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Synergistic activation of yeast-expressed rat androgen receptor by modulators of protein kinase-A.
We have employed a yeast (Saccharomyces cerevisiae) based rat androgen receptor expression system to examine the cross-talk between different signalling pathways. We report here the synergistic modulation of androgen regulated transcriptional activation of beta-galactosidase reporter activity by the activators of protein kinase-A, like forskolin and 8-bromo-cyclic AMP. A similar ligand-dependent enhancement of reporter activity compared to a DHT treated control has been noticed with okadaic acid, which is a potent inhibitor of protein phosphatase. The activation could be blocked by protein kinase-A/C inhibitor, H7. Forskolin treatment neither altered levels of receptor mRNA nor [3H]R1881 binding to the receptor. Although it promotes binding of receptor to an androgen response element, forskolin was unable to activate subsequent interaction with the transcription machinery in the absence of androgen. Additionally, the synergistic actions of these activators were independent of the degree of androgen response element occupancy. Anti-androgens, cyproterone acetate and flutamide, which failed to exhibit antagonistic behaviour with yeast expressed receptor, were able to antagonize only the forskolin mediated augmentation of reporter activity. Finally, analyses of mutants established the role of DNA and steroid binding domains of receptor for this synergism.
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