Effects of alternate RNA splicing on glucokinase isoform activities in the pancreatic islet, liver, and pituitary

Pancreatic β-cell heterogeneity in health and diabetes: classes, sources, and subtypes

Pancreatic β-cells perform glucose-stimulated insulin secretion, a process at the center of type 2 diabetes etiology. Efforts to understand how β-cells behave in healthy and stressful conditions have revealed a wide degree of morphological, functional, and transcriptional heterogeneity. Sources of heterogeneity include β-cell topography, developmental origin, maturation state, and stress response. Advances in sequencing and imaging technologies have led to the identification of β-cell subtypes, which play distinct roles in the islet niche. This review examines β-cell heterogeneity from morphological, functional, and transcriptional perspectives, and considers the relevance of topography, maturation, development, and stress response. It also discusses how these factors have been used to identify β-cell subtypes, and how heterogeneity is impacted by diabetes. We examine open questions in the field and discuss recent technological innovations that could advance understanding of β-cell heterogeneity in health and disease.

Glucokinase intrinsically regulates glucose sensing and glucagon secretion in pancreatic alpha cells

The secretion of glucagon by pancreatic alpha cells is regulated by a number of external and intrinsic factors. While the electrophysiological processes linking a lowering of glucose concentrations to an increased glucagon release are well characterized, the evidence for the identity and function of the glucose sensor is still incomplete. In the present study we aimed to address two unsolved problems: (1) do individual alpha cells have the intrinsic capability to regulate glucagon secretion by glucose, and (2) is glucokinase the alpha cell glucose sensor in this scenario. Single cell RT-PCR was used to confirm that glucokinase is the main glucose-phosphorylating enzyme expressed in rat pancreatic alpha cells. Modulation of glucokinase activity by pharmacological activators and inhibitors led to a lowering or an increase of the glucose threshold of glucagon release from single alpha cells, measured by TIRF microscopy, respectively. Knockdown of glucokinase expression resulted in a loss of glucose control of glucagon secretion. Taken together this study provides evidence for a crucial role of glucokinase in intrinsic glucose regulation of glucagon release in rat alpha cells.

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.

Hypothalamic arcuate nucleus glucokinase regulates insulin secretion and glucose homeostasis

AIMS

To investigate the role of arcuate glucokinase (GK) in the regulation of glucose homeostasis.

MATERIALS AND METHODS

A recombinant adeno-associated virus expressing either GK or an antisense GK construct was used to alter GK activity specifically in the hypothalamic arcuate nucleus (arc). GK activity in this nucleus was also increased by stereotactic injection of the GK activator, compound A. The effect of altered arc GK activity on glucose homeostasis was subsequently investigated using glucose and insulin tolerance tests.

RESULTS

Increased GK activity specifically within the arc increased insulin secretion and improved glucose tolerance in rats during oral glucose tolerance tests. Decreased GK activity in this nucleus reduced insulin secretion and increased glucose levels during the same tests. Insulin sensitivity was not affected in either case. The effect of arc GK was maintained in a model of type 2 diabetes.

CONCLUSIONS

These results demonstrate a role for arc GK in systemic glucose homeostasis.

All mixed up: defining roles for β-cell subtypes in mature islets

Following differentiation during fetal development, β cells further adapt to their postnatal role through functional maturation. While adult islets are thought to contain functionally mature β cells, recent analyses of transgenic rodent and human pancreata reveal a number of novel heterogeneity markers in mammalian β cells. The marked heterogeneity long after maturation raises the prospect that diverse populations harbor distinct roles aside from glucose-stimulated insulin secretion. In this review, we outline our current understanding of the β-cell maturation process, emphasize recent literature on novel heterogeneity markers, and offer perspectives on reconciling the findings from these two areas.

TMG-123, a novel glucokinase activator, exerts durable effects on hyperglycemia without increasing triglyceride in diabetic animal models

Glucokinase (GK) plays a critical role for maintaining glucose homeostasis with regulating glucose uptake in liver and insulin secretion in pancreas. GK activators have been reported to decrease blood glucose levels in patients with type 2 diabetes mellitus. However, clinical development of GK activators has failed due to the loss of glucose-lowering effects and increased plasma triglyceride levels after chronic treatment. Here, we generated a novel GK activator, TMG-123, examined its in vitro and in vivo pharmacological characteristics, and evaluated its risks of aforementioned clinical issues. TMG-123 selectively activated GK enzyme activity without increasing Vmax. TMG-123 improved glucose tolerance without increasing plasma insulin levels in both insulin-deficient (Goto-Kakizaki rats) and insulin-resistant (db/db mice) models. The beneficial effect on glucose tolerance was greater than results observed with clinically available antidiabetic drugs such as metformin and glibenclamide in Zucker Diabetic Fatty rats. TMG-123 also improved glucose tolerance in combination with metformin. After 4 weeks of administration, TMG-123 reduced the Hemoglobin A1c levels without affecting liver and plasma triglyceride levels in Goto-Kakizaki rats and Diet-Induced Obesity mice. Moreover, TMG-123 sustained its effect on Hemoglobin A1c levels even after 24 weeks of administration without affecting triglycerides. Taken together, these data indicate that TMG-123 exerts glucose-lowering effects in both insulin-deficient and -resistant diabetes, and sustains reduced Hemoglobin A1c levels without affecting hepatic and plasma triglycerides even after chronic treatment. Therefore, TMG-123 is expected to be an antidiabetic drug that overcomes the concerns previously reported with other GK activators.

Ischemic preconditioning maintains the immunoreactivities of glucokinase and glucokinase regulatory protein in neurons of the gerbil hippocampal CA1 region following transient cerebral ischemia

Glucokinase (GK) is involved in the control of blood glucose homeostasis. In the present study, the effect of ischemic preconditioning (IPC) on the immunoreactivities of GK and its regulatory protein (GKRP) following 5 min of transient cerebral ischemia was investigated in gerbils. The gerbils were randomly assigned to four groups (sham-operated group, ischemia-operated group, IPC + sham-operated group and IPC + ischemia-operated group). IPC was induced by subjecting the gerbils to 2 min of ischemia, followed by 1 day of recovery. In the ischemia-operated group, a significant loss of neurons was observed in the stratum pyramidale (SP) of the hippocampal CA1 region (CA1) at 5 days post-ischemia; however, in the IPC+ischemia-operated group, the neurons in the SP were well protected. Following immunohistochemical investigation, the immunoreactivities of GK and GKRP in the neurons of the SP were markedly decreased in the CA1, but not the CA2/3, from 2 days post-ischemia, and were almost undetectable in the SP 5 days post-ischemia. In the IPC + ischemia-operated group, the immunoreactivities of GK and GKRP in the SP of the CA1 were similar to those in the sham-group. In brief, the findings of the present study demonstrated that IPC notably maintained the immunoreactivities of GK and GKRP in the neurons of the SP of CA1 following ischemia-reperfusion. This indicated that GK and GKRP may be necessary for neuron survival against transient cerebral ischemia.

The role of tanycytes in hypothalamic glucosensing

Tanycytes are elongated hypothalamic glial cells that cover the basal walls of the third ventricle; their apical regions contact the cerebrospinal fluid (CSF), and their processes reach hypothalamic neuronal nuclei that control the energy status of an organism. These nuclei maintain the balance between energy expenditure and intake, integrating several peripheral signals and triggering cellular responses that modify the feeding behaviour and peripheral glucose homeostasis. One of the most important and well-studied signals that control this process is glucose; however, the mechanism by which this molecule is sensed remains unknown. We along with others have proposed that tanycytes play a key role in this process, transducing changes in CSF glucose concentration to the neurons that control energy status. Recent studies have demonstrated the expression and function of monocarboxylate transporters and canonical pancreatic β cell glucose sensing molecules, including glucose transporter 2 and glucokinase, in tanycytes. These and other data, which will be discussed in this review, suggest that hypothalamic glucosensing is mediated through a metabolic interaction between tanycytes and neurons through lactate. This article will summarize the recent evidence that supports the importance of tanycytes in hypothalamic glucosensing, and discuss the possible mechanisms involved in this process. Finally, it is important to highlight that a detailed analysis of this mechanism could represent an opportunity to understand the evolution of associated pathologies, including diabetes and obesity, and identify new candidates for therapeutic intervention.

Cloning and characterization of feline islet glucokinase

BACKGROUND

Glucokinase (GK) is a metabolic enzyme encoded by the GCK gene and expressed in glucose-sensitive tissues, principally pancreatic islets cell and hepatocytes. The GK protein acts in pancreatic islets as a "glucose sensor" that couples fluctuations in the blood glucose concentration to changes in cellular function and insulin secretion. GCK and GK have proposed importance in the development and progression of diabetes mellitus and are potential therapeutic targets for diabetes treatment. The study was undertaken to determine the nucleotide sequence of feline pancreatic GK cDNA, predict the amino acid sequence and structure of the feline GK protein, and perform comparative bioinformatic analysis of feline cDNA and protein. Routine PCR techniques were used with cDNA from feline pancreas. Clones were assembled to obtain the full length cDNA. Protein prediction and modeling were performed using bioinformatic tools.

RESULTS

Full-length feline pancreatic GK cDNA contains a 1398 nucleotide coding sequence with high identity to other pancreatic GK cDNAs. The deduced 465 amino acid feline protein has 15 amino acid substitutions not found in other mammalian GK proteins but maintains high structural homology with human GK. Feline pancreatic GK is highly conserved at nucleotide and protein levels. Residues crucial for substrate binding and catalysis are completely conserved in the feline protein.

CONCLUSION

Molecular analysis predicts that feline pancreatic GK functions similarly to other mammalian GK proteins.

Nutritional regulation of glucokinase: a cross-species story

The glucokinase (GK) enzyme (EC 2.7.1.1.) is essential for the use of dietary glucose because it is the first enzyme to phosphorylate glucose in excess in different key tissues such as the pancreas and liver. The objective of the present review is not to fully describe the biochemical characteristics and the genetics of this enzyme but to detail its nutritional regulation in different vertebrates from fish to human. Indeed, the present review will describe the existence of the GK enzyme in different animal species that have naturally different levels of carbohydrate in their diets. Thus, some studies have been performed to analyse the nutritional regulation of the GK enzyme in humans and rodents (having high levels of dietary carbohydrates in their diets), in the chicken (moderate level of carbohydrates in its diet) and rainbow trout (no carbohydrate intake in its diet). All these data illustrate the nutritional importance of the GK enzyme irrespective of feeding habits, even in animals known to poorly use dietary carbohydrates (carnivorous species).

Dynamic localization of glucokinase and its regulatory protein in hypothalamic tanycytes

Glucokinase (GK), the hexokinase involved in glucose sensing in pancreatic β cells, is also expressed in hypothalamic tanycytes, which cover the ventricular walls of the basal hypothalamus and are implicated in an indirect control of neuronal activity by glucose. Previously, we demonstrated that GK was preferentially localized in tanycyte nuclei in euglycemic rats, which has been reported in hepatocytes and is suggestive of the presence of the GK regulatory protein, GKRP. In the present study, GK intracellular localization in hypothalamic and hepatic tissues of the same rats under several glycemic conditions was compared using confocal microscopy and Western blot analysis. In the hypothalamus, increased GK nuclear localization was observed in hyperglycemic conditions; however, it was primarily localized in the cytoplasm in hepatic tissue under the same conditions. Both GK and GKRP were next cloned from primary cultures of tanycytes. Expression of GK by Escherichia coli revealed a functional cooperative protein with a S_0.5 of 10 mM. GKRP, expressed in Saccharomyces cerevisiae, inhibited GK activity in vitro with a K_i 0.2 µM. We also demonstrated increased nuclear reactivity of both GK and GKRP in response to high glucose concentrations in tanycyte cultures. These data were confirmed using Western blot analysis of nuclear extracts. Results indicate that GK undergoes short-term regulation by nuclear compartmentalization. Thus, in tanycytes, GK can act as a molecular switch to arrest cellular responses to increased glucose.

Factors affecting insulin-regulated hepatic gene expression

Obesity has become a major concern of public health. A common feature of obesity and related metabolic disorders such as noninsulin-dependent diabetes mellitus is insulin resistance, wherein a given amount of insulin produces less than normal physiological responses. Insulin controls hepatic glucose and fatty acid metabolism, at least in part, via the regulation of gene expression. When the liver is insulin-sensitive, insulin can stimulate the expression of genes for fatty acid synthesis and suppress those for gluconeogenesis. When the liver becomes insulin-resistant, the insulin-mediated suppression of gluconeogenic gene expression is lost, whereas the induction of fatty acid synthetic gene expression remains intact. In the past two decades, the mechanisms of insulin-regulated hepatic gene expression have been studied extensively and many components of insulin signal transduction pathways have been identified. Factors that alter these pathways, and the insulin-regulated hepatic gene expression, have been revealed and the underlying mechanisms have been proposed. This chapter summarizes the recent progresses in our understanding of the effects of dietary factors, drugs, bioactive compounds, hormones, and cytokines on insulin-regulated hepatic gene expression. Given the large amount of information and progresses regarding the roles of insulin, this chapter focuses on findings in the liver and hepatocytes and not those described for other tissues and cells. Typical insulin-regulated hepatic genes, such as insulin-induced glucokinase and sterol regulatory element-binding protein-1c and insulin-suppressed cytosolic phosphoenolpyruvate carboxyl kinase and insulin-like growth factor-binding protein 1, are used as examples to discuss the mechanisms such as insulin regulatory element-mediated transcriptional regulation. We also propose the potential mechanisms by which these factors affect insulin-regulated hepatic gene expression and discuss potential future directions of the area of research.

Roles of Vitamin A Metabolism in the Development of Hepatic Insulin Resistance

The increase in the number of people with obesity- and noninsulin-dependent diabetes mellitus has become a major public health concern. Insulin resistance is a common feature closely associated with human obesity and diabetes. Insulin regulates metabolism, at least in part, via the control of the expression of the hepatic genes involved in glucose and fatty acid metabolism. Insulin resistance is always associated with profound changes of the expression of hepatic genes for glucose and lipid metabolism. As an essential micronutrient, vitamin A (VA) is needed in a variety of physiological functions. The active metablite of VA, retinoic acid (RA), regulates the expression of genes through the activation of transcription factors bound to the RA-responsive elements in the promoters of RA-targeted genes. Recently, retinoids have been proposed to play roles in glucose and lipid metabolism and energy homeostasis. This paper summarizes the recent progresses in our understanding of VA metabolism in the liver and of the potential transcription factors mediating RA responses. These transcription factors are the retinoic acid receptor, the retinoid X receptor, the hepatocyte nuclear factor 4α, the chicken ovalbumin upstream promoter-transcription factor II, and the peroxisome proliferator-activated receptor β/δ. This paper also summarizes the effects of VA status and RA treatments on the glucose and lipid metabolism in vivo and the effects of retinoid treatments on the expression of insulin-regulated genes involved in the glucose and fatty acid metabolism in the primary hepatocytes. I discuss the roles of RA production in the development of insulin resistance in hepatocytes and proposes a mechanism by which RA production may contribute to hepatic insulin resistance. Given the large amount of information and progresses regarding the physiological functions of VA, this paper mainly focuses on the findings in the liver and hepatocytes and only mentions the relative findings in other tissues and cells.

Decreased glucokinase protein expression in the aged gerbil hippocampus

Glucokinase (GK) and its regulatory protein (GKRP) play roles in glucose utilization as well as glucose-sensing process in the brain. In the present study, we compared GK and GKRP protein expressions in the hippocampus of adult (postnatal month 6) and aged (postnatal month 24) gerbils using immunohistochemistry and western blot analysis. Both GK and GKRP immunoreactivities were observed primarily in the pyramidal cells of the hippocampus proper and in the granule cells of the dentate gyrus of the adult and aged hippocampus. GK, not GKRP, immunoreactivity was apparently decreased in the pyramidal and granule cells of the aged group compared with that in the adult group. In addition, western blot analysis also showed that the GK, not GKRP, protein level was significantly decreased in the aged hippocampus. These results indicate that the decrease of GK may be closely related to the reduction of glucose utilization and uptake, although the ability for regulation of GK is maintained in the aged hippocampus.

LRH-1-dependent glucose sensing determines intermediary metabolism in liver

Liver receptor homolog 1 (LRH-1), an established regulator of cholesterol and bile acid homeostasis, has recently emerged as a potential drug target for liver disease. Although LRH-1 activation may protect the liver against diet-induced steatosis and insulin resistance, little is known about how LRH-1 controls hepatic glucose and fatty acid metabolism under physiological conditions. We therefore assessed the role of LRH-1 in hepatic intermediary metabolism. In mice with conditional deletion of Lrh1 in liver, analysis of hepatic glucose fluxes revealed reduced glucokinase (GCK) and glycogen synthase fluxes as compared with those of wild-type littermates. These changes were attributed to direct transcriptional regulation of Gck by LRH-1. Impaired glucokinase-mediated glucose phosphorylation in LRH-1-deficient livers was also associated with reduced glycogen synthesis, glycolysis, and de novo lipogenesis in response to acute and prolonged glucose exposure. Accordingly, hepatic carbohydrate response element-binding protein activity was reduced in these animals. Cumulatively, these data identify LRH-1 as a key regulatory component of the hepatic glucose-sensing system required for proper integration of postprandial glucose and lipid metabolism.

Roles of vitamin A status and retinoids in glucose and fatty acid metabolism

The rising prevalence of metabolic diseases, such as obesity and diabetes, has become a public health concern. Vitamin A (VA, retinol) is an essential micronutrient for a variety of physiological processes, such as tissue differentiation, immunity, and vision. However, its role in glucose and lipid metabolism has not been clearly defined. VA activities are mediated by the metabolite of retinol catabolism, retinoic acid, which activates the retinoic acid receptor and retinoid X receptor (RXR). Since RXR is an obligate heterodimeric partner for many nuclear receptors involved in metabolism, it is reasonable to assume that VA status and retinoids contribute to glucose and lipid homeostasis. To date, the impacts of VA and retinoids on energy metabolism in animals and humans have been demonstrated in some basic and clinical investigations. This review summarizes the effects of VA status and retinoid treatments on metabolism of the liver, adipocytes, pancreatic β-cells, and skeletal muscle. It proposes a mechanism by which the dietary and hormonal signals converge on the promoter of sterol regulatory element-binding protein 1c gene to induce its expression, and in turn, the expression of lipogenic genes in hepatocytes. Future research projects relevant to the VA's roles in metabolic diseases are also discussed.

Mutations in pancreatic ß-cell Glucokinase as a cause of hyperinsulinaemic hypoglycaemia and neonatal diabetes mellitus

Glucokinase is a key enzyme involved in regulating insulin secretion from the pancreatic ß-cell. The unique role of glucokinase in human glucose physiology is illustrated by the fact that genetic mutations in glucokinase can either cause hyperglycaemia or hypoglycaemia. Heterozygous inactivating mutations in glucokinase cause maturity-onset diabetes of the young (MODY), homozygous inactivating in glucokinase mutations result in permanent neonatal diabetes whereas heterozygous activating glucokinase mutations cause hyperinsulinaemic hypoglycaemia.

Glucokinase and glucokinase regulatory proteins are functionally coexpressed before birth in the rat brain

Our previous description of functional glucokinase (GK) isoforms and their interactions with glucokinase regulatory protein (GKRP) in adult rat and human brains suggested that both participate in glucose sensing in the central nervous system. To determine whether both proteins are coexpressed and active before birth or during early post-natal life, we characterised these molecules in the brains of foetal and post-natal pup rats. We found GK and GKRP mRNAs that were similar to those previously reported in the adult rat brain. Likewise, GK and GKRP gene expression gave rise to proteins of 52 and 69 kDa, respectively. Immunohistochemistry experiments showed the colocalisation of both GK and GKRP proteins in the same brain cells of 21-day-old rat foetuses. Furthermore, coprecipitation of GK and GKRP in the presence of fructose 6-phosphate suggests interactions between both proteins. The presence of GK phosphorylating activity was detected in different brain areas of 21-day-old foetuses with a contribution to the total glucose-phosphorylating activity of between 17.2 +/- 1.7% and 12.4 +/- 3.7%, with the hypothalamus being the region of maximum activity. The hypothalamic GK activity in 21-day-old foetuses has a high apparent K(m) for glucose and no product inhibition by glucose 6-phosphate. Our findings indicate that both proteins may be functionally active before birth and that they can act within a glucose sensor system involved in controlling food intake.

A glucose sensor role for glucokinase in anterior pituitary cells

Enzymatic activity of glucokinase was demonstrated, quantitated, and characterized kinetically in rat and mouse pituitary extracts using a highly specific and sensitive spectrometric assay. A previously proposed hypothesis that the glucokinase gene might be expressed in the pituitary corticotrophic cells was therefore reexamined using mRNA in situ hybridization and immunohistochemical techniques. No evidence was found that corticotrophs are glucokinase positive, and the identity of glucokinase-expressing cells remains to be determined. The findings do, however, suggest a novel hypothesis that a critical subgroup of anterior pituitary cells might function as glucose sensor cells and that direct fuel regulation of such cells may modify the classical indirect neuroendocrine pathways that are known to control hormone secretion from anterior pituitary cells.

Glucokinase regulatory protein is associated with mitochondria in hepatocytes

The association of glucokinase with liver mitochondria has been reported [Danial et al. (2003) BAD and glucokinase reside in a mitochondrial complex that integrates glycolysis and apoptosis. Nature 424, 952-956]. We confirmed association of glucokinase immunoreactivity with rat liver mitochondria using Percoll gradient centrifugation and demonstrated its association with the 68 kDa regulatory protein (GKRP) but not with the binding protein phosphofructokinase-2/fructose bisphosphatase-2. Substrates and glucagon induced adaptive changes in the mitochondrial glucokinase/GKRP ratio suggesting a regulatory role for GKRP. Combined with previous observations that GKRP overexpression partially inhibits glycolysis [de la Iglesia et al. (2000) The role of the regulatory protein of glucokinase in the glucose sensory mechanism of the hepatocyte. J. Biol. Chem. 275, 10597-10603] these findings suggest that there may be distinct glycolytic pools of glucokinase.

FoxO1 regulates multiple metabolic pathways in the liver: effects on gluconeogenic, glycolytic, and lipogenic gene expression

FoxO transcription factors are important targets of insulin action. To better understand the role of FoxO proteins in the liver, we created transgenic mice expressing constitutively active FoxO1 in the liver using the alpha1-antitrypsin promoter. Fasting glucose levels are increased, and glucose tolerance is impaired in transgenic (TGN) versus wild type (WT) mice. Interestingly, fasting triglyceride and cholesterol levels are reduced despite hyperinsulinemia, and post-prandial changes in triglyceride levels are markedly suppressed in TGN versus WT mice. Activation of pro-lipogenic signaling pathways (atypical protein kinase C and protein kinase B) and the ability to suppress beta-hydroxybutyrate levels are not impaired in TGN. In contrast, de novo lipogenesis measured with (3)H(2)O is suppressed by approximately 70% in the liver of TGN versus WT mice after refeeding. Gene-array studies reveal that the expression of genes involved in gluconeogenesis, glycerol transport, and amino acid catabolism is increased, whereas genes involved in glucose utilization by glycolysis, the pentose phosphate shunt, lipogenesis, and sterol synthesis pathways are suppressed in TGN versus WT. Studies with adenoviral vectors in isolated hepatocytes confirm that FoxO1 stimulates expression of gluconeogenic genes and suppresses expression of genes involved in glycolysis, the shunt pathway, and lipogenesis, including glucokinase and SREBP-1c. Together, these results indicate that FoxO proteins promote hepatic glucose production through multiple mechanisms and contribute to the regulation of other metabolic pathways important in the adaptation to fasting and feeding in the liver, including glycolysis, the pentose phosphate shunt, and lipogenic and sterol synthetic pathways.

Glucokinase in chicken (Gallus gallus). Partial cDNA cloning, immunodetection and activity determination

Chickens are more hyperglycaemic and insulin-resistant than mammals, and in efforts to understand their glucose metabolism we investigated whether glucokinase (GK) is present in chicken liver or pancreas. This enzyme plays a major role in glucose-sensing in mammals and we have examined whether it also contributes to glucose homeostasis in chickens. Using RT-PCR, we cloned and sequenced a partial cDNA fragment (750 bp) from liver and pancreas that showed a high degree of identity with mammalian GK. Using antibodies directed towards human GK, we immunodetected a 50 kDa band in chicken liver and pancreas. The molecular mass of the band and its specific interaction with the antibody suggest that this protein corresponds to a chicken homologue of human GK. We also determined by spectrophotometry a glucokinase-like activity in crude liver homogenates with an apparent half saturating concentration for glucose of 8.6 mM. GK gene and protein expression did not differ between fed and 24 h fasted states but GK-like activity was significantly increased in fed chickens. In conclusion, our study provides evidence for the presence of GK gene and protein in chicken liver and pancreas and shows that the liver enzyme is active.

Glucokinase, glucose homeostasis, and diabetes mellitus

The enzyme glucokinase (GK) regulates the rate of glucose metabolism in many tissues, including liver, the pancreatic b cells, certain neurons, enteroendocrine cells, and the pituitary, serving as a glucose sensor in many of these. Thus, GK plays a critical role in glucose homeostasis. Spontaneous mutants of GK in humans result in autosomal-dominant hypo- and hyperglycemia syndromes described as "GK disease." GK activator drugs have been discovered that lower blood glucose in normal and diabetic animals and promise to be useful in the treatment of type 2 diabetes mellitus. There is no question that the GK molecule and related issues will continue to be a fruitful topic for future research.

Expression of glucose transporter isoform GLUT-2 and glucokinase genes in human brain

The glucose transporter isoform-2 (GLUT-2) and glucokinase are considered to be components of a glucose sensor system controlling several key processes, and hence may modulate feeding behaviour. We have found GLUT-2 and glucokinase mRNAs in several brain regions, including the ventromedial and arcuate nuclei of the hypothalamus. GLUT-2, glucokinase and glucokinase regulatory protein mRNAs and proteins were present in these areas as determined by biochemical approaches. In addition, glucose-phosphorylating activity with a high apparent Km for glucose that displayed no product inhibition by glucose-6-phosphate was observed. Increased glycaemia after meals may be recognized by specific hypothalamic neurones due to the high Km of GLUT-2 and glucokinase. This enzyme is considered to be the true glucose sensor because it catalyses the rate-limiting step of glucose catabolism its activity being regulated by interaction with glucokinase regulatory protein, that functions as a metabolic sensor.

A glucokinase-like enzyme induced in Mule duck livers by overfeeding

The Mule duck develops a fatty liver in response to overfeeding, which results from a dramatic increase in de novo liver lipogenesis, and thus raises questions regarding the role of glucokinase (GK), a key enzyme regulating carbohydrate metabolism in mammals. However, the presence of GK in avian species is still a matter of debate. The aim of the present study was to characterize a GK-like protein (using an immunological technique) and a GK-like activity (using an enzymatic assay) in duck liver and to measure their respective variations during various stages of overfeeding. Duck liver protein cross-reacted with antibodies directed against mammalian GK yielding a band at 50 kDa, i.e., the same molecular weight as mammalian GK. The intensity of the signal varied significantly between overfed and control ducks but in opposing ways according to the GK antibodies used, which suggests the presence of 2 isoforms of GK in the duck liver as in mammals. Enzymatic analysis demonstrated the presence of glucose phosphorylation activity sensitive to high and low glucose concentrations (high/low ratio between 1.7 and 3.7) in the soluble and particulate fractions of liver homogenates. Glucokinase-like activity per milligram protein was strongly induced by overfeeding, and plasma insulin levels increased concomitantly. More than 80% of total GK-like activity was concentrated in the soluble component from 1 to 13 d of overfeeding. These results suggest that a GK-like enzyme may actively contribute to glucose disposal throughout the overfeeding period in Mule ducks fed a carbohydrate-rich diet.

Glucose-induced toxicity in insulin-producing pituitary cells that coexpress GLUT2 and glucokinase. Implications for metabolic engineering

We have shown that intermediate lobe (IL) pituitary cells can be engineered to produce sufficient amounts of insulin (ins) to cure diabetes in nonobese diabetic mice but, unlike transplanted islets, ILins cells evade immune attack. To confer glucose-sensing capabilities into these cells, they were further modified with recombinant adenoviruses to express high levels of GLUT2 and the beta-cell isoform of glucokinase (GK). Although expression of GLUT2 alone had negligible effects on glucose usage and lactate production, expression of GK alone resulted in approximately 2-fold increase in glycolytic flux within the physiological (3-20 mm) glucose range. GLUT2/GK coexpression further increased glycolytic flux at 20 mm glucose but disproportionately increased flux at 3 mm glucose. Despite enhanced glycolytic fluxes, GLUT2/GK-coexpressing cells showed glucose dose-dependent accumulation of hexose phosphates, depletion of intracellular ATP, and severe apoptotic cell death. These studies demonstrate that glucose-sensing properties can be introduced into non-islet cells by the single expression of GK and that glucose responsiveness can be augmented by the coexpression of GLUT2. However, in the metabolic engineering of surrogate beta cells, it is critical that the levels of the components be closely optimized to ensure their physiological function and to avoid the deleterious consequences of glucose-induced toxicity.

Cloning and functional characterization of the 5' regulatory region of the human mitochondrial glycerol-3-phosphate dehydrogenase gene. Lack of 3,5,3'-triiodothyronine responsiveness in adipose tissue

We report data on the structural and functional characterization of the 5' flanking region of the human mitochondrial glycerol-3-phosphate dehydrogenase (mtGPDH) gene. We found two regions upstream of 5'-untranslated sequences exhibiting promoter activity in transient transfection assays. Transcription start sites and potential regulatory sites in both promoter regions were defined. The proximal promoter was approximately sevenfold more active than the distal one in most cell lines, but it was only twice as active in a neuroblastoma cell line. These observations seem to indicate that the rate of transcription, as well as the tissue-specific expression of the human mtGPDH gene, is the result of a combinatorial effect of transcription factors on at least two promoters. 3,5,3'-Triiodothyronine failed to alter the transcriptional activity of human mtGPDH promoter(s) constructs in transient transfection assays. Although this finding seems to be in conflict with the reported effect of 3,5,3'-triiodothyronine in rodents, it is consistent with our observation of 3,5, 3'-triiodothyronine stimulation of mtGPDH activity in primary cultures of rat adipocytes, but not human cultured adipocytes, suggesting distinct regulation of this gene in both species.

A novel cytosolic dual specificity phosphatase, interacting with glucokinase, increases glucose phosphorylation rate

A novel protein was cloned from a rat liver cDNA library by interaction with the liver glucokinase. This protein contained 339 residues and possessed a canonical consensus sequence for a dual specificity phosphatase. The recombinant protein was able to dephosphorylate phosphotyrosyl and phosphoseryl/threonyl substrates. We called this protein the glucokinase-associated phosphatase (GKAP). The GKAP partially dephosphorylated the recombinant glucokinase previously phosphorylated, in vitro, by protein kinase A. The GKAP fused with green fluorescent protein was located in the cytosol, where glucokinase phosphorylates glucose, and not in the nucleus where the glucokinase is retained inactive by the glucokinase regulatory protein. More importantly, the GKAP accelerated the glucokinase activity in a dose-dependent manner and with a stoichiometry compatible with a physiological mechanism. This strongly suggested that the interaction between GKAP and glucokinase had a functional significance. The cloning of this novel protein with a dual specificity phosphatase activity allows the description of a possible new regulatory step in controlling the glycolysis flux.

Functional glucokinase isoforms are expressed in rat brain

Recently, the description of glucokinase mRNA in certain neuroendocrine cells has opened new ways to characterize this enzyme in the rat brain. In this study, we found glucokinase mRNA and a similar RNA splicing pattern of the glucokinase gene product in rat hypothalamus and pancreatic islets; the mRNA that codes for B1 isoform was the most abundant, with minor amounts of those coding for the B2, P1, P2, P1/B2, and P2/B2 isoforms. Glucokinase gene expression in rat brain gave rise to a protein of 52 kDa with a high apparent Km for glucose and no product inhibition by glucose 6-phosphate, with a contribution to the total glucose phosphorylating activity of between 40 and 14%; the hypothalamus and cerebral cortex were the regions of maximal activity. Low and high Km hexokinases were characterized by several criteria. Also, using RT-PCR analysis we found a glucokinase regulatory protein mRNA similar to that previously reported in liver. These findings indicate that the glucokinase present in rat brain should facilitate the adaptation of this organ to fluctuations in blood glucose concentrations, and the expression of glucokinase and GLUT-2 in the same hypothalamic neurons suggests a role in glucose sensing.

Nuclear import of hepatic glucokinase depends upon glucokinase regulatory protein, whereas export is due to a nuclear export signal sequence in glucokinase

Hepatic glucokinase (GK) moves between the nucleus and cytoplasm in response to metabolic alterations. Here, using heterologous cell systems, we have found that at least two different mechanisms are involved in the intracellular movement of GK. In the absence of the GK regulatory protein (GKRP) GK resides only in the cytoplasm. However, in the presence of GKRP, GK moves to the nucleus and resides there in association with this protein until changes in the metabolic milieu prompt its release. GK does not contain a nuclear localization signal sequence and does not enter the nucleus in a GKRP-independent manner because cells treated with leptomycin B, a specific inhibitor of leucine-rich NES-dependent nuclear export, do not accumulate GK in the nucleus. Instead, entry of GK into the nucleus appears to occur via a piggy-back mechanism that involves binding to GKRP. Nuclear export of GK, which occurs after its release from GKRP, is due to a leucine-rich nuclear export signal within the protein ((300)ELVRLVLLKLV(310)). Thus, GKRP appears to function as both a nuclear chaperone and metabolic sensor and is a critical component of a hepatic GK translocation cycle for regulating the activity of this enzyme in response to metabolic alterations.

Cloning and analysis of unique human glutaminase isoforms generated by tissue-specific alternative splicing

Three human glutaminase (hGA) isoforms were identified, two of which represent isoforms previously unidentified in any species. One isoform contains an open reading frame with high homology with the rat kidney-type glutaminase, suggesting that this isoform represents the human kidney-type glutaminase, hKGA. A second isoform, termed hGAC, contains an open reading frame that matches hKGA except for a unique COOH-terminal amino acid sequence. In addition, a third human glutaminase isoform was identified from a computer search and on further analysis was found to represent an additional unique isoform, hGAM. hKGA is expressed predominantly in brain and kidney but not in liver, hGAC is expressed principally in cardiac muscle and pancreas but not in liver or brain, and hGAM is expressed solely in cardiac and skeletal muscle. hGAC is the predominant isoform expressed by a human breast cancer cell line that exhibits a high rate of glutamine utilization and glutaminase activity. Genomic Southern analysis as well as isolation and analysis of five glutaminase genomic clones suggested that all three hGA isoforms originate from the same locus and therefore represent mRNA species that are produced by tissue-specific alternative splicing of a single pre-mRNA. Furthermore, an RT-PCR assay was developed that can be used to easily differentiate between hKGA and hGAC mRNA species.

An unusual high-Km hexokinase is expressed in the mhAT3F hepatoma cell line

In most hepatoma cells, the high-Km GLUT2/glucokinase proteins are replaced by the ubiquitous low-Km GLUT1/hexokinase type I proteins. In the mhAT3F hepatoma cells, the stimulatory effect of glucose on gene expression and glycogen accumulation was not maximal at 5 mmol/liter glucose. This response to high glucose is observed in mhAT3F cells, where GLUT2 was expressed, but not glucokinase (assessed by Northern blotting and reverse transcription-polymerase chain reaction). A low-Km hexokinase activity (19.6 +/- 3.8 milliunits/mg of protein) was present, but a high-Km (40 mmol/liter) hexokinase activity (13.9 +/- 2.5 milliunits/mg) was also detected in mhAT3F cells. The high-Km hexokinase activity was dependent on both ATP (or PPi) and glucose in the assay and was recovered in a 10-50-kDa fraction after filtration. A 30-kDa protein was detected using an anti-glucokinase antibody and localized by confocal microscopy at the same sites as glucokinase in hepatocytes. In FAO cells, the high-Km hexokinase activity and 30-kDa protein were not found. We conclude that a high-Km hexokinase activity is present in mhAT3F cells. This might explain why the effects of glucose on gene expression were not maximal at a glucose concentration of 5 mmol/liter. A 30-kDa protein identified using an anti-glucokinase antibody may be responsible for this activity present in mhAT3F cells.

Quantitative imaging of green fluorescent protein in cultured cells: comparison of microscopic techniques, use in fusion proteins and detection limits

To determine the application limits of green fluorescent protein (GFP) as a reporter gene or protein tag, we expressed GFP by itself and with fusion protein partners, and used three different imaging methods to identify GFP fluorescence. In conventional epifluorescence photomicroscopy, GFP expressed in cells could be distinguished as a bright green signal over a yellow-green autofluorescence background. In quantitative fluorescence microscopy, however, the GFP signal is contaminated by cellular autofluorescence. Improved separation of GFP signal from HeLa cell autofluorescence was achieved by the combination of confocal scanning laser microscopy using 488-nm excitation, a rapid cut-on dichroic mirror and a narrow-bandpass emission filter. Two-photon excitation of GFP fluorescence at the equivalent of approximately 390 nm provided better absorption than did 488-nm excitation. This resulted in increased signal/background but also generated a different autofluorescence pattern and appeared to increase GFP photobleaching. Fluorescence spectra similar to those of GFP alone were observed when GFP was expressed as a fusion protein either with glutathione-S-transferase (GST) or with glucokinase. Furthermore, purified GST.GFP fusion protein displayed an extinction coefficient and quantum yield consistent with values previously reported for GFP alone. In HeLa cells, the cytoplasmic GFP concentration must be greater than approximately 1 microM to allow quantifiable discrimination over autofluorescence. However, lower expression levels may be detectable if GFP is targeted to discrete subcellular compartments, such as the plasma membrane, organelles or nucleus.

Transgenic knockouts reveal a critical requirement for pancreatic beta cell glucokinase in maintaining glucose homeostasis

The secretion of insulin is controlled by the rate of glucose metabolism in the pancreatic beta cells. As phosphorylation by glucokinase (GLK) appears to be the rate-limiting step for glucose catabolism in beta cells, this enzyme may be the glucose sensor. To test this possibility and to resolve the relative roles of liver and beta cell GLK in maintaining glucose levels, we have generated mice completely deficient in GLK and transgenic mice in which GLK is expressed only in beta cells. In mice with only one GLK allele, blood glucose levels are elevated and insulin secretion is reduced. GLK-deficient mice die perinatally with severe hyperglycemia. Expression of GLK in beta cells in the absence of expression in the liver is sufficient for survival. These mice demonstrate the critical need for beta cell GLK in maintaining normal glucose levels and provide a novel model for one form of noninsulin-dependent diabetes.

Mutations in gld-1, a female germ cell-specific tumor suppressor gene in Caenorhabditis elegans, affect a conserved domain also found in Src-associated protein Sam68

The gld-1 gene of Caenorhabditis elegans is a germ-line-specific tumor suppressor gene that is essential for oogenesis. We have cloned the gld-1 gene and find that it encodes two proteins that differ by 3 amino acids. The predicted proteins contain a approximately 170-amino-acid region that we term the GSG domain (GRP33/Sam68/GLD-1), on the basis of significant similarity between GLD-1, GRP33 from shrimp, and the Src-associated protein Sam68 from mouse (also described as GAPap62 from humans). A conserved structural motif called the KH domain is found within the larger GSG domain, suggesting a biochemical function for GLD-1 protein in binding RNA. The importance of the GSG domain to the function of gld-1 in vivo is revealed by mutations that affect 5 different conserved GSG domain residues. These include missense mutations in an absolutely conserved residue of the KH domain that eliminate the tumor suppressor function of gld-1.

Age-related changes in pancreatic islet cell gene expression

Previous studies have indicated that insulin secretion in response to glucose diminishes with age but insulin synthesis and gene transcription do not. To determine whether expression of genes other than those that encode insulin are subject to age-related changes that could alter pancreatic islet function, mRNAs for insulins I and II, amylin, glucose transporter 2 (GluT2), glucagon, and glucokinase were quantified in 2-, 6-, 12-, and 24-month-old Fischer 344 rats using species-specific ribonuclease (RNase) protection assays. There was only a modest (1.2- to 1.3-fold) increase in insulin I and insulin II mRNAs between ages 2 and 12 months. There were no statistically significant changes in levels of glucokinase mRNA with age. In contrast, the abundances of amylin, GluT2, and glucagon mRNAs all doubled during the same period. Variance in values from 24-month-old rats was too great to allow conclusions, except that the ratio of insulin II mRNA to insulin I mRNA increased with age. This change was not related to islet mass or total insulin mRNA abundance because it persisted at age 24 months, when total mRNA abundance had decreased. These results indicate that aging is associated with significant alterations in the relative proportion of expression of pancreatic islet cell genes implicated in insulin secretion and in intraislet glucose metabolism.

Positive and negative regulatory elements are involved in transcriptional control of the rat glucokinase gene in the insulin producing cell line HIT M2.2.2

Nested deletion mutants of the 5' flanking region of the beta-cell transcription unit of the rat glucokinase gene (r beta GK) were fused to the CAT-reporter gene. Transient expression studies in HIT M2.2.2 and BHK21 cells revealed a distal (upstream of -359) and a proximal promoter region (between -278/-49) harbouring positive and negative regulatory elements. DNaseI footprinting revealed three protected areas between nucleotides -190 and -60. DNA-elements playing a crucial role in transcriptional control of the insulin genes (IEB- and CT-motifs) have been detected within the proximal promoter region and contribute to beta-cell specific gene regulation. 3' deletion analysis revealed that DNA-elements located downstream from transcription initiation sites (up to +123) contribute to transcriptional regulation.

Hypoglycemia but not hyperglycemia induces rapid changes in pancreatic beta-cell gene transcription

The purpose of these studies was to quantify several mRNAs expressed specifically in pancreatic islet cells and known or postulated to be important for insulin release after acute well defined alterations in levels of plasma glucose. Glucose levels were maintained at 50, 120, or 180 mg/dl (2.8, 6.7, or 10 mM) for 3 h in conscious unrestrained rats. Hypoglycemia (for 3 h) caused significant decreases in pancreatic content of mRNAs for insulin 2 and GLUT-2 to 55 and 34% of control values, respectively. There were no significant changes in insulin 1, amylin, glucokinase, or glucagon mRNAs. Unprocessed insulin 1 and 2 mRNA precursors were decreased to 17 and 10% of levels in controls, consistent with effects of short-term hypoglycemia on new mRNA synthesis. Hyperglycemia (for 3 h) caused no increase in pancreatic content of any mRNA measured. To discriminate between effects of hypoglycemia and hyperinsulinemia in the hypoglycemic animals, rats were made hypoglycemic by infusion with etomoxir, a carnitine palmitoyltransferase I inhibitor that lowers glucose in the fasted (glycogen-depleted) state by inhibiting hepatic gluconeogenesis. A single dose of this agent caused a decrease in glucose from 120 mg/dl (6.7 mM) to 80 mg/dl (4.4 mM) and significantly decreased insulin mRNA and pre-mRNA. These results are consistent with the hypothesis that glucose modulates islet cell gene transcription directly. They indicate that the range of glucose concentrations that modulate gene transcription differs from the levels of glucose that alter both insulin biosynthetic and secretion rates.

Insulin signalling and regulation of glucokinase gene expression in cultured hepatocytes

In cultured rat hepatocytes, transcription of the glucokinase gene is turned on by insulin and turned off by glucagon/cAMP, the latter being the dominant effector system. It is thus possible that in the absence of hormones the gene is maintained in a repressed state by the basal level of cAMP and that insulin turns on transcription by relieving cAMP repression, for instance via activation of a cyclic-nucleotide phosphodiesterase. Three inhibitors of this class of enzymes were tested for their effect on the insulin-dependent induction of the glucokinase gene in hepatocytes. Isobutyl methylxanthine, the prototype inhibitor, abrogated the gene response to insulin, as shown by run-on transcription assay. Among the drugs investigated, Ly186126, a preferential inhibitor of type-III phosphodiesterase, proved the most potent in inhibiting insulin-induced accumulation of glucokinase mRNA. Type-III phosphodiesterase is inhibited by cGMP. Induction of glucokinase mRNA was prevented in hepatocytes challenged with insulin in presence of 8-bromoguanosine-3',5'-phosphate. These results are consistent with the involvement of type-III phosphodiesterase in transduction of the insulin signal to the glucokinase gene. However, we were unable to detect significant decreases in total cellular cAMP level or cAMP-dependent-protein-kinase ratio after the addition of insulin to hepatocytes. Many effects of glucagon are mediated via cAMP-dependent protein-kinase phosphorylation of regulatory proteins and, conversely, insulin effects are often accompanied by protein dephosphorylation. A specific inhibitor of protein phosphatases PP1 and PP2A, okadaic acid, was shown to abolish the transcriptional response of the glucokinase gene to insulin. Thus, interference of insulin with the cAMP signal transduction pathway at several steps may be a critical aspect of insulin action on hepatic glucokinase gene expression. In addition, insulin induction of glucokinase mRNA was suppressed by inhibitors of protein synthesis. The underlying mechanism was a severe inhibition of the transcriptional effect of insulin, rather than mRNA destabilization, as demonstrated by run-on transcription assays with nuclei from cycloheximide-treated or pactamycin-treated cells. Transcription of the glucokinase gene may therefore depend on de novo synthesis of the product of an early-response gene induced by insulin, or may require a short-lived trans-acting or accessory factor of transcription. Alternatively, insulin signalling may be compromised in hepatocytes by a mechanism indirectly related to the arrest of protein synthesis.

Differential role of four cysteines on the activity of a low M(r) phosphotyrosine protein phosphatase

In this paper we describe the construction of five mutants of a bovine liver low M(r) phosphotyrosine protein phosphatase (PTPase) expressed as a fusion protein with the maltose binding protein in E. coli. Almost no changes in the kinetic parameters were observed in the fusion protein with respect to the native PTPase. Using oligonucleotide-directed mutagenesis Cys-17, Cys-62 and Cys-145 were converted to Ser while Cys-12 was converted to both Ser and Ala. The kinetic properties of the mutants, using p-nitrophenyl phosphate as substrate, were compared with those of the normal protein fused with the maltose binding protein of E. coli; both of the Cys-12 mutants showed a complete loss of enzymatic activity while the specific activity of the Cys-17 mutant was greatly decreased (200-fold). The Cys-62 mutant showed a 2.5-fold decrease in specific activity, while the Cys-145 mutant remained almost unchanged. These data confirm the involvement of Cys-12 and Cys-17 in the catalytic site and suggest that Cys-62 and Cys-145 mutations may destabilise the structure of the enzyme.

Tissue-specific regulation of glucokinase gene expression

Glucokinase contributes to the maintenance of blood glucose homeostasis by catalyzing the high Km phosphorylation of glucose in the liver and the pancreatic beta cell, the only two tissues known to express this enzyme. Molecular biological studies of the glucokinase gene and its products have advanced our understanding of how this gene is differentially regulated in the liver and beta cell. The production of an active glucokinase isoform is determined by both transcriptional and post-transcriptional events. Two different promoter regions that are widely separated in a single glucokinase gene are used to produce glucokinase mRNAs in the liver, pancreatic beta cell, and pituitary. The different transcription control regions are tissue-specific in their expression and are differentially regulated. In liver, glucokinase gene expression is regulated by insulin and cAMP, whereas in the beta cell it is regulated by glucose. The upstream glucokinase promoter region, which gives rise to the glucokinase mRNA in pituitary and pancreas, is structurally and functionally different from the downstream promoter region, which gives rise to the glucokinase mRNA in liver. The use of distinct promoter regions in a single glucokinase gene enables a different set of transcription factors to be utilized in the liver and islet, thus allowing a functionally similar gene product to be regulated in a manner consistent with the different functions of these two tissues. In addition, the splicing of the glucokinase pre-mRNA is regulated in a tissue-specific manner and can affect the activity of the gene product.(ABSTRACT TRUNCATED AT 250 WORDS)