Future prospect of gallbladder therapy using Hedgehog signaling inhibitor

Research into Hedgehog (Hh) signaling in gallbladder cancer is in its infancy, and clinical trials are now only being conducted. Importantly, Hh signaling inhibitors work very well in vitro and in vivo mice models; however, as described by Takebe et al. (1), clinical trials of vismodegib, a Smoothened (SMO) inhibitor, has never met the clinical endpoint (2), and thus we must consider this discrepancy.

To account for this anomaly, we must study the tumor microenvironment; in particular, hypoxia. The in vivo environment, especially at local tumor sites, exists under hypoxic conditions (3,4). The oxygen tension in local tumor sites is <1.3% O₂ compared with 5.3% in mixed venous blood and 3.3–7.9% in well-vascularized organs (4,5). Therefore, different modes of signaling transduction may occur under hypoxic conditions.

Hh signaling is activated under hypoxic conditions through upregulation of SMO transcription (6). As shown in our previous study (7), inhibition of transcription using siRNA was significantly more effective than suppression of SMO by cyclopamine treatment in gallbladder cancer, demonstrating the importance of its regulation at the transcription level. Many Hh inhibitors are protein or antibody inhibitors, and they may not regulate Hh signaling at the transcriptional level. Under hypoxic conditions, SMO may be constitutively supplied, overwhelming the activity of Hh inhibitors. We believe that analysis of the mechanism of SMO transcription upregulation under hypoxia is seriously required. Recently, it has been shown that transcriptional regulator, recombination signal binding protein for immunoglobulin-kappa-J region (RBPJ), and transcriptional co-activator, mastermind-like 3 (MAML3), contribute to hypoxia-induced upregulation of SMO (8). Such a study may improve the efficacy of Hh inhibitors in clinical use.

As described in Mittal et al. (9), whether Hh signaling acts in an autocrine or in a paracrine manner is also an important issue. In addition, we must consider cross-talk signaling. GLI1 is located downstream of Hh signaling as a target gene and transcriptional factor, and therefore, it is a milestone of Hh signaling activation. It has been reported that GLI1 is also activated through other signaling cascades such as the EWS/FLI1 (10), PKC-delta (11), PI3k-AKT (12), and RAS-MEK1 pathways (13), but not through canonical Hh signaling. This infers that SMO inhibition alone is not sufficient to suppress Hh signaling.

GLI2 and GLI3 are also members of the GLI transcription factor family (9). Both occur as inhibitory truncated forms and activated full-length forms. It is difficult for us to discriminate between the two clinically. Previously, we have shown that GLI3 but not GLI1 has a pivotal role in inducing the tumorigenicity of colon cancer (14). Therefore, studies examining the roles of GLI2 and GLI3 in cancer should be conducted in the future.

Acknowledgements

The author thanks Ms Kaori Nomiyama for skillful technical assistance.

Funding: This study was supported by Japan Society for the Promotion of Science KAKENHI grant number 26293289.

Footnote

Conflicts of Interest: The author has no conflicts of interest to declare.

References

1. Takebe N, Yang SX. Sonic hedgehog signaling pathway and gallbladder cancer: targeting with precision medicine approach. Chin Clin Oncol 2016;5:1. [PubMed]
2. Catenacci DV, Junttila MR, Karrison T, et al. Randomized Phase Ib/II Study of Gemcitabine Plus Placebo or Vismodegib, a Hedgehog Pathway Inhibitor, in Patients With Metastatic Pancreatic Cancer. J Clin Oncol 2015;33:4284-92. [Crossref] [PubMed]
3. Koong AC, Mehta VK, Le QT, et al. Pancreatic tumors show high levels of hypoxia. Int J Radiat Oncol Biol Phys 2000;48:919-22. [Crossref] [PubMed]
4. Caldwell CC, Kojima H, Lukashev D, et al. Differential effects of physiologically relevant hypoxic conditions on T lymphocyte development and effector functions. J Immunol 2001;167:6140-9. [Crossref] [PubMed]
5. Höckel S, Schlenger K, Vaupel P, et al. Association between host tissue vascularity and the prognostically relevant tumor vascularity in human cervical cancer. Int J Oncol 2001;19:827-32. [PubMed]
6. Onishi H, Kai M, Odate S, et al. Hypoxia activates the hedgehog signaling pathway in a ligand-independent manner by upregulation of Smo transcription in pancreatic cancer. Cancer Sci 2011;102:1144-50. [Crossref] [PubMed]
7. Matsushita S, Onishi H, Nakano K, et al. Hedgehog signaling pathway is a potential therapeutic target for gallbladder cancer. Cancer Sci 2014;105:272-80. [Crossref] [PubMed]
8. Onishi H, Yamasaki A, Kawamoto M, et al. Hypoxia but not normoxia promotes Smoothened transcription through upregulation of RBPJ and Mastermind-like 3 in pancreatic cancer. Cancer Lett 2016;371:143-50. [Crossref] [PubMed]
9. Mittal B, Yadav S. Targeting the hedgehog pathway for gallbladder cancer therapy? Chin Clin Oncol 2016;5:2. [PubMed]
10. Joo J, Christensen L, Warner K, et al. GLI1 is a central mediator of EWS/FLI1 signaling in Ewing tumors. PLoS One 2009;4:e7608. [Crossref] [PubMed]
11. Riobo NA, Lu K, Emerson CP Jr. Hedgehog signal transduction: signal integration and cross talk in development and cancer. Cell Cycle 2006;5:1612-5. [Crossref] [PubMed]
12. Riobó NA, Lu K, Ai X, et al. Phosphoinositide 3-kinase and Akt are essential for Sonic Hedgehog signaling. Proc Natl Acad Sci U S A 2006;103:4505-10. [Crossref] [PubMed]
13. Nolan-Stevaux O, Lau J, Truitt ML, et al. GLI1 is regulated through Smoothened-independent mechanisms in neoplastic pancreatic ducts and mediates PDAC cell survival and transformation. Genes Dev 2009;23:24-36. [Crossref] [PubMed]
14. Iwasaki H, Nakano K, Shinkai K, et al. Hedgehog Gli3 activator signal augments tumorigenicity of colorectal cancer via upregulation of adherence-related genes. Cancer Sci 2013;104:328-36. [Crossref] [PubMed]