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Hedgehog Signaling Pathway and Lung Cancer

      Signaling pathways responsible for embryogenesis play a critical role in the maintenance of stem cells in adult life and cellular responses to injury. Dysfunction of the developmental signaling pathways during adult homeostasis leads to various events resulting in the development of neoplasia. We review the biology of the hedgehog signaling pathway and its potential role in the development of lung cancer.

      Key Words

      Signaling pathways responsible for embryogenesis appear to play a critical role in the maintenance of stem cells in adult life and cellular responses to injury. Dysregulation of these signaling pathways during adult homeostasis can lead to various events resulting in the development of neoplasia (Figure 1).
      Figure thumbnail gr1
      FIGURE 1Role of developmental signaling pathways in the Tumorigenesis.
      Hedgehog (Hh) signaling pathway is one such pathway that is crucial in the embryogenesis. Hh pathway also plays a central role in the repair and regeneration of adult tissue. The Hh signaling pathway was first studied in drosophilae. During embryonic development, drosophilae with a mutation in the Hh gene were covered with pointed denticles, resembling a hedgehog, hence the name. Several studies have recently demonstrated that dysregulation of the Hh signaling plays a role in several cancers including the brain, skin, gastrointestinal tract, pancreas, and lung.
      • Goodrich LV
      • Milenkovic L
      • Higgins KM
      • Scott MP
      Altered neural cell fates and medulloblastoma in mouse patched mutants.
      • Berman DM
      • Karhadkar SS
      • Hallahan AR
      • et al.
      Medulloblastoma growth inhibition by hedgehog pathway blockade.
      • Sanchez P
      • Hernandez AM
      • Stecca B
      • et al.
      Inhibition of prostate cancer proliferation by interference with SONIC HEDGEHOG-GLI1 signaling.
      • Thayer SP
      • di Magliano MP
      • Heiser PW
      • et al.
      Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis.
      • Bak M
      • Hansen C
      • Tommerup N
      • Larsen LA
      The Hedgehog signaling pathway—implications for drug targets in cancer and neurodegenerative disorders.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.

      Hh SIGNALING

      Mammalian Hh signaling pathway (Figure 2) constitutes (a) Hh ligand with three variants: desert (Dhh), Indian (Ihh), and sonic (Shh); (b) a transmembrane receptor-patched homolog 1 and 2 (Ptch1 and 2); (c) smoothened (Smo), a G protein–coupled receptor; and (d) a cytoplasmic complex that regulates the cubitus interruptus (Ci) or glioma-associated oncogene homolog (Gli) family of transcriptional effectors. All the three ligands bind to the same receptors and elicit similar responses. Shh is the most extensively characterized variant and is widely expressed during embryogenesis. Shh acts as a morphogen and plays an important role in the formation of the neural tube, axial skeleton, primitive gut, and the tracheobronchial tree.
      • Mahlapuu M
      • Enerback S
      • Carlsson P
      Haploinsufficiency of the forkhead gene Foxf1, a target for sonic hedgehog signaling, causes lung and foregut malformations.
      • Bellusci S
      • Furuta Y
      • Rush MG
      • Henderson R
      • Winnier G
      • Hogan BL
      Involvement of sonic hedgehog (Shh) in mouse embryonic lung growth and morphogenesis.
      Figure thumbnail gr2
      FIGURE 2Hedgehog signaling pathway.
      Autocatalytic cleavage and coupling of cholesterol are the essential posttranslational processes that maintain the signaling capability of the Hh ligands.
      • Gallet A
      • Rodriguez R
      • Ruel L
      • Therond PP
      Cholesterol modification of hedgehog is required for trafficking and movement, revealing an asymmetric cellular response to hedgehog.
      The secretion of the functional Hh ligand by the Hh-secreting cell is dependent on the availability of dispatched (Disp), a transmembrane protein with homology to patched (Ptch), which is an Hh receptor on the Hh responsive cell.
      • Burke R
      • Nellen D
      • Bellotto M
      • et al.
      Dispatched, a novel sterol-sensing domain protein dedicated to the release of cholesterol-modified hedgehog from signaling cells.
      • Tian H
      • Jeong J
      • Harfe BD
      • Tabin CJ
      • McMahon AP
      Mouse Disp1 is required in sonic hedgehog-expressing cells for paracrine activity of the cholesterol-modified ligand.
      The Ptch 1 and 2 are membrane receptors for Hh ligands. Ptch 1 is more widely expressed and well characterized. Binding of Hh ligand with the Ptch alters the interactions of Ptch with Smo, resulting in the activation of Smo. This initiates a cascade of events resulting in the Ci and Gli entering the nucleus and acting as transcriptional activators. It is unclear how the activation of Smo communicates with the cytoplasmic Ci/Gli transcription factor complex. Gli bind to the DNA through zinc finger domains directed to particular target genes regulating key cell survival and differentiation functions. Gli1 is a transcription activator, and Gli2 and Gli3 are both activators and repressors of transcription. Gli3 and Ci regulate transcription by binding to the CREB-binding protein, which is a transcription coactivator. Cyclin D and cyclin E are known transcriptional targets of Hh signaling, and these proteins are vital in the G1-to-S transition in the cell cycle.
      • Pasca di Magliano M
      • Hebrok M
      Hedgehog signalling in cancer formation and maintenance.
      Hh signaling activates the mitosis promoting factor by increasing the intranuclear availability of cyclin B.
      • Barnes EA
      • Kong M
      • Ollendorff V
      • Donoghue DJ
      Patched1 interacts with cyclin B1 to regulate cell cycle progression.
      Hh signaling also opposes normal stimuli for epithelial cell cycle arrest (by inhibiting P21) and promotes cell growth.
      • Fan H
      • Khavari PA
      Sonic hedgehog opposes epithelial cell cycle arrest.
      Hh signaling inhibits a well-known regulator of apoptosis, the p53 tumor suppressor gene.

      Yoshinori A, Oda-Sato E, Tobiume K, et al. 2006. The negative regulation of p53 by hedgehog signaling. Presented at the AACR Annual Meeting, Washington, DC, 2006. (Abstract# 1135-b).

      When the Hh ligand is unavailable Ptch1 inhibits the activity of Smo, thus repressing the downstream signaling events. When the Hh signaling is lacking, Gli proteins are bound to microtubules in the cytosol along with a multiprotein complex consisting of Fused (Fu) and suppressor of Fu (SuFu).
      The Hh operates through a series of inhibitory steps. The availability of the Hh ligand for signaling is regulated by the expression of Hh interacting protein (HIP) on the cell surface of Hh responsive cell. The HIP is a membrane glycoprotein that binds to Hh ligands with an affinity similar to that of the membrane protein Ptch1. HIP lacks signal transduction capacity and acts to internalize and degrade the Hh ligand.
      • Chuang PT
      • McMahon AP
      Vertebrate hedgehog signalling modulated by induction of a hedgehog-binding protein.
      Activation of the Hh pathway causes an increased expression of the HIP via a negative feedback mechanism, thus serving as an inducible antagonist of Shh signaling.
      • Chuang PT
      • McMahon AP
      Vertebrate hedgehog signalling modulated by induction of a hedgehog-binding protein.
      The Hh signaling is regulated at various levels, indicating the importance of tight control of Hh signaling. Several inhibitors of the pathway like Ptch and HIP are transcriptional target genes and Hh activation induces negative feedback, reducing the intensity of Hh signaling. Gli genes are regulated by complex mechanisms at both the posttranslational and transcriptional level. Hh signaling up-regulates Gli1 expression while repressing Gli3 expression.
      • Bak M
      • Hansen C
      • Tommerup N
      • Larsen LA
      The Hedgehog signaling pathway—implications for drug targets in cancer and neurodegenerative disorders.
      Dysregulation of the Hh signaling can occur from ligand-dependent and -independent mechanisms (Table 1).
      TABLE 1Dysregulation of Hh Pathway in Cancer
      Pathway ComponentType of Cancer
      Increased Hh ligandBasal cell carcinoma,1,31 medulloblastoma, small cell lung cancer,6 digestive tract tumors,32–34 ovarian tumors,35 prostate cancer36
      Reduced expression of Hh interacting proteinPancreatic cancer,37 liver,23 lung,23 digestive tract tumors,23 prostate cancer36
      Inactivating mutations in the PtchMedulloblastoma,1 basal cell carcinoma,38 rhabdomyosarcoma39
      Activating mutations in the transmembrane helices of SmoBasal cell carcinoma,40 ovarian tumors35
      Overexpression of the Gli proteinsBasal cell carcinoma,41 small cell lung cancer,6 esophageal cancers,33 gastric cancers34
      Loss of function mutations of SuFuMedulloblastoma,42 rhabdomyosarcoma,39 prostate cancer36
      Hh, hedgehog; Ptch, transmembrane receptor-patched homolog; Smo, smoothened a G protein–coupled receptor; Gli, glioma-associated oncogene homolog; SuFu, suppressor of fused.

      Hh SIGNALING IN THE DEVELOPMENT OF NORMAL LUNG

      The lungs develop from an outpouching of the primitive endodermal tube into the surrounding mesenchyme. In the developing lung in mouse models, an elevated Shh expression was detected in the tracheal diverticulum and in the trachea and lung endoderm.
      • Litingtung Y
      • Lei L
      • Westphal H
      • Chiang C
      Sonic hedgehog is essential to foregut development.
      Studies indicate that the Hh signaling pathway is essential for the growth and differentiation of the trachea and lung, and aberrations in the signaling components may be involved in abnormal development of the lung.
      • Mahlapuu M
      • Enerback S
      • Carlsson P
      Haploinsufficiency of the forkhead gene Foxf1, a target for sonic hedgehog signaling, causes lung and foregut malformations.
      • Bellusci S
      • Furuta Y
      • Rush MG
      • Henderson R
      • Winnier G
      • Hogan BL
      Involvement of sonic hedgehog (Shh) in mouse embryonic lung growth and morphogenesis.
      • Litingtung Y
      • Lei L
      • Westphal H
      • Chiang C
      Sonic hedgehog is essential to foregut development.
      Natural teratogens like cyclopamine and jervine (extracted from of corn lilies) are inhibitors of Hh signaling. Pregnant animals treated with these inhibitors of Hh signaling at an early gestation period results in multiple developmental anomalies including abnormal lung development.
      • Cooper MK
      • Porter JA
      • Young KE
      • Beachy PA
      Teratogen-mediated inhibition of target tissue response to Shh signaling.
      The airway epithelial progenitor (stem) cells play an important role in the development of the respiratory epithelium. The differentiation of these progenitor cells to form the neuroendocrine or non-neuroendocrine (ciliated, mucous, clara, or basal cells) component of the respiratory epithelium is tightly regulated by a complex bipotential notch signaling.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      • Watkins DN
      • Berman DM
      • Baylin SB
      Hedgehog signaling: progenitor phenotype in small-cell lung cancer.
      Notch and Wnt signaling are evolutionary conserved signaling pathways tightly regulating cell death, cell movement, and cell division and differentiation during embryogenesis. During development and repair of the lung epithelium, Hh signaling maintains this bipotential notch signaling.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      • Watkins DN
      • Berman DM
      • Baylin SB
      Hedgehog signaling: progenitor phenotype in small-cell lung cancer.
      Hh and Wnt pathways possibly play an important role in the maintenance and the expansion of the progenitor (stem) cells during development and can mediate lung growth by signaling to adjacent lung mesenchyme.
      • Taipale J
      • Beachy PA
      The hedgehog and Wnt signalling pathways in cancer.

      Hh SIGNALING AND LUNG CANCER

      Hh signaling is possibly inactive in the human adult lung epithelium except in the epithelial progenitor (stem) cells. This persistence of Hh signaling in the epithelial progenitor (stem) cells could help maintain these cells and play a critical role in the response to airway epithelial injury.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      • Watkins DN
      • Berman DM
      • Baylin SB
      Hedgehog signaling: progenitor phenotype in small-cell lung cancer.
      • Watkins DN
      • Peacock CD
      Hedgehog signalling in foregut malignancy.
      Studies on animal lung airway epithelial injury/regeneration model suggest that persistent injury to the airway is a potent stimulus for the activation of the Hh signaling, and this helps the expansion of airway epithelial progenitor cells.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      • Watkins DN
      • Berman DM
      • Baylin SB
      Hedgehog signaling: progenitor phenotype in small-cell lung cancer.
      • Watkins DN
      • Peacock CD
      Hedgehog signalling in foregut malignancy.
      Shh and Gli1 are expressed in the regenerating lung airway epithelium.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      Studies on cell lines showed that all the seven small cell lung cancer (SCLC) and seven non-small cell lung cancer (NSCLC) cell lines expressed Shh protein. Five of seven SCLC cell lines expressed both Shh and Gli1 in contrast to NSCLC, which expressed only Shh but not Gli1.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      Analysis of clinical samples of human lung cancer tissue demonstrated 50% (five of 10) of SCLC expressed both Shh and Gli1 compared to only 10% (four of 40) of NSCLC.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      Another study investigating the expression of Gli1 in SCLC tissue reported that 85% (34 of 40) of SCLC express Gli1 and more than 60% have a medium to strong expression correlating with increased Hh signaling.
      • Vestergaard J
      • Pedersen MW
      • Pedersen N
      • et al.
      Hedgehog signaling in small-cell lung cancer: frequent in vivo but a rare event in vitro.
      It thus appears that lung cancer cells retain aspects of the Hh signaling seen in the primitive lung endodermal cells. However, the degree of dependence on this signaling varies ahmong the subtypes of lung cancer. Inhibition of Shh ligand activity using monoclonal antibody and cyclopamine resulted in the significant growth inhibition in SCLC cell lines expressing both Shh and Gli but not NSCLC cell lines (which do not express both Shh and Gli).
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      Similar results were found in in vivo studies on lung cancer xenografts in nude mice.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      HIP is a natural antagonist of Hh signaling as discussed above. Reduced expression of HIP as been reported in lung cancer A549 cell line xenograft in nude mice and a decrease in the expression of HIP was seen in five of 10 human NSCLC tissues.
      • Olsen CL
      • Hsu PP
      • Glienke J
      • Rubanyi GM
      • Brooks AR
      Hedgehog-interacting protein is highly expressed in endothelial cells but down-regulated during angiogenesis and in several human tumors.
      Experimental models studying HIP knockout mice confirmed increased Hh signaling.
      • Kawahira H
      • Ma NH
      • Tzanakakis ES
      • McMahon AP
      • Chuang PT
      • Hebrok M
      Combined activities of hedgehog signaling inhibitors regulate pancreas development.
      • Chuang PT
      • Kawcak T
      • McMahon AP
      Feedback control of mammalian hedgehog signaling by the hedgehog-binding protein, Hip1, modulates Fgf signaling during branching morphogenesis of the lung.
      There appears to be down-regulation of HIP in endothelial cells during angiogenesis.
      • Olsen CL
      • Hsu PP
      • Glienke J
      • Rubanyi GM
      • Brooks AR
      Hedgehog-interacting protein is highly expressed in endothelial cells but down-regulated during angiogenesis and in several human tumors.
      These findings suggest that reduced expression of HIP could potentially enhance the Hh signaling and possibly facilitate angiogenesis. Hh signaling thus appears to play a role in proliferation of malignant cells and promote angiogenesis.

      Hh SIGNALING PATHWAY AS A THERAPEUTIC TARGET

      Therapeutic inactivation of the Hh signaling offers a potential treatment for cancer. Inactivation of the Hh signaling can be done at various levels, mainly (1) extracellular blocking of the Shh ligands using Shh antibodies, (2) activation of Smo in the cell membrane, (3) modulating intracytoplasmic regulators of Hh signaling like protein kinase A and SuFu, and (4) altering the intranuclear functioning of Gli.
      Curis Inc. developed a Shh antagonist that showed promising results in preclinical models.

      Hua Tian DM, Ahn C, Modrusan Z, et al. 2006. Characterization of a Hedgehog pathway antagonist in a mouse medulloblastoma allograft model (abstract 5639). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

      Tang TT, Dongwei L, Reich M, et al. Inhibition of the Hedgehog pathway as a therapeutic approach for the treatment of basal cell carcinomas (abstract 3809). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

      Antibodies to Shh are currently being evaluated in phase I clinical trials for basal cell carcinoma. Cyclopamine is another potential molecule of interest for the inhibition of Hh signaling.

      Hua Tian DM, Ahn C, Modrusan Z, et al. 2006. Characterization of a Hedgehog pathway antagonist in a mouse medulloblastoma allograft model (abstract 5639). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

      Tang TT, Dongwei L, Reich M, et al. Inhibition of the Hedgehog pathway as a therapeutic approach for the treatment of basal cell carcinomas (abstract 3809). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

      Cyclopamine has demonstrated a good safety profile in mice.
      • Berman DM
      • Karhadkar SS
      • Hallahan AR
      • et al.
      Medulloblastoma growth inhibition by hedgehog pathway blockade.
      • Thayer SP
      • di Magliano MP
      • Heiser PW
      • et al.
      Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis.
      • Watkins DN
      • Berman DM
      • Burkholder SG
      • Wang B
      • Beachy PA
      • Baylin SB
      Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
      Several other compounds that bind to Smo and inhibit the downstream events have been identified (KAAD-cyclop, SANT1-4, CUR61414).
      • Chen JK
      • Taipale J
      • Young KE
      • Maiti T
      • Beachy PA
      Small molecule modulation of smoothened activity.
      However, patients with downstream alterations in the Hh signaling could be resistant to the treatment with Shh antagonist and Smo targeted therapies. Another potential mechanism for blocking Gli activity is the use of protein kinase A agonists like forskolin, which maintain the Gli in inactive state.
      • Shindo N
      • Sakai A
      • Arai D
      • Matsuoka O
      • Yamasaki Y
      • Higashinakagawa T
      The ESC-E(Z) complex participates in the hedgehog signaling pathway.
      Antisense oligonucleotides targeting Gli RNA also provide a viable option to prevent Gli-mediated activation of target genes.
      • Sanchez P
      • Hernandez AM
      • Stecca B
      • et al.
      Inhibition of prostate cancer proliferation by interference with SONIC HEDGEHOG-GLI1 signaling.

      Chang DZ. Synthetic miRNAs targeting the GLI-1 transcription factor inhibit division and induce apoptosis in pancreatic tumor cells (abstract 2718). Presented at the 97th AACR Annual Meeting, 2006, Washington, DC.

      It is important to clarify the role of activation of Hh pathway in the process of carcinogenesis and progression in lung cancer. Numerous mechanisms have been implicated in the development, proliferation, and progression of lung cancer; it is critical to understand how the Hh pathway interacts with the other pathways implicated in lung cancer. There have been virtually no significant advances in the systemic therapy of SCLC in the past three decades. With the current trend toward developing targeted therapies, the Hh pathway modulators offer a potential new avenue in the treatment of lung cancer.

      REFERENCES

        • Goodrich LV
        • Milenkovic L
        • Higgins KM
        • Scott MP
        Altered neural cell fates and medulloblastoma in mouse patched mutants.
        Science. 1997; 277: 1109-1113
        • Berman DM
        • Karhadkar SS
        • Hallahan AR
        • et al.
        Medulloblastoma growth inhibition by hedgehog pathway blockade.
        Science. 2002; 297: 1559-1561
        • Sanchez P
        • Hernandez AM
        • Stecca B
        • et al.
        Inhibition of prostate cancer proliferation by interference with SONIC HEDGEHOG-GLI1 signaling.
        Proc Natl Acad Sci U S A. 2004; 101: 12561-12566
        • Thayer SP
        • di Magliano MP
        • Heiser PW
        • et al.
        Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis.
        Nature. 2003; 425: 851-856
        • Bak M
        • Hansen C
        • Tommerup N
        • Larsen LA
        The Hedgehog signaling pathway—implications for drug targets in cancer and neurodegenerative disorders.
        Pharmacogenomics. 2003; 4: 411-429
        • Watkins DN
        • Berman DM
        • Burkholder SG
        • Wang B
        • Beachy PA
        • Baylin SB
        Hedgehog signalling within airway epithelial progenitors and in small-cell lung cancer.
        Nature. 2003; 422: 313-317
        • Mahlapuu M
        • Enerback S
        • Carlsson P
        Haploinsufficiency of the forkhead gene Foxf1, a target for sonic hedgehog signaling, causes lung and foregut malformations.
        Development. 2001; 128: 2397-2406
        • Bellusci S
        • Furuta Y
        • Rush MG
        • Henderson R
        • Winnier G
        • Hogan BL
        Involvement of sonic hedgehog (Shh) in mouse embryonic lung growth and morphogenesis.
        Development. 1997; 124: 53-63
        • Gallet A
        • Rodriguez R
        • Ruel L
        • Therond PP
        Cholesterol modification of hedgehog is required for trafficking and movement, revealing an asymmetric cellular response to hedgehog.
        Dev Cell. 2003; 4: 191-204
        • Burke R
        • Nellen D
        • Bellotto M
        • et al.
        Dispatched, a novel sterol-sensing domain protein dedicated to the release of cholesterol-modified hedgehog from signaling cells.
        Cell. 1999; 99: 803-815
        • Tian H
        • Jeong J
        • Harfe BD
        • Tabin CJ
        • McMahon AP
        Mouse Disp1 is required in sonic hedgehog-expressing cells for paracrine activity of the cholesterol-modified ligand.
        Development. 2005; 132: 133-142
        • Pasca di Magliano M
        • Hebrok M
        Hedgehog signalling in cancer formation and maintenance.
        Nat Rev Cancer. 2003; 3: 903-911
        • Barnes EA
        • Kong M
        • Ollendorff V
        • Donoghue DJ
        Patched1 interacts with cyclin B1 to regulate cell cycle progression.
        EMBO J. 2001; 20: 2214-2223
        • Fan H
        • Khavari PA
        Sonic hedgehog opposes epithelial cell cycle arrest.
        J Cell Biol. 1999; 147: 71-76
      1. Yoshinori A, Oda-Sato E, Tobiume K, et al. 2006. The negative regulation of p53 by hedgehog signaling. Presented at the AACR Annual Meeting, Washington, DC, 2006. (Abstract# 1135-b).

        • Chuang PT
        • McMahon AP
        Vertebrate hedgehog signalling modulated by induction of a hedgehog-binding protein.
        Nature. 1999; 397: 617-621
        • Litingtung Y
        • Lei L
        • Westphal H
        • Chiang C
        Sonic hedgehog is essential to foregut development.
        Nat Genet. 1998; 20: 58-61
        • Cooper MK
        • Porter JA
        • Young KE
        • Beachy PA
        Teratogen-mediated inhibition of target tissue response to Shh signaling.
        Science. 1998; 280: 1603-1607
        • Watkins DN
        • Berman DM
        • Baylin SB
        Hedgehog signaling: progenitor phenotype in small-cell lung cancer.
        Cell Cycle. 2003; 2: 196-198
        • Taipale J
        • Beachy PA
        The hedgehog and Wnt signalling pathways in cancer.
        Nature. 2001; 411: 349-354
        • Watkins DN
        • Peacock CD
        Hedgehog signalling in foregut malignancy.
        Biochem Pharmacol. 2004; 68: 1055-1060
        • Vestergaard J
        • Pedersen MW
        • Pedersen N
        • et al.
        Hedgehog signaling in small-cell lung cancer: frequent in vivo but a rare event in vitro.
        Lung Cancer. 2006; 52: 281-290
        • Olsen CL
        • Hsu PP
        • Glienke J
        • Rubanyi GM
        • Brooks AR
        Hedgehog-interacting protein is highly expressed in endothelial cells but down-regulated during angiogenesis and in several human tumors.
        BMC Cancer. 2004; 4: 43
        • Kawahira H
        • Ma NH
        • Tzanakakis ES
        • McMahon AP
        • Chuang PT
        • Hebrok M
        Combined activities of hedgehog signaling inhibitors regulate pancreas development.
        Development. 2003; 130: 4871-4879
        • Chuang PT
        • Kawcak T
        • McMahon AP
        Feedback control of mammalian hedgehog signaling by the hedgehog-binding protein, Hip1, modulates Fgf signaling during branching morphogenesis of the lung.
        Genes Dev. 2003; 17: 342-347
      2. Hua Tian DM, Ahn C, Modrusan Z, et al. 2006. Characterization of a Hedgehog pathway antagonist in a mouse medulloblastoma allograft model (abstract 5639). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

      3. Tang TT, Dongwei L, Reich M, et al. Inhibition of the Hedgehog pathway as a therapeutic approach for the treatment of basal cell carcinomas (abstract 3809). Presented at the 97th AACR Annual Meeting, Washington, DC, 2006.

        • Chen JK
        • Taipale J
        • Young KE
        • Maiti T
        • Beachy PA
        Small molecule modulation of smoothened activity.
        Proc Natl Acad Sci U S A. 2002; 99: 14071-14076
        • Shindo N
        • Sakai A
        • Arai D
        • Matsuoka O
        • Yamasaki Y
        • Higashinakagawa T
        The ESC-E(Z) complex participates in the hedgehog signaling pathway.
        Biochem Biophys Res Commun. 2005; 327: 1179-1187
      4. Chang DZ. Synthetic miRNAs targeting the GLI-1 transcription factor inhibit division and induce apoptosis in pancreatic tumor cells (abstract 2718). Presented at the 97th AACR Annual Meeting, 2006, Washington, DC.