Hedgehog/GLI Signaling Pathway: Transduction, Regulation, and Implications for Disease

Abstract

The Hh/GLI signaling pathway was originally discovered in Drosophila as a major regulator of segment patterning in development. This pathway consists of a series of ligands (Shh, Ihh, and Dhh), transmembrane receptors (Ptch1 and Ptch2), transcription factors (GLI1-3), and signaling regulators (SMO, HHIP, SUFU, PKA, CK1, GSK3β, etc.) that work in concert to repress (Ptch1, Ptch2, SUFU, PKA, CK1, GSK3β) or activate (Shh, Ihh, Dhh, SMO, GLI1-3) the signaling cascade. Not long after the initial discovery, dysregulation of the Hh/GLI signaling pathway was implicated in human disease. Activation of this signaling pathway is observed in many types of cancer, including basal cell carcinoma, medulloblastoma, colorectal, prostate, pancreatic, and many more. Most often, the activation of the Hh/GLI pathway in cancer occurs through a ligand-independent mechanism. However, in benign disease, this activation is mostly ligand-dependent. The upstream signaling component of the receptor complex, SMO, is bypassed, and the GLI family of transcription factors can be activated regardless of ligand binding. Additional mechanisms of pathway activation exist whereby the entirety of the downstream signaling pathway is bypassed, and PTCH1 promotes cell cycle progression and prevents caspase-mediated apoptosis. Throughout this review, we summarize each component of the signaling cascade, non-canonical modes of pathway activation, and the implications in human disease, including cancer.

Keywords: GLI; Hh pathway inhibitors; SUFU; cancer; canonical and non-canonical activation; hedgehog.

Figure 1

Activation of the Hh/GLI signaling pathway. Left panel: In the absence of ligand binding, PTCH exerts repressive effects on SMO. GLI transcription factors are sequestered by SUFU and phosphorylated by PKA, CK1, and GSK3β, marking them for proteolytic cleavage. The cleavage of the C-terminal domain creates GLIr, the repressor form of the transcription factor. GLIr then translocates into the nucleus and represses the transcription of Hh/GLI target genes. Right panel: Hh ligand binding to the extracellular domain of PTCH inhibits the receptor, relieving the repressive effects on SMO. SMO then inhibits the sequestration by SUFU and phosphorylation by PKA, CK1, and GSK3β, sparing GLI from proteolytic cleavage. The full-length form of GLI is a transcriptional activator that translocates into the nucleus and promotes the transcription of Hh/GLI target genes such as PTCH1, GLI1, BCL2, Cyclin D1, IGFL1, HOXD8, and WNT. The canonical Hh/GLI signaling pathway is most typically restricted to the primary cilium, however aberrant activation of this pathway may occur in alternate cellular compartments such as the cell membrane.

Figure 2

GLI1–3 regulatory and protein-interacting domains, post-translational modification sites, and interactions with co-regulators. (A) GLI1–3 proteins and their relevant regulatory sequences and protein-interacting domains. RD (blue) represents the repressor domain, D (orange) represents degron sequence, SB (cyan) represents the SUFU binding site, ZF (green) represents the zinc finger domain, NLS (purple) represents the nuclear localization signal, NES (yellow) represents the nuclear export signal, and TAD (red) represents the transcriptional activator domain. (B) GLI1 protein amino acids required for post-translation modifications. Serine 84 is recognized and phosphorylated by mTOR (pink). Serine 102, Serine 408, and Threonine 1074 are recognized and phosphorylated by AMPK (red). Threonine 374 is recognized and phosphorylated by PKA (blue). Serine 408 is also recognized and phosphorylated by DYRK1A (green). MEKK1 (yellow) recognizes and phosphorylates a series of amino acids between Serine 461 and Threonine 1014. (C) GLI1 protein-interacting domains for several co-regulators. SUFU (blue) binds the SUFU binding domain. GLI2 (purple) and STAT3 (green) can complex and bind to the zinc finger domain. PCAF (orange) and SMAD2/4 (red) can bind near the degron. SMARCA2 (yellow) can bind in the trans-activator domain.

Figure 3

Non-canonical activation of the Hh/GLI pathway. TGFβ Signaling: Ligand binding in the extracellular domain induces the catalytic activity of the receptor on the intracellular domain. R-SMADs can be phosphorylated and complex with co-SMADs, then translocate into the nucleus. Inside the nucleus, SMADs can recruit and bind with GLI1 to activate the transcription of Hh/GLI target genes. KRAS signaling: Constitutively active KRAS will phosphorylate Raf, which in turn phosphorylates MEK, which then phosphorylates ERK. Activated ERK will then translocate into the nucleus and activate a variety of transcription factors, including GLI1/2. Wnt/β-catenin: Extracellular Wnt ligand binding promotes sequestration of the Axin complex to the intracellular side of the receptor. This inhibits phosphorylation of β-catenin, allowing for its nuclear translocation and recruitment of additional transcription factors, including GLI1, for transcriptional activation. Figure created with BioRender.