Abstract LB-043: MRTF|Profilin is an important signaling axis for metastatic outgrowth of triple negative breast cancer cells

Treatment of triple negative breast cancer (TNBC) is particularly difficult due to lack of molecular targets. The vast majority of breast cancer patient mortality is a result of emergence of disseminated cancer cells from a so-called dormancy-like state allowing subsequent aggressive growth at the metastatic sites. Actin assembly and important regulators of actin assembly play a key role in promoting the dormancy to proliferation switch of extravasated breast cancer cells. MRTF (myocardin-related transcription factor - an important family of transcription cofactors for SRF)/SRF transcriptional axis plays a major role in transcriptional regulation of many major molecular components of actin cytoskeletal system. We hypothesize that MRTF plays a critical role in post-extravasation survival and outgrowth of TNBC cells. To test this hypothesis, we first performed matrigel-on-top assay (3D culture experiments which successfully predicts the post-extravasation pulmonary metastatic outgrowth competency of breast cancer cells) and found that loss-of-function (LOF) of MRTF, either by knockdown or small molecule inhibitor CCG-1423 (and its analog CCG-203971), significantly retards the outgrowth of isolated TNBC cells. In this model, LOF of MRTF also dramatically suppresses the progression of established outgrowth (an in vitro mimic of micro-to-macrometastasis progression) of TNBC cells. Translating these findings in vivo, in intracardiac-injection model of experimental metastasis, administration of MRTF inhibitor reduces the metastatic burden of TNBC cells in immunocompetent mice. These experimental data are consistent with clinical findings demonstrating significantly shorter progression-free survival with MRTF upregulation in breast cancer patients. To obtain further insight into the mechanism behind MRTF’s regulation of metastatic outgrowth, we performed RNAseq from TNBC grown in 3D culture treated with CCG and identified a number of differentially expressed genes related to cell proliferation/survival and invasion. In addition to these findings, we also found that LOF of MRTF causes a dramatic reduction of intracellular content of actin-binding protein Profilin-1 (Pfn1), an important regulator of actin cytoskeletal dynamics. Using a tetracycline-inducible Pfn1 knockdown system, we further demonstrated that acute Pfn1 depletion alone is sufficient to cause a major reduction of metastatic colonization of TNBC cells in vivo. Collectively, these findings suggest that MRTF-Pfn1 is an important signaling axis for metastatic outgrowth of TNBC cells. Citation Format: David M. Gau, Souvik Chakraborty, David Boone, Alan Wells, Partha Roy. MRTF|Profilin is an important signaling axis for metastatic outgrowth of triple negative breast cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr LB-043.