E2F1Promotes Immune Escaping Of Prostate Cancer Cells Through Down-regulating ICAM-1

pRB deficiency or mutation is a hallmark of most human cancers. pRB deficiency could lead to the activation of E2F1. Transcription factor E2F1is a key regulator of cell cycle, proliferation and apoptosis through modulating the expression and function of many downstream target genes. Its activity is strictly controlled by pRB/E2F pathway. E2F1is involved in regulation of several transcription factors such as NF-κB and SP1. However, the function of E2F1besides cell cycle and apoptosis is still not clear.Intercellular adhesion molecule-1(ICAM-1) is a cell surface glycoprotein belonging to the immunoglobulin superfamily. It is associated with a wide range of inflammatory and immune responses. The interaction between ICAM-1and its ligand LFA-1acts as a co-stimulatory signal in the activation of CTL and NK cells, and stimulates immune activation. The promoter of ICAM-1gene contains a number of binding sites for transcription factors including NF-κB, which is most important for ICAM-1expression. The regulation of ICAM-1gene expression is complex and cell type-specific. In prostate cancer cells, the phenomenon and molecular mechanism of E2F1regulating ICAM-1are still not clear.We first established the E2F1-knockdown stable cell line with a human prostate cancer cell line DU145. By gene microarray assay, we found that E2F1can down-regulate ICAM-1expression in DU145cells. However, the regulation of ICAM-1by E2F1is independent on the transactivation domain of E2F1. It suggested that E2F1may regulate ICAM-1expression through other transcription factors. Luciferase reporter gene assay showed that E2F1down-regulating expression of ICAM-1requires the expression and nuclear translocation of NF-KB/p65as well as the presence of two kB binding motifs on the ICAM-1promoter. Electrophoretic mobility shift assay (EMSA) and Chromatin immunoprecipitation (ChIP) indentified that E2F1can suppress the expression of ICAM-1by blocking the interaction between NF-KB/p65and ICAM-1promoter instead of binding to the ICAM-1promoter directly. Functional experiments showed that E2F1-knockdown in DU145cells increased the ability of T lymphocytes to adhere to and kill target cells. However, E2F1did not affect T lymphocytes adhering or killing ability when the expression of ICAM-1was inhibited or the binding of ICAM-1with its ligand was blocked. These results suggested that E2F1could inhibit T lymphocytes adhesion and cytotoxicity to prostate cancer cells through down-regulating ICAM-1expression.Taken together, our work explains the molecular mechanism of E2F1transcriptional regulating ICAM-1expression and reveals a new function of E2F1in modulation of immune escaping of tumor cells besides cell cycle and apoptosis.