Forkhead transcription factors in the regulation of VEGF in breast cancer

Abstract

High levels of the major angiogenic factor VEGF, have been reported in a number of cancer cell lines and in clinical specimens derived from breast. The forkhead transcription factors important for the regulation of many different physiological processes have been implicated in VEGF regulation in breast cancer. In this study, we have shown the interplay between FOXO3a and FOXM1 in breast cancer, with FOXO3a acting as a direct transcriptional repressor of VEGF. The mode of action of FOXO3a on the promoter of VEGF is dictated by events such as the competition with the VEGF transcriptional activator FOXM1, and the subsequent recruitment of a FOXO3a/HDAC2 complex on the exact binding site. This action results in the repression of VEGF transcription and the decrease of VEGF expression and cell migration. Mutating the putative forkhead responsive element affects promoter activity, and silencing FOXO3a results in up-regulation of VEGF expression. Apart overexpression of FOXO3a also results in the repression of FOXM1 expression, by its direct binding to the FOXM1 promoter. This event is also involved, indirectly, in the regulation of VEGF repression. Apart from FOXO3a and FOXM1, two other forkhead transcription factors that are implicated in breast cancer, FOXA1 and FOXC2, are also involved in the regulation of VEGF. FOXA1, a good prognosis factor in breast cancer, seems to inhibit the expression of FOXC2, a poor prognosis factor. FOXA1 is directly recruited on its binding site of the FOXC2 promoter, affecting its transcription and conferring a significantly low expression. Silencing FOXA1 results in high FOXC2 protein levels. The mode of action of these two factors between them affects the expression of VEGF. These findings provide information on the cross-talk between different forkhead transcription factors and a crucial factor of tumour migration, invasion, angiogenesis and metastasis