may contribute to hyperinsulinemia in PCOS.

Increased glycolysis affects β-cell function and identity in aging and diabetes

Objective

Age is a risk factor for type 2 diabetes (T2D). We aimed to elucidate whether β-cell glucose metabolism is altered with aging and contributes to T2D.

Methods

We used senescence-accelerated mice (SAM), C57BL/6J (B6) mice, and ob/ob mice as aging models. As a diabetes model, we used db/db mice. The glucose responsiveness of insulin secretion and the [U-¹³C]-glucose metabolic flux were examined in isolated islets. We analyzed the expression of β-cell-specific genes in isolated islets and pancreatic sections as molecular signatures of β-cell identity. β cells defective in the malate-aspartate (MA) shuttle were previously generated from MIN6-K8 cells by the knockout of Got1, a component of the shuttle. We analyzed Got1 KO β cells as a model of increased glycolysis.

Results

We identified hyperresponsiveness to glucose and compromised cellular identity as dysfunctional phenotypes shared in common between aged and diabetic mouse β cells. We also observed a metabolic commonality between aged and diabetic β cells: hyperactive glycolysis through the increased expression of nicotinamide mononucleotide adenylyl transferase 2 (Nmnat2), a cytosolic nicotinamide adenine dinucleotide (NAD)-synthesizing enzyme. Got1 KO β cells showed increased glycolysis, β-cell dysfunction, and impaired cellular identity, phenocopying aging and diabetes. Using Got1 KO β cells, we show that attenuation of glycolysis or Nmnat2 activity can restore β-cell function and identity.

Conclusions

Our study demonstrates that hyperactive glycolysis is a metabolic signature of aged and diabetic β cells, which may underlie age-related β-cell dysfunction and loss of cellular identity. We suggest Nmnat2 suppression as an approach to counteract age-related T2D.

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Glucose hypersensitivity and impaired identity are common features of aged and diabetic β cells.

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Metabolic tracing reveals increased glycolysis and altered NAD production in aged and diabetic β cells.

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Increased glycolysis induces β-cell dysfunction and loss of identity.

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NAD production by Nmnat2 can be targeted to restore β-cell phenotypes.

Characterization of gustatory receptor 7 in the brown planthopper reveals functional versatility

Insect pests consume tastants as their necessary energy and nutrient sources. Gustatory receptors play important roles in insect life and can form within an extremely complicated regulatory network. However, there are still many gustatory genes that have a significant impact on insect physiology, but their functional mechanism is still unknown. Here, we purified and characterized a gustatory receptor (protein) coding gene, NlGr7, from the brown planthopper (BPH) Nilaparvata lugens, which is an important insect pest of rice. Our results revealed that NlGr7 has an active association with various ligands, such as lectins, lipids (phospho- and sphingolipid) and copper. The mass-spectrometry result showed that NlGr7 is a sugar receptor, and NlGr7 is validated by different types of insoluble polysaccharides and a varied range of tastants. Further, we observed that NlGr7-bound ATP hydrolysed on the ATPase activity assay, which indicated that NlGr7 may be associated with important biological functions in the BPH. Furthermore, an injection of NlGr7 (protein), into newly emerged female adults of BPH, showed the reduced vitellogenin in ovary. The important NlGr7 for chemoreception has now been characterized in the BPH. We showed that NlGr7 in the BPH is required for various protein-ligands, as well as protein-sugars interactions, and for regulation of fecundity marker to play crucial roles in this pest. This study will provide valuable information for further functional studies of chemoreception mechanisms in this important agricultural pest.

Amelioration by quercetin of insulin resistance and uterine GLUT4 and ERα gene expression in rats with polycystic ovary syndrome (PCOS)

Insulin resistance (IR) and infertility are two major complications of polycystic ovary syndrome (PCOS), which are the results of changes in certain parts of the reproductive and metabolic systems. We aimed to observe the effect of quercetin on dehydroepiandrosterone (DHEA)-induced PCOS and insulin resistance in rats. All animals were divided into five groups and DHEA was used to induce PCOS. Bodyweight and ovarian morphology of all groups were observed. Fasting blood glucose and insulin levels were analysed. The homeostasis model assessment of insulin resistance (HOMA-IR) method was used for IR level determination. The expression of oestrogen receptor α (ERα) and glucose transporter 4 (GLUT4) genes in the uterus was examined by real-time polymerase chain reaction. Liver hexokinase (HK) and glucokinase (GK) activity was determined using spectrophotometry. Quercetin significantly improved the IR state in PCOS rats. PCOS resulted in a decrease in liver GK and an increase in liver HK specific activity, whereas quercetin increased both liver HK and GK activity. Our data also showed a significant reduction in uterine ERα and GLUT4 expression in the PCOS group, which was increased by quercetin. A remarkable effect of quercetin was the intensive reduction of PCOS-IR and significant induction of uterine GLUT4 and ERα gene expression; it could thus be a possible effective treatment for PCOS and its complications, IR and infertility.

The two major glucokinase isoforms show conserved functionality in β-cells despite different subcellular distribution

Glucokinase (GCK) is crucial to regulating glucose metabolism in the liver and in pancreatic β-cells. There are two major GCK isoforms, hepatic and pancreatic GCKs, which differ only in exon 1. However, the functional differences between the two GCK isoforms remain poorly understood. Here, we used a β-cell-targeted gene transfer vector to determine the impact of isoform-specific GCK overexpression on β-cells in vitro and in vivo. We showed that pancreatic GCK had a nuclear localization signal unique to the pancreatic isoform, facilitating its nuclear distribution in β-cells. Despite the difference in subcellular distribution, overexpression of GCK isoforms similarly enhanced glucose uptake and β-cell proliferation in vitro. Overexpression of hepatic or pancreatic GCK also similarly enhanced β-cell proliferation in normal diet mice without affecting fasting glucose and intraperitoneal glucose tolerance tests (IPGTT). Our further study on human GCK sequences identified disproportional GCK amino acid variants in exon 1, while mutations linked to maturity onset diabetes of the young type 2 (MODY2) were disproportionally found in exons 2 through 10. Our results therefore indicate functional conservation between the two major GCK isoforms despite their distinct subcellular distribution.

Impaired β-cell glucokinase as an underlying mechanism in diet-induced diabetes

High-fat diet (HFD)-fed mouse models have been widely used to study early type 2 diabetes. Decreased β-cell glucokinase (GCK) expression has been observed in HFD-induced diabetes. However, owing to its crucial roles in glucose metabolism in the liver and in islet β-cells, the contribution of decreased GCK expression to the development of HFD-induced diabetes is unclear. Here, we employed a β-cell-targeted gene transfer vector and determined the impact of β-cell-specific increase in GCK expression on β-cell function and glucose handling in vitro and in vivo Overexpression of GCK enhanced glycolytic flux, ATP-sensitive potassium channel activation and membrane depolarization, and increased proliferation in Min6 cells. β-cell-targeted GCK transduction did not change glucose handling in chow-fed C57BL/6 mice. Although adult mice fed a HFD showed reduced islet GCK expression, impaired glucose tolerance and decreased glucose-stimulated insulin secretion (GSIS), β-cell-targeted GCK transduction improved glucose tolerance and restored GSIS. Islet perifusion experiments verified restored GSIS in isolated HFD islets by GCK transduction. Thus, our data identify impaired β-cell GCK expression as an underlying mechanism for dysregulated β-cell function and glycemic control in HFD-induced diabetes. Our data also imply an etiological role of GCK in diet-induced diabetes.This article has an associated First Person interview with the first author of the paper.

Phosphorus ingestion improves oral glucose tolerance of healthy male subjects: a crossover experiment

Background

Fasting serum phosphorus (P) was reported to be inversely related to serum glucose and insulin, while the impact of P ingestion is not well documented. The effect of P intake with or before glucose ingestion on postprandial glucose and insulin statuses was investigated.

Method

Two cross over experiments using healthy male subjects were conducted. Experiment 1: Overnight fasted subjects (n = 7) randomly received: 500 mg of P tablets, glucose (75 g) solution with placebo or 500 mg of P tablets. Experiment 2: Overnight fasted subjects (n = 8) underwent similar procedures to those of experiment 1, except that placebo or 500 mg P tablets were given 60 min prior to glucose ingestion.

Results

In both experiments, serum P decreased following glucose ingestion. Co-ingestion of P with glucose improved, at time 60 min, postprandial glucose (P < 0.05), insulin (P < 0.05), and insulin sensitivity index (p < 0.006), while P pre-ingestion failed to exert similar effect.

Conclusion

This study suggests that postprandial glucose and insulin are affected by exogenous P supply, especially when co-ingested with glucose.

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.

Functional diversification and specialization of cytosolic 70-kDa heat shock proteins

A fundamental question in molecular evolution is how protein functional differentiation alters the ability of cells and organisms to cope with stress and survive. To answer this question we used two paralogous Hsp70s from mouse and explored whether these highly similar cytosolic molecular chaperones, which apart their temporal expression have been considered functionally interchangeable, are differentiated with respect to their lipid-binding function. We demonstrate that the two proteins bind to diverse lipids with different affinities and therefore are functionally specialized. The observed lipid-binding patterns may be related with the ability of both Hsp70s to induce cell death by binding to a particular plasma-membrane lipid, and the potential of only one of them to promote cell survival by binding to a specific lysosomal-membrane lipid. These observations reveal that two seemingly identical proteins differentially modulate cellular adaptation and survival by having acquired specialized functions via sequence divergence. Therefore, this study provides an evolutionary paradigm, where promiscuity, specificity, sub- and neo-functionalization orchestrate one of the most conserved systems in nature, the cellular stress-response.

Glucose homeostasis and the enteroinsular axis in the horse: a possible role in equine metabolic syndrome

One of the principal components of equine metabolic syndrome (EMS) is hyperinsulinaemia combined with insulin resistance. It has long been known that hyperinsulinaemia occurs after the development of insulin resistance. But it is also known that hyperinsulinaemia itself can induce insulin resistance and obesity and might play a key role in the development of metabolic syndrome. This review focuses on the physiology of glucose and insulin metabolism and the pathophysiological mechanisms in glucose homeostasis in the horse (compared with what is already known in humans) in order to gain insight into the pathophysiological principles underlying EMS. The review summarizes new insights on the oral uptake of glucose by the gut and the enteroinsular axis, the role of diet in incretin hormone and postprandial insulin responses, the handling of glucose by the liver, muscle and fat tissue, and the production and secretion of insulin by the pancreas under healthy and disrupted glucose homeostatic conditions in horses.

Simultaneous analysis of intracellular pH and Ca²⁺ from cell populations

Although changes in both pH(in) and [Ca(2+)](i) have been observed in response to a variety of agonists, it is not clear whether these ionic events work independently or are coordinated to lead to a specific physiological response. One of the fundamental problems in studying these ionic events is that changes in pH(in) modify Ca(2+) regulatory mechanisms and changes in Ca(2+) may modify pH regulation. It is desirable to use a technique that allows concomitant monitoring of these two ions in cell populations with high time resolution. Furthermore, like many Ca(2+) binding proteins, all Ca(2+)-sensitive fluoroprobes are inherently sensitive to pH owing to competition of H(+) for the Ca(2+)-binding sites. This chapter describes experimental paradigms that provide optimum conditions for simultaneous measurement of pH from the fluorescence emission of snarf-1, and Ca(2+) using fura-2. The fluorescence spectra of these compounds are sufficiently different to allow simultaneous measurement of pH and Ca(2+) both in vitro and in vivo. Moreover, the ratio of the H(+)-sensitive wavelengths of snarf-1 is unaffected by Ca(2+), or the concomitant presence of fura-2 in cells. Although the fluorescence ratio of fura-2 is insensitive to the presence of snarf-1, it is affected by pH, as indicated above. We describe procedures to correct for this effect and to obtain calibration parameters for fura-2 and snarf-1 required to facilitate analysis of pH and Ca(2+) concentrations within cell populations.

Characterization of the gene expression profile of heterozygous liver-specific glucokinase knockout mice at a young age

In the liver, glucokinase (GCK) facilitates hepatic glucose uptake during hyperglycemia and is essential for the regulation of a network of glucose-responsive genes involved in glycolysis, glycogen synthesis, and lipogenesis. To better understand the consequences of changes in response to a liver-specific deficiency of GCK function, we examined the expression profiles of genes involved in glucose metabolism in the liver, pancreas, muscle and adipose tissue in heterozygous liver-specific Gck knockout (Gck(w/-)) mice. Our results showed that with the development of a liver GCK deficiency, significant decreases in the mRNA levels for insulin receptor and Glut2 were observed in the liver, and HkII in muscle, while glucagon mRNA increased markedly in the pancreas. The levels of circulating glucagon hormone levels increased with increased mRNA levels. Depite a decrease in muscle HkII levels, the hexokinase activity level did not change. Our findings suggest that in liver-specific Gck(w/-) mice, peripheral tissues use different strategies to tackle with hyperglycemia even at a young age. By identifying the specific changes that occur in different tissues at an early stage of glucokinase deficiency, potentially we can develop interventions to prevent further progression to diabetes.

Activation of protein kinase C-ζ in pancreatic β-cells in vivo improves glucose tolerance and induces β-cell expansion via mTOR activation

OBJECTIVE

PKC-ζ activation is a key signaling event for growth factor-induced β-cell replication in vitro. However, the effect of direct PKC-ζ activation in the β-cell in vivo is unknown. In this study, we examined the effects of PKC-ζ activation in β-cell expansion and function in vivo in mice and the mechanisms associated with these effects.

RESEARCH DESIGN AND METHODS

We characterized glucose homeostasis and β-cell phenotype of transgenic (TG) mice with constitutive activation of PKC-ζ in the β-cell. We also analyzed the expression and regulation of signaling pathways, G1/S cell cycle molecules, and β-cell functional markers in TG and wild-type mouse islets.

RESULTS

TG mice displayed increased plasma insulin, improved glucose tolerance, and enhanced insulin secretion with concomitant upregulation of islet insulin and glucokinase expression. In addition, TG mice displayed increased β-cell proliferation, size, and mass compared with wild-type littermates. The increase in β-cell proliferation was associated with upregulation of cyclins D1, D2, D3, and A and downregulation of p21. Phosphorylation of D-cyclins, known to initiate their rapid degradation, was reduced in TG mouse islets. Phosphorylation/inactivation of GSK-3β and phosphorylation/activation of mTOR, critical regulators of D-cyclin expression and β-cell proliferation, were enhanced in TG mouse islets, without changes in Akt phosphorylation status. Rapamycin treatment in vivo eliminated the increases in β-cell proliferation, size, and mass; the upregulation of cyclins Ds and A in TG mice; and the improvement in glucose tolerance-identifying mTOR as a novel downstream mediator of PKC-ζ-induced β-cell replication and expansion in vivo. CONCLUSIONS PKC:-ζ, through mTOR activation, modifies the expression pattern of β-cell cycle molecules leading to increased β-cell replication and mass with a concomitant enhancement in β-cell function. Approaches to enhance PKC-ζ activity may be of value as a therapeutic strategy for the treatment of diabetes.

Restoration of hepatic glucokinase expression corrects hepatic glucose flux and normalizes plasma glucose in zucker diabetic fatty rats

OBJECTIVE

We examined in 20-week-old Zucker diabetic fatty (ZDF) rats whether restoration of hepatic glucokinase (GK) expression would alter hepatic glucose flux and improve hyperglycemia.

RESEARCH DESIGN AND METHODS

ZDF rats were treated at various doses with an adenovirus that directs the expression of rat liver GK (AdvCMV-GKL) dose dependently, and various metabolic parameters were compared with those of nondiabetic lean littermates (ZCL rats) before and during a hyperglycemic clamp. Viral infection per se did not affect hepatic GK activity, since expression of a catalytically inactive form of GK did not alter endogenous hepatic GK activity.

RESULTS

ZDF rats compared with ZCL rats have lower hepatic GK activity (11.6 +/- 1.9 vs. 32.5 +/- 3.2 mU/mg protein), marked hyperglycemia (23.9 +/- 1.2 vs. 7.4 +/- 0.3 mmol/l), higher endogenous glucose production (80 +/- 3 vs. 38 +/- 3 micromol x kg(-1) x min(-1)), increased glucose-6-phosphatase flux (150 +/- 11 vs. 58 +/- 8 micromol x kg(-1) x min(-1)), and during a hyperglycemic clamp, a failure to suppress endogenous glucose production (80 +/- 7 vs. -7 +/- 4 micromol x kg(-1) x min(-1)) and promote glucose incorporation into glycogen (15 +/- 5 vs. 43 +/- 3 micromol/g liver). Treatment of ZDF rats with different doses of AdvCMV-GKL, which restored hepatic GK activity to one to two times that of ZCL rats, normalized plasma glucose levels and endogenous glucose production. During a hyperglycemic clamp, glucose production was suppressed and glucose incorporation into glycogen was normal.

CONCLUSIONS

Alteration of hepatic GK activity in ZDF rats has profound effects on plasma glucose and hepatic glucose flux.

Detailed molecular analysis of the induction of the L-PK gene by glucose

Glucose has powerful effects on gene expression and participates in the fasted-to-fed transition of the liver. However, the molecular mechanism of glucose-regulated gene expression has not been completely described. In the present study, we performed a detailed analysis of the molecular events of the insulin-independent glucose response of the liver-type pyruvate kinase (L-PK) gene. L-PK mRNA was increased by glucose at the transcriptional level as determined by real-time RT-PCR, mRNA stability measurements, and nuclear run-on assays. LY294002 and LY303511 inhibited the glucose response of the L-PK gene at the transcriptional level. Histones H3 and H4 associated with the L-PK gene promoter were hyperacetylated and HNF4alpha was constitutively bound in low and high glucose. Treatment with 20mM glucose increased recruitment of ChREBP, additional HNF4alpha, and RNA polymerase II. Glucose-stimulated the phosphorylation of the C-terminal domain of RNA polymerase II, with increased Ser5 phosphorylation near the transcription start site and increased Ser2 phosphorylation near the termination signal. LY294002 and LY303511 blocked the recruitment of RNA polymerase II to the L-PK gene, reducing the rate of transcription. The results of these studies demonstrate fundamental details of the molecular mechanism of glucose activated gene expression.

A role for PFK-2/FBPase-2, as distinct from fructose 2,6-bisphosphate, in regulation of insulin secretion in pancreatic β-cells

PFK-2/FBPase-2 (6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase) catalyses the formation and degradation of fructose 2,6-P2 (fructose 2,6-bisphosphate) and is also a glucokinase-binding protein. The role of fructose 2,6-P2 in regulating glucose metabolism and insulin secretion in pancreatic β-cells is unresolved. We down-regulated the endogenous isoforms of PFK-2/FBPase-2 with siRNA (small interfering RNA) and expressed KA (kinase active) and KD (kinase deficient) variants to distinguish between the role of PFK-2/FBPase-2 protein and the role of its product, fructose 2,6-P2, in regulating β-cell function. Human islets expressed the PFKFB2 (the gene encoding isoform 2 of the PFK2/FBPase2 protein) and PFKFB3 (the gene encoding isoform 3 of the PFK2/FBPase2 protein) isoforms and mouse islets expressed PFKFB2 at the mRNA level [RT–PCR (reverse transcription–PCR)]. Rat islets expressed PFKFB2 lacking the C-terminal phosphorylation sites. The glucose-responsive MIN6 and INS1E cell lines expressed PFKFB2 and PFKFB3. PFK-2 activity and the cell content of fructose 2,6-P2 were increased by elevated glucose concentration and during pharmacological activation of AMPK (AMP-activated protein kinase), which also increased insulin secretion. Partial down-regulation of endogenous PFKFB2 and PFKFB3 in INS1E by siRNA decreased PFK-2/FBPase-2 protein, fructose 2,6-P2 content, glucokinase activity and glucoseinduced insulin secretion. Selective down-regulation of glucose-induced fructose 2,6-P2 in the absence of down-regulation of PFK-2/FBPase-2 protein, using a KD PFK-2/FBPase-2 variant, resulted in sustained glycolysis and elevated glucose-induced insulin secretion, indicating an over-riding role of PFK-2/FBPase-2 protein, as distinct from its product fructose 2,6-P2, in potentiating glucose-induced insulin secretion. Whereas down-regulation of PFK-2/FBPase-2 decreased glucokinase activity, overexpression of PFK-2/FBPase-2 only affected glucokinase distribution. It is concluded that PFK-2/FBPase-2 protein rather than its product fructose 2,6-P2 is the over-riding determinant of glucose-induced insulin secretion through regulation of glucokinase activity or subcellular targeting.

Trefoil factor 3 stimulates human and rodent pancreatic islet beta-cell replication with retention of function

Both major forms of diabetes involve a decline in beta-cell mass, mediated by autoimmune destruction of insulin-producing cells in type 1 diabetes and by increased rates of apoptosis secondary to metabolic stress in type 2 diabetes. Methods for controlled expansion of beta-cell mass are currently not available but would have great potential utility for treatment of these diseases. In the current study, we demonstrate that overexpression of trefoil factor 3 (TFF3) in rat pancreatic islets results in a 4- to 5-fold increase in [(3)H]thymidine incorporation, with full retention of glucose-stimulated insulin secretion. This increase was almost exclusively due to stimulation of beta-cell replication, as demonstrated by studies of bromodeoxyuridine incorporation and co-immunofluorescence analysis with anti-bromodeoxyuridine and antiinsulin or antiglucagon antibodies. The proliferative effect of TFF3 required the presence of serum or 0.5 ng/ml epidermal growth factor. The ability of TFF3 overexpression to stimulate proliferation of rat islets in serum was abolished by the addition of epidermal growth factor receptor antagonist AG1478. Furthermore, TFF3-induced increases in [3H]thymidine incorporation in rat islets cultured in serum was blocked by overexpression of a dominant-negative Akt protein or treatment with triciribine, an Akt inhibitor. Finally, overexpression of TFF3 also caused a doubling of [3H]thymidine incorporation in human islets. In summary, our findings reveal a novel TFF3-mediated pathway for stimulation of beta-cell replication that could ultimately be exploited for expansion or preservation of islet beta-cell mass.

Inhibition of glucokinase translocation by AMP-activated protein kinase is associated with phosphorylation of both GKRP and 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase

The rate of glucose phosphorylation in hepatocytes is determined by the subcellular location of glucokinase and by its association with its regulatory protein (GKRP) in the nucleus. Elevated glucose concentrations and precursors of fructose 1-phosphate (e.g., sorbitol) cause dissociation of glucokinase from GKRP and translocation to the cytoplasm. In this study, we investigated the counter-regulation of substrate-induced translocation by AICAR (5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside), which is metabolized by hepatocytes to an AMP analog, and causes activation of AMP-activated protein kinase (AMPK) and depletion of ATP. During incubation of hepatocytes with 25 mM glucose, AICAR concentrations below 200 microM activated AMPK without depleting ATP and inhibited glucose phosphorylation and glucokinase translocation with half-maximal effect at 100-140 microM. Glucose phosphorylation and glucokinase translocation correlated inversely with AMPK activity. AICAR also counteracted translocation induced by a glucokinase activator and partially counteracted translocation by sorbitol. However, AICAR did not block the reversal of translocation (from cytoplasm to nucleus) after substrate withdrawal. Inhibition of glucose-induced translocation by AICAR was greater than inhibition by glucagon and was associated with phosphorylation of both GKRP and the cytoplasmic glucokinase binding protein, 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) on ser-32. Expression of a kinase-active PFK2 variant lacking ser-32 partially reversed the inhibition of translocation by AICAR. Phosphorylation of GKRP by AMPK partially counteracted its inhibitory effect on glucokinase activity, suggesting altered interaction of glucokinase and GKRP. In summary, mechanisms downstream of AMPK activation, involving phosphorylation of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase and GKRP are involved in the ATP-independent inhibition of glucose-induced glucokinase translocation by AICAR in hepatocytes.

Contributions of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 to the elevated glycolysis in hepatocytes from Zucker fa/fa rats

The insulin-resistant Zucker fa/fa rat has elevated hepatic glycolysis and activities of glucokinase and phosphofructokinase-2/fructose bisphosphatase-2 (PFK2). The latter catalyzes the formation and degradation of fructose-2,6-bisphosphate (fructose-2,6-P2) and is a glucokinase-binding protein. The contributions of glucokinase and PFK2 to the elevated glycolysis in fa/fa hepatocytes were determined by overexpressing these enzymes individually or in combination. Metabolic control analysis was used to determine enzyme coefficients on glycolysis and metabolite concentrations. Glucokinase had a high control coefficient on glycolysis in all hormonal conditions tested, whereas PFK2 had significant control only in the presence of glucagon, which phosphorylates PFK2 and suppresses glycolysis. Despite the high control strength of glucokinase, the elevated glycolysis in fa/fa hepatocytes could not be explained by the elevated glucokinase activity alone. In hepatocytes from fa/fa rats, glucokinase translocation between the nucleus and the cytoplasm was refractory to glucose but responsive to glucagon. Expression of a kinase-active PFK2 variant reversed the glucagon effect on glucokinase translocation and glucose phosphorylation, confirming the role for PFK2 in sequestering glucokinase in the cytoplasm. Glucokinase had a high control on glucose-6-phosphate content; however, like PFK2, it had a relative modest effect on the fructose-2,6-P2 content. However, combined overexpression of glucokinase and PFK2 had a synergistic effect on fructose-2,6-P2 levels, suggesting that interaction of these enzymes may be a prerequisite for formation of fructose-2,6-P2. Cumulatively, this study provides support for coordinate roles for glucokinase and PFK2 in the elevated hepatic glycolysis in fa/fa rats.

Cell biology assessment of glucokinase mutations V62M and G72R in pancreatic beta-cells: evidence for cellular instability of catalytic activity

Mutations in the glucokinase (GK) gene cause defects in blood glucose homeostasis. In some cases (V62M and G72R), the phenotype cannot be explained by altered enzyme kinetics or protein instability. We used transient and stable expression of green fluorescent protein (GFP) GK chimaeras in MIN6 beta-cells to study the phenotype defect of V62M and G72R. GK activity in lysates of MIN6 cell lines stably expressing wild-type or mutant GFP GK showed the expected affinity for glucose and response to pharmacological activators, indicating the expression of catalytically active enzymes. MIN6 cells stably expressing GFP V62M or GFP G72R had a lower GK activity-to-GK immunoreactivity ratio and GK activity-to-GK mRNA ratio but not GK immunoreactivity-to-GK mRNA ratio than wild-type GFP GK. Heterologous expression of liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2/FDP2) in cell lines increased GK activity for wild-type GK and V62M but not for G72R, whereas expression of liver GK regulatory protein (GKRP) increased GK activity for wild type but not V62M or G72R. Lack of interaction of these mutants with GKRP was also evident in hepatocyte transfections from the lack of nuclear accumulation. These results suggest that cellular loss of GK catalytic activity rather than impaired translation or enhanced protein degradation may account for the hyperglycemia in subjects with V62M and G72R mutations.

Increased sensitivity of glycogen synthesis to phosphorylase-a and impaired expression of the glycogen-targeting protein R6 in hepatocytes from insulin-resistant Zucker fa/fa rats

Hepatic insulin resistance in the leptin-receptor defective Zucker fa/fa rat is associated with impaired glycogen synthesis and increased activity of phosphorylase-a. We investigated the coupling between phosphorylase-a and glycogen synthesis in hepatocytes from fa/fa rats by modulating the concentration of phosphorylase-a. Treatment of hepatocytes from fa/fa rats and Fa/? controls with a selective phosphorylase inhibitor caused depletion of phosphorylase-a, activation of glycogen synthase and stimulation of glycogen synthesis. The flux-control coefficient of phosphorylase on glycogen synthesis was glucose dependent and at 10 mm glucose was higher in fa/fa than Fa/? hepatocytes. There was an inverse correlation between the activities of glycogen synthase and phosphorylase-a in both fa/fa and Fa/? hepatocytes. However, fa/fa hepatocytes had a higher activity of phosphorylase-a, for a corresponding activity of glycogen synthase. This defect was, in part, normalized by expression of the glycogen-targeting protein, PTG. Hepatocytes from fa/fa rats had normal expression of the glycogen-targeting proteins G(L) and PTG but markedly reduced expression of R6. Expression of R6 protein was increased in hepatocytes from Wistar rats after incubation with leptin and insulin. Diminished hepatic R6 expression in the leptin-receptor defective fa/fa rat may be a contributing factor to the elevated phosphorylase activity and/or its high control strength on glycogen synthesis.

The role of glucose 6-phosphate in mediating the effects of glucokinase overexpression on hepatic glucose metabolism

Pharmacological activation or overexpression of glucokinase in hepatocytes stimulates glucose phosphorylation, glycolysis and glycogen synthesis. We used an inhibitor of glucose 6-phosphate (Glc6P) hydrolysis, namely the chlorogenic derivative, 1-[2-(4-chloro-phenyl)-cyclopropylmethoxy]-3, 4-dihydroxy-5-(3-imidazo[4,5-b]pyridin-1-yl-3-phenyl-acryloyloxy)-cyclohexanecarboxylic acid (also known as S4048), to determine the contribution of Glc6P concentration, as distinct from glucokinase protein or activity, to the control of glycolysis and glycogen synthesis by glucokinase overexpression. The validity of S4048 for testing the role of Glc6P was supported by its lack of effect on glucokinase binding and its nuclear/cytoplasmic distribution. The stimulation of glycolysis by glucokinase overexpression correlated strongly with glucose phosphorylation, whereas glycogen synthesis correlated strongly with Glc6P concentration. Metabolic control analysis was used to determine the sensitivity of glycogenic flux to glucokinase or Glc6P at varying glucose concentrations (5-20 mm). The concentration control coefficient of glucokinase on Glc6P (1.4-1.7) was relatively independent of glucose concentration, whereas the flux control coefficients of Glc6P (2.4-1.0) and glucokinase (3.7-1.8) on glycogen synthesis decreased with glucose concentration. The high sensitivity of glycogenic flux to Glc6P at low glucose concentration is consistent with covalent modification by Glc6P of both phosphorylase and glycogen synthase. The high control strength of glucokinase on glycogenic flux is explained by its concentration control coefficient on Glc6P and the high control strength of Glc6P on glycogen synthesis. It is suggested that the regulatory strength of pharmacological glucokinase activators on glycogen metabolism can be predicted from their effect on the Glc6P content.

Dual role of phosphofructokinase-2/fructose bisphosphatase-2 in regulating the compartmentation and expression of glucokinase in hepatocytes

Hepatic glucokinase is regulated by a 68-kDa regulatory protein (GKRP) that is both an inhibitor and nuclear receptor for glucokinase. We tested the role of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK2) in regulating glucokinase compartmentation in hepatocytes. PFK2 catalyzes formation or degradation of the regulator of glycolysis fructose 2,6-bisphosphate (fructose 2,6-P2), depending on its phosphorylation state (ser-32), and is also a glucokinase-binding protein. Incubation of hepatocytes at 25 mmol/l glucose causes translocation of glucokinase from the nucleus to the cytoplasm and an increase in fructose 2,6-P2. Glucagon caused phosphorylation of PFK2-ser-32, lowered the fructose 2,6-P2 concentration, and inhibited glucose-induced translocation of glucokinase. These effects of glucagon were reversed by expression of a kinase-active PFK2 mutant (S32A/H258A) that overrides the suppression of fructose 2,6-P2 but not by overexpression of wild-type PFK2. Overexpression of PFK2 potentiated glucokinase expression in hepatocytes transduced with an adenoviral vector-encoding glucokinase by a mechanism that does not involve stabilization of glucokinase protein from degradation. It is concluded that PFK2 has a dual role in regulating glucokinase in hepatocytes: it potentiates glucokinase protein expression by posttranscriptional mechanisms and favors its cytoplasmic compartmentation. Thus, it acts in a complementary mechanism to GKRP, which also regulates glucokinase protein expression and compartmentation.

Beta-cell secretory dysfunction in the pathogenesis of low birth weight-associated diabetes: a murine model

Low birth weight (LBW) is an important risk factor for type 2 diabetes. We have developed a mouse model of LBW resulting from undernutrition during pregnancy. Restriction of maternal food intake from day 12.5 to 18.5 of pregnancy results in a 23% decrease in birth weight (P < 0.001), with normalization after birth. However, offspring of undernutrition pregnancies develop progressive, severe glucose intolerance by 6 months. To identify early defects that are responsible for this phenotype, we analyzed mice of undernutrition pregnancies at age 2 months, before the onset of glucose intolerance. Fed insulin levels were 1.7-fold higher in mice of undernutrition pregnancies (P = 0.01 vs. controls). However, insulin sensitivity was normal in mice of undernutrition pregnancies, with normal insulin tolerance, insulin-stimulated glucose disposal, and isolated muscle and adipose glucose uptake. Although insulin clearance was mildly impaired in mice of undernutrition pregnancies, the major metabolic phenotype in young mice of undernutrition pregnancies was dysregulation of insulin secretion. Despite normal beta-cell mass, islets from normoglycemic mice of undernutrition pregnancies showed basal hypersecretion of insulin, complete lack of responsiveness to glucose, and a 2.5-fold increase in hexokinase activity. Taken together, these data suggest that, at least in mice, primary beta-cell dysfunction may play a significant role in the pathogenesis of LBW-associated type 2 diabetes.

Increased potency and efficacy of combined phosphorylase inactivation and glucokinase activation in control of hepatocyte glycogen metabolism

Glucokinase and phosphorylase both have a high control strength over hepatocyte glycogen metabolism and are potential therapeutic targets for type 2 diabetes. We tested whether combined phosphorylase inactivation and glucokinase activation is a more effective strategy for controlling hepatic glycogen metabolism than single-site targeting. Activation of glucokinase by enzyme overexpression combined with selective dephosphorylation of phosphorylase-a by an indole carboxamide that favors the T conformation of phosphorylase caused a greater stimulation of glycogen synthesis than the sum of either treatment alone. This result is explained by the complementary roles of elevated glucose-6-phosphate (G6P; a positive modulator) and depleted phosphorylase-a (a negative modulator) in activating glycogen synthase and also by synergistic inactivation of phosphorylase-a by glucokinase activation and the indole carboxamide. Inactivation of phosphorylase-a by the indole carboxamide was counteracted by 5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside, which is metabolized to an AMP analog; this effect was reversed by G6P. Our findings provide further evidence for the inverse roles of G6P and AMP in regulating the activation state of hepatic phosphorylase. It is proposed that dual targeting of glucokinase and phosphorylase-a enables improved potency and efficacy in controlling hepatic glucose metabolism.

Developmental expression of glucokinase in rat hypothalamus

Neurons in the hypothalamus sense changes in glucose concentration. Glucokinase (GK), a key enzyme for pancreatic (beta)-cell glucose sensing, was found in both the embryonic and adult hypothalamus. GK activity accounted for approximately 20% of total hexokinase (HK) activity in both embryonic and adult hypothalamus with no activity measured in cortical samples, indicating that glucose sensing in the hypothalamus initiates early in development and precedes the maturation of glucose signaling in liver.

The Glycogenic Action of Protein Targeting to Glycogen in Hepatocytes Involves Multiple Mechanisms Including Phosphorylase Inactivation and Glycogen Synthase Translocation

Expression of the glycogen-targeting protein PTG promotes glycogen synthase activation and glycogen storage in various cell types. In this study, we tested the contribution of phosphorylase inactivation to the glycogenic action of PTG in hepatocytes by using a selective inhibitor of phosphorylase (CP-91149) that causes dephosphorylation of phosphorylase a and sequential activation of glycogen synthase. Similar to CP-91194, graded expression of PTG caused a concentration-dependent inactivation of phosphorylase and activation of glycogen synthase. The latter was partially counter-acted by the expression of muscle phosphorylase and was not additive with the activation by CP-91149, indicating that it is in part secondary to the inactivation of phosphorylase. PTG expression caused greater stimulation of glycogen synthesis and translocation of glycogen synthase than CP-91149, and the translocation of synthase could not be explained by accumulation of glycogen, supporting an additional role for glycogen synthase translocation in the glycogenic action of PTG. The effects of PTG expression on glycogen synthase and glycogen synthesis were additive with the effects of glucokinase expression, confirming the complementary roles of depletion of phosphorylase a (a negative modulator) and elevated glucose 6-phosphate (a positive modulator) in potentiating the activation of glycogen synthase. PTG expression mimicked the inactivation of phosphorylase caused by high glucose and counteracted the activation caused by glucagon. The latter suggests a possible additional role for PTG on phosphorylase kinase inactivation.

Leucine culture reveals that ATP synthase functions as a fuel sensor in pancreatic beta-cells

Our goal was to investigate whether leucine culture affects beta-cell glucose sensing. One-day culture of rat islets with 10 mM leucine had no effect on glucose-induced insulin secretion. One-week leucine culture decreased the threshold for glucose-induced insulin secretion and increased maximal insulin secretion at 30 mM glucose. Glucose-induced cytosolic free Ca(2+) was increased at 1 week but not at 1 day of leucine culture. Without glucose, ATP content was not different with or without leucine culture for 1 week. With 20 mM glucose, ATP content was higher by 1.5-fold in islets cultured for 1 week with leucine than those without leucine. Microarray experiments indicated that culture of RINm5F cells with leucine increased expression of ATP synthase beta subunit 3.2-fold, which was confirmed by real time reverse transcription-PCR analysis (3.0- +/- 0.4-fold) in rat islets at 1 week but not after 1 day with leucine culture. Down-regulation of ATP synthase beta subunit by siRNA decreased INS1 cell ATP content and insulin secretion with 20 mM glucose. Overexpression of ATP synthase beta subunit in INS1 cell increased insulin secretion in the presence of 5 and 20 mM glucose. In conclusion, one-week leucine culture of rat islets up-regulated ATP synthase and increased ATP content, which resulted in elevated [Ca(2+)] levels and more insulin exocytosis by glucose. Depletion of ATP synthase beta subunit with siRNA produced opposite effects. These data reveal the fuel-sensing role of mitochondrial ATP synthase in the control of ATP production from glucose and the control of glucose-induced insulin secretion.

An adenovirus vector for efficient RNA interference-mediated suppression of target genes in insulinoma cells and pancreatic islets of langerhans

Silencing gene expression by RNA interference (RNAi) can provide insight into gene function but requires efficient delivery of small interfering RNAs (siRNAs) into cells. Introduction of exogenous nucleic acids can be especially difficult in cultured pancreatic islets. This article describes a method for making recombinant adenoviruses that efficiently drive expression of siRNAs in islet beta-cells and a beta-cell-derived cell line. Transduction with a virus expressing an siRNA specific for GLUT2 reduced GLUT2 mRNA and protein levels by 80% in the INS-1-derived beta-cell line, 832/13, and GLUT2 protein levels by >90% in primary rat islets. Another virus expressing an siRNA specific for glucokinase (GK) caused 80% suppression of GK mRNA and 50% suppression of GK protein levels in 832/13 cells. These experiments validate recombinant adenoviral RNAi vectors as a useful tool for suppression of the expression of specific genes in pancreatic islets and beta-cell lines. Advantages of this approach include 1) the high efficiency of adenovirus-mediated gene transfer in insulinoma cell lines and rat islets and 2) the rapidity with which RNAi constructs can be prepared and tested relative to stable-transfection strategies.

A modest glucokinase overexpression in the liver promotes fed expression levels of glycolytic and lipogenic enzyme genes in the fasted state without altering SREBP-1c expression

Hepatic genes crucial for carbohydrate and lipid homeostasis are regulated by insulin and glucose metabolism. However, the relative contributions of insulin and glucose to the regulation of metabolic gene expression are poorly defined in vivo. To address this issue, adenovirus-mediated hepatic overexpression of glucokinase was used to determine the effects of increased hepatic glucose metabolism on gene expression in fasted or ad libitum fed rats. In the fasted state, a 3 fold glucokinase overexpression was sufficient to mimic feeding-induced increases in pyruvate kinase and acetyl CoA carboxylase mRNA levels, demonstrating a primary role for glucose metabolism in the regulation of these genes in vivo. Conversely, glucokinase overexpression was unable to mimic feeding-induced alterations of fatty acid synthase, glucose-6-phosphate dehydrogenase, carnitine palmitoyl transferase I or PEPCK mRNAs, indicating insulin as the primary regulator of these genes. Interestingly, glucose-6-phosphatase mRNA was increased by glucokinase overexpression in both the fasted and fed states, providing evidence, under these conditions, for the dominance of glucose over insulin signaling for this gene in vivo. Importantly, glucokinase overexpression did not alter sterol regulatory element binding protein 1-c mRNA levels in vivo and glucose signaling did not alter the expression of this gene in primary hepatocytes. We conclude that a modest hepatic overexpression of glucokinase is sufficient to alter expression of metabolic genes without changing the expression of SREBP-1c.

Glucose 6-phosphate causes translocation of phosphorylase in hepatocytes and inactivates the enzyme synergistically with glucose

The role of glucose 6-P (glucose 6-phosphate) in regulating the activation state of glycogen synthase and its translocation is well documented. In the present study, we investigated the effects of glucose 6-P on the activation state and compartmentation of phosphorylase in hepatocytes. Glucose 6-P levels were modulated in hepatocytes by glucokinase overexpression or inhibition with 5-thioglucose and the effects of AMP were tested using AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolized to an AMP analogue. Inhibition of glucokinase partially counteracted the effect of glucose both on the inactivation of phosphorylase and on the translocation of phosphorylase a from a soluble to a particulate fraction. The increase in glucose 6-P caused by glucokinase overexpression caused translocation of phosphorylase a to the pellet and had additive effects with glucose on inactivation of phosphorylase. It decreased the glucose concentration that caused half-maximal inactivation from 20 to 11 mM, indicating that it acts synergistically with glucose. AICAR activated phosphorylase and counteracted the effect of glucose 6-P on phosphorylase inactivation. However, it did not counteract translocation of phosphorylase by glucose 6-P. Glucose 6-P and AICAR had opposite effects on the activation state of glycogen synthase, but they had additive effects on translocation of the enzyme to the pellet. There was a direct correlation between the translocation of phosphorylase a and of glycogen synthase to the pellet, suggesting that these enzymes translocate in tandem. In conclusion, glucose 6-P causes both translocation of phosphorylase and inactivation, indicating a more complex role in the regulation of glycogen metabolism than can be explained from regulation of glycogen synthase alone.

Understanding of basic mechanisms of beta-cell function and survival: prelude to new diabetes therapies

Type 1 and type 2 diabetes are both diseases of insulin insufficiency, although they develop by distinct pathways. The recent surge in the incidence of type 2 diabetes and the chronic ailments confronted by patients with either form of the disease highlight the need for better understanding of beta-cell biology. In this review, we present recent work focused on this goal. Our hope is that basic research being conducted in this and other laboratories will ultimately contribute to the development of methods for enhancing beta-cell function and survival in the context of both major forms of diabetes. Our strategy for understanding the beta-cell involves a multidisciplinary approach in which tools from the traditional fields of biochemistry, enzymology, and physiology are teamed with newer technologies from the fields of molecular biology, gene discovery, cell and developmental biology, and biophysical chemistry. We have focused on two important aspects of beta-cell biology in our studies: beta-cell function, specifically the metabolic regulatory mechanisms involved in glucose-stimulated insulin secretion, and beta-cell resistance to immune attack, with emphasis on resistance to inflammatory cytokines and reactive oxygen species.

Oxidative stress is a mediator of glucose toxicity in insulin-secreting pancreatic islet cell lines

Pancreatic beta cells secrete insulin in response to changes in the extracellular glucose. However, prolonged exposure to elevated glucose exerts toxic effects on beta cells and results in beta cell dysfunction and ultimately beta cell death (glucose toxicity). To investigate the mechanism of how increased extracellular glucose is toxic to beta cells, we used two model systems where glucose metabolism was increased in beta cell lines by enhancing glucokinase (GK) activity and exposing cells to physiologically relevant increases in extracellular glucose (3.3-20 mm). Exposure of cells with enhanced GK activity to 20 mm glucose accelerated glycolysis, but reduced cellular NAD(P)H and ATP, caused accumulation of intracellular reactive oxygen species (ROS) and oxidative damage to mitochondria and DNA, and promoted apoptotic cell death. These changes required both enhanced GK activity and exposure to elevated extracellular glucose. A ROS scavenger partially prevented the toxic effects of increased glucose metabolism. These results indicate that increased glucose metabolism in beta cells generates oxidative stress and impairs cell function and survival; this may be a mechanism of glucose toxicity in beta cells. The level of beta cell GK may also be critical in this process.

Suppression of beta cell energy metabolism and insulin release by PGC-1alpha

beta cell dysfunction is an important component of type 2 diabetes, but the molecular basis for this defect is poorly understood. The transcriptional coactivator PGC-1alpha mRNA and protein levels are significantly elevated in islets from multiple animal models of diabetes; adenovirus-mediated expression of PGC-1alpha to levels similar to those present in diabetic rodents produces a marked inhibition of glucose-stimulated insulin secretion from islets in culture and in live mice. This inhibition coincides with changes in metabolic gene expression associated with impaired beta cell function, including the induction of glucose-6-phosphatase and suppression of GLUT2, glucokinase, and glycerol-3-phosphate dehydrogenase. These changes result in blunting of the glucose-induced rise in cellular ATP levels and membrane electrical activity responsible for Ca(2+) influx and insulin exocytosis. These results strongly suggest that PGC-1alpha plays a key functional role in the beta cell and is involved in the pathogenesis of the diabetic phenotype.

Hepatic expression of a targeting subunit of protein phosphatase-1 in streptozotocin-diabetic rats reverses hyperglycemia and hyperphagia despite depressed glucokinase expression

Glycogen-targeting subunits of protein phosphatase-1 (PP-1) are scaffolding proteins that facilitate the regulation of key enzymes of glycogen metabolism by PP-1. In the current study, we have tested the effects of hepatic expression of GMDeltaC, a truncated version of the muscle-targeting subunit isoform, in rats rendered insulin-deficient via injection of a single moderate dose of streptozotocin (STZ). Three key findings emerged. First, GMDeltaC expression in liver was sufficient to fully normalize blood glucose levels (from 335 +/- 31 mg/dl prior to viral injection to 109 +/- 28 mg/dl 6 days after injection) and liver glycogen content in STZ-injected rats. Second, this normalization occurred despite very low levels of liver glucokinase expression in the insulin-deficient STZ-injected rats. Finally, the hyperphagia induced by STZ injection was completely reversed by GMDeltaC expression in liver. In contrast to these findings with GMDeltaC, overexpression of another targeting subunit, GL, in STZ-injected rats caused a large increase in liver glycogen stores but only a transient decrease in food intake and blood glucose levels. The surprising demonstration of a glucose-lowering effect of GMDeltaC in the background of depressed hepatic glucokinase expression suggests that controlled stimulation of liver glycogen storage may be an effective mechanism for improving glucose homeostasis, even when normal pathways of glucose disposal are impaired.

Glucose 6-phosphate regulates hepatic glycogenolysis through inactivation of phosphorylase

High glucose concentration suppresses hepatic glycogenolysis by allosteric inhibition and dephosphorylation (inactivation) of phosphorylase-a. The latter effect is attributed to a direct effect of glucose on the conformation of phosphorylase-a. Although glucose-6-phosphate (G6P), like glucose, stimulates dephosphorylation of phosphorylase-a by phosphorylase phosphatase, its physiological role in regulating glycogenolysis in intact hepatocytes has not been tested. We show in this study that metabolic conditions associated with an increase in G6P, including glucokinase overexpression and incubation with octanoate or dihydroxyacetone, cause inactivation of phosphorylase. The latter conditions also inhibit glycogenolysis. The activity of phosphorylase-a correlated inversely with the G6P concentration within the physiological range. The inhibition of glycogenolysis and inactivation of phosphorylase-a caused by 10 mmol/l glucose can be at least in part counteracted by inhibition of glucokinase with 5-thioglucose, which lowers G6P. In conclusion, metabolic conditions that alter the hepatic G6P content affect glycogen metabolism not only through regulation of glycogen synthase but also through regulation of the activation state of phosphorylase. Dysregulation of G6P in diabetes by changes in activity of glucokinase or glucose 6-phosphatase may be a contributing factor to impaired suppression of glycogenolysis by hyperglycemia.

Stimulus/secretion coupling factors in glucose-stimulated insulin secretion: insights gained from a multidisciplinary approach

There is a growing appreciation for the complexity of the pathways involved in glucose-stimulated insulin secretion (GSIS) from pancreatic islet beta-cells. In our laboratory, this has stimulated the development of an interdisciplinary approach to the problem. In this study, we review recent studies combining the tools of recombinant adenovirus for gene delivery, the development of novel cell lines that exhibit either robust or weak GSIS, and nuclear magnetic resonance imaging for metabolic fingerprinting of glucose-stimulated cells. Using these tools, we demonstrate a potentially important role for pyruvate carboxylase-mediated pyruvate cycling pathways in the control of GSIS, and discuss potential coupling factors produced by such pathways.

While tinkering with the beta-cell...metabolic regulatory mechanisms and new therapeutic strategies: American Diabetes Association Lilly Lecture, 2001

A common feature of the two major forms of human diabetes is the partial or complete loss of insulin secretion from beta-cells in the pancreatic islets of Langerhans. In this article, we review the development of a set of tools for studying beta-cell biology and their application to understanding of fuel-mediated insulin secretion and enhancement of beta-cell survival. Insights into these basic issues are likely to be useful for the design of new drug and cell-based diabetes therapies.

Regulation of secretory granule pH in insulin-secreting cells

Luminal acidification is important for the maturation of secretory granules, yet little is known regarding the regulation of pH within them. A pH-sensitive green fluorescent protein (EGFP) was targeted to secretory granules in RIN1046-38 insulinoma cells by using a construct in which the EGFP gene was preceded by the nucleotide sequence for human growth hormone. Stimulatory levels of glucose doubled EGFP secretion from cell cultures, and potentiators of glucose-induced insulin secretion enhanced EGFP release. Thus this targeted EGFP is useful for population measurements of secretion. However, less than ~4% of total cell EGFP was released after 1.5 h of stimulation. Consequently, when analyzed in single cells, fluorescence of the targeted EGFP acts as an indicator of pH within secretory granules. Glucose elicited a decrease in granule pH, whereas inhibitors of the V-type H(+)-ATPase increased pH and blocked the glucose effect. Granule pH also was modified by effectors of the protein kinase A pathway, with activation eliciting granule alkalinization, suggesting that potentiation of peptide release by cAMP may involve regulated changes in secretory granule pH.

Mitochondrial metabolism sets the maximal limit of fuel-stimulated insulin secretion in a model pancreatic beta cell: a survey of four fuel secretagogues

The precise metabolic steps that couple glucose catabolism to insulin secretion in the pancreatic beta cell are incompletely understood. ATP generated from glycolytic metabolism in the cytosol, from mitochondrial metabolism, and/or from the hydrogen shuttles operating between cytosolic and mitochondrial compartments has been implicated as an important coupling factor. To identify the importance of each of these metabolic pathways, we have compared the fates of four fuel secretagogues (glucose, pyruvate, dihydroxyacetone, and glycerol) in the INS1-E beta cell line. Two of these fuels, dihydroxyacetone and glycerol, are normally ineffective as secretagogues but are enabled by adenovirus-mediated expression of glycerol kinase. Comparison of these two particular fuels allows the effect of redox state on insulin secretion to be evaluated since the phosphorylated products dihydroxyacetone phosphate and glycerol phosphate lie on opposite sides of the NADH-consuming glycerophosphate dehydrogenase reaction. Based upon measurements of glycolytic metabolites, mitochondrial oxidation, mitochondrial matrix calcium, and mitochondrial membrane potential, we find that insulin secretion most tightly correlates with mitochondrial metabolism for each of the four fuels. In the case of glucose stimulation, the high control strength of glucose phosphorylation sets the pace of glucose metabolism and thus the rate of insulin secretion. However, bypassing this reaction with pyruvate, dihydroxyacetone, or glycerol uncovers constraints imposed by mitochondrial metabolism, each of which attains a similar maximal limit of insulin secretion. More specifically, we found that the hyperpolarization of the mitochondrial membrane, related to the proton export from the mitochondrial matrix, correlates well with insulin secretion. Based on these findings, we propose that fuel-stimulated secretion is in fact limited by the inherent thermodynamic constraints of proton gradient formation.

Inclusion of low amounts of fructose with an intraduodenal glucose load markedly reduces postprandial hyperglycemia and hyperinsulinemia in the conscious dog

Intraportal infusion of small amounts of fructose markedly augmented net hepatic glucose uptake (NHGU) during hyperglycemic hyperinsulinemia in conscious dogs. In this study, we examined whether the inclusion of catalytic amounts of fructose with a glucose load reduces postprandial hyperglycemia and the pancreatic beta-cell response to a glucose load in conscious 42-h-fasted dogs. Each study consisted of an equilibration (-140 to -40 min), control (-40 to 0 min), and test period (0-240 min). During the latter period, glucose (44.4 micromol x kg(-1) x min(-1)) was continuously given intraduodenally with (2.22 micromol x kg(-1) x min(-1)) or without fructose. The glucose appearance rate in portal vein blood was not significantly different with or without the inclusion of fructose (41.3 +/- 2.7 vs. 37.3 +/- 8.3 micromol x kg(-1) x min(-1), respectively). In response to glucose infusion without the inclusion of fructose, the net hepatic glucose balance switched from output to uptake (from 10 +/- 2 to 11 +/- 4 micromol x kg(-1) x min(-1)) by 30 min and averaged 17 +/- 6 micromol x kg(-1) x min(-1). The fractional extraction of glucose by the liver during the infusion period was 7 +/- 2%. Net glycogen deposition was 2.44 mmol glucose equivalent/kg body wt; 49% of deposited glycogen was synthesized via the direct pathway. Net hepatic lactate production was 1.4 mmol/kg body wt. Arterial blood glucose rose from 4.1 +/- 0.2 to 7.3 +/- 0.4 mmol/l, and arterial plasma insulin rose from 42 +/- 6 to 258 +/- 66 pmol/l at 30 min, after which they decreased to 7.0 +/- 0.5 mmol/l and 198 +/- 66 pmol/l, respectively. Arterial plasma glucagon decreased from 54 +/- 7 to 32 +/- 3 ng/l. In response to intraduodenal glucose infusion in the presence of fructose, net hepatic glucose balance switched from 9 +/- 1 micromol x kg(-1) x min(-1) output to 12 +/- 3 and 28 +/- 5 micromol x kg(-1) x min(-1) uptake by 15 and 30 min, respectively. The average NHGU (28 +/- 5 micromol x kg(-1) x min(-1)) and fractional extraction during infusion period (12 +/- 2%), net glycogen deposition (3.68 mmol glucose equivalent/kg body wt), net hepatic lactate production (3.27 mmol/kg), and glycogen synthesis via the direct pathway (68%) were significantly higher (P < 0.05) compared to that in the absence of fructose. The increases in arterial blood glucose (from 4.4 +/- 0.1 to 6.4 +/- 0.2 mmol/l at 30 min) and arterial plasma insulin (from 48 +/- 6 to 126 +/- 30 pmol/l at 30 min) were significantly smaller (P < 0.05). In summary, the inclusion of small amounts of fructose with a glucose load augmented NHGU, increased hepatic glycogen synthesis via the direct pathway, and augmented hepatic glycolysis. As a result, postprandial hyperglycemia and insulin release by the pancreatic beta-cell were reduced. In conclusion, catalytic amounts of fructose have the ability to improve glucose tolerance.

Glycogen-targeting subunits and glucokinase differentially affect pathways of glycogen metabolism and their regulation in hepatocytes

Overexpression of the glucose-phosphorylating enzyme glucokinase (GK) or members of the family of glycogen-targeting subunits of protein phosphatase-1 increases hepatic glucose disposal and glycogen synthesis. This study was undertaken to evaluate the functional properties of a novel, truncated glycogen-targeting subunit derived from the skeletal muscle isoform G(M)/R(Gl) and to compare pathways of glycogen metabolism and their regulation in cells with overexpressed targeting subunits and GK. When overexpressed in hepatocytes, truncated G(M)/R(Gl) (G(M)DeltaC) was approximately twice as potent as full-length G(M)/R(Gl) in stimulation of glycogen synthesis, but clearly less potent than GK or two other native glycogen-targeting subunits, G(L) and PTG. We also found that cells with overexpressed G(M)DeltaC are unique in that glycogen was efficiently degraded in response to lowering of media glucose concentrations, stimulation with forskolin, or a combination of both maneuvers, whereas cells with overexpressed G(L), PTG, or GK exhibited impairment in one or both of these glycogenolytic signaling pathways. (2)H NMR analysis of purified glycogen revealed that hepatocytes with overexpressed GK synthesized a larger portion of their glycogen from triose phosphates and a smaller portion from tricarboxylic acid cycle intermediates than cells with overexpressed glycogen-targeting subunits. Additional evidence for activation of distinct pathways of glycogen synthesis by GK and targeting subunits is provided by the additive effect of co-overexpression of the two types of proteins upon glycogen synthesis and a much larger stimulation of glucose utilization, glucose transport, and lactate production elicited by GK. We conclude that overexpression of the novel targeting subunit G(M)DeltaC confers unique regulation of glycogen metabolism. Furthermore, targeting subunits and GK stimulate glycogen synthesis by distinct pathways.

Adenoviral vector-mediated insulin gene transfer in the mouse pancreas corrects streptozotocin-induced hyperglycemia

Therapy for type 1 diabetes consists of tight blood glucose (BG) control to minimize complications. Current treatment relies on multiple insulin injections or an insulin pump placement, beta-cell or whole pancreas transplantation. All approaches have significant limitations and have led to the realization that novel treatment strategies are needed. Pancreatic acinar cells have features that make them a good target for insulin gene transfer. They are not subject to autoimmune attack, a problem with pancreas or islets transplantation, they are avidly transduced by recombinant adenoviral vectors, and capable of exporting a variety of peptides into the portal circulation. Recombinant adenoviral vectors were engineered to express either wild-type or furin-modified human insulin cDNA (AdCMVhInsM). Immunodeficient mice were made diabetic with streptozotocin and injected intrapancreatically with the vectors. BG and blood insulin levels have normalized after administration of AdCMVhInsM. Immunohistochemistry and electron microscopy showed the presence of insulin in acinar cells throughout the pancreas and localization of insulin molecules to acinar cell vesicles. The data clearly establish a relationship between intrapancreatic vector administration, decreased BG and elevated blood insulin levels. The findings support the use of pancreatic acinar cells to express and secrete insulin into the blood stream.

Hepatic glycogen synthesis is highly sensitive to phosphorylase activity: evidence from metabolic control analysis

We used metabolic control analysis to determine the flux control coefficient of phosphorylase on glycogen synthesis in hepatocytes by titration with a specific phosphorylase inhibitor (CP-91149) or by expression of muscle phosphorylase using recombinant adenovirus. The muscle isoform was used because it is catalytically active in the b-state. CP-91149 inactivated phosphorylase with sequential activation of glycogen synthase. It increased glycogen synthesis by 7-fold at 5 mm glucose and by 2-fold at 20 mm glucose with a decrease in the concentration of glucose causing half-maximal rate (S(0.5)) from 26 to 19 mm. Muscle phosphorylase was expressed in hepatocytes mainly in the b-state. Low levels of phosphorylase expression inhibited glycogen synthesis by 50%, with little further inhibition at higher enzyme expression, and caused inactivation of glycogen synthase that was reversed by CP-91149. At endogenous activity, phosphorylase has a very high (greater than unity) negative control coefficient on glycogen synthesis, regardless of whether it is determined by enzyme inactivation or overexpression. This high control is attenuated by glucokinase overexpression, indicating dependence on other enzymes with high control. The high control coefficient of phosphorylase on glycogen synthesis affirms that phosphorylase is a strong candidate target for controlling hyperglycemia in type 2 diabetes in both the absorptive and postabsorptive states.

Overexpression of the P46 (T1) translocase component of the glucose-6-phosphatase complex in hepatocytes impairs glycogen accumulation via hydrolysis of glucose 1-phosphate

The final step of gluconeogenesis and glycogenolysis is catalyzed by the glucose-6-phosphatase (Glc-6-Pase) enzyme complex, located in the endoplasmic reticulum. The complex consists of a 36-kDa catalytic subunit (P36), a 46-kDa glucose 6-phosphate translocase (P46), and putative glucose and inorganic phosphate transporters. Mutations in the genes encoding P36 or P46 have been linked to glycogen storage diseases type Ia and type Ib, respectively. However, the relative roles of these two proteins in control of the rate of glucose 6-phosphate hydrolysis have not been defined. To gain insight into this area, we have constructed a recombinant adenovirus containing the cDNA encoding human P46 (AdCMV-P46) and treated rat hepatocytes with this virus, or a virus encoding P36 (AdCMV-P36), or the combination of both viruses, resulting in large and equivalent increases in expression of the transgenes within 8-24 h of viral treatment. The overexpressed P46 protein was appropriately targeted to hepatocyte microsomes and caused a 58% increase in glucose 6-phosphate hydrolysis in nondetergent-treated (intact) microsomal preparations relative to controls, whereas overexpression of P36 caused a 3.6-fold increase. Overexpression of P46 caused a 50% inhibition of glycogen accumulation in hepatocytes from fasted rats incubated at 25 mm glucose relative to cells treated with a control virus (AdCMV-betaGAL). Furthermore, in hepatocytes from fed rats cultured at 25 mm glucose and then exposed to 15 mm glucose, AdCMV-P46 treatment activated glycogenolysis, as indicated by a 50% reduction in glycogen content relative to AdCMV-betaGAL-treated controls. In contrast, overexpression of P46 had only small effects on glycolysis, whereas overexpression of P36 had large effects on both glycogen metabolism and glycolysis, even in the presence of co-overexpressed glucokinase. Finally, P46 overexpression enhanced glucose 1-phosphate but not fructose 6-phosphate hydrolysis in intact microsomes, providing a mechanism by which P46 overexpression may exert its preferential effects on glycogen metabolism.

A novel glucokinase regulator in pancreatic beta cells: precursor of propionyl-CoA carboxylase beta subunit interacts with glucokinase and augments its activity

A glucokinase regulatory protein has been reported to exist in the liver, which suppresses enzyme activity in a complex with fructose 6-phosphate, whereas no corresponding protein has been found in pancreatic beta cells. To search for such a protein in pancreatic beta cells, we screened for a cDNA library of the HIT-T15 cell line with the cDNA of glucokinase from rat islet by the yeast two hybrid system. We detected a cDNA encoding the precursor of propionyl-CoA carboxylase beta subunit (pbetaPCCase), and glutathione S-transferase pull-down assay illustrated that pbetaPCCase interacted with recombinant rat islet glucokinase and with glucokinase in rat liver and islet extracts. Functional analysis indicated that pbetaPCCase decreased the K(m) value of recombinant islet glucokinase for glucose by 18% and increased V(max) value by 23%. We concluded that pbetaPCCase might be a novel activator of glucokinase in pancreatic beta cells.

Regulation of gene expression by glucose in pancreatic beta -cells (MIN6) via insulin secretion and activation of phosphatidylinositol 3'-kinase

Increases in glucose concentration control the transcription of the preproinsulin (PPI) gene and several other genes in the pancreatic islet beta-cell. Although recent data have demonstrated that secreted insulin may regulate the PPI gene (Leibiger, I. B., Leibiger, B., Moede, T., and Berggren, P. O. (1998) Mol. Cell 1, 933-938), the role of insulin in the control of other beta-cell genes is unexplored. To study the importance of insulin secretion in the regulation of the PPI and liver-type pyruvate kinase (L-PK) genes by glucose, we have used intranuclear microinjection of promoter-luciferase constructs into MIN6 beta-cells and photon-counting imaging. The activity of each promoter was increased either by 30 (versus 3) mm glucose or by 1-20 nm insulin. These effects of insulin were not due to enhanced glucose metabolism since culture with the hormone had no impact on the stimulation of increases in intracellular ATP concentration caused by 30 mm glucose. Furthermore, the islet-specific glucokinase promoter and cellular glucokinase immunoreactivity were unaffected by 30 mm glucose or 20 nm insulin. Inhibition of insulin secretion with the Ca(2+) channel blocker verapamil, the ATP-sensitive K(+) channel opener diazoxide, or the alpha(2)-adrenergic agonist clonidine blocked the effects of glucose on L-PK gene transcription. Similarly, 30 mm glucose failed to induce the promoter after inhibition of phosphatidylinositol 3'-kinase activity with LY294002 and the expression of dominant negative-acting phosphatidylinositol 3'-kinase (Deltap85) or the phosphoinositide 3'-phosphatase PTEN (phosphatase and tensin homologue). LY294002 also diminished the activation of the L-PK gene caused by inhibition of 5'-AMP-activated protein kinase with anti-5'-AMP-activated protein kinase alpha2 antibodies. Conversely, stimulation of insulin secretion with 13 mm KCl or 10 microm tolbutamide strongly activated the PPI and L-PK promoters. These data indicate that, in MIN6 beta-cells, stimulation of insulin secretion is important for the activation by glucose of L-PK as well as the PPI promoter, but does not cause increases in glucokinase gene expression or glucose metabolism.