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.

Salicylates are among the oldest and most widely used medications, used to reduce fever, pain and inflammation. The major oral salicylates are aspirin and salsalate, both of which are rapidly metabolized to salicylate in vivo. Due to its acetyl group, aspirin irreversibly inhibits cyclooxygenases and thus blocks platelet aggregation, while salsalate has been used for treatment of inflammatory diseases such as rheumatoid arthritis. Recently, beneficial effects of salicylates in type 2 diabetes and cancer have been proposed. This has led to renewed interest in understanding how these simple molecules have such diverse and multifaceted effects. Here we discuss the idea that AMP-activated protein kinase (AMPK) might mediate some effects of salicylate-based drugs, particularly by modulating cellular metabolism. KeywordsAMPK; aspirin; salicylate; inflammation; diabetes; cancer Salicylates -origin and discovery of medicinal propertiesSalicylates are hormones produced by plants in response to infection, which are critical in their defence against attack by pathogens [1]. Salicylate and derivatives such as salicin (a β-glucosyl ester, Fig. 1) are transported to neighbouring tissues, triggering defensive responses; volatile derivatives such as methyl salicylate can even spread to neighbouring plants. Salicylates can therefore be regarded as the equivalent of cytokines operating in the plant version of the innate immune system. They are produced by essentially all plants, but are named after the willow Salix alba due to the high levels of salicin found in its bark. The medicinal effects of willow were known to ancient humans, making natural salicylates almost certainly the oldest drugs known to mankind (Box 1). A synthetic derivative, acetyl salicylic acid (Fig. 1) Recently, evidence has accumulated that salicylates are effective not only in their original roles of reducing pain, fever, inflammation and blood clotting, but also for treating insulin resistance and protecting against cancer. It seems unlikely that all of these beneficial effects can be explained by a single molecular target, and we discuss below the idea that some of these effects might be mediated by AMP-activated protein kinase (AMPK), a recently discovered new target [3,4]. Mechanism of action of salicylates: inhibition of cyclo-oxygenases?In 1971, Vane showed that aspirin inhibited prostaglandin synthesis [5]. The first two steps in this pathway are catalysed by cyclo-oxygenases that occur as two major isoforms, the ubiquitous COX1 form and COX2, whose expression is induced at sites of inflammation. Interestingly, the acetyl group of aspirin is transferred to a serine residue in the active site of both isoforms, causing irreversible inhibition [6]. Inhibition of synthesis of the prothrombotic prostanoid, thromboxane, elegantly explains the ability of aspirin to inhibit blood clotting triggered by platelets; because acetylation is irreversible and platelets cannot resynthesize COX1, the inhibition lasts for the full lifetime of the platelet [6]. ...

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.

AMPK (AMP-activated protein kinase) is a sensor of cellular energy status that appears to have arisen during early eukaryotic evolution. In the unicellular eukaryote Saccharomyces cerevisiae, the AMPK orthologue is activated by glucose starvation and is required for the switch from glycolysis (fermentation) to oxidative metabolism when glucose runs low. In mammals, rapidly proliferating cells (including tumour cells) and immune cells involved in inflammation both tend to utilize rapid glucose uptake and glycolysis (termed the Warburg effect or aerobic glycolysis) rather than oxidative metabolism to satisfy their high demand for ATP. Since mammalian AMPK, similar to its yeast orthologue, tends to promote the more energy-efficient oxidative metabolism at the expense of glycolysis, it might be expected that drugs that activate AMPK would inhibit cell proliferation and and hence cancer, as well as exerting anti-inflammatory effects. Evidence supporting this view is discussed, including our findings that AMPK is activated by the classic anti-inflammatory drug salicylate.

The AMP-activated protein kinase (AMPK) is activated by energy stress and restores homeostasis by switching on catabolism, while switching off cell growth and proliferation. Findings that AMPK acts downstream of the tumor suppressor LKB1 have suggested that AMPK might also suppress tumorigenesis. In mouse models of B and T cell lymphoma in which genetic loss of AMPK occurred before tumor initiation, tumorigenesis was accelerated, confirming that AMPK has tumor-suppressor functions. However, when loss of AMPK in a T cell lymphoma model occurred after tumor initiation, or simultaneously with tumor initiation in a lung cancer model, the disease was ameliorated. Thus, once tumorigenesis has occurred, AMPK switches from tumor suppression to tumor promotion. Analysis of alterations in AMPK genes in human cancers suggests similar dichotomies, with some genes being frequently amplified while others are mutated. Overall, while AMPK-activating drugs might be effective in preventing cancer, in some cases AMPK inhibitors might be required to treat it.