Modulation of androgen receptor action by the cochaperone BAG-1L

Abstract

Androgen receptor (AR) is a cytoplasmic protein that becomes translocated into the nucleus after hormone binding and induces or represses the expression of its target genes. Several factors have been identified as interacting partners and modulators of the action of the AR. One of them is BAG-1L, a member of a family of co-chaperones, nucleotide exchange factor for the molecular chaperone Hsp70. BAG-1L enhances hormone-dependent transactivation by AR in prostate cancer cells. In the present work the molecular mechanism of the BAG-1L-mediated regulation of the AR has been studied. Deletion studies revealed that the τ5 region in the N-terminus of the AR is the target for BAG-1L action, though the C-terminus of the AR also contribute to the BAG-1L-mediated enhancement of the transactivation function of the AR. On the part of the BAG-1L, experiments with deletion mutants showed that both the last 47 C-terminal and the first 70 N-terminal amino acids are important for the BAG-1L-mediated increase in transactivation by the AR. This N-terminal region of BAG-1L contains a nuclear localization signal, but it was determined that in addition to nuclear targeting, this region of BAG-1L is also actively involved in its function. In vitro binding studies demonstrated that the N-terminal sequences of BAG-1L interact with the C-terminus of the AR, and the C-terminus of BAG-1L with the N-terminus of the AR to a region between amino acids 360 and 554 (also termed τ5 domain). Thus these results identify BAG-1L as a bridging molecule for the N- and C-termini interaction of the AR, which is known to be important in the function of the receptor. As the C-terminus of BAG-1L also binds to the molecular chaperone Hsp70, involvement of this region of BAG-1L in the enhancement of the AR function was also analyzed. Overexpression of the Hsp70 increased the enhancing effect of BAG-1L on transactivation by the AR. Furthermore, chromatin immunoprecipitaion experiment showed the hormone dependent recruitment of Hsp70, BAG-1L and AR to the prostate specific antigene promoter in the AR positive 22Rv1 prostate carcinoma cells. These studies therefore provide a model for the regulation of the transactivation function of the AR by a cochaperone, where the cochaperone BAG-1L and the molecular chaperone Hsp70 form a ternary complex with the AR at an androgen-regulated promoter.