Abstract 5193: p38 gamma is a novel driver of breast cancer metastasis by modulating cellular motility

Breast cancer is the second leading cause of cancer-related deaths in women in the US each year. The vast majority of these fatalities are not caused by primary tumor burden but rather by metastases to vital organs. The clinical shift from localized to metastatic breast cancer entails a requirement that cancer cells activate an invasive program and be able to adapt to changing extracellular stimuli. The p38 mitogen activated protein kinase (MAPK) pathway represents a potential signaling switch for the transition from primary to metastatic cancer. p38 is a member of the MAPK family of stress and mitogen-responsive protein kinases and consists of four closely related isoforms: alpha, beta, gamma, and delta. p38 serves as a major signaling hub in the cell, integrating signals from a variety of signaling pathways and channeling these stimuli into cellular responses through an array of effector proteins. The four isoforms show unique expression patterns in normal tissue: alpha, beta, and delta are the most ubiquitously expressed, while gamma is restricted primarily to skeletal muscle. Surprisingly, we found high levels of phosphorylated p38 gamma in a myoepithelial-derived aggressive breast cancer cell line, MDA-MB-231, compared to the non-tumorigenic mammary epithelial cell line MCF-10A, despite both cell lines having similar levels of total p38 gamma. This led us to hypothesize that, based on its normal function in skeletal muscle, p38 gamma may play a role in mediating metastatic behavior in basally-derived breast cancers. To test this hypothesis, we used isoform-specific shRNA to knock down p38 gamma expression in MDA-MB-231. Inhibition of p38 gamma (referred to as shGamma) resulted in decreased cell motility and invasion, two hallmarks of cancer metastasis. Analysis of individual cells revealed a change in the mode of motility from mesenchymal-like in control cells to lamellipodia-driven motility in the shGamma cells. This altered motility appears to be the result of a dramatically modified actin cytoskeleton, characterized by loss of lengthwise stress fibers and bundled, rather than branched, actin in the lamellipodia. When we investigated gene expression changes responsible for this phenotype we discovered that expression of the metastatic oncogene RhoC GTPase is significantly reduced at the protein, but not mRNA, level in shGamma cells due to decreased RhoC protein stability. These findings have great potential clinical significance, as early evidence indicates that p38 gamma is overexpressed in cases of invasive breast cancer, and appears to correlate with RhoC expression in these samples. Our current and future work includes determining which breast cancer subtypes utilize p38 gamma for invasion and metastasis, and characterizing in more detail the dynamic actin cytoskeletal changes affected by p38 gamma. 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 5193.