Hedgehog Signaling: From Basic Biology to Cancer Therapy

Abstract

The Hedgehog (HH) signaling pathway was discovered originally as a key pathway in embryonic patterning and development. Since its discovery, it has become increasingly clear that the HH pathway also plays important roles in a multitude of cancers. Therefore, HH signaling has emerged as a therapeutic target of interest for cancer therapy. In this review, we provide a brief overview of HH signaling and the key molecular players involved and offer an up-to-date summary of our current knowledge of endogenous and exogenous small molecules that modulate HH signaling. We discuss experiences and lessons learned from the decades-long efforts toward the development of cancer therapies targeting the HH pathway. Challenges to develop next-generation cancer therapies are highlighted.

Keywords: GLI; basal cell carcinoma; cancer therapy; cholesterol; drug resistance; endogenous smoothened regulation; hedgehog signaling; medulloblastoma; smoothened; stem cell.

Figure 1.. Schematic Illustrations of the Mammalian HH Signaling Pathway

(A) In the absence of HH ligand, PTCH1 localizes at the base of the PC (a subcellular membrane extension with high levels of PI4P (blue) but low levels of PI(4,5)P₂ (red)), and inhibits SMO accumulation in the PC and consequently SMO activity. The GLI transcription factors GLI2 and GLI3 are sequestered in the cytoplasm by SUFU and phosphorylated by PKA, CK1, and GSK3β. GPR161, a ciliary G-protein-coupled receptor localized at the base of the PC, can activate PKA through increasing the cAMP levels, promoting the phosphorylation of GLI2 and GLI3. Phosphorylated GLI2 and GLI3 are processed by the proteasome into repressor forms (GLI2^R and GLI3^R). (B) Upon ligand binding, PTCH1 and GPR161 are displaced from the PC and SMO interacts with DLG5 and translocates into the PC. Within the PC, SMO forms a complex with EVC and EVC2 to transduce the HH signaling response. Activated SMO relieves SUFU-mediated suppression of GLI2 and GLI3 within the PC. GLI2 and GLI3 maintain their full-length status and bypass phosphorylation, as PKA activity is restrained by a decreased level of cAMP induced by the exit of GPR161 from PC and the degradation of cAMP by phosphodiesterase 4 (PDE4). The activator forms of GLI2 and GLI3 (GLI2^A and GLI3^A) translocate to the nucleus and induce the expression of HH target genes. Movement of GLI2 and GLI3 proteins within the PC occurs in conjunction with KIF7, a member of the kinesin family of anterograde motor proteins.

Figure 2.. Distinct Mechanisms of HH Pathway Activation in Cancer

(A) Ligand-independent activation due to inactivating mutations in the negative regulators PTCH1 or SUFU, or activating mutations in the positive regulator SMO, or amplification of GLI activators. (B and C) Ligand-dependent activation occurs through paracrine (B) or “reverse paracrine” mechanisms (C). (B) Paracrine signaling occurs when the tumor cells secrete the ligand and surrounding cells (e.g., stromal cells for epithelial tumors) respond and secrete tumor supporting signals. These include IL6, VEGF, IGF, and Wnts depending on the HH target tissue and tumor. (C) “Reverse paracrine” signaling occurs when tumor cells receive HH secreted by adjacent non-tumor cells (e.g., stromal cells) to promote tumor growth.

Figure 3.. Current Strategies for Modulating Activity of the HH Signaling Pathway

The HH signaling pathway can be modulated at many different levels, including interfering with the interaction of HH ligand and receptor through anti-HH antibodies or robotnikinin; modulating SMO activity; regulating ciliogenesis and the ciliary localization of pathway components; targeting GLI transcription factors directly or indirectly by modulating GLI interacting or regulatory factors.

Figure 4.. Mechanisms of Drug Resistance to HH Pathway Inhibitors and Potential Approaches to Overcome Tumor Resistance

Drug resistance to SMO inhibitors is attributed in part to SMO mutations, the amplification of GLI transcription factors, upregulation of PI3K-mTOR signaling, aPKC-ι/λ activation, BRD4 activation, and PDE4 activation. Consequently, drugs targeting these components may circumvent the acquired resistance.

Figure 5.. A Hypothetical Model for Endogenous SMO Modulation by PTCH1

(Left) In the absence of HH ligand, PTCH1 suppresses SMO activity by retaining PI4P, limiting its access to SMO for activation, and potentially fluxing a steroidal antagonist (red diamonds) that binds SMO to its CRD and/or TM sites. (Right) When HH ligand binds PTCH1, its secretion of a potential antagonist might be blocked. PI4P is released from PTCH1 and enriched in the PC, which binds the SMO cytoplasmic tail and activates SMO by promoting phosphorylation and dimerization. Meanwhile, an ubiquitous agonist such as cholesterol or an agonist also subject to PTCH1 regulation might function synergistically in SMO activation via engaging the CRD and/or the TM domains.