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J. Biol. Chem., Vol. 279, Issue 47, 49120-49130, November 19, 2004

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cAMP-dependent Protein Kinase Regulates Ubiquitin-Proteasome-mediated Degradation and Subcellular Localization of the Nuclear Receptor Coactivator GRIP1^*

Tuyen Hoang, Ingvild S. Fenne, Carol Cook, Bente Børud¶, Marit Bakke¶, Ernst Asbjørn Lien||, and Gunnar Mellgren||**

From the Hormone Laboratory, Section of Endocrinology, Institute of Medicine, ^¶Division of Anatomy and Cell Biology, Department of Biomedicine, University of Bergen and ^||Haukeland University Hospital, Bergen N-5021, Norway

Nuclear receptors and their coactivators are key regulators of numerous physiological functions. GRIP1 (glucocorticoid receptor-interacting protein) is a member of the steroid receptor coactivator family. Here, we show that GRIP1 is regulated by cAMP-dependent protein kinase (PKA) that induces its degradation through the ubiquitin-proteasome pathway. GRIP1 was down-regulated in transiently transfected COS-1 cells after treatment with 8-para-chlorophenylthio-cAMP or forskolin and 3-isobutyl-1-methylxanthine and in adrenocortical Y1 cells after incubation with adrenocorticotropic hormone. Pulse-chase experiments with transiently transfected COS-1 cells demonstrated that the half-life of GRIP1 was markedly reduced in cells overexpressing the PKA catalytic subunit, suggesting that activation of PKA increases the turnover of GRIP1 protein. The proteasome inhibitors MG132 and lactacystin abolished the PKA-mediated degradation of GRIP1. Using ts20 cells, a temperature-sensitive cell line that contains a thermolabile ubiquitin-activating E1 enzyme, it was confirmed that PKA-mediated degradation of GRIP1 is dependent upon the ubiquitin-proteasome pathway. Coimmunoprecipitation studies of COS-1 cells transfected with expression vectors encoding GRIP1 and ubiquitin using anti-GRIP1 and anti-ubiquitin antibodies showed that the ubiquitination of GRIP1 was increased by overexpression of PKA. Finally, we show that PKA regulates the intracellular distribution pattern of green fluorescent protein-GRIP1 and stimulates recruitment of GRIP1 to subnuclear foci that are colocalized with the proteasome. Taken together, these data demonstrate that GRIP1 is ubiquitinated and degraded through activation of the PKA pathway. This may represent a novel regulatory mechanism whereby hormones down-regulate a nuclear receptor coactivator.

Received for publication, August 25, 2004 , and in revised form, September 3, 2004.

^* This work was supported by grants from the Research Council of Norway, the Norwegian Cancer Society, Helse Vest HF, and Meltzerfondet. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Both authors contributed equally to this work.

^** To whom correspondence should be addressed: Hormone Laboratory, Institute of Medicine, University of Bergen, Haukeland University Hospital, N-5021 Bergen, Norway. Tel.: 47-55-97-43-66; Fax: 47-55-97-58-14; E-mail: gunnar.mellgren{at}med.uib.no.

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