The NR4A orphan nuclear receptor NOR1 is induced by platelet-derived growth factor and mediates vascular smooth muscle cell proliferation.

Members of the nuclear hormone receptor superfamily function as key transcriptional regulators of inflammation and proliferation in cardiovascular diseases. In addition to the ligand-dependent peroxisome proliferator-activated receptors and liver X receptors, this family of transcription factors includes a large number of orphan receptors, and their role in vascular diseases remains to be investigated. The neuron-derived orphan receptor-1 (NOR1) belongs to the ligand-independent NR4A subfamily, which has been implicated in cell proliferation, differentiation, and apoptosis. In this study, we demonstrate NOR1 expression in vascular smooth muscle cells (SMC) of human atherosclerotic lesions. In response to mitogenic stimulation with platelet-derived growth factor (PDGF), SMC rapidly express NOR1 through an ERK-MAPK-dependent signaling pathway. 5'-deletion analysis, site-directed mutagenesis, and transactivation experiments demonstrate that PDGF-induced NOR1 expression is mediated through a cAMP-response element-binding protein (CREB)-dependent transactivation of the NOR1 promoter. Consequently, short interfering RNA-mediated depletion of CREB abolished PDGF-induced NOR1 expression in SMC. Furthermore, PDGF induced Ser-133 phosphorylation of CREB and subsequent binding to the CRE sites of the endogenous NOR1 promoter. Functional analysis demonstrated that PDGF induces NOR1 transactivation of its consensus NGFI-B-response elements (NBRE) in SMC. We finally demonstrate that SMC isolated from NOR1-deficient mice exhibit decreased cell proliferation and characterize cyclin D1 and D2 as NOR1 target genes in SMC. These experiments indicate that PDGF-induced NOR1 transcription in SMC is mediated through CREB-dependent transactivation of the NOR1 promoter and further demonstrate that NOR1 functions as a key transcriptional regulator of SMC proliferation.