Targeting Notch, Hedgehog, and Wnt pathways in cancer stem cells: clinical update

Abstract

During the past decade, cancer stem cells (CSCs) have been increasingly identified in many malignancies. Although the origin and plasticity of these cells remain controversial, tumour heterogeneity and the presence of small populations of cells with stem-like characteristics is established in most malignancies. CSCs display many features of embryonic or tissue stem cells, and typically demonstrate persistent activation of one or more highly conserved signal transduction pathways involved in development and tissue homeostasis, including the Notch, Hedgehog (HH), and Wnt pathways. CSCs generally have slow growth rates and are resistant to chemotherapy and/or radiotherapy. Thus, new treatment strategies targeting these pathways to control stem-cell replication, survival and differentiation are under development. Herein, we provide an update on the latest advances in the clinical development of such approaches, and discuss strategies for overcoming CSC-associated primary or acquired resistance to cancer treatment. Given the crosstalk between the different embryonic developmental signalling pathways, as well as other pathways, designing clinical trials that target CSCs with rational combinations of agents to inhibit possible compensatory escape mechanisms could be of particular importance. We also share our views on the future directions for targeting CSCs to advance the clinical development of these classes of agents.

Figure 1

The canonical Notch signalling pathway and relevant pharmacological inhibitors under development in cancer. DLL1, DLL3 and DLL4, and Jagged ligands (JAG1 and JAG2) expressed on the cell surface can induce signalling in adjacent cells expressing their cognate receptors Notch1–4. Ligand binding promotes sequential cleavage of the Notch receptors by ADAM/TACE enzymes (S2 cleavage) and then γ-secretase (S3 cleavage), resulting in release the NICD, which interacts with transcriptional regulators in the nucleus to instigate a Notch gene-expression profile. Notch target genes, in turn, regulate pivotal cell-fate choices, including differentiation, cell-cycle progression and survival. The final phenotypic effect is dependent on the specific signalling context, paralogue, ligand and dosage. Under many conditions, and in several types of cancer stem-like cells, Notch signalling can delay differentiation, and maintain proliferative and survival potential. Potential therapeutic inhibitors of targets involved in the Notch signalling include soluble decoy receptors, mAbs targeting the Notch ligands or receptors in the extracellular space, and small-molecules or mAb inhibitors targeting the γ-secretase complex. Abbreviations: ADAM, a disintegrin and metalloproteinase; APH-1/2, anterior pharynx-defective-1/2; CSL, CBF1/Su(H)/Lag-1; DLL, delta-like ligand; HAT, histone acetyltransferase; HES, hairy and enhancer of split-1; JAG1, Jagged-1; JAG2, Jagged-2; mAb, monoclonal antibody; MAML1, Mastermind-like 1; NICD, Notch intracellular domain; NRARP, Notch-regulated ankyrin-repeated protein; SKIP, ski-interacting protein; TACE, TNF-α-converting enzyme (also known as ADAM17).

Figure 2

The canonical HH-signalling pathway and pharmacological inhibitors targeting this pathway that are under ongoing development as anticancer therapies. The HH-processing pathway involves HHC autocatalysis, and SKN and Dispatched proteins, which mediate the release of HHN ligands (IHH, DHH and SHH). In the absence of HHN binding, PTCH interacts with and inhibits the activity of SMO; HHN binding to PTCH releases its inhibitory effects on SMO, resulting in SMO accumulation and sequestration of COS and SUFU proteins in cilia, which releases the GLI transcription factors to exert their effects in the nucleus. KIF3A and β-arrestin are required for localization of SMO to cilia. GLI1/2 promote a gene-expression pattern relevant to tumorigenesis. Development of investigational anticancer agents that inhibit SMO activation is of great interest. In addition, other potential targets, such as extracellular HHN ligands, GLI1/2 activation, or inhibition of gene transcription are under preclinical investigation. Besides the FDA-approved agent vismodegib, a number of small-molecule inhibitors of SMO are currently under clinical investigation. Abbreviations: COS, Costal; DHH, Desert hedgehog; HH, Hedgehog; HHC, Hedgehog C-terminal domain; HHN, Hedgehog N-terminal domain; HIP, Hedgehog interacting protein; IHH, Indian hedgehog; Ptch, Patched; SHH, Sonic hedgehog; SKN, Skinny hedgehog; SMO, Smoothened; SUFU, suppressor of fused.

Figure 3

The canonical Wnt signalling pathway and pharmacological inhibitors under investigation in cancer. Various proteins, including Porcupine and Wntless, regulate the secretion on Wnt proteins. LGK974 is an investigational agent that targets Porcupine to inhibit Wnt-ligand secretion from the endoplasmic reticulum. Once released, Wnt protein binding to Fz-family receptors on neighbouring cells results in intracellular signal transduction and gene expression with diverse consequences of relevance to cancer. Small molecules and mAbs targeting either the Wnt proteins or Fz receptor complexes have been developed to inhibit ligand–receptor interactions. The anti-Fz mAb vantictumab and the Wnt decoy receptor OMP-54F28 are currently being tested in clinical trials. Activation of the canonical Wnt signalling pathway leads to β-catenin accumulation as a result of disruption of a multiprotein destruction complex (dashed arrow), mediated by phospho-Dvl, which enables β-catenin to influence gene-expression patterns that determine cell differentiation. Small molecules that stabilize the multiprotein destruction complex and, thus, promote β-catenin degradation, such as tankyrase inhibitors, are in preclinical development. PRI-724 is an investigational agent to disrupt β-catenin–CBP complex, which might shift the balance from β-catenin-mediated gene-expression patterns that block differentiation (and thus promote cell ‘stemness’) to those that promote differentiation, is currently undergoing testing in clinical trials. Abbreviations: APC, adenomatous polyposis coli protein; BCL9, B-cell lymphoma 9; CBP, cyclic AMP response element-binding protein; CK1, casein kinase 1; DKKs, dickkopfs; Dvl, Dishevelled; Fz, Frizzled; GSK3β, glycogen synthase kinase 3β; LRP5/6, low-density lipoprotein receptor-related protein 5/6; mAbs, monoclonal antibodies; PPARG, peroxisome proliferator-activated receptor γ; Pygo, Pygopus; sFRP, secreted Frizzled-related protein; TCF/LEF, T-cell-specific transcription factor/lymphoid enhancer-binding factor; WIF-1, Wnt inhibitory factor 1.