PGC-1α is induced by parathyroid hormone and coactivates Nurr1-mediated promoter activity in osteoblasts

Parathyroid hormone (PTH) potently activates cAMP-protein kinase A (PKA)-driven molecular cascades in ostcoblasts. The NR4A/NGFI-B orphan nuclear receptor (NR) Nurr1 is a PTH-induced, cAMP-responsive primary response gene (PRG) that transactivates osteocalcin (Ocn) expression through a putative NGFI-B response element (NBRE) in the proximal promoter. As a true orphan NR, Nurr1's expression level and coactivator recruitment regulate its transactivation capacity. We postulated that Nurr1's induction through cAMP-PKA signaling might favor a coactivator that is likewise cAMP-dependent. A possible candidate is the cAMP-inducible coactivator PPAR-gamma coactivator-1 alpha. (PGC-1 alpha). We hypothesize that PGC-1 alpha. is a PTH-induced PRG that synergizes with Nurr1 to induce target gene transcription in osteoblasts. We show that 10 nM PTH for 2 h maximally induced PGC-1 alpha mRNA in primary mouse osteoblasts (MOBs) and calvariae. Selective signaling agonists and antagonists demonstrated that PTH induced PGC-1 alpha mRNA primarily through the cAMP-PKA pathway. Protein synthesis inhibition sustained PTH-induced PGC-1 alpha expression. PGC-1 alpha enhanced Nurr1-induced transactivation of a consensus 3xNBRE-luciferase construct and the rat (-1050)Ocn promoter-luciferase construct from 3.7- to 9.6- and 10.1-fold, respectively. This synergy required Nurrl-DNA binding, since a mutation of the Ocn promoter NBRE abolished both Nurr1- and Nurr1-PGC-1 alpha-induced transactivation. Using GST pull-down assays, PGC-1 alpha directly interacted with in vitro-generated and nuclear Nurr1. We conclude that PGC-lot is a PTH-induced, cAMP-dependent PRG that directly synergizes with Nurr1 to transactivate target genes in osteoblasts. Taken together with published data, our findings suggest that Nurr1 and PGC-1 alpha may be pivotal mediators of cAMP-induced osteoblast gene expression and osteoblast function. (C) 2006 Elsevier Inc. All rights reserved.