In the absence of HH
In the absence of HH ligand, HH signaling is actively maintained in a repressed state by the activity of the HH receptor Patched (PTCH). PTCH is a 12-transmembrane domain protein whose intracellular localization is concentrated at the A-740003 of the primary cilium, a single antenna-like structure that protrudes from the cell surface of many adherent cell types and functions as a signal transduction compartment (). Although the detailed mechanism of pathway repression by PTCH has not been elucidated, recent studies have shown that unliganded PTCH, which can act catalytically (), prevents the translocation of the seven-transmembrane domain protein and essential pathway effector Smoothened (SMO) into the primary cilium (). This leads to proteolytic cleavage of the latent zinc finger transcription factors GLI3—and to some extent also of GLI2—into C-terminally truncated repressor forms (GLIR) (; ). GLIR formation involves preceding and sequential phosphorylation by protein kinase A (PKA), glycogen synthase kinase 3-beta (GSK), and casein kinase I (CKI) (Price and Kalderon, 2002) as well as a functional primary cilium (; ). Following processing, GLIR translocates to the nucleus to bind to HH target gene promoters and repress target gene expression (). GLI signals are also negatively regulated by proteasome-mediated degradation of GLI and by binding to Suppressor of Fused (SUFU), which sequesters GLI proteins in the cytoplasm (Fig. 2.1, left) (reviewed in ; Teglund and Toftgard, 2010). Paracrine or autocrine activation of HH signaling is initiated by binding of secreted and posttranslationally modified HH proteins, that is, Sonic (SHH), Indian (IHH), or Desert (DHH) Hedgehog, to their receptor PTCH (Gallet, 2011; ). The interaction of HH protein with PTCH removes PTCH from the primary cilium, thus allowing SMO to enter the cilium via lateral transport and activate downstream signaling events (; ). The translocation and activation of SMO involve association of SMO with β-arrestins and the GPCR kinase GKR2, respectively (; ; ; ). Active SMO localized in the primary cilium (i) interferes with GLI repressor formation and (ii) triggers release of GLI from SUFU (; ). As a result of these complex regulatory steps, the full-length activator form of GLI, hitherto referred to as GLIA, can translocate to the nucleus, where it binds to target gene promoters and activates transcription of HH target genes including GLI1. Being a strong transcriptional activator, induction of GLI1 in response to canonical PTCH/SMO-dependent pathway activation causes amplification of the GLIA signal (; ; ). In essence, one can therefore state that the precise regulation of the GLIA/GLIR ratio is the most critical parameter for proper development and homeostasis. This is supported by the fact that the balance of GLIA/GLIR forms determines not only the strength and output of HH signaling but also the fate of a cell. High GLIA/GLIR ratios are mainly associated with proliferation, increased survival, and stem cell self-renewal, while low ratios favor differentiation and quiescence (). During the past years, numerous studies have unraveled a fundamental role of the HH/GLI pathway in a wide variety of human cancers including solid tumors and hematological malignancies. Current estimates suggest that up to 25% of human cancers display aberrant HH/GLI signaling (Teglund and Toftgard, 2010). These cancers are characterized by uncontrolled and persistent activation of the HH/GLI pathway leading to an increase in GLIA forms, mostly GLI1, at the expense of GLIR forms. High GLIA/GLIR ratios can be the result of (i) loss-of-function mutations in pathway repressors such as PTCH and SUFU, (ii) gain-of-function mutations and amplifications in SMO and GLI2/GLI1, respectively, or (iii) overexpression of and sustained exposure to HH ligands (; Stecca and Ruiz, 2010). While it is clear that aberrant activation of HH/GLI signaling is an etiologic factor in many cancers, the identity of the HH signal-receiving cell is more complex as there is evidence for both the tumor cell itself and the tumor-associated stromal cell being the target of HH pathway activation (for detailed reviews, see Scales and de Sauvage, 2009; Teglund and Toftgard, 2010).