Regulation of the Osteopontin Gene by the Orphan Nuclear Receptor NURR1 in Osteoblasts

Abstract

The orphan nuclear receptor Nurr1 is mainly expressed in the central nervous system but is also detected in certain peripheral tissues such as bone. To elucidate the role of Nurr1 in bone, we examined the ability of Nurr1 to regulate osteopontin (OPN) expression in osteoblastic cell lines. Transfection of Nurr1 in osteoblastic cells increased OPN mRNA expression. A dominant negative Nurr1 variant abolished the ability of PTH to induce OPN expression, suggesting that Nurr1 is involved in mediating the regulation of OPN by PTH. Nurr1 efficiently transactivated a luciferase reporter construct driven by the -857/+191 fragment of the mouse OPN promoter. The activation of the OPN promoter was mediated by the monomeric form of Nurr1, required direct binding of Nurr1 to the OPN promoter, and was dependent on the amino-terminal transactivation function-1. The OPN promoter is also regulated by vitamin D receptor and estrogen-related receptors. We show that Nurr1 and vitamin D activate the OPN promoter in a synergistic fashion, whereas Nurr1-mediated transactivation of the OPN promoter is repressed by estrogen-related receptors. In conclusion, Nurr1 activates the OPN promoter directly in osteoblastic cells, suggesting a role for Nurr1 in the regulation of bone homeostasis.

FGF-8 stimulates the expression of NR4A orphan nuclear receptors in osteoblasts

“…The induction of Nurr1 expression by FGF-8b demonstrated in our study links these two pathways. Nurr1 regulates the expression M a n u s c r i p t 16 of several osteoblastic differentiation markers such as alkaline phosphatase (ALP), collagen type I alpha I (COL1A1), osteocalcin (OCN), and osteopontin (OPN) (Lammi et al, 2004;Pirih et al, 2004;Lee et al, 2006). FGF-8b treatment increases ALP activity and enhances bone formation in mouse bone marrow cell cultures (Valta et al, 2006).…”

“…receptor expression is rapidly increased in response to parathyroid hormone (PTH) (Tetradis et al, 2001a;Tetradis et al, 2001b;Lammi et al, 2004;Pirih et al, 2005). Regulation of the M a n u s c r i p t 4 expression levels of NR4A receptors is most likely an important mechanism modulating their activities.…”

“…FGF-8b binds to and activates FGFR2c, FGFR3c, and FGFR4 that are expressed in osteoblasts (MacArthur et al, 1995;Valta et al, 2006 A c c e p t e d M a n u s c r i p t 5 NR4A receptors have been shown to be expressed in osteoblasts isolated from mouse calvariae and long bones. NR4A receptors are also expressed in several osteoblastic cell lines (Mages et al, 1994;Lammi et al, 2004, Pirih et al, 2005. Nurr1 expression has been reported in the developing limb but the expression pattern of NR4A receptors in developing or mature bone has not yet been analyzed (Zetterström et al, 1996).…”

“…regulates osteoblastic differentiation and stimulates the expression of osteopontin and osteocalcin genes (Lammi et al, 2004;Pirih et al, 2004;Pirih et al, 2005;Lee et al, 2006).…”

Estrogen receptor-related receptor α (ERRα) regulates osteopontin expression through a non-canonical ERRα response element in a cell context-dependent manner

“…Among those regulated in osteoblasts were osteopontin (OPN; Bonnelye et al 2001), a secreted phosphorylated multifunctional glycoprotein whose regulation depends on a number of NRs with roles in bone biology (Noda et al 1990, Craig & Denhardt 1991, Vanacker et al 1998, Lee et al 2000, Bonnelye et al 2001, Denhardt et al 2001, Ogawa et al 2005, Shen & Christakos 2005. However, there are conflicting reports on the regulation of OPN by ERRa in osteoblastic cells, with some reporting upregulation (Vanacker et al 1998, Bonnelye et al 2001 and others reporting downregulation, the latter via interference of ERRa with the orphan NR nuclear receptor related 1 (Nurr1) (Lammi et al 2004). We therefore readdressed OPN regulation by ERRa and report that mOPN regulation by ERRa is cell context dependent and does not involve the previously identified ERRE, but rather a non-canonical site overlapping the inverted CAAT box.…”