Abstract 1454: LPA1 regulated pathways that mediate breast cancer dormancy in the liver and lung
Authors: Nakayama Joji , Marshall Jean-Claude A. , Murone Maximilien , Steeg Patricia S. 
Published: June 2012

Loss of expression of Nm23 significantly contributes to metastasis progression without effect on primary tumor growth. This is a hallmark of metastasis suppressor genes (MSGs). Translation of MSG basic research to the clinic is difficult since it is impossible to re-express a gene in every tumor cell. Previous studies identified a G-protein coupled receptor for the bioactive lipid lysophophatidic acid (LPA1) as a being inversely correlated with Nm23-H1. LPA1 inhibitor (Debio 0719) treatment significantly decreased metastasis progression in two model systems of breast cancer, the murine mammary cancer cell line 4T1 and human breast cancer cell line MDA-MB-231. Reduced metastasis in response to 0719 was thought to result from induction of dormancy, characterized by a decrease in phospho-Erk and an increase in phospho-p38 levels. No concurrent change in mammary fat pad tumor expression of either phosphorylated protein was found.

We hypothesize the in vivo site specific induction of dormancy results from the regulation of LPA1 signaling by the metastatic microenvironment. The effects of 0719 treatment on the expression of reported p38 activating cell surface receptors was determined. Quantitative RT-PCR showed an increased of 4-fold in PDGFRα, 2-fold in IGF-1Rβ and 1.8-fold in TNFR1 expression in 4T1 cells treated with 0719. This increase was validated by flow cytometry showing a 29.2%, 32.6% and 34.2% increases in cell surface expression of these receptors, respectively. To determine if 0719 increased expression of these receptors in vivo, immunoflourescent staining was conducted on liver and lung metastases from mice treated with 0719 or vehicle. Increased staining for PDGFRα, IGF-1Rβ and TNFR1 was observed in 0719 treated tissues compared to vehicle control mice.

Based on these results, we hypothesize that site specific dormancy may result from the overexpression of ligands for p38 activating receptors in metastatic site tissues. 0719 or LPA1 knockdown stimulates expression of the cognate receptor, resulting in a stress response and dormancy. To determine if these cytokines were differentially expressed in primary versus metastatic sites, we analyzed conditioned medium from primary cultures of mouse mammary fat pad, lung and liver tissues. The presence of PDGFAA and BB, IGF1 and TNFα were only found in conditioned medium isolated from the lung and liver primary cultures, not the mammary fat pad. Using neutralizing antibodies to PDGFRα, we decreased the activation of phospho-p38 by 0719 treatment in the presence of liver conditioned medium. These results indicate that a microenvironmental stimulus is necessary for the induction of dormancy in metastasis by an LPA1 inhibitor.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1454. doi:10.1158/1538-7445.AM2011-1454