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

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 PKC␤II 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 androgentreated 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.

Expression profile of protein kinase C isozymes in preimplantation mouse development

“…An example of the latter is AKAP 79, which binds to PKCs a and bII (Klauck et al 1996), as well as at least two other signal transducers, protein kinase A and calcineurin (a Ca 2þ /calmodulindependent phosphatase). RACK1 is a non-substrate receptor for PKC bII, although it also binds other PKC isoforms and other molecules (Jaken & Parker 2000, Rigas et al 2003. Since RACK1 is involved in the translocation and association of PKC bII with a normal substrate in cardiac myocytes (Ron & Mochly-Rosen 1995, Csukai et al 1997, it is interesting that RACK1 and PKC bII distributions differ in preimplantation embryos.…”

Regulation of the androgen receptor by post-translational modifications

“…Furthermore, Tyr534-mutant AR is not stimulated to translocate to the nucleus by Src activation (Guo et al 2006). The scaffold protein GNB2L1 (RACK1), which interacts with the AR to facilitate ligand-independent AR nuclear translocation (Rigas et al 2003), can also modulate AR tyrosine phosphorylation and its interaction with Src (Kraus et al 2006). Tyr534 phosphorylation is increased in refractory xenograft models and is present in refractory human samples of PC.…”

Androgen receptor corepressors and prostate cancer

“…As such, it is proposed that calreticulin may act as a general repressor of nuclear receptor action. In addition, Rack1 has been associated with decreased AR promoter occupancy through its activation of the protein kinase C (PKC) pathway (Rigas et al 2003). Together, these corepressors limit AR function by inhibiting AR nuclear translocation and/ or DNA binding.…”