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Targeting the Hedgehog Pathway in Cancer

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Targeting the Hedgehog Pathway in Cancer REVIEWS Targeting the Hedgehog pathway in cancer Lee L. Rubin* and Frederic J. de Sauvage‡ Abstract | Several key signalling pathways, such as Hedgehog, Notch, Wnt and BMP–TGFβ– Activin (bone morphogenetic protein–transforming growth factor-β–Activin), are involved in most processes essential to the proper development of an embryo. It is also becoming increasingly clear that these pathways can have a crucial role in tumorigenesis when reactivated in adult tissues through sporadic mutations or other mechanisms. We will focus here on the Hedgehog pathway, which is abnormally activated in most basal cell carcinomas, and discuss potential therapeutic opportunities offered by the progress made in understanding this signalling pathway. Holoprosencephaly During embryonic development, the Hedgehog (Hh) Mechanisms of Hedgehog signal transduction A birth defect that can result as signalling pathway regulates proliferation and differen- ‘Hedgehog’ proteins (Hh) are secreted signalling pro- a consequence of insufficient tiation in a time- and position-dependent fashion (for a teins (FIG. 1) that were first discovered in Drosophila, Hh signalling. It relates to a review see REF. 1) so that developing tissues reach their along with many components of their signal transduc- failure of the brain to properly correct size with the appropriate cell types and adequate tion machinery5. They are highly hydrophobic proteins, separate into two halves and in its extreme form can be degrees of vascularization and innervation. The crucial which, after secretion, can diffuse and establish gradients associated with cyclopia, the developmental function of Hh signalling is illustrated by in tissues, which have a paramount role in the proper presence of a single centrally the dramatic consequences in human foetuses of defects development of the embryo1. In the context of embry- located eye. in the signalling pathways, such as holoprosencephaly in onic development, the cells that synthesize Hh ligands Sonic Hedgehog (SHH) mutants2. are distinct from the responsive cells, which are some- It is not difficult to imagine how a pathway that times juxtaposed to the producing cells, but can be at a regulates both proliferation and differentiation, when significant distance. mutated or otherwise deregulated, could contribute Although some crucial differences exist, the signal- to the onset of tumorigenesis or accelerate the rate of ling mechanisms are generally well conserved between tumour growth. Several other key signalling pathways Drosophila and higher organisms. Three Hh homologues that have crucial roles during embryonic development with different spatial and temporal distribution patterns have been found to play a role in tumorigenesis; for have been identified in humans: Sonic hedgehog (SHH), example, Notch1 mutations are found in most T-cell Indian hedgehog (IHH) and Desert hedgehog (DHH). acute lymphoblastic leukaemias (T-ALL)3, and the Wnt The Hh signalling cascade is initiated by Hh binding pathway is activated in the great majority of colon ade- to the Patched 1 protein (PTCH1 in humans, Ptch1 nocarcinoma cases, often through APC (adenomatous in mouse and Ptc in Drosophila) on the target cell (for polyposis coli) gene mutations4. a review see REF. 6). In the absence of the Hh ligand, *Harvard Stem Cell Institute, The Hh pathway has a well-recognized involvement PTCH1 represses the activity of Smoothened (SMOH Biolabs, Room 150, in two types of cancer with an unmet need for drug in humans and Smo in mouse/Drosophila), a G-protein- 16 Divinity Avenue, development: basal cell carcinoma, the most common coupled receptor (GPCR)-like receptor, presumably by Cambridge, Massachusetts cancer in the Western world, and medulloblastoma, a preventing its localization to the cell surface7 (FIG. 1). 02138, USA. ‡Department of Molecular childhood cancer with an invariably poor prognosis. Binding of Hh ligand to PTCH1 causes inhibition of Biology, Genentech, Inc., This review provides a brief overview of Hh signalling, SMOH to be relieved and the Hh signal is transmitted via South San Francisco, discusses the recent advances in the design of small- a protein complex that includes, in Drosophila, pathway- California 94080, USA. molecule and antibody-based inhibitors targeting this specific components such as the atypical kinesin-like Correspondence to L.L.R.: e-mail: signalling cascade, and summarizes relatively new con- protein Costal 2 (Cos2), Fused (Fu), a serine-threonine [email protected] nections between the Hh pathway and a variety of other protein kinase, Suppressor of Fused (SuFu) and the doi:10.1038/nrd2086 solid tumours. zinc-finger transcription factor cubitus interruptus (Ci). 1026 | DECEMBER 2006 | VOLUME 5 www.nature.com/reviews/drugdisc © 2006 Nature Publishing Group REVIEWS Ci is regulated at several levels via phosphorylation a Cilium through kinases such as protein kinase A (PKA), glycogen synthase kinase 3β (GSK3β) and casein kinase 1α (CK1α), which have a crucial role in the processing, activity and nuclear localization of Ci. In the absence of Hh signalling, Ci is cleaved, and its amino-terminal frag- PKA ment acts as an inhibitor of Hh target gene transcription. CK1α Upon Hh signalling, the cleavage of Ci is prevented and PTCH1 GSK3β the full-length molecule becomes a labile activator of Hh target gene transcription8. SMOH However, the situation in higher organisms is not clear, because Cos2 and Fu, which are crucial for signal- ling in the fly, do not seem to be conserved in mammals. (FIG. 1) P In contrast, mammalian Hh signalling requires P Nucleus 9–11 Endogenous P βTrCP the presence of non-motile cilia to which SMOH and ligand? GLI1 GLI2 GLI3 GLIR other downstream pathway components need to transit in order to achieve activation of GLI transcription fac- 12 Hh target tors, the Ci orthologues . Furthermore, the activator and genes repressor forms of Ci seem to have evolved into three SUFU GLIA separate zinc-finger proteins, with GLI1 and GLI2 func- 13 Iguana tioning mostly as activators and GLI3 as a repressor . The mechanism by which this signalling cascade regulates proliferation is now relatively well under- b Endogenous SMOH ligand, Cilium stood and involves the activation of cyclins and cyclin- SMOH agonist dependent kinases14. The control of differentiation Synthetic might be more complex, occurring via the production antagonist of other secreted proteins, including neurotrophic and α SMOH 15 Soluble -Hh angiogenic factors . receptor monoclonal In general terms, it seems clear that the degree of Hh antibody signalling in the adult is much reduced compared with PTCH1 G protein? that in the embryo or neonate, at least in the mouse, and GLI1 GLI2 GLI3 has only conclusively been detected in a few sites such Hedgehog as CNS neural stem cells16–19 (which are responsive to Nucleus Hh activation and express detectable levels of Ptch and 20 Inhibitors A Gli), as well as in gut epithelium . This does not imply of Hh GLI processing that the Hh pathway is without essential function in the adult, however. In addition, evidence is increasing that Internalized Hh target genes some of the same processes and pathways that are so prominent during development are reactivated by tis- sue damage and stimulate tissue repair15,21,22, including (PTCH1, GLI1) peripheral nerve regeneration23. However, in the past few years it has become clear that aberrant activation of the Figure 1 | A schematic of the vertebrate Hh signalling pathway in the absence or Hh signalling pathway can lead to cancer. presence of Hh ligands. The model illustrated here is based on current understanding of the Hedgehog (Hh) pathway in vertebrates. In the unliganded state (no Hh; a), Patched 1 Hh pathway mutations and cancer (PTCH1), a 12-transmembrane (12-TM) domain protein, is located, at least in part, on the The first connection between aberrant Hh signalling and plasma membrane, and the 7-TM protein GPCR-like receptor Smoothened (SMOH) is predominantly located in the membrane of intracellular endosomes. It is proposed that an cancer was the discovery that the rare condition Gorlin 24,25 endogenous intracellular small molecule that acts as an agonist for SMOH (as is often true syndrome is caused by a mutation in PTCH1 . Gorlin for GPCRs) is transported outside the cell by PTCH1 so that it is not able to bind to SMOH. patients develop numerous basal cell carcinomas (BCCs) Under these circumstances, different kinases phosphorylate GLI2/3, creating a repressor during their lifetimes and are predisposed to other kinds form of this transcription factor (GLIR). Iguana and SUFU prevent the active form of GLI (GLIA) of cancer as well, especially medulloblastoma, a tumour from transactivating Hh-responsive genes in a manner that is still not completely of cerebellar granule neuron progenitor cells, and rhab- understood. Upon binding an Hh ligand (b)72–74, PTCH1 is internalized, and apparently domyosarcoma, a muscle tumour. More importantly, destabilized, so that it can no longer transport the endogenous agonist molecules outwards. it was further determined that a large majority of spo- This allows them to accumulate intracellularly and activate SMOH, which itself translocates radically occurring BCCs also involve hyper-activated to the plasma membrane, apparently concentrating in cilia in at least some types of cells. Hh signalling, as judged by high levels of mRNA of the SMOH then engages as-yet-undefined components of the signalling machinery, 26,27 culminating in the appearance of activator forms of GLI that then regulate the expression of Hh target genes GLI1 and PTCH1 in tumour cells . Hh target genes. The known synthetic small-molecule SMOH agonists and antagonists bind Inactivating mutations in PTCH1 occur most commonly to the same site as the putative endogenous ligand. Modified, with permission, from REF. 75 in these tumours, with activating mutations in SMOH © (2006) The Company of Biologists. CK1α, casein kinase 1α; GPCR, G-protein-coupled found in about 10% of all BCCs28,29. In addition, one- receptor; GSK3β, glycogen synthase kinase 3β; PKA, protein kinase A.
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