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Liposuction: Extracellular Fat Removal Promotes Proliferation
Please cite this article as: Oikawa et al., Nature’s Strategy for Catalyzing Diels-Alder Reaction, Cell Chemical Biology (2016), http://dx.doi.org/10.1016/ j.chembiol.2016.04.003
Cell Chemical Biology
Previews
Liposuction: Extracellular Fat Removal Promotes Proliferation Robert A. Egnatchik1,* and Ralph J. DeBerardinis1,* 1Children’s Medical Center Research Institute at the University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75390, USA *Correspondence: [email protected] (R.A.E.), [email protected] (R.J.D.) http://dx.doi.org/10.1016/j.chembiol.2016.04.003
In this issue of Cell Chemical Biology, Yao et al. (2016) investigate the makeup of lipid membranes of both cancer and non-transformed cells to reveal that doubling cells preferentially use exogenous fatty acids over de novo synthesis to proliferate. The construction of daughter cells requires duplicating the macromolecular biomass of the parent. Anabolic metabolism converts raw carbohydrate and amino acid material into protein, nucleic acid, and lipid building blocks required for new cells. In particular, the membranes that surround cells and organelles require significant amounts of fatty acid precursors to generate phospholipid bilayers. In this issue of Cell Chemical Biology, Yao et al. (2016) report that proliferating fibroblasts and cancer cells scavenge a majority of the membrane precursor fatty acids from extracellular sources. These findings constitute an advance over many previous studies which focused on the role of de novo fatty acid synthesis for lipid production and have implications for the therapeutic potential of inhibiting fatty acid synthesis in cancer. Cellular membranes are composed of diverse phospholipid species, and synthesis of these species is limited by the availability of fatty acids. To accumulate enough fatty acids to duplicate membrane biomass, the cell has two options. First, dividing cells utilize de novo fatty acid synthesis pathways to accumulate lipid membrane precursors. This process sequentially combines two carbon units from acetyl-CoA to generate the 16 carbon fatty acid palmitate. The source of this acetyl-CoA is dependent on substrate availability, metabolic enzyme levels, and oxygen tension. Normally glucose provides the majority of raw material for acetyl-CoA generation (Lee et al., 1995); however, glutamine can contribute to the lipogenic acetyl-CoA pool under hypoxia (Wise et al., 2011; Metallo et al., 2012), mitochondrial defects (Mullen et al.,
2012), and suppression of pyruvate dehydrogenase (Rajagopalan et al., 2015). Acetate and branched-chain amino acids also serve as alternative acetyl-CoA sources under some circumstances (Schug et al., 2015; Green et al., 2016). Although these studies helped define the contributions of various fatty acid precursors, the fractional contribution of de novo fatty acid synthesis as opposed to fatty acid/ lipid scavenging in proliferating and quiescent cells remains an open question. If doubling cells import fatty acids and other lipid intermediates, as has been observed in some systems (Kamphorst et al., 2013), this might allow them to bypass the need for lipogenic acetylCoA and resist inhibition of fatty acid synthesis. As a model to understand lipid synthesis under proliferative and non-proliferative states, Yao et al. employed fibroblasts which can proliferate rapidly but undergo reversible quiescence due to contact inhibition. They employed an arsenal of 13C analytical techniques to investigate how membrane synthesis differs between proliferating and quiescent fibroblasts. First, the authors fed U-13C6 glucose and U-13C5 glutamine in parallel experiments to determine the net contribution of these carbon sources to the lipid pool. Solid-state NMR was used to measure 13C incorporation into various macromolecular pools, including intact lipids. The authors report that glucose makes a higher contribution to the lipid pool in proliferating as opposed to nonproliferating cells. Glutamine, in contrast, makes a relatively consistent contribution to these same pools independently of the cellular growth rate. The authors report instead that the elevated rate of
glutamine consumption in proliferating cells provides raw material for protein rather than lipid synthesis. This finding is consistent with a recent report indicating that glutamine carbons are primarily fated to enter protein biomass in proliferating cells (Hosios et al., 2016). Yao et al. (2016) suggest that although glutamine is a versatile source of acetyl-CoA, the switch from quiescent to proliferative metabolism occurs without substantive changes in glutamine’s relative contribution to lipogenesis. Next, the authors investigated mechanisms of lipogenesis that do not require de novo fatty acid synthesis. Strikingly, the free fatty acid palmitate scavenged from the medium was by far the most prominent source of fatty acids in quiescent fibroblasts. In proliferating cells, palmitate consumption rose even further to meet the rising demand for lipids, such that approximately 90% of the total palmitate pool was scavenged rather than synthesized. Yao et al. (2016) note that the ability to switch between de novo lipogenesis and net lipid import is not a function of oncogenic transformation; that is, both proliferating fibroblasts and cancer cells modulate the flux through these two pathways as a function of extracellular lipid availability. This is in stark contrast to quiescent fibroblasts, which engage in lipid degradation to supply energy and membrane maintenance in the absence of the high rates of glucose catabolism that accompanies proliferation. The finding that extracellular fatty acids support the proliferative demand for biomass has therapeutic implications for cancer cell proliferation in tumors. Inhibitors have been developed to target Cell Chemical Biology 23, April 21, 2016 431
Please cite this article as: Oikawa et al., Nature’s Strategy for Catalyzing Diels-Alder Reaction, Cell Chemical Biology (2016), http://dx.doi.org/10.1016/ j.chembiol.2016.04.001
Cell Chemical Biology
Previews
key enzymes in the de novo lipogenic pathway. These therapeutics are predicted to attenuate unwanted proliferation by reducing lipid biomass accumulation. However, these drugs would be largely ineffective if proliferating cells circumvent de novo lipogenesis by increasing fatty acid import or if the default pathway of lipogenesis relies heavily on fatty acid scavenging. Interestingly, supplying proliferating cells with an excess of palmitate reduced their glucose uptake and modestly improved their ability to tolerate glycolytic inhibition, adding further evidence that palmitate is preferred over glucose as a lipogenic precursor in the cells studied by Yao et al. Overall, this new work adds to a growing body of literature which states that proliferating cells are efficient scavengers of many available raw materials, including macromolecules, for both biomass and bioenergetics (Commisso et al., 2013). These studies have broadened the metabolic repertoire with which cells can fuel anabolic pathways beyond simple nutrients like glucose and glutamine. It will be interesting
and potentially clinically important to determine whether targeting these scavenging pathways can be used to suppress pathological cell proliferation in cancer and other diseases.
Lee, W.N.P., Byerley, L.O., Bassilian, S., Ajie, H.O., Clark, I., Edmond, J., and Bergner, E.A. (1995). Anal. Biochem. 226, 100–112. Metallo, C.M., Gameiro, P.A., Bell, E.L., Mattaini, K.R., Yang, J., Hiller, K., Jewell, C.M., Johnson, Z.R., Irvine, D.J., Guarente, L., et al. (2012). Nature 481, 380–384.
ACKNOWLEDGMENTS R.J.D. is on the Advisory Boards of Peloton Therapeutics and Agios Pharmaceuticals, two companies with an interest in cancer metabolism. REFERENCES Commisso, C., Davidson, S.M., Soydaner-Azeloglu, R.G., Parker, S.J., Kamphorst, J.J., Hackett, S., Grabocka, E., Nofal, M., Drebin, J.A., Thompson, C.B., et al. (2013). Nature 497, 633–637. Green, C.R., Wallace, M., Divakaruni, A.S., Phillips, S.A., Murphy, A.N., Ciaraldi, T.P., and Metallo, C.M. (2016). Nat. Chem. Biol. 12, 15–21. Hosios, A.M., Hecht, V.C., Danai, L.V., Johnson, M.O., Rathmell, J.C., Steinhauser, M.L., Manalis, S.R., and Vander Heiden, M.G. (2016). Dev. Cell 36, 540–549. Kamphorst, J.J., Cross, J.R., Fan, J., de Stanchina, E., Mathew, R., White, E.P., Thompson, C.B., and Rabinowitz, J.D. (2013). Proc. Natl. Acad. Sci. USA 110, 8882–8887.
Mullen, A.R., Wheaton, W.W., Jin, E.S., Chen, P.-H., Sullivan, L.B., Cheng, T., Yang, Y., Linehan, W.M., Chandel, N.S., and DeBerardinis, R.J. (2012). Nature 481, 385–388. Rajagopalan, K.N., Egnatchik, R.A., Calvaruso, M.A., Wasti, A.T., Padanad, M.S., Boroughs, L.K., Ko, B., Hensley, C.T., Acar, M., Hu, Z., et al. (2015). Cancer Metab. 3, 7. Schug, Z.T., Peck, B., Jones, D.T., Zhang, Q., Grosskurth, S., Alam, I.S., Goodwin, L.M., Smethurst, E., Mason, S., Blyth, K., et al. (2015). Cancer Cell 27, 57–71. Wise, D.R., Ward, P.S., Shay, J.E.S., Cross, J.R., Gruber, J.J., Sachdeva, U.M., Platt, J.M., DeMatteo, R.G., Simon, M.C., and Thompson, C.B. (2011). Proc. Natl. Acad. Sci. USA 108, 19611– 19616. Yao, C., Grider, R.F., Mahieu, N.G., Liu, G., Chen, Y., Wang, R., Singh, M., Potter, G.S., Gross, R.W., Schaefer, J., et al. (2016). Cell Chem. Biol. 23, this issue, 483–493.
Novel Diubiquitin Probes Expand the Chemical Toolkit to Study DUBs Yogesh Kulathu1,* 1MRC Protein Phosphorylation and Ubiquitylation Unit, School of Life Sciences, University of Dundee, Dow Street, Dundee DD1 5EH, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chembiol.2016.04.001
Linkage-specific DUBs employ different mechanisms to recognize and cleave polyubiquitin chains of specific linkage types. In this issue of Cell Chemical Biology, Flierman et al. (2016) develop a new family of novel nonhydrolyzable diubiquitin probes that will be valuable tools to study how DUBs achieve specificity. The posttranslational modification of proteins with one or several ubiquitin molecules can alter the fate, activity, localization, and protein-protein interactions of the modified protein. Polyubiquitin chains of eight different linkage types can be generated and the type of linkage between the ubiquitins can determine the outcome of ubiquitylation. Thousands of cellular proteins are modified by ubiquitin and as a consequence, ubiquitylation reg432 Cell Chemical Biology 23, April 21, 2016
ulates a wide range of cellular processes in eukaryotes. Deubiquitinases (DUBs) are proteases that reverse this modification by hydrolysing the isopeptide bond between ubiquitin and the target protein, and thereby function as important regulators of ubiquitylation. How DUBs specifically recognize and hydrolyze different polyubiquitin modifications is poorly understood. In this issue, Flierman et al. (2016) describe novel ubiquitin
probes that will be valuable tools to study how DUBs recognize ubiquitin chains of different linkage types. There are 100 DUBs encoded in the human genome that can be classified into five different families: ubiquitin C-terminal hydrolase (UCH), ubiquitin-specific proteases (USPs), ovarian tumor (OTU), Josephins, and JAB1/MPDN+/MOV34 (JAMM). With the exception of JAMM family DUBs, which are metallo-proteases, the
Cell Chemical Biology
Previews
Liposuction: Extracellular Fat Removal Promotes Proliferation Robert A. Egnatchik1,* and Ralph J. DeBerardinis1,* 1Children’s Medical Center Research Institute at the University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd, Dallas, TX 75390, USA *Correspondence: [email protected] (R.A.E.), [email protected] (R.J.D.) http://dx.doi.org/10.1016/j.chembiol.2016.04.003
In this issue of Cell Chemical Biology, Yao et al. (2016) investigate the makeup of lipid membranes of both cancer and non-transformed cells to reveal that doubling cells preferentially use exogenous fatty acids over de novo synthesis to proliferate. The construction of daughter cells requires duplicating the macromolecular biomass of the parent. Anabolic metabolism converts raw carbohydrate and amino acid material into protein, nucleic acid, and lipid building blocks required for new cells. In particular, the membranes that surround cells and organelles require significant amounts of fatty acid precursors to generate phospholipid bilayers. In this issue of Cell Chemical Biology, Yao et al. (2016) report that proliferating fibroblasts and cancer cells scavenge a majority of the membrane precursor fatty acids from extracellular sources. These findings constitute an advance over many previous studies which focused on the role of de novo fatty acid synthesis for lipid production and have implications for the therapeutic potential of inhibiting fatty acid synthesis in cancer. Cellular membranes are composed of diverse phospholipid species, and synthesis of these species is limited by the availability of fatty acids. To accumulate enough fatty acids to duplicate membrane biomass, the cell has two options. First, dividing cells utilize de novo fatty acid synthesis pathways to accumulate lipid membrane precursors. This process sequentially combines two carbon units from acetyl-CoA to generate the 16 carbon fatty acid palmitate. The source of this acetyl-CoA is dependent on substrate availability, metabolic enzyme levels, and oxygen tension. Normally glucose provides the majority of raw material for acetyl-CoA generation (Lee et al., 1995); however, glutamine can contribute to the lipogenic acetyl-CoA pool under hypoxia (Wise et al., 2011; Metallo et al., 2012), mitochondrial defects (Mullen et al.,
2012), and suppression of pyruvate dehydrogenase (Rajagopalan et al., 2015). Acetate and branched-chain amino acids also serve as alternative acetyl-CoA sources under some circumstances (Schug et al., 2015; Green et al., 2016). Although these studies helped define the contributions of various fatty acid precursors, the fractional contribution of de novo fatty acid synthesis as opposed to fatty acid/ lipid scavenging in proliferating and quiescent cells remains an open question. If doubling cells import fatty acids and other lipid intermediates, as has been observed in some systems (Kamphorst et al., 2013), this might allow them to bypass the need for lipogenic acetylCoA and resist inhibition of fatty acid synthesis. As a model to understand lipid synthesis under proliferative and non-proliferative states, Yao et al. employed fibroblasts which can proliferate rapidly but undergo reversible quiescence due to contact inhibition. They employed an arsenal of 13C analytical techniques to investigate how membrane synthesis differs between proliferating and quiescent fibroblasts. First, the authors fed U-13C6 glucose and U-13C5 glutamine in parallel experiments to determine the net contribution of these carbon sources to the lipid pool. Solid-state NMR was used to measure 13C incorporation into various macromolecular pools, including intact lipids. The authors report that glucose makes a higher contribution to the lipid pool in proliferating as opposed to nonproliferating cells. Glutamine, in contrast, makes a relatively consistent contribution to these same pools independently of the cellular growth rate. The authors report instead that the elevated rate of
glutamine consumption in proliferating cells provides raw material for protein rather than lipid synthesis. This finding is consistent with a recent report indicating that glutamine carbons are primarily fated to enter protein biomass in proliferating cells (Hosios et al., 2016). Yao et al. (2016) suggest that although glutamine is a versatile source of acetyl-CoA, the switch from quiescent to proliferative metabolism occurs without substantive changes in glutamine’s relative contribution to lipogenesis. Next, the authors investigated mechanisms of lipogenesis that do not require de novo fatty acid synthesis. Strikingly, the free fatty acid palmitate scavenged from the medium was by far the most prominent source of fatty acids in quiescent fibroblasts. In proliferating cells, palmitate consumption rose even further to meet the rising demand for lipids, such that approximately 90% of the total palmitate pool was scavenged rather than synthesized. Yao et al. (2016) note that the ability to switch between de novo lipogenesis and net lipid import is not a function of oncogenic transformation; that is, both proliferating fibroblasts and cancer cells modulate the flux through these two pathways as a function of extracellular lipid availability. This is in stark contrast to quiescent fibroblasts, which engage in lipid degradation to supply energy and membrane maintenance in the absence of the high rates of glucose catabolism that accompanies proliferation. The finding that extracellular fatty acids support the proliferative demand for biomass has therapeutic implications for cancer cell proliferation in tumors. Inhibitors have been developed to target Cell Chemical Biology 23, April 21, 2016 431
Please cite this article as: Oikawa et al., Nature’s Strategy for Catalyzing Diels-Alder Reaction, Cell Chemical Biology (2016), http://dx.doi.org/10.1016/ j.chembiol.2016.04.001
Cell Chemical Biology
Previews
key enzymes in the de novo lipogenic pathway. These therapeutics are predicted to attenuate unwanted proliferation by reducing lipid biomass accumulation. However, these drugs would be largely ineffective if proliferating cells circumvent de novo lipogenesis by increasing fatty acid import or if the default pathway of lipogenesis relies heavily on fatty acid scavenging. Interestingly, supplying proliferating cells with an excess of palmitate reduced their glucose uptake and modestly improved their ability to tolerate glycolytic inhibition, adding further evidence that palmitate is preferred over glucose as a lipogenic precursor in the cells studied by Yao et al. Overall, this new work adds to a growing body of literature which states that proliferating cells are efficient scavengers of many available raw materials, including macromolecules, for both biomass and bioenergetics (Commisso et al., 2013). These studies have broadened the metabolic repertoire with which cells can fuel anabolic pathways beyond simple nutrients like glucose and glutamine. It will be interesting
and potentially clinically important to determine whether targeting these scavenging pathways can be used to suppress pathological cell proliferation in cancer and other diseases.
Lee, W.N.P., Byerley, L.O., Bassilian, S., Ajie, H.O., Clark, I., Edmond, J., and Bergner, E.A. (1995). Anal. Biochem. 226, 100–112. Metallo, C.M., Gameiro, P.A., Bell, E.L., Mattaini, K.R., Yang, J., Hiller, K., Jewell, C.M., Johnson, Z.R., Irvine, D.J., Guarente, L., et al. (2012). Nature 481, 380–384.
ACKNOWLEDGMENTS R.J.D. is on the Advisory Boards of Peloton Therapeutics and Agios Pharmaceuticals, two companies with an interest in cancer metabolism. REFERENCES Commisso, C., Davidson, S.M., Soydaner-Azeloglu, R.G., Parker, S.J., Kamphorst, J.J., Hackett, S., Grabocka, E., Nofal, M., Drebin, J.A., Thompson, C.B., et al. (2013). Nature 497, 633–637. Green, C.R., Wallace, M., Divakaruni, A.S., Phillips, S.A., Murphy, A.N., Ciaraldi, T.P., and Metallo, C.M. (2016). Nat. Chem. Biol. 12, 15–21. Hosios, A.M., Hecht, V.C., Danai, L.V., Johnson, M.O., Rathmell, J.C., Steinhauser, M.L., Manalis, S.R., and Vander Heiden, M.G. (2016). Dev. Cell 36, 540–549. Kamphorst, J.J., Cross, J.R., Fan, J., de Stanchina, E., Mathew, R., White, E.P., Thompson, C.B., and Rabinowitz, J.D. (2013). Proc. Natl. Acad. Sci. USA 110, 8882–8887.
Mullen, A.R., Wheaton, W.W., Jin, E.S., Chen, P.-H., Sullivan, L.B., Cheng, T., Yang, Y., Linehan, W.M., Chandel, N.S., and DeBerardinis, R.J. (2012). Nature 481, 385–388. Rajagopalan, K.N., Egnatchik, R.A., Calvaruso, M.A., Wasti, A.T., Padanad, M.S., Boroughs, L.K., Ko, B., Hensley, C.T., Acar, M., Hu, Z., et al. (2015). Cancer Metab. 3, 7. Schug, Z.T., Peck, B., Jones, D.T., Zhang, Q., Grosskurth, S., Alam, I.S., Goodwin, L.M., Smethurst, E., Mason, S., Blyth, K., et al. (2015). Cancer Cell 27, 57–71. Wise, D.R., Ward, P.S., Shay, J.E.S., Cross, J.R., Gruber, J.J., Sachdeva, U.M., Platt, J.M., DeMatteo, R.G., Simon, M.C., and Thompson, C.B. (2011). Proc. Natl. Acad. Sci. USA 108, 19611– 19616. Yao, C., Grider, R.F., Mahieu, N.G., Liu, G., Chen, Y., Wang, R., Singh, M., Potter, G.S., Gross, R.W., Schaefer, J., et al. (2016). Cell Chem. Biol. 23, this issue, 483–493.
Novel Diubiquitin Probes Expand the Chemical Toolkit to Study DUBs Yogesh Kulathu1,* 1MRC Protein Phosphorylation and Ubiquitylation Unit, School of Life Sciences, University of Dundee, Dow Street, Dundee DD1 5EH, UK *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chembiol.2016.04.001
Linkage-specific DUBs employ different mechanisms to recognize and cleave polyubiquitin chains of specific linkage types. In this issue of Cell Chemical Biology, Flierman et al. (2016) develop a new family of novel nonhydrolyzable diubiquitin probes that will be valuable tools to study how DUBs achieve specificity. The posttranslational modification of proteins with one or several ubiquitin molecules can alter the fate, activity, localization, and protein-protein interactions of the modified protein. Polyubiquitin chains of eight different linkage types can be generated and the type of linkage between the ubiquitins can determine the outcome of ubiquitylation. Thousands of cellular proteins are modified by ubiquitin and as a consequence, ubiquitylation reg432 Cell Chemical Biology 23, April 21, 2016
ulates a wide range of cellular processes in eukaryotes. Deubiquitinases (DUBs) are proteases that reverse this modification by hydrolysing the isopeptide bond between ubiquitin and the target protein, and thereby function as important regulators of ubiquitylation. How DUBs specifically recognize and hydrolyze different polyubiquitin modifications is poorly understood. In this issue, Flierman et al. (2016) describe novel ubiquitin
probes that will be valuable tools to study how DUBs recognize ubiquitin chains of different linkage types. There are 100 DUBs encoded in the human genome that can be classified into five different families: ubiquitin C-terminal hydrolase (UCH), ubiquitin-specific proteases (USPs), ovarian tumor (OTU), Josephins, and JAB1/MPDN+/MOV34 (JAMM). With the exception of JAMM family DUBs, which are metallo-proteases, the