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

DEVIN ROSENTHAL; HARISH IYER; SILVIA ESCUDERO; LIWEI BAO; ZHIFEN WU; ALEJANDRA C VENTURA; ELLEN M ARRUDA; KRISHNA GARIKIPATI; SOFIA D MERAJVER

Conferencia; 2011 AACR Annual Meeting; 2011

Understanding the molecular alterations that confer metastatic properties to otherwise benign cells is essential to controlling breastcancer and significantly improving patient survival.p38γ is a member of the p38 MAPK family and is normally expressed predominantly in muscle tissue. Developmentally, theprimary role of p38γ is to promote myoblast differentiation into myotubes. Viewing the function of p38γ through the perspective ofits developmental role led us to hypothesize that p38γ is involved in enabling mesenchymal-like behavior in breast cancer cells bycontrolling their motility properties.To elucidate the role of p38γ in breast cancer progression we used RNAi to knock down (KD) p38γ in three aggressive breastcancer cell lines. We then subjected the KD cells to a panel of in vitro assays targeting the hallmarks of cancer progression. Weinvestigated in detail the relationship between cytoskeletal defects and motility in p38γ KD cells using an innovative fusion ofmathematical modeling and experimental cell biology. To evaluate the in vivo relevance of our in vitro observations, stable p38γKD cells were orthotopically xenografted into athymic nude mice. Finally, we analyzed p38γ and RhoC expression in over 300clinical breast cancer samples.We found that p38γ phosphorylation is elevated in all three breast cancer cell lines compared to two control cell lines. p38γ KDalso significantly impairs cell motility and invasion while increasing proliferation, and also drastically alters actin cytoskeletalarchitecture. By developing an in silico mechanical model of cell motility we show that the modified cytoskeletal architecture ofp38γ KD cells is a driving force behind the observed altered motility. Our analysis also uncovered a novel leading edge behaviorused by both p38γ KD and control cells to accommodate varying cytoskeletal architectures. Biochemically, p38γ concurrentlyalters RhoC expression, and rescuing RhoC expression restores aggressiveness to p38γ KD cells. When xenografted, p38γ KD cellsare significantly less metastatic than scrambled control cells despite forming larger primary tumors. p38γ and RhoC expressioncorrelate 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 tumorproliferation and metastatic cell motility and thus presumably between ductal carcinoma in situ and invasive breast cancer. p38γelicits its effects, at least in part, through cytoskeletal remodeling and by modulating expression of another known metastaticoncogene, RhoC. These findings have important therapeutic implications as p38γ and RhoC expression were found to beconcurrently altered in patient tissue.