A Rac1/PAK1 cascade controls β-catenin activation in colon cancer cells

Abstract

P21-activated kinase 1 (PAK1) is associated with colon cancer progression and metastasis, whereas the molecular mechanism remains elusive. Here, we show that downregulation of PAK1 in colon cancer cells reduces total b-catenin level, as well as cell proliferation. Mechanistically, PAK1 directly phosphorylates b-catenin proteins at Ser675 site and this leads to more stable and transcriptional active b-catenin. Corroborating these results, PAK1 is required for full Wnt signaling, and superactivation of b-catenin is achieved by simultaneous knockdown of adenomatous polyposis coli protein and activation of PAK1. Moreover, we show that Rac1 functions upstream of PAK1 in colon cancer cells and contributes to b-catenin phosphorylation and accumulation. We conclude that a Rac1/PAK1 cascade controls b-catenin S675 phosphorylation and full activation in colon cancer cells. Supporting this conclusion, overexpression of PAK1 is observed in 70% of colon cancer samples and is correlated with massive b-catenin accumulation.

The guanine nucleotide exchange factor Net1 facilitates the specification of dorsal cell fates in zebrafish embryos by promoting maternal β-catenin activation

“…In addition to phosphorylation by GSK-3β and CK1α at the N terminus, β-catenin can also be phosphorylated at S552, Y654, S663 and S675, resulting in enhanced protein stability or transcriptional activity [20][21][22][23][24]. However, the developmental roles of these C-terminal modifications of β-catenin in vertebrate dorsal axis formation have not yet been elucidated.…”

“…Phosphorylation of β-catenin at the C-terminal tyrosine and serine residues has been shown to increase its biological activity [21][22][23][24]. Specifically, the phosphorylation of S675 effectively modulates transcriptional activity of β-catenin in a variety of cells and tissues, but its effect on β-catenin stability is cell type-dependent [22,23,60]. We aimed to investigate whether β-catenin S675 phosphorylation is regulated by Net1 expression.…”

“…Injection of 200 pg mRNA of wild-type net1, but not its loss-of-function mutants, could restore phosphorylation level of β-catenin at S675 in net1 MO1-injected embryos ( Figure 4F), indicating the requirement for Net1 GEF activity for β-catenin S675 phosphorylation. To verify the functional relevance of S675 phosphorylation in dorsal axis formation, we overexpressed β-catenin S675D and S675A mutants, which respectively mimic phosphorylated and unphosphorylated forms of the protein and retain the same cellular localization as the wild-type [23]. In situ hybridization assays revealed that only injection of β-catenin S675D mRNA could significantly expand the expression domain of boz and chd in wild-type embryos ( Figure 4G).…”

“…Since S675 of β-catenin has been shown to be a major target of PAK1 and protein kinase A (PKA) [22,23], we first examined the potential involvement of these two kinases during dorsal axis formation in zebrafish. Embryos injected with the mRNA encoding a specific inhibitory protein for PKA (PKI) [22,62] or treated with the PKA selective chemical inhibitor H89 [63] exhibited normal expression of the dorsal markers and the phospho-β-catenin (S675; Supplementary information, Figure S11A and S11B), thus excluding a major role for PKA in zebrafish dorsalization.…”

“…It has been shown that Rac1 activates JNK2 that in turn phosphorylates β-catenin at Ser191 and Ser605 and controls β-catenin nuclear translocation in the mouse bone marrow-derived ST2 cell line [27]. Rac1 also activates p21-activated kinase 1 (PAK1) to promote β-catenin S675 phosphorylation and activation in colon cancer cells [23]. A number of GEFs for the Rho family of small GTPases also play vital role in regulating Wnt/β-catenin signal.…”