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Activation of cAMP-Dependent Protein Kinase Increases the Protein Level of Steroidogenic Factor-1

Department of Anatomy and Cell Biology (R.A., G.M., J.L.), The Hormone Laboratory, Department of Clinical Biochemistry, (G.M.), University of Bergen, N-5009 Bergen, Norway; and Department of Developmental Biology, National Institute for Basic Biology, and Core Research for Evolutional Science and Technology, Japan Science and Technology (K.-I.M.), Okazaki, 444-8585 Japan

Address all correspondence and requests for reprints to: Reidun Aesøy, Department of Anatomy and Cell Biology, University of Bergen, Aarstadveien 19, N-5009 Bergen, Norway. E-mail: reidun.asoy{at}pki.uib.no

The orphan nuclear receptor steroidogenic factor 1 (SF-1) is an essential regulator of endocrine organogenesis, sexual differentiation, and steroidogenisis. SF-1 is a transcriptional regulator of cAMP responsive genes, but the exact mechanisms by which cAMP-dependent PKA modulates SF-1 dependent transcription leading to increased steroidogenic output have not been determined. In this report the effects of PKA activation on SF-1 in living cells have been examined by the use of full-length SF-1 cDNA fused to the cDNA encoding green fluorescent protein (GFP). The GFP-SF-1 fusion protein localized to the nucleus of both steroidogenic Y1 cells and nonsteroidogenic COS-1 cells, and the functional properties of wild-type SF-1 were conserved. When the catalytic subunit of PKA was coexpressed with GFP-SF-1, we observed that the fluorescence emission was markedly elevated. These findings were confirmed by Western blot analysis, showing that stimulation of PKA increased SF-1 protein levels. The PKA- induced expression of SF-1 protein was not accompanied by an increase in SF-1 mRNA levels. However, pulse-chase studies showed a decrease in SF-1 degradation rate in response to activation of PKA, indicating that PKA elevates the level of SF-1 by increasing the stability of SF-1 protein.