Stable overexpression of PML alters regulation of cell cycle progression in HeLa cells

Our previous studies demonstrated that PML is a growth suppressor thatsuppresses oncogenic transformation of NIH/3T3 cells and rat embryofibroblasts. PML is a nuclear matrix-associated phosphoprotein whoseexpression is regulated during the cell cycle. Disruption of PML functionby t(15;17) in acute promyelocytic leukemia (APL) plays a critical role inleukemogenesis. To further study the role of PML in the control of cellgrowth, we have stably overexpressed PML protein in the HeLa cell line.This overexpression of PML significantly reduced the growth rate of HeLacells and suppressed anchorage-independent growth in soft agar. Weconsequently investigated several parameters correlated with cell growthand cell cycle progression. We found that, in comparison with the parentalHeLa cells, HeLa/PML stable clones showed proportionally more cells in G1phase, fewer cells in S phase and about the same number in G2/M phase. TheHeLa/PML clones showed a significantly longer doubling time as a result ofa lengthening of the G1 phase. No effect on apoptosis was found in HeLacells overexpressing PML. This observation indicates that PML suppressescell growth by increasing cell cycle duration as a result of G1 elongation.To further understand the mechanism of the effect of PML on HeLa cells,expression of cell cycle-related proteins in HeLa/PML and parental HeLacells was analyzed. We found that Rb phosphorylation was significantlyreduced in PML stable clones. Expression of cyclin E, Cdk2 and p27 proteinswas also significantly reduced. These studies indicate that PML affectscell cycle progression by mediating expression of several key proteins thatnormally control cell cycle progression. These results further extend ourcurrent understanding of PML function in human cells and its important rolein cell cycle regulation.