Role of Protein Phosphorylation in Post-translational Regulation of Protein B23 during Programmed Cell Death in the Prostate Gland

Abstract

Protein B23 is a nucleolar and nuclear matrix-associated phosphoprotein that is involved in ribosome synthesis. Its expression and phosphorylation in rat ventral prostate, an androgen target organ, are profoundly influenced by androgens. Induction of programmed cell death (apoptosis) in the prostatic epithelium by androgen deprivation in the animal induces an early decline in protein B23 in the absence of a corresponding loss of protein B23 mRNA. We have now demonstrated that prostatic nuclei retain the ability to transcribe the B23 mRNA and that a significant amount of this mRNA persists even after 7 days of androgen deprivation when >80% of the prostatic epithelial cells have undergone apoptosis. The B23 mRNA from these nuclei is also translatable in vitro. However, the majority of the B23 mRNA is associated with free and short-stretch polysomes, which may account for the castration-induced decline in synthesis of protein B23 in vivo. In addition, the mechanism of down-regulation of protein B23 in apoptotic prostatic cells appears to relate to two coordinate signals, which include loss of phosphorylation of the protein as well as the expression of a protease active toward dephosphorylated protein B23, under these conditions.Programmed cell death or apoptosis has been described in diverse biological systems mediated by a variety of signals (1-3), and the response of the prostatic glandular epithelium to androgen withdrawal is one of the frequently studied models (3, 4). Androgen withdrawal via orchiectomy in the rat induces an energy-dependent cascade of biochemical and morphological changes that lead to the death of 80% of the secretory epithelium between days 2 and 6 (4). Morphologically, the nucleolus dissolves (5), and the chromatin is condensed and fragmented to form the membrane-bound apoptotic bodies that appear in appreciable numbers after 2 days of androgen deprivation (6). Biochemically, there is an increase in intracellular calcium and enhancement of calcium/magnesium-dependent endonuclease activity that reaches its maximum 4 -5 days after androgen withdrawal (7). Apoptosis in the prostate is associated with modulation of gene expression so that the expression of some genes is repressed and that of others is enhanced. Among the latter are c-fos, c-myc, heat shock proteins (8), glutathione S-transferase (9), and TRPM-2/sulfated glycoproteins (10). On the other hand, the synthesis of ribosomes, especially their assembly into polysomes, markedly declines after androgen deprivation (11,12). Also, prostatic ribosomes from animals treated with 5␣-dihydrotestosterone support a significantly higher incorporation of radiolabeled amino acids into proteins than do ribosomes isolated from castrated animal controls (11).We have been interested in the mechanisms underlying the decline in prostatic rRNA synthesis and assembly after androgen deprivation. Protein B23, a conserved phosphoprotein that is localized to the granular and fibrillar regions of the nucleolus where rRNA synthesis and assembly take place (13, 1...

A Potential Role of Nuclear Matrix-associated Protein Kinase CK2 in Protection against Drug-induced Apoptosis in Cancer Cells

“…Some of the apoptosis-inducing chemical agents are known to induce DNA damage as an initial step, which is a distinguishing feature from receptor-mediated apoptosis (21-23). Our previous work employing receptor-mediated apoptosis in the prostate epithelial cells has shown a distinct response of NM-associated CK2 such that a rapid loss of CK2 from the NM compartment temporally precedes the appearance of apoptosis (1,9,11,14,15,24). However, no such studies have been reported on the response of the CK2 signal in relation to chemical-mediated apoptosis in cancer cells.…”

“…In the same paradigm, androgen withdrawal evoked a rapid loss of nuclear CK2 that was most apparent in the NM fraction. The treatment of castrated rats with androgen, and consequent induction of gene activity and cell growth in the prostatic epithelial cells, concurred with a rapid translocation of the CK2 from cytoplasm to the nuclear compartment such that the enhancement in the NM-associated CK2 was the most prominent (1,2,9,(11)(12)(13).…”

“…The association of CK2 in these compartments is profoundly responsive to the status of growth stimuli (1,9,(11)(12)(13). To that end, we have employed androgen action in the prostate epithelial cells that is mediated via the androgen-receptor system as an experimental model.…”

Regulation of chicken protein tyrosine phosphatase 1 and human protein tyrosine phosphatase 1B activity by casein kinase II- and p56lck-mediated phosphorylation

“…Phosphorylation of CPTP1-2 and HPTP1B was blocked with the use of known CKII inhibitors, heparin, or poly(Glu.Tyr) (4:1) (Tawfic et al, 1995;Wang et al, 1999) in order to confirm whether the enhancement of phosphatase activity was mainly dependent on the phosphorylation process. The extent of CPTP1-2 phosphorylation was found to be about ten fold lower in the heparin (30 µg/ml) treated CPTP1-2 ( Figure 3A, upper panel, lane 2) in comparison with control.…”

Altered expression of nuclear matrix proteins in etoposide induced apoptosis in HL-60 cells

“…Accordingly, nuclear lamins A/C or B have been found to decrease in apoptotic thymocytes [6], T cells [7], and carcinoma cell line [8], [9]. The nucleolar protein B23, an obscure matrix protein has been reported to be degraded early in apoptotic prostate cells [10]. The above mentioned studies were mainly based on the use of immunocytochemical andultra-structural analysis [11], [12].…”