The Lipogenic Phenotype in Melanoma and its Contribution to the Metastatic Process

Metastatic melanoma causes a very poor prognosis with a median survival time of only eight months. Recently we have screened clonal melanoma tumors and identified MET receptor signaling as a mechanism to drive melanoma metastasis. Additionally, we observed up-regulation of enzymes involved in the steroid metabolism as well as accumulation of lipids in metastatic melanoma cells. Therefore, this project aims at elucidating the molecular regulation of lipogenesis by the MET affected mechanistic target of rapamycin (mTOR) and its consequences for the metastatic process. We hypothesize that specifically accumulation of cholesterol, either by increased uptake or synthesis leads to enhanced transmembrane receptor signaling in lipid rafts and increased activation of small GTPases. We will test whether cholesterol accumulation is mTOR dependent and whether this process induces a metastatic phenotype. This project has broad implications for the treatment of this fatal disease because i) the role of cholesterol and its metabolites is not well established during induction of metastasis and ii) cholesterol biosynthesis as well as mTOR signaling can be controlled by readily available pharmaceuticals. Hence this project has the potential to uncover novel mechanisms during metastasis and offers the identification of druggable targets essentially involved in this deadly disease.

In our research project we investigated cholesterol homeostasis in developmental systems and used this knowledge to analyze the role of cholesterol and its receptors in cancer. We identified the cholesterol transporting, high density lipoprotein receptor (named SR-BI) as an important biomarker in malignant melanoma. Next we asked the question whether the sole presence of SR-BI or its selective cholesterol transporting activity were responsible for driving melanoma aggressiveness. We used metastatic melanoma cells and screened for the loss of disease associated pathways after either treatment with an pharmacologic inhibitor blocking SR-BI function or we applied a genetic approach to completely remove the SR-BI protein. Importantly we identified expression of SR-BI protein to be responsible for maintaining pathways known to be essential for the metastatic process. Interestingly observed effects where independent of the cholesterol transporting function of SR-BI, indicating that the presence of SR-BI within cells is sufficient for enabling metastasis associated processes. In particular the protein glycosylation pathway was affected by SR-BI loss leading to decreased STAT5 signaling and reduced epithelial mesenchymal transition phenotype. We could show this effects not only in normal cell culture, but also in three dimensional invitro metastasis models. Finally we could also establish an association between SR-BI expression in human melanoma patients and STAT5 protein amounts. Our study provides evidence for an SR-BI governed gene expression signature, which enables tumor cell metastasis. Therefore we conclude that SR-BI is a novel biomarker associated with metastatic cancer spread in melanoma. Interestingly we could additionally show that in prostate cancer SR-BI expression is also correlated with a worsened prognosis. In summary, for the first time we could show that SR-BI has important intracellular function and regulates protein glycosylation at the endoplasmatic reticulum and Golgi interface. We established SR- BI as a novel target for future melanoma therapy, which will hopefully increase the chances to cure this disease.