Apoptosis induced by growth factor withdrawal in fibroblasts overproducing fructose 2,6-bisphosphate

Role of AMPK activation in oxidative cell damage: Implications for alcohol-induced liver disease

Chronic alcohol consumption is a well-known risk factor for liver disease. Progression of alcohol-induced liver disease (ALD) is a multifactorial process that involves a number of genetic, nutritional and environmental factors. Experimental and clinical studies increasingly show that oxidative damage induced by ethanol contributes in many ways to the pathogenesis of alcohol hepatoxicity. Oxidative stress appears to activate AMP-activated protein kinase (AMPK) signaling system, which has emerged in recent years as a kinase that controls the redox-state and mitochondrial function. This review focuses on the most recent insights concerning the activation of AMPK by reactive oxygen species (ROS), and describes recent evidences supporting the hypothesis that AMPK signaling pathways play an important role in promoting cell viability under conditions of oxidative stress, such as during alcohol exposure. We suggest that AMPK activation by ROS can promote cell survival by inducing autophagy, mitochondrial biogenesis and expression of genes involved in antioxidant defense. Hence, increased intracellular concentrations of ROS may represent a general mechanism for enhancement of AMPK-mediated cellular adaptation, including maintenance of redox homeostasis. On the other hand, AMPK inhibition in the liver by ethanol appears to play a key role in the development of steatosis induced by chronic alcohol consumption. Although more studies are needed to assess the functions of AMPK during oxidative stress, AMPK may be a possible therapeutic target in the particular case of alcohol-induced liver disease.

Differential effects of energy stress on AMPK phosphorylation and apoptosis in experimental brain tumor and normal brain

Background

AMP-activated protein kinase (AMPK) is a known physiological cellular energy sensor and becomes phosphorylated at Thr-172 in response to changes in cellular ATP levels. Activated AMPK acts as either an inducer or suppressor of apoptosis depending on the severity of energy stress and the presence or absence of certain functional tumor suppressor genes.

Results

Here we show that energy stress differentially affects AMPK phosphorylation and cell-death in brain tumor tissue and in tissue from contra-lateral normal brain. We compared TSC2 deficient CT-2A mouse astrocytoma cells with syngeneic normal astrocytes that were grown under identical condition in vitro. Energy stress induced by glucose withdrawal or addition of 2-deoxyglucose caused more ATP depletion, AMPK phosphorylation and apoptosis in CT-2A cells than in the normal astrocytes. Under normal energy conditions pharmacological stimulation of AMPK caused apoptosis in CT-2A cells but not in astrocytes. TSC2 siRNA treated astrocytes are hypersensitive to apoptosis induced by energy stress compared to control cells. AMPK phosphorylation and apoptosis were also greater in the CT-2A tumor tissue than in the normal brain tissue following implementation of dietary energy restriction. Inefficient mTOR and TSC2 signaling, downstream of AMPK, is responsible for CT-2A cell-death, while functional LKB1 may protect normal brain cells under energy stress.

Conclusion

Together these data demonstrates that AMPK phosphorylation induces apoptosis in mouse astrocytoma but may protect normal brain cells from apoptosis under similar energy stress condition. Therefore, using activator of AMPK along with glycolysis inhibitor could be a potential therapeutic approach for TSC2 deficient human malignant astrocytoma.

Increase in Fru-2,6-P(2) levels results in altered cell division in Schizosaccharomyces pombe

Mitogenic response to growth factors is concomitant with the modulation they exert on the levels of Fructose 2,6-bisphosphate (Fru-2,6-P2), an essential activator of the glycolytic flux. In mammalian cells, decreased Fru-2,6-P2 concentration causes cell cycle delay, whereas high levels of Fru-2,6-P2 sensitize cells to apoptosis. In order to analyze the cell cycle consequences due to changes in Fru-2,6-P2 levels, the bisphosphatase-dead mutant (H258A) of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase enzyme was over-expressed in Schizosaccharomyces pombe cells and the variation in cell phenotype was studied. The results obtained demonstrate that the increase in Fru-2,6-P2 levels results in a defective division of S. pombe, as revealed by an altered multisepted phenotype. The H258A-expressing cells showed impairment of cytokinesis, but normal nuclear division. In order to identify cellular mediators responsible for this effect, we transformed different S. pombe strains and observed that the cytokinetic defect was absent in cells defective for Wee1 kinase function. Therefore, in S. pombe, Wee1 integrates the metabolic signal emerging from changes in Fru-2,6-P2 content, thus coupling metabolism with cell proliferation. As the key regulators of the cell cycle checkpoints are conserved throughout evolution, these results may help to understand the experimental evidences obtained by manipulation of Fru-2,6-P2 levels in mammalian cells.

AMPK, the metabolic syndrome and cancer

The fuel-sensing enzyme 5'-AMP-activated protein kinase (AMPK) has a major role in the regulation of cellular lipid and protein metabolism in response to stimuli such as exercise, changes in fuel availability and the adipocyte-derived hormones leptin and adiponectin. Recent studies indicate that abnormalities in cellular lipid metabolism are involved in the pathogenesis of the metabolic syndrome, possibly because of dysregulation of AMPK and malonyl-CoA, a closely related molecule. As we discuss in this article, several findings also point to a link between AMPK and the growth and/or survival of some cancer cells. Thus, it has been demonstrated recently that the tumor suppressor LKB1 is a kinase that has a major role in phosphorylating and activating AMPK, and that another tumor suppressor, tuberous sclerosis complex 2, is phosphorylated and activated by AMPK. In addition, other studies indicate that mammalian homolog of target of rapamycin (mTOR), which has been implicated in the pathogenesis of insulin resistance and many types of cancer, is inhibited by AMPK.

Acadesine activates AMPK and induces apoptosis in B-cell chronic lymphocytic leukemia cells but not in T lymphocytes

Acadesine, 5-aminoimidazole-4-carboxamide (AICA) riboside, induced apoptosis in B-cell chronic lymphocytic leukemia (B-CLL) cells in all samples tested (n = 70). The half-maximal effective concentration (EC(50)) for B-CLL cells was 380 +/- 60 microM (n = 5). The caspase inhibitor Z-VAD.fmk completely blocked acadesine-induced apoptosis, which involved the activation of caspase-3, -8, and -9 and cytochrome c release. Incubation of B-CLL cells with acadesine induced the phosphorylation of adenosine monophosphate-activated protein kinase (AMPK), indicating that it is activated by acadesine. Nitrobenzylthioinosine (NBTI), a nucleoside transport inhibitor, 5-iodotubercidin, an inhibitor of adenosine kinase, and adenosine completely inhibited acadesine-induced apoptosis and AMPK phosphorylation, demonstrating that incorporation of acadesine into the cell and its subsequent phosphorylation to AICA ribotide (ZMP) are necessary to induce apoptosis. Inhibitors of protein kinase A and mitogen-activated protein kinases did not protect from acadesine-induced apoptosis in B-CLL cells. Moreover, acadesine had no effect on p53 levels or phosphorylation, suggesting a p53-independent mechanism in apoptosis triggering. Normal B lymphocytes were as sensitive as B-CLL cells to acadesine-induced apoptosis. However, T cells from patients with B-CLL were only slightly affected by acadesine at doses up to 4 mM. AMPK phosphorylation did not occur in T cells treated with acadesine. Intracellular levels of ZMP were higher in B-CLL cells than in T cells when both were treated with 0.5 mM acadesine, suggesting that ZMP accumulation is necessary to activate AMPK and induce apoptosis. These results suggest a new pathway involving AMPK in the control of apoptosis in B-CLL cells and raise the possibility of using acadesine in B-CLL treatment.

5-Aminoimidazole-4-carboxamide riboside induces apoptosis in Jurkat cells, but the AMP-activated protein kinase is not involved

5-Aminoimidazole-4-carboxamide (AICA) riboside, a precursor of purine nucleotide biosynthesis, induces apoptosis in Jurkat cells. Incorporation of AICAriboside into the cells is necessary for this effect since addition of nitrobenzylthioinosine, a nucleoside-transport inhibitor, completely protects Jurkat cells from apoptosis. Adenosine, but not other nucleosides, also protects Jurkat cells from AICAriboside-induced apoptosis. The apoptotic effect is caspase-dependent since caspases 9 and 3 are activated and the caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone (Z-VAD.fmk) blocks apoptosis. Furthermore, AICAriboside induces mitochondrial cytochrome c release. AICAriboside, when phosphorylated to AICAribotide (ZMP), is a specific activator of the AMP-activated protein kinase (AMPK) in certain cell types. However, AICAriboside does not activate AMPK in Jurkat cells. Moreover, 5-iodotubercidin, an inhibitor of AICAriboside phosphorylation, does not inhibit apoptosis in Jurkat cells. These results indicate that AICAriboside induces apoptosis independently of ZMP synthesis and AMPK activation in Jurkat cells.

An E2F-binding site mediates the activation of the proliferative isoform of 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase by phosphatidylinositol 3-kinase

In the present study, we demonstrate that E2F is implicated in the regulation of the glycolytic enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (6PF2K/Fru-2,6-BPase) during cell division. The expression of this enzyme is induced during the G(1)/S transition of the cell cycle. We identified and monitored the E2F-pocket protein complexes that bind to the E2F site of the F-type promoter during cell-cycle entry, and we analysed their contribution to the phosphatidylinositol 3-kinase (PI 3-kinase)-mediated regulation of the promoter. We found that the predominant E2F complex bound to the F-type promoter in unstimulated/quiescent cells contains E2F4, DP1 and p130 proteins. In serum-stimulated (S-phase) cells, the composition of the complex switched to E2F1/4, DP1 and p107, together with cyclin A and cyclin-dependent kinase 2. Treatment with the PI 3-kinase specific inhibitor LY 294002 prevented the formation of the S-phase complex, suggesting that activation of the PI 3-kinase pathway is essential for the formation of this complex. Further supporting this idea, we obtained results showing that treatment of cycling NIH 3T3 cells with either wortmannin or LY 294002 induces the accumulation of the transcriptionally repressive p130-E2F4-DP1 complex. Using the Rat-1 ER-E2F1 cell line where E2F1 activity can be conditionally induced, we demonstrated that E2F activity is involved in the in vivo transcriptional regulation of the F-type 6PF2K/Fru-2, 6-BPase gene. Taken together, our results show that the F-type 6PF2K/Fru-2, 6-BPase is a genuine E2F-regulated gene, and that its regulation by the PI 3-kinase pathway is at least partially mediated through the E2F transcription factor.

Enhanced antioxidant defenses and resistance to TNF-alpha in a glycolysis-depleted lung epithelial cell line

Glycolysis-depleted cells, obtained by stable transfection of fructose 2,6-bisphosphatase in mink lung epithelial cells (Mv1Lu), were less sensitive to serum withdrawal- and TNF-alpha-induced apoptosis than cells transfected with the empty vector pcDNA3 (control cells). We compared the differences in the redox status of the two transfectants and the changes produced by TNF-alpha treatment. The activities of the antioxidant enzymes catalase and glutathione peroxidase, as well as the content of reduced glutathione (GSH) and the activity of the nuclear transcription factor kappa B (NF-kappa B), were higher in pFBPase-2 clones than in control cells in all the conditions tested. TNF-alpha challenge sharpened the differences in glutathione peroxidase activity, GSH/GSSG ratios, and NF-kappa B activation between transfectants. These data indicate that glycolysis restriction at the PFK step protects cells against apoptotic stimuli by increasing the GSH content and NF-kappa B activity. This acquired feature may compromise antineoplastic treatments based on glycolytic depletion.

Pyruvate kinase type M2: a crossroad in the tumor metabolome

Cell proliferation is a process that consumes large amounts of energy. A reduction in the nutrient supply can lead to cell death by ATP depletion, if cell proliferation is not limited. A key sensor for this regulation is the glycolytic enzyme pyruvate kinase, which determines whether glucose carbons are channelled to synthetic processes or used for glycolytic energy production. In unicellular organisms pyruvate kinase is regulated by ATP, ADP and AMP, by ribose 5-P, the precursor of the nucleic acid synthesis, and by the glycolytic intermediate fructose 1,6-P2 (FBP), thereby adapting cell proliferation to nutrient supply. The mammalian pyruvate kinase isoenzyme type M2 (M2-PK) displays the same kinetic properties as the pyruvate kinase enzyme from unicellular organisms. The mammalian M2-PK isoenzyme can switch between a less active dimeric form and a highly active tetrameric form which regulates the channeling of glucose carbons either to synthetic processes (dimeric form) or to glycolytic energy production (tetrameric form). Tumor cells are usually characterized by a high amount of the dimeric form leading to a strong accumulation of all glycolytic phosphometabolites above pyruvate kinase. The tetramer-dimer ratio is regulated by ATP, FBP and serine and by direct interactions with different oncoproteins (pp60v-src, HPV-16 E7). In solid tumors with sufficient oxygen supply pyruvate is supplied by glutaminolysis. Pyruvate produced in glycolysis and glutaminolysis is used for the synthesis of lactate, glutamate and fatty acids thereby releasing the hydrogen produced in the glycolytic glyceraldehyde 3-phosphate dehydrogenase reaction.

Identification of Rhus verniciflua Stokes compounds that exhibit free radical scavenging and anti-apoptotic properties

Rhus verniciflua Stokes (RVS) is a widely used herbal plant with various biological properties. Our previous study using cultured neuronal cells showed that an ethanol extract of RVS had strong antioxidant properties. In this study, we characterized the antioxidant activity of the RVS ethanol extract and identified the active compounds responsible for this activity. From the RVS ethanol extract, we derived three water-eluted fractions and another three fractions eluted by organic solvents, and determined that the water-eluted fractions are what protect against reactive oxygen species (ROS) generated by iron and enzymes. Water-eluted fraction F(2) was the most efficient antioxidant. Moreover, DNA fragmentation and terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL) staining experiments revealed that F(2) also protects against thymocyte apoptosis mediated by hydroxyl radicals. Finally, EI-MS, (1)H-NMR, and (13)C-NMR spectra signals confirmed that the fraction contained flavonoid derivatives, including fustin, quercetin, butein, and sulfuretin. These results suggest that the flavonoid derivatives in F(2) are the compounds in the RVS ethanol extract that act as antioxidants.

Hyperglycemia-induced apoptosis in human umbilical vein endothelial cells: inhibition by the AMP-activated protein kinase activation

Apoptosis has been observed in vascular cells, nerve, and myocardium of diabetic humans and experimental animals, although whether it contributes to or is a marker of complications in these tissues is unclear. Previous studies have shown that incubation of human umbilical vein endothelial cells (HUVECs) with 30 vs. 5 mmol/l glucose for 72 h causes a significant increase in apoptosis, possibly related to an increase in oxidative stress. We report here that this increase in apoptosis (assessed morphologically by TdT-mediated dUTP nick- end labeling staining) is preceded (24 h of incubation) by inhibition of fatty acid oxidation, by increases in diacylglycerol synthesis, the concentration of malonyl CoA, and caspase-3 activity, and by decreases in mitochondrial membrane potential and cellular ATP content. In addition, the phosphorylation of Akt in the presence of 150 microU/ml insulin was impaired. No increases in ceramide content or its de novo synthesis were observed. AMP-activated protein kinase (AMPK) activity was not diminished; however, incubation with the AMPK activator 5-aminoimidazole-4-carboxamide-riboside increased AMPK activity twofold and completely prevented all of these changes. Likewise, expression of a constitutively active AMPK in HUVEC prevented the increase in caspase-3 activity. The results indicate that alterations in fatty-acid metabolism, impaired Akt activation by insulin, and increased caspase-3 activity precede visible evidence of apoptosis in HUVEC incubated in a hyperglycemic medium. They also suggest that AMPK could play an important role in protecting the endothelial cell against the adverse effects of sustained hyperglycemia.

AMP-activated protein kinase: the energy charge hypothesis revisited

The AMP-activated protein kinase cascade is a sensor of cellular energy charge, and its existence provides strong support for the energy charge hypothesis first proposed by Daniel Atkinson in the 1960s. The system is activated in an ultrasensitive manner by cellular stresses that deplete ATP (and consequently elevate AMP), either by inhibiting ATP production (e.g., hypoxia), or by accelerating ATP consumption (e.g., exercise in muscle). Once activated, it switches on catabolic pathways, both acutely by phosphorylation of metabolic enzymes and chronically by effects on gene expression, and switches off many ATP-consuming processes. Recent work suggests that activation of AMPK is responsible for many of the effects of physical exercise, both the rapid metabolic effects and the adaptations that occur during training. Dominant mutations in regulatory subunit isoforms (gamma2 and gamma3) of AMPK, which appear to increase the basal activity in the absence of AMP, lead to hypertrophy of cardiac and skeletal muscle respectively.

AMP-activated protein kinase (AMPK) is activated in muscle of subjects with type 2 diabetes during exercise

Insulin-stimulated GLUT4 translocation is impaired in people with type 2 diabetes. In contrast, exercise results in a normal increase in GLUT4 translocation and glucose uptake in these patients. Several groups have recently hypothesized that exercise increases glucose uptake via an insulin-independent mechanism mediated by the activation of AMP-activated protein kinase (AMPK). If this hypothesis is correct, people with type 2 diabetes should have normal AMPK activation in response to exercise. Seven subjects with type 2 diabetes and eight matched control subjects exercised on a cycle ergometer for 45 min at 70% of maximum workload. Biopsies of vastus lateralis muscle were taken before exercise, after 20 and 45 min of exercise, and at 30 min postexercise. Blood glucose concentrations decreased from 7.6 to 4.77 mmol/l with 45 min of exercise in the diabetic group and did not change in the control group. Exercise significantly increased AMPK alpha2 activity 2.7-fold over basal at 20 min in both groups and remained elevated throughout the protocol, but there was no effect of exercise on AMPK alpha1 activity. Subjects with type 2 diabetes had similar protein expression of AMPK alpha1, alpha2, and beta1 in muscle compared with control subjects. AMPK alpha2 was shown to represent approximately two-thirds of the total alpha mRNA in the muscle from both groups. In conclusion, people with type 2 diabetes have normal exercise-induced AMPK alpha2 activity and normal expression of the alpha1, alpha2 and beta1 isoforms. Pharmacological activation of AMPK may be an attractive target for the treatment of type 2 diabetes.

The AMP-activated protein kinase prevents ceramide synthesis de novo and apoptosis in astrocytes

Fatty acids induce apoptosis in primary astrocytes by enhancing ceramide synthesis de novo. The possible role of the AMP-activated protein kinase (AMPK) in the control of apoptosis was studied in this model. Long-term stimulation of AMPK with 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) prevented apoptosis. AICAR blunted fatty acid-mediated induction of serine palmitoyltransferase and ceramide synthesis de novo, without affecting fatty acid synthesis and oxidation. Prevention of ceramide accumulation by AICAR led to a concomitant blockade of the Raf-1/extracellular signal-regulated kinase cascade, which selectively mediates fatty acid-induced apoptosis. Data indicate that AMPK may protect cells from apoptosis induced by stress stimuli.

Molecular cloning, genomic organization, and mapping of PRKAG2, a heart abundant gamma2 subunit of 5'-AMP-activated protein kinase, to human chromosome 7q36

5'-AMP-activated protein kinase (AMPK) acts as a major regulator of cellular ATP levels and protects cells against stresses that cause ATP depletion. AMPK is a protein heterotrimer composed of a catalytic alpha subunit and two regulatory subunits, beta and gamma. In the present study, a homologue of the AMPK gamma1-subunit cDNA with an open reading frame encoding 328 amino acids was identified. The putative protein sequence is about 76% identical to the 331-amino-acid gamma1 subunit and also has four consecutive cystathionine-beta-synthase (CBS) domains, a characteristic structure of AMPK gamma subunits from various species. This cDNA (tentatively termed PRKAG2-b) is identical to a recently reported cDNA (tentatively termed PRKAG2-a) of human AMPK gamma subunits except in their 5'-end regions, suggesting that these two cDNAs are two different transcripts of the same gene. To determine the expression pattern of the gene, two probes, one from the 3'-UTR of PRKAG2-b and the other from the 5'- unique region of PRKAG2-a, were used to hybridize MTN membranes. Three transcripts (3.8, 3.0, and 2.4 kb) were observed when the first probe was used, whereas only 3.8- and 3.0-kb transcripts were seen when the second probe was used. Thus, the PRKAG2-b corresponded to the 2.4-kb transcript, which is ubiquitously expressed except in liver and thymus. The highest level was detected in heart, while abundant expression also existed in placenta and testis. The expression pattern of PRKAG2-b is completely different from those of PRKAG2-a and PRKAG1, whose expression patterns were also determined in the current study. The PRKAG2 gene was located to human chromosome 7q36 between markers D7S2439 and D7S2462 by radiation hybrid mapping. The genomic organization of PRKAG2-b was identified by comparing its cDNA sequence with two genomic sequences AC006358 and AC006966, which showed that PRKAG2-b spanned an approximately 80-kb region and was composed of 12 exons.

Overexpression of fructose 2,6-bisphosphatase decreases glycolysis and delays cell cycle progression

The ability to overexpress 6-phosphofructo-2-kinase/fructose 2, 6-bisphosphatase (PFK-2)/(FBPase-2) or a truncated form of the enzyme with only the bisphosphatase domain allowed us to analyze the relative role of the kinase and the bisphosphatase activities in regulating fructose 2,6-bisphosphate (Fru-2,6-P(2)) concentration and to elucidate their differential metabolic impact in epithelial Mv1Lu cells. The effect of overexpressing PFK-2/FBPase-2 resulted in a small increase in the kinase activity and in the activity ratio of the bifunctional enzyme, increasing Fru-2,6-P(2) levels, but these changes had no major effects on cell metabolism. In contrast, expression of the bisphosphatase domain increased the bisphosphatase activity, producing a significant decrease in Fru-2,6-P(2) concentration. The fall in the bisphosphorylated metabolite correlated with a decrease in lactate production and ATP concentration, as well as a delay in cell cycle. These results provide support for Fru-2,6-P(2) as a regulator of glycolytic flux and point out the role of glycolysis in cell cycle progression.

Lavandulylflavonoids: a new class of in vitro apoptogenic agents from Sophora flavescens

The root of Sophora flavescens has been reported to possess antitumor activity in Sarcoma 180, lymphoid leukemia 1210 and melanotic melanoma. We have isolated four cytotoxic flavonoids with a lavandulyl side-chain at C8 and tested for their effects on human myeloid leukemia HL-60 cells and human hepatocarcinoma HepG2 cells, in terms of inhibition of proliferation and induction of apoptosis. They showed potent antiproliferative effects with IC(50) values from 11.3 microM to 18.5 microM in HL60 cells and from 13.3 microM to 36. 2 microM in HepG2 cells. Treatment of HL-60 cells with the lavandulylflavonoids induced apoptosis in a dose-dependent manner. Apoptosis was judged by the detection of DNA fragmentation by agarose gel electrophoresis and the degree of apoptosis was quantified by a sandwich enzyme immunoassay. The hydration of C4"'C5"' double bond with or without C3 hydroxylation caused a complete loss of cytotoxicity. These results suggest that the lavandulyl side-chain is essential for the activity of the flavonoids isolated from S. flavescens which may be used as cancer chemotherapeutic and chemopreventive agents.

Polymethoxyflavonoids from Vitex rotundifolia inhibit proliferation by inducing apoptosis in human myeloid leukemia cells

Three polymethoxyflavonoids from the fruit of Vitex rotundifolia, namely 2',3',5-trihydroxy-3,6,7-trimethoxyflavone (Vx-1), vitexicarpin (Vx-5) and artemetin (Vx-6), were tested for their antiproliferative activity in human myeloid leukemia HL-60 cells. They showed a dose-dependent decrease in the growth of HL-60 cells. The concentrations required for 50% inhibition of the growth (IC50) after 96 h were 4.03 microM, 0.12 microM and 30.98 microM for Vx-1, Vx-5 and Vx-6, respectively. Treatment of HL-60 cells with the flavonoids induced morphological changes that are characteristic of apoptosis. We judged the induction of apoptosis by the detection of DNA fragmentation in agarose gel electrophoresis and the degree of apoptosis was quantified by a double-antibody sandwich ELISA and by flow cytometric analysis. The C-3 hydroxyl and C-8 methoxyl groups were found not to be essential for the activity, but the C-3' methoxyl instead of hydroxyl group lowered the antiproliferative and apoptosis inducing activity. These results suggest that the polymethoxyflavonoids isolated from V. rotundifolia may be used as potential chemopreventive and chemotherapeutic agents.

Cells overexpressing fructose-2,6-bisphosphatase showed enhanced pentose phosphate pathway flux and resistance to oxidative stress

Changes in the content of fructose-2,6-bisphosphate, a modulator of glycolytic flux, also affect other metabolic fluxes such as the non-oxidative pentose phosphate pathway. Since this is the main source of precursors for biosynthesis in proliferating cells, PFK-2/FBPase-2 has been proposed as a potential target for neoplastic treatments. Here we provide evidence that cells with a low content of fructose-2,6-bisphosphate have a lower energy status than controls, but they are also less sensitive to oxidative stress. This feature is related to the activation of the oxidative branch of the pentose phosphate pathway and the increased production of NADPH.

The effect of AMP-activated protein kinase and its activator AICAR on the metabolism of human umbilical vein endothelial cells

In several non-vascular tissues in which it has been studied, AMP-activated protein kinase (AMPK) appears to modulate the cellular response to stresses such as ischemia. In liver and muscle, it phosphorylates and inhibits acetyl CoA carboxylase (ACC), leading to an increase in fatty acid oxidation; and in muscle, its activation is associated with an increase in glucose transport. Here we report the presence of both AMPK and ACC in human umbilical vein endothelial cells (HUVEC). Incubation of HUVEC with 2 mM AICAR, an AMPK activator, caused a 5-fold activation of AMPK, which was accompanied by a 70% decrease in ACC activity and a 2-fold increase in fatty acid oxidation. Surprisingly, glucose uptake and glycolysis, the dominant energy-producing pathway in HUVEC, were diminished by 40-60%. Despite this, cellular ATP levels were increased by 35%. Thus activation of AMPK by AICAR is associated with major alterations in endothelial cell energy balance. Whether these alterations protect the endothelium during ischemia or other stresses remains to be determined.