Wnt, Notch, and Hedgehog are three highly conserved signaling pathways that control cell fate decisions, embryonic development, and tissue homeostasis. Their dysregulation is a common feature of many cancers.
Wnt Signaling
Wnt proteins are lipid-modified secreted glycoproteins that signal through the Frizzled family of receptors and the LRP5 and LRP6 coreceptors. In the absence of Wnt, the pathway is off. Beta-catenin, the key transcriptional coactivator, is continuously synthesized and degraded. The destruction complex, containing APC, Axin, GSK3-beta, and CK1, phosphorylates beta-catenin, targeting it for ubiquitination and proteasomal degradation.
Wnt binding to Frizzled and LRP recruits the destruction complex to the membrane. The protein Dishevelled inhibits GSK3-beta activity, allowing beta-catenin to accumulate and translocate to the nucleus. In the nucleus, beta-catenin binds TCF-LEF transcription factors and activates target genes including c-Myc, cyclin D1, and Axin2. The pathway is further modulated by secreted antagonists including Dickkopf proteins that bind LRP and prevent complex formation, and secreted Frizzled-related proteins that sequester Wnt ligands.
Canonical and Non-Canonical Wnt
The beta-catenin-dependent pathway described above is called canonical Wnt signaling. Non-canonical Wnt pathways are beta-catenin-independent and include the planar cell polarity pathway, which regulates cytoskeletal organization and cell movement through small GTPases such as Rho and Rac, and the Wnt-calcium pathway, which increases intracellular calcium and activates CaM kinase and PKC. Non-canonical pathways can antagonize canonical signaling.
Wnt in Development and Disease
Wnt signaling is essential for embryonic patterning, stem cell maintenance, and tissue regeneration. In the intestinal crypt, Wnt maintains stem cell identity and drives proliferation. Aberrant Wnt activation causes cancer. APC mutation is an initiating event in familial adenomatous polyposis and most sporadic colorectal cancers, causing constitutive beta-catenin accumulation. Beta-catenin itself is mutated in many cancers. Wnt pathway inhibitors are being developed as cancer therapeutics.
Notch Signaling
Notch signaling is unique because the receptor itself is cleaved to release a transcription factor, requiring no second messengers. Notch receptors are single-pass transmembrane proteins that undergo proteolytic processing during maturation. Notch ligands, Delta and Jagged in mammals, are also transmembrane proteins. Signaling requires direct cell-cell contact.
Ligand binding induces two sequential proteolytic cleavages of the Notch receptor. The extracellular domain is shed by ADAM metalloproteases, then gamma-secretase cleaves within the transmembrane domain, releasing the Notch intracellular domain. NICD translocates to the nucleus, where it binds the CSL transcription factor and recruits coactivators, displacing corepressors. Target genes include the Hairy-Enhancer of Split family of transcriptional repressors and Myc.
Notch controls binary cell fate decisions and boundary formation in development. In the immune system, Notch directs T cell lineage specification. In the intestine, Notch maintains the progenitor state and suppresses secretory cell differentiation. Notch signaling is frequently hyperactivated in T-cell acute lymphoblastic leukemia through NOTCH1 mutations. Gamma-secretase inhibitors block Notch signaling and are being tested in clinical trials.
Hedgehog Signaling
The Hedgehog family includes Sonic, Indian, and Desert Hedgehog in vertebrates. Hedgehog proteins undergo autocatalytic cleavage and cholesterol modification to generate the active signaling form. The receptor is Patched, a 12-pass transmembrane protein that normally represses the pathway by inhibiting Smoothened, a GPCR-like protein.
In the absence of Hedgehog, Patched inhibits Smoothened, preventing its accumulation in the primary cilium. The Gli transcription factors are processed by the proteasome to generate repressor forms that enter the nucleus and repress target genes. Hedgehog binding to Patched relieves Smoothened inhibition, allowing Smoothened to accumulate in the cilium and activate Gli. Activated Gli proteins accumulate as full-length activators and induce target genes including Gli1, Ptch1, and cyclin D and E.
Hedgehog signaling patterns the developing embryo, specifying cell types in the neural tube, limb buds, and somites. Sonic Hedgehog from the notochord induces ventral neural tube cell fates. In adults, Hedgehog maintains stem cell populations. Aberrant Hedgehog signaling causes basal cell carcinoma, medulloblastoma, and rhabdomyosarcoma. Vismodegib and sonidegib, Smoothened inhibitors, treat advanced basal cell carcinoma.
