BRAF activates and physically interacts with PAK to regulate cell motility

Abstract

Increased p21-activated kinase (PAK) signaling and expression have been identified in the invasive fronts of aggressive papillary thyroid cancers (PTCs), including those withRET/PTC, BRAFV600E, and mutantRASexpression. Functionally, thyroid cancer cell motilityin vitrois dependent on group 1 PAKs, particularly PAK1. In this study, we hypothesize that BRAF, a central kinase in PTC tumorigenesis and invasion, regulates thyroid cancer cell motility in part through PAK activation. Using three well-characterized human thyroid cancer cell lines, we demonstrated in all cell lines thatBRAFknockdown reduced PAK phosphorylation of direct downstream targets. In contrast, inhibition of MEK activity either pharmacologically or with siRNA did not reduce PAK activity, indicating MEK is dispensable for PAK activity. Inhibition of cell migration through BRAF loss is rescued by overexpression of either constitutive active MEK1 or PAK1, demonstrating that both signaling pathways are involved in BRAF-regulated cell motility. To further characterize BRAF–PAK signaling, immunofluorescence and immunoprecipitation demonstrated that both exogenously overexpressed and endogenous PAK1 and BRAF co-localize and physically interact, and that this interaction was enhanced in mitosis. Finally, we demonstrated that acute induction of BRAFV600E expressionin vivoin murine thyroid glands results in increased PAK expression and activity confirming a positive signaling relationshipin vivo. In conclusion, we have identified a signaling pathway in thyroid cancer cells which BRAF activates and physically interacts with PAK and regulates cell motility.