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Waste Not, Want Not: Lactate Oxidation Fuels the TCA Cycle
Cell Metabolism
Previews Waste Not, Want Not: Lactate Oxidation Fuels the TCA Cycle Inmaculada Martı´nez-Reyes1 and Navdeep S. Chandel1,* 1Department of Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA *Correspondence: [email protected] https://doi.org/10.1016/j.cmet.2017.11.005
Previous studies have demonstrated that mitochondrial respiration is essential for tumorigenesis. Hui et al. (2017) and Faubert et al. (2017) demonstrate that lactate, traditionally viewed as a waste product of anaerobic and aerobic glycolysis, is a major carbon source to fuel the mitochondrial TCA cycle in normal tissue and in tumors. Tumors require metabolic rewiring to efficiently utilize nutrients needed to support uncontrolled cell proliferation (DeBerardinis and Chandel, 2016). The reprograming of metabolic pathways in cancer cells is driven by genetic mutations and environmental factors. Previous studies have shown that mitochondrial metabolism is essential for tumorigenesis in vivo (Weinberg et al., 2010). The carbon molecules that fuel mitochondrial metabolism in vivo are not fully understood. Two recent studies by Hui et al. in Nature and Faubert et al. in Cell indicate that lactate fuels normal tissue and cancer mitochondria in mice and in human lung tumors, respectively. In the 1920s, Otto Warburg first observed the high rate of production of lactate in cancer tissues ex vivo, i.e., the Warburg effect or aerobic glycolysis. This observation led many investigators to surmise that mitochondrial metabolism in tumors was a negligible contributor to macromolecule synthesis needed for tumor cell growth and proliferation. However, multiple studies have indicated that increased flux through glycolysis and TCA cycle generates essential metabolites for macromolecule synthesis. Indeed, a previous study by DeBerardinis and colleagues showed evidence of enhanced glycolysis and TCA cycle in human lung tumors when the patients were infused with 13 C-glucose tracer (Hensley et al., 2016). In a follow-up study that examined metabolism of 13C-lactate tracers in lung cancer patients, Faubert et al. discovered that lactate also fuels the TCA cycle in molecularly heterogeneous tumors (Faubert et al., 2017). Tumors displaying import and metabolism of lactate from the plasma also had higher expression of the lactate
transporters MCT1 and MCT4, as well as the lactate dehydrogenases LDHA and LDHB. Mouse xenograft models using human lung cancer cells infused with 13 C-lactate revealed high levels of labeled lactate in the tumors as well as TCA cycle intermediates. Lactate can be transported in and out of the cells by the protoncoupled monocarboxylate transporters (MCTs 1–4). Indeed, knocking out MCT1 diminished lactate uptake by tumors in vivo. Collectively, these human and mouse infusions with lactate isotopic tracer demonstrate that lactate is a prominent fuel for mitochondrial metabolism. Furthermore, the authors also observed that markers of lactate metabolism in patient tumors are associated with disease progression, including the appearance of distant metastases. This suggests that methods to assess lactate utilization might help predict oncological aggressiveness. In the study by Hui et al. in Nature, the authors systematically investigated the fluxes of metabolites that are present at high concentrations in the blood by infusing 13 -C-labeled metabolites into the circulation of mice at a constant rate until the steady-state labeling was achieved (Hui et al., 2017). Surprisingly, lactate showed a 2.5-fold higher circulatory turnover flux in fasted mice when compared to that of glucose, previously considered the predominant circulating carbon source. Further analysis indicated that a high fraction of pyruvate in the normal tissues is derived from circulating lactate. Moreover, lactate infusion yielded the highest labeling of TCA cycle intermediates in all tissues except the brain. To assess the contribution of nutrients to replenish the TCA cycle in tumors, the authors used three genetically engineered mouse
models, two for lung cancer carrying different genetic mutations (KrasLSL-G12D/+ Trp53 / and KrasG12D/+Stk11 / ) and one for pancreas cancer (KrasLSL-G12D/+ Trp53 / Ptf1aCRE/+). Interestingly, the contribution of lactate to the TCA cycle was higher than that of glucose in the two different lung cancer mouse models and in normal lung tissue, whereas glutamine contributed more in the pancreas and the pancreatic cancer, highlighting the similar preferences of cancer cells and their niche. An intriguing aspect of these studies is the preference for lactate labeling of TCA cycle when compared to glucose even when they contribute equally to the labeling of tumor pyruvate. Lactate conversion to pyruvate is coupled to the conversion of NAD+ to NADH by LDH (Figure 1). In the cytosol, GAPDH, a key enzyme in glycolysis, also requires NAD+. Thus, an intriguing implication of the current studies is that the conversion of lactate to pyruvate might occur inside of mitochondria by LDH, where there is a high pool of NAD+ due to a functional mitochondrial complex I. Likewise, in certain tissues like retina, the presence of lactate has recently been shown to inhibit glucose consumption (Kanow et al., 2017). Further investigations are needed to determine the source of lactate in circulation. In the Cori cycle, muscle produces lactate that is taken up by the liver for gluconeogenesis. It is likely that muscle-generated lactate enters the circulation to feed the TCA cycle in many tissues (Figure 1). Therapeutically, mitochondria have emerged as a site to target cancer metabolism pathways. Specifically, cancer therapy-resistant cells are highly dependent on mitochondrial metabolism (Viale et al., 2014). Going forward,
Cell Metabolism 26, December 5, 2017 ª 2017 Elsevier Inc. 803
Cell Metabolism
Previews
Figure 1. Circulating Lactate Is a Primary Carbon Source to Replenish the TCA Cycle in Normal and Cancer Cell Mitochondria In the Cori cycle, muscle produces lactate by anaerobic glycolysis. Circulating lactate travels to the liver, where it is converted back to glucose by gluconeogenesis. Normal and cancer cells in different tissues uptake lactate from the blood to feed the mitochondrial TCA cycle. Monocarboxylate transporters (MCTs) and GLUT import lactate and glucose from the circulation to the inside of the cell, respectively. Pyruvate can be produced by glycolysis or by oxidation of lactate to enter the TCA cycle in mitochondria. Lactate might be imported and converted to pyruvate inside the mitochondria to feed the TCA cycle. The metabolic intermediates generated in glycolysis and mitochondria are essential to sustain tumor growth and proliferation through the synthesis of macromolecules.
it will be of interest to elucidate whether lactate is a fuel for these cells. Both studies highlight the increased level of complexity that the microenvironment adds when trying to understand tumor metabolism (Gui et al., 2016). In cultured cancer cells, glycolysis converts glucose into pyruvate, which is reduced to lactate by lactate dehydrogenase and secreted. The secretion of lactate helps maintain the intracellular pH by eliminating protons through the MCTs, and the pyruvate to lactate reaction recycles NAD+, allowing the glycolysis to persist. These two facts have largely contributed to our previous view of lactate as a byproduct waste of glycolysis and undervalue its potential role in tumorigenesis. However, the new studies by Hui et al. and Faubert et al. add evidence to previous data pointing out the importance of lactate metabolism in cancer cells and the value of studying cancer metabolism in vivo (Chen et al., 2016; Kennedy et al.,
804 Cell Metabolism 26, December 5, 2017
2013). They confirm the power of the isotope tracers to uncover important metabolic pathways in vivo and further affirm that targeting proteins within mitochondria is likely to be important for cancer therapy. ACKNOWLEDGMENTS N.S.C. is supported by NIH 5R35CA197532-02, 5P01AG049665, and 5P01HL071643-13.
E., Thomas, C.J., and Vander Heiden, M.G. (2016). Cell Metab. 24, 716–727. Hensley, C.T., Faubert, B., Yuan, Q., Lev-Cohain, N., Jin, E., Kim, J., Jiang, L., Ko, B., Skelton, R., Loudat, L., et al. (2016). Cell 164, 681–694. Hui, S., Ghergurovich, J.M., Morscher, R.J., Jang, C., Teng, X., Lu, W., Esparza, L.A., Reya, T., Le Zhan, Yanxiang Guo, J., et al. (2017). Nature 551, 115–118. Kanow, M.A., Giarmarco, M.M., Jankowski, C.S., Tsantilas, K., Engel, A.L., Du, J., Linton, J.D., Farnsworth, C.C., Sloat, S.R., Rountree, A., et al. (2017). Elife 6, https://doi.org/10.7554/eLife.28899.
REFERENCES Chen, Y.J., Mahieu, N.G., Huang, X., Singh, M., Crawford, P.A., Johnson, S.L., Gross, R.W., Schaefer, J., and Patti, G.J. (2016). Nat. Chem. Biol. 12, 937–943. DeBerardinis, R.J., and Chandel, N.S. (2016). Sci. Adv. 2, e1600200. Faubert, B., Li, K.Y., Cai, L., Hensley, C.T., Kim, J., Zacharias, L.G., Yang, C., Do, Q.N., Doucette, S., Burguete, D., et al. (2017). Cell 171, 358–371.e9. Gui, D.Y., Sullivan, L.B., Luengo, A., Hosios, A.M., Bush, L.N., Gitego, N., Davidson, S.M., Freinkman,
Kennedy, K.M., Scarbrough, P.M., Ribeiro, A., Richardson, R., Yuan, H., Sonveaux, P., Landon, C.D., Chi, J.T., Pizzo, S., Schroeder, T., and Dewhirst, M.W. (2013). PLoS One 8, e75154. Viale, A., Pettazzoni, P., Lyssiotis, C.A., Ying, H., Sa´nchez, N., Marchesini, M., Carugo, A., Green, T., Seth, S., Giuliani, V., et al. (2014). Nature 514, 628–632. Weinberg, F., Hamanaka, R., Wheaton, W.W., Weinberg, S., Joseph, J., Lopez, M., Kalyanaraman, B., Mutlu, G.M., Budinger, G.R., and Chandel, N.S. (2010). Proc. Natl. Acad. Sci. USA 107, 8788–8793.
Previews Waste Not, Want Not: Lactate Oxidation Fuels the TCA Cycle Inmaculada Martı´nez-Reyes1 and Navdeep S. Chandel1,* 1Department of Medicine, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA *Correspondence: [email protected] https://doi.org/10.1016/j.cmet.2017.11.005
Previous studies have demonstrated that mitochondrial respiration is essential for tumorigenesis. Hui et al. (2017) and Faubert et al. (2017) demonstrate that lactate, traditionally viewed as a waste product of anaerobic and aerobic glycolysis, is a major carbon source to fuel the mitochondrial TCA cycle in normal tissue and in tumors. Tumors require metabolic rewiring to efficiently utilize nutrients needed to support uncontrolled cell proliferation (DeBerardinis and Chandel, 2016). The reprograming of metabolic pathways in cancer cells is driven by genetic mutations and environmental factors. Previous studies have shown that mitochondrial metabolism is essential for tumorigenesis in vivo (Weinberg et al., 2010). The carbon molecules that fuel mitochondrial metabolism in vivo are not fully understood. Two recent studies by Hui et al. in Nature and Faubert et al. in Cell indicate that lactate fuels normal tissue and cancer mitochondria in mice and in human lung tumors, respectively. In the 1920s, Otto Warburg first observed the high rate of production of lactate in cancer tissues ex vivo, i.e., the Warburg effect or aerobic glycolysis. This observation led many investigators to surmise that mitochondrial metabolism in tumors was a negligible contributor to macromolecule synthesis needed for tumor cell growth and proliferation. However, multiple studies have indicated that increased flux through glycolysis and TCA cycle generates essential metabolites for macromolecule synthesis. Indeed, a previous study by DeBerardinis and colleagues showed evidence of enhanced glycolysis and TCA cycle in human lung tumors when the patients were infused with 13 C-glucose tracer (Hensley et al., 2016). In a follow-up study that examined metabolism of 13C-lactate tracers in lung cancer patients, Faubert et al. discovered that lactate also fuels the TCA cycle in molecularly heterogeneous tumors (Faubert et al., 2017). Tumors displaying import and metabolism of lactate from the plasma also had higher expression of the lactate
transporters MCT1 and MCT4, as well as the lactate dehydrogenases LDHA and LDHB. Mouse xenograft models using human lung cancer cells infused with 13 C-lactate revealed high levels of labeled lactate in the tumors as well as TCA cycle intermediates. Lactate can be transported in and out of the cells by the protoncoupled monocarboxylate transporters (MCTs 1–4). Indeed, knocking out MCT1 diminished lactate uptake by tumors in vivo. Collectively, these human and mouse infusions with lactate isotopic tracer demonstrate that lactate is a prominent fuel for mitochondrial metabolism. Furthermore, the authors also observed that markers of lactate metabolism in patient tumors are associated with disease progression, including the appearance of distant metastases. This suggests that methods to assess lactate utilization might help predict oncological aggressiveness. In the study by Hui et al. in Nature, the authors systematically investigated the fluxes of metabolites that are present at high concentrations in the blood by infusing 13 -C-labeled metabolites into the circulation of mice at a constant rate until the steady-state labeling was achieved (Hui et al., 2017). Surprisingly, lactate showed a 2.5-fold higher circulatory turnover flux in fasted mice when compared to that of glucose, previously considered the predominant circulating carbon source. Further analysis indicated that a high fraction of pyruvate in the normal tissues is derived from circulating lactate. Moreover, lactate infusion yielded the highest labeling of TCA cycle intermediates in all tissues except the brain. To assess the contribution of nutrients to replenish the TCA cycle in tumors, the authors used three genetically engineered mouse
models, two for lung cancer carrying different genetic mutations (KrasLSL-G12D/+ Trp53 / and KrasG12D/+Stk11 / ) and one for pancreas cancer (KrasLSL-G12D/+ Trp53 / Ptf1aCRE/+). Interestingly, the contribution of lactate to the TCA cycle was higher than that of glucose in the two different lung cancer mouse models and in normal lung tissue, whereas glutamine contributed more in the pancreas and the pancreatic cancer, highlighting the similar preferences of cancer cells and their niche. An intriguing aspect of these studies is the preference for lactate labeling of TCA cycle when compared to glucose even when they contribute equally to the labeling of tumor pyruvate. Lactate conversion to pyruvate is coupled to the conversion of NAD+ to NADH by LDH (Figure 1). In the cytosol, GAPDH, a key enzyme in glycolysis, also requires NAD+. Thus, an intriguing implication of the current studies is that the conversion of lactate to pyruvate might occur inside of mitochondria by LDH, where there is a high pool of NAD+ due to a functional mitochondrial complex I. Likewise, in certain tissues like retina, the presence of lactate has recently been shown to inhibit glucose consumption (Kanow et al., 2017). Further investigations are needed to determine the source of lactate in circulation. In the Cori cycle, muscle produces lactate that is taken up by the liver for gluconeogenesis. It is likely that muscle-generated lactate enters the circulation to feed the TCA cycle in many tissues (Figure 1). Therapeutically, mitochondria have emerged as a site to target cancer metabolism pathways. Specifically, cancer therapy-resistant cells are highly dependent on mitochondrial metabolism (Viale et al., 2014). Going forward,
Cell Metabolism 26, December 5, 2017 ª 2017 Elsevier Inc. 803
Cell Metabolism
Previews
Figure 1. Circulating Lactate Is a Primary Carbon Source to Replenish the TCA Cycle in Normal and Cancer Cell Mitochondria In the Cori cycle, muscle produces lactate by anaerobic glycolysis. Circulating lactate travels to the liver, where it is converted back to glucose by gluconeogenesis. Normal and cancer cells in different tissues uptake lactate from the blood to feed the mitochondrial TCA cycle. Monocarboxylate transporters (MCTs) and GLUT import lactate and glucose from the circulation to the inside of the cell, respectively. Pyruvate can be produced by glycolysis or by oxidation of lactate to enter the TCA cycle in mitochondria. Lactate might be imported and converted to pyruvate inside the mitochondria to feed the TCA cycle. The metabolic intermediates generated in glycolysis and mitochondria are essential to sustain tumor growth and proliferation through the synthesis of macromolecules.
it will be of interest to elucidate whether lactate is a fuel for these cells. Both studies highlight the increased level of complexity that the microenvironment adds when trying to understand tumor metabolism (Gui et al., 2016). In cultured cancer cells, glycolysis converts glucose into pyruvate, which is reduced to lactate by lactate dehydrogenase and secreted. The secretion of lactate helps maintain the intracellular pH by eliminating protons through the MCTs, and the pyruvate to lactate reaction recycles NAD+, allowing the glycolysis to persist. These two facts have largely contributed to our previous view of lactate as a byproduct waste of glycolysis and undervalue its potential role in tumorigenesis. However, the new studies by Hui et al. and Faubert et al. add evidence to previous data pointing out the importance of lactate metabolism in cancer cells and the value of studying cancer metabolism in vivo (Chen et al., 2016; Kennedy et al.,
804 Cell Metabolism 26, December 5, 2017
2013). They confirm the power of the isotope tracers to uncover important metabolic pathways in vivo and further affirm that targeting proteins within mitochondria is likely to be important for cancer therapy. ACKNOWLEDGMENTS N.S.C. is supported by NIH 5R35CA197532-02, 5P01AG049665, and 5P01HL071643-13.
E., Thomas, C.J., and Vander Heiden, M.G. (2016). Cell Metab. 24, 716–727. Hensley, C.T., Faubert, B., Yuan, Q., Lev-Cohain, N., Jin, E., Kim, J., Jiang, L., Ko, B., Skelton, R., Loudat, L., et al. (2016). Cell 164, 681–694. Hui, S., Ghergurovich, J.M., Morscher, R.J., Jang, C., Teng, X., Lu, W., Esparza, L.A., Reya, T., Le Zhan, Yanxiang Guo, J., et al. (2017). Nature 551, 115–118. Kanow, M.A., Giarmarco, M.M., Jankowski, C.S., Tsantilas, K., Engel, A.L., Du, J., Linton, J.D., Farnsworth, C.C., Sloat, S.R., Rountree, A., et al. (2017). Elife 6, https://doi.org/10.7554/eLife.28899.
REFERENCES Chen, Y.J., Mahieu, N.G., Huang, X., Singh, M., Crawford, P.A., Johnson, S.L., Gross, R.W., Schaefer, J., and Patti, G.J. (2016). Nat. Chem. Biol. 12, 937–943. DeBerardinis, R.J., and Chandel, N.S. (2016). Sci. Adv. 2, e1600200. Faubert, B., Li, K.Y., Cai, L., Hensley, C.T., Kim, J., Zacharias, L.G., Yang, C., Do, Q.N., Doucette, S., Burguete, D., et al. (2017). Cell 171, 358–371.e9. Gui, D.Y., Sullivan, L.B., Luengo, A., Hosios, A.M., Bush, L.N., Gitego, N., Davidson, S.M., Freinkman,
Kennedy, K.M., Scarbrough, P.M., Ribeiro, A., Richardson, R., Yuan, H., Sonveaux, P., Landon, C.D., Chi, J.T., Pizzo, S., Schroeder, T., and Dewhirst, M.W. (2013). PLoS One 8, e75154. Viale, A., Pettazzoni, P., Lyssiotis, C.A., Ying, H., Sa´nchez, N., Marchesini, M., Carugo, A., Green, T., Seth, S., Giuliani, V., et al. (2014). Nature 514, 628–632. Weinberg, F., Hamanaka, R., Wheaton, W.W., Weinberg, S., Joseph, J., Lopez, M., Kalyanaraman, B., Mutlu, G.M., Budinger, G.R., and Chandel, N.S. (2010). Proc. Natl. Acad. Sci. USA 107, 8788–8793.