MEKK2 mediates an alternative β-catenin pathway that promotes bone formation

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.T ight control of the activity of the β-catenin transcription factor has emerged as a key determinant of osteoblast differentiation and activity. Removal of β-catenin in mature osteoblasts leads to osteopenia associated with increased osteoclast activity (1, 2). Similarly, deletion of β-catenin in the mesenchymal progenitors of osteoblasts leads to a substantial reduction in the mineralization of both intramembranous and endochondral bones (3, 4). At a cellular level, deletion of β-catenin activity reduces osteoblast mineralization capacity in vitro. Thus, maintaining β-catenin activity is critical for the development and maintenance of bone mass. However, increased β-catenin activity also has deleterious effects. Activating β-catenin in osteoblasts by conditional deletion of the third exon containing the inhibitory glycogen synthase kinase 3β (GSK3β) phosphorylation sites leads to both osteoblast-intrinsic osteopetrosis and the development of spontaneous acute myeloid leukemia resulting from altered hematopoietic niche function (1, 5). Taken together, these findings emphasize that β-catenin activity in osteoblasts must be tailored carefully to maintain homeostasis of skeletal and hematopoietic systems. However, how this tailoring is accomplished in osteoblasts has yet to be fully elucidated.β-Catenin activity is regulated primarily by stimulation with a subset of the WNT family of ligands, the so-called "canonical" WNTs. Activation of these ligands leads to the suppression of a constitutively active destruction complex containing the adaptor proteins anaphase-promoting complex (APC) and Axin alongside the kinases GSK3β and casein kinase 1α (CK1α) (6). CK1α phosphorylates β-catenin S45, priming subsequent GSK3β phosphorylation of S33, S37, and T41. GSK3β...

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Targeting skeletal endothelium to ameliorate bone loss

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