Timeless Is a Novel Estrogen Receptor Co-activator Involved in Multiple Signaling Pathways in MCF-7 Cells.

Activation of estrogen receptor α (ERα) stimulates cell division and tumor growth by modulating the expression of ERα target genes. This activation involves the recruitment of specific proteins with activities that are still not fully understood. Timeless, the human homolog of the Drosophila gene involved in circadian rhythm, was previously shown to be a strong predictor of tamoxifen relapse, and is involved in genomic stability and cell cycle control. In this study, we investigated the interplay between Timeless and ERα, and showed that human Timeless is an ERα coactivator. Timeless binds to ERα and enhances its transcriptional activity. Overexpressing Timeless increases PARP1 expression and enhances ERα-induced gene regulation through the proximal LXXLL motif on Timeless protein and ERα PARylation. Finally, Timeless is recruited with ERα on the GREB1 and cMyc promoters. These data, the first to link Timeless to steroid hormone function, provide a mechanistic basis for its clinical association with tamoxifen resistance. Thus, our results identify Timeless as another key regulator of ERα in controlling ERα transactivation.