Fig. 2

1. The Wnt signaling pathway is initiated by the binding of the specific Wnt ligand to the seven-pass transmembrane Wnt receptors known as Frizzled (FZD). LRP5/6 co-receptors facilitate binding of ligand-receptor which are inhibited by DKK proteins. 1.a. in the canonical Wnt signaling pathway, following binding of the receptor and ligand, Dvl protein is activated which in turn inhibits GSK-3. This results in inhibition of β-catenin protein degradation and rising of nuclear β-catenin concentration which increases osteogenic gene expression. 1.b. the noncanonical Wnt signaling consists of a Ca²⁺ dependent and a Ca²⁺ independent pathway. Activation of both these pathways as well as the canonical Wnt pathway contributes to upregulation of Runx2 gene expression [22]. 2. BMPs are members of the transforming growth factor-beta (TGF-β) superfamily. BMP signaling is initiated as BMPs bind to the heterodimeric Type I/Type II BMP transmembrane receptors. This results in phosphorylation of the receptor Smads (Smad1/5/8) and their binding with Smad4. The Smad complex is then translocated to the nucleus, increasing osteogenic gene expression. Smurf1 is a negative regulator of BMP-Smad signaling which is responsible for degradation and ubiquitination of Smad1 and Smad 5. The Smad-independent BMP pathway also contributes to osteogenesis [23]. 3.a. Estrogen signaling is a signaling pathway known to induce osteogenesis by enhancing BMP signaling. This enhancement is mediated through upregulation of BMP type II receptor and Smad1/5/8 as well as inhibition of the inhibitory Smad6/7. 3.b. Moreover, estradiol has been found to exert an inhibitory effect on miRNA 30-b. This miRNA is an inhibitor of the Runx2 gene [24]. 3.c. Estrogen signaling is also known to induce osteogenesis through crosstalk with Wnt signaling pathway. The mechanism suggested for this effect is thought to be upregulation of two key elements in estrogen signaling. Wnt ligands upregulate the estrogen receptors as well as aromatase, the important enzyme in estrogen synthesis. 4. Four and a half LIM domains protein 2 (FHL-2), is a transcriptional coregulator crucial for osteogenic differentiation, which is increased by dexamethasone. Increasing FHL-2 levels contributes to nuclear translocation of β-catenin protein and enhancement of subsequent gene transcription [25]