Examining the roles of the pyruvate kinase isoforms, PKMI1 and PKM2, in systemic metabolism and tumor development

Abstract

Alternative splicing of the Pkm gene product generates the PKM1 and PKM2 isoforms of pyruvate kinase, and PKM2 expression is closely linked to embryogenesis, tissue regeneration, and cancer. PKM1 expression, on the other hand, is restricted mostly to skeletal muscle, heart, and brain. To interrogate the functional requirement for PKM1 or PKM2 during development and tissue homeostasis, we generated germline PKM1 (Pkm1-/-) or PKM2 null mice (Pkm2-/-). Unexpectedly, despite being the primary isoform expressed in most wild-type adult tissues, we found that Pkm2' mice are viable and fertile. Thus, PKM2 is not required for embryonic or postnatal development. Loss of PKM2 leads to compensatory expression of PKM1 in the tissues that normally express PKM2. We found that Pkm1-/- mice are also viable and fertile. Thus, neither PKM isoform is required for embryonic or postnatal development. In Pkm1-/- mice, loss of PKM1 leads to compensatory expression of PKM2 in the tissues that normally express PKM1. Aside from distinct changes in the plasma metabolite profiles of Pkm1-/- mice compared to wild-type (WT) mice, germline loss of PKMI appears to be of little phenotypic consequence. In contrast, PKM2 loss leads to a striking phenotype: spontaneous development of hepatocellular carcinoma (HCC) with high penetrance that is accompanied by progressive changes in systemic metabolism characterized by altered systemic glucose homeostasis, inflammation, and hepatic steatosis. Therefore, in addition to its role in cancer metabolism, PKM2 plays a role in controlling systemic metabolic homeostasis and inflammation, thereby preventing HCC by a non-cell-autonomous mechanism. To interrogate the cell-autonomous functional requirement for PKM2 during tumor initiation, we used a conditional Pkm2 allele (Pkm2f/) to abolish PKM2 expression in the context of two Kras driven mouse models - for lung adenocarcinoma and soft-tissue sarcoma (STS). In the sarcoma model, where the presumed tumor cell-of-origin expresses PKM1, deletion of PKM2 led to delayed tumor formation. In contrast, in the lung cancer model the presumed tumor cell-of-origin expresses PKM2 and deletion of PKM2 had no effect on tumor latency or tumor area. PKM2-null sarcomas expressed PKM1 and contained a high number of infiltrating PKM2+ stromal cells. Metabolite analysis of sarcoma cell lines generated from PKM2-null and wild-type tumors revealed metabolic changes in the PKM2-null tumors. These results argue that the consequences of PKM2 loss during tumor initiation depend on the tumor type and that the requirement for PKM2 expression can be overcome through metabolic adaptation. Taken together, the work presented in this thesis provides key insights into the pleiotropic roles played by PKM1 and PKM2 in the contexts of normal and malignant proliferation and in tumor and systemic metabolism. Our findings contribute to a more complete understanding of the distinct cell-intrinsic and cell-extrinsic roles of PKM isoform expression in the context of cancer, and may potentially inform strategies that target metabolism for the treatment of cancer.