Reprogramming of endothelial gene expression by tamoxifen inhibits angiogenesis and ER-negative tumor growth

Rationale: 17 beta-estradiol (E2) can directly promote the growth of ER alpha-negative cancer cells through activation of endothelial ER alpha in the tumor microenvironment, thereby increasing a normalized tumor angiogenesis. ER alpha acts as a transcription factor through its nuclear transcriptional AF-1 and AF-2 transactivation functions, but membrane ER alpha plays also an important role in endothelium. The present study aims to decipher the respective roles of these two pathways in ER alpha-negative tumor growth. Moreover, we delineate the actions of tamoxifen, a Selective Estrogen Receptor Modulator (SERM) in ER alpha-negative tumors growth and angiogenesis, since we recently demonstrated that tamoxifen impacts vasculature functions through complex modulation of ER alpha activity.Methods: ER alpha-negative B16K1 cancer cells were grafted into immunocompetent mice mutated for ER alpha-subfunctions and tumor growths were analyzed in these different models in response to E2 and/or tamoxifen treatment. Furthermore, RNA sequencings were analyzed in endothelial cells in response to these different treatments and validated by RT-qPCR and western blot.Results: We demonstrate that both nuclear and membrane ER alpha actions are required for the pro-tumoral effects of E2, while tamoxifen totally abrogates the E2-induced in vivo tumor growth, through inhibition of angiogenesis but promotion of vessel normalization. RNA sequencing indicates that tamoxifen inhibits the E2-induced genes, but also initiates a specific transcriptional program that especially regulates angiogenic genes and differentially regulates glycolysis, oxidative phosphorylation and inflammatory responses in endothelial cells.Conclusion: These findings provide evidence that tamoxifen specifically inhibits angiogenesis through a reprogramming of endothelial gene expression via regulation of some transcription factors, that could open new promising strategies to manage cancer therapies affecting the tumor microenvironment of ER alpha-negative tumors.