Cooperation of E2F-p130 and Sp1-pRb Complexes in Repression of the Chinese Hamster dhfr Gene

Abstract

In mammalian cells reiterated binding sites for Sp1 and two overlapping and inverted E2F sites at the transcription start site regulate the dhfr promoter during the cell growth cycle. Here we have examined the contributions of the dhfr Sp1 and E2F sites in the repression of dhfr gene expression. In serum-starved cells or during serum stimulation, the Chinese hamster dhfr gene was not derepressed by trichostatin A (TSA), an inhibitor of histone deacetylases (HDAC). Immunoprecipitation experiments showed that HDAC1 and hypophosphorylated retinoblastoma protein (pRb) are associated with Sp1 in serum-starved CHOC400 cells. In transfection experiments, reporter plasmids containing the reiterated dhfr Sp1 sites were stimulated 10-fold by TSA, while a promoter containing four dhfr E2F sites and a TATA box was responsive to E2F but was completely unaffected by TSA. HDAC1 did not coprecipitate with p130-E2F DNA binding complexes, the predominant E2F binding activity in cell extracts after serum starvation, suggesting that p130 imposes a TSA-insensitive state on the dhfr promoter. In support of this notion, recruitment of GAL4-p130 to a dihydrofolate reductase-GAL4 reporter rendered the promoter insensitive to TSA, while repression by GAL4-pRb was sensitive to TSA. Upon phosphorylation of pRb and p130 after serum stimulation, the Sp1-pRb and p130-E2F interactions were lost while the Sp1-HDAC1 interaction persisted into S phase. Together these studies suggest a dynamic model for the cooperation of pRb and p130 in repression of dhfr gene expression during withdrawal from the cell cycle. We propose that, during initial phases of cell cycle withdrawal, the binding of dephosphorylated pRb to Sp1-HDAC1 complexes and complexes of E2F-1 -to -3 with DP results in transient, HDAC-dependent suppression of dhfr transcription. Upon withdrawal of cells into G 0 , recruitment of p130 to E2F-4-DP-1 complexes at the transcription start site results in a TSA-insensitive complex that cooperates with Sp1-HDAC-pRb complexes to stably repress dhfr promoter activity in quiescent cells.

p53-Independent Induction of G1 Arrest and p21WAF1/CIP1 Expression by Ascofuranone, an Isoprenoid Antibiotic, through Downregulation of c-Myc

“…2E and 3F to I, ascofuranone had no effect on c-Myc transcription in these cell lines. These results suggest that c-Myc is an upstream repressor of p21 WAF1/CIP1 expression in ascofuranone-treated cells, as has been shown in cells treated with transforming growth factor-β, phorbol ester, and genotoxins (32,33).…”

Combination effect of oncolytic adenovirotherapy and TRAIL gene therapy in syngeneic murine breast cancer models

“…Under starvation synchronized growth arrest conditions, endogenous E2F may be released and activated. 16,17,18 The result showed that starvation dramatically induced E2F activity in MDA-MB-231 but not in MDA-MB-435 (Figure 2b (Figure 4). In comparison, combinations of Delta24 with Ad/CMV-GFP or Ad/TRAIL-F/RGD with Ad/CMV-GFP did not improve the tumor suppression when compared with groups treated by Delta24 or Ad/TRAIL-F/RGD alone (P40.05).…”

Deciphering Transcriptional Regulatory Elements That Encode Specific Cell-Cycle Phasing by Comparative Genomics Analysis

“…The role of all three TFs in cell cycle regulation is well established. 4,9,37,38 In the promoters presented in Figure 1, biologically active BSs emerge as islands of conservation, easily distinguishable from the surrounding sequences. Unfortunately, in most cases the identification of TFBSs is more difficult, either because of differences between the orthologous BSs, or because the BSs lie within long stretches of highly conserved promoter regions.…”