Protein Kinase CK2 Controls the Stability of Prostate Derived ETS Factor

ABSTRACT

Prostate Derived ETS Factor (PDEF) is an ETS transcription factor expressed in prostate epithelial cells, and diminished PDEF protein accumulation is correlated with prostate cancer progression. PDEF interacts with the tumor suppressor NKX3.1, and this interaction abolishes the ability of PDEF to activate the Prostate-Specific Antigen promoter. NKX3.1 stability is known to be regulated by Protein Kinase CK2 and the E3 ubiquitin ligase TOPORS. To determine if PDEF and NKX3.1 are coordinately regulated in prostate cancer cells, the effect of CK2 inhibition on steady-state PDEF levels was explored. Inhibition of CK2 activity with apigenin or 4,5,6,7-tetrabromo-benzimidazole (TBB) reduced steadystate levels of PDEF in LNCaP cells, and this effect was reversed by inhibiting the 26S proteasome. siRNA-mediated knockdown of CK2α′ in LNCaP cells also reduced PDEF accumulation. Mass spectrometric analysis of phosphorylated recombinant PDEF revealed that Thr144, Ser151 and Ser187 are CK2 phosphoacceptor sites in vitro. PDEF was also robustly polyubiquitinated by TOPORS in vitro. These results suggest that PDEF and NKX3.1 are coordinately regulated by CK2 phosphorylation that inhibits their proteasomal degradation in prostate cancer cells.