c-Fos oncogene regulator Elk-1 interacts with BRCA1 splice variants BRCA1a/1b and enhances BRCA1a/1b-mediated growth suppression in breast cancer cells

Elk-1, a c-Fos protooncogene regulator, which belongs to the ETS-domain fam ily of transcriptional factors, plays an important role in the induction of immediate early gene expression in response to a variety of extracellular signals. In this study, we demonstrate for the first time the in, vitro and in vivo interaction of Elk-1 with BRCA1 splice variants BRCA1a and BRCA1b using GST-pull down assays, co-imunoprecipitations/Western blot analysis of cell extracts from breast cancer cells and mammalian two-hybrid assays. We have localized the BRCA1 interaction domain of Elk-1 protein to the conser ved ETS domain, a motif involved in DNA binding and protein-protein interac tions. We also observed binding of BRCA1 proteins to other ETS-domain trans cription factors SAP1, ETS-1, ERG-2 and Fli-1 but not to Elk-1 splice varia nt Delta Elk-1 and c-Fos protooncogene, Both BRCA1a and BRCA1b splice varia nts function as growth suppressors of human breast cancer cells. interestin gly, our studies reveal that although both Elk-1 and SAP-1 are highly homol ogous members of a subfamily of ETS domain proteins called ternary complex factors, it is only Elk-1 but not SAP-I that can augment the growth suppres sive function of BRCA1a/1b proteins in breast cancer cells, Thus Elk-1 coul d be a potential downstream target of BRCA1 in its growth control pathway, Furthermore, we hare observed inhibition of c-Fos promoter activity in BRCA 1a transfected stable breast cancer cells and over expression of BRCA1a/1b attenuates MEK-induced SRE activation in vivo. These results demonstrate fo r the first time a link between the growth suppressive function of BRCA1a/1 b proteins and signal transduction pathway involving Elk-1 protein, All the se results taken together suggest that one of the mechanisms by which BRCA1 a/1b proteins function as growth/tumor suppressors is through inhibition of the expression of Elk-1 target genes like c-Fos.