Ursolic acid-induced AMP-activated protein kinase (AMPK) activation contributes to growth inhibition and apoptosis in human bladder cancer T24 cells

Ursolic acid (UA) has shown the anti-tumor properties against a number of human cancers both in vivo and in vitro, however, its effect in bladder cancer and the corresponding mechanisms of action remain largely unknown. Here we found that UA dose-dependently induced growth inhibition and apoptosis in human bladder cancer T24 cells, and activation of AMP-activated protein kinase (AMPK) may contribute to the process. Our Western-blot results demonstrated a significant AMPK activation after UA treatment in T24 cells. Notably, knockdown of AMPKα by the targeted shRNA largely inhibited UA-induced T24 cell growth inhibition and apoptosis, while an AMPK activator 5-aminoimidazole-4-carboxamide-1-β-D-ribofuranoside (AICAR) or a constitutively active form of AMPK mimic UA's effect. We found the ceramide level was increased after UA treatment in T24 cells, and UA-induced AMPK activation and T24 cell apoptosis were inhibited by ceramide synthase inhibitor fumonisin B1, and was enhanced by exogenously adding cell permeable short-chain ceramide (C6), suggesting that ceramide might serve as an upstream signal for AMPK activation. Further, activation of AMPK by UA promoted c-Jun N-terminal kinase (JNK) activation, but inhibited mTOR complex 1 (mTORC1) signaling to cause survivin down-regulation. Our study suggests that activation of AMPK by UA contributes to growth inhibition and apoptosis in human bladder cancer cells.     

AMP-activated protein kinase (AMPK) activation is involved in chrysin-induced growth inhibition and apoptosis in cultured A549 lung cancer cells

Here we show that chrysin induces growth inhibition and apoptosis in cultured lung cancer A549 cells, and activation of AMP-activated protein kinase (AMPK) may contribute to this process. Our Western-blots results demonstrated a significant AMPK activation after chrysin treatment in A549 cells. Inhibition of AMPK by shRNA-mediated gene silencing, or by its inhibitor, diminished chrysin-induced A549 cell growth inhibition and apoptosis. Forced activation of AMPK by introducing a constitutively active form of AMPKα (CA-AMPKα), or by its activators, mimicked chrysin's effect. For mechanism analysis, we found chrysin inhibited Akt/mammalian target of rapamycin (mTOR) activation, and knocking-down of AMPK by shRNA almost reversed this effect. Finally, we observed that a relative low dose of chrysin enhanced doxorubicin-induced AMPK activation to promote A549 cell apoptosis. Our study suggests that activation of AMPK by chrysin contributes to Akt suppression, growth inhibition and apoptosis in human lung cancer cells, and agents that could activate AMPK may serve as useful adjuvants for traditional chemotherapy against lung cancer.     

Plumbagin-induced apoptosis of human breast cancer cells is mediated by inactivation of NF-kappaB and Bcl-2

Breast cancer remains the major cause of cancer-related deaths in women world-wide. The heterogeneity of breast cancer has further complicated the progress of target-based therapies. Triple negative breast cancers, lacking estrogen receptor, progesterone receptor and the Her-2/neu (ErbB2), represent a highly aggressive breast cancer subtype, that are difficult to treat. Pleiotropic agents, such as those found in nature, can target receptor-positive as well as receptor-negative cancer cells, suggesting that such agents could have significant impact in breast cancer prevention and/or therapy. Plumbagin (5-hydroxy-2-methyl-1, 4-naphthoquinone) is one such agent which has anti-tumor activity against several cancers. However, its mechanism of action against breast cancer is not clearly understood. We hypothesized that plumbagin may act as an effective agent against breast cancer especially triple negative breast cancer. We tested our hypothesis using ER-positive MCF-7 and ER-negative MDA-MB-231 (triple negative) breast cancer cells, and we found that plumbagin significantly inhibits the growth of breast cancer cells with no effect on normal breast epithelial cells. We also found that plumbagin induces apoptosis with concomitant inactivation of Bcl-2 and the DNA binding activity of NF-kappaB. Bcl-2 over-expression resulted in attenuation of plumbagin-induced effects, suggesting that the inhibition of cell growth and induction of apoptosis by plumbagin is in part due to inactivation of NF-kappaB/Bcl-2 pathway. To our knowledge, this is the first report, showing mechanistic and cancer cell specific apoptosis-inducing effects of plumbagin in breast cancer cells, suggesting the potential role of plumbagin in the prevention and/or treatment of breast cancer.     

Activation of AMP-activated protein kinase is involved in vincristine-induced cell apoptosis in B16 melanoma cell

The molecular basis for induction of apoptosis in melanoma cells by vincristine remains unknown. Here we tested the potential involvement of AMP-activated protein kinase (AMPK) in this process. We found for the first time that vincristine induces AMPK activation (AMPKα, Thr 172) and Acetyl-CoA carboxylase (ACC, Ser 79) (a downstream molecular target of AMPK) phosphorylation in cultured melanoma cells in vitro. Reactive oxygen species (ROS) dependent LKB1 activation serves as the upstream signal for AMPK activation. AMPK inhibitor (compound C) or AMPKα siRNA knockdown inhibits vincristine induced B16 melanoma cell apoptosis, while AMPK activator 5-aminoimidazole-4-carboxamide-1-β-riboside (AICAR) enhances it. AMPK activation is involved in vincristine induced p53 phosphorylation and stabilization, the latter is known to mediate melanoma cell apoptosis. Further, activation of AMPK by vincristine inhibits mTOR Complex 1 (mTORC1) in B16 melanoma cells, which serves as another important mechanism to induce melanoma cell apoptosis. Our study provides new insights into understanding the cellular and molecular mechanisms of vincristine induced cancer cell death/apoptosis. We suggest that combining AMPK activator AICAR with vincristine may have potential to be used as a new therapeutic intervention against melanoma.     

Identification of an Annonaceous Acetogenin Mimetic,AA005, as an AMPK Activator and Autophagy Inducer in Colon Cancer Cells

Annonaceous acetogenins, a large family of naturally occurring polyketides isolated from various species of the plant genus Annonaceae, have been found to exhibit significant cytotoxicity against a variety of cancer cells. Previous studies showed that these compounds could act on the mitochondria complex-I and block the corresponding electron transport chain and terminate ATP production. However, more details of the mechanisms of action remain ambiguous. In this study we tested the effects of a set of mimetics of annonaceous acetogenin on some cancer cell lines, and report that among them AA005 exhibits the most potent antitumor activity. AA005 depletes ATP, activates AMP-activated protein kinase (AMPK) and inhibits mTOR complex 1 (mTORC1) signal pathway, leading to growth inhibition and autophagy of colon cancer cells. AMPK inhibitors compound C and inosine repress, while AMPK activator AICAR enhances, AA005-caused proliferation suppression and subsequent autophagy of colon cancer cells. AA005 enhances the ATP depletion and AMPK activation caused by 2-deoxyglucose, an inhibitor of mitochondrial respiration and glycolysis. AA005 also inhibits chemotherapeutic agent cisplatin-triggered up-regulation of mTOR and synergizes with this drug in suppression of proliferation and induction of apoptosis of colon cancer cells. These data indicate that AA005 is a new metabolic inhibitor which exhibits therapeutic potentials in colon cancer.     

Activation of AMP-activated protein kinase by temozolomide contributes to apoptosis in glioblastoma cells via p53 activation and mTORC1 inhibition

Methylating drugs such as temozolomide (TMZ) are widely used in the treatment of brain tumors including malignant glioblastoma. The mechanism of TMZ-induced glioblastoma cell death and apoptosis, however, is not fully understood. Here, we tested the potential involvement of AMP-activated protein kinase (AMPK) in this process. We found that methylating agents TMZ and N-methyl-N'-nitro-N-nitrosoguanidine induce AMPK activation in primary cultured human glioblastoma and glioblastoma cell lines. TMZ-induced O(6)-methylguanine production is involved in AMPK activation. O(6)-benzylguanine, an O(6)-methylguanine-DNA methyltransferase inhibitor, enhances TMZ-induced O(6)-methylguanine production, leading to enhanced reactive oxygen species production, which serves as an upstream signal for AMPK activation. Activation of AMPK is involved in TMZ-induced glioblastoma cell death and apoptosis. AMPK inhibitor (Compound C) or AMPKα siRNA knockdown inhibits TMZ-induced glioblastoma cell death and apoptosis, whereas AMPK activator 5-aminoimidazole-4-carboxamide-1-β-d-ribofuranoside enhances it. In further studies, we found that activation of AMPK is involved in TMZ-induced p53 activation and subsequent p21, Noxa, and Bax up-regulation. Activation of AMPK by TMZ also inhibits mTOR complex 1 (mTORC1) signaling and promotes anti-apoptosis protein Bcl-2 down-regulation, which together mediate TMZ-induced pro-cell apoptosis effects. Our study suggests that activation of AMPK by TMZ contributes to glioblastoma cell apoptosis, probably by promoting p53 activation and inhibiting mTORC1 signaling.     

Capsaicin induces apoptosis in human osteosarcoma cells through AMPK-dependent and AMPK-independent signaling pathways

Recent studies have focused on the anti-tumor activity of capsaicin. However, the potential effects of capsaicin in osteosarcoma cells and the underlying mechanisms are not fully understood. In the current study, we observed that capsaicin-induced growth inhibition and apoptosis in cultured osteosarcoma cells (U2OS and MG63), which were associated with a significant AMP-activated protein kinase (AMPK) activation. AMPK inhibition by compound C or RNA interference suppressed capsaicin-induced cytotoxicity, while AMPK activators (AICAR and A769662) promoted osteosarcoma cell death. For the mechanism study, we found that AMPK activation was required for capsaicin-induced mTORC1 (mTOR complex 1) inhibition, B cell lymphoma 2 (Bcl-2) downregulation and Bax upregulation in MG63 cells. Capsaicin administration induced p53 activation, mitochondrial translocation and Bcl-2 killer association, such effects were dependent on AMPK activation. Interestingly, we observed a significant pro-apoptotic c-Jun NH2-terminal kinases activation by capsaicin in MG63 cells, which appeared to be AMPK independent. In conclusion, capsaicin possessed strong efficacy against human osteosarcoma cells. Molecular studies revealed that capsaicin activated AMPK-dependent and AMPK-independent signalings to mediate cell apoptosis. The results of this study should have significant translational relevance in managing this deadly malignancy.     

Berberine inhibits human colon cancer cell migration via AMP-activated protein kinase-mediated downregulation of integrin β1 signaling

Colon cancer is associated with a poor prognosis, motivating strategies to prevent its development. An encouraging preventative strategy is the use of nutraceuticals; however, scientific verification of therapeutic functions and mechanisms of biological activity are necessary for the acceptance of dietary supplements in cancer treatment. Berberine is a benzylisoquinoline alkaloid extracted from many kinds of medicinal plants that has been extensively used as a Chinese traditional medicine. Recently, berberine has been reported to possess antitumoral activities. Among the various cellular targets of berberine is AMP-activated protein kinase (AMPK), which regulates tumor progression and metastasis. However, the specific role of berberine-induced AMPK activation and its effects on the metastatic potential of colon cancer remain largely unknown. The present study investigated berberine-induced activation of AMPK and its effects on colon cancer cell migration. Berberine decreased the migration of SW480 and HCT116 cells. We found that berberine activated AMPK in human colon cancer cell lines. Notably, berberine-induced activation of AMPK reduced the integrin β1 protein levels and decreased the phosphorylation of integrin β1 signaling targets. Knockdown of AMPKα1 subunits using small interfering RNA significantly attenuated berberine-induced downregulation of integrin β1 and inhibition of tumor cell migration. Collectively, our results suggest that berberine-induced AMPK activation inhibits the metastatic potential of colon cancer cells by decreasing integrin β1 protein levels and downstream signaling.     

Cryptotanshinone induces G1 cell cycle arrest and autophagic cell death by activating the AMP-activated protein kinase signal pathway in HepG2 hepatoma

AMP-activated protein kinase (AMPK) performs a pivotal function in energy homeostasis via the monitoring of intracellular energy status. Once activated under the various metabolic stress conditions, AMPK regulates a multitude of metabolic pathways to balance cellular energy. In addition, AMPK also induces cell cycle arrest or apoptosis through several tumor suppressors including LKB1, TSC2, and p53. LKB1 is a direct upstream kinase of AMPK, while TSC2 and p53 are direct substrates of AMPK. Therefore, it is expected that activators of AMPK signal pathway might be useful for treatment or prevention of cancer. In the present study, we report that cryptotanshinone, a natural compound isolated from Salvia miltiorrhiza, robustly activated AMPK signaling pathway, including LKB1, p53, TSC2, thereby leading to suppression of mTORC1 in a number of LKB1-expressing cancer cells including HepG2 human hepatoma, but not in LKB1-deficient cancer cells. Cryptotanshinone induced HepG2 cell cycle arrest at the G1 phase in an AMPK-dependent manner, and a portion of cells underwent apoptosis as a result of long-term treatment. It also induced autophagic HepG2 cell death in an AMPK-dependent manner. Cryptotanshinone significantly attenuated tumor growth in an HCT116 cancer xenograft in vivo model, with a substantial activation of AMPK signal pathways. Collectively, we demonstrate for the first time that cryptotanshinone harbors the therapeutic potential for the treatment of cancer through AMPK activation.     

AMP-activated protein kinase (AMPK)/Ulk1-dependent autophagic pathway contributes to C6 ceramide-induced cytotoxic effects in cultured colorectal cancer HT-29 cells

Colorectal cancer is the second leading cause of cancer-related deaths. Drug resistance and/or off-target toxicity against normal cells limit the effectiveness of current chemotherapies for the treatment of colorectal cancer. In the current study, we studied the potential cytotoxic effects of short-chain and cell-permeable C6 ceramide in cultured colorectal cancer HT-29 cells and focused on the underlying mechanisms. We observed that C6 ceramide-induced HT-29 cell death and growth inhibition in a dose- and time-dependent manner. However, no significant apoptosis was observed in C6 ceramide-treated HT-29 cells. Our data support that autophagy contributed to C6 ceramide-induced cytotoxic effects, as autophagy inhibitors, 3-methyladenine (3-MA) and hydroxychloroquine, inhibited C6 ceramide’s effect; however, autophagy activators, everolimus (RAD001) and temsirolimus, mimicked C6 ceramide effects and induced HT-29 cell death. Further, we indentified that AMP-activated protein kinase (AMPK)/Ulk1 signaling was required for autophagy induction by C6 ceramide, and AMPK silencing by a specific short hairpin RNA suppressed C6 ceramide-induced autophagy and cytotoxic effects. Reversely, forced activation of AMPK by its activator AICAR or by genetic manipulation caused autophagic death in HT-29 cells, which was inhibited by 3-MA. Our results suggest that autophagy, but not apoptosis, is a major contributor for C6 ceramide-induced cytotoxic effects in HT-29 cells, and activation of AMPK/Ulk1 is required for the process.     

Prevention of cytokine-induced apoptosis by insulin-like growth factor-I is independent of cell adhesion molecules in HT29-D4 colon carcinoma cells-evidence for a NF-kappaB-dependent survival mechanism

We have previously established that insulin-like growth factor (IGF)-I, -II and insulin exert a strong protective effect against tumor necrosis factor-alpha (TNF)-induced apoptosis in interferon-gamma (IFN)-sensitized HT29-D4 human colon carcinoma cells. In this study, we report that this effect was still operative when cells were cultured in the absence of integrin- and E-cadherin-mediated cell-extracellular matrix and cell-cell interactions. In this model, IGF-I did not activate the focal adhesion kinase, whereas it induced tyrosine phosphorylation of the insulin receptor substrate-1 and activation of the extracellular signal-related kinase 1 and 2, p38, phosphatidylinositol 3'-kinase and protein kinase B/Akt. However, the use of specific inhibitors indicated that these pathways did not play a role in the adhesion-independent IGF-I anti-apoptotic signal. In contrast, inhibition of the NF-kappaB activation induced a complete reversal of the IGF-I anchorage-independent protective effect. Correspondingly, IGF-I markedly enhanced the TNF- and IFN/TNF-induced NF-kappaB-dependent interleukin-8 production. Our results provide evidence that IGF-I induces resistance against cytokine-induced cell death even in the absence of cell adhesion-mediated signaling. NF-kappaB appears to be a key mediator of this anti-apoptotic effect that should contribute to the resistance of colon cancer cells to immune-destruction during metastasis.     

The role of short-chain fatty acids in orchestrating two types of programmed cell death in colon cancer

Short-chain fatty acids are the major by-products of bacterial fermentation of undigested fibers in the colon. SCFAs, mostly propionate and butyrate, induce differentiation, growth arrest, and apoptosis in colon cancer cells. The anticancer effect of SCFAs is also supported by epidemiological studies suggesting an inverse relationship between the level of dietary fibers and the incidence of human colon cancer. Dietary components influence the risk of human colon cancer including colon cancer through diverse mechanisms, which include the activation or inhibition of autophagy (type II programmed cell death (PCD)). Herein we demonstrate that propionate and butyrate induce autophagy in human colon cancer cells to dampen apoptosis, whereas inhibition of autophagy potentiates SCFA-induced apoptosis. The propionate-induced autophagy originates from mitochondria defects associated with cellular ATP depletion and ROS generation, both of which contribute to AMPK activation and consequential mTOR inhibition. Remarkably, when autophagy is suppressed through either pharmacological or genetic approaches, the colon cancer cells become sensitized toward propionate-induced apoptotic cell death (type I PCD). Our study is the first report characterizing this novel role of SCFAs in orchestrating two types of programmed cell death. The observed pro-survival effects of autophagy in retarding mitochondria-mediated apoptosis suggest that application of an autophagy inhibitor might improve the therapeutic efficacy of SCFAs in inducing colon cancer suppression.     

AMPK activation by ASP4132 inhibits non-small cell lung cancer cell growth

Activation of adenosine monophosphate-activated protein kinase (AMPK) is able to produce significant anti-non-small cell lung cancer (NSCLC) cell activity. ASP4132 is an orally active and highly effective AMPK activator. The current study tested its activity against NSCLC cells. In primary NSCLC cells and established cell lines (A549 and NCI-H1944) ASP4132 potently inhibited cell growth, proliferation and cell cycle progression as well as cell migration and invasion. Robust apoptosis activation was detected in ASP4132-treated NSCLC cells. Furthermore, ASP4132 treatment in NSCLC cells induced programmed necrosis, causing mitochondrial p53-cyclophilin D (CyPD)-adenine nucleotide translocase 1 (ANT1) association, mitochondrial depolarization and medium lactate dehydrogenase release. In NSCLC cells ASP4132 activated AMPK signaling, induced AMPKα1-ACC phosphorylation and increased AMPK activity. Furthermore, AMPK downstream events, including mTORC1 inhibition, receptor tyrosine kinases (PDGFRα and EGFR) degradation, Akt inhibition and autophagy induction, were detected in ASP4132-treated NSCLC cells. Importantly, AMPK inactivation by AMPKα1 shRNA, knockout (using CRISPR/Cas9 strategy) or dominant negative mutation (T172A) almost reversed ASP4132-induced anti-NSCLC cell activity. Conversely, a constitutively active AMPKα1 (T172D) mimicked and abolished ASP4132-induced actions in NSCLC cells. In vivo, oral administration of a single dose of ASP4132 largely inhibited NSCLC xenograft growth in SCID mice. AMPK activation, mTORC1 inhibition and EGFR-PDGFRα degradation as well as Akt inhibition and autophagy induction were detected in ASP4132-treated NSCLC xenograft tumor tissues. Together, activation of AMPK by ASP4132 potently inhibits NSCLC cell growth in vitro and in vivo.Subject terms: Targeted therapies, Non-small-cell lung cancer     

Ultraviolet enhances the sensitivity of pancreatic cancer cells to gemcitabine by activation of 5' AMP-activated protein kinase

Although gemcitabine is recognized as the standard drug for the treatment of advanced pancreatic cancer, the clinical outcome is not satisfactory. We recently reported that relatively high dose ultraviolet-C (UV-C; 200 J) inhibits cell growth by desensitization of epidermal growth factor receptor (EGFR) in human pancreatic cancer cells. In the present study, we investigated the combination effects of low dose UV-C (10 J) and gemcitabine on apoptosis and cell growth in these cells. UV-C enhanced gemcitabine-induced suppression of cell viability. In addition, the combination use clearly induced apoptosis, while neither UV-C nor gemcitabine alone did. Concurrently, combination use caused the decrease in the EGFR protein level and reduced EGF-induced activation of Akt pathway, subsequently resulting in accumulation of β-catenin. The order of the treatment with UV-C and gemcitabine did not affect their synergistic effects on apoptosis and cell growth. Interestingly, combination use synergistically induced phosphorylation of 5′ AMP-activated protein kinase (AMPK) alpha at Thr172 and acetyl-CoA carboxylase at Ser79 as a downstream molecular target of AMPK. AMPK activator, 5-aminoimidazole-4-carboxamide-1-β-riboside, induced apoptosis and suppressed cell growth in these cells, thus suggesting that combination effects of UV-C and gemcitabine is due to the activation of AMPK. Together, our findings could provide a new aspect of pancreatic cancer therapy.     

TAK1 activates AMPK‐dependent cytoprotective autophagy in TRAIL‐treated epithelial cells

The capacity of tumour necrosis factor‐related apoptosis‐inducing ligand (TRAIL) to trigger apoptosis preferentially in cancer cells, although sparing normal cells, has motivated clinical development of TRAIL receptor agonists as anti‐cancer therapeutics. The molecular mechanisms responsible for the differential TRAIL sensitivity of normal and cancer cells are, however, poorly understood. Here, we show a novel signalling pathway that activates cytoprotective autophagy in untransformed human epithelial cells treated with TRAIL. TRAIL‐induced autophagy is mediated by the AMP‐activated protein kinase (AMPK) that inhibits mammalian target of rapamycin complex 1, a potent inhibitor of autophagy. Interestingly, the TRAIL‐induced AMPK activation is refractory to the depletion of the two known AMPK‐activating kinases, LKB1 and Ca(2+)/calmodulin‐dependent kinase kinase‐β, but depends on transforming growth factor‐β‐activating kinase 1 (TAK1) and TAK1‐binding subunit 2. As TAK1 and AMPK are ubiquitously expressed kinases activated by numerous cytokines and developmental cues, these data are most likely to have broad implications for our understanding of cellular control of energy homoeostasis as well as the resistance of untransformed cells against TRAIL‐induced apoptosis.     

Involvement of Akt2/protein kinase B β (PKBβ) in the 8‐Cl‐cAMP‐induced cancer cell growth inhibition

8‐chloro‐cyclic AMP (8‐Cl‐cAMP), which induces differentiation, growth inhibition, and apoptosis in various cancer cells, has been investigated as a putative anti‐cancer drug. However, the exact mechanism of 8‐Cl‐cAMP functioning in cancer cells is not fully understood. Akt/protein kinase B (PKB) genes (Akt1, Akt2, and Akt3) encode enzymes belonging to the serine/threonine‐specific protein kinase family. It has been suggested that Akt/PKB enhances cell survival by inhibiting apoptosis. Recently, we showed that 8‐Cl‐cAMP and 5‐aminoimidazole‐4‐carboxamide ribonucleoside (AICAR) inhibited cancer cell growth through the activation of AMPK and p38 MAPK. Therefore, we anticipated that the phosphorylation of Akt/PKB would be decreased upon treatment with 8‐Cl‐cAMP. However, treatment with 8‐Cl‐cAMP and AICAR induced the phosphorylation of Akt/PKB, which was inhibited by ABT702 (an adenosine kinase inhibitor) and NBTI (an adenosine transporter inhibitor). Furthermore, whereas Compound C (an AMPK inhibitor), AMPK‐DN (AMPK‐dominant negative) mutant, and SB203580 (a p38 MAPK inhibitor) did not block the 8‐Cl‐cAMP‐induced phosphorylation of Akt/PKB, TCN (an Akt1/2/3 specific inhibitor) and an Akt2/PKBβ‐targeted siRNA inhibited the 8‐Cl‐cAMP‐ and AICAR‐mediated phosphorylation of AMPK and p38 MAPK. TCN also reversed the growth inhibition mediated by 8‐Cl‐cAMP and AICAR. Moreover, an Akt1/PKBα‐targeted siRNA did not reduce the phosphorylation of AMPK and p38 MAPK after treatment with 8‐Cl‐cAMP. These results suggest that Akt2/PKBβ activation promotes the phosphorylation of AMPK and p38 MAPK during the 8‐Cl‐cAMP‐ and AICAR‐induced growth inhibition. J. Cell. Physiol. 228: 890–902, 2013. © 2012 Wiley Periodicals, Inc.     

Activation of adenosine monophosphate activated protein kinase inhibits growth of multiple myeloma cells

The role of adenosine monophosphate activated protein kinase (AMPK) in regulating multiple myeloma (MM) cell growth is not yet clear. In this study, we show that the AMPK activators 5-aminoimidazole-4-carboxamide riboside (AICAr) and D942 inhibit cell growth in MM cell lines. AICAr also induced an S-phase cell cycle arrest in all four tested cell lines and led to phosphorylation and thus activation of AMPK. Furthermore, the inhibition of a nucleoside transporter by nitrobenzyl-thio-9-beta-d-ribofuranosylpurine (NBTI), inhibition of the adenosine kinase by iodotubericidine and inhibition of AMPK by AMPKI Compound C reversed AICAr effects, indicating that the cellular effects of AICAr were mediated by AMPK. Activation of AMPK inhibited basal extracellular signal-regulated kinase (ERK), mammalian target of rapamycin (mTOR) and P70S6 kinase (P70S6K) as well as AKT phosphorylation, and blocked IL-6, IGF-1, and HS-5 stromal cell conditioned medium-induced increase of cell growth. Troglitazone, which has previously been shown to activate AMPK, similarly inhibited MM cell growth, activated AMPK, and decreased ERK and P70S6K phosphorylation. Our results suggest that activation of AMPK inhibits MM cell growth despite stimulation with IL-6, IGF-1, or HS-5 stromal cell conditioned medium and represents a potential new target in the therapy of MM.     

Involvement of AMP-activated protein kinase and p38 mitogen-activated protein kinase in 8-Cl-cAMP-induced growth inhibition

8-Cl-cAMP (8-chloro-cyclic AMP), which induces differentiation, growth inhibition and apoptosis in various cancer cells, has been investigated as a putative anti-cancer drug. Although we reported that 8-Cl-cAMP induces growth inhibition via p38 mitogen-activated protein kinase (MAPK) and a metabolite of 8-Cl-cAMP, 8-Cl-adenosine mediates this process, the action mechanism of 8-Cl-cAMP is still uncertain. In this study, it was found that 8-Cl-cAMP-induced growth inhibition is mediated by AMP-activated protein kinase (AMPK). 8-Cl-cAMP was shown to activate AMPK, which was also dependent on the metabolic degradation of 8-Cl-cAMP. A potent agonist of AMPK, 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) could also induce growth inhibition and apoptosis. To further delineate the role of AMPK in 8-Cl-cAMP-induced growth inhibition and apoptosis, we used two approaches: pharmacological inhibition of the enzyme with compound C and expression of a dominant negative mutant (a kinase-dead form of AMPKalpha2, KD-AMPK). AICAR was able to activate p38 MAPK and pre-treatment with AMPK inhibitor or expression of KD-AMPK blocked this p38 MAPK activation. Cell growth inhibition was also attenuated. Furthermore, p38 MAPK inhibitor attenuated 8-Cl-cAMP- or AICAR-induced growth inhibition but had no effect on AMPK activation. These results demonstrate that 8-Cl-cAMP induced growth inhibition through AMPK activation and p38 MAPK acts downstream of AMPK in this signaling pathway.