Deletion Of PKM2 Research Articles

Pyruvate Kinase Isoform Expression Alters Nucleotide Synthesis to Impact Cell Proliferation

Metabolic regulation influences cell proliferation. The influence of pyruvate kinase isoforms on tumor cells has been extensively studied, but whether PKM2 is required for normal cell proliferation is unknown. We examine how PKM2 deletion affects proliferation and metabolism in nontransformed, nonimmortalized PKM2-expressing primary cells. We find that deletion of PKM2 in primary cells results in PKM1 expression and proliferation arrest. PKM1 expression, rather than PKM2 loss, is responsible for this effect, and proliferation arrest cannot be explained by cell differentiation, senescence, death, changes in gene expression, or prevention of cell growth. Instead, PKM1 expression impairs nucleotide production and the ability to synthesize DNA and progress through the cell cycle. Nucleotide biosynthesis is limiting, as proliferation arrest is characterized by severe thymidine depletion, and supplying exogenous thymine rescues both nucleotide levels and cell proliferation. Thus, PKM1 expression promotes a metabolic state that is unable to support DNA synthesis.

Abstract BS02-2: Alterations in tumor metabolism to support breast tumor growth and progression

Abstract Breast cancer cells must adapt metabolism to support anabolic processes and allow the accumulation of biomass to allow proliferation. However, tumor cells also experience periods of stress, and metabolic plasticity to shift metabolism to maximize efficiency and survive nutrient limitation is also needed for cancer progression. The M2 pyruvate kinase isoform (PKM2) is expressed in cancers and plays a role in regulating metabolism. To determine whether PKM2 is required for the development and growth of primary breast tumors, we generated mice harboring a conditional allele for PKM2. PKM2 deletion selectively abolishes PKM2 protein production while continued production of PKM1, the other major pyruvate kinase isoform. We crossed PKM2 conditional mice to a model of human breast cancer driven by loss of the Brca1 tumor suppressor gene. Contrary to prior expectation, mammary tumor formation and progression was accelerated despite PKM2 deletion. PKM2-null tumors displayed heterogeneous compensatory PKM1 expression, and expression of PKM1 was observed only in non-proliferating tumor cells. In contrast, proliferating cells in the tumor had no detectable pyruvate kinase expression. This suggests that PKM2 protein is not necessary for tumor cell proliferation and implies that the inactive state of PKM2 is associated with the proliferating cell population within tumors, while cells with active pyruvate kinase are involved in aspects of tumor progression that do not involve proliferation. Consistent with these findings, variable expression of PKM2 was observed across a panel of human breast tumors, and recurrent mutations resulting in disruption of one PKM2 allele are found in multiple human cancers. Together, these data support a model where regulation of PKM2 activity allows the enzyme to support the differential metabolic requirements of both proliferating and non-proliferating tumor cells. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr BS02-2.

Differential Dependence On Aerobic Glycolysis In Normal and Malignant Hematopoietic Stem and Progenitor Cells To Sustain Daughter Cell Production

How glucose is metabolized can influence cell function, but whether differences in glucose metabolism reflect, or dictate, cell state is not clear and is of particular interest given the association of cancer with aerobic glycolysis. Studies on cancer cell lines have indicated that increased glucose uptake with lactate production regardless of oxygen concentration, a phenomenon also known as the Warburg effect, is promoted in part by expression of the M2 isoform of pyruvate kinase (PKM2) and the muscle form of lactate dehydrogenase A (LDHA). Normal somatic cells thought to also preferentially use glycolytic metabolism are tissue stem cells, particularly the self-renewing hematopoietic stem cells (HSC) resident in the hypoxic microenvironment of the bone marrow. It remains to be defined, however whether proliferating hematopoietic progenitor cells rely on aerobic glycolysis and whether malignant and normal hematopoietic cells are dependent on the same metabolic regulation. We observed that PKM2 and LDHA are the predominant isoforms expressed by all BM hematopoietic cells. To further understand the role of glycolytic metabolism in hematopoiesis and hematological malignancy, we utilized a mouse strain that allows conditional deletion of the PKM2 specific exon 10. Deletion of PKM2 in hematopoetic cells leads to expression of PKM1, accompanied with partial inhibition of lactate production and decreased glycolytic intermediates in the hematopoietic stem/progenitor cell (HSPC) population. Loss of PKM2 compromises the long-term repopulation capacity of HSPCs as revealed by serial transplantation assay. Interestingly, the repopulating defects resulting from PKM2 depletion appear to involve progenitors, perhaps due to inadequate biomass generation necessary for robust cell proliferation. To confirm that the effect of PKM2 deletion on HSPC function is due to metabolic changes rather than other putative PKM2 functions, we engineered a mouse strain that allowed conditional knockout of LDHA to more potently impair aerobic glycolysis. LDHA deletion completely inhibited lactate production, enhanced ROS levels in hematopoietic cells and impaired long-term BM repopulating activity. In contrast to PKM2 deletion that affects progenitor but not stem cells, LDHA depletion impacts both stem cell maintenance and progenitor cell proliferation. Deletion of either PKM2 or LDHA markedly suppressed leukemia initiation by either putative stem cell (BCR-ABL) or progenitor (MLL-AF9) transforming alleles. Therefore, modulating aeroblic glycolysis has effects on normal hematopoietic cells that depend upon cell state and negatively impacts leukemic growth regardless of cell state. The differential sensitivity of normal and malignant cells to modulation of aerobic glycolysis suggests a potential therapeutic opportunity for leukemia intervention. Disclosures:No relevant conflicts of interest to declare.

PKM2 Isoform-Specific Deletion Reveals a Differential Requirement for Pyruvate Kinase in Tumor Cells

The pyruvate kinase M2 isoform (PKM2) is expressed in cancer and plays a role in regulating anabolic metabolism. To determine whether PKM2 is required for tumor formation or growth, we generated mice with a conditional allele that abolishes PKM2 expression without disrupting PKM1 expression. PKM2 deletion accelerated mammary tumor formation in a Brca1-loss-driven model of breast cancer. PKM2 null tumors displayed heterogeneous PKM1 expression, with PKM1 found in nonproliferating tumor cells and no detectable pyruvate kinase expression in proliferating cells. This suggests that PKM2 is not necessary for tumor cell proliferation and implies that the inactive state of PKM2 is associated with the proliferating cell population within tumors, whereas nonproliferating tumor cells require active pyruvate kinase. Consistent with these findings, variable PKM2 expression and heterozygous PKM2 mutations are found in human tumors. These data suggest that regulation of PKM2 activity supports the different metabolic requirements of proliferating and nonproliferating tumor cells.