Two Sp sites are important cis elements regulating the upstream promoter region of the gene for rat type I hexokinase

Aiding Cancer's "Sweet Tooth": Role of Hexokinases in Metabolic Reprogramming

Hexokinases (HKs) convert hexose sugars to hexose-6-phosphate, thus trapping them inside cells to meet the synthetic and energetic demands. HKs participate in various standard and altered physiological processes, including cancer, primarily through the reprogramming of cellular metabolism. Four canonical HKs have been identified with different expression patterns across tissues. HKs 1-3 play a role in glucose utilization, whereas HK 4 (glucokinase, GCK) also acts as a glucose sensor. Recently, a novel fifth HK, hexokinase domain containing 1 (HKDC1), has been identified, which plays a role in whole-body glucose utilization and insulin sensitivity. Beyond the metabolic functions, HKDC1 is differentially expressed in many forms of human cancer. This review focuses on the role of HKs, particularly HKDC1, in metabolic reprogramming and cancer progression.

Glucose metabolism in cancer: importance of transcription factor-DNA interactions within a short segment of the proximal region og the type II hexokinase promoter

A common signature of many cancers is a high glucose catabolic rate frequently dependent on the overexpression of Type II hexokinase (HKII), a mitochondrial bound enzyme that also suppresses cell death. As the tumor HKII promoter plays a significant role in HKII overexpression, studies reported here were undertaken to identify both the major regions and transcription factors involved under tumor-like conditions. Reporter gene assays following transfection of hepatoma cells with decreasing segments of the HKII promoter traced its known strength to the proximal region (-281 to -35). Mutational analyses showed that in this short region GC boxes 1, 2, 5, and 6, a CCAAT box, an inverted CCAAT box, and CRE are involved in promoter activation. Other studies demonstrated binding of transcription factors Sp1, Sp2, and Sp3 to GC boxes 1 and 6, Sp1 and Sp2 to GC boxes 2 and 5, NF-Y to CCAAT boxes, and CREB, ATF1, and CREM to CRE. In addition, transfection studies involving Sp1, Sp2, Sp3, CREB, and NFY (dominant negative form) provided evidence that these proteins are promoter activators. Finally, alignment of available HK proximal promoters showed strong conservation only among HKII sequences. These findings implicate signaling pathways directed to a short segment of the proximal region of the HKII promoter as major contributors to HKII overexpression in many cancers.

Isozymes of mammalian hexokinase: structure, subcellular localization and metabolic function

The first step in metabolism of glucose (Glc) is usually phosphorylation, catalyzed by hexokinase. However, the Glc-6-P produced can then enter one or more of several alternative pathways. Selective expression of isozymic forms of hexokinase, differing in catalytic and regulatory properties as well as subcellular localization, is likely to be an important factor in determining the pattern of Glc metabolism in mammalian tissues/cells. Despite their overall structural similarity, the Type I, Type II and Type III isozymes differ in important respects. All three isozymes are inhibited by the product, Glc-6-P, but with the Type I isozyme, this inhibition is antagonized by P(I), whereas with the Type II and Type III isozymes, P(i) actually causes additional inhibition. Reciprocal changes in intracellular levels of Glc-6-P and P(i) are closely associated with cellular energy status, and it is proposed that the response of the Type I isozyme to these effectors adapts it for catabolic function, introducing Glc into glycolytic metabolism for energy production. In contrast, the Type II, and probably the Type III, isozymes are suggested to serve primarily anabolic functions, e.g. to provide Glc-6-P for glycogen synthesis or metabolism via the pentose phosphate pathway for lipid synthesis. Type I hexokinase binds to mitochondria through interaction with porin, the protein that forms channels through which metabolites traverse the outer mitochondrial membrane. Several experimental approaches have led to the conclusion that the Type I isozyme, bound to actively phosphorylating mitochondria, selectively uses intramitochondrial ATP as substrate. Such interactions are thought to facilitate coordination of the introduction of Glc into glycolysis, via the hexokinase reaction, with the terminal oxidative stages of Glc metabolism occurring in the mitochondria, thus ensuring an overall rate of Glc metabolism commensurate with cellular energy demands and avoiding excessive production of lactate. The Type II isozyme also binds to mitochondria. Whether such coupling occurs with mitochondrially bound Type II hexokinase in normal tissues, and how it might be related to the proposed anabolic role of this isozyme, remain to be determined. The Type III isozyme lacks the hydrophobic N-terminal sequence known to be critical for binding of the Type I and Type II isozymes to mitochondria. Immunolocalization studies have indicated that, in many cell types, the Type III has a perinuclear localization, the possible metabolic consequences of which remain unclear.

High glucose induces type 1 hexokinase gene expression in isolated glomeruli of diabetic rats and in mesangial cells

BACKGROUND

Diabetic nephropathy, which is characterized by renal hypertrophy and accumulation of extracellular matrix, is one of the leading causes for end-stage renal disease. Pathophysiological changes, which finally lead to the development of diabetic nephropathy, act through an increase in the intracellular NADH/NAD ratio and the activation of the polyol and protein kinase C pathways. The first rate-limiting enzymes in intracellular glucose metabolism are the hexokinases, which catalyze the phosphorylation of glucose. Therefore, in order to examine a possible link between increased glucose metabolism and the development of diabetic nephropathy mRNA and protein expression as well as enzyme activity of type 1 hexokinase were examined in kidneys of control and diabetic rats and in mesangial cells.

METHODS

Diabetes in rats was induced by intravenous injection of streptozotocin and animals were treated or not treated with insulin. RNA or protein was extracted from isolated glomeruli at different time intervals. In addition, glomerular mesangial cells were incubated in high glucose medium and hexokinase expression determined along with enzyme activity.

RESULTS

The experiments demonstrate a significant increase in gene and protein expression of type 1 hexokinase in glomeruli of diabetic rats throughout a three week observation period. Insulin therapy reduced glomerular type 1 hexokinase mRNA expression. Gene expression and hexokinase enzyme activity were also increased in mesangial cells grown in high glucose medium.

CONCLUSION

The present experiments demonstrate that the expression of type 1 hexokinase is increased in isolated glomeruli of diabetic animals and is regulated by high ambient glucose concentrations. These results add further evidence to the fact that the kidney is one of the tissues most sensitive to high glucose levels in diabetes.

The human gene for the type III isozyme of hexokinase: structure, basal promoter, and evolution

The structure of the gene for the Type III isozyme of human hexokinase is nearly identical to that of previously characterized genes for other isozymes of hexokinase. The most striking difference is that the 5'-untranslated sequence and the initial coding sequence are contained in two exons in the Type III hexokinase gene but in a single exon in genes for the other isozymes. Sequence at the transcriptional start site for rat Type III hexokinase (S. Sebastian, J. A. White, and J. E. Wilson, 1999, J. Biol. Chem. 274, 31700-31706) is conserved in the human gene, as is an Oct-1 site, in reverse orientation, approximately 30 bp upstream from the start site. This site has been shown to regulate transcription of both human and rat genes for Type III hexokinase. Comparison of the genes for the various mammalian isozymes of hexokinase indicates that a major feature in the evolution of this isozyme family has been acquisition of alternative first exons.

Characterization of the rat type III hexokinase gene promoter. A functional octamer 1 motif is critical for basal promoter activity

A 1532-base pair 5'-flanking region of the gene encoding rat type III hexokinase has been cloned and sequenced. The total sequence includes positions -1548 to -17 (A of the translational start ATG as position +1). Using luciferase reporter constructs transfected into PC12 (rat pheochromocytoma) and L2 (rat lung) cells, basal promoter activity has been associated with sequence between -182 and -89. This includes a single transcriptional start site, an adenine at position -134 identified by primer extension. Together with previously cloned cDNA sequence, this accounts for an mRNA of approximately 3.9 kilobases, found by Northern blotting of RNA from rat lung and kidney. Sequence upstream of the transcriptional start site was devoid of canonical TATA and CAAT elements. An octamer 1 (Oct-1) binding site, located between positions -166 and -159 was shown by deletion analysis and site-directed mutation to be critical for promoter activity. Nuclear extracts from PC12 cells contained a protein (or proteins) specifically binding the octamer sequence, and supershift experiments with anti-Oct-1 indicated involvement of this ubiquitously expressed transcription factor in the complex. Sequence including the Oct-1 site and immediately adjacent regions was protected from DNase I digestion in footprinting experiments with nuclear extracts from PC12 cells. Reverse transcription polymerase chain reaction indicated that levels of type III hexokinase mRNA in rat tissues increased in the order brain < liver < lung approximately kidney; immunoblotting indicated that type III hexokinase protein in these tissues increased in a similar manner.