Overexpression of proliferating cell nuclear antigen in mammalian cells negates growth arrest by serum starvation and cell contact.

Proliferating cell nuclear antigen (PCNA) functions as a processivity factor for DNA polymerase delta, and is expressed at high levels in growing normal and tumor cells. To clarify the relationship between cell proliferation and PCNA expression, we generated NIH-3T3 cells that overexpress PCNA and analyzed the phenotype of these cells. The resulting 3T3-PCNA cells, which overexpressed PCNA, were found to proliferate beyond the saturation density of the parental NIH-3T3 cells. Although NIH-3T3 cell proliferation is arrested under serum starvation conditions, 3T3-PCNA cell proliferation is not arrested by serum starvation. The expression levels of cdk2, cdk4 and cdk6 were the same in 3T3-PCNA and NIH-3T3 cells. The activity of cdk4 was identical for both cell types. However, the activity of cdk2 was higher in serum-starved 3T3-PCNA cells than in NIH-3T3 cells, although the expression of cyclin E decreased in both types of cells, suggesting that increases in cdk2 activity are related to negation of growth arrest in 3T3-PCNA cells. These results indicate that increases in PCNA expression lead to the disruption of growth control and may lead to malignant transformation.