The role of PCK2 in lung cancer metabolism and growth

Cancer cells are re-programmed to utilize glucose (dextrose) at high rates. Consequently, glucose levels may decrease substantially in the microenvironment of solid cancers, such as lung cancer. Gluconeogenesis is a metabolic pathway for the synthesis of glucose from smaller carbon substrates. Recently, the key enzyme (molecule) for gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK, PCK2) has been identified by our group to enhance survival of lung cancer cells under low glucose conditions. PEPCK (PCK2) is not only responsible for the generation of glucose e.g. in liver cells, but also plays a role in the synthesis of different cellular building blocks. However, its role in cancer cell metabolism is poorly understood. In the present project we will address the question, which substrates can be metabolized via the PCK2 pathway. Furthermore, we will analyze, whether and which cellular components are synthesized through the action of PCK2. This would show, for the first time, that in cancer cells the well-known glucose degrading pathway (glycolysis) may be reversed under low glucose conditions, challenging the present paradigm. In order to understand the underlying mechanisms in regulating this pathway and ultimately in the adaptation of lung cancer cells to low glucose, we will aim to identify the intracellular signaling molecules regulating PCK2. Finally, we will analyze the role of the novel metabolic pathway in cancer growth and response to chemotherapy in a mouse model in vivo. The study will thus help to clarify, whether PCK2 is a potential therapeutic target for the treatment of lung cancer.

Cancer cells show high rates of proliferation (cell doubling) and thus rewire their metabolism and biosynthetic activity to promote the formation of biomass. If metabolic precursors, like glucose (sugar) are scarce due to a limited supply, metabolic adaptation in cancer cells is necessary, which is at present poorly understood. We previously found that an enzyme that is usually active in the liver to produce glucose (sugar), PCK2, is activated in lung cancer cells lacking glucose. In this study, we sought to determine whether PCK2 is critical for the generation of biomass in lung cancer cells under low nutrient stress and to clarify, whether inhibition of this enzyme affects cancer growth. To this end we inhibited PCK2 by genetic tools (RNA interference) and monitored metabolism by stable isotopic tracers and mass spectrometry. We found that PCK2 is required for the conversion of non-sugar precursors to a component of cellular lipids. Moreover, we found that PCK2 inhibition deprived cancer cells of these lipids, if glucose availability was low. Importantly, tumor growth was stopped at a microscopic size when PCK2 was blocked. In a study using almost 500 samples of human non-small cell lung cancer, a frequent and aggressive human cancer, we discovered that the main route for biomass precursor production from sugars, glycolysis, as well as the PCK2 rescue pathway are both co-activated in most tumors, albeit PCK2 is also present in normal lungs. We found an unexpected localization of PCK2 at the tumor margin. We figured out that low oxygen conditions, which are frequently found in the tumor center, favor the glycolysis pathway in cancer cells, which might explain the irregular distribution of PCK2. This co-existence of both pathways in lung cancer has not been addressed before in larger studies and should be taken into account in future research on biomass-generation pathways in cancer. Using the methods established in that study we identified another pathway that is hyper-activated in human lung cancer: the generation of a specific cellular lipid, phosphatidylethanolamine. Together, the results of this collaborative study significantly contributed to the understanding of the role of the gluconeogenesis enzyme PCK2 in cancer cells and of the mechanisms underlying metabolic adaptation in cancer cells. Potentially, pharmaceutical inhibition of PCK2 could be considered in future studies to influence cancer metabolism and inhibit cancer growth.