E3 Ubiquitin Ligase-Mediated Regulation of Osteoblast Differentiation and Bone Formation

Abstract

The ubiquitin-proteasome system (UPS) is an essential pathway that regulates the homeostasis and function of intracellular proteins and is a crucial protein-degradation system in osteoblast differentiation and bone formation. Abnormal regulation of ubiquitination leads to osteoblast differentiation disorders, interfering with bone formation and ultimately leading to osteoporosis. E3 ubiquitin ligases (E3) promote addition of a ubiquitin moiety to substrate proteins, specifically recognizing the substrate and modulating tyrosine kinase receptors, signaling proteins, and transcription factors involved in the regulation of osteoblast proliferation, differentiation, survival, and bone formation. In this review, we summarize current progress in the understanding of the function and regulatory effects of E3 ligases on the transcription factors and signaling pathways that regulate osteoblast differentiation and bone formation. A deep understanding of E3 ligase-mediated regulation of osteoblast differentiation provides a scientific rationale for the discovery and development of novel E3-targeting therapeutic strategies for osteoporosis.

Keywords: E3 ubiquitin ligase; bone formation; osteoblast differentiation; signaling pathway; therapeutic target; transcription factor.

FIGURE 1

E3 ubiquitin ligase-mediated signaling pathways in osteoblast differentiation. Various E3 ubiquitin ligases and signaling pathways function in a coordinated manner to ensure osteoblast differentiation. The involved signaling pathways are described below. (A) Hedgehog signaling: SCF^β–TrCP directly interacts with phosphorylated Gli2 and promotes Gli2 ubiquitination and degradation, negatively regulating HH signaling. Spop, part of the Cul3 ubiquitin ligase complex, negatively regulates HH signaling by targeting Gli3R for ubiquitination and degradation. (B) Notch signaling: Itch binds the N-terminal portion of the NICD and promotes NICD ubiquitination and degradation, negatively regulating Notch signaling. NEDD4 forms a complex with Numb and PTEN, leading to the ubiquitin-mediated proteasomal degradation of PTEN and positively regulating Notch signaling. (C) Canonical Wnt signaling: RNF185 targets Dvl2, promotes Dvl2 ubiquitination and degradation, and negatively regulates WNT signaling. Tankyrase stimulates the ubiquitination and degradation of Axin and positively regulates WNT signaling. Smurf1 mediates Lys29-nonproteolytic polyubiquitination degradation of Axin, physically interacts with β-catenin, promotes β-catenin ubiquitination and degradation, and negatively regulates WNT signaling. SCF^β–TrCP specifically recognizes GSK3β-phosphorylated β-catenin, promotes β-catenin ubiquitination and degradation, and negatively regulates WNT signaling. (D) BMP signaling: Smurf1 targets Smad1, MEKK2, and JunB, leading to their ubiquitination and degradation and negatively regulating BMP signaling. Smurf2 can also target Smad1 for ubiquitination similar to, but independent of, Smurf1. WWP1 (under TNF-induced conditions) and Itch can also target JunB and promote JunB ubiquitination and degradation, negatively regulating BMP signaling. (E) FGF signaling: Cbl can mediate the ubiquitination and degradation of FGFR2, subsequently inhibiting ERK1/2 and PI3K and negatively regulating FGF signaling. This figure is based on and modeled after Figure 4 in the review by Salhotra et al. (2020).