Cytokine Induced Nuclear Localization Of Pyruvate Kinase M2 In Acute Myeloid Leukemia

Abstract

A common characteristic among nearly all cancers, including leukemia, is the cell's metabolic proclivity for glycolysis over the more energy efficient process of oxidative phosphorylation (OXPHOS) in the presence of oxygen. This altered state of aerobic glycolysis was observed in tumor cells by Otto Warburg over fifty years ago (Warburg, 1956) and continues to be intensely investigated in hopes of ultimately exploiting this “Warburg effect” in the treatment of cancer (Vander Heiden et al. 2009). Recent studies have revealed that the M2 isoform of the enzyme pyruvate kinase (PKM2) plays a critical role in the maintenance of aerobic glycolysis in tumor cells and is important for their growth and development (Christofk et al. 2008). Pyruvate kinase produces pyruvate and one molecule of adenosine 5'-triphosphate (ATP) in the rate-limiting step of glycolysis. Pyruvate kinase coded for by PKM has two splice isoforms, the constitutively active PKM1 that exists only as a tetramer and PKM2 that can shift between a more active tetramer and less active dimers or monomers. The dynamic enzymatic activity of PKM2 is key to its preferential expression in tumor cells. By utilizing the less active form of PKM2, tumor cells can limit the levels of pyruvate available for OXPHOS and instead shunt glycolytic carbons towards anabolic processes. However, recent studies have revealed novel activities of PKM2 outside the realm of energy metabolism that also contribute to tumor formation, maintenance, and growth. The less active PKM2 dimer, whose structure is favored upon phosphorylation at Tyr105 (Hitosugi et al. 2009), can also translocate to the nucleus and act as a transcription factor for cell cycle associated genes like MYC and CCND1 upon stimulation with epidermal growth factor in certain cancer cell lines (Gao et al. 2012). Acute myeloid leukemia (AML) is a malignancy of hematopoietic progenitor cells characterized by the extraordinarily rapid growth of abnormal myeloid cells, making the proliferative influences of PKM2 an intriguing target for therapy. We have found that PKM2 is abundantly expressed in AML cell lines and primary AML patient samples and that low basal levels of PKM2 can be detected in their nuclei. Interestingly, stimulation with various cytokines such as IL-6 or GM-CSF can induce the nuclear translocation of PKM2 and association with histone H3 in these cells and concomitant treatment with PKM2 activating compounds that have been shown to promote its tetrameric structure and suppress tumor growth (Anastasiou et al. 2012) can inhibit this effect. These data show that the role of PKM2 in regulating transcription in addition to its metabolic activity may be important for the proliferation and maintenance of hematopoietic malignancies. Using fluorescence-activated cell sorting to isolate specific sub-populations of primary AML patient cells and elucidating PKM2's interaction with protein kinases involved in known signaling pathways like JAK/STAT, ERK1/2, and FLT3, we show that the proliferative influences of PKM2 function and activity differ between AML cell phenotypes. For example, cells from AML patient samples sorted based on high or low levels of reactive oxygen species (ROS) differ in relative phosphorylation of PKM2 at Tyr105. These data, along with reports that the PKM2 dimer specifically plays a role in tumor cell antioxidant response (Anastasiou, et al. 2011) suggests that PKM2 may contribute to the maintenance of phenotypically ROS-low leukemia stem cells that are thought to contribute to patient relapse after achieving remission (Hope et al. 2004). Our data suggests that the broad cellular functions of PKM2 employed by AML cells and its direct influence on tumor growth and survival make it a promising potential target for therapy. Disclosures:No relevant conflicts of interest to declare.