- Email: [email protected]
Autophagy Mediates Tumor Suppression via Cellular Senescence
TICB 1196 No. of Pages 3
Spotlight
Autophagy Mediates Tumor Suppression via Cellular Senescence Lorenzo Galluzzi,1,2,3,4,5,* José Manuel Bravo-San[7_TD$IF] Pedro,1,2,3,4,5 and Guido Kroemer1,2,3,4,6,7,8,* Autophagy not only constitutes a robust barrier against malignant transformation at the cell-intrinsic level, but also contributes to the organismal control of potentially oncogenic cells. Recent data provide molecular insights into the mechanisms whereby oncogene hyperactivation induces autophagy to establish a permanent proliferative arrest commonly known as cellular senescence.
(Figure 1A). Taken together, these two cell-intrinsic functions of autophagy explain why it generally counteracts malignant transformation (the conversion of a healthy cell into a neoplastic precursor) [1–3]. Autophagy also preserves organismal homeostasis and mediates oncosuppressive functions as it participates in the communication of unrecoverable states of cellular distress to the whole organism (Figure 1A). Thus, while autophagy-proficient malignant cells secrete ATP as a danger signal in the course of so-called [9_TD$IF]‘immunogenic cell death,[10_TD$IF]’ which results in the activation of an adaptive immune response against dying cells, their autophagy-deficient counterparts fail to do so and hence go unnoticed by the immune system, culminating in deficient anticancer immunosurveillance [4]. Along similar lines, the downregulation of essential components of the autophagic machinery like ATG5 or ATG7 prevents human diploid fibroblasts (HDFs) to undergo a permanent proliferative arrest commonly known as cellular senescence following oncogene hyperactivation [5,6]. Importantly, although the latter may appear as a purely cell-intrinsic oncosuppressive mechanism, it is not. Indeed, senescent cells not only secrete a wide panel of cytokines and chemokines that alert the organism of a situation of danger, but also express increased amounts of ligands for activatory natural killer (NK)-cell receptors, resulting in the activation of the innate arm of the immune system [7]. Oncogeneinduced senescence (OIS) not only relies on autophagy [5,6], but also proceeds along with the degradation of lamin B1 (LMNB1), a component of the nuclear lamina, and the consequent activation of tumor protein 53 (TP53) and retinoblastoma 1 (RB1) [8]. Until recently, however, the mechanistic links between autophagy and LMNB1 degradation in the course of OIS were elusive.
Macroautophagy (hereafter referred to as autophagy) is an evolutionary ancient process involving the sequestration of endogenous or exogenous cytoplasmic material within a double-membraned organelle (called autophagosome), followed by its lysosomal degradation. One of the functions of autophagy consists in the preservation of cellular homeostasis in physiological conditions. Indeed, autophagy continuously operates at baseline levels to ensure the disposal of potentially dangerous structures that may accumulate as a consequence of normal cellular functions, like damaged mitochondria or noxious protein aggregates. Moreover, the autophagic machinery is connected to sensors that detect perturbations of the intracellular or extracellular microenvironment. Thus, chemical, physical, metabolic, or infectious cues can all promote an increase in autophagic flux, and the pri- Now, a Nature Letter from Shelley Bergmary goal of such an adaptive response is er's laboratory has shed new light on this the restoration of cellular homeostasis process [9]. Dou et al. observed that
LMNB1, but not lamin A/C and B2, physically binds the lipidated form of microtubule-associated protein 1 light chain 3 beta (MAP1LC3B, best known as LC3B) and other autophagic adaptors in the nucleus of HDFs maintained in baseline conditions, and precisely mapped interaction domains to residues S393, S395, S396, R397, and V398 on LMNB1, and to residues R10 and R11 on LC3 [9]. Upon the expression of HRASG12V[8_TD$IF], an oncogenic variant of Harvey rat sarcoma viral oncogene homolog that promotes OIS in normal cells, LMNB1 was exported from the nucleus together with heterochromatin domains known as cytoplasmic chromatin fragments (CCFs) [10], and these LMNB1- and chromatin-containing buds were processed for degradation by the autophagic machinery. Importantly, when autophagy was inhibited by the depletion of ATG7, HRASG12V expression failed to stimulate the degradation of cytoplasmic LMNB1 and the consequent transition of HDFs into senescence. Similarly, autophagy was required for the induction of cellular senescence by two other stimuli, namely, the DNA-damaging agent etoposide and the pro-oxidant hydrogen peroxide. Moreover, when the physical interaction between LMNB1 and LC3 was impeded by mutating key residues on LMNB1 or LC3, as well as by expressing an LMNB1-derived peptide that competes with endogenous LMNB1 for binding to LC3, HRASG12V expression did not cause the translocation of LMNB1 to the cytosol, nor the appearance of CCFs, and failed to result in LMNB1 degradation and cellular senescence [9]. Taken together, these findings indicate that LMNB1 and LC3 must interact for the former to be exported from the nucleus and then to be degraded by autophagy in the course of OIS. Of note, subjecting HDFs to nutrient deprivation or treating them with a pharmacological inhibitor of mechanistic target of rapamycin (MTOR), which are two potent means to activate autophagy, failed to
Trends in Cell Biology, Month Year, Vol. xx, No. yy
1
TICB 1196 No. of Pages 3
(A)
(B) Cellular homeostasis
Stress
Nucleus
Restored cellular homeostasis
Oncogene hyperacvaon
LC3 LMNB1 Physiological condions
Basal autophagy levels
Stress-induced autophagy
CCF LMNB1 LC3
LAD Budding of CCFs
Autolysosome
LC3 LMNB1
Cellular adaptaon to stress
Physiological condions
Lysosome Organismal homeostasis
Restored organismal homeostasis
Stress
Cellular homeostasis Stress-induced autophagy
Cell eliminaon
Basal autophagy levels Physiological condions
Phagophore
Autophagic degradaon of LMNB1
Organismal adaptaon to stress
SASP Permanent proliferave arrest
Cell-intrinsic tumor suppression
Oncogene-induced senescence
Acvatory NK-cell receptor ligands
Cell-extrinsic tumor suppression
Figure 1. Cell-Intrinsic and –Extrinsic Tumor Suppression by Autophagy. [3_TD$IF](A[4_TD$IF]) Autophagy operates at baseline levels to remove potentially dangerous structures that may accumulate as a consequence of normal cellular functions. Moreover, perturbations of the intracellular or extracellular microenvironment can cause adaptive autophagic responses, which aim to restore cellular homeostasis. Thus, autophagy efficiently counteracts malignant transformation in a cell-intrinsic manner, by preserving the fitness of healthy tissues. Autophagy also mediates cell-extrinsic oncosuppressive functions. In particular, autophagy participates in various forms of danger signaling, such as the communication of an unrecoverable loss of cellular homeostasis to the organism. Danger signals normally trigger an innate or adaptive immune response that aims to preserve organismal homeostasis in conditions in which normal cellular functions are compromised beyond recovery. [5_TD$IF](B[4_TD$IF]) Lipidated LC3 constitutively interacts with LMNB1 at the nuclear lamina. In response to oncogene hyperactivation, LMNB1-, LC3- and heterochromatin-containing structures known as cytoplasmic chromatin fragments (CCFs) are extruded by the nucleus and get degraded by the autophagic machinery. This process depends on the constitutive interaction between LMNB1 and LC3, and is critically required for the establishment of oncogene-induced senescence. Oncogene-induced senescence is associated with a permanent proliferative arrest (hence exerting cell-intrinsic oncosuppressive effects), as well as with the upregulation of activatory natural killer (NK)-cell receptor ligands, and with the secretion of several cytokines (hence mediating cell-extrinsic oncosuppressive effects).[6_TD$IF] Abbreviations: LAD, lamin-associated domain; SASP, senescence-associated secretory phenotype.
trigger LMNB1 degradation and senescence [9]. This suggests that, although the activation of autophagy is required for the establishment of OIS, it is not sufficient to trigger senescence. Moreover, lipidated LC3 turned out to resemble LMNB1 in its ability to interact with transcriptionally inactive heterochromatin zones called lamin-associated domains (LADs) at the genome-wide scale [9]. However, Dou et al. did not rule out the possibility that LADs associate to LC3 via LMNB1 (while they did demonstrate that LADs bind to LMNB1 independent of LC3). The authors proposed therefore a model in which LC3 binds LMNB1 as well as LADs in baseline conditions. Such an
2
Trends in Cell Biology, Month Year, Vol. xx, No. yy
interaction appears to be required for the budding of LMNB1- and heterochromatin-containing structures destined to autophagic degradation following oncogene hyperactivation, a key step in OIS [9].
RB1 in response to LMNB1 degradation? Third, how does global transcription change in cells experiencing the loss of heterochromatin fragments in the course of OIS? Fourth, are mutations in LMNB1 or LC3 that affect their mutual interaction associated with some forms of cancer in humans? Answering these and other questions will ameliorate our understanding of OIS as a form of autophagydependent tumor suppression[1_TD$IF].
The findings by Dou et al. provide novel mechanistic insights into the ability of autophagy to mediate OIS, a form of tumor suppression that operates at both the cell-intrinsic and the cell-extrinsic levels (Figure 1B). Several important questions, however, remain to be addressed. Acknowledgments First, what is the actual trigger that initiates [12_TD$IF]The authors are supported by the Ligue contre le Canthe expulsion of LMNB1 and CCFs from cer (équipe labelisée); Agence National de la Recherche the nucleus in the course of OIS? Second, (ANR) – Projets blancs; ANR under the frame of which is the signal that activates TP53 and E-Rare-2, the ERA-Net for Research on Rare Diseases;
TICB 1196 No. of Pages 3
Association pour la recherche sur le cancer (ARC); Cancéropôle Ile-de-France; Institut National du Cancer (INCa); Fondation Bettencourt-Schueller; Fondation de France; Fondation pour la Recherche Médicale (FRM); the European Commission (ArtForce); the European Research Council (ERC); the LabEx Immuno-Oncology; the SIRIC Stratified Oncology Cell DNA Repair and Tumor Immune Elimination (SOCRATE); the SIRIC Cancer Research and Personalized Medicine (CARPEM); and the Paris Alliance of Cancer Research Institutes (PACRI).
3
Université Paris Descartes/Paris V, Sorbonne Paris Cité, 75006 Paris, France 4 Université Pierre et Marie Curie/Paris VI, 75006 Paris, France 5 Gustave Roussy Comprehensive Cancer Institute, 94805 Villejuif, France 6 Metabolomics and Cell Biology Platforms, Gustave Roussy Comprehensive Cancer Institute, 94805 Villejuif, France 7
Pôle de Biologie, Hôpital Européen Georges Pompidou, AP-HP, 75015 Paris, France 8 Department of Women's and Children's Health, Karolinska University Hospital, 17176 Stockholm, Sweden
Disclaimer Statement
*Correspondence: [email protected] (L. Galluzzi) and [email protected] (G. Kroemer).
The authors declare no conflicts of interests.
http://dx.doi.org/10.1016/j.tcb.2015.11.001
1
Equipe 11 labellisée Ligue contre le Cancer, Centre de Recherche des Cordeliers, 75006 Paris, France 2 INSERM, U1138, 75006 Paris, France
References 1. Galluzzi, L. et al. (2014) Metabolic control of autophagy. Cell 159, 1263–1276
2. Sica, V. et al. (2015) Organelle-specific initiation of autophagy. Mol. Cell 59, 522–539 3. Galluzzi, L. et al. (2015) Autophagy in malignant transformation and cancer progression. EMBO J. 34, 856–880 4. Kroemer, G. et al. (2013) Immunogenic cell death in cancer therapy. Annu. Rev. Immunol. 31, 51–72 5. Liu, H. et al. (2013) Down-regulation of autophagy-related protein 5 (ATG5) contributes to the pathogenesis of earlystage cutaneous melanoma. Sci. Transl. Med. 5, ra123 6. Young, A.R. et al. (2009) Autophagy mediates the mitotic senescence transition. Genes Dev. 23, 798–803 7. Iannello, A. et al. (2013) p53-dependent chemokine production by senescent tumor cells supports NKG2D-dependent tumor elimination by natural killer cells. J. Exp. Med. 210, 2057–2069 8. Shimi, T. et al. (2011) The role of nuclear lamin B1 in cell proliferation and senescence. Genes Dev. 25, 2579–2593 9. Dou, Z. et al. (2015) Autophagy mediates degradation of nuclear lamina. Nature In press 10. Ivanov, A. et al. (2013) Lysosome-mediated processing of chromatin in senescence. J. Cell Biol. 202, 129–143
Trends in Cell Biology, Month Year, Vol. xx, No. yy
3
Spotlight
Autophagy Mediates Tumor Suppression via Cellular Senescence Lorenzo Galluzzi,1,2,3,4,5,* José Manuel Bravo-San[7_TD$IF] Pedro,1,2,3,4,5 and Guido Kroemer1,2,3,4,6,7,8,* Autophagy not only constitutes a robust barrier against malignant transformation at the cell-intrinsic level, but also contributes to the organismal control of potentially oncogenic cells. Recent data provide molecular insights into the mechanisms whereby oncogene hyperactivation induces autophagy to establish a permanent proliferative arrest commonly known as cellular senescence.
(Figure 1A). Taken together, these two cell-intrinsic functions of autophagy explain why it generally counteracts malignant transformation (the conversion of a healthy cell into a neoplastic precursor) [1–3]. Autophagy also preserves organismal homeostasis and mediates oncosuppressive functions as it participates in the communication of unrecoverable states of cellular distress to the whole organism (Figure 1A). Thus, while autophagy-proficient malignant cells secrete ATP as a danger signal in the course of so-called [9_TD$IF]‘immunogenic cell death,[10_TD$IF]’ which results in the activation of an adaptive immune response against dying cells, their autophagy-deficient counterparts fail to do so and hence go unnoticed by the immune system, culminating in deficient anticancer immunosurveillance [4]. Along similar lines, the downregulation of essential components of the autophagic machinery like ATG5 or ATG7 prevents human diploid fibroblasts (HDFs) to undergo a permanent proliferative arrest commonly known as cellular senescence following oncogene hyperactivation [5,6]. Importantly, although the latter may appear as a purely cell-intrinsic oncosuppressive mechanism, it is not. Indeed, senescent cells not only secrete a wide panel of cytokines and chemokines that alert the organism of a situation of danger, but also express increased amounts of ligands for activatory natural killer (NK)-cell receptors, resulting in the activation of the innate arm of the immune system [7]. Oncogeneinduced senescence (OIS) not only relies on autophagy [5,6], but also proceeds along with the degradation of lamin B1 (LMNB1), a component of the nuclear lamina, and the consequent activation of tumor protein 53 (TP53) and retinoblastoma 1 (RB1) [8]. Until recently, however, the mechanistic links between autophagy and LMNB1 degradation in the course of OIS were elusive.
Macroautophagy (hereafter referred to as autophagy) is an evolutionary ancient process involving the sequestration of endogenous or exogenous cytoplasmic material within a double-membraned organelle (called autophagosome), followed by its lysosomal degradation. One of the functions of autophagy consists in the preservation of cellular homeostasis in physiological conditions. Indeed, autophagy continuously operates at baseline levels to ensure the disposal of potentially dangerous structures that may accumulate as a consequence of normal cellular functions, like damaged mitochondria or noxious protein aggregates. Moreover, the autophagic machinery is connected to sensors that detect perturbations of the intracellular or extracellular microenvironment. Thus, chemical, physical, metabolic, or infectious cues can all promote an increase in autophagic flux, and the pri- Now, a Nature Letter from Shelley Bergmary goal of such an adaptive response is er's laboratory has shed new light on this the restoration of cellular homeostasis process [9]. Dou et al. observed that
LMNB1, but not lamin A/C and B2, physically binds the lipidated form of microtubule-associated protein 1 light chain 3 beta (MAP1LC3B, best known as LC3B) and other autophagic adaptors in the nucleus of HDFs maintained in baseline conditions, and precisely mapped interaction domains to residues S393, S395, S396, R397, and V398 on LMNB1, and to residues R10 and R11 on LC3 [9]. Upon the expression of HRASG12V[8_TD$IF], an oncogenic variant of Harvey rat sarcoma viral oncogene homolog that promotes OIS in normal cells, LMNB1 was exported from the nucleus together with heterochromatin domains known as cytoplasmic chromatin fragments (CCFs) [10], and these LMNB1- and chromatin-containing buds were processed for degradation by the autophagic machinery. Importantly, when autophagy was inhibited by the depletion of ATG7, HRASG12V expression failed to stimulate the degradation of cytoplasmic LMNB1 and the consequent transition of HDFs into senescence. Similarly, autophagy was required for the induction of cellular senescence by two other stimuli, namely, the DNA-damaging agent etoposide and the pro-oxidant hydrogen peroxide. Moreover, when the physical interaction between LMNB1 and LC3 was impeded by mutating key residues on LMNB1 or LC3, as well as by expressing an LMNB1-derived peptide that competes with endogenous LMNB1 for binding to LC3, HRASG12V expression did not cause the translocation of LMNB1 to the cytosol, nor the appearance of CCFs, and failed to result in LMNB1 degradation and cellular senescence [9]. Taken together, these findings indicate that LMNB1 and LC3 must interact for the former to be exported from the nucleus and then to be degraded by autophagy in the course of OIS. Of note, subjecting HDFs to nutrient deprivation or treating them with a pharmacological inhibitor of mechanistic target of rapamycin (MTOR), which are two potent means to activate autophagy, failed to
Trends in Cell Biology, Month Year, Vol. xx, No. yy
1
TICB 1196 No. of Pages 3
(A)
(B) Cellular homeostasis
Stress
Nucleus
Restored cellular homeostasis
Oncogene hyperacvaon
LC3 LMNB1 Physiological condions
Basal autophagy levels
Stress-induced autophagy
CCF LMNB1 LC3
LAD Budding of CCFs
Autolysosome
LC3 LMNB1
Cellular adaptaon to stress
Physiological condions
Lysosome Organismal homeostasis
Restored organismal homeostasis
Stress
Cellular homeostasis Stress-induced autophagy
Cell eliminaon
Basal autophagy levels Physiological condions
Phagophore
Autophagic degradaon of LMNB1
Organismal adaptaon to stress
SASP Permanent proliferave arrest
Cell-intrinsic tumor suppression
Oncogene-induced senescence
Acvatory NK-cell receptor ligands
Cell-extrinsic tumor suppression
Figure 1. Cell-Intrinsic and –Extrinsic Tumor Suppression by Autophagy. [3_TD$IF](A[4_TD$IF]) Autophagy operates at baseline levels to remove potentially dangerous structures that may accumulate as a consequence of normal cellular functions. Moreover, perturbations of the intracellular or extracellular microenvironment can cause adaptive autophagic responses, which aim to restore cellular homeostasis. Thus, autophagy efficiently counteracts malignant transformation in a cell-intrinsic manner, by preserving the fitness of healthy tissues. Autophagy also mediates cell-extrinsic oncosuppressive functions. In particular, autophagy participates in various forms of danger signaling, such as the communication of an unrecoverable loss of cellular homeostasis to the organism. Danger signals normally trigger an innate or adaptive immune response that aims to preserve organismal homeostasis in conditions in which normal cellular functions are compromised beyond recovery. [5_TD$IF](B[4_TD$IF]) Lipidated LC3 constitutively interacts with LMNB1 at the nuclear lamina. In response to oncogene hyperactivation, LMNB1-, LC3- and heterochromatin-containing structures known as cytoplasmic chromatin fragments (CCFs) are extruded by the nucleus and get degraded by the autophagic machinery. This process depends on the constitutive interaction between LMNB1 and LC3, and is critically required for the establishment of oncogene-induced senescence. Oncogene-induced senescence is associated with a permanent proliferative arrest (hence exerting cell-intrinsic oncosuppressive effects), as well as with the upregulation of activatory natural killer (NK)-cell receptor ligands, and with the secretion of several cytokines (hence mediating cell-extrinsic oncosuppressive effects).[6_TD$IF] Abbreviations: LAD, lamin-associated domain; SASP, senescence-associated secretory phenotype.
trigger LMNB1 degradation and senescence [9]. This suggests that, although the activation of autophagy is required for the establishment of OIS, it is not sufficient to trigger senescence. Moreover, lipidated LC3 turned out to resemble LMNB1 in its ability to interact with transcriptionally inactive heterochromatin zones called lamin-associated domains (LADs) at the genome-wide scale [9]. However, Dou et al. did not rule out the possibility that LADs associate to LC3 via LMNB1 (while they did demonstrate that LADs bind to LMNB1 independent of LC3). The authors proposed therefore a model in which LC3 binds LMNB1 as well as LADs in baseline conditions. Such an
2
Trends in Cell Biology, Month Year, Vol. xx, No. yy
interaction appears to be required for the budding of LMNB1- and heterochromatin-containing structures destined to autophagic degradation following oncogene hyperactivation, a key step in OIS [9].
RB1 in response to LMNB1 degradation? Third, how does global transcription change in cells experiencing the loss of heterochromatin fragments in the course of OIS? Fourth, are mutations in LMNB1 or LC3 that affect their mutual interaction associated with some forms of cancer in humans? Answering these and other questions will ameliorate our understanding of OIS as a form of autophagydependent tumor suppression[1_TD$IF].
The findings by Dou et al. provide novel mechanistic insights into the ability of autophagy to mediate OIS, a form of tumor suppression that operates at both the cell-intrinsic and the cell-extrinsic levels (Figure 1B). Several important questions, however, remain to be addressed. Acknowledgments First, what is the actual trigger that initiates [12_TD$IF]The authors are supported by the Ligue contre le Canthe expulsion of LMNB1 and CCFs from cer (équipe labelisée); Agence National de la Recherche the nucleus in the course of OIS? Second, (ANR) – Projets blancs; ANR under the frame of which is the signal that activates TP53 and E-Rare-2, the ERA-Net for Research on Rare Diseases;
TICB 1196 No. of Pages 3
Association pour la recherche sur le cancer (ARC); Cancéropôle Ile-de-France; Institut National du Cancer (INCa); Fondation Bettencourt-Schueller; Fondation de France; Fondation pour la Recherche Médicale (FRM); the European Commission (ArtForce); the European Research Council (ERC); the LabEx Immuno-Oncology; the SIRIC Stratified Oncology Cell DNA Repair and Tumor Immune Elimination (SOCRATE); the SIRIC Cancer Research and Personalized Medicine (CARPEM); and the Paris Alliance of Cancer Research Institutes (PACRI).
3
Université Paris Descartes/Paris V, Sorbonne Paris Cité, 75006 Paris, France 4 Université Pierre et Marie Curie/Paris VI, 75006 Paris, France 5 Gustave Roussy Comprehensive Cancer Institute, 94805 Villejuif, France 6 Metabolomics and Cell Biology Platforms, Gustave Roussy Comprehensive Cancer Institute, 94805 Villejuif, France 7
Pôle de Biologie, Hôpital Européen Georges Pompidou, AP-HP, 75015 Paris, France 8 Department of Women's and Children's Health, Karolinska University Hospital, 17176 Stockholm, Sweden
Disclaimer Statement
*Correspondence: [email protected] (L. Galluzzi) and [email protected] (G. Kroemer).
The authors declare no conflicts of interests.
http://dx.doi.org/10.1016/j.tcb.2015.11.001
1
Equipe 11 labellisée Ligue contre le Cancer, Centre de Recherche des Cordeliers, 75006 Paris, France 2 INSERM, U1138, 75006 Paris, France
References 1. Galluzzi, L. et al. (2014) Metabolic control of autophagy. Cell 159, 1263–1276
2. Sica, V. et al. (2015) Organelle-specific initiation of autophagy. Mol. Cell 59, 522–539 3. Galluzzi, L. et al. (2015) Autophagy in malignant transformation and cancer progression. EMBO J. 34, 856–880 4. Kroemer, G. et al. (2013) Immunogenic cell death in cancer therapy. Annu. Rev. Immunol. 31, 51–72 5. Liu, H. et al. (2013) Down-regulation of autophagy-related protein 5 (ATG5) contributes to the pathogenesis of earlystage cutaneous melanoma. Sci. Transl. Med. 5, ra123 6. Young, A.R. et al. (2009) Autophagy mediates the mitotic senescence transition. Genes Dev. 23, 798–803 7. Iannello, A. et al. (2013) p53-dependent chemokine production by senescent tumor cells supports NKG2D-dependent tumor elimination by natural killer cells. J. Exp. Med. 210, 2057–2069 8. Shimi, T. et al. (2011) The role of nuclear lamin B1 in cell proliferation and senescence. Genes Dev. 25, 2579–2593 9. Dou, Z. et al. (2015) Autophagy mediates degradation of nuclear lamina. Nature In press 10. Ivanov, A. et al. (2013) Lysosome-mediated processing of chromatin in senescence. J. Cell Biol. 202, 129–143
Trends in Cell Biology, Month Year, Vol. xx, No. yy
3