The Hedgehog/GLI signaling pathway activates transcription of Slug (Snail2) in melanoma cells.

In melanoma and other cancers, invasion, epithelial-to-mesenchymal transition, metastasis and cancer stem cell maintenance are regulated by transcription factors including the Snail family. Slug (Snail2) protein generally supports migration and apoptosis resistance. However, its role in melanoma is not completely understood. The present study investigated the transcriptional regulation of the SLUG gene in melanoma. It demonstrated that SLUG is under the control of the Hedgehog/GLI signaling pathway and is activated predominantly by the transcription factor GLI2. The SLUG gene promoter contains a high number of GLI-binding sites. Slug expression is activated by GLI factors in reporter assays and inhibited by GANT61 (GLI inhibitor) and cyclopamine (SMO inhibitor). SLUG mRNA levels are lowered by GANT61 as assessed by reverse transcription-quantitative PCR. Chromatin immunoprecipitation revealed abundant binding of factors GLI1-3 in the four subregions of the proximal SLUG promoter. Notably, melanoma-associated transcription factor (MITF) is an imperfect activator of the SLUG promoter in reporter assays, and downregulation of MITF had no effect on endogenous Slug protein levels. Immunohistochemical analysis confirmed the above findings and showed MITF-negative regions in metastatic melanoma that were positive for GLI2 and Slug. Taken together, the results demonstrated a previously unrecognized transcriptional activation mechanism of the SLUG gene, which may represent its main regulation of expression in melanoma cells.

Importance of Aberrantly Activated Hedgehog/Gli Pathway in Tumour Progression.

BACKGROUND: Cancer is the second most common cause of death in the Czech Republic. The treatment of this disease is very exhausting for the patients and the treatment has often limited success only. The disease often relapses after a period of remission. Moreover, metastases often appear in lungs, liver or other organs and worsen patients prognosis and probability of survival. The Hedgehog (Hh) signaling pathway is one of the important pathways that affects initiation and maintenance of various types of tumours. When aberrantly activated, Hh signaling pathway helps cells escape apoptosis, disturbs cell energy metabolism, influences the process of epithelial-mesenchymal transition, helps to escape immune system, maintains cancer stem cells and supports metastasis. The role of Hh signaling cascade in tumour initiation, maintenance and progression is intensively studied. Several types of inhibitors of this pathway were developed. The most intensively studied were inhibitors of the receptor Smoothened. Due to commonly occurring resistance, the research of other groups of inhibitors is in the centre of interest. These new drugs do not target receptor Smoothened but proteins standing downstream of Smoothened (inhibition of final Gli transcription factors). The drugs could give new hope to patients whose treatment fails. PURPOSE: This review summarizes the findings about the role of Hh signaling pathway in tumour development and describes the progress in the development of targeted inhibitors of this pathway.