The Hedgehog Signaling Pathway: A Viable Target in Breast Cancer?

Abstract

The hedgehog (Hh) pathway plays a key role in embryonic development and stem cell programs. Deregulation of the Hh pathway is a key driver of basal cell carcinoma, and therapeutic targeting led to approval of Hh inhibitor, vismodegib, in the management of this cancer. The Hh pathway is implicated in other malignancies including hormone receptor (HR+) positive and triple negative breast cancer (TNBC). Hh signaling, which is activated in human mammary stem cells, results in activation of glioma-associated oncogene (GLI) transcription factors. High GLI1 expression correlates with worse outcomes in breast cancer. Non-canonical GLI1 activation is one mechanism by which estrogen exposure promotes breast cancer stem cell proliferation and epithelial-mesenchymal transition. Tamoxifen resistant cell lines show aberrant activation of Hh signaling, and knockdown of Hh pathway inhibited growth of tamoxifen resistant cells. As in other cancers Hh signaling is activated by the PI3K/AKT pathway in these endocrine resistant cell lines. Hh pathway activation has also been reported to mediate chemotherapy resistance in TNBC via various mechanisms including paracrine signaling to tumor micro-environment and selective proliferation of cancer stem cells. Co-activation of Hh and Wnt signaling pathways is a poor prognostic marker in TNBC. Early phase clinical trials are evaluating the combination of smoothened (SMO) inhibitors and chemotherapy in TNBC. In addition to SMO inhibitors like vismodegib and sonidegib, which are in clinical use for basal cell carcinoma, GLI1 inhibitors like GANT58 and GANT61 are in preclinical drug development and might be an effective mechanism to overcome drug resistance in breast cancer. Gene signatures predictive of Hh pathway activation could enrich for patients likely to respond to these agents.

Keywords: GLI1; breast cancer; hedgehog.

Figure 1

Figure depicts canonical hedgehog signaling pathway. In absence of hedgehog ligands, sonic hedgehog (SHH), Indian hedgehog (IHH) and desert hedgehog (DHH) bind to Patched (PTCH) protein, which inhibits Smoothened (SMO) and its downstream signaling events. Binding of Hh ligands to PTCH inhibits the PTCH, leading to the dis-inhibition of SMO, which then inhibits suppressor of fused (SUFU), thereby leading to release and nuclear translocation of glioma-associated oncogene (GLI)1/2 proteins and transcriptional upregulation of various GLI target genes. GLI3 is a transcriptional repressor, that when associated with SUFU in a trimolecular complex with GSK3b, undergoes further processing to generate repressor GLI. Activation of SMO leads to dissociation of SUFU/GLI3/GSK3b complex. Inhibitors of the pathway, including vismodegib, cyclopamine, itraconazole, GANT58 and GANT61 and the sites of inhibition are indicated.

Figure 2

Figure depicts pathways that cross-talk with Hh pathway leading to its non-canonical activation. EGF(Epidermal growth factor) signaling pathway activates GLI mediated transcription through pAKT, as well as MAPK activation (shown in brown/red). TGFβ activation is mediated through SMAD2/3/4 complex (shown in blue) and Wnt signaling pathway activation is through b-catenin/TCF-LEF (shown in green).