BAG-1L Protein Enhances Androgen Receptor Function*
Abstract
BAG-1 is a regulator of heat shock protein (Hsp) 70/Hsc70 family proteins that interacts with steroid hormone receptors. The recently identified BAG-1 long (BAG-1L) protein, an isoform of BAG-1 that arises from translation initiation at a noncanonical CUG codon, was co-immunoprecipitated with androgen receptors (AR) from LNCaP prostate cancer cells and other cell lysates, whereas the shorter originally identified BAG-1 and BAG-1M (RAP 46) proteins were not. BAG-1L, but not BAG-1 or BAG-1M (RAP46), also markedly enhanced the ability of AR to transactivate reporter gene plasmids containing an androgen response element (ARE) in PC3 prostate cancer and other cell lines. A C-terminal region deletion mutant of BAG-1L failed to co-immunoprecipitate with AR and functioned as a trans-dominant inhibitor of BAG-1L, impairing AR-induced transactivation of ARE-containing reporter plasmids. In addition, BAG-1L significantly reduced the concentrations of 5α-dihydrotestosterone (DHT) required for AR activity but did not induce ligand-independent transactivation. BAG-1L also markedly improved the ability of AR to transactivate reporter genes when cells were cultured with DHT in combination with the anti-androgen cyproterone acetate. The effects of BAG-1L on AR could not be explained by detectable alterations in the DHT-induced translocation of AR from cytosol to nucleus, nor by BAG-1L-induced increases in the amounts of AR protein. These findings implicate BAG-1L in the regulation of AR function and may have relevance to mechanisms of prostate cancer resistance to hormone-ablative and anti-androgen therapy.
Footnotes
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↵* This work was supported by NCI, National Institutes of Health, Grant CA-67329, the Swiss Science National Foundation, and Cancer Research Switzerland Grant BIL KFS 19891995.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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↵‡ To whom correspondence should be addressed: Scientific Director, The Burnham Institute, 10901 N. Torrey Pines Rd., La Jolla, CA 92037. Tel.: 619-646-3140; Fax: 619-646-3194; E-mail:jreed{at}burnham-inst.org.
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↵1 The abbreviations used are: AR, androgen receptor(s); ARE, androgen response element; DHT, 5α-dihydrotestosterone; CAT, chloramphenicol acetyltransferase; Hsp, heat shock protein; PAGE, polyacrylamide gel electrophoresis; FCS, fetal calf serum; CPA, cyproterone acetate; PBS, phosphate-buffered saline; CMV, cytomegalovirus.
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↵2 S. Takayama, S. Krajewski, M. Krajewska, S. Kitada, J. Zapata, K. Kochel, D. Knee, D. Scudiero, G. Tudor, G. J. Miller, M. Yamada, T. Miyashita, and J. Reed, submitted for publication.
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↵3 B. A. Froesch, S. Takayama, and J. C. Reed, unpublished data.
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- Received November 26, 1997.
- Revision received February 20, 1998.
- The American Society for Biochemistry and Molecular Biology, Inc.
