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N. Watkins
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MO04 - Lung Cancer Biology I (ID 86)
- Event: WCLC 2013
- Type: Mini Oral Abstract Session
- Track: Biology
- Presentations: 1
- Moderators:G. Sozzi, D.C. Lam
- Coordinates: 10/28/2013, 16:15 - 17:45, Bayside 103, Level 1
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MO04.12 - DISCUSSANT (ID 3895)
17:20 - 17:35 | Author(s): N. Watkins
- Abstract
- Presentation
Abstract not provided
Only Members that have purchased this event or have registered via an access code will be able to view this content. To view this presentation, please login, select "Add to Cart" and proceed to checkout. If you would like to become a member of IASLC, please click here.
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MS02 - Stem Cells and Epigenetics in Lung Cancer (ID 19)
- Event: WCLC 2013
- Type: Mini Symposia
- Track: Biology
- Presentations: 1
- Moderators:G. Sozzi, J. Minna
- Coordinates: 10/28/2013, 14:00 - 15:30, Bayside 103, Level 1
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MS02.2 - Targeting Hedgehog Signaling in Small Cell Lung Cancer (ID 463)
14:25 - 14:45 | Author(s): N. Watkins
- Abstract
- Presentation
Abstract
Small cell lung cancer (SCLC) is a malignant neuroendocrine tumour responsible for 20% of all lung cancer deaths. Despite the effectiveness of platinum-based chemotherapy, the overwhelming majority of patients succumb to a chemoresistant recurrence within 2 years of diagnosis. Therefore, the discovery of novel strategies to prevent disease recurrence may have a significant impact on outcome. Many groups, including our own, have identified the importance of embryonic signaling pathways in promoting tumor-regeneration through the regulation of self-renewal. Activation of self-renewal pathways such as Notch, Hedgehog (Hh) and WNT is also thought to contribute to the survival of cancer cells with innate resistance to chemotherapeutic agents. Since the naturally occurring Hh inhibitor cyclopamine can block self-renewal in SCLC cells in vitro, we postulated that this pathway might be targetable following platinum-based chemotherapy. The Hh pathway is a highly conserved signaling system that specifies cell fate and self-renewal in development and homeostasis. The Hh ligand Sonic Hh (Shh) binds to, and inactivates, the receptor Patched (Ptch). This prevents Ptch from inhibiting Smoothened (Smo), the molecular target of the small molecule Hh inhibitors. Smo activation requires translocation to the tip of the primary cilium, a single, immotile, tubulin-based organelle present on most vertebrate cells. The ciliary motor protein Kif3a is essential for Smo translocation to the ciliary tip, and is required for Smo signaling in development. The importance of primary cilia in cancer is poorly understood. In addition, the formation of cilia is normally restricted to cells in the G0 of G1 phases of the cell cycle. In order to better understand how Hh signaling is regulated in SCLC, we investigated pathway activation in the context of cilia formation, and the expression of Smo protein in these cilia. Using a genetic mouse model of SCLC, we observed that approximately 25% of tumour cells express primary cilia, with a variable number expressing Smo at the cilia tip. Clonal growth of these tumour cells could be inhibited by blocking the Shh ligand with a monoclonal antibody, or by inactivating Smo with the small molecule LDE225 (Novartis). These data suggest that activation of Smo by Hh ligand at the level of the primary cilium is a crucial step in the initiation and self-renewal of SCLC. By contrast, cilia were rarely observed in human SCLC cells, both in vitro and in vivo. Based on our hypothesis that Hh signaling may be important in the regeneration of SCLC stem-like cells following chemotherapy, we employed a primary xenograft model in which we could identify minimal residual disease following treatment with single-agent carboplatin. Following chemotherapy, more than 50% of the residual tumour cells expressed primary cilia, and in the majority of these, Smo could be detected in the tip. In addition, marked upregulation of Shh ligand expression was observed. However, when these tumours were allowed to regrow to their original size, expression of Shh and cilia returned to the same pattern as seen in the treatment naive tumour, once again supporting a role for cilia-dependent activation of Smo by Hh ligand in SCLC stem-like cells. Furthermore, treatment of mice following chemotherapy with several small molecule inhibitors of Smo, including LDE225, delayed the regeneration of these tumours in vivo. One major controversy surrounding the role Hh signaling in cancer relates to the role of tumour stroma as a target of Shh signaling. To exclude this as a potential mechanism, we used two approaches. First, we crossed the mouse genetic model of SCLC referred to above with a reporter mouse in which LacZ activity can be used to measure Hh pathway activation. In this model, heterogeneous LacZ expression was clearly seen in tumour cells, but not stromal cells. Second, we recreated the carboplatin regeneration model of human SCLC in vitro using a three dimensional, stroma-free clonogenic assay. Transient manipulation of Hh signaling at all levels of the pathway by antibody blockade, siRNA, transfection or small molecule treatment dramatically affected long term cloning capacity in SCLC cells. Moreover, SCLC cells that survived an LD~95~ treatment with carboplatin in vitro demonstrated a marked increase in clonal capacity that was even more sensitive to Hh pathway blockade. Confocal immunofluoresence imaging of these cells revealed expression of Smo in the tips of numerous primary cilia, demonstrating that cell autonomous Hh pathway activation could be observed in a subset of innately chemoresistant, stem-like cells. In the last 5 years, targeted deletion of different components of the primary cilium in mice has dramatically expanded our understanding of the role of these structures in cell signaling. Once considered vestigial, cilia are now recognised as key signaling nodes for Hh, WNT, Notch, PDGF and mTOR signaling. In addition, we have identified heterogeneous expression of primary cilia in several other tumour models, most strikingly in osteosarcoma. More recently, we have shown that knockdown of KIF3a, an essential component of cilia assembly, causes a more dramatic loss of cloning capacity than inhibition of Hh signaling alone. These data suggest that in a subset of self-renewing tumour cells, cilia-dependent signaling pathways in addition to Hh are of importance, and may represent novel therapeutic targets. We are currently using genetically modified mouse models of cancer in which we can conditionally knockout Kif3a to address this question.Only Members that have purchased this event or have registered via an access code will be able to view this content. To view this presentation, please login, select "Add to Cart" and proceed to checkout. If you would like to become a member of IASLC, please click here.
