Extracellular receptor kinase and cAMP response element binding protein activation in the neonatal rat heart after perinatal cocaine exposure

The Melatonin Signaling Pathway in a Long-Term Memory In Vitro Study

The activation of cyclic adenosine monophosphate (cAMP) response element-binding protein (CREB) via phosphorylation in the hippocampus is an important signaling mechanism for enhancing memory processing. Although melatonin is known to increase CREB expression in various animal models, the signaling mechanism between melatonin and CREB has been unknown in vitro. Thus, we confirmed the signaling pathway between the melatonin receptor 1 (MT1) and CREB using melatonin in HT-22 cells. Melatonin increased MT1 and gradually induced signals associated with long-term memory processing through phosphorylation of Raf, ERK, p90RSK, CREB, and BDNF expression. We also confirmed that the calcium, JNK, and AKT pathways were not involved in this signaling pathway by melatonin in HT-22 cells. Furthermore, we investigated whether melatonin regulated the expressions of CREB-BDNF associated with long-term memory processing in aged HT-22 cells. In conclusion, melatonin mediated the MT1-ERK-p90RSK-CREB-BDNF signaling pathway in the in vitro long-term memory processing model and increased the levels of p-CREB and BDNF expression in melatonin-treated cells compared to untreated HT-22 cells in the cellular aged state. Therefore, this paper suggests that melatonin induces CREB signaling pathways associated with long-term memory processing in vitro.

Orphan Adhesion GPCR GPR64/ADGRG2 Is Overexpressed in Parathyroid Tumors and Attenuates Calcium-Sensing Receptor-Mediated Signaling

Abnormal feedback of serum calcium to parathyroid hormone (PTH) secretion is the hallmark of primary hyperparathyroidism (PHPT). Although the molecular pathogenesis of parathyroid neoplasia in PHPT has been linked to abnormal expression of genes involved in cell growth (e.g., cyclin D1, retinoblastoma, and β-catenin), the molecular basis of abnormal calcium sensing by calcium-sensing receptor (CaSR) and PTH hypersecretion in PHPT are incompletely understood. Through gene expression profiling, we discovered that an orphan adhesion G protein-coupled receptor (GPCR), GPR64/ADGRG2, is expressed in human normal parathyroid glands and is overexpressed in parathyroid tumors from patients with PHPT. Using immunohistochemistry, Western blotting, and coimmunoprecipitation, we found that GPR64 is expressed on the cell surface of parathyroid cells, is overexpressed in parathyroid tumors, and physically interacts with the CaSR. By using reporter gene assay and GPCR second messenger readouts we identified Gαs, 3',5'-cyclic adenosine monophosphate (cAMP), protein kinase A, and cAMP response element binding protein (CREB) as the signaling cascade downstream of GPR64. Furthermore, we found that an N-terminally truncated human GPR64 is constitutively active and a 15-amino acid-long peptide C-terminal to the GPCR proteolysis site (GPS) of GPR64 activates this receptor. Functional characterization of GPR64 demonstrated its ability to increase PTH release from human parathyroid cells at a range of calcium concentrations. We discovered that the truncated constitutively active, but not the full-length GPR64 physically interacts with CaSR and attenuates the CaSR-mediated intracellular Ca²⁺ signaling and cAMP suppression in HEK293 cells. Our results indicate that GPR64 may be a physiologic regulator of PTH release that is dysregulated in parathyroid tumors, and suggest a role for GPR64 in pathologic calcium sensing in PHPT. © 2016 American Society for Bone and Mineral Research.

Effect of bombesin receptor subtype-3 and its synthetic agonist on signaling, glucose transport and metabolism in myocytes from patients with obesity and type 2 diabetes

Bombesin receptor subtype-3 (BRS-3) is an orphan G-protein-coupled receptor (GPCR) member of the bombesin receptor family. Several studies have suggested an association between obesity, alterations in glucose metabolism, diabetes and the BRS-3 receptor. In this study, we focused on patients simultaneously diagnosed with obesity and type 2 diabetes (OB/T2D). The analysis of BRS-3 expression in the skeletal muscle of these patients revealed a marked decrease in the expression of BRS-3 at the mRNA (23.6±1.3-fold downregulation, p<0.0001) and protein level (49±7% decrease, p<0.05) compared to the normal patients (no obesity and diabetes). Moreover, in cultured primary myocytes from patients with OB/T2D, the synthetic BRS-3 agonist, [D-Try⁶,β-Ala¹¹,Phe¹³,Nle¹⁴]bombesin_6–14, significantly increased the phosphorylation levels of mitogen-activated protein kinase (MAPK), p90RSK1, protein kinase B (PKB) and p70s6K. Specifically, the ligand at 10⁻¹¹ M induced the maximal phosphorylation of MAPKs (p42, 159±15% of the control; p44, 166±11% of the control; p<0.0001) and p90RSK1 (148±2% of the control, p<0.0001). The basal phosphorylation levels of all kinases were reduced (p<0.05) in the patients with OB/T2D compared to the normal patients. Furthermore, the BRS-3 agonist stimulated glucose transport, which was already detected at 10⁻¹² M (133±9% of the control), reached maximal levels at 10⁻¹¹ M (160±9%, p<0.0001) and was maintained at up to 10⁻⁸ M (overall mean, 153±7%; p<0.007). This effect was less promiment than that attained with 10⁻⁸ M insulin (202±9%, p=0.009). The effect of the agonist on glycogen synthase a activity achieved the maximum effect at 10⁻¹¹ M (165±16% of the control; p<0.0001), which did not differ from that observed with higher concentrations of the agonist. These results suggest that muscle cells isolated from patients with OB/T2D have extremely high sensitivity to the synthetic ligand, and the effects are particularly observed on MAPK and p90RSK1 phosphorylation, as well as glucose uptake. Moreover, our data indicate that BRS-3 may prove to be useful as a potential therapeutic target for the treatment of patients with OB/T2D.

Human BRS-3 receptor: functions/role in cell signaling pathways and glucose metabolism in obese or diabetic myocytes

Several studies showed that the orphan Bombesin Receptor Subtype-3 (BRS-3) - member of the bombesin receptor family - has an important role in glucose homeostasis (v.g.: BRS-3-KO mice developed mild obesity, and decreased levels of BRS-3 mRNA/protein have been described in muscle from obese (OB) and type 2 diabetic (T2D) patients). In this work, to gain insight into BRS-3 receptor cell signaling pathways, and its implication on glucose metabolism, primary cultured myocytes from normal subjects, OB or T2D patients were tested using high affinity ligand - [d-Tyr(6),β-Ala(11),Phe(13),Nle(14)]bombesin6-14. In muscle cells from all metabolic conditions, the compound significantly increased not only MAPKs, p90RSK1, PKB and p70s6K phosphorylation levels, but also PI3K activity; moreover, it produced a dose-response stimulation of glycogen synthase a activity and glycogen synthesis. Myocytes from OB and T2D patients were more sensitive to the ligand than normal, and T2D cells even more than obese myocytes. These results widen the knowledge of human BRS-3 cell signaling pathways induced by a BRS-3 agonist, described its insulin-mimetic effects on glucose metabolism, showed the role of BRS-3 receptor in glucose homeostasis, and also propose the employing of BRS-3/ligand system, as participant in the obese and diabetic therapies.

Sustained intracellular acidosis activates the myocardial Na(+)/H(+) exchanger independent of amino acid Ser(703) and p90(rsk)

The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) is a ubiquitously expressed pH-regulatory membrane protein that functions in the myocardium and other tissues. It is an important mediator of the myocardial damage that occurs after ischemia-reperfusion injury and is implicated in heart hypertrophy. Regulation of NHE1 has been proposed as a therapeutic target for cardioprotection. We therefore examined mechanisms of control of NHE1 in the myocardium. Several different amino acids have been implicated as a being critical to NHE1 regulation in a number of tissues including Ser(703), Ser(770), and Ser(771). In the myocardium, NHE1 is activated in response to a variety of stimuli including activation by an ERK-dependent sustained intracellular acidosis. In this study, we determined whether Ser(703) and p90(rsk) activity are critical in activation of NHE1 by sustained intracellular acidosis. In vitro phosphorylation of NHE1 C-terminal fusion proteins determined that ERK-dependent phosphorylation of the cytoplasmic region was not dependent on Ser(703); however, phosphorylation by p90(rsk) required Ser(703). A Ser703Ala mutation decreased basal NHE1 activity in CHO cells but not in cardiomyocytes. NHE1 with a Ser703Ala mutation was activated in response to sustained intracellular acidosis in CHO cells. In addition, sustained intracellular acidosis also activated the Ser703Ala mutant protein in isolated cardiomyocytes and phosphorylation levels were also increased by acidosis. The presence of a dominant-negative p90(rsk) kinase also did not prevent activation and phosphorylation of NHE1 by sustained intracellular acidosis in isolated cardiomyocytes. We conclude that Ser(703) and p90(rsk) are not required for activation by sustained intracellular acidosis and that p90(rsk) phosphorylation of Ser(703) is independent of this type of activation.

Phenylephrine and sustained acidosis activate the neonatal rat cardiomyocyte Na+/H+ exchanger through phosphorylation of amino acids Ser770 and Ser771

The mammalian Na(+)/H(+) exchanger isoform 1 (NHE1) is a ubiquitously expressed membrane protein that regulates intracellular pH in the myocardium. NHE1 is also important in mediating myocardial hypertrophy, and the blockage of NHE1 activity prevents hypertrophy and reduces ischemia-reperfusion injury in animal models. We recently demonstrated that extracellular-regulated kinase (ERK)-mediated activation of NHE1 occurs during ischemia-reperfusion of the myocardium. To understand the regulation of NHE1 in the myocardium by phosphorylation, we expressed a series of adenoviruses that express wild-type and mutant cDNA for NHE1. All exogenous cDNA for NHE1 had additional mutations [Leu(163)Phe/Gly(174)Ser], which increases NHE1 resistance to EMD-87580 (a specific blocker of NHE1) 100-fold, and allowed the measurement of exogenous NHE1 while inhibiting endogenous NHE1. By examining the effects of a series of mutations of the NHE1 cytosolic region, we determined that the amino acids Ser(770) and Ser(771) were essential for the acute activation of NHE1 activity in rat cardiomyocytes. The specific mutation of either residue prevented the rapid activation of exchanger activity by a sustained intracellular acidosis through ERK-dependent pathways. The same amino acids were critical to phenylephrine-mediated, ERK-dependent activation of NHE1 activity and increased the phosphorylation in intact rat cardiomyocytes. The results demonstrate that both sustained intracellular acidosis and phenylephrine rapidly activate the NHE1 protein in intact cardiac cells through ERK-dependent pathways that act on a common pathway mediated by amino acids Ser(770) and Ser(771) of the cytosolic tail of the protein.

Effects of acute cocaine on ERK and DARPP-32 phosphorylation pathways in the caudate-putamen of Fischer rats

Activation of extracellular signal-regulated kinase (ERK) and dopamine- and cAMP-regulated phosphoprotein (DARPP-32) pathways has been implicated in biochemical and behavioral effects induced by various drugs of abuse. In this study, we investigated the phosphorylation pathways of these two proteins in response to acute cocaine administration. A single cocaine administration (30 mg/kg) increased ERK-mediated signaling proteins, phosphoryation of cAMP response element-binding protein (CREB) kinase, pp90 ribosomal S6 kinase (RSK), and c-Fos protein levels in the caudate/putamen of Fischer rats. Acute cocaine administration also induced phosphorylation of the striatal-enriched protein tyrosine phosphatase (STEP) and decreased the phosphorylation of DARPP-32 protein at the Thr-75 site. The phosphorylation states of these inhibitors of ERK and DARPP-32 proteins may thus contribute to the effects of cocaine on ERK- and DARPP-32-mediated cascades, on gene expression and on behaviors.

Cocaine-induced inhibition of ATP-sensitive K+ channels in rat ventricular myocytes and in heart-derived H9c2 cells

Cocaine use may cause coronary artery spasm and acute myocardial ischaemia/infarction. However, its effects on ATP-sensitive K+ (KATP) channel, an ion channel responsible for ischaemic preconditioning, remain unknown. In isolated rat ventricular myocytes with whole-cell experiments, cocaine can reverse action potential shortening and increased K+ current caused by the openers of ATP-sensitive K+ (KATP) channels. In inside-out patches, cocaine applied to intracellular surface suppressed KATP-channel activity in a concentration-dependent manner with an IC50 value of 9.2 microM; however, it did not modify the single-channel conductance of this channel. The change in the kinetic behaviour of KATP channels caused by cocaine is primarily the result of an increase in mean closed time and a decrease in mean open time. Cocaine-induced inhibition of KATP channels is independent of change in intracellular ATP concentrations. In heart-derived H9c2 cells, cocaine is also capable of suppressing KATP-channel activity. The present study provides evidence that cocaine can produce a depressant action on KATP channels in cardiac myocytes, and thus disturb ischaemic preconditioning in clinical settings.

CREB expression in cardiac fibroblasts and CREM expression in ventricular myocytes

Activation of gene expression by the cAMP-dependent signaling pathway is regulated by members of the cAMP response element binding protein (CREB) family consisting of CREB, CREM, and ATF-1. It is decisively for the understanding of the heart function as to which type of heart cells expresses CREB and/or CREM. Ventricular myocytes and fibroblasts of young (3 months) and old (24 months) rat hearts were separately investigated to analyse CREB, CREM, and phospho-CREB. Western blot showed CREB expression exclusively in fibroblasts but CREM was predominantly detected in ventricular myocytes. CREB-positive nuclei in heart sections were only revealed in fibroblasts. CREB was activated by forskolin (10 microM), PMA (500 nM), and cyclical mechanical strain (1 Hz, 5% elongation) in fibroblasts. The number of CREB-positive myocytes in old rats was larger than in young rats. But CREB could not be activated by forskolin (10 microM) in all myocytes. Our results suggest that the expression of CREB depends on the cell type and the age of the animal. We discuss that modulation of gene expression as it occurs with a age could be affected by the change within the CREB family members.