Glucose catabolism in cancer cells: regulation of the Type II hexokinase promoter by glucose and cyclic AMP

High glucose inhibits autophagy and promotes the proliferation and metastasis of colorectal cancer through the PI3K/AKT/mTOR pathway

BACKGROUND

Colorectal cancer (CRC) ranks among the most prevalent malignancies worldwide, characterized by its complex etiology and slow research progress. Diabetes, as an independent risk factor for CRC, has been widely certified. Consequently, this study centers on elucidating the intricacies of CRC cells initiation and progression within a high-glucose environment.

METHODS

A battery of assays was employed to assess the proliferation and metastasis of CRC cells cultured under varying glucose concentrations. Optimal glucose levels conducive to cells' proliferation and migration were identified. Western blot analyses were conducted to evaluate alterations in apoptosis, autophagy, and EMT-related proteins in CRC cells under high-glucose conditions. The expression of PI3K/AKT/mTOR pathway-associated proteins was assessed using western blot. The effect of high glucose on xenograft growth was investigated in vivo by MC38 cells, and changes in inflammatory factors (IL-4, IL-13, TNF-α, IL-5, and IL-12) were measured via serum ELISA.

RESULTS

Our experiments demonstrated that elevated glucose concentrations promoted both the proliferation and migration of CRC cells; the most favorable glucose dose is 20 mM. Western blot analyses revealed a decrease in apoptotic proteins, such as Bim, Bax, and caspase-3 with increasing glucose levels. Concurrently, the expression of EMT-related proteins, including N-cadherin, vimentin, ZEB1, and MMP9, increased. High-glucose cultured cells exhibited elevated levels of PI3K/AKT/mTOR pathway proteins. In the xenograft model, tumor cells stimulated by high glucose exhibited accelerated growth, larger tumor volumes, and heightened KI67 expression of immunohistochemistry. ELISA experiments revealed higher expression of IL-4 and IL-13 and lower expression of TNF-α and IL-5 in the serum of high-glucose-stimulated mice.

CONCLUSION

The most favorable dose and time for tumor cells proliferation and migration is 20 mM, 48 h. High glucose fosters CRC cell proliferation and migration while suppressing autophagy through the activation of the PI3K/AKT/mTOR pathway.

Hexokinase-2-Linked Glycolytic Overload and Unscheduled Glycolysis-Driver of Insulin Resistance and Development of Vascular Complications of Diabetes

The recent discovery of the glucose-induced stabilization of hexokinase-2 (HK2) to proteolysis in cell dysfunction in model hyperglycemia has revealed a likely key initiating factor contributing to the development of insulin resistance and vascular complications in diabetes. Consequently, the increased flux of glucose metabolism without a change in the expression and activity of glycolytic enzymes produces a wave of increased glycolytic intermediates driving mitochondrial dysfunction and increased reactive oxygen species (ROS) formation, the activation of hexosamine and protein kinase C pathways, the increased formation of methylglyoxal-producing dicarbonyl stress, and the activation of the unfolded protein response. This is called HK2-linked glycolytic overload and unscheduled glycolysis. The conditions required to sustain this are GLUT1 and/or GLUT3 glucose uptake and the expression of HK2. A metabolic biomarker of its occurrence is the abnormally increased deposition of glycogen, which is produced by metabolic channeling when HK2 becomes detached from mitochondria. These conditions and metabolic consequences are found in the vasculature, kidneys, retina, peripheral nerves, and early-stage embryo development in diabetes and likely sustain the development of diabetic vascular complications and embryopathy. In insulin resistance, HK2-linked unscheduled glycolysis may also be established in skeletal muscle and adipose tissue. This may explain the increased glucose disposal by skeletal uptake in the fasting phase in patients with type 2 diabetes mellitus, compared to healthy controls, and the presence of insulin resistance in patients with type 1 diabetes mellitus. Importantly, glyoxalase 1 inducer—trans-resveratrol and hesperetin in combination (tRES-HESP)—corrected HK2-linked glycolytic overload and unscheduled glycolysis and reversed insulin resistance and improved vascular inflammation in overweight and obese subjects in clinical trial. Further studies are now required to evaluate tRES-HESP for the prevention and reversal of early-stage type 2 diabetes and for the treatment of the vascular complications of diabetes.

Metabolic Strategies for Inhibiting Cancer Development

The tumor microenvironment is a complex mix of cancerous and noncancerous cells (especially immune cells and fibroblasts) with distinct metabolisms. These cells interact with each other and are influenced by the metabolic disorders of the host. In this review, we discuss how metabolic pathways that sustain biosynthesis in cancer cells could be targeted to increase the effectiveness of cancer therapies by limiting the nutrient uptake of the cell, inactivating metabolic enzymes (key regulatory ones or those linked to cell cycle progression), and inhibiting ATP production to induce cell death. Furthermore, we describe how the microenvironment could be targeted to activate the immune response by redirecting nutrients toward cytotoxic immune cells or inhibiting the release of waste products by cancer cells that stimulate immunosuppressive cells. We also examine metabolic disorders in the host that could be targeted to inhibit cancer development. To create future personalized therapies for targeting each cancer tumor, novel techniques must be developed, such as new tracers for positron emission tomography/computed tomography scan and immunohistochemical markers to characterize the metabolic phenotype of cancer cells and their microenvironment. Pending personalized strategies that specifically target all metabolic components of cancer development in a patient, simple metabolic interventions could be tested in clinical trials in combination with standard cancer therapies, such as short cycles of fasting or the administration of sodium citrate or weakly toxic compounds (such as curcumin, metformin, lipoic acid) that target autophagy and biosynthetic or signaling pathways.

SLC45A4 promotes glycolysis and prevents AMPK/ULK1-induced autophagy in TP53 mutant pancreatic ductal adenocarcinoma

Background

Somatic mutations of the TP53 gene occur frequently in pancreatic ductal adenocarcinoma (PDA). Solute carrier family 45 member A4 (SLC45A4) is a H⁺‐dependent sugar cotransporter. The role of SLC45A4 in PDA, especially in TP53 mutant PDA, remains poorly understood.

Methods

We explored the TCGA datasets to identify oncogenes in TP53 mutant PDA. MTS [3‐(4,5‐dimethylthiazol‐2‐yl)‐5‐(3‐carboxymethoxyphenyl)‐2‐(4‐sulfophenyl)‐2H‐tetrazolium], colony formation and 5‐ethynyl‐2′‐deoxyuridine (Edu) assays were performed to investigate the function of SLC45A4 in vitro. Glucose consumption, lactate production and ATP production were detected to evaluate glucose utilization. Extracellular acidification rate and oxygen consumption rate assays were used to evaluate glycolysis and oxidative phosphorylation. The subcutaneous xenotransplantation models were conducted to explore the function of SLC45A4 in vivo. RNA‐sequencing and gene set enrichment analysis were employed to explore the biological alteration caused by SLC45A4 knockdown. Western blotting was performed to evaluate the activation of glycolysis, as well as the AMPK pathway and autophagy.

Results

SLC45A4 was overexpressed in PDA for which the expression was significantly higher in TP53 mutant PDA than that in wild‐type PDA tissues. Moreover, high level of SLC45A4 expression was tightly associated with poor clinical outcomes in PDA patients. Silencing SLC45A4 inhibited proliferation in TP53 mutant PDA cells. Knockdown of SLC45A4 reduced glucose uptake and ATP production, which led to activation of autophagy via AMPK/ULK1 pathway. Deleting SLC45A4 in TP53 mutant HPAF‐II cells inhibited the growth of xenografts in nude mice.

Conclusions

The present study found that SLC45A4 prevents autophagy via AMPK/ULK1 axis in TP53 mutant PDA, which may be a promising biomarker and therapeutic target in TP53 mutant PDA.

High glucose: an emerging association between diabetes mellitus and cancer progression

The association of cancer and diabetes mellitus (DM) has been studied for decades. Hyperglycemia and the imbalance of hormones are factors that contribute to the molecular link between DM and carcinogenesis and cancer progression. Hyperglycemia alone or in combination with hyperinsulinemia are key factors that promote cancer aggressiveness. Many preclinical studies suggest that high glucose induces abnormal energy metabolism and aggressive cancer via several mechanisms. As evidenced by clinical studies, hyperglycemia is associated with poor clinical outcomes in patients who have comorbid DM. The prognoses of cancer patients with DM are improved when their plasma glucose levels are controlled. This suggests that high glucose level maybe be involved in the molecular mechanism that causes the link between DM and cancer and may also be useful for prognosis of cancer progression. This review comprehensively summarizes the evidence from recent pre-clinical and clinical studies of the impact of hyperglycemia on cancer advancement as well as the underlying molecular mechanism for this impact. Awareness among clinicians of the association between hyperglycemia or DM and cancer progression may improve cancer treatment outcome in patients who have DM.

Tristetraprolin-mediated hexokinase 2 expression regulation contributes to glycolysis in cancer cells

Hexokinase 2 (HK2) catalyzes the first step of glycolysis and is up-regulated in cancer cells. The mechanism has not been fully elucidated. Tristetraprolin (TTP) is an AU-rich element (ARE)-binding protein that inhibits the expression of ARE-containing genes by enhancing mRNA degradation. TTP expression is down-regulated in cancer cells. We demonstrated that TTP is critical for down-regulation of HK2 expression in cancer cells. HK2 mRNA contains an ARE within its 3'-UTR. TTP binds to HK2 3'-UTR and enhances degradation of HK2 mRNA. TTP overexpression decreased HK2 expression and suppressed the glycolytic capacity of cancer cells, measured as glucose uptake and production of glucose-6-phosphate, pyruvate, and lactate. TTP overexpression reduced both the extracellular acidification rate (ECAR) and the oxygen consumption rate (OCR) of cancer cells. Ectopic expression of HK2 in cancer cells attenuated the reduction in glycolytic capacity, ECAR, and OCR from TTP. Taken together, these findings suggest that TTP acts as a negative regulator of HK2 expression and glucose metabolism in cancer cells.

Antitumoural activity of a cytotoxic peptide of Lactobacillus casei peptidoglycan and its interaction with mitochondrial-bound hexokinase

Supplemental Digital Content is available in the text.

In a previous study, we reported the cytotoxic activity against various tumour cells of the peptidoglycan of Lactobacillus casei. To isolate the most active components, we performed column-chromatography separation of the peptidoglycan complex and tested the related fractions for their cytotoxic activity. The most active fractions were then lyophilized and the residue was analysed by gas chromatography for its amino acid content and composition. On the basis of the known chemical formula of the basic peptidic component of the peptidoglycan complex of L. casei, a peptide was then synthesized [Europ. (CH-DE-FR-GB) Patent number 1217005; IT number 01320177] and its cytotoxicity was tested against tumoural and normal cells. The synthetic peptide was found to impair the entire metabolism of cultured tumour cells and to restore the apoptotic process. By contrast, normal cells appeared to be stimulated rather than inhibited by the peptide, whereas primary mouse embryo fibroblasts behaved similarly to tumour cells. On the basis of these results, L. casei peptidoglycan fragments and their constituent basic peptide might be applicable as potent antitumour agents.

High glucose enhances progression of cholangiocarcinoma cells via STAT3 activation

Epidemiological studies have indicated diabetes mellitus (DM) as a risk of cholangiocarcinoma (CCA), however, the effects and mechanisms of high glucose on progression of CCA remain unclear. This study reports for the first time of the enhancing effects of high glucose on aggressive phenotypes of CCA cells via STAT3 activation. CCA cells cultured in high glucose media exerted significantly higher rates of cell proliferation, adhesion, migration and invasion than those cultured in normal glucose. The phosphokinase array revealed STAT3 as the dominant signal activated in response to high glucose. Increased nuclear STAT3, p-STAT3 and its downstream target proteins, cyclin D1, vimentin and MMP2, were shown to be underling mechanisms of high glucose stimulation. The link of high glucose and STAT3 activation was confirmed in tumor tissues from CCA patients with DM that exhibited higher STAT3 activation than those without DM. Moreover, the levels of STAT3 activation were correlated with the levels of blood glucose. Finally, decreasing the level of glucose or using a STAT3 inhibitor could reduce the effects of high glucose. These findings suggest that controlling blood glucose or using a STAT3 inhibitor as an alternative approach may improve the therapeutic outcome of CCA patients with DM.

The combination of the expression of hexokinase 2 and pyruvate kinase M2 is a prognostic marker in patients with pancreatic cancer

Metabolism may determine the biologically malignant behavior of pancreatic cancer. To investigate the significance and prognostic value of cancer metabolism in cancer patients, we investigated the expression of two key enzymes in anaerobic glycolysis, hexokinase 2 (HK2) and pyruvate kinase isoenzyme type M2 (PKM2), in surgical specimens obtained from 36 patients who underwent curative resection of pancreatic ductal carcinoma. The hk2-glycolysis axis is a key system in the clinical imaging of tumors via positron emission tomography. Immunohistochemical staining for hk2 and pkm2 was performed and the data were statistically analyzed to evaluate their prognostic power. The expression of hk2 and pkm2 was associated with clinicopathological variables and patient prognosis, including overall survival, local recurrence-free survival and distant metastasis-free survival. Staining for hk2 was negative and positive in 42 and 58% of the patients, respectively, whereas staining for pkm2 was negative and positive in 56 and 44%, respectively; hk2-positive staining was correlated with progressive pathological tumor stage (pT3 vs. pT1 and pT2; P=0.017). In the univariate analysis, the positive expression of hk2 and pkm2, pathological stage (pT3 vs. pT1 and pT2) and nodal metastasis were significantly correlated with poor prognosis (P<0.03). In the multivariate analysis, pathological nodal metastasis was an independent prognostic factor for overall survival, whereas the positive expression of hk2 and pkm2 exhibited borderline significance (P=0.08 and 0.12, hazard ratio = 2.57 and 2.16, respectively). In addition, the combination of high expression of hk2 as well as pkm2 was found to be significant (P<0.05). These results suggested that the expression of hk2 and pkm2, particularly their combination, in surgical specimens obtained during curative resection, may predict an unfavorable clinical outcome in patients with pancreatic cancer.

Recent developments in multilayered polymeric particles – from fabrication techniques to therapeutic formulations

Multilayered particles are emerging as a powerful platform in pharmaceutics, especially for targeted, triggered and sustained drug delivery.

Nuclear receptors and the Warburg effect in cancer

In 1927 Otto Warburg established that tumours derive energy primarily from the conversion of glucose to lactic acid and only partially through cellular respiration involving oxygen. In the 1950s he proposed that all causes of cancer reflected different mechanisms of disabling cellular respiration in favour of fermentation (now termed aerobic glycolysis). The role of aberrant glucose metabolism in cancer is now firmly established. The shift away from oxidative phosphorylation towards the metabolically expensive aerobic glycolysis is somewhat counter-intuitive given its wasteful nature. Multiple control processes are in place to maintain cellular efficiency and it is likely that these mechanisms are disrupted to facilitate the shift to the reliance on aerobic glycolysis. One such process of cell control is mediated by the nuclear receptor superfamily. This large family of transcription factors plays a significant role in sensing environmental cues and controlling decisions on proliferation, differentiation and cell death for example, to regulate glucose uptake and metabolism and to modulate the actions of oncogenes and tumour suppressors. In this review we highlight mechanisms by which nuclear receptors actions are altered during tumorigenic transformation and can serve to enhance the shift to aerobic glycolysis. At the simplest level, a basic alteration in NR behaviour can serve to enhance glycolytic flux thus providing a basis for enhanced survival within the tumour micro-environment. Ameliorating the enhanced NR activity in this context may help to sensitize cancer cells to Warburg targeted therapies and may provide future drug targets.

Genetic insights into OXPHOS defect and its role in cancer

Warburg proposed that cancer originates from irreversible injury to mitochondrial oxidative phosphorylation (mtOXPHOS), which leads to an increase rate of aerobic glycolysis in most cancers. However, despite several decades of research related to Warburg effect, very little is known about the underlying genetic cause(s) of mtOXPHOS impairment in cancers. Proteins that participate in mtOXPHOS are encoded by both mitochondrial DNA (mtDNA) as well as nuclear DNA. This review describes mutations in mtDNA and reduced mtDNA copy number, which contribute to OXPHOS defects in cancer cells. Maternally inherited mtDNA renders susceptibility to cancer, and mutation in the nuclear encoded genes causes defects in mtOXPHOS system. Mitochondria damage checkpoint (mitocheckpoint) induces epigenomic changes in the nucleus, which can reverse injury to OXPHOS. However, irreversible injury to OXPHOS can lead to persistent mitochondrial dysfunction inducing genetic instability in the nuclear genome. Together, we propose that "mitocheckpoint" led epigenomic and genomic changes must play a key role in reversible and irreversible injury to OXPHOS described by Warburg. These epigenetic and genetic changes underlie the Warburg phenotype, which contributes to the development of cancer.

shRNA-mediated silencing of hexokinase II inhibits proliferation but promotes apoptosis in SGC7901 cells

AIM: To observe the changes in cell proliferation and apoptosis in gastric cancer cell line SGC7901 after silencing the hexokinase II (HK-II) gene with an HK-II-specific shRNA, and to assess the potential application of HK-II-targeted gene therapy for gastric cancer.

METHODS: Gastric cancer cell line SGC7901 and gastric epithelial cell line GES-1 were used in this study. Both SGC7901 cells and GES-1 cells were divided into 5 groups: HK-II shRNA group, positive control group, negative control group, empty liposome group, and blank control group. After transfection of HK-II-specific shRNA into SGC7901 and GES-1 cells, the change in the expression of HK-II mRNA was detected by reverse transcription-polymerase chain reaction (RT-PCR), and the changes in cell proliferation and apoptosis were assessed by MTT assay and flow cytometry, respectively.

RESULTS: The expression level of HK-II mRNA in SGC7901 cells was significantly higher than that in GES-1 cells (t = 12.119, P < 0.01). The expression of HK-II mRNA was obviously silenced after transfection of HK-II-specific shRNA (P < 0.01). HK-II knockdown could significantly inhibit proliferation (F = 159.811, P < 0.01) and promote apoptosis (χ² = 21.324, P < 0.01) in SGC7901 cells, but had no significant effect on proliferation and apoptosis in GES-1 cells (F = 0.704, P = 0.592; χ² = 1.007, P = 0.909).

CONCLUSION: HK-II knockdown significantly inhibits proliferation and promotes apoptosis in SGC7901 cells, but has no significant impact in GES-1 cells. HK-II may be a potential target for the treatment of gastric cancer.

Glucose-triggered drug delivery from borate mediated layer-by-layer self-assembly

In this study, we report a novel approach for glucose-triggered anticancer drug delivery from the self-assembly of neutral poly(vinyl alcohol) (PVA) and chitosan. In the present study, we have fabricated multilayer thin film of PVA-borate and chitosan on colloidal particle (MF particle) and monitored the layer-by-layer growth using Zeta potential measurements. Formation of multilayer membrane on MF particle has been further characterized with transmission electron microscopy (TEM) and confocal laser scanning microscopy (CLSM). Subsequently, disintegration of multilayer thin film and microcapsules was observed in presence of glucose. We investigated the disassembly of PVA-borate and chitosan self-assembly under CLSM and atomic force microscopy. These results suggest that this multilayer thin film is very efficient for encapsulation and release of DOX molecules above certain concentration of glucose (25 mM). This glucose-sensitive self-assembly is relevant for the application of anticancer therapeutic drug delivery.

Hexokinase-2 bound to mitochondria: cancer's stygian link to the "Warburg Effect" and a pivotal target for effective therapy

The most common metabolic hallmark of malignant tumors, i.e., the "Warburg effect" is their propensity to metabolize glucose to lactic acid at a high rate even in the presence of oxygen. The pivotal player in this frequent cancer phenotype is mitochondrial-bound hexokinase [Bustamante E, Pedersen PL. High aerobic glycolysis of rat hepatoma cells in culture: role of mitochondrial hexokinase. Proc Natl Acad Sci USA 1977;74(9):3735-9; Bustamante E, Morris HP, Pedersen PL. Energy metabolism of tumor cells. Requirement for a form of hexokinase with a propensity for mitochondrial binding. J Biol Chem 1981;256(16):8699-704]. Now, in clinics worldwide this prominent phenotype forms the basis of one of the most common detection systems for cancer, i.e., positron emission tomography (PET). Significantly, HK-2 is the major bound hexokinase isoform expressed in cancers that exhibit a "Warburg effect". This includes most cancers that metastasize and kill their human host. By stationing itself on the outer mitochondrial membrane, HK-2 also helps immortalize cancer cells, escapes product inhibition and gains preferential access to newly synthesized ATP for phosphorylating glucose. The latter event traps this essential nutrient inside the tumor cells as glucose-6-P, some of which is funneled off to serve as carbon precursors to help promote the production of new cancer cells while much is converted to lactic acid that exits the cells. The resultant acidity likely wards off an immune response while preparing surrounding tissues for invasion. With the re-emergence and acceptance of both the "Warburg effect" as a prominent phenotype of most clinical cancers, and "metabolic targeting" as a rational therapeutic strategy, a number of laboratories are focusing on metabolite entry or exit steps. One remarkable success story [Ko YH, Smith BL, Wang Y, Pomper MG, Rini DA, Torbenson MS, et al. Advanced cancers: eradication in all cases using 3-bromopyruvate therapy to deplete ATP. Biochem Biophys Res Commun 2004;324(1):269-75] is the use of the small molecule 3-bromopyruvate (3-BP) that selectively enters and destroys the cells of large tumors in animals by targeting both HK-2 and the mitochondrial ATP synthasome. This leads to very rapid ATP depletion and tumor destruction without harm to the animals. This review focuses on the multiple roles played by HK-2 in cancer and its potential as a metabolic target for complete cancer destruction.

Causes and consequences of increased glucose metabolism of cancers

In this review we examine the mechanisms (causes) underlying the increased glucose consumption observed in tumors within a teleological context (consequences). In other words, we will ask not only "How do cancers have high glycolysis?" but also, "Why?" We believe that the insights gained from answering the latter question support the conclusion that elevated glucose consumption is a necessary component of carcinogenesis. Specifically we propose that glycolysis is elevated because it produces acid, which provides an evolutionary advantage to cancer cells vis-à-vis normal parenchyma into which they invade.

Hexokinase II: cancer's double-edged sword acting as both facilitator and gatekeeper of malignancy when bound to mitochondria

A key hallmark of many cancers, particularly the most aggressive, is the capacity to metabolize glucose at an elevated rate, a phenotype detected clinically using positron emission tomography (PET). This phenotype provides cancer cells, including those that participate in metastasis, a distinct competitive edge over normal cells. Specifically, after rapid entry of glucose into cancer cells on the glucose transporter, the highly glycolytic phenotype is supported by hexokinase (primarily HK II) that is overexpressed and bound to the outer mitochondrial membrane via the porin-like protein voltage-dependent anion channel (VDAC). This protein and the adenine nucleotide transporter move ATP, newly synthesized by the inner membrane located ATP synthase, to active sites on HK II. The abundant amounts of HK II bind both the ATP and the incoming glucose producing the product glucose-6-phosphate, also at an elevated rate. This critical metabolite then serves both as a biosynthetic precursor to support cell proliferation and as a precursor for lactic acid, the latter exiting cancer cells causing an unfavorable environment for normal cells. Although helping facilitate this chemical warfare, HK II via its mitochondrial location also suppresses the death of cancer cells, thus increasing their possibility for metastasis and the ultimate death of the human host. For these reasons, targeting this key enzyme is currently being investigated in several laboratories in a strategy to develop novel therapies that may turn the tide on the continuing struggle to find effective cures for cancer. One such candidate is 3-bromopyruvate that has been shown recently to eradicate advanced stage, PET positive hepatocellular carcinomas in an animal model without apparent harm to the animals.

Hypoxia stimulates proliferation of human hepatoma cells through the induction of hexokinase II expression

BACKGROUND/AIMS

In a hypoxic state, a glycolytic system is operating as a salvage pathway of generating ATP, and hexokinase II, the first enzyme in this system, might be over-expressed in hepatocellular carcinomas (HCCs). This study was to evaluate if hexokinase II is participating in HCC cell survival in a hypoxic state, and to analyze the mechanism of cell death caused by hexokinase II-specific inhibition.

METHODS

Human hepatoma cell lines were grown either in a normoxic or hypoxic condition. Hexokinase II and hypoxia-inducible factor-1alpha (HIF-1alpha) expression were evaluated using immunoblot techniques. Cell growth was assessed using the MTS assay. Apoptotic signaling cascades were explored by immunoblot analysis.

RESULTS

Hypoxia stimulated HCC cellular growth through HIF-1alpha-dependent induction of hexokinase II expression. The hexokinase II-specific inhibitor, 3-bromopyruvate, significantly suppressed cellular growth in a hypoxic state compared to cells in a normoxic condition. This suppression was due to the induction of apoptosis through activating mitochondrial apoptotic signaling cascades.

CONCLUSIONS

This study demonstrates that hypoxia stimulates HCC cellular growth through hexokinase II induction, and its inhibition induces apoptotic cell death. Therefore, hexokinase II induction may participate in HCC progression and the blockage of this enzyme may therapeutically be efficacious in human HCCs.

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.

Glucose metabolism in cancer. Evidence that demethylation events play a role in activating type II hexokinase gene expression

One of the "signature" phenotypes of highly malignant, poorly differentiated tumors, including hepatomas, is their remarkable propensity to utilize glucose at a much higher rate than normal cells, a property frequently dependent on the marked overexpression of type II hexokinase (HKII). As the expression of the gene for this enzyme is nearly silent in liver tissue, we tested the possibility that DNA methylation/demethylation events may be involved in its regulation. Initial studies employing methylation restriction endonuclease analysis provided evidence for differential methylation patterns for the HKII gene in normal hepatocytes and hepatoma cells, the latter represented by a highly glycolytic model cell line (AS-30D). Subsequently, sequencing following sodium bisulfite treatment revealed 18 methylated CpG sites within a CpG island (-350 to +781 bp) in the hepatocyte gene but none in that of the hepatoma. In addition, treatment of a hepatocyte cell line with the DNA methyltransferase inhibitors, 5'-azacytidine and 5'-aza-2'-deoxycytidine, activated basal expression levels of HKII mRNA and protein. Finally, stably transfecting the hepatocyte cell line with DNA demethylase also resulted in activating the basal expression levels of HKII mRNA and protein. These novel observations indicate that one of the initial events in activating the HKII gene during either transformation or tumor progression may reside at the epigenetic level.

Mitochondrial bound type II hexokinase: a key player in the growth and survival of many cancers and an ideal prospect for therapeutic intervention

Despite more than 75 years of research by some of the greatest scientists in the world to conquer cancer, the clear winner is still cancer. This is reflected particularly by liver cancer that worldwide ranks fourth in terms of mortality with survival rates of no more than 3-5%. Significantly, one of the earliest discovered hallmarks of cancer had its roots in Bioenergetics as many tumors were found in the 1920s to exhibit a high glycolytic phenotype. Although research directed at unraveling the underlying basis and significance of this phenotype comprised the focus of cancer research for almost 50 years, these efforts declined greatly from 1970 to 1990 as research into the molecular and cell biology of this disease gained center stage. Certainly, this change was necessary as the new knowledge obtained about oncogenes, gene regulation, and programmed cell death once again placed Bioenergetics in the limelight of cancer research. Thus, we now have a much better molecular understanding of the high glycolytic phenotype of many cancers, the pivotal roles that Type II hexokinase-mitochondrial interactions play in this process to promote tumor cell growth and survival, and how this new knowledge can lead to improved therapies that may ultimately turn the tide on our losing war on cancer.

Elevated expression of hexokinase II protects human lung epithelial-like A549 cells against oxidative injury

Increased glucose utilization and hexokinase (HK)-II expression are adaptive features of lung cells exposed to hypoxia or hyperoxia. HK-II is the most regulated isoform of HK. Whether its overexpression could be protective against oxidative stress was explored in human lung epithelial-like (A549) cells. HK-II was overexpressed in A549 cells in a tetracycline-repressible retroviral vector system. Elevated expression of HK-II was confirmed by Western blot and activity measurements. Cell death caused by exposure to hyperoxia was decreased in HK-II-overexpressing cells. This effect was reversed when HK-II expression was suppressed with doxycycline. A similar protective effect was observed in HK-II-overexpressing cells after treatment with 1 mM hydrogen peroxide for 48 h. At baseline, fluorescence microscopy showed that overexpressed HK-II was localized to mitochondria. Electron microscopic studies showed that hyperoxia-exposed HK-II overexpressors had better-preserved and quantitatively smaller mitochondria than those in which the HK-II expression was suppressed or in the nontransduced A549 cells. Mitochondrial membrane potential was increased in HK-II-overexpressing cells exposed to hyperoxia compared with the nontransduced control cells under similar conditions. The present study demonstrates that HK-II protects human lung epithelial-like A549 cells against oxidative insults by protecting the mitochondria.

Glucose catabolism in cancer cells: identification and characterization of a marked activation response of the type II hexokinase gene to hypoxic conditions

One of the most common signatures of highly malignant tumors is their capacity to metabolize more glucose to lactic acid than their tissues of origin. Hepatomas exhibiting this phenotype are dependent on the high expression of type II hexokinase, which supplies such tumors with abundant amounts of glucose 6-phosphate, a significant carbon and energy source especially under hypoxic conditions. Here we report that the distal region of the hepatoma type II hexokinase promoter displays consensus motifs for hypoxia-inducible factor (HIF-1) that overlap E-box sequences known to be related in other gene promoters to glucose response. Moreover, we show that subjecting transfected hepatoma cells to hypoxic conditions activates the type II hexokinase promoter almost 3-fold, a value that approaches 7-fold in the presence of glucose. Consistent with these findings is the induction under hypoxic conditions of the HIF-1 protein. Reporter gene analyses with a series of nested deletion mutants of the hepatoma type II hexokinase promoter show that a significant fraction of the total activation observed under hypoxic conditions localizes to the distal region where the overlapping HIF-1/E-box sequences are located. Finally, DNase I footprint analysis with a segment of the promoter containing these elements reveals the binding of several nuclear proteins. In summary, these novel studies identify and characterize a marked glucose-modulated activation response of the type II hexokinase gene to hypoxic conditions within highly glycolytic hepatoma cells, a property that may help assure that such cells exhibit a growth and survival advantage over their parental cells of origin.

Hexokinase type II: a novel tumor-specific promoter for gene-targeted therapy differentially expressed and regulated in human cancer cells

The use of tissue- or tumor-selective promoters in targeted gene therapy for cancer depends on strong and selective activity. Hexokinase type II (HK II) catalyzes the first committed step of glycolysis and is overexpressed in tumors, where it is no longer responsive to normal physiological inhibitors, e.g., glucagon. We show, in a reporter gene assay, activation of HK II in non-small cell lung carcinomas NCI-H661 and NCI-H460 at 61 and 40%, respectively, relative to the activation observed with a constitutive promoter, while it was only 0.9% in different preparations of primary normal human bronchial epithelial cells (NHBECs). Similar results were observed in a variety of normal and tumor cells. Moreover, treatment of the transfectants with glucagon did not inhibit promoter activation in the transformed H661 cells, while endogenous HK II in NHBECs is suppressed by glucagon. H460 and H661 cells infected with a recombinant adenovirus carrying an HK II/LacZ expression cassette, AdHexLacZ, demonstrated beta-galactosidase activity that correlated with the level of HK II promoter activation in these cells. Under similar conditions, no enzyme activity was observed in NHBECs. Cells were then infected with AdHexTk and treated with GCV. Our results demonstrate selectivity in toxicity, with a 10- to 100-fold increase in IC50 between lung cancer cell lines H661 and H460, respectively, and NHBECs. There was also a 100-fold increase in IC50 in NHMECs relative to breast carcinoma cell line MCF-7. In HepG2 cells, an IC50 of 1 microg/ml was observed, comparable to that of other tumor cell lines. This represents a novel use of the hexokinase type II as a selective promoter in cancer gene therapy.

Inhibition of the oxidative and nonoxidative pentose phosphate pathways by somatostatin: a possible mechanism of antitumor action

Long-acting somatostatin analogs have recently become supplemental drugs in the treatment of neurofibroma because of their marked tumor growth inhibitory effect. Somatostatin is currently under extended evaluation in other cancers as a possible supplemental drug to the treatment protocols in use. The mode of action is not known. Somatostatin has been shown to cause glucose intolerance by inhibiting glucose-6-phosphate dehydrogenase (G6PD) in fish liver. Recent data generated in our laboratory indicate that it is this pathway and the transketolase reactions of the pentose cycle (PC) which are directly involved in the ribose synthesis process of pancreatic adenocarcinoma cells. In cell culture, somatostatin alone inhibited glucose carbon recycling through the PC by 5.7%, which was increased to 19.8% in combination with oxythiamine, a competitive inhibitor of transketolase. Oxythiamine produced strong apoptosis in in-vitro hosted tumor cells. We hypothesize that somatostatin- and oxythiamine-induced antiproliferative action is mediated by the inhibition of G6PD, transketolase, or both.

Properties and purification of a glucose-inducible human fatty acid synthase mRNA-binding protein

Glucose stabilizes the mRNA for human fatty acid synthase (FAS), an enzyme relevant to diverse human disorders, including hyperlipidemia, obesity, and malignancy. To determine the underlying mechanisms, RNA gel mobility shift assays were used to demonstrate that human Hep G2 cells contain a cytoplasmic factor that binds specifically to the 3'-terminus of the human FAS mRNA. D-Glucose increased RNA-binding activity by 2.02-fold (P = 0.0033), with activity peaking 3 h after glucose feeding. Boiling or treatment of extracts with proteinase K abolished binding. Ultraviolet cross-linking of the FAS mRNA-binding factor followed by SDS-PAGE resolved a proteinase K-sensitive band with an apparent molecular mass of 178 +/- 7 kDa. The protein was purified to homogeneity using nondenaturing polyacrylamide gels as an affinity matrix. Acid phosphatase treatment of the protein prevented binding to the FAS mRNA, but binding activity was unaffected by modification of sulfhydryl groups and was not Mg2+ or Ca2+ dependent. Deletion and RNase T1 mapping localized the binding site of the protein to 37 nucleotides characterized by the repetitive motif ACCCC and found within the first 65 bases of the 3'-UTR. Hybridization of the FAS transcript with an oligonucleotide antisense to this sequence abolished binding. These findings indicate that a 178-kDa glucose-inducible phosphoprotein binds to an (ACCCC)n-containing sequence in the 3'-UTR of the FAS mRNA within the same time frame that glucose stabilizes the FAS message. This protein may participate in the posttranscriptional control of FAS gene expression.

Quantitative determinations of the steady state transcript levels of hexokinase isozymes and glucose transporter isoforms in normal rat tissues and the malignant tumor cell line AH130

The steady state transcript levels of the four hexokinase (HK) isozymes and four glucose transporter (GLUT) isoforms were determined quantitatively by Northern analysis of RNA samples from rat tissues using synthetic fragments of the RNAs encoding the HK isozymes and GLUT isoforms. Results showed that the levels of HK isozyme transcripts were low in rat tissues, the level of that most highly expressed, the type I isozyme (HKI), in the brain being 0.025% of the total poly(A)+ RNA. A good correlation was found between the reported HK activities and the total amounts of transcripts encoding all HK isozymes in various tissues, showing that the HK activities in tissues can be estimated from the total amount of transcripts encoding HK isozymes. The proposed associated expressions of HK isozymes and GLUT isoforms in particular tissues were confirmed at their transcript levels. The steady state transcript levels of type II HK and the type 1 GLUT isoform in the malignant tumor cell line AH130 were also determined quantitatively.

Nonoxidative pentose phosphate pathways and their direct role in ribose synthesis in tumors: is cancer a disease of cellular glucose metabolism?

Pentose phosphate pathways (PPP) are considered important in tumor proliferation processes because of their role in supplying tumor cells with reduced NADP and carbons for intracellular anabolic processes. Direct involvement of PPP in tumor DNA/RNA synthesis is not considered as significant as in lipid and protein syntheses. Currently, PPP activity in tumor cells is measured by lactate production, which shows a moderate activity: about 4% to 7% compared with glycolysis. Recent data generated in our laboratory indicate that PPP are directly involved in ribose synthesis in pancreatic adenocarcinoma cells, through oxidative steps (< 31%) and transketolase reactions (69%). These findings raise serious questions about the adequacy of lactate in measuring PPP activity in tumors. We hypothesize that ribose, not lactate, is the major product of PPP in tumor cells. Control of both oxidative and nonoxidative PPP may be critical in the treatment of cancer. PPP are substantially involved in the proliferation of human tumors, which raises the prospect of new treatment strategies targeting specific biochemical reactions of PPP by hormones related to glucose metabolism, controlling thiamine intake, the cofactor of the nonoxidative transketolase PPP reaction, or treating cancer patients with antithiamine analogues.

Aberrant glycolytic metabolism of cancer cells: a remarkable coordination of genetic, transcriptional, post-translational, and mutational events that lead to a critical role for type II hexokinase

For more than two-thirds of this century we have known that one of the most common and profound phenotypes of cancer cells is their propensity to utilize and catabolize glucose at high rates. This common biochemical signature of many cancers, particularly those that are poorly differentiated and proliferate rapidly, has remained until recently a "metabolic enigma." However, with many advances in the biological sciences having been applied to this problem, cancer cells have begun to reveal their molecular strategies in maintaining an aberrant metabolic behavior. Specifically, studies performed over the past two decades in our laboratory demonstrate that hexokinase, particularly the Type II isoform, plays a critical role in initiating and maintaining the high glucose catabolic rates of rapidly growing tumors. This enzyme converts the incoming glucose to glucose-6-phosphate, the initial phosphorylated intermediate of the glycolytic pathway and an important precursor of many cellular "building blocks." At the genetic level the tumor cell adapts metabolically by first increasing the gene copy number of Type II hexokinase. The enzyme's gene promoter, in turn, shows a wide promiscuity toward the signal transduction cascades active within tumor cells. It is activated by glucose, insulin, low oxygen "hypoxic" conditions, and phorbol esters, all of which enhance the rate of transcription. Also, the tumor cell uses the tumor suppressor p53, which is usually modified by mutations to debilitate cell cycle controls, to further activate hexokinase gene transcription. This results in both enhanced levels of the enzyme, which binds to mitochondrial porins thus gaining preferential access to mitochondrially generated ATP, and in a decreased susceptibility to product inhibition and proteolytic degradation. Significantly, these multiple strategies all work together to enable tumor cells to develop a metabolic strategy compatible with rapid proliferation and prolonged survival.

Source of ATP for hexokinase-catalyzed glucose phosphorylation in tumor cells: dependence on the rate of oxidative phosphorylation relative to that of extramitochondrial ATP generation

We isolated highly intact and tightly coupled mitochondria from the rat ascites hepatoma cell line AH130 by disruption of the cell membrane by nitrogen cavitation. These isolated mitochondria were found to have essentially the same functional properties as rat liver mitochondria, but unlike the latter, hexokinase (HK) was bound to their membrane. Using the tumor mitochondrial preparation, we examined the source of ATP for phosphorylation of glucose by HK under conditions in which intra- and extramitochondrial ATP-generation systems operated separately or together. Results showed that the membrane-bound HK utilized ATP derived from the most efficiently operating ATP generation system, i.e., oxidative phosphorylation. However, when the rate of extramitochondrial ATP generation was much greater than that of oxidative phosphorylation, HK used ATP from the extramitochondrial ATP-generation system.