PAR, a putative chromosomal passenger protein involved in cell cycle

Proc Amer Assoc Cancer Res, Volume 47, 2006 4910 Recently we reported the isolation of a novel gene called PAR, which has been shown to be involved in proliferation of normal and tumor cells. The gene is ubiquitously expressed in normal cells and is overexpressed in most of their malignant counterparts. The ectopic overexpression of PAR induced malignant phenotype in NIH3T3 cells. Here we present our data on the role of this gene in cell cycle. The PAR level of expression during the cell cycle was studied using Western blots and flow cytometry. In DU145 cells synchronized by double thymidine incorporation in G1/S phase PAR level was low in G1/S phase, increased as the cells progressed through S phase, reached the maximum in G2/M phase and returned to low levels when the cells re entered the G1 phase. The presence of 2 D box sequences at the amino and carboxy terminus of PAR molecule suggests a role for ubiquitination/ proteasome degradation at the end of the mitosis. The localization of PAR protein during mitosis was studied by immunofluorescence using and antiPAR antibody. In prophase and metaphase PAR protein is localized in the centrosome and in the spindle poles. Then, in anaphase PAR translocates to the spindle midzone. At late ana/telophase and cytokinesis PAR accumulates in the midbody. During mitosis, a fraction of PAR protein is distributed around the cell periphery. In addition, after the onset of anaphase a portion of PAR is closely associated with the cell membrane, at the cleavage furrow. This localization during cell cycle suggests that PAR might be related with chromosomal passenger proteins (CPP). This is also supported by confocal microscopy studies showing that PAR co-localizes with Aurora at centrosomes and with Survivin at the midbody. Furthermore, immunoprecipitation with anti PAR Ab indicated that PAR forms complexes with Aurora and Survivin. PAR, like the other CPP, could be involved in various aspects of the cell cycle, including centrosomes segregation, spindle formation, and cytokinesis, which could become aberrant when PAR expression changes. Transfection of DU145 cells with PAR antisense oligonucleotides resulted in defects at multiple stages of the cell cycle: increased polyploidy, cells with multipolar spindles and aberrant mitosis. Further work is pursued in our laboratory to expand these findings and to get more information about the function and mechanism of PAR action. Evaluating the amplification and expression of PAR in a range of human tumors will further define its contribution to tumor development. If these studies will confirm our previous findings on PAR over expression in tumors, this protein could be used as a new marker for malignant cell phenotype. In addition, PAR could become a potential target for the treatment of different cancers.