Non-canonical Hedgehog Signaling Pathway in Cancer: Activation of GLI Transcription Factors Beyond Smoothened

Abstract

The Hedgehog-GLI (HH-GLI) pathway is a highly conserved signaling that plays a critical role in controlling cell specification, cell-cell interaction and tissue patterning during embryonic development. Canonical activation of HH-GLI signaling occurs through binding of HH ligands to the twelve-pass transmembrane receptor Patched 1 (PTCH1), which derepresses the seven-pass transmembrane G protein-coupled receptor Smoothened (SMO). Thus, active SMO initiates a complex intracellular cascade that leads to the activation of the three GLI transcription factors, the final effectors of the HH-GLI pathway. Aberrant activation of this signaling has been implicated in a wide variety of tumors, such as those of the brain, skin, breast, gastrointestinal, lung, pancreas, prostate and ovary. In several of these cases, activation of HH-GLI signaling is mediated by overproduction of HH ligands (e.g., prostate cancer), loss-of-function mutations in PTCH1 or gain-of-function mutations in SMO, which occur in the majority of basal cell carcinoma (BCC), SHH-subtype medulloblastoma and rhabdomyosarcoma. Besides the classical canonical ligand-PTCH1-SMO route, mounting evidence points toward additional, non-canonical ways of GLI activation in cancer. By non-canonical we refer to all those mechanisms of activation of the GLI transcription factors occurring independently of SMO. Often, in a given cancer type canonical and non-canonical activation of HH-GLI signaling co-exist, and in some cancer types, more than one mechanism of non-canonical activation may occur. Tumors harboring non-canonical HH-GLI signaling are less sensitive to SMO inhibition, posing a threat for therapeutic efficacy of these antagonists. Here we will review the most recent findings on the involvement of alternative signaling pathways in inducing GLI activity in cancer and stem cells. We will also discuss the rationale of targeting these oncogenic pathways in combination with HH-GLI inhibitors as a promising anti-cancer therapies.

Keywords: GLI; Hedgehog; cancer; non-canonical; oncogene; targeted therapy; tumor suppressor.

FIGURE 1

Canonical activation of HH-GLI signaling. In absence of the HH ligand (A), PTCH inhibits SMO, and therefore GLI2 and GLI3 are phosphorylated by PKA, CK1 and GSK3β, which create binding sites for the E3 ubiquitin ligase β-TrCP. GLI3 and GLI2 undergo partial proteasome degradation, leading to the formation of repressor forms (GLI3/2^R), that translocate into the nucleus where they inhibit the transcription of HH target genes. Upon HH ligand binding (B), the repression of SMO by PTCH is relieved, allowing accumulation and activation of SMO. Thus, SMO promotes a signaling cascade that ultimately leads to translocation of full length (FL) activated forms of GLI (GLI^A) into the nucleus, where they induce transcription of HH target genes. CK1, caseine kinase 1; GLI2/3^R, GLI2/3 repressors; GLI^A, GLI activators; GLI^FL, GLI full length; GSK3β, glycogen synthase kinase 3β; Hh, Hedgehog; PKA, protein kinase A; PTCH, Patched; SMO, Smoothened; SUFU, Suppressor of Fused.

FIGURE 2

Schematic representation of human GLI1, GLI2, and GLI3 isoforms. See text for details.

FIGURE 3

Non-canonical activation of HH-GLI signaling by oncogenic pathways. Schematic diagram of GLI1/2 and their positive (green) and negative regulators (red). EWS/FLI, SMAD3, miR-326 and miR-361 are only direct transcriptional regulators of GLI1/2. BRD4, HDACs and SRF-MKL1 regulate both GLI1/2 expression and transcriptional activity. See text for details. Abbreviations: AKT, protein kinase B; aPKCι/λ, atypical protein kinase Cι/λ; BRD4, bromodomain-containing protein 4; β-TrCP, β-transducin repeat-containing protein; DYRK1/2, dual specificity tyrosine-phosphorylation-regulated kinase 1/2; EWS/FLI, Ewing Sarcoma/Friend Leukemia Integration 1; GSK3β, glycogen synthase kinase 3β; HDAC, histone deacetylase; IKKβ, inhibitor of nuclear factor kappa-B kinase subunit β; MEK, MAPK (Mitogen-activated protein kinase)/ERK (extracellular signal-regulated kinase) kinase; PI3K, phosphoinositide 3-kinase; PRMT1/5, protein arginine methyltransferases 1/5; SRF-MKL1, serum response factor-megakaryoblastic leukemia 1; S6K1, ribosomal protein S6 kinase 1; WIP1, wild-type p53-induced phosphatase 1.