br Hedgehog HH signaling plays an important role
Hedgehog (HH) signaling plays an important role both during embryonic development and adult life. It is involved in the regulation of cell differentiation, cell proliferation and tissue polarity, as well as in the maintenance of stem cells, tissue repair, and regeneration (, ). Three ligands, Indian, Sonic, and Desert HH, can activate this pathway. Binding of HH ligands to their receptor, PTCH1 () lift its inhibition on SMO, resulting in activation and nuclear translocation of GLI transcription factors (). The vertebrate gene family is composed of three distinct genes , , and , encoding Krüppel-like transcription factors. GLI proteins exhibit distinct regulations, biochemical properties, and target genes. GLI3 acts as the main repressor of the pathway in the absence of HH ligands, whereas, in their presence, GLI2 is the main HH effector that drives the expression of GLI1 (). The direct role of the HH signaling pathway in tumorigenesis was first established through the identification of loss-of-function mutations in the gene in patients with familial and sporadic basal cell carcinomas of the skin and in medulloblastoma in children (). HH pathway activation, often estimated as elevated GLI1 expression, has since been described in an ever-growing number of tumors, including esophageal squamous cell sarcomas; bladder, ovarian, gastrointestinal, lung and pancreatic carcinomas; and cutaneous melanoma (). Targeting the HH pathway for cancer treatment by means of SMO antagonists has shown remarkable efficacy against tumors with identified mutations in the upstream components of the pathway. In particular, US Food and Drug Administration approves two SMO inhibitors (Sonidegib/LDE225/Odomzo and GDC-0449/Vismodegib) for basal cell carcinoma treatment. However, a number of tumors are oblivious to HH signaling inhibition, even though they exhibit high expression of GLI1, suggesting the existence of alternate pathways that lead to expression of downstream HH mediators and/or targets, such as GLI1. We identified GLI2 as a direct transcriptional target of the TGF-β pathway (, ). The latter promotes tumor progression through the regulation of several cellular processes, including induction of EMT and evasion from immune surveillance (). TGF-β, which are secreted abundantly by tumor CMX001 as well as by the local microenvironment, also facilitate tumor cell proliferation and dissemination by stimulating both peritumoral angiogenesis and remodeling of the tumor stroma (). We demonstrated that induction of mRNA by TGF-β is rapid and independent of the HH pathway, as demonstrated by the persistence of GLI2 induction by TGF-β in presence of the HH pathway inhibitor cyclopamine. Importantly, we also found that TGF-β–induced GLI2 expression leads to HH-independent GLI1 expression (). Remarkably, pharmacologic inhibition of autocrine TGF-β signaling efficiently slowed the growth of cyclopamine-resistant pancreatic cancer cell lines, while also reducing GLI2 expression (). These experiments were the first to identify TGF-β signaling as a relevant target for therapeutic intervention in a cellular context characterized by high GLI1 expression and lack of responsiveness to therapeutic HH targeting. There is ample experimental evidence from mouse studies that GLI2 plays a direct functional role in the development of solid tumors (reviewed in ). For example, overexpression of Gli2 in mouse skin by use of a Keratin 5 promoter is sufficient to produce basal cell carcinomas (). Inversely, knockdown in prostate cancer cells delays tumor xenograft growth in vivo (). We established that high GLI2 expression in highly invasive melanoma cells depends largely upon autocrine TGF-β signaling and is associated with a mesenchymal transition and loss of E-cadherin expression, events associated with enhanced cell motility and capacity to metastasize (, ).
Introduction The Hedgehog (HH) signaling pathway regulates growth and patterning in multiple tissues in a variety of metazoan embryos (reviewed in Wilson and Chuang, 2010). Secreted HH ligands can spread over several cell diameters, eliciting both short and long-range effects (Chamberlain et al., 2008, Li et al., 2006, Nahmad and Stathopoulos, 2009, Sanders et al., 2013). HH-receiving cells respond by modulating the activity of the GLI transcription factors (GLI1-3, homologs of Ci in Drosophila). In the absence of HH ligand, GLIs are partially degraded by the proteasome, forming a truncated protein that functions as a transcriptional repressor (GLI-R). Conversely, in the presence of HH ligand, processing of GLI proteins is inhibited, permitting the formation of GLI activators (GLI-A) (Aza-blanc et al., 1997, Méthot and Basler, 1999, Pan et al., 2006, Wang et al., 2000).