Hexokinase and 'glucokinase' in liver metabolism

Hexokinase domain-containing protein-1 in metabolic diseases and beyond

Glucose phosphorylation by hexokinases (HKs) traps glucose in cells and facilitates its usage in metabolic processes dependent on cellular needs. HK domain-containing protein-1 (HKDC1) is a recently discovered protein with wide expression containing HK activity, first noted through a genome-wide association study (GWAS) to be linked with gestational glucose homeostasis during pregnancy. Since then, HKDC1 has been observed to be expressed in many human tissues. Moreover, studies have shown that HKDC1 plays a role in glucose homeostasis by which it may affect the progression of many pathophysiological conditions such as gestational diabetes mellitus (GDM), nonalcoholic steatohepatitis (NASH), and cancer. Here, we review the key studies contributing to our current understanding of the roles of HKDC1 in human pathophysiological conditions and potential therapeutic interventions.

Control of pancreatic β-cell bioenergetics

The canonical model of glucose-stimulated insulin secretion (GSIS) by pancreatic β-cells predicts a glucose-induced rise in the cytosolic ATP/ADP ratio. Such bioenergetic sensitivity to metabolic fuel is unusual as it implies that ATP flux is governed, to a significant extent, by ATP supply, while it is predominantly demand-driven in other cell types. Metabolic control is generally shared between different processes, but potential control of ATP consumption over β-cell bioenergetics has been largely ignored to date. The present paper offers a brief overview of experimental evidence that demonstrates ATP flux control by glucose-fuelled oxidative phosphorylation. Based on old and new data, it is argued that ATP supply does not hold exclusive control over ATP flux, but shares it with ATP demand, and that the distribution of control is flexible. Quantification of the bioenergetic control distribution will be important from basic and clinical perspectives, but precise measurement of the cytosolic ATP/ADP ratio is complicated by adenine nucleotide compartmentalisation. Metabolic control analysis of β-cell bioenergetics will likely clarify the mechanisms by which glucose and fatty acids amplify and potentiate GSIS, respectively. Moreover, such analysis may offer hints as to how ATP flux control shifts from ATP supply to ATP demand during the development of type 2 diabetes, and why prolonged sulfonylurea treatment causes β-cell deterioration.

Energy metabolism in tumor cells

In early studies on energy metabolism of tumor cells, it was proposed that the enhanced glycolysis was induced by a decreased oxidative phosphorylation. Since then it has been indiscriminately applied to all types of tumor cells that the ATP supply is mainly or only provided by glycolysis, without an appropriate experimental evaluation. In this review, the different genetic and biochemical mechanisms by which tumor cells achieve an enhanced glycolytic flux are analyzed. Furthermore, the proposed mechanisms that arguably lead to a decreased oxidative phosphorylation in tumor cells are discussed. As the O(2) concentration in hypoxic regions of tumors seems not to be limiting for the functioning of oxidative phosphorylation, this pathway is re-evaluated regarding oxidizable substrate utilization and its contribution to ATP supply versus glycolysis. In the tumor cell lines where the oxidative metabolism prevails over the glycolytic metabolism for ATP supply, the flux control distribution of both pathways is described. The effect of glycolytic and mitochondrial drugs on tumor energy metabolism and cellular proliferation is described and discussed. Similarly, the energy metabolic changes associated with inherent and acquired resistance to radiotherapy and chemotherapy of tumor cells, and those determined by positron emission tomography, are revised. It is proposed that energy metabolism may be an alternative therapeutic target for both hypoxic (glycolytic) and oxidative tumors.

DNA polymerase lambda, a novel DNA repair enzyme in human cells

DNA polymerase lambda (pol lambda) is a novel family X DNA polymerase that has been suggested to play a role in meiotic recombination and DNA repair. The recent demonstration of an intrinsic 5'-deoxyribose-5-phosphate lyase activity in pol lambda supports a function of this enzyme in base excision repair. However, the biochemical properties of the polymerization activity of this enzyme are still largely unknown. We have cloned and purified human pol lambda to homogeneity in a soluble and active form, and we present here a biochemical description of its polymerization features. In support of a role in DNA repair, pol lambda inserts nucleotides in a DNA template-dependent manner and is processive in small gaps containing a 5'-phosphate group. These properties, together with its nucleotide insertion fidelity parameters and lack of proofreading activity, indicate that pol lambda is a novel beta-like DNA polymerase. However, the high affinity of pol lambda for dNTPs (37-fold over pol beta) is consistent with its possible involvement in DNA transactions occurring under low cellular levels of dNTPs. This suggests that, despite their similarities, pol beta and pol lambda have nonredundant in vivo functions.

The glucose sensor protein glucokinase is expressed in glucagon-producing alpha-cells

Expression of glucokinase in hepatocytes and pancreatic 6-cells is of major physiologic importance to mammalian glucose homeostasis. Liver glucokinase catalyzes the first committed step in the disposal of glucose, and beta-cell glucokinase catalyzes a rate-limiting step required for glucose-regulated insulin release. The present study reports the expression of glucokinase in rat glucagon-producing alpha-cells, which are negatively regulated by glucose. Purified rat alpha-cells express glucokinase mRNA and protein with the same transcript length, nucleotide sequence, and immunoreactivity as the beta-cell isoform. Glucokinase activity accounts for more than 50% of glucose phosphorylation in extracts of alpha-cells and for more than 90% of glucose utilization in intact cells. The glucagon-producing tumor MSL-G-AN also contained glucokinase mRNA, protein, and enzymatic activity. These data indicate that glucokinase may serve as a metabolic glucose sensor in pancreatic alpha-cells and, hence, mediate a mechanism for direct regulation of glucagon release by extracellular glucose. Since these cells do not express Glut2, we suggest that glucose sensing does not necessarily require the coexpression of Glut2 and glucokinase.

Intracellular pH governs the subcellular distribution of hexokinase in a glioma cell line

Hexokinase plays a key role in regulating cell energy metabolism. Hexokinase is mainly particulate, bound to the mitochondrial outer membrane in brain and tumour cells. We hypothesized that the intracellular pH (pH1) controls the intracellular distribution of hexokinase. Using the SNB-19 glioma cell line, pH1 variations were imposed by incubating cells in a high-K+ medium at different pH values containing specific ionophores (nigericin and valinomycin), without affecting cell viability. Subcellular fractions of cell homogenates were analysed for hexokinase activity. Imposed pH1 changes were verified microspectrofluorimetrically by using the pH1-sensitive probe SNARF-1-AM (seminaphtho-rhodafluor-1-acetoxymethyl ester). Imposition of an acidic pH1 for 30 min strongly decreased the particulate/total hexokinase ratio, from 63% in the control sample to 31%. Conversely, when a basic pH1, was imposed, the particulate/total hexokinase ratio increased to 80%. The glycolytic parameters, namely lactate/pyruvate ratio, glucose 6-phosphate and ATP levels, were measured concomitantly. Lactate/pyruvate ratio and ATP level were both markedly decreased by acidic pH1 and increased by basic pH1. Conversely, the glucose 6-phosphate level was increased by acidic pH1 and decreased by basic pH1. To demonstrate that the change of hexokinase distribution was not due to altered metabolite levels of glycolysis, a pH1 was imposed for a 5 min incubation time. Modification of the hexokinase distribution was similar to that noted after a 30 min incubation, whereas metabolite levels of glycolysis were not affected. These results provide evidence that the intracellular distribution of hexokinase is highly sensitive to variations of the pH1, and regulates hexokinase activity.

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.