Abstract 4941: Profilin-1 is a key determinant for tumorigenic potential of breast cancer cells

A hallmark of oncogenic transformation is disruption of the actin cytoskeleton, which is partly attributed to altered expression and/or activity of proteins that bind to and regulate the state of polymerization of actin in cells. Profilin-1 (Pfn1) is a key regulator of actin polymerization that is downregulated in human breast cancer, and reduced Pfn1 levels were shown to facilitate dissemination-promoting activities of certain types of breast cancer cells. The objective of the present study was to investigate whether Pfn1 modulation affects tumor initiating capacity of breast cancer cells. Using a MMTV-PyMT transgenic mouse model, we demonstrated that mice engineered for conditional Cre-mediated homozygous deletion of Pfn1 (Pfn1−/−) in the mammary gland develop fewer spontaneous mammary tumors and exhibit longer survival than either wild-type or heterozygous Pfn1-knockout littermates. Haplo-insufficiency of Pfn1 (Pfn1+/−) causes the expected ∼50% downregulation of Pfn1 expression in tumors; however, mammary tumors isolated from Pfn1−/− mice exhibit residual Pfn1 expression that is at least comparable to the level found in tumors derived from Pfn1+/− mice. This scenario is consistent with a competitive disadvantage of Pfn1-null tumor cells for autonomous growth and/or survival, allowing selective outgrowth of tumor cell populations that have incomplete Cre-mediated gene excision of Pfn1, likely resulting from mosaic activation of Cre-driving MMTV-promoter in mammary gland. Acute deletion of floxed-Pfn1 alleles in normal mouse mammary epithelial cells by Cre-adenovirus transduction dramatically impairs their ability to outgrow when seeded sparsely on 3D matrigel. Likewise, human breast cancer cells exhibit retarded outgrowth on 3D matrigel upon stable silencing of Pfn1 expression even though their proliferation on 2D tissue-culture substrate was not affected. Correlated with these phenotypic changes, Pfn1 knockdown suppresses stem-cell-like behavior of breast cancer cells as marked by a significant reduction in both aldehyde dehydrogenase activity and mammosphere forming ability. Collectively, these findings suggest that a minimum level of Pfn1 is essential for tumor initiation of breast cancer cells. Interestingly, the outgrowth-deficient phenotype of Pfn1-depleted breast cancer cells on 3D-matrigel can be fully rescued by addition of collagen-I which suggests that alteration in cell-matrix adhesion and downstream signaling underlies the growth-related phenotypic changes induced by loss of Pfn1. Finally, the outgrowth-deficient phenotype of breast cancer cells can also be elicited in vitro and in vivo by Pfn1 overexpression, further suggesting that an optimum range of Pfn1 expression is conducive for tumor-initiating capacity of breast cancer cells. Based on these results, we conclude that Pfn1 expression level is a major determinant for tumorigenic potential of breast cancer cells. Citation Format: Chang Jiang, Zhijie Ding, Marion Joy, Laura Vollmer, Dave Gau, Su Hyeong Kim, Shivendra Singh, Andreas Vogt, Partha Roy. Profilin-1 is a key determinant for tumorigenic potential of breast cancer cells. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4941. doi:10.1158/1538-7445.AM2015-4941