The Scaffolding Protein RACK1 Interacts with Androgen Receptor and Promotes Cross-talk through a Protein Kinase C Signaling Pathway

Anastasia C Rigas,Daniel M Ozanne,David E Neal,Craig N Robson

doi:10.1074/jbc.m306219200

Anastasia C Rigas, Daniel M Ozanne + Show 2 more

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https://doi.org/10.1074/jbc.m306219200

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Abstract

The androgen receptor (AR), a member of the nuclear hormone receptor superfamily, functions as a ligand-dependent transcription factor that regulates genes involved in cell proliferation and differentiation. Using a C-terminal region of the human AR in a yeast two-hybrid screen, we have identified RACK1 (receptor for activated C kinase-1) as an AR-interacting protein. In this report we found that RACK1, which was previously shown to be a protein kinase C (PKC)-anchoring protein that determines the localization of activated PKCbetaII isoform, facilitates ligand-independent AR nuclear translocation upon PKC activation by indolactam V. We also observed RACK1 to suppress ligand-dependent and -independent AR transactivation through PKC activation. In chromatin immunoprecipitation assays, we demonstrate a decrease in AR recruitment to the AR-responsive prostate-specific antigen (PSA) promoter following stimulation of PKC. Furthermore, prolonged exposure to indolactam V, a PKC activator, caused a reduction in PSA mRNA expression in prostate cancer LNCaP cells. Finally, we found PKC activation to have a repressive effect on AR and PSA protein expression in androgen-treated LNCaP cells. Our data suggest that RACK1 may function as a scaffold for the association and modification of AR by PKC enabling translocation of AR to the nucleus but rendering AR unable to activate transcription of its target genes.

Highlights

The androgen receptor (AR),1 a member of the steroid hormone nuclear receptor superfamily, is a ligand-dependent transcription factor that regulates gene expression required for proliferation and differentiation of cells within the prostate and has a role in the development and progression of prostate cancer
In this report we found that RACK1, which was previously shown to be a protein kinase C (PKC)-anchoring protein that determines the localization of activated PKC␤II isoform, facilitates ligand-independent AR nuclear translocation upon PKC activation by indolactam V
There is mounting evidence that ligand-independent activation of AR may play a role in hormone refractory prostate cancer. cAMP-dependent kinase (PKA) has been reported to mediate ligand-independent AR activation and cross-talk between AR and PKA signaling pathways resulting in androgen-independent induction of prostate-specific antigen (PSA) gene expression [3, 4]

Summary

Introduction

The androgen receptor (AR), a member of the steroid hormone nuclear receptor superfamily, is a ligand-dependent transcription factor that regulates gene expression required for proliferation and differentiation of cells within the prostate and has a role in the development and progression of prostate cancer. Pyk interacts with ARA55 protein and represses AR transactivation via phosphorylation of ARA55 [10] These alternative pathways may affect AR translocation to the nucleus or transcriptional activity via interaction with scaffolding proteins that orchestrate the precise compartmentalization of signal transduction components. It is thought scaffolding proteins cluster signaling proteins allowing a tight control of cellular pathways as well as cross-talk between different cascades. We have found RACK1 to have a role in ligand-independent AR movement and transactivation via the PKC signaling pathway suggesting a possible function for this scaffolding protein in AR cross-talk with other signaling pathways

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