news and views
Targeting self-renewal, an Achilles’ heel of cancer stem cells Max S Wicha
npg
© 2014 Nature America, Inc. All rights reserved.
Many tumors display a hierarchical organization that is maintained by a self-renewing ‘cancer stem cell’ population. A new study in mice shows that targeting the self-renewal regulator BMI-1 abrogates the tumorigenic capacity of colon cancer stem cells, providing a new therapeutic strategy (pages 29–36). There is compelling evidence that the majority of human malignancies are initiated and maintained by a population of cells that display stem cell properties1,2. Such properties include self-renewal and the ability to differentiate into the non–self-renewing cell populations that form the tumor bulk. These cells, which have been referred to as tumor-initiating cells or cancer stem cells (CSCs), may mediate tumor metastasis. In addition, by virtue of their resistance to cytotoxic chemotherapy and radiation, CSCs may contribute to tumor relapse and, ultimately, death. These negative clinical outcomes highlight the importance of developing therapeutic strategies to target the CSC population. In a new study in this issue of Nature Medicine, Kreso et al.3 demonstrate that by targeting BMI1, a gene that lies at the heart of stem cells’ self-renewal machinery, they can effectively eliminate human colon cancer stem cells in mouse xenografts. They further show that a small-molecule BMI-1 inhibitor blocks tumor growth and metastasis in the absence of systemic toxicity, illustrating the feasibility of targeting self-renewal as a new strategy for treatment of colon cancer. Although there has been substantial progress in the treatment of colon cancer, metastatic disease remains largely incurable. Accumulating evidence suggests that colon cancers are driven by a cellular subpopulation that displays stem cell properties that mediate metastasis and treatment resistance4. Elegant lineage tracing and mouse xenograft models have helped to identify and characterize normal and malignant colon stem cells5. These cells are characterized Max S. Wicha is at the University of Michigan Comprehensive Cancer Center, Ann Arbor, Michigan, USA. e-mail: [email protected]
14
by the expression of markers including LGR5 and CD44, which are downstream targets of Wnt signaling, a pathway deregulated in virtually all colon carcinomas6. These colon cancer stem-like cells are also characterized by expression of the polycomb gene BMI1. This gene forms an essential component of polycomb regulatory complex 1 (PRC1). PRC1 has a fundamental role in the organization of chromatin structure, which, in turn, regulates the expression of a cassette of genes involved in stem cell behavior. These genes include the INK4A locus and CDKN2A (encoding p16INK4a and P14ARF (ref. 7)). Proteins in the PRC1 complex cooperate with PRC 2-complex proteins to serve as transcriptional repressors by binding to histones displaying histone H3 lysine (H3K27) trimethylation. BMI-1 may also cooperate with TWIST1 to regulate the epithelialto-mesenchymal transition (EMT), a state associated with CSCs and metastasis. The coordination of these pathways involves miRNAs such as mir-200C, which negatively regulates BMI-1 expression8 (Fig. 1). BMI-1 expression has been associated with aggressive disease and poor patient outcomes in a wide variety of malignancies, including colon cancer9. Furthermore, BMI-1 has been shown to play a pivotal part in the regulation of normal and malignant stem cell self-renewal in both the hematopoietic system10 and the breast, where BMI-1 is activated by the Hedgehog pathway11. BMI-1 has also been shown to mediate chemotherapy and radiation resistance12. The work of Kreso et al.3 now demonstrates the feasibility of targeting colon cancer– initiating cells (CICs) by inhibiting BMI-1. To validate BMI-1 as a therapeutic target, the authors first used a genetic approach to show that knockdown of BMI1 transcript
levels impairs the self-renewal capacity of human CICs, thus reducing their frequency. This results in substantial inhibition of tumor growth in mouse xenografts of human colon cancer cells. The researchers then used a BMI-1 reporter system to conduct high-throughput screening to identify selective BMI-1 inhibitors. This approach enabled them to identify PTC-209, a small molecule capable of inhibiting BMI-1 expression at submicromolar concentrations. Kreso et al.3 then showed that this compound blocks CIC self-renewal in vitro and effectively blocks tumor growth in mouse xenografts. Tumors failed to grow back following withdrawal of treatment, indicating that PTC-209 has irreversible effects. Furthermore, PTC-209 inhibited tumor growth in the absence of apparent systemic toxicity. This study adds to a growing body of literature13,14 that demonstrates the feasibility of developing therapeutic agents that target CSCs. An important concern for the development of these agents has been their potential toxicities resulting from inhibition of pathways used by normal tissue stem cells. In fact, Bmi1-knockout mice display severe hematopoietic and gastrointestinal abnormalities10. The lack of systemic toxicity of PTC-209 might be related to the more limited pharmacological reduction of BMI-1 expression by PTC-209 compared to genetic knockdown. Moreover, the substantial reduction in CICs in the absence of demonstrable systemic toxicity suggests that agents targeting self-renewal pathways may display a ‘therapeutic index’ that allows differentiation between normal and malignant stem cells. Further studies will be required to determine whether CSCs display ‘oncogene addiction’. This phenomenon is characterized by an absolute dependence of tumor cells on continued oncogene expression
volume 20 | number 1 | january 2014 nature medicine
news and views
Notch
SMO
FZ
miR-200c
Notch RTK
Hh
Wnt
PTC-209
Akt
underway. Agents that target the self-renewal machinery of CSCs, such as PTC-209, add to this growing armamentarium. The relative rarity of CSC populations in many tumors highlights the need to develop new clinical trial designs and biomarkers to assess the utility of CSC-targeting agents. Ultimately, randomized clinical trials will be required to determine whether effective targeting of CSCs improves patient outcome.
BMI-1 PRC1
COMPETING FINANCIAL INTERESTS The author declares competing financial interests: details are available in the online version of the paper.
PRC2
me3
me3
me3
H3K27
H3K27
H3K27
OFF
INK4A/ARF HOX P16INK4a P14ARF
npg
© 2014 Nature America, Inc. All rights reserved.
TWIST
EMT CSC self-renewal Tumorigenesis
Figure 1 BMI-1–mediated regulation of CSC self-renewal. Expression of the polycomb protein BMI-1 is regulated by multiple signal transduction pathways, including the Akt, Wnt, Notch, Hedgehog (Hh) and receptor tyrosine kinase (RTK) pathways. BMI-1 expression is negatively regulated by miRNAs such as mir-200C. BMI-1 complexes with other PRC1 and PRC2 proteins that repress the expression of genes bearing H3K27 histone marks, including those encoding INK4A/ARF and HOX. These pathways interact with EMT transcription factors including TWIST to regulate EMT and CSC self-renewal. Kreso et al.1 now identify a small molecule, PTC-209, that inhibits BMI-1 expression, thus inhibiting CSC self-renewal and tumorigenesis. SMO, Smoothened; FZ, Frizzled.
for their survival, and it has been proposed that other tumorigenic cell populations show this dependence. In addition, previous preclinical studies have suggested that CSC-targeting agents will have the greatest efficacy when they are administered early or in an adjuvant setting
alongside a primary therapy15. It will therefore be interesting to explore the efficacy of PTC209 in such settings. The CSC model has profound clinical implications. Several dozen early-phase clinical trials aimed at targeting CSCs are now
1. Beck, B. & Blanpain, C. Nat. Rev. Cancer 13, 727–738 (2013). 2. Enderling, H., Hlatky, L. & Hahnfeldt, P. Front. Oncol. 3, 76 (2013). 3. Kreso, A. et al. Nat. Med. 20, 29–36 (2014). 4. Meacham, C.E. & Morrison, S.J. Nature 501, 328–337 (2013). 5. Schepers, A.G. et al. Science 337, 730–735 (2012). 6. Vermeulen, L. et al. Nat. Cell Biol. 12, 468–476 (2010). 7. Cao, L. et al. J. Cell. Biochem. 112, 2729–2741 (2011). 8. Shimono, Y. et al. Cell 138, 592–603 (2009). 9. Glinsky, G.V., Berezovska, O. & Glinskii, A.B. J. Clin. Invest. 115, 1503–1521 (2005). 10. Rizo, A., Dontje, B., Vellenga, E., de Haan, G. & Schuringa, J.J. Blood 111, 2621–2630 (2008). 11. Liu, S. et al. Cancer Res. 66, 6063–6071 (2006). 12. Siddique, H.R. et al. PLoS ONE 8, e60664 (2013). 13. Schott, A.F. et al. Clin. Cancer Res. 19, 1512–1524 (2013). 14. Chen, K., Huang, Y.H. & Chen, J.L. Acta Pharmacol. Sin. 34, 732–740 (2013). 15. Korkaya, H. & Wicha, M.S. Cancer Res. 73, 3489–3493 (2013).
Good guys gone bad: exTreg cells promote autoimmune arthritis Nicole Joller & Vijay K Kuchroo Under most circumstances, Foxp3+ regulatory T (Treg) cells are a stable T cell population essential for maintaining self-tolerance. A study now shows that the inflammatory environment in autoimmune arthritis induces conversion of a subset of Foxp3+ T cells into interleukin-17–producing cells that contribute to disease pathogenesis (pages 62–68). CD4+Foxp3+ Treg cells suppress aberrant and excessive immune responses that induce tissue inflammation and autoimmunity. Under physio logical conditions, a number of mechanisms ensure sustained Foxp3 expression and Treg Nicole Joller and Vijay K. Kuchroo are at the Center for Neurologic Diseases, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA. e-mail: [email protected]
cell stability1,2; however, during inflammation, some Treg cells may lose their ability to express Foxp3 as well as their regulatory function, thus acquiring characteristics of effector T cells3. Many autoimmune diseases, including rheumatoid arthritis, multiple sclerosis and type 1 diabetes, are characterized by loss of Treg cell numbers or their function4. In this setting, proinflammatory T helper type 1 (TH1) and/ or TH17 effector cells accumulate while Treg cell function is lost, resulting in an unfavorable shift
nature medicine volume 20 | number 1 | january 2014
in the Treg cell/effector T cell balance. In this issue of Nature Medicine, Komatsu et al.5 show that the local inflammatory milieu shifts this balance not only by promoting TH17 cell differentiation but also by converting a Foxp3+ subset into IL-17 producers that thereby contribute to pathogenesis in autoimmune arthritis (Fig. 1). The investigators found double-positive IL-17+Foxp3+ T cells in the synovial membrane of individuals with rheumatoid arthritis (RA)4. Using Foxp3-fate reporter mice, they identified 15
Targeting self-renewal, an Achilles’ heel of cancer stem cells Max S Wicha
npg
© 2014 Nature America, Inc. All rights reserved.
Many tumors display a hierarchical organization that is maintained by a self-renewing ‘cancer stem cell’ population. A new study in mice shows that targeting the self-renewal regulator BMI-1 abrogates the tumorigenic capacity of colon cancer stem cells, providing a new therapeutic strategy (pages 29–36). There is compelling evidence that the majority of human malignancies are initiated and maintained by a population of cells that display stem cell properties1,2. Such properties include self-renewal and the ability to differentiate into the non–self-renewing cell populations that form the tumor bulk. These cells, which have been referred to as tumor-initiating cells or cancer stem cells (CSCs), may mediate tumor metastasis. In addition, by virtue of their resistance to cytotoxic chemotherapy and radiation, CSCs may contribute to tumor relapse and, ultimately, death. These negative clinical outcomes highlight the importance of developing therapeutic strategies to target the CSC population. In a new study in this issue of Nature Medicine, Kreso et al.3 demonstrate that by targeting BMI1, a gene that lies at the heart of stem cells’ self-renewal machinery, they can effectively eliminate human colon cancer stem cells in mouse xenografts. They further show that a small-molecule BMI-1 inhibitor blocks tumor growth and metastasis in the absence of systemic toxicity, illustrating the feasibility of targeting self-renewal as a new strategy for treatment of colon cancer. Although there has been substantial progress in the treatment of colon cancer, metastatic disease remains largely incurable. Accumulating evidence suggests that colon cancers are driven by a cellular subpopulation that displays stem cell properties that mediate metastasis and treatment resistance4. Elegant lineage tracing and mouse xenograft models have helped to identify and characterize normal and malignant colon stem cells5. These cells are characterized Max S. Wicha is at the University of Michigan Comprehensive Cancer Center, Ann Arbor, Michigan, USA. e-mail: [email protected]
14
by the expression of markers including LGR5 and CD44, which are downstream targets of Wnt signaling, a pathway deregulated in virtually all colon carcinomas6. These colon cancer stem-like cells are also characterized by expression of the polycomb gene BMI1. This gene forms an essential component of polycomb regulatory complex 1 (PRC1). PRC1 has a fundamental role in the organization of chromatin structure, which, in turn, regulates the expression of a cassette of genes involved in stem cell behavior. These genes include the INK4A locus and CDKN2A (encoding p16INK4a and P14ARF (ref. 7)). Proteins in the PRC1 complex cooperate with PRC 2-complex proteins to serve as transcriptional repressors by binding to histones displaying histone H3 lysine (H3K27) trimethylation. BMI-1 may also cooperate with TWIST1 to regulate the epithelialto-mesenchymal transition (EMT), a state associated with CSCs and metastasis. The coordination of these pathways involves miRNAs such as mir-200C, which negatively regulates BMI-1 expression8 (Fig. 1). BMI-1 expression has been associated with aggressive disease and poor patient outcomes in a wide variety of malignancies, including colon cancer9. Furthermore, BMI-1 has been shown to play a pivotal part in the regulation of normal and malignant stem cell self-renewal in both the hematopoietic system10 and the breast, where BMI-1 is activated by the Hedgehog pathway11. BMI-1 has also been shown to mediate chemotherapy and radiation resistance12. The work of Kreso et al.3 now demonstrates the feasibility of targeting colon cancer– initiating cells (CICs) by inhibiting BMI-1. To validate BMI-1 as a therapeutic target, the authors first used a genetic approach to show that knockdown of BMI1 transcript
levels impairs the self-renewal capacity of human CICs, thus reducing their frequency. This results in substantial inhibition of tumor growth in mouse xenografts of human colon cancer cells. The researchers then used a BMI-1 reporter system to conduct high-throughput screening to identify selective BMI-1 inhibitors. This approach enabled them to identify PTC-209, a small molecule capable of inhibiting BMI-1 expression at submicromolar concentrations. Kreso et al.3 then showed that this compound blocks CIC self-renewal in vitro and effectively blocks tumor growth in mouse xenografts. Tumors failed to grow back following withdrawal of treatment, indicating that PTC-209 has irreversible effects. Furthermore, PTC-209 inhibited tumor growth in the absence of apparent systemic toxicity. This study adds to a growing body of literature13,14 that demonstrates the feasibility of developing therapeutic agents that target CSCs. An important concern for the development of these agents has been their potential toxicities resulting from inhibition of pathways used by normal tissue stem cells. In fact, Bmi1-knockout mice display severe hematopoietic and gastrointestinal abnormalities10. The lack of systemic toxicity of PTC-209 might be related to the more limited pharmacological reduction of BMI-1 expression by PTC-209 compared to genetic knockdown. Moreover, the substantial reduction in CICs in the absence of demonstrable systemic toxicity suggests that agents targeting self-renewal pathways may display a ‘therapeutic index’ that allows differentiation between normal and malignant stem cells. Further studies will be required to determine whether CSCs display ‘oncogene addiction’. This phenomenon is characterized by an absolute dependence of tumor cells on continued oncogene expression
volume 20 | number 1 | january 2014 nature medicine
news and views
Notch
SMO
FZ
miR-200c
Notch RTK
Hh
Wnt
PTC-209
Akt
underway. Agents that target the self-renewal machinery of CSCs, such as PTC-209, add to this growing armamentarium. The relative rarity of CSC populations in many tumors highlights the need to develop new clinical trial designs and biomarkers to assess the utility of CSC-targeting agents. Ultimately, randomized clinical trials will be required to determine whether effective targeting of CSCs improves patient outcome.
BMI-1 PRC1
COMPETING FINANCIAL INTERESTS The author declares competing financial interests: details are available in the online version of the paper.
PRC2
me3
me3
me3
H3K27
H3K27
H3K27
OFF
INK4A/ARF HOX P16INK4a P14ARF
npg
© 2014 Nature America, Inc. All rights reserved.
TWIST
EMT CSC self-renewal Tumorigenesis
Figure 1 BMI-1–mediated regulation of CSC self-renewal. Expression of the polycomb protein BMI-1 is regulated by multiple signal transduction pathways, including the Akt, Wnt, Notch, Hedgehog (Hh) and receptor tyrosine kinase (RTK) pathways. BMI-1 expression is negatively regulated by miRNAs such as mir-200C. BMI-1 complexes with other PRC1 and PRC2 proteins that repress the expression of genes bearing H3K27 histone marks, including those encoding INK4A/ARF and HOX. These pathways interact with EMT transcription factors including TWIST to regulate EMT and CSC self-renewal. Kreso et al.1 now identify a small molecule, PTC-209, that inhibits BMI-1 expression, thus inhibiting CSC self-renewal and tumorigenesis. SMO, Smoothened; FZ, Frizzled.
for their survival, and it has been proposed that other tumorigenic cell populations show this dependence. In addition, previous preclinical studies have suggested that CSC-targeting agents will have the greatest efficacy when they are administered early or in an adjuvant setting
alongside a primary therapy15. It will therefore be interesting to explore the efficacy of PTC209 in such settings. The CSC model has profound clinical implications. Several dozen early-phase clinical trials aimed at targeting CSCs are now
1. Beck, B. & Blanpain, C. Nat. Rev. Cancer 13, 727–738 (2013). 2. Enderling, H., Hlatky, L. & Hahnfeldt, P. Front. Oncol. 3, 76 (2013). 3. Kreso, A. et al. Nat. Med. 20, 29–36 (2014). 4. Meacham, C.E. & Morrison, S.J. Nature 501, 328–337 (2013). 5. Schepers, A.G. et al. Science 337, 730–735 (2012). 6. Vermeulen, L. et al. Nat. Cell Biol. 12, 468–476 (2010). 7. Cao, L. et al. J. Cell. Biochem. 112, 2729–2741 (2011). 8. Shimono, Y. et al. Cell 138, 592–603 (2009). 9. Glinsky, G.V., Berezovska, O. & Glinskii, A.B. J. Clin. Invest. 115, 1503–1521 (2005). 10. Rizo, A., Dontje, B., Vellenga, E., de Haan, G. & Schuringa, J.J. Blood 111, 2621–2630 (2008). 11. Liu, S. et al. Cancer Res. 66, 6063–6071 (2006). 12. Siddique, H.R. et al. PLoS ONE 8, e60664 (2013). 13. Schott, A.F. et al. Clin. Cancer Res. 19, 1512–1524 (2013). 14. Chen, K., Huang, Y.H. & Chen, J.L. Acta Pharmacol. Sin. 34, 732–740 (2013). 15. Korkaya, H. & Wicha, M.S. Cancer Res. 73, 3489–3493 (2013).
Good guys gone bad: exTreg cells promote autoimmune arthritis Nicole Joller & Vijay K Kuchroo Under most circumstances, Foxp3+ regulatory T (Treg) cells are a stable T cell population essential for maintaining self-tolerance. A study now shows that the inflammatory environment in autoimmune arthritis induces conversion of a subset of Foxp3+ T cells into interleukin-17–producing cells that contribute to disease pathogenesis (pages 62–68). CD4+Foxp3+ Treg cells suppress aberrant and excessive immune responses that induce tissue inflammation and autoimmunity. Under physio logical conditions, a number of mechanisms ensure sustained Foxp3 expression and Treg Nicole Joller and Vijay K. Kuchroo are at the Center for Neurologic Diseases, Brigham and Women’s Hospital, Harvard Medical School, Boston, Massachusetts, USA. e-mail: [email protected]
cell stability1,2; however, during inflammation, some Treg cells may lose their ability to express Foxp3 as well as their regulatory function, thus acquiring characteristics of effector T cells3. Many autoimmune diseases, including rheumatoid arthritis, multiple sclerosis and type 1 diabetes, are characterized by loss of Treg cell numbers or their function4. In this setting, proinflammatory T helper type 1 (TH1) and/ or TH17 effector cells accumulate while Treg cell function is lost, resulting in an unfavorable shift
nature medicine volume 20 | number 1 | january 2014
in the Treg cell/effector T cell balance. In this issue of Nature Medicine, Komatsu et al.5 show that the local inflammatory milieu shifts this balance not only by promoting TH17 cell differentiation but also by converting a Foxp3+ subset into IL-17 producers that thereby contribute to pathogenesis in autoimmune arthritis (Fig. 1). The investigators found double-positive IL-17+Foxp3+ T cells in the synovial membrane of individuals with rheumatoid arthritis (RA)4. Using Foxp3-fate reporter mice, they identified 15
