The LKB1/AMPK signaling pathway has tumor suppressor activity in acute myeloid leukemia through the repression of mTOR-dependent oncogenic mRNA translation

Abstract

Finding an effective treatment for acute myeloid leukemia (AML) remains a challenge, and all cellular processes that are deregulated in AML cells should be considered in the design of targeted therapies. We show in our current study that the LKB1/AMPK/TSC tumor suppressor axis is functional in AML and can be activated by the biguanide molecule metformin, resulting in a specific inhibition of mammalian target of rapamycin (mTOR) catalytic activity. This induces a multisite dephosphorylation of the key translation regulator, 4E-BP1, which markedly inhibits the initiation step of mRNA translation. Consequently, metformin reduces the recruitment of mRNA molecules encoding oncogenic proteins to the polysomes, IntroductionIn acute myeloid leukemia (AML), the oncogenic deregulation of mRNA translation markedly contributes to the malignant phenotype. However, the efficacy of clinically available therapies that target this process, such as rapamycin, remains moderate due to the induction of multiple resistance mechanisms, 1 and new approach should be considered to bypass these processes.The liver kinase B1 (LKB1) serine/threonine kinase is encoded by the tumor-suppressor gene, STK11, which harbors germ-line mutations in the inherited cancer predisposition, Peutz-Jeghers syndrome, and somatic mutations in sporadic cancers. 2 The search for substrates of LKB1 that mediate its tumor-suppressor function led to the identification of the LKB1/AMPK/TSC adenosine monophosphate-activated protein kinase (AMPK) as a direct LKB1 substrate. 3 AMPK is a heterotrimeric complex comprising a catalytic ␣-and 2 regulatory ␤-and ␥-subunits, and LKB1 enhances AMPK activity through the phosphorylation of the ␣-subunit at T 172 . 4 AMPK is allosterically activated by the accumulation of AMP molecules, due to metabolic stresses that inhibit adenosine triphosphate (ATP) production (eg, hypoxia, glucose deprivation) or stimulate ATP consumption 5 and is also activated by drugs used to treat type 2 diabetes, including metformin. 6 AMPK is therefore the main cellular energy sensor, acting as a central negative regulator of metabolic pathways, such as fatty acid oxidation and glucose consumption. 7 The LKB1/AMPK pathway also regulates the protein synthesis rate through the control of the serine/threonine kinase mammalian target of rapamycin (mTOR), 8 a process that is consistently deregulated in AML cells. 9 When activated, AMPK stimulates tuberous sclerosis complex 1/2 (TSC 1/2 ), which comprises the TSC1/hamartin and TSC2/tuberin proteins, through AMPK-mediated TSC2 phosphorylation at the T 1227 and S 1345 residues. 10 This stimulates the GTPase-activating protein (GAP) function of TSC2 toward the small G-protein Rheb (Ras homolog enriched in brain), and increases the pool of guanosine diphosphate (GDP)-bound Rheb molecules. Although the molecular mechanism underlying mTOR activation by Rheb-guanosine triphosphate (GTP) is still under debate, [11][12][13] it is well established that TSC2 activation switches off Rheb, resulting in the inhibition o...

AMP-dependent kinase/mammalian target of rapamycin complex 1 signaling in T-cell acute lymphoblastic leukemia: therapeutic implications

“…42 Moreover, metformin spared normal hematopoiesis ex vivo, while it affected the clonogenic activity of AML CD34 þ cells. 20 Taken together, these findings suggest that targeting the Figure 5 Metformin targets the SP in T-ALL cell lines. Cells were treated with metformin (5 mM) for 24 h, then processed for SP analysis by staining with Hoechst 33343 dye (5 mg/ml) followed by flow cytometry.…”

“…12 In this study, we tested the anti-leukemic activity of metformin on T-ALL cells, as recent data have highlighted that this drug targets 4E-BP1-dependent translation in AML. 20 Metformin demonstrated marked antileukemic effects in vitro against both T-ALL cell lines and patients lymphoblasts. The cytotoxic effects were specific to leukemic cells, as metformin only slightly affected the proliferation of CD4 þ T-lymphocytes from healthy donors, in agreement with a previous report, in which the AMPK activator acadesine (AICAR) was tested on healthy T-lymphocytes.…”

“…30 Using Jurkat cells, we compared the effects of rapamycin and metformin on eIF4F assembly by pull-down assays using 7methyl-GTP-Sepharose beads that mimic the cap structure of mRNA. 20 While exposure to metformin dissociated eIF4G from eIF4E and increased the amount of 4E-BP1 bound to eIF4E, indicating inhibition of eIF4F complex assembly, treatment with rapamycin did not change the amount of eIF4G or 4E-BP1 bound to eIF4E (Figure 4c). Western blot analysis …”

“…18 In light of these findings, metformin is now being tested in several clinical trials in patients with solid tumors, especially breast cancer. 19 A recent paper has highlighted the cytotoxicity of metformin in preclinical models of acute myelogenous leukemia (AML), 20 emphasizing the effects of metformin on the translation of oncogenic proteins. We have recently demonstrated that inhibition of mTORC1-mediated oncogenic protein translation occurs also in T-ALL cells treated with mTORC1/mTORC2 ATP-competitive inhibitors, but not with rapamycin.…”

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

“…This inhibition suppresses the translation of the 'oncogenic' mRNAs. 149 The antiviral drug ribavirin can block the binding of eIF4E to mRNA. 150,151 Thus, the effects of this compound were examined in some patients with relapsed M4/M5 AML who were no longer eligible for chemotherapy.…”