p38γ drives cell motility in aggressive breast cancers by modulating cytoskeletal architecture

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

Simposio; Annual CMB Fall Symposium; 2010

Understanding the molecular alterations that confer metastatic properties to otherwise benign cells isessential to controlling breast cancer and significantly improving patient survival. p38γ is a member ofthe 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 p38γthrough the perspective of its developmental role led us to hypothesize that p38γ is involved in enablingmesenchymal-like behavior in breast cancer cells by controlling their motility properties. To elucidatethe role of p38γ in breast cancer progression we used RNAi to knock down (KD) p38γ in three aggressivebreast cancer cell lines. We then subjected the KD cells to a panel of in vitro assays targeting thehallmarks of cancer progression. We investigated in detail the relationship between cytoskeletal defectsand motility in p38γ KD cells using an innovative fusion of mathematical modeling and experimental cellbiology. To evaluate the in vivo relevance of our in vitro observations, stable p38γ KD cells wereorthotopically xenografted into athymic nude mice. Finally, we analyzed p38γ and RhoC expression inover 300 clinical breast cancer samples. We found that p38γ phosphorylation is elevated in all threebreast cancer cell lines compared to two control cell lines. p38γ KD also significantly impairs cell motilityand invasion while increasing proliferation, and also drastically alters actin cytoskeletal architecture. Bydeveloping an in silico mechanical model of cell motility we show that the modified cytoskeletalarchitecture of p38γ KD cells is a driving force behind the observed altered motility. Our analysis alsouncovered a novel leading edge behavior used by both p38γ KD and control cells to accommodatevarying cytoskeletal architectures. Biochemically, p38γ concurrently alters RhoC expression, andrescuing RhoC expression restores aggressiveness to p38γ KD cells. When xenografted, p38γ KD cells aresignificantly less metastatic than scrambled control cells despite forming larger primary tumors. p38γand RhoC expression correlate across clinical breast cancer specimens and may be predictive of diseaseseverity. We demonstrate for the first time that p38γ is a metastatic oncogene that specifically acts as aswitch between primary tumor proliferation and metastatic cell motility and thus presumably betweenductal carcinoma in situ and invasive breast cancer. p38γ elicits its effects, at least in part, throughcytoskeletal 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 beconcurrently altered in patient tissue.