br Materials and methods br Results

Materials and methods
Results
Discussion In this study, we discover that Sufu stabilizes Gli1 protein levels by antagonizing Numb/Itch-mediated protein degradation. Together with previous work that demonstrates control of Gli2/3 protein levels by Sufu (Chen et al., 2009, Jia et al., 2009, Wang et al., 2010), our new finding indicates that regulation of Gli protein levels by Sufu is a major general mechanism in modulating Hh responses. Interestingly, our analysis of Sufu-deficient lungs led to the unexpected observation that while canonical Hh targets such as Ptch1 are upregulated, Pdgfra, a direct target of Gli1, is downregulated. We propose that Hh targets do not respond uniformly to Hh signaling under different genetic perturbations and this could also vary from tissue to tissue. We speculate that this may be due to differential levels of individual Gli proteins that control common and distinct sets of Hh targets in a given tissue (Fig. 5D). As a result, some Hh targets are activated while others are concomitantly downregulated. Perhaps this enables Hh signaling to generate complex outputs by regulating the availability of different Gli proteins. Testing and generalizing this hypothesis in diverse tissues will significantly increase our understanding of how Hh signaling leads to diverse phenotypic consequences in nmda receptor and disease.
Acknowledgments We thank David Ornitz for providing Dermo1Cre mice, Brian Black for the modified pGL-Basic vector, Vivian Chen and Chen-Che Huang for technical assistance, members of the Chuang laboratory for discussion and Ross Metzger for critical reading of the manuscript. This work was supported by grants from the Canadian Cancer Society Research Institute (Grant No. 2011-700774) to C.C. H. and the National Institutes of Health (R01 HL091915) to P.T. C.
Hedgehog Introduction The Hedgehog (HH)/GLI pathway was originally identified as a key signaling system that controls multiple processes during invertebrate and vertebrate embryonic development (Ingham and McMahon, 2001; Nusslein-Volhard and Wieschaus, 1980; Teglund and Toftgard, 2010). More recently, the pathway has also been implicated in the control of tissue homeostasis, regeneration, and healing in adult organisms where signaling contributes to the regulation of stem cell activation and self-renewal (; ). Precise control of the onset, strength, and termination of HH/GLI signaling is a critical requirement for proper pattern formation, cell proliferation, survival, differentiation, and morphogenesis. In line with its pivotal regulatory role in these processes, aberrant activation of HH/GLI signaling by either genetic alterations or uncontrolled expression of selected pathway effectors turned out to be an etiological factor in the initiation and growth of numerous human cancer entities including a wide spectrum of solid tumors and hematological malignancies (reviewed in ; ). Targeted inhibition of inappropriate HH/GLI signaling in cancer patients has therefore become a major interest of biotech and pharmaceutical companies with more than half a dozen novel Hedgehog pathway inhibitors (HPIs) currently evaluated for their therapeutic benefit in several clinical trials (for details, see Table 2.1; Epstein, 2008; Scales and de Sauvage, 2009; ).
Regulation of Canonical HH Signaling and Its Implication in Cancer Regulation of Hedgehog signaling is a complex process and a detailed review of the mechanisms of pathway regulation would be beyond the scope of this chapter (for detailed reviews, see Huangfu and Anderson, 2006; ; Varjosalo and Taipale, 2007). We will provide in the following an overview of the key steps and players in canonical HH signaling relayed via the Patched/Smoothened/GLI route and subsequently elaborate on the noncanonical Smoothened-independent signals that have been implicated in the control of the GLI zinc finger transcription factors controlling Hedgehog target gene expression.