Proliferating cell nuclear antigen in malignancy

Abstract

The development of a malignancy is a multistep process that is not clearly understood. Nonmalignant cells undergo a transformation process resulting in aberrantly proliferating malignant cells suggesting that the replication machinery of malignant cells is altered. In this study the DNA replication apparatus (the DNA synthesome) was examined to identify proteins altered in malignant cells. Analysis of malignant and nonmalignant cells by two dimensional gel electrophoresis (2D PAGE) demonstrated that many malignant cell types contain a unique, acidic form of proliferating cell nuclear antigen (PCNA). This protein is a 36 kD nuclear protein required for DNA replication and DNA repair. The unique form of PCNA was found in malignant breast, prostate, colon, cervical, brain and leukemia cell lines and in malignant human breast tumors and chronic myelogenous leukemia specimens. Serum collected from a breast cancer patient was analyzed and found to contain the cancer specific form of PCNA. Analysis of nonmalignant breast tissue and serum collected from cancer free individuals demonstrated that the cancer specific form of PCNA was not present. Further experiments were performed to characterize the unique form of PCNA. 2D PAGE analysis was performed on nonmalignant, transformed breast cell lines overexpressing the oncogenes c-myc (A1N4 myc) and SV40T (A1N4T). The cancer specific form of PCNA was present in these transformed cells. It was also determined that the acidic form of PCNA was not the result of growth stimulation or genetic mutation, suggesting that differential post-translational modification may be responsible. Although PCNA does undergo poly (ADP-ribosylation), 2D PAGE analysis demonstrated that the cancer specific form of PCNA was unmodified. The cancer specific form of PCNA appears to be a fundamental characteristic of malignancy and its role in tumorigenesis needs to be examined. These results suggest that epigenetic changes may contribute to the development of cancer.