Expression of the human glucokinase gene: important roles of the 5' flanking and intron 1 sequences

BMAL1 Regulates Glucokinase Expression Through E-Box Elements In Vitro

The organization of a circadian system includes an endogenous pacemaker system, input pathways for environmental synchronizing (entraining) stimuli, and output pathways through which the clock regulates physiological and behavioral processes, for example, the glucose-sensing mechanism in the liver. The liver is the central regulator of metabolism and one of our peripherals clocks. In mammals, central to this pacemaker are the transcription factors Circadian Locomotor Output Cycles Kaput (CLOCK) and BMAL1 (Brain and Muscle ARNT-Like 1). BMAL1 dimerizes with CLOCK, and this heterodimer then binds to the E-box promoter elements (CACGTG) present in clock and clock-controlled genes (CCGs). However, we are just beginning to understand how output pathways and regulatory mechanisms of CCGs are involved in rhythmic physiological processes. Glucokinase (GCK) is a fundamental enzyme in glucose homeostasis, catalyzing the high Km phosphorylation of glucose and allowing its storage. Moreover, gck is a dependent circadian gene. This study aims to determine the contribution of clock genes to hepatic gck expression and to define the specific role of E-box sequences on the circadian regulation of hepatic gck. Results showed that gck expression follows a circadian rhythm in rat hepatocytes in vitro. Accordingly, bmal1 expression induces the glucokinase circadian rhythmic expression in hepatocytes and the analysis of human and rat gck promoters, indicating the presence of E-box regions. Moreover, the basal activity of gck promoter was increased by clock/bmal1 co-transfection but inhibited by Period1/Period2 (per1/per2) co-transfection. Thus, the data suggest that the clock proteins tightly regulate the transcriptional activity of the gck promoter.

Low-Frequency Genetic Variant in the Hepatic Glucokinase Gene Is Associated With Type 2 Diabetes and Insulin Resistance in Chinese Population

Glucokinase (GCK) regulates insulin secretion and hepatic glucose metabolism, and its inactivating variants could cause diabetes. We aimed to evaluate the association of a low-frequency variant of GCK (rs13306393) with type 2 diabetes (T2D), prediabetes, or both (impaired glucose regulation [IGR]) in a Chinese population. An association study was first conducted in a random cluster sampling population (sample 1: 537 T2D, 768 prediabetes, and 1,912 control), and then another independent population (sample 2: 3,896 T2D, 2,301 prediabetes, and 868 control) was used to confirm the findings in sample 1. The A allele of rs13306393 was associated with T2D (odds ratio 3.08 [95% CI 1.77-5.36], P = 0.00007) in sample 1; rs13306393 was also associated with prediabetes (1.67 [1.05-2.65], P = 0.03) in sample 2. In a pooled analysis of the two samples, the A allele increased the risk of T2D (1.57 [1.15-2.15], P = 0.005), prediabetes (1.83 [1.33-2.54], P = 0.0003) or IGR (1.68 [1.26-2.25], P = 0.0004), insulin resistance estimated by HOMA (β = 0.043, P = 0.001), HbA_1c (β = 0.029, P = 0.029), and urinary albumin excretion (β = 0.033, P = 0.025), irrespective of age, sex, and BMI. Thus, the Chinese-specific low-frequency variant increased the risk of T2D through reducing insulin sensitivity rather than islet β-cell function, which should be considered in the clinical use of GCK activators in the future.

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.

Role of glucokinase in the subcellular localization of glucokinase regulatory protein

Glucokinase (GCK) is the rate-limiting enzyme of liver glucose metabolism. Through protein-protein interactions, glucokinase regulatory protein (GCKR) post-transcriptionally regulates GCK function in the liver, and causes its nuclear localization. However the role of GCK in regulating GCKR localization is unknown. In the present study, using in vitro and in vivo models, we examined the levels of GCK and GCKR, and their subcellular localization. We found that total cellular levels of GCKR did not vary in the in vivo models, but its subcellular localization did. In animals with normal levels of GCK, GCKR is mainly localized to the nuclei of hepatocytes. In seven-day old rats and liver-specific Gck gene knockout mice (animals that lack or have reduced levels of GCK protein), GCKR was found primarily in the cytoplasm. The interaction of GCK with GCKR was further examined using in vitro models where we varied the levels of GCK and GCKR. Varying the level of GCK protein had no effect on total cellular GCKR protein levels. Taken together, our results indicate that GCK is important for the localization of GCKR to the nucleus and raises the possibility that GCKR may have functions in addition to those regulating GCK activity in the cytoplasm.

Normal glucose metabolism in carnivores overlaps with diabetes pathology in non-carnivores

Carnivores, such as the dolphin and the domestic cat, have numerous adaptations that befit consumption of diets with high protein and fat content, with little carbohydrate content. Consequently, nutrient metabolism in carnivorous species differs substantially from that of non-carnivores. Important metabolic pathways known to differ between carnivores and non-carnivores are implicated in the development of diabetes and insulin resistance in non-carnivores: (1) the hepatic glucokinase (GCK) pathway is absent in healthy carnivores yet GCK deficiency may result in diabetes in rodents and humans, (2) healthy dolphins and cats are prone to periods of fasting hyperglycemia and exhibit insulin resistance, both of which are risk factors for diabetes in non-carnivores. Similarly, carnivores develop naturally occurring diseases such as hemochromatosis, fatty liver, obesity, and diabetes that have strong parallels with the same disorders in humans. Understanding how evolution, environment, diet, and domestication may play a role with nutrient metabolism in the dolphin and cat may also be relevant to human diabetes.

Production of glucose-6-phosphate by glucokinase coupled with an ATP regeneration system

A process of glucose-6-phosphate (G-6-P) production coupled with an adenosine triphosphate (ATP) regeneration system was constructed that utilized acetyl phosphate (ACP) via acetate kinase (ACKase). The genes glk and ack from Escherichia coli K12 were amplified and cloned into pET-28a(+), then transformed into E. coli BL21 (DE3) and the recombinant strains were named pGLK and pACK respectively. Glucokinase (glkase) in pGLK and ACKase in pACK were both overexpressed in soluble form. G-6-P was efficiently produced from glucose and ACP using a very small amount of ATP. The conversion yield was greater than 97 % when the reaction solution containing 10 mM glucose, 20 mM ACP-Na₂, 0.5 mM ATP, 5 mM Mg²⁺, 50 mM potassium phosphate buffer (pH 7.0), 4.856 U glkase and 3.632 U ACKase were put into 37 °C water bath for 1 h.

Evolution of glucose utilization: glucokinase and glucokinase regulator protein

Glucose is an essential nutrient that must be distributed throughout the body to provide energy to sustain physiological functions. Glucose is delivered to distant tissues via be blood stream, and complex systems have evolved to maintain the levels of glucose within a narrow physiological range. Phosphorylation of glucose, by glucokinase, is an essential component of glucose homeostasis, both from the regulatory and metabolic point-of-view. Here we review the evolution of glucose utilization from the perspective of glucokinase. We discuss the origin of glucokinase, its evolution within the hexokinase gene family, and the evolution of its interacting regulatory partner, glucokinase regulatory protein (GCKR). Evolution of the structure and sequence of both glucokinase and GCKR have been necessary to optimize glucokinase in its role in glucose metabolism.

Evolution of hepatic glucose metabolism: liver-specific glucokinase deficiency explained by parallel loss of the gene for glucokinase regulatory protein (GCKR)

BACKGROUND

Glucokinase (GCK) plays an important role in the regulation of carbohydrate metabolism. In the liver, phosphorylation of glucose to glucose-6-phosphate by GCK is the first step for both glycolysis and glycogen synthesis. However, some vertebrate species are deficient in GCK activity in the liver, despite containing GCK genes that appear to be compatible with function in their genomes. Glucokinase regulatory protein (GCKR) is the most important post-transcriptional regulator of GCK in the liver; it participates in the modulation of GCK activity and location depending upon changes in glucose levels. In experimental models, loss of GCKR has been shown to associate with reduced hepatic GCK protein levels and activity.

METHODOLOGY/PRINCIPAL FINDINGS

GCKR genes and GCKR-like sequences were identified in the genomes of all vertebrate species with available genome sequences. The coding sequences of GCKR and GCKR-like genes were identified and aligned; base changes likely to disrupt coding potential or splicing were also identified.

CONCLUSIONS/SIGNIFICANCE

GCKR genes could not be found in the genomes of 9 vertebrate species, including all birds. In addition, in multiple mammalian genomes, whereas GCKR-like gene sequences could be identified, these genes could not predict a functional protein. Vertebrate species that were previously reported to be deficient in hepatic GCK activity were found to have deleted (birds and lizard) or mutated (mammals) GCKR genes. Our results suggest that mutation of the GCKR gene leads to hepatic GCK deficiency due to the loss of the stabilizing effect of GCKR.