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.