Serine 232 of the alpha(2A)-adrenergic receptor is a protein kinase C-sensitive effector coupling switch

A2B adenosine receptor activation and modulation by protein kinase C

Protein kinase C (PKC) isoforms regulate many important signaling pathways. Here, we report that PKC activation by phorbol 12-myristate 13-acetate (PMA) enhanced A2B adenosine receptor (AR)-mediated, but not β2-adrenergic receptor-mediated, cAMP accumulation, in H9C2 cardiomyocyte-like and HEK293 cells. In addition to enhancement, PKC (PMA-treatment) also activated A2BAR with low Emax (H9C2 and NIH3T3 cells endogenously expressing A2BAR), or with high Emax (A2BAR-overexpressing HEK293 cells) to induce cAMP accumulation. A2BAR activation induced by PKC was inhibited by A2BAR and PKC inhibitors but enhanced by A2BAR overexpression. Gαi isoforms and PKCγ isoform were found to be involved in both enhancement of A2BAR function and A2BAR activation. Thus, we establish PKC as an endogenous modulator and activator of A2BAR, involving Giα and PKCγ. Depending on signaling pathway, PKC could activate and enhance, or alternatively inhibit A2BAR activity. These findings are relevant to common functions of A2BAR and PKC, e.g. cardioprotection and cancer progression/treatment.

Regulation of alpha2AR trafficking and signaling by interacting proteins

The continuing discovery of new G protein-coupled receptor (GPCR) interacting proteins and clarification of the functional consequences of these interactions has revealed multiple roles for these events. Some of these interactions serve to scaffold GPCRs to particular cellular micro-compartments or to tether them to defined signaling molecules, while other GPCR-protein interactions control GPCR trafficking and the kinetics of GPCR-mediated signaling transduction. This review provides a general overview of the variety of GPCR-protein interactions reported to date, and then focuses on one prototypical GPCR, the alpha(2)AR, and the in vitro and in vivo significance of its reciprocal interactions with arrestin and spinophilin. It seems appropriate to recognize the life and career of Arthur Hancock with a summary of studies that both affirm and surprise our preconceived notions of how nature is designed, as his career-long efforts similarly affirmed the complexity of human biology and attempted to surprise pathological changes in that biology with novel, discovery-based therapeutic interventions. Dr. Hancock's love of life, of family, and of commitment to making the world a better place are a model of the life well lived, and truly missed by those who were privileged to know, and thus love, him.

Impaired trafficking of choline transporter-like protein-1 at plasma membrane and inhibition of choline transport in THP-1 monocyte-derived macrophages

The present study investigates choline transport processes and regulation of choline transporter-like protein-1 (CTL1) in human THP-1 monocytic cells and phorbol myristate 13-acetate (PMA)-differentiated macrophages. Choline uptake is saturable and therefore protein-mediated in both cell types, but its transport characteristics change soon after treatments with PMA. The maximal rate of choline uptake intrinsic to monocytic cells is greatly diminished in differentiated macrophages as demonstrated by alterations in Vmax values from 1,973 ± 118 to 380 ± 18 nmol·mg−1·min−1, when the binding affinity did not change significantly ( Km values 56 ± 8 and 53 ± 6 μM, respectively). Treatments with hemicholinim-3 effectively inhibit most of the choline uptake, establishing that a choline-specific transport protein rather than a general transporter is responsible for the observed kinetic parameters. mRNA screening for the expression of various transporters reveals that CTL1 is the most plausible candidate that possesses the described kinetic and inhibitory properties. Fluorescence-activated cell sorting analyses at various times after PMA treatments further demonstrate that the disappearance of CTL1 protein from the cell surface follows the same trend as the reduction in choline uptake. Importantly, the loss of functional CTL1 from the cell surface occurs without significant changes in total CTL1 protein or its mRNA level indicating that an impaired CTL1 trafficking is the key contributing factor to the reduced choline uptake, subsequent to the PMA-induced THP-1 differentiation to macrophages.

Biochemical and pharmacological control of the multiplicity of coupling at G-protein-coupled receptors

For decades, it has been generally proposed that a given receptor always interacts with a particular GTP-binding protein (G-protein) or with multiple G-proteins within one family. However, for several G-protein-coupled receptors (GPCR), it now becomes generally accepted that simultaneous functional coupling with distinct unrelated G-proteins can be observed, leading to the activation of multiple intracellular effectors with distinct efficacies and/or potencies. Multiplicity in G-protein coupling is frequently observed in artificial expression systems where high densities of receptors are obtained, raising the question of whether such complex signalling reveals artefactual promiscuous coupling or is a genuine property of GPCRs. Multiple biochemical and pharmacological evidence in favour of an intrinsic property of GPCRs were obtained in recent studies. Thus, there are now many examples showing that the coupling to multiple signalling pathways is dependent on the agonist used (agonist trafficking of receptor signals). In addition, the different couplings were demonstrated to involve distinct molecular determinants of the receptor and to show distinct desensitisation kinetics. Such multiplicity of signalling at the level of G-protein coupling leads to a further complexity in the functional response to agonist stimulation of one of the most elaborate cellular transmission systems. Indeed, the physiological relevance of such versatility in signalling associated with a single receptor requires the existence of critical mechanisms of dynamic regulation of the expression, the compartmentalisation, and the activity of the signalling partners. This review aims at summarising the different studies that support the concept of multiplicity of G-protein coupling. The physiological and pharmacological relevance of this coupling promiscuity will be discussed.

Allosteric activation of plasma membrane receptors--physiological implications and structural origins

Allosteric modulation of receptors has physiological not just pharmacological significance. Thus, the chemical context in which an agonist signal is received can have a major impact on the nature of the physiological response by modifying receptor sensitivity and/or maximal activity-even the nature of the signalling response. In addition, recognising that an endogenous activator is the allosteric modulator of a known receptor, rather than the agonist of a novel receptor, has the potential to solve, in dramatic fashion, key physiological questions. What is an allosteric modulator and why are allosteric effects on receptors so diverse and frequently complex? What is the scope of allosteric effects? Can the existence of endogenous modulators be predicted from a receptor's amino acid sequence? How should screening for endogenous allosteric modulators be undertaken? These questions form the framework of this mini-review on physiological and structural aspects of receptor allostery.

Phosphorylation of Ser360 in the third intracellular loop of the alpha2A-adrenoceptor during protein kinase C-mediated desensitization

The alpha2A-adrenoceptor undergoes desensitization in response to protein kinase C activation. Using mutagenesis and recombinant expression we sought to determine the specific sites within the receptor which are phosphorylated by protein kinase C and are responsible for this desensitization. Ser/Thr in the third intracellular loop were substituted with Ala to create mutant receptors T272A, S258A, S324A and S360A. These mutations had no effect on ligand binding or functional coupling to inositol phosphate accumulation and intracellular Ca2+ release. Three of the four mutant receptors displayed wild-type desensitization (approximately 60% loss of function) in response to 0.1 microM phorbol-12-myristate-13-acetate (PMA) exposure for 2 min. However, the S360A mutant had only approximately 24% desensitization. In whole cell phosphorylation studies, S360A failed to undergo detectable PMA promoted phosphorylation. We conclude that protein kinase C-mediated desensitization of alpha2A-adrenoceptor function is primarily due to phosphorylation of Ser at amino acid 360. This thus represents one mechanism by which these receptors undergo regulation by heterologous means, such as pathologic processes which activate protein kinase C or crosstalk with other receptors.