Impact of endosomal cholesterol trafficking on melanoma metastasis

Patients suffering from metastatic melanoma are confronted with a very poor prognosis. Despite recent success of targeted therapies against the mutated BRAF oncogene, the mean 5-year overall survival rate for stage IV melanoma patients is alarmingly low. Here, we propose that late endosomal cholesterol transport is linked with the potential for melanoma to metastasize, since our preliminary data show that distinct cholesterol transporters are selectively over-expressed in metastatic melanoma cells. Moreover, cholesterol transporter expression negatively correlates with patient survival. Interestingly, addition of exogenous cholesterol to non-metastatic melanoma cells induces S6 as well as PI3K/AKT activity, which dependents on late endosomal cholesterol flux. Thus, our central hypothesis is that blocking late endosomal cholesterol export reduces the ability of melanoma to metastasize. We aim in this project to characterize altered cholesterol homeostasis as part of the metastatic signature in malignant melanoma. We will dissect lipid status, cholesterol transporter expression and cholesterol trafficking in melanoma skin tumors and melanoma lymph node metastasis in mouse models that faithfully reflect human melanoma progression (BRAFV600E PTEN-/- and N-RasQ61K Ink4a-/- mice). In-vitro studies with different mouse and human primary melanoma derived cell lines will assess the mechanisms of the pro-metastatic effect of cholesterol, focusing on the S6, PI3K/AKT and MAPK pathway. In addition, we will analyze markers of proliferation, invasion and metastasis in response to exogenous cholesterol. Inhibitors of intracellular cholesterol trafficking will be tested for their potential to revert these effects and to exert synergistic effects with already established drugs. Finally, selected cholesterol trafficking inhibitors alone or in combination with established drugs will be tested in melanoma xenograft mouse models to suppress tumor progression and metastasis. Overall, our findings will raise knowledge on cholesterol metabolism during melanoma metastasis. Novel therapeutic approaches targeting metabolic changes during melanoma progression might emerge from this project.

Melanoma is the most deadly kind of skin cancer and its incidence rises worldwide. Once melanoma cells from the primary tumor spread to lymph nodes, patient survival rates drop sharply. Melanoma is hard to treat and even recent advances in novel therapies that activate the patients immune system to destroy the tumors work in a limited subset of patients only. Therefore, in-depth knowledge of the processes that determine melanoma malignancy are urgently needed as a basis for new therapeutic options. Within this project, we identified two cellular processes that characterize melanoma cells that are able to metastasize: reprogramming of cholesterol metabolism and adaption to endoplasmic reticulum stress (ER stress). Like every cell in the human body, melanoma cells require the lipid cholesterol. We have found that melanoma cells make use of a molecule called SR-B1, which is a receptor for HDL, the so-called good cholesterol. Our results show that malignant melanoma cells contain abundant amounts of SR-B1. This results of an increased migratory potential of the melanoma cells and in a poor prognosis for tumor patients. SR-B1 might thus be a potential marker and therapeutic target in melanoma. Interestingly we have shown that reprogramming of cholesterol metabolism in another tumor type, colorectal cancer, is associated with the resistance to therapy. Another metabolic adaption of melanoma cells is the way that cells cope with ER stress. The ER, the central site for protein synthesis and modification, is frequently challenged in tumor cells, causing stress in this important organelle. Normal cells, which are confronted with huge levels of ER stress, undergo programmed cell death. However, we show that melanoma cells can adapt to this stress response avoiding cell death. Instead, melanoma cells can even exploit this stress to secrete growth factors that trigger their own malignancy. Importantly, these oncogenic growth factors can be reduced by a chemical compound that reduces ER stress. While our experiments show that this compound reduces melanoma malignancy in isolated tumor cells from patients, it fails to do so in animal models of melanoma. Future work on similar compounds that can reduce ER stress in animal models and in patients might be a promising approach to treat melanoma.