Gastroenterology
Volume 137, Issue 2 , Pages 422-424, August 2009

New Insights into the Role of Hedgehog Signaling in Gastrointestinal Development and Cancer

  • Byeong–Moo Kim
    • ,
  • Michael Y. Choi

      Affiliations

    • Corresponding Author InformationReprint requests Address requests for reprints to: Michael Y. Choi, MD, Massachusetts General Hospital, Jackson 822, 55 Fruit Street, Boston, Massachusetts 02114

Department of Medicine/GI Unit, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts

published online 29 June 2009.

Article Outline

 

See “Paracrine Hedgehog signaling in stomach and intestine: new roles for hedgehog in gastrointestinal patterning” by Kolterud Å, Grosse AS, Zacharias WJ, et al, on page 618; and “Reduced level of smoothened suppresses intestinal tumorigenesis by down-regulation of Wnt signaling” by Arimura S, Matsunaga A, Kitamura T, et al, on page 629.

Since Christiane Nusslein-Volhard and Eric Wieschaus identified and named the Hedgehog (Hh) gene in 1980 to describe mutant Drosophila larvae entirely covered with bristles,1 resembling a hedgehog, the signaling pathway driven by this protein has gained a center stage in the field of developmental biology. Its critical functions in early development, differentiation, and patterning have been demonstrated in numerous species and their various organ systems. In vertebrates, 3 Hh homologs, Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh), have been described.2 Hh initiates signaling cascade by binding to its 12-span transmembrane protein receptor Patched (Ptch), and relieves Ptch from repressing another transmembrane protein Smoothened (Smo). De-repressed Smo relays signaling pathway downstream, ultimately resulting in activation of glioma-associated (Gli) family of zinc finger transcription factors Gli1, Gli2, and Gli3, which translocate to the nucleus and regulate transcription of several target genes including Ptch1 and Gli1.3, 4 However, this concise description is a simplified version of the actual signaling cascade, and the true biology of this pathway is context driven, incorporates several additional molecules, and intersects with other pivotal pathways. Several aspects of this signaling cascade are poorly understood, including its role in gastrointestinal development and cancer, although recent advances have provided insights.

Hh signaling is crucial in gastrointestinal tract development. Shh and Ihh are both expressed in the embryonic gut endoderm.5 Both Shh- and Ihh-null mice have extensive gastrointestinal abnormalities.6, 7 Mutations in this pathway are responsible for both foregut and hindgut anomalies, including congenital malformations such as tracheoesophageal fistula in mice and imperforated anus in humans.8 Moreover, Hh pathway is intimately involved in the genesis and maintenance of several different types of tumors including basal cell carcinoma, medulloblastoma, rhabdomyosarcoma, glioma, and tumors of the breast, esophagus, stomach, pancreas, prostate, lung, biliary tract, bladder, and oral cavity.9 However, Hh pathway's role in colon cancer has not been clear.

In this issue of Gastroenterology, 2 articles shed new light on Hh signaling in early gut development and intestinal tumors. Kolterud et al10 describe precise embryonic and early postnatal gut expression patterns of Hh signaling molecules Ptch1, Gli1, and Gli2 using LacZ reporter mice and in situ hybridization. By analyzing gastrointestinal tissues from several time points, including E10.5, E14.5, E16.5, P0, and P10, dynamic expression patterns of Hh signaling molecules over time are delineated. As expected, the time, place, and intensity of gene expression rapidly change during this critical developmental period in the gastrointestinal tract. For example, only at E10.5 do epithelial cells of the hindgut and tailgut express Ptch1—at no other tested time points does epithelial compartment express Ptch1, Gli1, or Gli2. Another finding is that the muscularis mucosa and villous cores contain Hh responsive cells in the postnatal intestine. Perhaps the most interesting data come from transgenic animals that overexpress Ihh in small intestinal epithelium by a villin promoter. These mice have expanded, hyperproliferated villous core smooth muscle cells that cause villi to become thicker.10 This observation is similar to a recent report of myofibroblast accumulation in the colonic mesenchyme of Ptch1 mutant mice that have constitutively active Hh signaling pathway.11

Findings from this article highlight the dynamic nature of Hh signaling molecule expression in the gastrointestinal tract during its key developmental period. Because Hh proteins are expressed only from the epithelial compartment, strictly mesenchymal expression of Ptch1, Gli1, and Gli2 implies that Hh signaling is most likely paracrine in nature during gut development, although we cannot exclude transient, autocrine activities that may occur between the tested time points. In any event, further investigation is necessary to understand the function and the molecular basis for these observations. Several questions immediately arise. Although a dramatic difference in the level of Hh signaling exists between the stomach and intestine at E16.5, how does Hh participate, if at all, in demarcating precise epithelial boundary between these 2 organs? What is the origin of villous core smooth muscle cells that proliferate with Hh signal? Do they migrate from muscularis mucosa or differentiate from mesenchymal cells already present in the villous core? Lineage-tracing experiments may resolve this issue.

Arimura et al12 explore the role of Hh signaling in a mouse model of intestinal tumor and human colon cancer cell lines (Figure 1). They first demonstrate that Smo is increased significantly in the intestinal adenoma epithelia of Apc+/Δ716 mice, and that Apc+/Δ716Smo+/− mice that express Smo at a much lower level have significantly decreased number of large polyps. Second, knocking down SMO in 2 human colon cancer cell lines SW480 and HCT116 suppress their proliferation by arresting their cell cycle at the G1/S phase. Lastly, SMO down-regulation does not affect the expression levels of Hh target genes such as PTCH1 or GLI1, but instead causes significant reduction of some Wnt target genes such as Cyclin D1 and PROX1. In fact, SMO knockdown reduces the amount of activated β-catenin level and induces its exclusion from the nucleus.12 Their data suggest that Smo activates Wnt signaling by a mechanism independent of the Gli-mediated Hh signaling.

  • View full-size image.
  • Figure 1. 

    Arimura et al propose that an activating signal from Smo, a pivotal molecule in Hh pathway, increases nuclear β-catenin level and consequently transcribes Wnt downstream targets.

Although positive regulation of the Wnt pathway by Hh signaling has been suggested before in tumor models of the pancreas, skin, and medulloblastoma,13, 14, 15 results from Arimura et al12 are still provocative and propose a novel signaling cascade connecting the 2 iconic pathways. Their data suggest that an activating signal from Smo, a pivotal molecule in Hh pathway, increases nuclear β-catenin level and consequently transcribes Wnt downstream targets. This possibility potentially contradicts some existing knowledge regarding Hh–Wnt cross-talk in the intestine. For example, using DLD-1 cancer cell line, van den Brink et al16 have reported possible negative regulation by the Hh signaling on Wnt pathway where overexpression of Ihh caused down-regulation of nuclear TCF4 and β-catenin. Some Wnt targets such as BMP4, cyclin D1, and c-Met were also down-regulated.16 The same group also demonstrated a depletion of proliferating colonic precursor cells and reduced nuclear β-catenin and its targets EphB2, EphB3, and CD44 in Ptch1 mutant mice that have activated Hh signaling pathway.11 Another report of possible negative regulation came from the Gumucio group, the authors of the other Hh paper published in this issue of Gastroenterology. They suppressed both Shh and Ihh in the intestine by generating villin–hedgehog interacting protein (Hhip) transgenic mice that overexpress the pan-Hh inhibitor Hhip in the intestinal epithelium. These mice displayed a highly proliferative intestinal epithelium with ectopic activation of the Wnt pathway where its targets were up-regulated and nuclear β-catenin was aberrantly present even in the villous tip epithelium.17

These seemingly conflicting results may be products of different experimental conditions. Although a detailed comparison of methods will not be discussed here, all of the data gathered by Arimura et al,12 the groups mentioned, and others have used various model systems, challenges, and readouts to deduce the Hh–Wnt relationship. Hh signaling is known to be highly context dependent, where either activating or repressing signals can culminate with subtle changes in the environment. Furthermore, Arimura et al12 conclude that Smo's activation of Wnt signaling is independent of the classic Hh pathway. This invokes an alternate possibility of Smo acting as a promiscuous signaling protein that plays a role in both Hh and Wnt pathways.

Arimura et al12 move the field forward by demonstrating a potentially novel connection between Smo and Wnt signaling. Their finding opens new avenues of investigation in developmental biology and cancer research. However, their results will still need to be substantiated with additional experiments using other model systems to ensure that their conclusion is not limited to Apc+/Δ716 mice and colon cancer cell lines. They report in their article of testing human colorectal adenocarcinoma samples for SMO expression. Although data are not shown, SMO expression is apparently increased in 9 out of 20 cancer samples compared with the normal epithelium of the same patients, which is encouraging. Another angle that needs to be investigated further is the mechanism of action between Smo and activated β-catenin. At this point, we do not know how these 2 molecules interact and what other proteins are involved in this potential cross-talk. It would be interesting to clarify the role of Gli3 because repressor forms of Gli3, generated in the absence of Hh signaling, have been shown to function as a negative regulator of Wnt signaling by interacting with β-catenin.18 Finally, their data naturally raise follow-up questions. For instance, what causes the expression level of Smo to be elevated in Apc+/Δ716 adenomatous epithelium? Because only the Apc gene is mutated in these mice, up-regulation of Smo must be a downstream consequence of the APC mutation. Hence, a signaling event from Wnt pathway to Smo may also be occurring.

Both Kolterud et al10 and Arimura et al12 give us new insights into the role of Hh signaling in gastrointestinal development and cancer. Yet, these new insights create a cascade of new questions, reminding us of what is not known than what we do know about Hh signaling.

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References 

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 Conflicts of interest The authors disclose no conflicts.

PII: S0016-5085(09)00990-1

doi:10.1053/j.gastro.2009.06.021

Refers to article:

  • Additional Online Content AvailableEditorial Accompanies Article Paracrine Hedgehog Signaling in Stomach and Intestine: New Roles for Hedgehog in Gastrointestinal Patterning , 14 May 2009

    Åsa Kolterud, Ann S. Grosse, William J. Zacharias, Katherine D. Walton, Katherine E. Kretovich, Blair B. Madison, Meghna Waghray, Jennifer E. Ferris, Chunbo Hu, Juanita L. Merchant, Andrzej A. Dlugosz, Andreas H. Kottmann, Deborah L. Gumucio
    Gastroenterology August 2009 (Vol. 137, Issue 2, Pages 618-628)

  • Additional Online Content AvailableEditorial Accompanies Article Reduced Level of Smoothened Suppresses Intestinal Tumorigenesis by Down-Regulation of Wnt Signaling , 08 May 2009

    Sumimasa Arimura, Akihiro Matsunaga, Takanori Kitamura, Koji Aoki, Masahiro Aoki, Makoto M. Taketo
    Gastroenterology August 2009 (Vol. 137, Issue 2, Pages 629-638)

Gastroenterology
Volume 137, Issue 2 , Pages 422-424, August 2009

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