Identification of a novel squalene epoxidase function in breast cancer brain metastasis.

Abstract

e13101 Background: Brain metastases are serious complications of breast cancer and there is no effective treatment. The major therapeutic issues can be attributed to the unique brain microenvironment as well as the difficulty of drug delivery to the brain due to the blood-brain barrier (BBB). BBB limits access of nutrients from the circulation and thereby makes the brain in a special condition that is hypoxic and composed of depleted metabolites, growth factors, and proteins. Nevertheless, tumor cells that are able to metastasize to the brain can necessitate metabolic adaptations for their nutrient availability and thereby succeed to colonize in the brain. However, it is still poorly understood how breast cancer cells alter their metabolic systems to manipulate brain metastasis by overcoming nutritional limitations. Methods: To probe the molecular mechanism of breast cancer to brain metastasis, we compared gene signatures in primary breast tumors and breast-to-brain metastatic tumors, and identified that SQLE (encodes squalene epoxidase, the second rate-limiting enzyme in the cholesterol biosynthesis) could be a leading-edge gene in breast-to-brain metastasis. Using the brain metastatic MDA-MB-231-BR (231-BR) and its parental MDA-MB-231 (231) cell line expressing SQLE shRNAs, we evaluated the effects of SQLE loss on cancer cell migration, invasion, and stemness by wound-healing, transwell invasion/migration, and tumorsphere formation assays. The in vitro cell-based BBB model and in vivo mouse models that develop 231-BR brain metastases were established to verify our hypothesis. Results: While loss of SQLE greatly attenuated cell invasiveness and stemness in both 231 and 231-BR cells, loss of SQLE could only affect the cell migration activity on 231 cells but not 231-BR cells. Since the ability to invade, migrate, and penetrate is critical for invasion of cancer cells, our results strongly imply the novel function of SQLE in breast cancer cell invasion, penetration, and colonization in the brain. Our RNA-seq data further identified a subset of SQLE-affected genes that is uniquely enriched in 231-BR cells and favors brain extravasation and colonization. The in vitro cell-based BBB model and in vivo mouse models that develop 231-BR brain extravasation and colonization finally verified our hypothesis that SQLE may play a unique function to promote breast cancer metastasis into the brain. Conclusions: Our findings provide new insights into contributions by SQLE in breast-to-brain extravasation and colonization and indicate that targeting SQLE may represent a therapeutic opportunity for breast cancer brain metastases.