A role of protein kinase C mu in signalling from the human adenosine A1 receptor to the nucleus

Adenosine A1 receptors link to smooth muscle contraction via CYP4a, protein kinase C-α, and ERK1/2

Adenosine A1 receptor (A1AR) activation contracts smooth muscle, although signaling mechanisms are not thoroughly understood. Activation of A1AR leads to metabolism of arachidonic acid, including the production of 20-hydroxyeicosatetraenoic acid (20-HETE) by cytochrome P4504a (CYP4a). The 20-HETE can activate protein kinase C-α (PKC-α), which crosstalks with extracellular signal-regulated kinase (ERK1/2) pathway. Both these pathways can regulate smooth muscle contraction, we tested the hypothesis that A1AR contracts smooth muscle through a pathway involving CYP4a, PKC-α, and ERK1/2. Experiments included isometric tension recordings of aortic contraction and Western blots of signaling molecules in wild type (WT) and A1AR knockout (A1KO) mice. Contraction to the A1-selective agonist 2-chloro-N cyclopentyladenosine (CCPA) was absent in A1KO mice aortae, indicating the contractile role of A1AR. Inhibition of CYP4a (HET0016) abolished 2-chloro-N cyclopentyladenosine-induced contraction in WT aortae, indicating a critical role for 20-HETE. Both WT and A1KO mice aortae contracted in response to exogenous 20-HETE. Inhibition of PKC-α (Gö6976) or ERK1/2 (PD98059) attenuated 20-HETE-induced contraction equally, suggesting that ERK1/2 is downstream of PKC-α. Contractions to exogenous 20-HETE were significantly less in A1KO mice; reduced protein levels of PKC-α, p-ERK1/2, and total ERK1/2 supported this observation. Our data indicate that A1AR mediates smooth muscle contraction via CYP4a and a PKC-α-ERK1/2 pathway.

The effect of adenosine on pro-inflammatory cytokine production by porcine T cells

Adenosine is a well described anti-inflammatory modulator of immune responses. The aim of the present study was to describe the role of common adenosine agonist 5'-N-ethylcarboxamidoadenosine (NECA) in cytokine production by main porcine T cell subpopulations. TNF-α, IFN-γ, IL-2 and IL-10 were detected by multicolor flow cytometry together with cell surface markers CD3, CD4 and CD8. It was found that NECA inhibits (in a dose-dependent manner) production of pro-inflammatory TNF-α and Th1-associated cytokines IFN-γ, IL-2 in all concanavalin A-stimulated T cell subpopulations. Moreover, production of IL-10 was potentiated in all T cell subpopulations tested. These corresponded well with the fact that all T cell subsets expressed mRNA for adenosine receptor (AR) subtypes to comparable extents. Contrary to concanavalin A-stimulated cells, NECA had a moderate effect on PMA-stimulated T cells, suggesting that AR in pigs acts via signaling pathways not associated with protein-kinase C. Non-selective antagonist CGS15943 as well as allosteric modulator SCH202676 failed to reverse the effect of NECA in pigs. In conclusion, NECA has an anti-inflammatory effect on porcine T cell subpopulations.

Enhancement of calcium transport in Caco-2 monolayer through PKCzeta-dependent Cav1.3-mediated transcellular and rectifying paracellular pathways by prolactin

Previous investigations suggested that prolactin (PRL) stimulated the intestinal calcium absorption through phosphoinositide 3-kinase (PI3K), protein kinase C (PKC), and RhoA-associated coiled-coil forming kinase (ROCK) signaling pathways. However, little was known regarding its detailed mechanisms for the stimulation of transcellular and voltage-dependent paracellular calcium transport. By using Ussing chamber technique, we found that the PRL-induced increase in the transcellular calcium flux and decrease in transepithelial resistance of intestinal-like Caco-2 monolayer were not abolished by inhibitors of gene transcription and protein biosynthesis. The PRL-stimulated transcellular calcium transport was completely inhibited by the L-type calcium channel blockers (nifedipine and verapamil) and plasma membrane Ca(2+)-ATPase (PMCA) inhibitor (trifluoperazine) as well as small interfering RNA targeting voltage-dependent L-type calcium channel Ca(v)1.3, but not TRPV6 or calbindin-D(9k). As demonstrated by (45)Ca uptake study, PI3K and PKC, but not ROCK, were essential for the PRL-enhanced apical calcium entry. In addition, PRL was unable to enhance the transcellular calcium transport after PKC(zeta) knockdown or exposure to inhibitors of PKC(zeta), but not of PKC(alpha), PKC(beta), PKC(epsilon), PKC(mu), or protein kinase A. Voltage-clamping experiments further showed that PRL markedly stimulated the voltage-dependent calcium transport and removed the paracellular rectification. Such PRL effects on paracellular transport were completely abolished by inhibitors of PI3K (LY-294002) and ROCK (Y-27632). It could be concluded that the PRL-stimulated transcellular calcium transport in Caco-2 monolayer was mediated by Ca(v)1.3 and PMCA, presumably through PI3K and PKC(zeta) pathways, while the enhanced voltage-dependent calcium transport occurred through PI3K and ROCK pathways.

CD44 is the signaling component of the macrophage migration inhibitory factor-CD74 receptor complex

The macrophage migration inhibitory factor (MIF) receptor (CD74) was cloned recently, but the signaling mechanism is not evident. We hypothesized that signaling requires an additional molecule such as CD44, which activates nonreceptor tyrosine kinases. We utilized the CD74- and CD44-deficient COS-7/M6 cell to create stable transfectants expressing CD74, CD44, and a truncated CD44 lacking its intracytoplasmic signaling domain. CD74 alone mediated MIF binding; however, MIF-induced ERK1 and ERK2 kinase phosphorylation required the coexpression of full-length CD44. MIF binding was associated with the serine phosphorylation of CD74 and CD44. Investigations that used siRNA or kinase inhibitors indicate that MIF-induced ERK1 and ERK2 activation through CD44 required the Src tyrosine kinase. Studies of CD74, CD44, and CD74-CD44 transformants and corresponding mutant cells showed that CD74 and CD44 were necessary for MIF protection from apoptosis. These data establish CD44 as an integral member of the CD74 receptor complex leading to MIF signal transduction.

A comparison of the antagonist affinities for the Gi- and Gs-coupled states of the human adenosine A1-receptor

The antagonist affinity for a given receptor is traditionally considered to be constant, reflecting the chemical nature of the specific ligand-receptor interaction. However, recent observations with all three beta-adrenoceptors have cast doubt on this basic pharmacological principle. The extent to which this finding applies to other G protein-coupled receptors and their interaction with different G proteins is unknown. Therefore, we studied the influence of different agonists on antagonist affinity measurements for Gi- and Gs-coupled conformations of the adenosine A1-receptor in Chinese hamster ovary cells stably expressing the human adenosine A1-receptor and a cAMP-response element (CRE)-secreted placental alkaline phosphatase reporter gene. Gi-coupled inhibition of [3H]cAMP accumulation via the A1-receptor was observed at low concentrations of agonist; however, a small increase in [3H]cAMP accumulation was also seen at higher agonist concentrations. This biphasic response was more evident for A1-stimulated CRE-gene transcription. The inhibitory component was abolished by pretreatment with pertussis toxin, whereas the stimulatory component was augmented, suggesting that the responses were due to an A1-Gi-coupled inhibition followed by an A1-Gs-coupled stimulation. However, the antagonist affinity values measured at the Gi-coupled and Gs-coupled conformations of the receptor were the same in both functional responses and whole-cell binding. Thus, in marked contrast to the beta-adrenoceptors, the A1-receptor conforms to the long-held principle of pharmacology that antagonist affinity measurements are constant regardless of the response being measured and the competing agonist used to stimulate that response. This was true even when the receptor was shown, in the same assay, to exist in two different conformational states coupled to two different G proteins.

Group I metabotropic glutamate receptors, mGlu1a and mGlu5a, couple to cyclic AMP response element binding protein (CREB) through a common Ca2+ - and protein kinase C-dependent pathway

Coupling of the group I metabotropic glutamate receptors, mGlu1a and mGlu5a, to the cAMP response element binding protein (CREB) has been studied in Chinese hamster ovary cell lines where receptor expression is under the control of an inducible promoter. Both receptors stimulate CREB phosphorylation with similar time courses, and agonist potency was also comparable between the two receptors. Stimulation of cells in Ca(2+)-free medium containing EGTA (100 microm), with or without the additional depletion of intracellular stores, caused marked decreases in agonist-mediated responses in both cell lines. Down-regulation of protein kinase C (PKC) activity by phorbol ester treatment, or treatment with the broad spectrum PKC inhibitor Ro 31-8220, partially attenuated both mGlu1a and mGlu5a receptor-mediated responses. Furthermore, stimulation of cells in the absence of extracellular Ca(2+) following prior PKC down-regulation resulted in additive inhibitory effects. The involvement of extracellular signal-regulated kinases (ERK1/2), Ca(2+)/calmodulin or Ca(2+)/calmodulin-dependent protein kinases was assessed using pharmacological inhibitors. Results indicated that coupling of the group I mGlu receptors to CREB phosphorylation occurs independently of these pathways. Thus, although the [Ca(2+)](i) signatures activated by these mGlu receptors differ, they couple to CREB with comparable potency and recruit similar downstream components to execute CREB phosphorylation.

OX1 orexin receptors couple to adenylyl cyclase regulation via multiple mechanisms

In this study, the mechanism of OX(1) orexin receptors to regulate adenylyl cyclase activity when recombinantly expressed in Chinese hamster ovary cells was investigated. In intact cells, stimulation with orexin-A led to two responses, a weak (21%), high potency (EC(50) approximately 1 nm) inhibition and a strong (4-fold), low potency (EC(50) = approximately 300 nm) stimulation. The inhibition was reversed by pertussis toxin, suggesting the involvement of G(i/o) proteins. Orexin-B was, surprisingly, almost equally as potent as orexin-A in elevating cAMP (pEC(50) = approximately 500 nm). cAMP elevation was not caused by Ca(2+) elevation or by Gbetagamma. In contrast, it relied in part on a novel protein kinase C (PKC) isoform, PKCdelta, as determined using pharmacological inhibitors. Yet, PKC stimulation alone only very weakly stimulated cAMP production (1.1-fold). In the presence of G(s) activity, orexins still elevated cAMP; however, the potencies were greatly increased (EC(50) of orexin-A = approximately 10 nm and EC(50) of orexin-B = approximately 100 nm), and the response was fully dependent on PKCdelta. In permeabilized cells, only a PKC-independent low potency component was seen. This component was sensitive to anti-Galpha(s) antibodies. We conclude that OX(1) receptors stimulate adenylyl cyclase via a low potency G(s) coupling and a high potency phospholipase C --> PKC coupling. The former or some exogenous G activation is essentially required for the PKC to significantly activate adenylyl cyclase. The results also suggest that orexin-B-activated OX(1) receptors couple to G(s) almost as efficiently as the orexin-A-activated receptors, in contrast to Ca(2+) elevation and phospholipase C activation, for which orexin-A is 10-fold more potent.

G16-mediated activation of nuclear factor kappaB by the adenosine A1 receptor involves c-Src, protein kinase C, and ERK signaling

The G(i)-linked adenosine A1 receptor has been shown to mediate anti-inflammatory actions, possibly via modulation of the transcription factor nuclear factor-kappaB (NFkappaB). Here we demonstrate that an adenosine A1 agonist, N(6)-cyclohexyladenosine (CHA), activated IKKalpha/beta phosphorylation through PTX-insensitive G proteins in human lymphoblastoma Reh cells. To delineate the mechanism of action, different PTX-insensitive G proteins were expressed in human embryonic kidney 293 cells. Only Galpha(16) supported the CHA-induced IKK phosphorylation and NFkappaB-driven luciferase activity in time-dependent, dose-dependent, and PTX-insensitive manners. Gbetagamma subunits also modulated IKK/NFkappaB, as indicated by the stimulatory actions of Gbeta(1)gamma(2) and the abrogation of CHA-induced response by transducin. The participation of phospholipase Cbeta, protein kinase C, and calmodulin-dependent kinase II in CHA-induced IKK/NFkappaB activation were demonstrated by employing specific inhibitors and dominant-negative mutants. Inhibition of c-Src and numerous intermediates along the extracellular signal-regulated (ERK) kinase cascade including Ras, Raf-1 kinase, and MEK1/2 abolished the CHA-induced IKK/NFkappaB activation. Although c-Jun N-terminal kinase and p38 MAPK were also activated by CHA, they were not required for the IKK/NFkappaB regulation. Similar results were obtained using Reh cells. These data suggest that the G(16)-mediated activation of IKK/NFkappaB by CHA required a complex signaling network composed of multiple intermediates.