Section 3.2.3: Hh signaling pathway in CSCs (from DOI: 10.1038/s41392-020-0110-5)
From publication: "Targeting cancer stem cell pathways for cancer therapy" published as Signal Transduct Target Ther; 2020 ; 5 8; DOI: https://doi.org/10.1038/s41392-020-0110-5
Section 3.2.3: Hh signaling pathway in CSCs
The Hh signaling pathway consists of ligands and receptors. The Hh signaling network is very complex, including extracellular Hh ligands, the transmembrane protein receptor PTCH, the transmembrane protein SMO, intermediate transduction molecules, and the downstream molecule GLI. The components of the Hh signaling pathway play different roles. The membrane protein SMO plays a positive regulatory role, while the transmembrane protein PTCH plays a negative regulatory role. PTCH has two subtypes, PTCH1 and PTCH2, and there is 73% homology between the two subtypes. GLI, an effector protein, has three subtypes, Gli1, Gli2, and Gli3, in vertebrates, and these effector proteins have different functions. Gli1 strongly activates transcription, while Gli3 inhibits transcription. Gli2 has dual functions of activating and inhibiting transcription but mainly functions as a transcriptional activator. Numerous studies have confirmed that Hh signaling is involved in embryonic development and the formation of the nervous system, skeleton, limbs, lung, heart, and gut. As an extracellular signaling pathway, in the presence of ligand signals, Hh ligands bind to PTCH receptors on target cell membranes and initiate a series of intracellular signal transduction processes. When there is no ligand signal, the transmembrane receptor PTCH on the target cell membrane binds to SMO and inhibits SMO activity, which prevents signaling. When the Hh ligand is present, it binds to PTCH, which changes the spatial conformation of PTCH, removing the inhibition of SMO activating the transcription factor GLI and inducing it to enter the cell nucleus, where GLI regulates cell growth, proliferation, and differentiation.
Studies have confirmed that abnormal activation of the Hh signaling pathway can be found in human cancers, such as breast cancer, lung cancer, bladder cancer, pancreatic cancer, chondrosarcoma, rhabdomyosarcoma, neuroblastoma, medulloblastoma, and gastric cancer. However, activation of Hh signaling is different in different tumors. Gorlin syndrome (basal cell nevus syndrome), an autosomal dominant condition, is associated with germline loss of the PTCH1 gene. This condition is very common in basal cell carcinoma, rhabdomyosarcoma, and medulloblastoma. Other Hh pathway components are also mutated in human cancers, such as Gli1 and Gli3 mutations in pancreatic adenocarcinoma, Gli1 gene amplification in glioblastoma, and SUFU (suppressor of fused) mutations in medulloblastoma. In addition, other genes also regulate the Hh signaling pathway. Speckle-type POZ protein, an E3 ubiquitin ligase adaptor, inhibits Hh signaling by accelerating Gli2 degradation in gastric cancer.
Figure 2: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7005297/bin/41392_2020_110_Fig2_HTML.jpg
Figure 2 caption: Hedgehog signaling pathway in cancer stem cells. The Hedgehog pathway plays a key role in stem maintenance, self-renewal, and regeneration of CSCs. The secreted Hh protein acts in a concentration- and time-dependent manner to initiate a series of cell responses, such as cell survival, proliferation, and differentiation. After receiving the Shh signal, the transmembrane protein receptor PTCH relieves the inhibition of the transmembrane protein SMO, which induces Gli1/2 to detach from SUFU and enter the nucleus to regulate downstream gene transcription. During activation of the Hh pathway, some proteins (IL-6, IL-27, Fbxl17 (F-box and leucine-rich repeat protein 17), PPKCI, RARalpha2, RUXN3, SCUBE2, HDAC6 (histone deacetylase 6), USP48, CK2alpha, WIP1, GALNT1, VASH2 (Vasohibin 2), BCL6, FOXC1 (forkhead box C1), and p65), microRNAs (miR-324-5p, miR-122, and miR-326), and the long noncoding RNA HDAC2 are involved in the Hedgehog pathway to affect CSC growth
Hh signaling plays distinct functions in different types of cancer. During tumor development, Hh signaling has three major roles: driving tumor development, promoting tumor growth, and regulating residual cancer cells after therapy. Based on these functions, the aberrant Hh pathway plays a causal role in CSCs (Fig. 2). The expression level of Hh signaling components is relatively high in CSCs. For example, Hh signaling promotes the maintenance, proliferation, self-renewal, and tumorigenicity of lung adenocarcinoma stem cells. In CD133+ glioma stem cells, SMO, GLI, and PTCH promote cell proliferation, self-renewal, migration, and invasion. The expression of Gli1, PTCH1, and PTCH2 is regulated by histone deacetylase 6. USP48 activates Gli-dependent transcription by stabilizing the Gli1 protein in glioma stem cells. The protein kinase CK2alpha enhances Gli1 expression and its transcriptional activity in lung CSCs. WIP1 (PPM1D), a nuclear Ser/Thr phosphatase, also enhances the function of Gli1 by increasing its transcriptional activity, protein stability, and nuclear localization in breast CSCs and medulloblastomas. F-box and leucine-rich repeat protein 17 mediates the release of Gli1 from SUFU for proper Hh signal transduction in medulloblastoma stem cells. Moreover, retinoic acid receptor alpha2 (RARalpha2) upregulates the expression of SMO and Gli1 in CD138+ multiple myeloma stem cells. PRKCI, which is regulated by miR-219 in tongue SCC, has a similar function as RARalpha2 in maintaining a stem-like phenotype in lung SCC cells. Interleukin-27 (IL-27) and IL-6 activate Hh signaling in CD133+ non-small-cell lung CSCs. During self-renewal and maintenance of stemness of BCMab1+CD44+ bladder CSCs, glycotransferase GALNT1-mediated glycosylation significantly activates Sonic Hh signaling by upregulating Gli1.
Furthermore, p63, a master regulator of normal epithelial stem cell maintenance, regulates the expression of Shh, Gli2, and PTCH1 by directly binding to their gene regulatory regions, which eventually contributes to the activation of Hh signaling in mammary CSCs. The N-terminal domain of forkhead box C1 binds directly to an internal region (amino acids (aa) 898-1168) of Gli2 to enhance transcriptional activation of Gli2 and determines the stem cell phenotype in breast CSCs. Through recruitment of the deubiquitinating enzyme ATXN3, tetraspanin-8 interacts with PTCH1 and inhibits the degradation of the SHH/PTCH1 complex. In addition, long noncoding microRNAs also activate Hh signaling. For example, lncHDAC2 promotes the self-renewal of liver CSCs by recruiting the NuRD complex onto the promoter of the PTCH1 gene to suppress its expression. In addition, the TME is crucial for the survival of CSCs. Consequently, breast CSCs secrete Shh, which upregulates cancer-associated fibroblasts (CAFs). Subsequently, CAFs secrete factors that promote the expansion and self-renewal of breast CSCs. Hh signaling also promotes self-renewal and metastasis of CSCs by upregulating the expression of related downstream markers of CSCs, such as Bmi-1, Wnt2, ALDH1, CD44, CCND1, Twist1, C-MYC, Nanog, Oct4, PDGFRalpha (platelet-derived factor receptor-alpha), Snail, Jagged 1, and C-MET.
Some proto-oncogenes and suppressor genes also directly or indirectly regulate Hh signaling in the proliferation and migration of CSCs. The signal peptide CUB EGF-like domain-containing protein 2 (SCUBE2), a member of the SCUBE family of proteins, inhibits cell proliferation and migration in glioma stem cells by downregulating Hh signaling. BCL6, a transcriptional repressor and lymphoma oncoprotein, directly represses the Sonic Hh effectors Gli1 and Gli2 in medulloblastoma stem cells. The transcription factor RUNX3 suppresses metastasis and the stemness of colorectal CSCs by promoting ubiquitination of Gli1 at the intracellular level. Vasohibin 2 suppresses Smo, Gli1, and Gli2 expression in pancreatic CSCs. beta-Catenin stably increases its physical interaction with Gli1, resulting in Gli1 degradation in medulloblastoma stem cells. In addition, microRNAs also target Hh signaling components to regulate CSC proliferation. For example, miR-324-5p significantly decreases SMO and Gli1 in myeloma stem cells. Mir-326 directly downregulates SMO and Gli2 in medulloblastoma stem cells. MiR-326 downregulates SMO in glioma stem cells. Mir-122 targets Shh and Gli1 in lung CSCs. These data demonstrate that amplified Hh signaling is important for the self-renewal, growth, and metastasis of CSCs.
