Regulatory Factor X1-induced Down-regulation of Transforming Growth Factor β2 Transcription in Human Neuroblastoma Cells

Regulatory factor X (RFX) proteins are transcription factors. Seven mammalian RFX proteins have been identified. RFX1 is the prototype RFX. However, its biological functions are not known. Here, RFX1 overexpression reduced fetal bovine serum-stimulated proliferation of SH-SY5Y cells, a human neuroblastoma cell line. This inhibition is associated with decreased transforming growth factor beta 2 (TGF beta 2) and phospho-extracellular signal-regulated kinase (ERK). Exogenous TGF beta 2 increased cell proliferation and phospho-ERK in cells overexpressing RFX1. An anti-TGF beta 2 antibody and PD98059, an ERK activation inhibitor, inhibited SH-SY5Y cell proliferation. TGF beta 2 promoter activity was decreased in cells overexpressing RFX1. Chromosome immunoprecipitation assay showed that RFX1 bound the TGF beta 2 promoter. RFX1 down-regulation increased TGF beta 2 in SH-SY5Y and HCN-1A cells, a normal human neuronal cell line. More importantly, TGF beta 2 concentrations were negatively correlated with RFX1 levels in human medulloblastoma tissues with a R-2 of 0.464. These results suggest that RFX1 reduces cell proliferation through inhibiting the TGF beta 2-ERK signaling pathway. RFX1 blocks TGF beta 2 expression through its direct action on TGF beta 2 transcription. This effect also appears in human brain tumor tissues. Because TGF beta is known to be involved in cancer development, our results provide initial evidence to suggest that RFX1 may play an important role in human tumor biology.