Abstract 1974: Novel function of PCNA in mammary tumor development and distant metastasis through tyrosine phosphorylation

Breast cancer is the most common cancer besides skin cancer and is the second leading cause of cancer-related death in women in the US. Despite intensive study, metastasis and tumor dormancy continue to be major hurdles to eradicate this disease. PCNA forms a homotrimer ring encircling the DNA double helix and acts as a sliding platform indispensable for DNA replication, damage repair, and chromatin remodeling. While a positive role of PCNA in is conceivable, how PCNA functions in tumor progression remains elusive due to the embryonic lethal phenotype in PCNA-deficient mice. We have shown that PCNA is regulated by phosphorylation at tyrosine 211 (Y211) meditated through multiple growth factor-stimulated signaling pathways. Y211 phosphorylation enhances chromatin-bound PCNA to promote cell proliferation and is frequently detected in multiple human cancers including breast cancer. To further dissect its biological functions, we have generated the PCNA211F/211F “knock-in” mice in which Y211 is replaced with a structurally similar but non-phosphorylatable amino acid phenylalanine (F). Comparing to wild-type mice, PCNA211F/211F mice have significantly reduced mammary tumor formation upon induction by the carcinogen DMBA. To understand its biological role in oncogene-induced mammary tumorigenesis, the PCNA211F/211F mice were crossed with the MMTV-PyMT mice in which mammary gland expression of the oncogene polyoma virus middle T antigen (PyMT) is driven by the mouse mammary tumor virus (MMTV) enhancer/promoter. PCNA211F/211F;MMTV-PyMT mice have statistically significant but only moderate decrease in mammary tumor development. However, their lung metastasis is dramatically ablated compared to PCNAWT/WT;MMTV-PyMT mice harboring primary tumors of similar size. These results suggest that pY211-PCNA has an essential function not only in tumorigenesis but more importantly in progression to distant metastasis. Consistently, mammary tumors derived from PCNAWT/WT;MMTV-PyMT mice exhibits more pronounced stromal activity than tumors of PCNA211F/211F;MMTV-PyMT mice. The suppression of metastasis and stromal activity in PCNA211F/211F;MMTV-PyMT mice is associated with reduced number of tumor-initiating cells (TICs) as determined by in vitro and in vivo analyses. Further study using syngeneic orthotopic model of transplanting murine cancer cells into the mammary glands of PCNAWT/WT and PCNA211F/211F mice unveils an unexpected function of pY211-PCNA in the stromal compartment on