BAG1L Enhances Trans-activation Function of the Vitamin D Receptor

doi:10.1074/jbc.M004977200

BAG1L Enhances Trans-activation Function of the Vitamin D Receptor*

From the ^‡Burnham Institute, La Jolla, California 92037 and ^§RIGEB, MAM-TÜBITAK, P. K. 21 Gebze 41 470, Kocaeli, Turkey

Abstract

The vitamin D receptor (VDR) is a member of the steroid/retinoid receptor superfamily of nuclear receptors that has potential tumor-suppressive functions. We show here that VDR interacts with and is regulated by BAG1L, a nuclear protein that binds heat shock 70-kDa (Hsp70) family molecular chaperones. Endogenous BAG1L can be co-immunoprecipitated with VDR from prostate cancer cells (ALVA31; LNCaP) in a ligand-dependent manner. BAG1L, but not shorter non-nuclear isoforms of this protein (BAG1; BAG1M/Rap46), markedly enhanced, in a ligand-dependent manner, the ability of VDR to trans-activate reporter gene plasmids containing a vitamin D response element in transient transfection assays. Mutant BAG1L lacking the C-terminal Hsc70-binding domain suppressed (in a concentration-dependent fashion) VDR-mediated trans-activation of vitamin D response element-containing reporter gene plasmids, without altering levels of VDR or endogenous BAG1L protein, suggesting that it operates as a trans-dominant inhibitor of BAG1L. Gene transfer-mediated elevations in BAG1L protein levels in a prostate cancer cell line (PC3), which is moderately responsive to VDR ligands, increased the ability of natural (1α,25(OH)₂ vitamin D₃) and synthetic (1α,25-dihydroxy-19-nor-22(E)-vitamin D₃) VDR ligands to induce expression of the VDR target gene, p21^Waf1, and suppress DNA synthesis. Thus, BAG1L is a direct regulator of VDR, which enhances its trans-activation function and improves tumor cell responses to growth-suppressive VDR ligands.

Footnotes

↵* This work was supported in part by Department of Defense Prostate Research Program Grant DAMD17-98-1-8584.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.
↵¶ Recipient of International Union Against Cancer fellowships from Yamagiwa-Yoshida Memorial and the American Cancer Society Young Investigators.
↵‖ To whom correspondence should be addressed: Burnham Inst., 10901 N. Torrey Pines Rd., La Jolla, CA 92037. Tel.: 858-646-3140; Fax: 858-646-3194; E-mail: jreed@burnham-inst.org.
Published, JBC Papers in Press, August 30, 2000, DOI 10.1074/jbc.M004977200
↵2 M. Guzey, S. Takayama, and J. C. Reed, unpublished observations.
Abbreviations:

VDR

vitamin D receptor

VDRE

vitamin D response element

RXR

retinoid X receptor

RAR

retinoic acid receptor

BSA

bovine serum albumin

PBS

phosphate-buffered saline

CMV

cytomegalovirus

tk

thymidine kinase

CAT

chloramphenicol acetyltransferase

PAGE

polyacrylamide gel electrophoresis

BrdUrd

bromodeoxyuridine

NR

nuclear receptor

AR

androgen receptor
- Received June 8, 2000.
- Revision received August 30, 2000.
The American Society for Biochemistry and Molecular Biology, Inc.