Chemistry & Biology
Previews with energies of highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital and electronegativities of the involved chemical species (Suksrichavalit et al., 2008). Do these findings imply that we need to revisit our thinking about the mechanism of action of at least some hydroxamic based HDAC inhibitors, including recently FDA-approved panobinostat (LBH-589), and consider that they might act also through HDAC-independent, catalase mimetic mechanisms? One important point to keep in mind about Olson et al.’s study is that all of the cell based assays were performed in the presence of high concentrations of compounds (usually 30 mM). These concentrations might not be relevant from a clinical perspective, given that, for example, in the case of panobinostat, this concentration is >10,000 higher than the biochemical IC50 calculated in vitro against HDACs. By itself, this does not necessarily mean that we should not think very carefully about whether a systematic re-evaluation of hydroxamic based HDAC inhibitors and their HDAC-inde-
pendent, catalase mimetic mechanisms is needed. But we pose that the critical question to keep in mind is whether the high concentrations need to achieve high catalase mimetic efficiency can be reached in vivo in patients, for example, in a chronic treatment setting. We think that this is unlikely because dose-limiting toxicity is readily achieved for all clinically tested HDAC inhibitors at doses well below the ‘‘catalase mimetic’’ range (Figure 1). For panobinostat, clinically used doses reach maximum values below 1 mM (Anne et al., 2013). Interestingly, Olson et al. (2015) suggest that HDAC inhibitors may act through a dual mechanism against oxidative stress: (1) inhibition of HDACs at low concentrations and (2) catalase mimetics at high concentrations. On the other hand, we think that the only clinically relevant concentrations for HDAC inhibitors are the ‘‘low’’ ones and that catalase mimetic mechanism might be of importance for those interested in exploring effects of HDAC inhibitors and oxidative stress in cell-based settings. The work by Olson et al. (2015) opens up several interesting opportunities for future
research. For example, it might be interesting to design new hydroxamic-acidbased compounds that would exhibit catalase mimetic activity at much lower concentrations. Those compounds would be more clinically attractive strategy to protect from oxidative stress via HDACdependent and -independent routes. REFERENCES Anne, M., Sammartino, D., Barginear, M.F., and Budman, D. (2013). Onco. Targets Ther. 6, 1613– 1624. Falkenberg, K.J., and Johnstone, R.W. (2014). Nat. Rev. Drug Discov. 13, 673–691. Lombardi, P.M., Cole, K.E., Dowling, D.P., and Christianson, D.W. (2011). Curr. Opin. Struct. Biol. 21, 735–743. Olson, D.E., Sleiman, S.F., Bourassa, M.W., Wagner, F.F., Gale, J.P., Zhang, Y.-L., Ratan, R.R., and Holson, E.B. (2015). Chem. Biol. 22, this issue, 439–445. Rong, Y., Doctrow, S.R., Tocco, G., and Baudry, M. (1999). Proc. Natl. Acad. Sci. USA 96, 9897– 9902. Suksrichavalit, T., Prachayasittikul, S., Piacham, T., Isarankura-Na-Ayudhya, C., Nantasenamat, C., and Prachayasittikul, V. (2008). Molecules 13, 3040–3056.
Macrophage Activation: On PAR with LPS Florian J. Bock1 and Paul Chang1,2,* 1Koch
Institute for Integrative Cancer Research, Massachusetts Institute of Technology, 500 Main Street, Cambridge, MA 02139, USA of Biology, Massachusetts Institute of Technology, 500 Main Street, Cambridge, MA 02139, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chembiol.2015.04.006 2Department
The inflammatory response is a critical component of the immune system that is activated by stimuli such as cytokines, foreign DNA, RNA, or other harmful substances. Krukenberg et al. (2015) identify poly(ADP-ribose) as a new signaling molecule that activates inflammation, thus providing yet another mechanism by which PARPs are involved in cellular stress responses. Multicellular organisms are in constant danger from harmful foreign substances, chemicals, and pathogens. Multiple defense mechanisms have therefore evolved to counter these threats. In mammals, the innate immune system acts as the front line defense to detect these stimuli and to mount an initial response that puts the immune system into a state of alert (Janeway and Medzhitov, 2002).
Stimuli such as foreign DNA, RNA, or other harmful substances are detected by macrophages that are subsequently activated. Upon activation, the macrophages secret cytokines that attract immune cells to the site of danger, activate immune cells, and present them with the molecular signature of the threat. Stimuli that activate macrophages are called PAMP (pathogen-associated molecular
432 Chemistry & Biology 22, April 23, 2015 ª2015 Elsevier Ltd All rights reserved
pattern) when derived from pathogens (e.g., DNA, RNA, or other molecules) or DAMP (damage-associated molecular pattern) during noninfectious inflammatory responses (e.g., harmful substances or normally intracellular host DNA, RNA, or proteins) (Tang et al., 2012). These signals are primarily recognized by pattern recognition receptors (PRR) such as members of the toll-like receptor (TLR)
Chemistry & Biology
Previews family found at the plasma vating the PARP(s) that membrane and intracellular are relevant in this new inflammembranes or the NOD-like matory signaling pathway or (NLRs) and RIG-I-like recepsystemic infusion of PAR could tors (RLR) found in the cytobe used to enhance the plasm. Upon engagement of immune response by providing these receptors, signaling an additional signal for activacascades are induced that tion of the immune system. It culminate in the activation of is already established that multiple stress-response PARP1 inhibitors have an pathways. The key pathway anti-inflammatory effect as a activated is NFkB, leading to result of regulation of tranincreased transcription and scription (Peralta-Leal et al., subsequent release of inflam2009). These inhibitors could Figure 1. PARP1 Enzymatic Activity Is Highly Upregulated in Cells matory cytokines. also impinge upon this extraUndergoing Necrosis Upon lysis of the necrotic cell, the PAR synthesized by PARP1 is released into Poly(ADP-ribose) (PAR) is a cellular mechanism of eliciting the extracellular space. Extracellular PAR can then be detected by TLR2 and posttranslational modification an inflammatory response. TLR4, which are endocytosed and signal the activation of NFkB, resulting in generated by poly(ADP)ribose Such inhibition could help the secretion of cytokines and activation of an inflammatory response. polymerases (PARPs) using dampen the immune response NAD+ as a substrate (Gibson by preventing activation of and Kraus, 2012). Most PARPs modify induce this response. Therefore PAR can macrophages by the released PAR and target proteins with monomers called be categorized as a novel DAMP (Figure 1). the following induction of proinflammatory mono(ADP-ribose); however, several The current study (Krukenberg et al., cytokines. Modulation of this response PARPs, including PARP1, 2, 5a, and 5b, 2015) has not identified the source of the could therefore be an interesting avenue are capable of generating PAR polymers. extracellular PAR that activates macro- for follow-up studies and the treatment of In addition to modifying target protein phages. One possible source is cells un- conditions of hyperactive inflammation. function by covalent attachment, PAR dergoing necrosis, a form of uncontrolled can also act as a signaling molecule by cell death known to induce an inflamma- ACKNOWLEDGEMENTS recruiting PAR binding proteins. Through tory response. PARP1 is hyperactivated this signaling function, PAR can alter during necrotic cell death, resulting in This work was supported by the National Institutes the localization, binding interactions, increased synthesis of PAR and a of Health (RO1GM087465) to P.C. F.B. was funded by a Ludwig Postdoctoral Fellowship. or activity of target proteins. PAR is corresponding depletion of NAD+ and primarily known to function in stress ATP (Zong et al., 2004). Following necrotic responses, but also plays stress-inde- lysis, the dying cells release high levels of REFERENCES pendent roles, in which it influences gene PAR, which can then be detected by Akira, S., and Takeda, K. (2004). Nat. Rev. Immuexpression and signal transduction (Vyas macrophages as described in the present nol. 4, 499–511. and Chang, 2014). PARP1 in particular study (Figure 1). Therefore, PAR could act Altmeyer, M., and Hottiger, M.O. (2009). Aging has been shown to modulate transcription as a signal for macrophages to engage (Albany NY) 1, 458–469. of a subset of NFkB target genes during the immune system at the site of necrosis. inflammation (Altmeyer and Hottiger, Other possibilities include the detection of Gibson, B.A., and Kraus, W.L. (2012). Nat. Rev. Mol. Cell Biol. 13, 411–424. stressed cells that could act as a signal 2009). To date, all of the functions identified that engagement of the immune system Janeway, C.A., Jr., and Medzhitov, R. (2002). Annu. Rev. Immunol. 20, 197–216. for PAR as a signaling molecule occur is needed. These cells could actively intracellularly. Krukenberg et al. (2015) secrete PAR, or PARylated proteins in Krukenberg, K.A., Kim, S., Tan, E.S., Maliga, Z., now show that PAR can act as an a yet unidentified manner as a means to and Mitchison, T.J. (2015). Chem. Biol. 22, this issue, 446–452. extracellular signaling molecule that is activate macrophages. Whether TLRs endocytosed into endosomes by macro- directly recognize, bind to, or endocytose Peralta-Leal, A., Rodrı´guez-Vargas, J.M., AguilarQuesada, R., Rodrı´guez, M.I., Linares, J.L., de Alphages, activating NFkB signaling. This PAR has not been investigated, but it is modo´var, M.R., and Oliver, F.J. (2009). Free Radic. process requires the PRRs TLR2 and an area of obvious interest given these Biol. Med. 47, 13–26. TLR4, which can detect various molecules results. TLRs require additional molecules Tang, D., Kang, R., Coyne, C.B., Zeh, H.J., and including the bacterial cell wall component (for instance CD14 in the case of LPS Lotze, M.T. (2012). Immunol. Rev. 249, 158–175. lipopolysaccharide (LPS). Upon PAR [Zanoni et al., 2011]) to detect their Vyas, S., and Chang, P. (2014). Nat. Rev. Cancer binding, TLR2 and TLR4 induce an in- targets, so the possibility exists that PAR 14, 502–509. flammatory response in a manner similar also requires additional factors to be Zanoni, I., Ostuni, R., Marek, L.R., Barresi, S., Barto pathogenic DNA and RNA (Akira and detected by TLRs. balat, R., Barton, G.M., Granucci, F., and Kagan, Given the widespread function of inflam- J.C. (2011). Cell 147, 868–880. Takeda, 2004). This new pathway of inflammatory response activation is specific mation in pathologies, the results from KruZong, W.X., Ditsworth, D., Bauer, D.E., Wang, to PAR, since single units of ADP-ribose kenberg et al. (2015) could have important Z.Q., and Thompson, C.B. (2004). Genes Dev. 18, or structurally similar molecules cannot clinical relevance. Once identified, acti- 1272–1282. Chemistry & Biology 22, April 23, 2015 ª2015 Elsevier Ltd All rights reserved 433
Previews with energies of highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital and electronegativities of the involved chemical species (Suksrichavalit et al., 2008). Do these findings imply that we need to revisit our thinking about the mechanism of action of at least some hydroxamic based HDAC inhibitors, including recently FDA-approved panobinostat (LBH-589), and consider that they might act also through HDAC-independent, catalase mimetic mechanisms? One important point to keep in mind about Olson et al.’s study is that all of the cell based assays were performed in the presence of high concentrations of compounds (usually 30 mM). These concentrations might not be relevant from a clinical perspective, given that, for example, in the case of panobinostat, this concentration is >10,000 higher than the biochemical IC50 calculated in vitro against HDACs. By itself, this does not necessarily mean that we should not think very carefully about whether a systematic re-evaluation of hydroxamic based HDAC inhibitors and their HDAC-inde-
pendent, catalase mimetic mechanisms is needed. But we pose that the critical question to keep in mind is whether the high concentrations need to achieve high catalase mimetic efficiency can be reached in vivo in patients, for example, in a chronic treatment setting. We think that this is unlikely because dose-limiting toxicity is readily achieved for all clinically tested HDAC inhibitors at doses well below the ‘‘catalase mimetic’’ range (Figure 1). For panobinostat, clinically used doses reach maximum values below 1 mM (Anne et al., 2013). Interestingly, Olson et al. (2015) suggest that HDAC inhibitors may act through a dual mechanism against oxidative stress: (1) inhibition of HDACs at low concentrations and (2) catalase mimetics at high concentrations. On the other hand, we think that the only clinically relevant concentrations for HDAC inhibitors are the ‘‘low’’ ones and that catalase mimetic mechanism might be of importance for those interested in exploring effects of HDAC inhibitors and oxidative stress in cell-based settings. The work by Olson et al. (2015) opens up several interesting opportunities for future
research. For example, it might be interesting to design new hydroxamic-acidbased compounds that would exhibit catalase mimetic activity at much lower concentrations. Those compounds would be more clinically attractive strategy to protect from oxidative stress via HDACdependent and -independent routes. REFERENCES Anne, M., Sammartino, D., Barginear, M.F., and Budman, D. (2013). Onco. Targets Ther. 6, 1613– 1624. Falkenberg, K.J., and Johnstone, R.W. (2014). Nat. Rev. Drug Discov. 13, 673–691. Lombardi, P.M., Cole, K.E., Dowling, D.P., and Christianson, D.W. (2011). Curr. Opin. Struct. Biol. 21, 735–743. Olson, D.E., Sleiman, S.F., Bourassa, M.W., Wagner, F.F., Gale, J.P., Zhang, Y.-L., Ratan, R.R., and Holson, E.B. (2015). Chem. Biol. 22, this issue, 439–445. Rong, Y., Doctrow, S.R., Tocco, G., and Baudry, M. (1999). Proc. Natl. Acad. Sci. USA 96, 9897– 9902. Suksrichavalit, T., Prachayasittikul, S., Piacham, T., Isarankura-Na-Ayudhya, C., Nantasenamat, C., and Prachayasittikul, V. (2008). Molecules 13, 3040–3056.
Macrophage Activation: On PAR with LPS Florian J. Bock1 and Paul Chang1,2,* 1Koch
Institute for Integrative Cancer Research, Massachusetts Institute of Technology, 500 Main Street, Cambridge, MA 02139, USA of Biology, Massachusetts Institute of Technology, 500 Main Street, Cambridge, MA 02139, USA *Correspondence: [email protected] http://dx.doi.org/10.1016/j.chembiol.2015.04.006 2Department
The inflammatory response is a critical component of the immune system that is activated by stimuli such as cytokines, foreign DNA, RNA, or other harmful substances. Krukenberg et al. (2015) identify poly(ADP-ribose) as a new signaling molecule that activates inflammation, thus providing yet another mechanism by which PARPs are involved in cellular stress responses. Multicellular organisms are in constant danger from harmful foreign substances, chemicals, and pathogens. Multiple defense mechanisms have therefore evolved to counter these threats. In mammals, the innate immune system acts as the front line defense to detect these stimuli and to mount an initial response that puts the immune system into a state of alert (Janeway and Medzhitov, 2002).
Stimuli such as foreign DNA, RNA, or other harmful substances are detected by macrophages that are subsequently activated. Upon activation, the macrophages secret cytokines that attract immune cells to the site of danger, activate immune cells, and present them with the molecular signature of the threat. Stimuli that activate macrophages are called PAMP (pathogen-associated molecular
432 Chemistry & Biology 22, April 23, 2015 ª2015 Elsevier Ltd All rights reserved
pattern) when derived from pathogens (e.g., DNA, RNA, or other molecules) or DAMP (damage-associated molecular pattern) during noninfectious inflammatory responses (e.g., harmful substances or normally intracellular host DNA, RNA, or proteins) (Tang et al., 2012). These signals are primarily recognized by pattern recognition receptors (PRR) such as members of the toll-like receptor (TLR)
Chemistry & Biology
Previews family found at the plasma vating the PARP(s) that membrane and intracellular are relevant in this new inflammembranes or the NOD-like matory signaling pathway or (NLRs) and RIG-I-like recepsystemic infusion of PAR could tors (RLR) found in the cytobe used to enhance the plasm. Upon engagement of immune response by providing these receptors, signaling an additional signal for activacascades are induced that tion of the immune system. It culminate in the activation of is already established that multiple stress-response PARP1 inhibitors have an pathways. The key pathway anti-inflammatory effect as a activated is NFkB, leading to result of regulation of tranincreased transcription and scription (Peralta-Leal et al., subsequent release of inflam2009). These inhibitors could Figure 1. PARP1 Enzymatic Activity Is Highly Upregulated in Cells matory cytokines. also impinge upon this extraUndergoing Necrosis Upon lysis of the necrotic cell, the PAR synthesized by PARP1 is released into Poly(ADP-ribose) (PAR) is a cellular mechanism of eliciting the extracellular space. Extracellular PAR can then be detected by TLR2 and posttranslational modification an inflammatory response. TLR4, which are endocytosed and signal the activation of NFkB, resulting in generated by poly(ADP)ribose Such inhibition could help the secretion of cytokines and activation of an inflammatory response. polymerases (PARPs) using dampen the immune response NAD+ as a substrate (Gibson by preventing activation of and Kraus, 2012). Most PARPs modify induce this response. Therefore PAR can macrophages by the released PAR and target proteins with monomers called be categorized as a novel DAMP (Figure 1). the following induction of proinflammatory mono(ADP-ribose); however, several The current study (Krukenberg et al., cytokines. Modulation of this response PARPs, including PARP1, 2, 5a, and 5b, 2015) has not identified the source of the could therefore be an interesting avenue are capable of generating PAR polymers. extracellular PAR that activates macro- for follow-up studies and the treatment of In addition to modifying target protein phages. One possible source is cells un- conditions of hyperactive inflammation. function by covalent attachment, PAR dergoing necrosis, a form of uncontrolled can also act as a signaling molecule by cell death known to induce an inflamma- ACKNOWLEDGEMENTS recruiting PAR binding proteins. Through tory response. PARP1 is hyperactivated this signaling function, PAR can alter during necrotic cell death, resulting in This work was supported by the National Institutes the localization, binding interactions, increased synthesis of PAR and a of Health (RO1GM087465) to P.C. F.B. was funded by a Ludwig Postdoctoral Fellowship. or activity of target proteins. PAR is corresponding depletion of NAD+ and primarily known to function in stress ATP (Zong et al., 2004). Following necrotic responses, but also plays stress-inde- lysis, the dying cells release high levels of REFERENCES pendent roles, in which it influences gene PAR, which can then be detected by Akira, S., and Takeda, K. (2004). Nat. Rev. Immuexpression and signal transduction (Vyas macrophages as described in the present nol. 4, 499–511. and Chang, 2014). PARP1 in particular study (Figure 1). Therefore, PAR could act Altmeyer, M., and Hottiger, M.O. (2009). Aging has been shown to modulate transcription as a signal for macrophages to engage (Albany NY) 1, 458–469. of a subset of NFkB target genes during the immune system at the site of necrosis. inflammation (Altmeyer and Hottiger, Other possibilities include the detection of Gibson, B.A., and Kraus, W.L. (2012). Nat. Rev. Mol. Cell Biol. 13, 411–424. stressed cells that could act as a signal 2009). To date, all of the functions identified that engagement of the immune system Janeway, C.A., Jr., and Medzhitov, R. (2002). Annu. Rev. Immunol. 20, 197–216. for PAR as a signaling molecule occur is needed. These cells could actively intracellularly. Krukenberg et al. (2015) secrete PAR, or PARylated proteins in Krukenberg, K.A., Kim, S., Tan, E.S., Maliga, Z., now show that PAR can act as an a yet unidentified manner as a means to and Mitchison, T.J. (2015). Chem. Biol. 22, this issue, 446–452. extracellular signaling molecule that is activate macrophages. Whether TLRs endocytosed into endosomes by macro- directly recognize, bind to, or endocytose Peralta-Leal, A., Rodrı´guez-Vargas, J.M., AguilarQuesada, R., Rodrı´guez, M.I., Linares, J.L., de Alphages, activating NFkB signaling. This PAR has not been investigated, but it is modo´var, M.R., and Oliver, F.J. (2009). Free Radic. process requires the PRRs TLR2 and an area of obvious interest given these Biol. Med. 47, 13–26. TLR4, which can detect various molecules results. TLRs require additional molecules Tang, D., Kang, R., Coyne, C.B., Zeh, H.J., and including the bacterial cell wall component (for instance CD14 in the case of LPS Lotze, M.T. (2012). Immunol. Rev. 249, 158–175. lipopolysaccharide (LPS). Upon PAR [Zanoni et al., 2011]) to detect their Vyas, S., and Chang, P. (2014). Nat. Rev. Cancer binding, TLR2 and TLR4 induce an in- targets, so the possibility exists that PAR 14, 502–509. flammatory response in a manner similar also requires additional factors to be Zanoni, I., Ostuni, R., Marek, L.R., Barresi, S., Barto pathogenic DNA and RNA (Akira and detected by TLRs. balat, R., Barton, G.M., Granucci, F., and Kagan, Given the widespread function of inflam- J.C. (2011). Cell 147, 868–880. Takeda, 2004). This new pathway of inflammatory response activation is specific mation in pathologies, the results from KruZong, W.X., Ditsworth, D., Bauer, D.E., Wang, to PAR, since single units of ADP-ribose kenberg et al. (2015) could have important Z.Q., and Thompson, C.B. (2004). Genes Dev. 18, or structurally similar molecules cannot clinical relevance. Once identified, acti- 1272–1282. Chemistry & Biology 22, April 23, 2015 ª2015 Elsevier Ltd All rights reserved 433
