The Orphan Nuclear Receptor, NOR-1, Is a Target of β-Adrenergic Signaling in Skeletal Muscle

Abstract

beta-Adrenergic receptor (beta-AR) agonists induce Nur77 mRNA expression in the C2C12 skeletal muscle cell culture model and elicit skeletal muscle hypertrophy. We previously demonstrated that Nur77 (NR4A1) is involved in lipolysis and gene expression associated with the regulation of lipid homeostasis. Subsequently it was demonstrated by another group that beta-AR agonists and cold exposure-induced Nur77 expression in brown adipocytes and brown adipose tissue, respectively. Moreover, NOR-1 (NR4A3) was hyperinduced by cold exposure in the nur77(-/-) animal model. These studies underscored the importance of understanding the role of NOR-1 in skeletal muscle. In this context we observed 30-480 min of beta-AR agonist treatment significantly and transiently increased expression of the orphan nuclear receptor NOR-1 in both mouse skeletal muscle tissue (plantaris) and C2C12 skeletal muscle cells. Specific beta(2)- and beta(3)-AR agonists had similar effects as the pan-agonist and were blocked by the beta-AR antagonist propranolol. Moreover, in agreement with these observations, isoprenaline also significantly increased the activity of the NOR-1 promoter. Stable exogenous expression of a NOR-1 small interfering RNA (but not the negative control small interfering RNA) in skeletal muscle cells significantly repressed endogenous NOR-1 mRNA expression and led to changes in the expression of genes involved in the control of lipid use and muscle mass underscored by a dramatic increase in myostatin mRNA expression. Concordantly the myostatin promoter was repressed by NOR-1 expression. In conclusion, NOR-1 is highly responsive to beta-adrenergic signaling and regulates the expression of genes controlling fatty acid use and muscle mass.

Synergistic Induction of NR4A Orphan Nuclear Receptors by LPS and cAMP in Murine Macrophage Cells

“…As in their response to LPS, levels of Nur77 and NOR1 mRNAs peaked at 1 and 2 h, respectively. In contrast to the responses to LPS, however, the mRNA levels did not very rapidly return to baseline but declined more slowly and remained somewhat elevated for at least 8 h. These response profiles are similar to the NR4A responses to 8-bromo-cAMP in murine hepatocytes [16] but differ from the rapid return to baseline for NOR-1 mRNA following treatment with a β-adrenergic agonist in a mouse myoblast line [13]. Maximum fold-induction of NOR1 by 8-bromo-cAMP was much greater than with LPS, but maximum fold-induction of Nur77 was similar with both agents (Figure 2).…”

“…By 4 h, the relative mRNA levels for Nur77 and [7-12, 20, 21], there has been increased interest in regulation of their expression by various inflammatory stimuli, including LPS [10]. Several studies have shown that NR4A expression can be induced by cAMP [13][14][15][16]19], whose intracellular levels can be increased in response to mediators of inflammation or stress such as prostaglandins or catecholamines. However, potential interactions between LPS and cAMP analogs or cAMP-elevating agents in regulating NR4A expression have not been reported previously.…”

“…During inflammation or infection in vivo, however, macrophages are exposed to multiple stimuli, which can include agents such as catecholamines or prostaglandins that stimulate intracellular cAMP synthesis, and the responses of specific genes to combinations of stimuli can vary significantly from their responses to individual stimuli. NR4A expression is induced by cAMP in liver, muscle and pituitary cells [13][14][15][16], but it has not been determined previously whether cAMP also induces expression of any of the NR4A isoforms in macrophages or, perhaps more importantly, whether cAMP alters their responses to inflammatory stimuli such as LPS. Therefore, this study was conducted to evaluate the potential impact of cAMP, alone or in combination with LPS, on expression of NR4A family members in macrophage cells.…”

Effects of β2-agonists and exercise on β2-adrenergic receptor signaling in skeletal muscles

“…Recently, Pearen et al 49) identified that the promoter region of NOR-1 is a target for β2-AR-mediated CRE activation in skeletal muscles. An increase in the expression of NOR-1, induced by β2-AR activation, is consistent with the involvement of PKA, p38 MAPK, and phosphorylated CREB in C2C12 cells 50) .…”

“…An increase in the expression of NOR-1, induced by β2-AR activation, is consistent with the involvement of PKA, p38 MAPK, and phosphorylated CREB in C2C12 cells 50) . Additionally, NOR-1 was found to be a negative regulator of myostatin (a member of the transforming growth factor-β superfamily and a potent negative regulator of muscle mass) 49) . Furthermore, Kawasaki et al 51) demonstrated that acute exercise increases the expression of NOR-1 mRNA in skeletal muscles.…”