The AMP-activated protein kinase (AMPK) acts as a cellular energy sensor. Once switched on by increases in cellular AMP : ATP ratios, it acts to restore energy homeostasis by switching on catabolic pathways while switching off cell growth and proliferation. The canonical AMP-dependent mechanism of activation requires the upstream kinase LKB1, which was identified genetically to be a tumour suppressor. AMPK can also be switched on by increases in intracellular Ca 2+ , by glucose starvation and by DNA damage via non-canonical, AMP-independent pathways. Genetic studies of the role of AMPK in mouse cancer suggest that, before disease arises, AMPK acts as a tumour suppressor that protects against cancer, with this protection being further enhanced by AMPK activators such as the biguanide phenformin. However, once cancer has occurred, AMPK switches to being a tumour promoter instead, enhancing cancer cell survival by protecting against metabolic, oxidative and genotoxic stresses. Studies of genetic changes in human cancer also suggest diverging roles for genes encoding subunit isoforms, with some being frequently amplified, while others are mutated.

Summary AMPK acts downstream of the tumor suppressor LKB1, yet its role in cancer has been controversial. AMPK is activated by biguanides, such as metformin and phenformin, and metformin use in diabetics has been associated with reduced cancer risk. However, whether this is mediated by cell-autonomous AMPK activation within tumor progenitor cells has been unclear. We report that T-cell-specific loss of AMPK-α1 caused accelerated growth of T cell acute lymphoblastic leukemia/lymphoma (T-ALL) induced by PTEN loss in thymic T cell progenitors. Oral administration of phenformin, but not metformin, delayed onset and growth of lymphomas, but only when T cells expressed AMPK-α1. This differential effect of biguanides correlated with detection of phenformin, but not metformin, in thymus. Phenformin also enhanced apoptosis in T-ALL cells both in vivo and in vitro . Thus, AMPK-α1 can be a cell-autonomous tumor suppressor in the context of T-ALL, and phenformin may have potential for the prevention of some cancers.

The AMP-activated protein kinase (AMPK) is activated by phosphorylation at Thr172, either by the tumor suppressor kinase LKB1 or by an alternate pathway involving the Ca 2þ /calmodulindependent kinase, CAMKK2. Increases in AMP:ATP and ADP:ATP ratios, signifying energy deficit, promote allosteric activation and net Thr172 phosphorylation mediated by LKB1, so that the LKB1-AMPK pathway acts as an energy sensor. Many tumor cells carry loss-of-function mutations in the STK11 gene encoding LKB1, but LKB1 reexpression in these cells causes cell-cycle arrest. Therefore, it was investigated as to whether arrest by LKB1 is caused by activation of AMPK or of one of the AMPK-related kinases, which are also dependent on LKB1 but are not activated by CAMKK2. In three LKB1-null tumor cell lines, treatment with the Ca 2þ ionophore A23187 caused a G 1 arrest that correlated with AMPK activation and Thr172 phosphorylation. In G361 cells, expression of a truncated, Ca 2þ /calmodulin-independent CAMKK2 mutant also caused G 1 arrest similar to that caused by expression of LKB1, while expression of a dominant-negative AMPK mutant, or a double knockout of both AMPK-a subunits, also prevented the cell-cycle arrest caused by A23187. These mechanistic findings confirm that AMPK activation triggers cell-cycle arrest, and also suggest that the rapid proliferation of LKB1-null tumor cells is due to lack of the restraining influence of AMPK. However, cell-cycle arrest can be restored by reexpressing LKB1 or a constitutively active CAMKK2, or by pharmacologic agents that increase intracellular Ca 2þ and thus activate endogenous CAMKK2.Implications: Evidence here reveals that the rapid growth and proliferation of cancer cells lacking the tumor suppressor LKB1 is due to reduced activity of AMPK, and suggests a therapeutic approach by which this block might be circumvented.

Many genotoxic cancer treatments activate AMP-activated protein kinase (AMPK), but the mechanisms of AMPK activation in response to DNA damage, and its downstream consequences, have been unclear. In this study, etoposide activates the a1 but not the a2 isoform of AMPK, primarily within the nucleus. AMPK activation is independent of ataxia-telangiectasia mutated (ATM), a DNA damage-activated kinase, and the principal upstream kinase for AMPK, LKB1, but correlates with increased nuclear Ca 2þ and requires the Ca 2þ /calmodulindependent kinase, CaMKK2. Intriguingly, Ca 2þ -dependent activation of AMPK in two different LKB1-null cancer cell lines caused G 1 -phase cell-cycle arrest, and enhanced cell viability/ survival after etoposide treatment, with both effects being abolished by knockout of AMPK-a1 and a2. The CDK4/6 inhibitor palbociclib also caused G 1 arrest in G361 but not HeLa cells and, consistent with this, enhanced cell survival after etoposide treatment only in G361 cells. These results suggest that AMPK activation protects cells against etoposide by limiting entry into S-phase, where cells would be more vulnerable to genotoxic stress.Implications: These results reveal that the a1 isoform of AMPK promotes tumorigenesis by protecting cells against genotoxic stress, which may explain findings that the gene encoding AMPK-a1 (but not -a2) is amplified in some human cancers. Furthermore, a1-selective inhibitors might enhance the anticancer effects of genotoxic-based therapies.

In the past few years, cell plasticity has emerged as a mode of targeted therapy evasion in prostate adenocarcinoma. When exposed to anticancer therapies, tumor cells may switch into a different histological subtype, such as the neuroendocrine phenotype which is associated with treatment failure and a poor prognosis. In this study, we demonstrated that long-term androgen signal depletion of prostate LNCaP cells induced a neuroendocrine phenotype followed by re-differentiation towards a “stem-like” state. LNCaP cells incubated for 30 days in charcoal-stripped medium or with the androgen receptor antagonist 2-hydroxyflutamide developed neuroendocrine morphology and increased the expression of the neuroendocrine markers βIII-tubulin and neuron specific enolase (NSE). When cells were incubated for 90 days in androgen-depleted medium, they grew as floating spheres and had enhanced expression of the stem cell markers CD133, ALDH1A1, and the transporter ABCB1A. Additionally, the pluripotent transcription factors Nanog and Oct4 and the angiogenic factor VEGF were up-regulated while the expression of E-cadherin was inhibited. Cell viability revealed that those cells were resistant to docetaxel and 2-hidroxyflutamide. Mechanistically, androgen depletion induced the decrease in AMP-activated kinase (AMPK) expression and activation and stabilization of the hypoxia-inducible factor HIF-1α. Overexpression of AMPK in the stem-like cells decreased the expression of stem markers as well as that of HIF-1α and VEGF while it restored the levels of E-cadherin and PGC-1α. Most importantly, docetaxel sensitivity was restored in stem-like AMPK-transfected cells. Our model provides a new regulatory mechanism of prostate cancer plasticity through AMPK that is worth exploring.