Matthew Blake Greenblatt^a,1,2, Dong Yeon Shin^a,1, Hwanhee Oh^a, Ki-Young Lee^b,c, Bo Zhai^d, Steven P. Gygi^d, Sutada Lotinun^e,f,g, Roland Baron^e,h, Dou Liu^i,j, Bing Su^i,j,k,l, Laurie H. Glimcher^m,2, and Jae-Hyuck Shim^a,2

 

^aDepartment of Pathology and Laboratory Medicine Weill Cornell Medicine, New York, NY 10065; 

^bDepartment of Molecular Cell Biology, Sungkyunkwan University School of Medicine, Suwon 440-746, Republic of Korea; 

^cSamsung Biomedical Research Institute, Sungkyunkwan University School of Medicine, Suwon 440-746, Republic of Korea; 

^dDepartment of Cell Biology, Harvard Medical School, Boston, MA 02115; 

^eDepartment of Oral Medicine, Infection and Immunity, Harvard School of Dental Medicine, Boston, MA 02115; 

^fDepartment of Physiology Faculty of Dentistry, Chulalongkorn University, Bangkok, 10330, Thailand; 

^gSTAR on Craniofacial and Skeletal Disorders, Faculty of Dentistry, Chulalongkorn University, Bangkok, 10330, Thailand; 

^hEndocrine Unit, Department of Medicine, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02215; 

ⁱDepartment of Immunobiology Yale Medical School, New Haven, CT 06520; 

^jVascular Biology and Therapeutics Program, Yale Medical School, New Haven, CT 06520; 

^kShanghai Institute of Immunology, Shanghai JiaoTong University School of Medicine, Shanghai 200025, China; 

^lDepartment of Immunology and Microbiology, Shanghai JiaoTong University School of Medicine, Shanghai 200025, China; 

^mDepartment of Medicine, Weill Cornell Medicine, New York, NY 10065 

Abstract

Proper tuning of β-catenin activity in osteoblasts is required for bone homeostasis, because both increased and decreased β-catenin activity have pathologic consequences. In the classical pathway for β-catenin activation, stimulation with WNT ligands suppresses constitutive phosphorylation of β-catenin by glycogen synthase kinase 3β, preventing β-catenin ubiquitination and proteasomal degradation. Here, we have found that mitogen-activated protein kinase kinase kinase 2 (MAP3K2 or MEKK2) mediates an alternative pathway for β-catenin activation in osteoblasts that is distinct from the canonical WNT pathway. FGF2 activates MEKK2 to phosphorylate β-catenin at serine 675, promoting recruitment of the deubiquitinating enzyme, ubiquitin-specific peptidase 15 (USP15). USP15 in turn prevents the basal turnover of β-catenin by inhibiting its ubiquitin-dependent proteasomal degradation, thereby enhancing WNT signaling. Analysis of MEKK2-deficient mice and genetic interaction studies between Mekk2- and β-catenin-null alleles confirm that this pathway is an important physiologic regulator of bone mass in vivo. Thus, an FGF2/MEKK2 pathway mediates an alternative nonclassical pathway for β-catenin activation, and this pathway is a key regulator of bone formation by osteoblasts. 

osteoblasts, beta-catenin, bone, MEKK2, MAPK

¹M.B.G. and D.Y.S. contributed equally to this work. 

²To whom correspondence may be addressed. 

Author contributions: M.B.G., D.Y.S., L.H.G., and J.-H.S. designed research; M.B.G., D.Y.S., H.O., K.-Y.L., B.Z., S.L., and J.-H.S. performed research; S.P.G., R.B., D.L., and B.S. contributed new reagents/analytic tools; M.B.G., D.Y.S., K.-Y.L., B.Z., S.P.G., S.L., R.B., and J.-H.S. analyzed data; and M.B.G., D.Y.S., and J.-H.S. wrote the paper.