HUVEC 3xHA.FoxO3.A3.ER +4-OHT vs vector +4-OHT

Forkhead Box O (FoxO) transcription factors are conserved proteins involved in the regulation of life span and age-related diseases, such as diabetes and cancer. Stress stimuli or growth factor deprivation promote nuclear localization and activation of FoxO proteins, which - depending on the cellular context - leads to cell cycle arrest or apoptosis. Moreover, FoxOs can control oxidative stress resistance and cell metabolism. In endothelial cells (ECs), they additionally regulate angiogenesis and may promote inflammation and vessel destabilization implicating a role of FoxOs in vascular diseases. In several cancers, FoxO transcription factors exert a tumor-suppressive function, due to their critical role in regulating proliferation and survival. Others and we have previously shown that FoxOs can regulate these processes via two different mechanisms: either by direct binding to FoxO-responsive elements (FRE) at the promoter of target genes or by a poorly understood alternative process that does not require direct DNA binding and regulates key targets in primary human ECs. Here we performed an interaction study in ECs to identify new nuclear FoxO3 interaction partners, which might contribute to FoxO-dependent gene regulation. Mass spectrometry analysis of FoxO3-interacting proteins revealed Transformation/Transcription Domain-Associated Protein (TRRAP), a member of multiple histone acetyltransferase (HAT) complexes, as novel binding partner of FoxO family proteins. TRRAP is required to support FoxO3 transactivation and FoxO3-dependent apoptosis in ECs via transcriptional activation of the proapoptotic Bcl-2 family member BIM. Moreover, FoxO-TRRAP interaction might explain FoxO-induced alternative gene regulation via TRRAP-dependent recruitment to target promoters lacking FRE sequences.