AMP-activated kinase (AMPK)-generated signals in malignant melanoma cell growth and survival

Extensive studies over the years have shown that the AMP-activated kinase (AMPK) exhibits negative regulatory effects on the activation of the mammalian target of rapamycin (mTOR) signaling cascade. We examined the potential involvement of AMPK in the regulation of growth and survival of malignant melanoma cells. In studies using the AMPK activators AICAR or metformin, we found potent inhibitory effects of AMPK activity on the growth of SK-MEL-2 and SK-MEL-28 malignant melanoma cells. Induction of AMPK activity was also associated with inhibition of the ability of melanoma cells to form colonies in an anchorage-independent manner in soft agar, suggesting an important role of the pathway in the control of malignant melanoma tumorigenesis. Furthermore, AICAR-treatment resulted in malignant melanoma cell death and such induction of apoptosis was further enhanced by concomitant statin-treatment. Taken together, our results provide evidence for potent inhibitory effects of AMPK on malignant melanoma cell growth and survival and raise the potential of AMPK manipulation as a novel future approach for the treatment of malignant melanoma.     

AMPK/mTOR信号通路的研究进展

mTOR是细胞生长和增殖的中枢调控因子。mTOR形成2个不同的复合物mTORC1和mTORC2。mTORC1受多种信号调节,如生长因子、氨基酸和细胞能量,同时,mTORC1调节许多重要的细胞过程,包括翻译、转录和自噬。AMPK作为一种关键的生理能量传感器,是细胞和有机体能量平衡的主要调节因子,协调多种代谢途径,平衡能量的供应和需求,最终调节细胞和器官的生长。能量代谢平衡调控是由多个与之相关的信号通路所介导,其中AMPK/mTOR信号通路在细胞内共同构成一个合成代谢和分解代谢过程的开关。此外,AMPK/mTOR信号通路还是一个自噬的重要调控途径。本文着重于目前对AMPK和mTOR信号传导之间关系的了解,讨论了AMPK/mTOR在细胞和有机体能量稳态中的作用。     

Reciprocal Regulation of AMP-activated Protein Kinase and Phospholipase D

AMP-activated protein kinase (AMPK), a critical sensor of energy sufficiency, acts as central metabolic switch in cell metabolism. Once activated by low energy status, AMPK phosphorylates key regulatory substrates and turns off anabolic biosynthetic pathways. In contrast, the mammalian/mechanistic target of rapamycin (mTOR) is active when there are sufficient nutrients for anabolic reactions. A critical factor regulating mTOR is phosphatidic acid (PA), a central metabolite of membrane lipid biosynthesis and the product of the phospholipase D (PLD)-catalyzed hydrolysis of phosphatidylcholine. PLD is a downstream target of the GTPase Rheb, which is turned off in response to AMPK via the tuberous sclerosis complex. Although many studies have linked AMPK with mTOR, very little is known about the connection between AMPK and PLD. In this report, we provide evidence for reciprocal regulation of PLD by AMPK and regulation of AMPK by PLD and PA. Suppression of AMPK activity led to an increase in PLD activity, and conversely, activation of AMPK suppressed PLD activity. Suppression of PLD activity resulted in elevated AMPK activity. Exogenously supplied PA abolished the inhibitory effects of elevated AMPK activity on mTOR signaling. In contrast, exogenously supplied PA could not overcome the effect AMPK activation if either mTOR or Raptor was suppressed, indicating that the inhibitory effects of PLD and PA on AMPK activity are mediated by mTOR. These data suggest a reciprocal feedback mechanism involving AMPK and the PLD/mTOR signaling node in cancer cells with therapeutic implications.     

Cadmium induction of reactive oxygen species activates the mTOR pathway, leading to neuronal cell death

Cadmium (Cd), a highly toxic environmental pollutant, induces neurodegenerative diseases. Recently we have demonstrated that Cd induces neuronal apoptosis in part through activation of the mammalian target of rapamycin (mTOR) pathway. However, the underlying mechanism is unknown. Here we show that Cd induces the generation of reactive oxygen species (ROS) by upregulating the expression of NADPH oxidase 2 and its regulatory proteins (p22(phox), p67(phox), p40(phox), p47(phox), and Rac1) in PC12 and SH-SY5Y cells. Cd induction of ROS contributed to the activation of mTOR signaling, as pretreatment with N-acetyl-l-cysteine (NAC), a ROS scavenger, prevented this event. Further studies reveal that Cd induction of ROS increased phosphorylation of the type I insulin-like growth factor receptor (IGFR) β subunit, which was abrogated by NAC. Wortmannin, a phosphoinositide 3'-kinase (PI3K) inhibitor, partially attenuated Cd-induced phosphorylation of Akt, p70 S6 kinase 1, and eukaryotic initiation factor 4E-binding protein 1, as well as apoptosis of the neuronal cells. In addition, overexpression of wild-type phosphatase and tensin homologue deleted on chromosome 10 (PTEN) or pretreatment with aminoimidazole carboxamide ribonucleotide, an AMP-activated protein kinase (AMPK) activator, partially prevented Cd-induced ROS and activation of the mTOR pathway, as well as cell death. The results indicate that Cd induction of ROS activates mTOR signaling, leading to neuronal cell death, in part by activating the positive regulators IGFR/PI3K and by inhibiting the negative regulators PTEN/AMPK. The findings suggest that inhibitors of PI3K and mTOR, activators of AMPK, or antioxidants may be exploited for the prevention of Cd-induced neurodegenerative diseases.     

GYY4137, a novel hydrogen sulfide-releasing molecule,likely protects against high glucose-induced cytotoxicity by activation of the AMPK/mTOR signal pathway in H9c2 cells

Diabetic cardiomyopathy (DCM) has become a major cause of diabetes-related morbidity and mortality. Increasing evidences have proved that hydrogen sulfide (H₂S) fulfills a positive role in regulating diabetic myocardial injury. The present study was designed to determine whether GYY4137, a novel H₂S-releasing molecule, protected H9c2 cells against high glucose (HG)-induced cytotoxicity by activation of the AMPK/mTOR signal pathway. H9c2 cells were incubated in normal glucose (5.5 mM), 22, 33, and 44 mM glucose for 24 h to mimic the hyperglycemia in DCM in vitro. Then we added 50, 100, and 200 μM GYY4137, and measured the cell viability, lactate dehydrogenase (LDH) enzyme activity, and mitochondrial membrane potential (MMP). 0.5 mM 5-amino-4-imidazole-carboxamide riboside (AICAR, an AMPK activator) and 1 mM adenine 9-β-d-arabinofuranoside (Ara-A, an AMPK inhibitor) were used to identity whether the AMPK/mTOR signal pathway was involved in GYY4137-mediated cardioprotection. We demonstrated that HG decreased cell viability and increased LDH enzyme activity in a concentration-dependent manner. 33 mM HG treatment for 24 h was chosen as our model group for further study. Both 100 and 200 μM GYY4137 treatments significantly attenuated HG-induced cell viability decrement, LDH enzyme activity increase, and MMP collapse. AICAR had similar effects to GYY4137 treatment while Ara-A attenuated GYY4137-mediated cardioprotection. Importantly, both GYY4137 and AICAR increased AMPK phosphorylation and decreased mTOR phosphorylation compared with the HG model group while Ara-A attenuated GYY4137-mediated AMPK phosphorylation increase and mTOR phosphorylation decrement. In conclusion, we propose that GYY4137 likely protects against HG-induced cytotoxicity by activation of the AMPK/mTOR signal pathway in H9c2 cells.     

雌激素通过GPR30-AMPK-mTOR通路诱导Ishikawa细胞自噬增强细胞活力

雌激素是子宫内膜癌发生发展的重要诱导因子,但关于其在子宫内膜癌中的作用机制目前仍不明确。自噬对细胞的存活具有重要的调节作用,研究发现其在子宫内膜癌发生发展的过程中起重要的调节作用。本文通过探讨雌激素对子宫内膜癌细胞自噬的影响,深入地了解雌激素促进子宫内膜发展的机制,并明确GPR30-MPK-mTOR 通路在其中的作用。MTT及透视电镜的结果显示,雌激素可以诱导细胞的自噬及增强细胞的活力,而这种作用具有一定的时间及浓度依赖性。同时,蛋白质印迹及实时定量PCR结果显示雌激素可以促进LC3、p-AMPK的表达,并且抑制P62、p-mTOR的表达,表明雌激素可以激活AMPK/mTOR通路。沉默G蛋白偶联受体30（GPR30）后,结果显示雌激素诱导细胞的自噬及细胞活力的作用被逆转,并且可以抑制AMPK/mTOR通路的激活,而G-1结果与之相反,表明雌激素通过GPR30激活AMPK/mTOR通路,诱导自噬及细胞活力。此外,加入AMPK抑制剂compound C,可以抑制雌激素诱导细胞的自噬及细胞活力的能力,并且促进P62、p-mTOR表达,降低LC3及p-AMPK表达,表明雌激素通过激活AMPK/mTOR激活细胞自噬及增强细胞活力。同时细胞预先加入自噬抑制剂3-MA或转染ATG5siRNA,可以降低雌激素增强细胞的活力,表明雌激素通过诱导自噬增强细胞活力。综合以上结果,雌激素通过GPR30-AMPK-mTOR通路诱导细胞的自噬增强细胞的活力。     

Chronic paraplegia-induced muscle atrophy downregulates the mTOR/S6K1 signaling pathway.

Ribosomal S6 kinase 1 (S6K1) is a downstream component of the mammalian target of rapamycin (mTOR) signaling pathway and plays a regulatory role in translation initiation, protein synthesis, and muscle hypertrophy. AMP-activated protein kinase (AMPK) is a cellular energy sensor, a negative regulator of mTOR, and an inhibitor of protein synthesis. The purpose of this study was to determine whether the hypertrophy/cell growth-associated mTOR pathway was downregulated during muscle atrophy associated with chronic paraplegia. Soleus muscle was collected from male Sprague-Dawley rats 10 wk following complete T(4)-T(5) spinal cord transection (paraplegic) and from sham-operated (control) rats. We utilized immunoprecipitation and Western blotting techniques to measure upstream [AMPK, Akt/protein kinase B (PKB)] and downstream components of the mTOR signaling pathway [mTOR, S6K1, SKAR, 4E-binding protein 1 (4E-BP1), and eukaryotic initiation factor (eIF) 4G and 2alpha]. Paraplegia was associated with significant soleus muscle atrophy (174 +/- 8 vs. 240 +/- 13 mg; P < 0.05). There was a reduction in phosphorylation of mTOR, S6K1, and eIF4G (P < 0.05) with no change in Akt/PKB or 4E-BP1 (P > 0.05). Total protein abundance of mTOR, S6K1, eIF2alpha, and Akt/PKB was decreased, and increased for SKAR (P < 0.05), whereas 4E-BP1 and eIF4G did not change (P > 0.05). S6K1 activity was significantly reduced in the paraplegic group (P < 0.05); however, AMPKalpha2 activity was not altered (3.5 +/- 0.4 vs. 3.7 +/- 0.5 pmol x mg(-1) x min(-1), control vs. paraplegic rats). We conclude that paraplegia-induced muscle atrophy in rats is associated with a general downregulation of the mTOR signaling pathway. Therefore, in addition to upregulation of atrophy signaling during muscle wasting, downregulation of muscle cell growth/hypertrophy-associated signaling appears to be an important component of long-term muscle loss.     

Therapeutic metformin/AMPK activation blocked lymphoma cell growth via inhibition of mTOR pathway and induction of autophagy

Adenosine monophosphate-activated protein kinase (AMPK) acts as a major sensor of cellular energy status in cancers and is critically involved in cell sensitivity to anticancer agents. Here, we showed that AMPK was inactivated in lymphoma and related to the upregulation of the mammalian target of rapamycin (mTOR) pathway. AMPK activator metformin potentially inhibited the growth of B- and T-lymphoma cells. Strong antitumor effect was also observed on primary lymphoma cells while sparing normal hematopoiesis ex vivo. Metformin-induced AMPK activation was associated with the inhibition of the mTOR signaling without involving AKT. Moreover, lymphoma cell response to the chemotherapeutic agent doxorubicin and mTOR inhibitor temsirolimus was significantly enhanced when co-treated with metformin. Pharmacologic and molecular knock-down of AMPK attenuated metformin-mediated lymphoma cell growth inhibition and drug sensitization. In vivo, metformin induced AMPK activation, mTOR inhibition and remarkably blocked tumor growth in murine lymphoma xenografts. Of note, metformin was equally effective when given orally. Combined treatment of oral metformin with doxorubicin or temsirolimus triggered lymphoma cell autophagy and functioned more efficiently than either agent alone. Taken together, these data provided first evidence for the growth-inhibitory and drug-sensitizing effect of metformin on lymphoma. Selectively targeting mTOR pathway through AMPK activation may thus represent a promising new strategy to improve treatment of lymphoma patients.     

Suppression of programmed cell death 4 (PDCD4) protein expression by BCR-ABL-regulated engagement of the mTOR/p70 S6 kinase pathway

There is accumulating evidence that mammalian target of rapamycin (mTOR)-activated pathways play important roles in cell growth and survival of BCR-ABL-transformed cells. We have previously shown that the mTOR/p70 S6 kinase (p70 S6K) pathway is constitutively activated in BCR-ABL transformed cells and that inhibition of BCR-ABL kinase activity by imatinib mesylate abrogates such activation. We now provide evidence for the existence of a novel regulatory mechanism by which BCR-ABL promotes cell proliferation, involving p70 S6K-mediated suppression of expression of programmed cell death 4 (PDCD4), a tumor suppressor protein that acts as an inhibitor of cap-dependent translation by blocking the translation initiation factor eIF4A. Our data also establish that second generation BCR-ABL kinase inhibitors block activation of p70 S6K and downstream engagement of the S6 ribosomal protein in BCR-ABL transformed cells. Moreover, PDCD4 protein expression is up-regulated by inhibition of the BCR-ABL kinase in K562 cells and BaF3/BCR-ABL transfectants, suggesting a mechanism for the generation of the proapoptotic effects of such inhibitors. Knockdown of PDCD4 expression results in reversal of the suppressive effects of nilotinib and imatinib mesylate on leukemic progenitor colony formation, suggesting an important role for this protein in the generation of antileukemic responses. Altogether, our studies identify a novel mechanism by which BCR-ABL may promote leukemic cell growth, involving sequential engagement of the mTOR/p70 S6K pathway and downstream suppression of PDCD4 expression.     

8-Chloroadenosine Sensitivity in Renal Cell Carcinoma Is Associated with AMPK Activation and mTOR Pathway Inhibition

The adenosine analog 8-chloroadenosine has been shown to deplete ATP and inhibit tumor growth in hematological malignancies as well as in lung and breast cancer cell lines. We investigated effects of 8-chloroadenosine on clear cell (cc) renal cell carcinoma (RCC) cell lines. 8-chloroadenosine was effective against ccRCC cell viability in vitro, with IC₅₀ ranging from 2 μM in the most sensitive CAKI-1 to 36 μM in the most resistant RXF-393. Proteomic analysis by reverse-phase protein array revealed that 8-chloroadenosine treatment leads to inhibition of the mTOR pathway. In time-course experiments, 8-chloroadenosine treatment rapidly activated AMPK, measured by AMPK and ACC phosphorylation, and subsequently caused dephosphorylation of p70S6K and ribosomal protein RPS6 in the sensitive cell lines. However, in the resistant cell lines, AMPK activity and the mTOR pathway were unaffected by the treatment. We also noted that the resistant cell lines had elevated basal levels of phospho RPS6 and AKT. Inhibition of PI3K pathway enhanced the efficacy of 8-chloroadenosine across all cell lines. Our observations indicate that 8-chloroadenosine activity is associated with inhibition of the mTOR pathway, and that phospho RPS6 and PI3K pathway activation status may determine resistance. Among solid tumors, RCC is one of the few susceptible to mTOR inhibition. We thus infer that 8-chloroadenosine may be effective in RCC by activating AMPK and inhibiting the mTOR pathway.     

Antagonistic control of muscle cell size by AMPK and mTORC1

Nutrition and physical activity have profound effects on skeletal muscle metabolism and growth. Regulation of muscle mass depends on a thin balance between growth-promoting and growth-suppressing factors. Over the past decade, the mammalian target of rapamycin (mTOR) kinase has emerged as an essential factor for muscle growth by mediating the anabolic response to nutrients, insulin, insulin-like growth factors and resistance exercise. As opposed to the mTOR signaling pathway, the AMP-activated protein kinase (AMPK) is switched on during starvation and endurance exercise to upregulate energy-conserving processes. Recent evidence indicates that mTORC1 (mTOR Complex 1) and AMPK represent two antagonistic forces governing muscle adaption to nutrition, starvation and growth stimulation. Animal knockout models with impaired mTORC1 signaling showed decreased muscle mass correlated with increased AMPK activation. Interestingly, AMPK inhibition in p70S6K-deficient muscle cells restores cell growth and sensitivity to nutrients. Conversely, muscle cells lacking AMPK have increased mTORC1 activation with increased cell size and protein synthesis rate. We also demonstrated that the hypertrophic action of MyrAkt is enhanced in AMPK-deficient muscle, indicating that AMPK acts as a negative feedback control to restrain muscle hypertrophy. Our recent results extend this notion by showing that AMPKα1, but not AMPKα2, regulates muscle cell size through the control of mTORC1 signaling. These results reveal the diverse functions of the two catalytic isoforms of AMPK, with AMPKα1 playing a predominant role in the control of muscle cell size and AMPKα2 mediating muscle metabolic adaptation. Thus, the crosstalk between AMPK and mTORC1 signaling is a highly regulated way to control changes in muscle growth and metabolic rate imposed by external cues.     

Akt activates the mammalian target of rapamycin by regulating cellular ATP level and AMPK activity

The serine/threonine kinase Akt is an upstream positive regulator of the mammalian target of rapamycin (mTOR). However, the mechanism by which Akt activates mTOR is not fully understood. The known pathway by which Akt activates mTOR is via direct phosphorylation and inhibition of tuberous sclerosis complex 2 (TSC2), which is a negative regulator of mTOR. Here we establish an additional pathway by which Akt inhibits TSC2 and activates mTOR. We provide for the first time genetic evidence that Akt regulates intracellular ATP level and demonstrate that Akt is a negative regulator of the AMP-activated protein kinase (AMPK), which is an activator of TSC2. We show that in Akt1/Akt2 DKO cells AMP/ATP ratio is markedly elevated with concomitant increase in AMPK activity, whereas in cells expressing activated Akt there is a dramatic decrease in AMP/ATP ratio and a decline in AMPK activity. Currently, the Akt-mediated phosphorylation of TSC2 and the inhibition of AMPK-mediated phosphorylation of TSC2 are viewed as two separate pathways, which activate mTOR. Our results demonstrate that Akt lies upstream of these two pathways and induces full inhibition of TSC2 and activation of mTOR both through direct phosphorylation and by inhibition of AMPK-mediated phosphorylation of TSC2. We propose that the activation of mTOR by Akt-mediated cellular energy and inhibition of AMPK is the predominant pathway by which Akt activates mTOR in vivo.     

AMPK- and p62/SQSTM1-dependent autophagy mediate Resveratrol-induced cell death in chronic myelogenous leukemia

Resveratrol (RSV) is an attractive candidate for cancer therapy via its ability to intervene at different levels in the AMPK/mTOR pathway. Indeed, RSV is unique in its capacity to inhibit both mTOR and S6 kinase and to activate AMPK. Our recent data reveals that RSV triggered autophagic cell death (ACD) in Chronic Myelogenous Leukemia (CML) cells, via both AMPK activation and JNK-mediated p62/SQSTM1 expression. Here we discuss how Resveratrol can mediate ACD in CML cells and the possibility of utilizing the AMPK/mTOR and JNK/p62 pathways via Resveratrol to combat CML and other hematopoietic malignancies.     

Compound C induces protective autophagy in cancer cells through AMPK inhibition-independent blockade of Akt/mTOR pathway

In the present study, we report that compound C, an inhibitor of a key intracellular energy sensor AMP-activated protein kinase (AMPK), can induce autophagy in cancer cells. The induction of autophagy in U251 human glioma cell line was demonstrated by acridine orange staining of intracellular acidic vesicles, Beclin 1 induction, p62 decrease and conversion of LC3-I to autophagosome-associated LC3-II in the presence of proteolysis inhibitors. The presence of autophagosome-like vesicles was confirmed by transmission electron microscopy. Compound C-mediated inhibition of AMPK and raptor in U251 cells was associated with paradoxical decrease in phosphorylation of AMPK/raptor-repressed mTOR, a major negative regulator of autophagy, and its downstream target p70S6K. The phosphorylation of an mTOR activator Akt and the PI3K-activating kinase Src was also impaired in compound C-treated cells. The siRNA-mediated AMPK silencing did not reduce the activity of the Akt/mTOR/p70S6K pathway and AMPK activators metformin and AIC AR failed to block compound C-induced autophagy. Autophagy inhibitors bafilomycin and chloroquine significantly increased the cytotoxicity of compound C towards U251 cells, as confirmed by increase in lactate dehydrogenase release, DNA fragmentation and caspase-3 activation. Similar effects of compound C were also observed in C6 rat glioma, L929 mouse fibrosarcoma and B16 mouse melanoma cell lines. Since compound C has previously been reported to suppress AMPK-dependent autophagy in different cell types, our findings suggest that the effects of compound C on autophagy might be dose-, cell type- and/or context-dependent. By demonstrating the ability of compound C to induce autophagic response in cancer cells via AMPK inhibition-independent downregulation of Akt/mTOR pathway, our results warrant caution when using compound C to inhibit AMPK-dependent cellular responses, but also support further exploration of compound C and related molecules as potential anticancer agents.     

Compound C induces protective autophagy in cancer cells through AMPK inhibition-independent blockade of Akt/mTOR pathway

In the present study, we report that compound C, an inhibitor of a key intracellular energy sensor AMP-activated protein kinase (AMPK), can induce autophagy in cancer cells. The induction of autophagy in U251 human glioma cell line was demonstrated by acridine orange staining of intracellular acidic vesicles, Beclin 1 induction, p62 decrease and conversion of LC3-I to autophagosome-associated LC3-II in the presence of proteolysis inhibitors. The presence of autophagosome-like vesicles was confirmed by transmission electron microscopy. Compound C-mediated inhibition of AMPK and raptor in U251 cells was associated with paradoxical decrease in phosphorylation of AMPK/raptor-repressed mTOR, a major negative regulator of autophagy, and its downstream target p70S6K. The phosphorylation of an mTOR activator Akt and the PI3K-activating kinase Src was also impaired in compound C-treated cells. The siRNA-mediated AMPK silencing did not reduce the activity of the Akt/mTOR/p70S6K pathway and AMPK activators metformin and AIC AR failed to block compound C-induced autophagy. Autophagy inhibitors bafilomycin and chloroquine significantly increased the cytotoxicity of compound C towards U251 cells, as confirmed by increase in lactate dehydrogenase release, DNA fragmentation and caspase-3 activation. Similar effects of compound C were also observed in C6 rat glioma, L929 mouse fibrosarcoma and B16 mouse melanoma cell lines. Since compound C has previously been reported to suppress AMPK-dependent autophagy in different cell types, our findings suggest that the effects of compound C on autophagy might be dose-, cell type- and/or context-dependent. By demonstrating the ability of compound C to induce autophagic response in cancer cells via AMPK inhibition-independent downregulation of Akt/mTOR pathway, our results warrant caution when using compound C to inhibit AMPK-dependent cellular responses, but also support further exploration of compound C and related molecules as potential anticancer agents.     

LOC100996425 acts as a promoter in prostate cancer by mediating hepatocyte nuclear factor 4A and the AMPK/mTOR pathway

The involvement of long non-coding RNAs (lncRNAs), differentially expressed genes and signals in prostate cancer (PCa) continues to be a subject of investigation. This study determined effects of LOC100996425 on human PCa by targeting hepatocyte nuclear factor 4A (HNF4A) via the AMPK/mTOR pathway. PCa and adjacent normal tissues were obtained to characterize expression pattern of LOC100996425, HNF4A and the AMPK/mTOR pathway-related genes. Then, the target gene of LOC100996425 was determined with lncRNA target prediction website and further verification was obtained through luciferase assay and ribonucleoprotein immunoprecipitation. After that, PCa cells were introduced with LOC100996425, HNF4A, siLOC100996425 or siHNF4A to explore the specific significance of LOC100996425 and HNF4A in PCa. The mechanism associated with AMPK/mTOR pathway was investigated using AMPK inhibitor or activator. LOC100996425 was up-regulated, while HNF4A was down-regulated in the PCa tissues. HNF4A was a target gene of LOC100996425. PCa cells transfected with either siLOC100996425 or HNF4A displayed reduced rates of PCa cell proliferation and migration while elevating cell apoptosis. HNF4A overexpression reversed the promotive effect of LOC100996425 overexpression on PCa. The activation of AMPK pathway involved in the cancer progression mediated by LOC100996425. Down-regulation of LOC100996425 retards progression of PCa through HNF4A-mediated AMPK/mTOR pathway.     

Time course changes in signaling pathways and protein synthesis in C2C12 myotubes following AMPK activation by AICAR

The role of the AMP-activated kinase (AMPK) as a metabolic sensor in skeletal muscle has been far better characterized for glucose and fat metabolism than for protein metabolism. Therefore, the studies presented here were designed to examine the effects of 5-aminoimidazole-4-carboxamide-1-beta-d-ribonucleoside (AICAR)-induced AMPK signaling on effector mechanisms of mRNA translation and protein synthesis in cultures of C(2)C(12) myotubes. The findings show that, following AICAR (2 mM) treatment, AMPK phosphorylation was increased within 15 min and remained elevated throughout a 60-min time course. In association with the increase in AMPK phosphorylation, global rates of protein synthesis declined to 90, 70, and 63% of the control values at the 15-, 30-, and 60-min time points, respectively. By 60 min, polysomes disaggregated into free ribosomal subunits, suggesting an inhibition of initiation of mRNA translation. However, phosphorylation of eukaryotic elongation factor 2 was increased at 15 and 30 min but then declined to control values by 60 min, suggesting a transient inhibition of translation elongation. The decline in protein synthesis and changes in mRNA translation were associated with a repression of the mammalian target of rapamycin (mTOR) signaling pathway, as indicated by increased association of Hamartin with Tuberin, increased association of regulatory associated protein of mTOR with mTOR, and dephosphorylation of the downstream targets ribosomal protein S6 kinase-1 and eukaryotic initiation factor 4E-binding protein-1. They were also associated with activation of the MAPK signaling pathway, as indicated by increased phosphorylation of MEK1/2 and ERK1/2 and the downstream target eIF4E. Overall, the data support the conclusion that AICAR-induced AMPK activation suppresses protein synthesis through concurrent repression of mTOR signaling and activation of MAPK signaling, the combination of which modulates transient changes in the initiation and elongation phases of mRNA translation.     

GHRH antagonist MZ-5-156 increases the expression of AMPK in A549 lung cancer cells

AMP-activated protein kinase (AMPK) regulates cellular proliferation, growth and metabolism. Targeted activation of AMPK is considered an important therapeutic strategy for cancer treatment. To evaluate the effect of growth hormone-releasing hormone (GHRH) and its antagonist MZ-5-156 on the phosphorylation of AMPK and other related regulatory intracellular proteins we employed human non-small cell lung cancer cell line A549, which expresses GHRH receptors. Treatment of A549 cells with GHRH antagonist decreased cell proliferation and activated AMPK as well as glycogen synthase kinase (GSK)3β. Furthermore, MZ-5-156 inhibited Akt, the mammalian target of rapamycin (mTOR) and its downstream target eIF4E which controls protein synthesis and cell growth. GHRH(1-29)NH2 counteracted all these effects. HeLa human endometrial cancer cells which do not express any GHRH receptors were used as a negative control and GHRH did not induce the AMPK activation in these cells. Our results demonstrate for the first time that GHRH antagonists can regulate the AMPK metabolic pathway, which is crucial for the growth of non-small cell lung cancer and other major cancers.     

AMPK/LKB1 Signaling in Epithelial Cell Polarity and Cell Division

Cells must coordinate diverse processes including cell division, cell migration, and cell polarity with the cell’s metabolic status. How single molecules coordinate these seemingly distinct cell biological events remains relatively unexplored. AMP-activated protein kinase (AMPK) sits at a unique position as a proposed energy sensor that can interface with diverse signaling molecules ranging from LKB1 to mammalian target of rapamycin (mTOR), affecting processes from ribosomal biogenesis to actin regulation. Determining biologically relevant direct kinase targets remains challenging. Alternatively, one can genetically inactivate a kinase and subsequently characterize cellular and whole animal phenotypes without the kinase’s activity. Recent genetic studies inactivating AMPK activity in Drosophila indicate unanticipated roles for AMPK as a regulator of epithelial polarity, consistent with known roles of an upstream activator, LKB1 as a PAR (partioning defective) mutant in Caenorhabditis elegans and polarity regulator. Additional genetic analyses demonstrate that both AMPK and LKB1 function are required for faithful chromosomal segregation during mitosis. At least some of these apparently divergent phenotypes may be mediated through myosin regulatory light chain, and presumably the acto-myosin complex, which can affect both polarity and cell division. Chromosomal integrity defects could also be consistent with LKB1’s role as a known human tumor suppressor gene. Elucidating the molecular players that interface with AMPK and their potential energy dependent regulation remains an important challenge to fully understand AMPK signaling.