The Mitogen-Activated Protein Kinase Kinase Kinase Kinase GCKR Positively Regulates Canonical and Noncanonical Wnt Signaling in B Lymphocytes

Abstract

Wnt ligands bind receptors of the Frizzled (Fz) family to control cell fate, proliferation, and polarity. Canonical Wnt/Fz signaling stabilizes β-catenin by inactivating GSK3β, leading to the translocation of β-catenin to the nucleus and the activation of Wnt target genes. Noncanonical Wnt/Fz signaling activates RhoA and Rac, and the latter triggers the activation of c-Jun N-terminal kinase (JNK). Here, we show that exposure of B-lymphocytes to Wnt3a-conditioned media activates JNK and raises cytosolic β-catenin levels. Both the Rac guanine nucleotide exchange factor Asef and the mitogen-activated protein kinase kinase kinase kinase germinal center kinase-related enzyme (GCKR) are required for Wnt-mediated JNK activation in B cells. In addition, we show that GCKR positively affects the β-catenin pathway in B cells. Reduction of GCKR expression inhibits Wnt3a-induced phosphorylation of GSK3β at serine 9 and decreases the accumulation of cytosolic β-catenin. Furthermore, Wnt signaling induces an interaction between GCKR and GSK3β. Our findings demonstrate that GCKR facilitates both canonical and noncanonical Wnt signaling in B lymphocytes.

We thank Mary Rust for excellent editorial assistance, Anthony Fauci and the National Institutes of Allergy and Infectious Diseases for their continued support, Tetsu Akiyama for providing us with the APC and Asef expression vectors and the APC and Asef RNA interface constructs, and Randall Moon for giving us the Super8XTOPFlash and Super8XFOPFlash luciferase reporter constructs.