The Vitamin D Receptor, Runx2, and the Notch Signaling Pathway Cooperate in the Transcriptional Regulation of Osteopontin

Abstract

Osteopontin (OPN), a glycosylated phosphoprotein that binds calcium, is present in bone extracellular matrix and has been reported to modulate both mineralization and bone resorption. Osteopontin (OPN)2 is a sialic acid-rich glycosylated phosphoprotein, comprising about 2% of the noncollagenous protein in bone (1, 2). OPN is produced by osteoblasts when they form bone matrix (1, 2). OPN is an extracellular matrix protein that contains arginine-glycineaspartate (RGD) integrin binding motifs and promotes attachment of bone cells to the bone surface through binding to OPN receptors such as the ␣ v ␤ 3 integrin and CD44 (1-3). OPN has been suggested to be involved in the attachment of osteoclasts during bone resorption, to play a role in osteogenesis by attachment of osteoblasts when they form bone matrix, and to act to regulate crystal size during bone mineralization (2). In addition, OPN has been suggested to be a mediator of bone remodeling in response to mechanical strain (4). OPN null mice are resistant to mineral loss and bone resorption upon estrogen deprivation and have impaired activation of osteoclasts (3, 5-7). Also, vascularization and resorption of bone discs have been reported to be significantly impaired in the absence of OPN (8). Although recent studies using OPN null mice have provided new insight into the role of OPN in vivo in bone metabolism, the factors that affect the regulation of OPN are not yet clearly defined. 1,25-Dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), the active form of vitamin D, is a major calcitropic hormone involved in calcium homeostasis (9). One of its functions in bone is to regulate the synthesis of the bone calcium-binding proteins osteocalcin (OC) and OPN (9). 1,25(OH) 2 D 3 modulates the expression of these genes through transcriptional regulation. The actions of 1,25(OH) 2 D 3 are mediated through the vitamin D receptor (VDR). Liganded VDR heterodimerizes with the retinoid X receptor and interacts with a vitamin D response element (VDRE). The VDRE in the mouse OPN promoter (at Ϫ757/ Ϫ743) is a perfect direct repeat of the motif GGTTCA spaced by three nucleotides (10). Transcription proceeds through the interaction of VDR with coactivators and coregulators, including SRC-1/NcoA1, SRC-2/GRIP-1 (GR-interacting protein)/NcoA2, SRC-3/ACTR, and the multisubunit DRIP (vitamin D receptor-interacting protein) complex (11). Although a VDRE has been identified in the mouse OPN promoter (10) and VDR null mice show marked inhibition of OPN expression in osteoblasts (12), the exact mechanisms, including proteinprotein and protein-DNA interactions, involved in 1,25(OH) 2 D 3 -regulated OPN transcription are not well understood.Runx2/Cbfa1 is a member of the runt/Cbfa family of transcription factors that was first identified as an osteoblast-specific transcription factor and a regulator of osteoblast differentiation (13,14). Runx2 Ϫ/Ϫ mice die shortly after birth and show a complete lack of mineralized bone tissue (13,14). Marked decreases in the expression of osteopontin and osteocalcin ar...

Association between osteopontin promoter variants and diastolic dysfunction in hypertensive heart in the Japanese population

“…The G base insertion at À156 site of OPN promoter generates another Runx2-binding site, 19,22 and Runx2 bindings are crucial for regulation of OPN transcription in bone tissue. 36 As discussed above, enhanced expression of OPN in heart potentially causes cardiac dysfunction. Together, we assume that OPN transcription in patients with À156G allele is more susceptible to Runx2-dependent upregulation and that patients carrying À156G allele demonstrated impaired diastolic function compared with À156del/À156del patients.…”

An architectural perspective of vitamin D responsiveness

“…Vitamin D treatment results in increased OP expression in osteoblastic cells as the VDR/RXR heterodimer binds to the OP promoter (−757 to −743) by recognizing a perfect DR3 motif [51]. The OP promoter also contains a Runx-binding site (−136 to −130) which is recognized by the Runx2 factor resulting in transcriptional enhancement [52].…”

“…Recent reports indicate that in osteoblastic cells the VDR and Runx2 cooperate to up-regulate OP transcription [52]. Moreover, vitamin D treatment results in increased binding of both Runx2 and the VDR to the OP promoter.…”

Abnormal Expression of the ERG Transcription Factor in Prostate Cancer Cells Activates Osteopontin

“…Several known cis-acting transcription factors have been described and most of them have been localized to a conserved region at 250 bp upstream of the proximal promoter. Potential binding sites for transcriptional regulators, such as AP-1, Myc, Oct-1, USF, v-Src, TGF-b/BMPs/Smad/Hox, Wnt/ b-catenin/APC/GSK-3b/Tcf-4, Ras/RRF, TF53, Runx2, and ETS family members, have been identified (14)(15)(16)(17)(18)(19)(20)(21).…”