Characterization of the human EPLIN (Epithelial Protein Lost in Neoplasm) gene reveals distinct promoters for the two EPLIN isoforms

EPLIN is a novel LIM domain protein that co-localizes to the actin stress fibers and focal adhesion plaques. We previously have demonstrated that two isoforms, the 600 aa EPLIN-α and the 759 aa EPLIN-β, are generated from a single gene. In the majority of human breast and prostate cancer cell lines, the expression of EPLIN-α is significantly reduced, while the expression of EPLIN-β is either up-regulated or unchanged. To understand the basis of this differential regulation, we have determined the organization of the human EPLIN gene. The human EPLIN gene spans >100 kb and consists of 11 exons. The EPLIN-β mRNA requires all 11 exons, while the EPLIN-α mRNA requires Exons 4–11. The transcriptional start sites of EPLIN-α were mapped within the third intron by 5′ RACE and S1 nuclease protection. Similarly, the 5′ ends of EPLIN-β were mapped upstream of Exon 1. The DNA sequences flanking the EPLIN-α or EPLIN-β transcriptional start sites were capable of stimulating the expression of promoter reporter constructs. Interestingly, the endogenous transcription of EPLIN-α, but not EPLIN-β, could be stimulated by serum, indicating that the expression of two EPLIN isoforms can be independently regulated. A consensus serum response element was present within 100 bp upstream of the transcriptional start sites of EPLIN-α. The activity of 0.7 kb EPLIN-α promoter reporter construct could be enhanced by activated RhoA, indicating that this serum response element is functional.