Abstract 1485: Identification of p38γ as a metastatic oncogene: Mechanism of action and clinical implications

Abstract

Abstract Understanding the molecular alterations that confer metastatic properties to otherwise benign cells is essential to controlling breast cancer and significantly improving patient survival. p38γ is a member of the p38 MAPK family and is normally expressed predominantly in muscle tissue. Developmentally, the primary role of p38γ is to promote myoblast differentiation into myotubes. Viewing the function of p38γ through the perspective of its developmental role led us to hypothesize that p38γ is involved in enabling mesenchymal-like behavior in breast cancer cells by controlling their motility properties. To elucidate the role of p38γ in breast cancer progression we used RNAi to knock down (KD) p38γ in three aggressive breast cancer cell lines. We then subjected the KD cells to a panel of in vitro assays targeting the hallmarks of cancer progression. We investigated in detail the relationship between cytoskeletal defects and motility in p38γ KD cells using an innovative fusion of mathematical modeling and experimental cell biology. To evaluate the in vivo relevance of our in vitro observations, stable p38γ KD cells were orthotopically xenografted into athymic nude mice. Finally, we analyzed p38γ and RhoC expression in over 300 clinical breast cancer samples. We found that p38γ phosphorylation is elevated in all three breast cancer cell lines compared to two control cell lines. p38γ KD also significantly impairs cell motility and invasion while increasing proliferation, and also drastically alters actin cytoskeletal architecture. By developing an in silico mechanical model of cell motility we show that the modified cytoskeletal architecture of p38γ KD cells is a driving force behind the observed altered motility. Our analysis also uncovered a novel leading edge behavior used by both p38γ KD and control cells to accommodate varying cytoskeletal architectures. Biochemically, p38γ concurrently alters RhoC expression, and rescuing RhoC expression restores aggressiveness to p38γ KD cells. When xenografted, p38γ KD cells are significantly less metastatic than scrambled control cells despite forming larger primary tumors. p38γ and RhoC expression correlate across clinical breast cancer specimens and may be predictive of disease severity. We demonstrate for the first time that p38γ is a metastatic oncogene that specifically acts as a switch between primary tumor proliferation and metastatic cell motility and thus presumably between ductal carcinoma in situ and invasive breast cancer. p38γ elicits its effects, at least in part, through cytoskeletal remodeling and by modulating expression of another known metastatic oncogene, RhoC. These findings have important therapeutic implications as p38γ and RhoC expression were found to be concurrently altered in patient tissue. 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 1485. doi:10.1158/1538-7445.AM2011-1485