The c-Jun N-terminal kinases (JNKs) are MAPK family members and are activated by stress, growth factors and cytokines. They are encoded by three separate genes (jnk 1, 2, and 3), spliced alternately creating 10 isoforms. JNK signaling promotes both cell death and cell survival in a stimuli and tissue specic manner and is also implicated in tumorigenesis. Using the Polyoma Virus Middle T Antigen (PyVMT) transgenic mouse model where jnk2 was either expressed or deleted, we found that the PyVMTjnk2-/- tumors expressed higher Epidermal Growth Factor Receptor Substrate 8 (EPS8) mRNA and protein. EPS8 regulates EGFR signaling from Ras to Rac and EGFR tracking via Rab5 and RN-Tre. EPS8 is a prime candidate for connecting the EGFR signaling to actin cytoskeleton remodeling, thus mediating cell migration, a critical step in metastasis. In migration assays, PyVMTjnk2+/+ cells migrated ve fold more than the PyVMTjnk2-/- cells. Re-expression of JNK2[alpha] in the PyVMTjnk2-/- cells rescued this phenotype. Expression of shRNA EPS8 in the PyVMTjnk2-/- cell increased migration in vitro. EPS8 localization at dorsal rues and internalization of EGF-EGFR complexes coincided with JNK2 expression. Expression of shEPS8 in the PyVMTjnk2-/- cells increased EGF internalization suggesting that in absence of JNK2, EPS8 participates in Rab5-RN-Tre complex that inhibits EGFR internalization. Finally, we report that in absence of JNK2, EPS8 protein stability is greatly increased, suggesting that JNK2 is essential for endosomal sorting and degradation of EGFR associated cargo, of which EPS8 is a critical part. In contrast, silencing JNK1 (p46) in 4T1.2 mammary tumor cells, consistently enhanced cell invasion and tumor growth. Tumors derived from orthotopic injection of the 4T1.2shJNK1 expressing cells into the mammary fat pad reached target volume signicantly earlier than non-silencing vector expressing tumors. When injected intravenously, signicantly higher lung metastasis was observed in the 4T1.2shJNK1 group. The more aggressive behavior of 4T1.2shJNK1 tumors was associated with an increase in CCR5 and pAkt as detected by microarray analysis. Taken together, our data suggest that JNK1 suppresses the expression of proteins associated with tumor growth and invasive phenotype, contributing to tumor progression. / text