Genetic And Pharmacological Proof-Of-Concept Studies Imply Profilin-1 As A Potential Target For Blocking Metastatic Colonization Of Breast Cancer Cells

Proceedings: AACR Annual Meeting 2014; April 5-9, 2014; San Diego, CA Profilin-1 (Pfn1), the founding member of Pfn family of proteins, is downregulated in human breast cancer, the exact significance of which is unclear. In xenograft models, Pfn1 downregulation promotes the disseminative potential of breast cancer cells but metastatic colonization of breast cancer cells absolutely requires Pfn1. This led us to hypothesize that loss of Pfn1 puts breast cancer and/or mammary epithelial cells (MEC) at a competitive disadvantage for growth but not for invasion. To test this hypothesis, we generated a mouse model with mammary-specific deletion of Pfn1 using MMTV-Cre. When evaluated at 6 weeks of age, mice harboring genetic deletion of Pfn1 exhibited accelerated ductal outgrowth compared to the wild-type controls. Mammary glands of Pfn1-/- mice revealed smaller acinar structure, and accordingly, primary MEC isolated from Pfn1-floxed mice exhibited defects in growth and morphogenesis in 3D-acini assay when Pfn1 was knocked out in vitro by adenovirus-mediated Cre transduction. Immunohistochemistry of mammary glands isolated from 6-week old mice showed that Pfn1+/- mice had the expected reduction in Pfn1 expression in almost entire MEC population, but only a small fraction of MEC showed evidence of loss of Pfn1 expression in Pfn1 -/- mice, implying competitive disadvantage of Pfn1 -/- cells for growth and/or survival relative to wild-type cells (likely those escaping Cre expression) during the course of development. In a tumor model, Pfn1-/- mice also developed fewer spontaneous PyMT-induced mammary tumors and displayed overall better survival compared to the wild-type controls. Consistent with these observations, outgrowth of sparsely seeded human breast cancer cells on 3D extracellular matrix (an in vitro representation of metastatic colonization) was dramatically inhibited when Pfn1 expression was silenced by RNAi. Interestingly, this growth-deficiency phenotype was also recapitulated when Pfn1 was overexpressed in breast cancer cells. These data not only suggest that an optimum range of Pfn1 expression is conducive to breast cancer cell growth, but open up opportunities for pharmacological agents capable of perturbing Pfn1 expression as potential therapeutics to block the colonization step of breast cancer metastasis. We have now identified small molecules that can either lead to near complete depletion or upregulation of Pfn1 expression, and further confirmed that Pfn1-inducing agent dramatically blocks outgrowth of breast cancer cells on 3D matrix in vitro, providing a compelling pharmacological proof-of-concept of our genetic data. These findings pave the way for future preclinical evaluation of Pfn1-perturbing compounds as a new line of anti-metastatic agents. Citation Format: Zhijie Ding, Marion Joy, Laura Vollmer, Andrew Stern, Andreas Vogt, Partha Roy. Genetic and pharmacological proof-of-concept studies imply profilin-1 as a potential target for blocking metastatic colonization of breast cancer cells. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 128. doi:10.1158/1538-7445.AM2014-128