Abstract CN06-04: The hedgehog signaling pathway in cancer.
Authors: Matsui William H. 
Published: June 2012

The highly conserved Hedgehog (Hh) signaling pathway is normally required for both embryonic development and adult tissue homeostasis and post-injury repair. In vertebrates, signaling is activated by the binding of one of the three ligands, Sonic (SHh), Desert (DHh), and Indian (IHh) Hedgehog, to Patched (Ptch), a 12-pass transmembrane spanning receptor. In the absence of ligand, Ptch represses Smoothened (Smo), a 7-pass transmembrane protein and positive regulator of pathway activity. Following ligand binding, Smo is de-pressed allowing the expression and post-translational processing of Gli transcription factors. Three forms of Gli exist. Gli1 induces and Gli3 represses target gene expression, whereas Gli2 can act either positively or negatively on transcription depending on post-transcriptional and post-translational modifications. Ultimately, the biological activity of the pathway is dictated by the balance between activating and repressive forms of the Glis at target gene promoters (1).

The identification of somatic PTCH1 mutations in patients with Gorlin syndrome and basal cell nevus syndrome who carry an increased risk of developing advanced basal cell carcinoma (BCC), medulloblastoma, and rhabdomyosarcoma initially implicated Hh pathway dysregulation in human carcinogenesis. Further support was provided by the detection of frequent Ptch1 and Smo mutations in sporadic BCCs and medulloblastoma as well as the ability of conditional Ptch loss-of-function or Smo and Gli gain-of-function mutations to recapitulate these diseases in transgenic mice (2). In addition to BCC and medulloblastoma, aberrant Hh signaling has been found in many other cancers, but pathway activity results from increased levels of Hh ligand rather than mutations within pathway components. More recently, multiple types of pathway activation have been described within these ligand-dependent tumors (3). Autocrine and/or juxacrine signaling in which tumor cells both produce and respond to Hh ligand represents the simplest model and has been described in small cell lung, pancreatic, colon, and metastatic prostate carcinomas as well as glioblastoma and melanoma. More recently, Hh signaling has been found to act in a paracrine fashion in which cells that produce Hh ligands are distinct from those responding with pathway activation. Paracrine signaling has been demonstrated in B cell lymphomas and multiple myeloma (MM) in which bone marrow or lymph node stromal cells produce Hh ligand that activates signaling in tumor cells (4). Increased Hh signaling in these settings has been found to promote a myriad of effects including enhanced tumor cell proliferation, survival, and chemoresistance. In pancreatic and colon cancers, an alternative mode of paracrine Hh signaling exists in which tumor cells secrete Hh ligands that induce pathway activity within non-malignant stromal cells. Here, Hh signaling within the tumor microenvironment leads to the secretion of growth factors and cytokines that in turn increase tumor cell survival or angiogenesis (5).

The Hh pathway has also been found to regulate cancer stem cells (CSCs) in a number of human diseases including glioblastoma, breast cancer, colon cancer, pancreatic adenocarcinoma, MM and chronic myeloid leukemia (CML). Self-renewal is a defining property of CSCs required for maintenance of the malignant clone, and studies involving both human tumors and mouse models of cancer have found that this process is dependent on active Hh signaling. In MM, we found that Hh signaling results in CSC expansion whereas pathway inhibition induces terminal differentiation and loss of clonogenic growth potential (6). Therefore, Hh pathway activity appears to regulate CSC fate decisions that include self-renewal and differentiation. Increasing data have also implicated a role for CSCs in the development of metastatic disease, and in pancreatic and colorectal carcinomas, active Hh signaling induces the epithelial to mesenchymal transition within CSCs to enhance their invasive and migratory properties (7).

The emerging role of Hh signaling in human cancers has fueled efforts to develop Hh pathway inhibitors. The discovery that cyclopamine, a steroidal alkyloid derived from Veratrum californicum, binds to Smo and inhibits Hh signaling provided early evidence that the pathway could be pharmacologically targeted (8). Large-scale compound screens have successfully identified several other Smo antagonists, and despite their unbiased nature, most agents bind to and inhibit Smo even though they are structurally distinct from one another and cyclopamine. These agents have moved into clinical testing and the first trial targeting the Hh pathway used one such Smo antagonist, vismodegib (GDC-0449; Genentech/Roche) in patients with advanced solid tumors. Efficacy in patients with advanced BCC was detected early in this trial, presumably because of the high frequency activating mutations, and it was subsequently expanded to specifically study BCC. Out of a total of 33 patients with locally advanced or metastatic BCC 55% demonstrated clinical responses that included 2 complete remissions (9). Reported toxicities were mild, and the most common events were mild loss of taste, hair loss, weight loss and hyponatremia. A transient subjective tumor response was also reported in a single medulloblastoma patient treated with vismodegib. Similar findings in advanced BCC and medulloblastoma have been reported with other Smo antagonists including IPI-926 (Infinity), LDE225 (Norvartis) and BMS-833923 (Bristol-Meyers Squibb/Exelixis).

Initial results with Smo antagonists have been encouraging, but alternative means of inhibiting Hh signaling may be useful for multiple reasons. These include the development of resistance mutations in Smo, the identification of mutations in Hh pathway components that act downstream of Smo (e.g., mutation of the negative pathway regulator Sufu or amplifications in Glis), and direct induction of Gli1 activity by transforming events (e.g., Ews-Fli in Ewing sarcoma or mutant KRas in pancreatic cancer). To this end, novel compounds targeting signal transduction components beyond Smo have been identified. Since mutations in Hh pathway components have not been described in most cancers, the inhibition of ligand binding may repress Hh signaling and the ligand neutralizing monoclonal antibody 5E1 has demonstrated anti-tumor activity in several disease models. In addition, the novel agent robotnikinin has been found to prevent Hh ligands from activating Ptch (10). Since pathway activation cumulates in transcriptional activation by Glis, small molecules modulating Gli activity (GANT-58 and GANT-61) may be useful (11). Furthermore, compounds inhibiting pathway activity by altering the post-translational processing and cellular localization of Gli1 and Gli2 (HPI-1, HPI-2, HPI-3, arsenic) or inhibiting the formation of primary cilia required for Hh signal transduction (HPI-4) have been identified (12, 13). Finally, it is possible that the interaction of other cellular pathways, such as PI3K/AKT, MAPK, TGF-β, and Liver × receptors may also target aberrant Hh signaling.

A role for aberrant Hh signaling pathway is emerging in a wide variety of human malignancies that include multiple roles in disease initiation, relapse, and progression. Therefore, improved understanding of the basic mechanisms that govern Hh signal transduction, the interaction of Hh signaling with other cellular pathways, and the precise role of Hh in specific malignancies should improve the anti-tumor activity of these novel approaches.

References:

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Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2011 Nov 12-16; San Francisco, CA. Philadelphia (PA): AACR; Mol Cancer Ther 2011;10(11 Suppl):Abstract nr CN06-04.