Targeting Hedgehog--a cancer stem cell pathway

Abstract

The Hedgehog (Hh) pathway has been implicated in a wide variety of human tumors, and early clinical trials with pathway antagonists have validated Hh signaling as a bona fide anticancer target. Despite these encouraging results, several issues surrounding the basic biology of the Hh pathway in human cancers remain unclear. These include the influence of specific oncogenic events on Hh signal transduction, the precise mode of Hh signaling (i.e., autocrine or paracrine) that occurs within human tumors, and the best means to inhibit aberrant pathway activity in the clinical setting. The cancer stem cell (CSC) hypothesis may explain a number of clinical phenomena, such as unchecked self-renewal and the development of metastatic disease, and to some extent, the Hh signaling pathway has been implicated in all of these processes. Therefore, Hh pathway inhibitors may also represent some of the first agents to formally examine the CSC hypothesis in the clinical setting. The diverse nature of Hh signaling in human cancers suggests that disease-specific factors must be carefully considered to identify the optimal use of novel pathway inhibitors.

Figure 1. Hedgehog signaling

A schematic of Hh pathway signal transduction derived from developmental and cancer models. (A) In the absence of Hh ligand, Ptch is located in the cilium and blocks Smo entry. Gli transcription factors exist in repressor forms that prevent transcription of target genes. (B) Three mammalian homologues of Hh (SHh, IHh, DHh) bind Ptch at the cell surface and allow it to move out of the primary cilium. Smo is derepressed and moves into the primary cilium where it can activate Gli transcription factors. During this process, the Gli transcription factors are processed to activator forms and translocated to the nucleus to induce the transcription of Hh target genes. Antibodies against the Hh ligands (5E1) and robotnikinin block pathway activation by preventing the interaction of Hh ligand with Ptch. Cyclopamine and novel antagonists of Smo directly bind and inhibit its function. Compounds such as HPI 2,3,4 block the transport of components in the signaling cascade. Direct Gli antagonists such as GANT block binding of Gli transcription factors to DNA.

Figure 2. Models of Hh signaling in cancer

(A) Non-ligand mediated signaling (mutational activation). Mutations in PTCH1, SMO, SUFU or amplification of GLI1 have all been reported in human cacners. i. Loss of PTCH1 activity may increase Hh pathway activity. ii. Overexpression or activating mutations in SMO are also depicted. (B) Ligand mediated autocrine signaling. Tumor cells produce Hh ligand that stimulates Hh pathway activity in tumor cells. (C) Ligand mediated paracrine signaling. Non-malignant stromal cells produces Hh ligand required by tumor cells for growth and survival. (D) Ligand mediated paracrine signaling. Tumor cells produce Hh ligand that activates Hh signaling in non-malignant stromal and endothelial cells. This results in the production of unknown factors within the microenvironment that ultimately supports tumor cell growth and survival as well as angiogenesis.

Figure 3. Hedgehog Signaling in Multiple Myeloma

Regulation of MM CSC by the HH signaling pathway. The inhibition of HH signaling (A) inhibits MM CSC displaying the side population phenotype by (B) inducing terminal plasma cells differentiation of MM CSC as indicated by the expression of CD138. (C) Multiple modes of signaling appear to be active in MM. Experimental data suggest that differentiated plasma cells can produce the ligand necessary for CSC survival and proliferation. Blocking signaling leads to CSC differentiation. Normal bone marrow stromal cells can also produce ligand and signal to myeloma cells to support their growth and survival. A possible role for tumor-to-stoma paracrine signaling may also take occur.

Figure 4. Hh signaling induces EMT and metastasis formation

Cells undergoing EMT under the influence of Hh signaling and become more motile and invasive as they acquire mesenchymal cell properties. This allows cells to escape from the primary tumor and circulate to distant sites. Once established at a distant site, Hh may be required for the clonogenic growth and self-renewal.