Search

Flow cytometric bivariate analysis was used to evaluate the expression of PCNA, p120, and p145 during the G0 reentry of CHO-K1 cells into the cell cycle. CHO-K1 cells were placed in a G0-like state using serum depletion and stimulated to reenter the cell cycle by replating into fresh, serum-containing medium. At discrete intervals after stimulation, replicate samples were stained for either PCNA, p120, or 145; stained for DNA (Coulter DNA-Prep); evaluated on the EPICS Profile I; and analyzed on the EPICS ELITE workstation. PCNA stained less than 10% of the G0 cells; in contrast, however, 30-35% of the G0 cells were positive for p120 and p145. Eight hours after stimulating G0 cells to reenter the cell cycle (during G0/G1), p120 reached 88% positivity, while p145 and PCNA were 63% and 30% positive, respectively. Cells in S phase (12 and 16 h following G0 stimulation) were greater than 90% positive for all three antigens. PCNA had the greatest change throughout the G0 reentry process, both in percentage positive and quantitatively (mean channel fluorescence). This report indicates that all three proliferation-associated antigens studied are differentially expressed during the reentry of G0 cells into the cell cycle. Furthermore, these antigens may be useful in the early detection of G0 recruitment.

Flow cytometric bivariate analysis was used to investigate the expression of PCNA, p120 and p145 during the cell cycle of a mammalian cell line (CHO-K1). Initially, aliquots of cells in exponential and plateau (G0) phase were analyzed for proliferation associated antigen expression. Expression of PCNA and p145 during G0 was markedly depressed (less than 12% positive) while 54% of the G0 cells stained positive for p120. The fluorescent intensity (mean channel fluorescence) of these G0 positive p120 cells, however, was only slightly above the mean channel fluorescence (MCF) of cells stained with a negative isotype control. In asynchronous cultures, all three antigens were expressed in greater than 70% of the cells, with PCNA staining being greater than 95%. Cells were then synchronized using mitotic selection (mitotic index of 97%) and antigen levels were measured as cells progressed synchronously through the cell cycle. From DNA analysis histograms, it appeared that the degree of synchrony was approximately 90% throughout the remainder of the cell cycle. The bivariate DNA/PCNA, DNA/p120, and DNA/p145 histograms for mitotic cells indicated that both p120 and p145 expression were elevated (percent positive and MCF) while PCNA levels were near controls (MCF). In early G1, all three markers were depressed (less than 12% positive); however PCNA levels rose precipitously in mid-G1 (greater than 50% positive). In late G1 to early S, p145 levels increased concomitantly with increases in p120. All three antigens were elevated throughout S phase and began to decline as cells moved from G2/M to G1 of the next cell cycle with p145 expression decreasing first. This report indicates that all three proliferation associated antigens studied are differentially expressed in the cell cycle and therefore may be useful in detecting and assessing the proliferation state.