Small inhibitory RNA duplexes for Sp1 mRNA block basal and estrogen-induced gene expression and cell cycle progression in MCF-7 breast cancer cells

Small interfering RNA duplexes containing 21-22 nucleotides that mediate sequence-specific mRNA degradation and inhibitory RNA (iRNA) for Sp1 mRNA were used in this study to investigate the role of Sp1 on basal and hormone-induced growth and transactivation in MCF-7 and ZR-75 human breast cancer cells. Transfection of Sp1 iRNA in MCF-7 or ZR-75 cells for 36-44 h decreased Sp1 protein (50-70%) in nuclear extracts, and immunohistochemical analysis showed that the Sp1 protein in transfected MCF-7 cells was barely detectable. In cell cycle progression studies in MCF-7 cells, decreased Spl protein was accompanied by a decrease in cells in the S phase and an increase in cells in G(0)/G(1), and estrogen-induced G(0)/G(1) --> S phase progression was inhibited in cells treated with iRNA for Spl. Spl iRNA also specifically blocked basal and estrogen-induced transactivation in cells transfected with a GC-rich construct linked to a luciferase reporter gene (pSp1(3)), and this was accompanied by decreased Spl binding to this GC-rich promoter as determined in gel mobility shift and chromatin immunoprecipitation assays. These results clearly demonstrate the key role of the Spl protein in basal and estrogen-induced growth and gene expression in breast cancer cells.