Protein B23/nucleophosmin/numatrin nuclear dynamics in relation to protein kinase CK2 and apoptotic activity in prostate cells

Protein B23/nucleophosmin/numatrin (hereafter, B23) is a conserved nucleolar (and nuclear matrix-associated) phosphoprotein that is localized to the granular and fibrillar regions of the nucleolus where rRNA synthesis and assembly take place (1, 2). Changes in rRNA synthesis are among the earliest responses to initiation or cessation of growth signals. For example, removal of the androgenic growth signal in the rat prostate, an organ strictly dependent on the availability of androgens for growth and proliferation, results in a rapid decline in rRNA synthesis. Conversely, in the same model, stimulation of prostatic growth (by androgen administration to castrated rats) results in a rapid synthesis of rRNA for which the availability of B23 is essential (3–9). Our previous data indicated that regulation of B23 expression in the prostate after androgen ablation in the animal was not at the transcriptional level since B23 mRNA was present at high level as late as 6 days following androgen ablation when extensive apoptosis in prostatic cells is apparent, suggesting that under these experimental conditions the observed changes in B23 were primarily at the post-transcriptional level (10, 11). Since B23 is phosphorylated by protein kinase CK2 (formerly casein kinase 2 or II), the concordant loss or increase of the two proteins in the nuclear fraction upon altered androgenic status in rat prostate further emphasized the involvement of the two proteins in the early expression of rRNA synthesis (10, 11). CK2 is a ubiquitous and highly conserved protein ser/thr kinase for which much evidence has emerged suggesting that it plays not only important roles in regulation of cell growth and proliferation but also in cell death by serving as a potent suppressor of apoptosis (12–16). CK2 is the key enzyme that mediates phosphorylation of B23 (10, 11, 17, 18), although a phosphorylation by p34cdc2 is also involved during mitosis (19). B23 has been implicated to have molecular chaperone activity which appears to be regulated by CK2-mediated phosphorylation (20, 21). Further, phosphorylation of B23 by CK2 regulates organization of nucleolar compartments so that CK2 plays a significant role in controlling the compartmentation of the rRNA-processing (22). These observations taken together with our previous studies (10, 11, 23) suggest a dynamic relationship between B23 and CK2 nuclear localization in response to altered growth stimuli in the cell; however, the nature of this relationship especially in response to altered cell growth and apoptotic activity in cells has not been studied. Here we have investigated the nature of CK2 mediated regulation of cellular B23 levels under conditions that are associated with modulation of CK2 and/or altered apoptotic activity by employing prostate cells as experimental model. For the biochemical experimental studies on B23 in relation to CK2, we have employed the nuclear matrix fraction which is known to include the nucleolar component (24). The choice of nuclear matrix (rather than whole nuclei) for the biochemical studies was based on the consideration that we have previously demonstrated the presence of B23 in the isolated nuclear matrix fraction (11), and importantly we have also demonstrated a profound rapid response of nuclear matrix associated CK2 to altered growth signals (25, 26). In the present work, our results demonstrate that downregulation of CK2 by various methods results in loss of nuclear associated B23 (as evidenced by its loss in the nuclear matrix) without a significant change in the cytoplasmic levels of the protein. Institution of a growth stimulus results in the translocation of B23 along with CK2 to the nucleus where both proteins demonstrate a colocalization as demonstrated by immunofluorescence staining studies of whole cells. Since rapid translocation of CK2 to the nucleus (where it shows differential association with subnuclear compartments) in response to growth stimulus has been documented (23, 25, 26), the coordinate shuttling of B23 and CK2 in and out of the nucleus and their colocalization in the nuclear compartment may represent an early event in their involvement in modulating responses to growth and apoptotic stimuli in the cell.