The role of Hedgehog (Hh) signaling as a developmental pathway is well established. Several recent studies have implicated a role for this pathway in multiple cancers. In this study we report that expression of GLI1 and osteopontin (OPN) increase progressively with the progression of melanoma from primary cutaneous cancer to metastatic melanoma in clinically derived specimens. We have further determined that OPN is a direct transcriptional target of GLI1. We have observed that OPN expression is stimulated in the presence of Hh ligands and inhibited in the presence of the Smoothened (SMO) inhibitor, cyclopamine. Transcriptional silencing of GLI1 negatively impacts OPN expression and compromises the ability of cancer cells to proliferate, migrate, and invade in vitro and interferes with their ability to grow as xenografts and spontaneously metastasize in nude mice. These altered attributes could be rescued by re-expressing OPN in the GLI1-silenced cells, suggesting that OPN is a critical downstream effector of active GLI1 signaling. Our observations lead us to conclude that the GLI1-mediated upregulation of OPN promotes malignant behavior of cancer cells.

The Hedgehog (Hh) pathway is well known for its involvement in angiogenesis and vasculogenesis during ontogeny. The ligand, Sonic hedgehog (SHH), plays an important role in vascular formation during development. However, SHH expression is upregulated on tumor cells and can impact the tumor microenvironment. We have investigated the effects of autocrine as well as paracrine Hh signaling on tumor cells as well as on endothelial cells, respectively.Upon constitutive expression of SHH, breast cancer cells showed aggressive behavior and rapid xenograft growth characterized by highly angiogenic tumors that were spontaneously metastatic. In these cells, SHH caused activation of the Hh transcription factor, GLI1, leading to upregulated expression of the potent pro-angiogenic secreted molecule, CYR61 (cysteine rich angiogenic inducer 61). Silencing of CYR61 from these SHH-expressing Hh activated cells blunted the malignant behavior of the tumor cells and resulted in reduced tumor vasculature and limited hematogenous metastases. Thus, CYR61 is a critical downstream contributor to the Hh influenced pro-angiogenic tumor microenvironment. We also observed concomitant upregulation of SHH and CYR61 transcripts in tumors from patients with advanced breast cancer, further ratifying the clinical relevance of our findings. In summary, we have defined a novel, VEGF-independent, clinically relevant, pro-angiogenic factor, CYR61, that is a transcriptional target of Hh-GLI signaling.

In addition to its role in development, the Hedgehog (Hh) pathway has been shown to be an active participant in cancer development, progression, and metastasis. While the pathway is activated by autocrine signaling by Hh ligands, it can also initiate paracrine signaling with cells in the microenvironment. This creates a network of Hh signaling that determines the malignant behavior of the tumor cells. As a result of paracrine signal transmission, the effect of Hh signaling most profoundly impacts the stromal cells that comprise the tumor microenvironment. The stromal cells in turn, produce factors that nurture the tumor. Thus, such a resonating crosstalk can amplify Hh signaling resulting in ‘molecular chatter’ that overall promotes tumor progression. Hh signaling or rather its inhibition has been the subject of intense research to uncover inhibitors. Several of these inhibitors are currently being evaluated in clinical trials. We have reviewed the role of the Hh pathway in impacting the signature characteristics of cancer cells that determine tumor development, progression and metastasis. This review condenses the latest findings on the signaling pathways activated and/or regulated via molecules generated from Hh signaling in cancer, promising clinical interventions, future directions to identify the appropriate patients for therapy, develop reliable markers of efficacy of treatment and finally combat resistance to Hh pathway inhibitors.