Myeloid derived suppressor cells (MDSCs) are significantly increased in hematopoietic organs of cancer patients and animals bearing tumors. The number of MDSCs is directly proportional to tumor malignancy. MDSCs infiltrate into tumors and facilitate tumor progression and metastasis. In searching for molecular mediators responsible for tumor metastasis, we used proteomics and mass spectrometry in combination with a mammary tumor model that includes two distinct tumor cell lines, 67NR and 4T1, which are derived from a single mammary tumor. While these cell lines form primary tumors with equivalent growth kinetics, they differ dramatically in their metastatic potential. MDSCs were isolated from spleens of tumor-bearing mice and compared for protein expression profiles using 2D-DIGE-MS. There was a significant reduction of a novel protein, neutrophilic granule protein (NGP), in MDSCs from 4T1- compared to 67NR- tumor-bearing mice. Further analysis showed that NGP was expressed in myeloid cells, but not in epithelial tumor cells. Addition of 4T1 tumor cell conditioned media decreased the gene expression in promyelocyte cells. Computational analysis of NGP indicates presence of a putative cystatin domain, a known cathepsin inhibitor site. Indeed, forced expression of NGP in cultured cells inhibited endogenous cathepsin B activity. Since cathepsins have been linked to tumor invasion and metastasis, we postulated that NGP has an anti-tumor role. To test this hypothesis, we generated 4T1 cells stably expressing NGP (4T1-NGP) or empty vector (4T1-Vec), and found that expression of NGP in tumor cells inhibited Matrigel invasion, but not cell proliferation. Furthermore, implantation of 4T1-NGP cells in mammary pad of syngeneic Balb/c mice resulted in a significant reduction of primary tumor growth and lung metastasis. Histological examination reveals reduced tumor angiogenesis and increased apoptosis. Collectively, this study identified a novel protein, NGP, in MDSCs acts as a negative mediator of tumor growth and metastasis, likely through inhibition of cathepsin activity. Down-regulation of NGP in the tumor microenvironment contributes to tumor growth and metastasis. NGP may act as a novel protease inhibitor and may be a therapeutic target for breast cancer.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1459.