Background: Breast cancer is one of the leading causes of cancer deaths in women worldwide. Significant advances have been made in the diagnosis and treatment of breast cancer, treatment of triple-negative and metastatic breast cancer poses significant challenge. Metastasis is a multi-step cascade that involves activation of proteases such as plasmin to facilitate the invasive escape of tumor cells to distant organs. The rate-limiting step in plasmin generation requires the interaction of plasminogen with cell surface plasminogen binding sites. Our laboratory first demonstrated that the plasminogen receptor, S100A10 (p11) was upregulated in many cancer cells and was responsible for much of their plasmin generation. Recently, it was reported that p11 is one of a few genes that are activated when human breast cancer cells metastasize from the primary tumor into the blood and is upregulated during the conversion of breast cancer cells to invasive phenotype. In the current study we have investigated the role of p11 in breast cancer tumor progression.
Methods: We have used MMTV-PyMT a mouse transgenic mammary tumor model to investigate the effects of loss of p11 on spontaneous tumor initiation, growth and progression to invasive carcinoma and metastasis. We used experimental metastasis assays to ascertain the role of stromal p11 in tumor cell extravasation and lung colonization. Genes and cytokines regulated by p11 in the PyMT tumors were assessed by microarray analysis and RT-qPCR. Finally, we employed gene profiling analysis and immunohistochemical staining of breast cancer patient tumors to correlate p11 expression to human breast cancer progression.
Results: Genetic deletion of p11 resulted in significantly decreased tumor onset, growth rate, and spontaneous pulmonary metastatic burden in the PyMT/p11-KO mice. This phenotype was accompanied by substantial reduction in Ki67 positivity, macrophage infiltration, decreased vascular density in the primary tumors and decrease in invasive carcinoma and pulmonary metastasis. Surprisingly, immunohistochemical analysis of wild-type MMTV-PyMT mice failed to detect p11 expression in the tumors or metastatic tumor cells and loss of p11 did not decrease plasmin generation in the PyMT tumors and cells. Furthermore, tumor cells expressing p11 displayed dramatically reduced lung metastasis when injected into p11-depleted mice, further strengthening the stromal role of p11 in tumor growth and metastasis. Transcriptome analysis of the PyMT tumors from p11-KO mice showed marked reduction in genes involved in breast cancer development, progression, and inflammation such as AREG, MUC1 and S100A8. The PyMT/p11- KO tumors displayed a remarkable increase in inflammatory cytokines such as IL-6, IL-10 and IFN-γ. Gene expression profiling from 176 primary breast cancer samples obtained through the CBCF tumor bank showed that p11 mRNA levels were significantly higher in tumors compared to normal tissues. P11 mRNA expression was significantly associated with poor patient prognosis (hazard ratio – 3.34) and significantly elevated in high grade, triple negative (TN) tumors, and tumors with high proliferative index. Evaluation of p11 protein expression in a NSHA cohort of patients revealed substantial upregulation of p11 in cancer tissues compared to normal controls.
Conclusions: This is the first study demonstrating the crucial role of p11 in breast tumor development and metastasis. The results emphasize the potential of p11 as a diagnostic and prognostic biomarker in breast cancer.