ROLE OF PROTEIN-PHOSPHORYLATION IN POSTTRANSLATIONAL REGULATION OF PROTEIN B23 DURING PROGRAMMED CELL-DEATH IN THE PROSTATE-GLAND

Protein B23 is a nucleolar and nuclear matrix-associated phosphoprotei n that is involved in ribosome synthesis, Its expression and phosphory lation in rat ventral prostate, an androgen target organ, are profound ly influenced by androgens, Induction of programmed cell death (apopto sis) in the prostatic epithelium by androgen deprivation in the animal induces an early decline in protein B23 in the absence of a correspon ding loss of protein B23 mRNA. We have now demonstrated that prostatic nuclei retain the ability to transcribe the B23 mRNA and that a signi ficant amount of this mRNA persists even after 7 days of androgen depr ivation when >80% of the prostatic epithelial cells have undergone apo ptosis. The B23 mRNA from these nuclei is also translatable in vitro. However, the majority of the B23 mRNA is associated with free and shor t stretch polysomes, which may account for the castration-induced decl ine in synthesis of protein B23 in vivo. In addition, the mechanism of down-regulation of protein B23 in apoptotic prostatic cells appears t o relate to two coordinate signals, which include loss of phosphorylat ion of the protein as well as the expression of a protease active towa rd dephosphorylated protein B23, under these conditions.