The transcription factor Sp1 modulates RNA polymerase III gene transcription by controlling and expression in human cells.

Specificity protein 1 (Sp1) is an important transcription factor implicated in numerous cellular processes. However, whether Sp1 is involved in the regulation of RNA polymerase III (Pol III)-directed gene transcription in human cells remains unknown. Here, we first show that filamin A (FLNA) represses Sp1 expression as well as expression of TFIIB-related factor 1 (BRF1) and general transcription factor III C subunit 2 (GTF3C2) in HeLa, 293T, and SaOS2 cell lines stably expressing -silencing shRNAs. Both promoter 4 (P4) and promoter 2 (P2) contain putative Sp1-binding sites, suggesting that Sp1 affects Pol III gene transcription by regulating and expression. We demonstrate that Sp1 knockdown inhibits Pol III gene transcription, and expression, and the proliferation of 293T and HeLa cells, whereas Sp1 overexpression enhances these activities. We obtained a comparable result in a cell line in which both and were depleted. These results indicate that Sp1 is involved in the regulation of Pol III gene transcription independently of FLNA expression. Reporter gene assays showed that alteration of Sp1 expression affects P4 and P2 activation, suggesting that Sp1 modulates Pol III-mediated gene transcription by controlling and gene expression. Further analysis revealed that Sp1 interacts with and thereby promotes the occupancies of TATA box-binding protein, TFIIAα, and p300 at both P4 and P2. These findings indicate that Sp1 controls Pol III-directed transcription and shed light on how Sp1 regulates cancer cell proliferation.