Instruct: Given a scientific claim, retrieve passages that support or refute the claim.
Query: aPKCz causes tumour suppression by affecting glutamine metabolism.
Tumor cells have high-energetic and anabolic needs and are known to adapt their metabolism to be able to survive and keep proliferating under conditions of nutrient stress. We show that PKCζ deficiency promotes the plasticity necessary for cancer cells to reprogram their metabolism to utilize glutamine through the serine biosynthetic pathway in the absence of glucose. PKCζ represses the expression of two key enzymes of the pathway, PHGDH and PSAT1, and phosphorylates PHGDH at key residues to inhibit its enzymatic activity. Interestingly, the loss of PKCζ in mice results in enhanced intestinal tumorigenesis and increased levels of these two metabolic enzymes, whereas patients with low levels of PKCζ have a poor prognosis. Furthermore, PKCζ and caspase-3 activities are correlated with PHGDH levels in human intestinal tumors. Taken together, this demonstrates that PKCζ is a critical metabolic tumor suppressor in mouse and human cancer.
Metabolic pathway reprogramming is a hallmark of cancer cell growth and survival and supports the anabolic and energetic demands of these rapidly dividing cells. The underlying regulators of the tumor metabolic program are not completely understood; however, these factors have potential as cancer therapy targets. Here, we determined that upregulation of the oncogenic transcriptional coregulator steroid receptor coactivator 2 (SRC-2), also known as NCOA2, drives glutamine-dependent de novo lipogenesis, which supports tumor cell survival and eventual metastasis. SRC-2 was highly elevated in a variety of tumors, especially in prostate cancer, in which SRC-2 was amplified and overexpressed in 37% of the metastatic tumors evaluated. In prostate cancer cells, SRC-2 stimulated reductive carboxylation of α-ketoglutarate to generate citrate via retrograde TCA cycling, promoting lipogenesis and reprogramming of glutamine metabolism. Glutamine-mediated nutrient signaling activated SRC-2 via mTORC1-dependent phosphorylation, which then triggered downstream transcriptional responses by coactivating SREBP-1, which subsequently enhanced lipogenic enzyme expression. Metabolic profiling of human prostate tumors identified a massive increase in the SRC-2-driven metabolic signature in metastatic tumors compared with that seen in localized tumors, further implicating SRC-2 as a prominent metabolic coordinator of cancer metastasis. Moreover, SRC-2 inhibition in murine models severely attenuated the survival, growth, and metastasis of prostate cancer. Together, these results suggest that the SRC-2 pathway has potential as a therapeutic target for prostate cancer.
Neurons and cancer cells use glucose extensively, yet the precise advantage of this adaptation remains unclear. These two seemingly disparate cell types also show an increased regulation of the apoptotic pathway, which allows for their long-term survival. Here we show that both neurons and cancer cells strictly inhibit cytochrome c-mediated apoptosis by a mechanism dependent on glucose metabolism. We report that the pro-apoptotic activity of cytochrome c is influenced by its redox state and that increases in reactive oxygen species (ROS) following an apoptotic insult lead to the oxidation and activation of cytochrome c. In healthy neurons and cancer cells, however, cytochrome c is reduced and held inactive by intracellular glutathione (GSH), generated as a result of glucose metabolism by the pentose phosphate pathway. These results uncover a striking similarity in apoptosis regulation between neurons and cancer cells and provide insight into an adaptive advantage offered by the Warburg effect for cancer cell evasion of apoptosis and for long-term neuronal survival.
Mammalian cells depend on growth factor signaling to take up nutrients; however, coordination of glucose and glutamine uptake has been a mystery. In this issue of Genes & Development, Wellen and colleagues (pp. 2784-2799) show that glucose flux through the hexosamine biosynthesis pathway regulates growth factor receptor glycosylation and enables glutamine consumption. This mechanism ensures that cells do not engage in anabolic metabolism when nutrients are limiting, and highlights how substrate availability for protein modifications can modulate cell signaling.
The oxidative pentose phosphate pathway (PPP) contributes to tumour growth, but the precise contribution of 6-phosphogluconate dehydrogenase (6PGD), the third enzyme in this pathway, to tumorigenesis remains unclear. We found that suppression of 6PGD decreased lipogenesis and RNA biosynthesis and elevated ROS levels in cancer cells, attenuating cell proliferation and tumour growth. 6PGD-mediated production of ribulose-5-phosphate (Ru-5-P) inhibits AMPK activation by disrupting the active LKB1 complex, thereby activating acetyl-CoA carboxylase 1 and lipogenesis. Ru-5-P and NADPH are thought to be precursors in RNA biosynthesis and lipogenesis, respectively; thus, our findings provide an additional link between the oxidative PPP and lipogenesis through Ru-5-P-dependent inhibition of LKB1-AMPK signalling. Moreover, we identified and developed 6PGD inhibitors, physcion and its derivative S3, that effectively inhibited 6PGD, cancer cell proliferation and tumour growth in nude mice xenografts without obvious toxicity, suggesting that 6PGD could be an anticancer target.
Metformin has been the mainstay of therapy for diabetes mellitus for many years; however, the mechanistic aspects of metformin action remained ill-defined. Recent advances revealed that this drug, in addition to its glucose-lowering action, might be promising for specifically targeting metabolic differences between normal and abnormal metabolic signalling. The knowledge gained from dissecting the principal mechanisms by which metformin works can help us to develop novel treatments. The centre of metformin's mechanism of action is the alteration of the energy metabolism of the cell. Metformin exerts its prevailing, glucose-lowering effect by inhibiting hepatic gluconeogenesis and opposing the action of glucagon. The inhibition of mitochondrial complex I results in defective cAMP and protein kinase A signalling in response to glucagon. Stimulation of 5′-AMP-activated protein kinase, although dispensable for the glucose-lowering effect of metformin, confers insulin sensitivity, mainly by modulating lipid metabolism. Metformin might influence tumourigenesis, both indirectly, through the systemic reduction of insulin levels, and directly, via the induction of energetic stress; however, these effects require further investigation. Here, we discuss the updated understanding of the antigluconeogenic action of metformin in the liver and the implications of the discoveries of metformin targets for the treatment of diabetes mellitus and cancer.
Metformin is a widely used drug for treatment of type 2 diabetes with no defined cellular mechanism of action. Its glucose-lowering effect results from decreased hepatic glucose production and increased glucose utilization. Metformin's beneficial effects on circulating lipids have been linked to reduced fatty liver. AMP-activated protein kinase (AMPK) is a major cellular regulator of lipid and glucose metabolism. Here we report that metformin activates AMPK in hepatocytes; as a result, acetyl-CoA carboxylase (ACC) activity is reduced, fatty acid oxidation is induced, and expression of lipogenic enzymes is suppressed. Activation of AMPK by metformin or an adenosine analogue suppresses expression of SREBP-1, a key lipogenic transcription factor. In metformin-treated rats, hepatic expression of SREBP-1 (and other lipogenic) mRNAs and protein is reduced; activity of the AMPK target, ACC, is also reduced. Using a novel AMPK inhibitor, we find that AMPK activation is required for metformin's inhibitory effect on glucose production by hepatocytes. In isolated rat skeletal muscles, metformin stimulates glucose uptake coincident with AMPK activation. Activation of AMPK provides a unified explanation for the pleiotropic beneficial effects of this drug; these results also suggest that alternative means of modulating AMPK should be useful for the treatment of metabolic disorders.
Glutaminase (GLS), which converts glutamine to glutamate, plays a key role in cancer cell metabolism, growth, and proliferation. GLS is being explored as a cancer therapeutic target, but whether GLS inhibitors affect cancer cell-autonomous growth or the host microenvironment or have off-target effects is unknown. Here, we report that loss of one copy of Gls blunted tumor progression in an immune-competent MYC-mediated mouse model of hepatocellular carcinoma. Compared with results in untreated animals with MYC-induced hepatocellular carcinoma, administration of the GLS-specific inhibitor bis-2-(5-phenylacetamido-1,3,4-thiadiazol-2-yl)ethyl sulfide (BPTES) prolonged survival without any apparent toxicities. BPTES also inhibited growth of a MYC-dependent human B cell lymphoma cell line (P493) by blocking DNA replication, leading to cell death and fragmentation. In mice harboring P493 tumor xenografts, BPTES treatment inhibited tumor cell growth; however, P493 xenografts expressing a BPTES-resistant GLS mutant (GLS-K325A) or overexpressing GLS were not affected by BPTES treatment. Moreover, a customized Vivo-Morpholino that targets human GLS mRNA markedly inhibited P493 xenograft growth without affecting mouse Gls expression. Conversely, a Vivo-Morpholino directed at mouse Gls had no antitumor activity in vivo. Collectively, our studies demonstrate that GLS is required for tumorigenesis and support small molecule and genetic inhibition of GLS as potential approaches for targeting the tumor cell-autonomous dependence on GLS for cancer therapy.
Successful generation of induced pluripotent stem cells entails a major metabolic switch from mitochondrial oxidative phosphorylation to glycolysis during the reprogramming process. The mechanism of this metabolic reprogramming, however, remains elusive. Here, our results suggest that an Atg5-independent autophagic process mediates mitochondrial clearance, a characteristic event involved in the metabolic switch. We found that blocking such autophagy, but not canonical autophagy, inhibits mitochondrial clearance, in turn, preventing iPSC induction. Furthermore, AMPK seems to be upstream of this autophagic pathway and can be targeted by small molecules to modulate mitochondrial clearance during metabolic reprogramming. Our work not only reveals that the Atg5-independent autophagy is crucial for establishing pluripotency, but it also suggests that iPSC generation and tumorigenesis share a similar metabolic switch.
The SNF1/AMP-activated protein kinase (AMPK) family maintains the balance between ATP production and consumption in all eukaryotic cells. The kinases are heterotrimers that comprise a catalytic subunit and regulatory subunits that sense cellular energy levels. When energy status is compromised, the system activates catabolic pathways and switches off protein, carbohydrate and lipid biosynthesis, as well as cell growth and proliferation. Surprisingly, recent results indicate that the AMPK system is also important in functions that go beyond the regulation of energy homeostasis, such as the maintenance of cell polarity in epithelial cells.
We previously observed that glucose deprivation induces cell death in multidrug-resistant human breast carcinoma cells (MCF-7/ADR). As a follow up we wished to test the hypothesis that metabolic oxidative stress was the causative process or at least the link between causative processes behind the cytotoxicity. In the studies described here, we demonstrate that mitogen-activated protein kinase (MAPK) was activated within 3 min of being in glucose-free medium and remained activated for 3 h. Glucose deprivation for 2-4 h also caused oxidative stress as evidenced by a 3-fold greater steady state concentration of oxidized glutathione and a 3-fold increase in pro-oxidant production. Glucose and glutamate treatment rapidly suppressed MAPK activation and rescued cells from cytotoxicity. Glutamate and the peroxide scavenger, pyruvate, rescued the cells from cell killing as well as suppressed pro-oxidant production. In addition the thiol antioxidant, N-acetyl-L-cysteine, rescued cells from glucose deprivation-induced cytotoxicity and suppressed MAPK activation. These results suggest that glucose deprivation-induced cytotoxicity and alterations in MAPK signal transduction are mediated by oxidative stress in MCF-7/ADR. These results also support the speculation that a common mechanism of glucose deprivation-induced cytotoxicity in mammalian cells may involve metabolic oxidative stress.
Despite being surrounded by diverse nutrients, mammalian cells preferentially metabolize glucose and free amino acids. Recently, Ras-induced macropinocytosis of extracellular proteins was shown to reduce a transformed cell's dependence on extracellular glutamine. Here, we demonstrate that protein macropinocytosis can also serve as an essential amino acid source. Lysosomal degradation of extracellular proteins can sustain cell survival and induce activation of mTORC1 but fails to elicit significant cell accumulation. Unlike its growth-promoting activity under amino-acid-replete conditions, we discovered that mTORC1 activation suppresses proliferation when cells rely on extracellular proteins as an amino acid source. Inhibiting mTORC1 results in increased catabolism of endocytosed proteins and enhances cell proliferation during nutrient-depleted conditions in vitro and within vascularly compromised tumors in vivo. Thus, by preventing nutritional consumption of extracellular proteins, mTORC1 couples growth to availability of free amino acids. These results may have important implications for the use of mTOR inhibitors as therapeutics.
The rapamycin-sensitive TOR signalling pathway in Saccharomyces cerevisiae activates a cell-growth program in response to nutrients such as nitrogen and carbon. The TOR1 and TOR2 kinases (TOR) control cytoplasmic protein synthesis and degradation through the conserved TAP42 protein. Upon phosphorylation by TOR, TAP42 binds and possibly inhibits type 2A and type-2A-related phosphatases; however, the mechanism by which TOR controls nuclear events such as global repression of starvation-specific transcription is unknown. Here we show that TOR prevents transcription of genes expressed upon nitrogen limitation by promoting the association of the GATA transcription factor GLN3 with the cytoplasmic protein URE2. The binding of GLN3 to URE2 requires TOR-dependent phosphorylation of GLN3. Phosphorylation and cytoplasmic retention of GLN3 are also dependent on the TOR effector TAP42, and are antagonized by the type-2A-related phosphatase SIT4. TOR inhibits expression of carbon-source-regulated genes by stimulating the binding of the transcriptional activators MSN2 and MSN4 to the cytoplasmic 14-3-3 protein BMH2. Thus, the TOR signalling pathway broadly controls nutrient metabolism by sequestering several transcription factors in the cytoplasm.
Calorie restriction (CR) extends lifespan in a wide spectrum of organisms and is the only regimen known to lengthen the lifespan of mammals. We established a model of CR in budding yeast Saccharomyces cerevisiae. In this system, lifespan can be extended by limiting glucose or by reducing the activity of the glucose-sensing cyclic-AMP-dependent kinase (PKA). Lifespan extension in a mutant with reduced PKA activity requires Sir2 and NAD (nicotinamide adenine dinucleotide). In this study we explore how CR activates Sir2 to extend lifespan. Here we show that the shunting of carbon metabolism toward the mitochondrial tricarboxylic acid cycle and the concomitant increase in respiration play a central part in this process. We discuss how this metabolic strategy may apply to CR in animals.
Cellular senescence is considered a major tumour-suppressor mechanism in mammals, and many oncogenic insults, such as the activation of the ras proto-oncogene, trigger initiation of the senescence programme. Although it was shown that activation of the senescence programme involves the up-regulation of cell-cycle regulators such as the inhibitors of cyclin-dependent kinases p16INK4A and p21CIP-1, the mechanisms underlying the senescence response remain to be resolved. In the case of stress-induced premature senescence, reactive oxygen species are considered important intermediates contributing to the phenotype. Moreover, distinct alterations of the cellular carbohydrate metabolism are known to contribute to oncogenic transformation, as is best documented for the phenomenon of aerobic glycolysis. These findings suggest that metabolic alterations are involved in tumourigenesis and tumour suppression; however, little is known about the metabolic pathways that contribute to these processes. Using the human fibroblast model of in vitro senescence, we analysed age-dependent changes in the cellular carbohydrate metabolism. Here we show that senescent fibroblasts enter into a metabolic imbalance, associated with a strong reduction in the levels of ribonucleotide triphosphates, including ATP, which are required for nucleotide biosynthesis and hence proliferation. ATP depletion in senescent fibroblasts is due to dysregulation of glycolytic enzymes, and finally leads to a drastic increase in cellular AMP, which is shown here to induce premature senescence. These results suggest that metabolic regulation plays an important role during cellular senescence and hence tumour suppression.
The MAP kinase kinase kinase TGFβ-activated kinase 1 (TAK1) is activated by TLRs, IL-1, TNF, and TGFβ and in turn activates IKK-NF-κB and JNK, which regulate cell survival, growth, tumorigenesis, and metabolism. TAK1 signaling also upregulates AMPK activity and autophagy. Here, we investigated TAK1-dependent regulation of autophagy, lipid metabolism, and tumorigenesis in the liver. Fasted mice with hepatocyte-specific deletion of Tak1 exhibited severe hepatosteatosis with increased mTORC1 activity and suppression of autophagy compared with their WT counterparts. TAK1-deficient hepatocytes exhibited suppressed AMPK activity and autophagy in response to starvation or metformin treatment; however, ectopic activation of AMPK restored autophagy in these cells. Peroxisome proliferator-activated receptor α (PPARα) target genes and β-oxidation, which regulate hepatic lipid degradation, were also suppressed in hepatocytes lacking TAK1. Due to suppression of autophagy and β-oxidation, a high-fat diet challenge aggravated steatohepatitis in mice with hepatocyte-specific deletion of Tak1. Notably, inhibition of mTORC1 restored autophagy and PPARα target gene expression in TAK1-deficient livers, indicating that TAK1 acts upstream of mTORC1. mTORC1 inhibition also suppressed spontaneous liver fibrosis and hepatocarcinogenesis in animals with hepatocyte-specific deletion of Tak1. These data indicate that TAK1 regulates hepatic lipid metabolism and tumorigenesis via the AMPK/mTORC1 axis, affecting both autophagy and PPARα activity.
Adenosine 5′-monophosphate–activated protein kinase (AMPK) is a pivotal regulator of metabolism at cellular and organismal levels. AMPK also suppresses inflammation. We found that pharmacological activation of AMPK rapidly inhibited the Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway in various cells. In vitro kinase assays revealed that AMPK directly phosphorylated two residues (Ser515 and Ser518) within the Src homology 2 domain of JAK1. Activation of AMPK enhanced the interaction between JAK1 and 14-3-3 proteins in cultured vascular endothelial cells and fibroblasts, an effect that required the presence of Ser515 and Ser518 and was abolished in cells lacking AMPK catalytic subunits. Mutation of Ser515 and Ser518 abolished AMPK-mediated inhibition of JAK-STAT signaling stimulated by either the sIL-6Rα/IL-6 complex or the expression of a constitutively active V658F-mutant JAK1 in human fibrosarcoma cells. Clinically used AMPK activators metformin and salicylate enhanced the inhibitory phosphorylation of endogenous JAK1 and inhibited STAT3 phosphorylation in primary vascular endothelial cells. Therefore, our findings reveal a mechanism by which JAK1 function and inflammatory signaling may be suppressed in response to metabolic stress and provide a mechanistic rationale for the investigation of AMPK activators in a range of diseases associated with enhanced activation of the JAK-STAT pathway.
The metabolic stress-sensing enzyme AMP-activated protein kinase (AMPK) is responsible for regulating metabolism in response to energy supply and demand. Drugs that activate AMPK may be useful in the treatment of metabolic diseases including type 2 diabetes. We have determined the crystal structure of AMPK in complex with its activator 5-(5-hydroxyl-isoxazol-3-yl)-furan-2-phosphonic acid (C2), revealing two C2-binding sites in the γ-subunit distinct from nucleotide sites. C2 acts synergistically with the drug A769662 to activate AMPK α1-containing complexes independent of upstream kinases. Our results show that dual drug therapies could be effective AMPK-targeting strategies to treat metabolic diseases.
The balance between oxidative and nonoxidative glucose metabolism is essential for a number of pathophysiological processes. By deleting enzymes that affect aerobic glycolysis with different potencies, we examine how modulating glucose metabolism specifically affects hematopoietic and leukemic cell populations. We find that a deficiency in the M2 pyruvate kinase isoform (PKM2) reduces the levels of metabolic intermediates important for biosynthesis and impairs progenitor function without perturbing hematopoietic stem cells (HSCs), whereas lactate dehydrogenase A (LDHA) deletion significantly inhibits the function of both HSCs and progenitors during hematopoiesis. In contrast, leukemia initiation by transforming alleles putatively affecting either HSCs or progenitors is inhibited in the absence of either PKM2 or LDHA, indicating that the cell-state-specific responses to metabolic manipulation in hematopoiesis do not apply to the setting of leukemia. This finding suggests that fine-tuning the level of glycolysis may be explored therapeutically for treating leukemia while preserving HSC function.
In this present work, we characterized the proteomes of pancreatic ductal adenocarcinoma (PDAC) cell line PANC-1 and normal pancreatic duct cells by mass spectrometry using LTQ-Orbitrap and identified more than 1700 proteins from each sample. On the basis of the spectra count label-free quantification approach, we identified a large number of differentially expressed metabolic enzymes and proteins involved in cytoskeleton, cell adhesion, transport, transcription, translation, and cell proliferation as well. The data demonstrated that metabolic pathways were altered in PANC-1, consistent with the Warburg effect. In addition, the comparative MS analysis unveiled anomalous metabolism of glutamine, suggesting that glutamine was largely consumed as a nitrogen donor in nucleotide and amino acid biosynthesis in PANC-1. Our analysis provides a potentially comprehensive picture of metabolism in PANC-1, which may serve as the basis of new diagnostics and treatment of PDAC.
The rat kidney ablation and infarction (A/I) model of subtotal or 5/6th nephrectomy is the most commonly studied model of nondiabetic chronic kidney disease (CKD). The A/I kidney at 1 wk exhibits reductions in kidney function, as determined by glomerular filtration rate, and diminished metabolic efficiency as determined by oxygen consumption per sodium transport (QO2/TNa). As renoprotective AMPK activity is affected by metabolic changes and cellular stress, we evaluated AMPK activity in this model system. We show that these early pathophysiological changes are accompanied by a paradoxical decrease in AMPK activity. Over time, these kidney parameters progressively worsen with extensive kidney structural, functional, metabolic, and fibrotic changes observed at 4 wk after A/I. We show that induction of AMPK activity with either metformin or 5-aminoimidazole-4-carboxamide ribonucleotide increases AMPK activity in this model and also corrects kidney metabolic inefficiency, improves kidney function, and ameliorates kidney fibrosis and structural alterations. We conclude that AMPK activity is reduced in the subtotal nephrectomy model of nondiabetic CKD, that altered regulation of AMPK is coincident with the progression of disease parameters, and that restoration of AMPK activity can suppress the progressive loss of function characteristic of this model. We propose that induction of AMPK activity may prove an effective therapeutic target for the treatment of nondiabetic CKD.
Cu/Zn superoxide dismutase (SOD1) is an abundant enzyme that has been best studied as a regulator of antioxidant defense. Using the yeast Saccharomyces cerevisiae, we report that SOD1 transmits signals from oxygen and glucose to repress respiration. The mechanism involves SOD1-mediated stabilization of two casein kinase 1-gamma (CK1γ) homologs, Yck1p and Yck2p, required for respiratory repression. SOD1 binds a C-terminal degron we identified in Yck1p/Yck2p and promotes kinase stability by catalyzing superoxide conversion to peroxide. The effects of SOD1 on CK1γ stability are also observed with mammalian SOD1 and CK1γ and in a human cell line. Therefore, in a single circuit, oxygen, glucose, and reactive oxygen can repress respiration through SOD1/CK1γ signaling. Our data therefore may provide mechanistic insight into how rapidly proliferating cells and many cancers accomplish glucose-mediated repression of respiration in favor of aerobic glycolysis.
Highly conserved among eukaryotic cells, the AMP-activated kinase (AMPK) is a central regulator of carbon metabolism. To map the complete network of interactions around AMPK in yeast (Snf1) and to evaluate the role of its regulatory subunit Snf4, we measured global mRNA, protein and metabolite levels in wild type, Deltasnf1, Deltasnf4, and Deltasnf1Deltasnf4 knockout strains. Using four newly developed computational tools, including novel DOGMA sub-network analysis, we showed the benefits of three-level ome-data integration to uncover the global Snf1 kinase role in yeast. We for the first time identified Snf1's global regulation on gene and protein expression levels, and showed that yeast Snf1 has a far more extensive function in controlling energy metabolism than reported earlier. Additionally, we identified complementary roles of Snf1 and Snf4. Similar to the function of AMPK in humans, our findings showed that Snf1 is a low-energy checkpoint and that yeast can be used more extensively as a model system for studying the molecular mechanisms underlying the global regulation of AMPK in mammals, failure of which leads to metabolic diseases.
Nitric oxide (NO) is a product of L-arginine metabolism that suppresses cellular oxidative metabolism through the inhibition of tricarboxylic acid cycle and electron transport chain enzymes. The impact of NO synthase (NOS) activity on specific pathways of glucose metabolism in freshly harvested and overnight-cultured rat resident peritoneal macrophages, at rest and after stimulation with zymosan, was investigated using radiolabeled glucose. NOS activity was modulated through the L-arginine concentration in culture media and the use of its specific inhibitor, NG-monomethyl-L-arginine, and quantitated using radiolabeled L-arginine. Results demonstrated that NOS activity was associated with increased glucose disappearance, glycolysis, and hexose monophosphate shunt activity and, in line with the known inhibition of oxidative metabolism associated with the production of NO, with a decrease in the flux of glucose and butyrate carbon through the tricarboxylic acid cycle. In addition, the relative increase in glucose utilization that follows zymosan stimulation was enhanced by treatments that suppressed NOS activity. These results demonstrate that the characteristics of glucose metabolism by macrophages are, to a significant extent, determined by products of NOS.
AMP-activated protein kinase (AMPK) is an energy-sensing enzyme whose activity is inhibited in settings of insulin resistance. Exposure to a high glucose concentration has recently been shown to increase phosphorylation of AMPK at Ser(485/491) of its α1/α2 subunit; however, the mechanism by which it does so is not known. Diacylglycerol (DAG), which is also increased in muscle exposed to high glucose, activates a number of signaling molecules including protein kinase (PK)C and PKD1. We sought to determine whether PKC or PKD1 is involved in inhibition of AMPK by causing Ser(485/491) phosphorylation in skeletal muscle cells. C2C12 myotubes were treated with the PKC/D1 activator phorbol 12-myristate 13-acetate (PMA), which acts as a DAG mimetic. This caused dose- and time-dependent increases in AMPK Ser(485/491) phosphorylation, which was associated with a ∼60% decrease in AMPKα2 activity. Expression of a phosphodefective AMPKα2 mutant (S491A) prevented the PMA-induced reduction in AMPK activity. Serine phosphorylation and inhibition of AMPK activity were partially prevented by the broad PKC inhibitor Gö6983 and fully prevented by the specific PKD1 inhibitor CRT0066101. Genetic knockdown of PKD1 also prevented Ser(485/491) phosphorylation of AMPK. Inhibition of previously identified kinases that phosphorylate AMPK at this site (Akt, S6K, and ERK) did not prevent these events. PMA treatment also caused impairments in insulin-signaling through Akt, which were prevented by PKD1 inhibition. Finally, recombinant PKD1 phosphorylated AMPKα2 at Ser(491) in cell-free conditions. These results identify PKD1 as a novel upstream kinase of AMPKα2 Ser(491) that plays a negative role in insulin signaling in muscle cells.