Regulation of corticotropin-releasing factor receptor type 2beta mRNA by mitogen-activated protein kinases in aortic smooth muscle cells

Resistin-induced cardiomyocyte hypertrophy is inhibited by apelin through the inactivation of extracellular signal-regulated kinase signaling pathway in H9c2 embryonic rat cardiomyocytes

It has been reported that resistin induces, whereas apelin inhibits cardiac hypertrophy. However, the underlying molecular mechanisms of apelin inhibiting resistin-induced cardiac hypertrophy remain unclear. The aim of the current study is to investigate the effects of apelin on resistin-induced cardiomyocyte hypertrophy and elucidate the underlying molecular mechanism. H9c2 cells were used in the present study, and cell surface area and protein synthesis were evaluated. Reverse transcription-quantitative polymerase chain reaction was performed to analyze the expression levels of hypertrophic markers, brain natriuretic peptide (BNP) and β-myosin heavy chain (β-MHC). In addition, western blotting was conducted to examine phosphorylation of extracellular signal-regulated kinase (ERK)1/2. Following treatment of H9c2 cells with resistin, cell surface area, protein synthesis, and BNP and β-MHC mRNA expression levels were increased. Subsequent to co-treatment of H9c2 cells with apelin and resistin, lead to the inhibition of resistin-induced hypertrophic effects by apelin. In addition, treatment with resistin increased phosphorylation of ERK1/2, whereas pretreatment with apelin decreased phosphorylation of ERK1/2, which was increased by resistin. These results indicate that resistin-induced cardiac hypertrophy is inhibited by apelin via inactivation of ERK1/2 cell signaling.

Estrogenic action on arterial smooth muscle: permissive for maintenance of CRHR2 expression

Urocortin (Ucn), a member of CRH family, has been implicated to be one of the endogenous regulators in the cardiovascular system and exerts its effects locally via an autocrine/paracrine fashion. Previous studies have shown the gender difference in CRH-induced vasodilation in human skin, which is related to the concentration of estrogens during the menstrual cycle. The aim of this study was to investigate whether estrogens modulate Ucn/CRH receptor type 2 (CRHR2) expression in vascular smooth muscle, thereby leading to vasodilation. We performed sham operation or bilateral ovariectomy (OVX) on female Sprague Dawley rats. OVX rats were sc administered 17β-estradiol (E₂) at a dose of 30 μg/kg·d or with placebo for 12 wk. Primary smooth muscle cells of aorta were used for the in vitro study. It was found that the Ucn-induced vasodilation and CRHR2 expression were decreased in OVX rats and restored by E₂ replacement treatment for 12 wk. E₂ increased the expression of CRHR2 in cultured smooth muscle cells, which was blocked by estrogen receptor-β antagonist. Ucn significantly suppressed the phenylephrine-induced phospholipase Cβ3 activation, inositol 1,4,5-trisphosphate (IP₃) production, and intracellular Ca²⁺ elevation. Ucn stimulated the expression of active GTP-bound Gαs protein and cAMP production. The suppressive effects of Ucn on phenylephrine-induced IP₃ production and intracellular Ca²⁺ elevation were blocked by the inhibitors of adenylate cyclase and protein kinase A. Our results demonstrate that estrogen maintains the expression of CRHR2 in aorta smooth muscle, thereby enhancing vasodilator actions of Ucn. Ucn exerts its vasorelaxant effects via Gαs-cAMP-protein kinase A signaling, leading to down-regulation of the phospholipase Cβ-IP₃-Ca²⁺ signaling pathway.

Intracellular mechanisms regulating corticotropin-releasing hormone receptor-2beta endocytosis and interaction with extracellularly regulated kinase 1/2 and p38 mitogen-activated protein kinase signaling cascades

Many important physiological roles of the urocortin (UCN) family of peptides as well as CRH involve the type 2 CRH receptor (CRH-R2) and downstream activation of multiple pathways. To characterize molecular determinants of CRH-R2 functional activity, we used HEK293 cells overexpressing recombinant CRH-R2beta and investigated mechanisms involved in attenuation of CRH-R2 signaling activity and uncoupling from intracellular effectors. CRH-R2beta-mediated adenylyl cyclase activation was sensitive to homologous desensitization induced by pretreatment with either UCN-II or the weaker agonist CRH. CRH-R2beta activation induced transient beta-arrestin1 and beta-arrestin2, as well as clathrin, recruitment to the plasma membrane. Beta-arrestin2 appeared to be the main beta-arrestin subtype associated with the receptor. This was followed by CRH-R2beta endocytosis in a mechanism that exhibited distinct agonist-dependent temporal characteristics. CRH-R2beta also induced transient activation of the ERK1/2 and p38MAPK signaling cascades that peaked at 5 min and returned to basal within 20-30 min. Unlike p38MAPK, activated ERK1/2 was localized both in the cytoplasm and nucleus. Experiments employing inhibitors of receptor endocytosis showed that CRH-R2beta-MAPK interaction does not require beta-arrestin, clathrin, or receptor endocytosis. Site-directed mutagenesis studies on CRH-R2beta C terminus showed that the amino acid cassette TAAV at the end of the C terminus is important for CRH-R2beta signaling because loss of a potential phospho-acceptor site in mutant receptors containing deletion or Ala substitution of the cassette TAAV resulted in reduced ERK1/2 activation and accelerated receptor internalization. These findings provide new insights about the signaling mechanisms regulating CRH-R2beta functional activity and determining its biological responses.

Differential regulation of CREB and ERK phosphorylation through corticotropin-releasing factor receptors type 1 and 2 in AtT-20 and A7r5 cells

The corticotropin-releasing factor (CRF) family of peptides generally exerts its biological actions by binding to two major subtypes of CRF receptors: CRF receptor type 1 (CRF1 receptor) and CRF receptor type 2 (CRF2 receptor). In this study, we investigated the mechanism by which three ligands altered phosphorylation of CREB and ERK 1/2, using AtT-20 cells (expressing CRF1 receptor) and A7r5 cells (expressing CRF2 receptor). Incubation with 100 nM of CRF, urocortin 1 (UCN 1), or UCN 2 increased CREB phosphorylation. The protein kinase A pathway was involved in the CRF- or UCN-mediated increase in CREB phosphorylation in both cell lines. Bisindolylmaleimide partially inhibited the CRF-mediated increase in CREB phosphorylation, but only in AtT-20 cells, suggesting that the protein kinase C pathway is involved in regulation of CREB phosphorylation via CRF1 receptor but not CRF2 receptor. CRF increased ERK phosphorylation in AtT-20 cells, whereas the UCNs decreased it in A7r5 cells. Bisindolylmaleimide partially inhibited the UCN-mediated decrease in ERK phosphorylation in A7r5 cells, suggesting that the protein kinase C pathway is partially involved in CRF2 receptor signal transduction. In AtT-20 cells, the mitogen-activated protein kinase kinase pathway regulated ERK phosphorylation following CRF1 receptor activation. These findings suggest differential regulation of CREB and ERK 1/2 phosphorylation through CRF receptors.

Modulation of Ca2+ influx by corticotropin-releasing factor (CRF) family of peptides via CRF receptors in rat pancreatic beta-cells

The actions of the corticotropin-releasing factor (CRF) family of peptides are mediated by the seven transmembrane-domain G-protein-coupled receptors, the CRF receptors type 1 (CRF1 receptor) and type 2 (CRF2 receptor). In a previous study, we reported that CRF, an endogenous ligand for CRF1 receptor, modulated Ca2+ influx in rat pancreatic beta-cells. In addition to CRF, other additional members of the family, urocortins, have been identified in mammals. Urocortin 1 (UCN 1), a peptide of the CRF family, binds both CRF1 receptor and CRF2 receptor with equal affinities. Urocortin 3 (UCN 3), a highly selective ligand for CRF2 receptor with little affinity for CRF1 receptor, has been shown in rat pancreatic beta-cells. The present study focused on the effects of the CRF family peptides on intracellular Ca2+ ([Ca2+]i) concentration via CRF receptors in rat pancreatic beta-cells. Microfluorimetric experiments showed that CRF (0.2 nM) and UCN 1 (0.2 nM) elevated [Ca2+]i levels. Both CRF and UCN 1 effects were attenuated by astressin, a non-selective CRF receptor antagonist. Antisauvagine-30, a selective CRF2 receptor antagonist, appeared to enhance the UCN 1 effect on the elevation of [Ca2+]i. The CRF effect on the elevation of [Ca2+]i was inhibited by the addition of UCN 3. Taken together, the activation of CRF2 receptor antagonizes the CRF1 receptor-stimulated Ca2+ influx.

Differential regulation of corticotropin-releasing factor receptor type 1 (CRF1 receptor) mRNA via protein kinase A and mitogen-activated protein kinase pathways in rat anterior pituitary cells

Corticotropin-releasing factor (CRF) receptor type 1 (CRF(1) receptor) mRNA levels are down-regulated by CRF via the cyclic AMP-protein kinase A (PKA) pathway. In this study, we focused on the involvement of both the mitogen-activated protein (MAP) kinase pathway and PKA in this regulation. Real-time PCR (RT-PCR) revealed that a MAP kinase, extracellular signal-regulated kinases 1/2, pathway was also involved in the down-regulation of CRF(1) receptor mRNA levels by CRF in the rat anterior pituitary (AP). Down-regulation of CRF(1) receptor mRNA levels was caused by a post-transcriptional system such as mRNA degradation, as incubation with CRF significantly decreased the half-life of CRF(1) receptor mRNA. Furthermore, pre-treatment with a PKA inhibitor completely blocked CRF-induced CRF(1) receptor mRNA destabilization, while pre-treatment with an extracellular signal-regulated kinases 1/2 inhibitor had no inhibitory effect. These results suggested that in the rat AP, down-regulation of CRF(1) receptor mRNA levels is caused by mRNA degradation via PKA, but not by the MAP kinase pathway.