Rhodopsin phosphorylation by transiently expressed human beta ARK1: a new method for drug development

Beta-arrestin 2 modulates resveratrol-induced apoptosis and regulation of Akt/GSK3ß pathways

BACKGROUND

Resveratrol is emerging as a novel anticancer agent. However, the mechanism(s) by which resveratrol exerts its effects on endometrial cancer (EC) are unknown. We previously reported that beta-arrestin 2 plays a critical role in cell apoptosis. The role of ß-arrestin 2 in resveratrol modulation of endometrial cancer cell apoptosis remains to be established.

SCOPE OF REVIEW

EC cells HEC1B and Ishikawa were transfected with either ß-arrestin 2 RNA interfering (RNAi) plasmid or beta-arrestin 2 full-length plasmid and control vector. The cells were then exposed to differing concentrations of resveratrol. Apoptotic cells were detected by TUNEL assay. Expression of total and phosphorylated Akt (p-Akt), total and phosphorylated glycogen synthase kinase 3 beta (p-GSK3ß), and caspase-3 were determined by Western blot analysis. Our data demonstrate that inhibition of ß-arrestin 2 increases the number of apoptotic cells and caspase-3 activation. Additionally ß-arrestin 2 exerted an additive effect on resveratrol-reduced levels of p-Akt and p-GSK3ß. Overexpression of ß-arrestin 2 decreased the percentage of apoptosis and caspase-3 activation and attenuated resveratrol-reduced levels of p-Akt and p-GSK3ß. Taken together, our studies demonstrate for the first time that ß-arrestin 2 mediated signaling plays a critical role in resveratrol-induced apoptosis in EC cells.

MAJOR CONCLUSIONS

Resveratrol primes EC cells to undergo apoptosis by modulating beta-arrestin 2 mediated Akt/GSK3ß signaling pathways.

GENERAL SIGNIFICANCE

These inspiring findings would provide a new molecular basis for further understanding of cell apoptotic mechanisms mediated by ß-arrestin 2 and may provide insights into a potential clinical relevance in EC.

Involvement of beta-arrestin-2 in modulation of the spinal antinociception induced by mu-opioid receptor agonists in the mouse

Beta-arrestins have been suggested to regulate mu-, delta-, and kappa-opioid receptor-mediated responses. In the present study, we examined the effects of pretreatment with beta-arrestin-2 antibody on tail-flick inhibition induced by opioid receptor agonists in the mouse spinal cord. Intrathecal (i.t.) pretreatment with beta-arrestin-2 antibody potentiated the antinociception induced by i.t.-administered mu-opioid receptor agonists [D-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) and endomorphin-1, but not endomorphin-2, the delta-opioid receptor agonist [D-Ala(2)]deltorphin II or the kappa-opioid receptor agonist U50,488H. The present result suggests that beta-arrestin-2 may tonically down-regulate a selected population of mu-opioid receptors activated by endomorphin-1 or DAMGO in the mouse spinal cord.

Direct and differential interaction of beta-arrestins with the intracellular domains of different opioid receptors

beta-arrestins have been shown to play important roles in regulation of signaling and desensitization of opioid receptors in many in vivo studies. The current study was carried out to measure the direct interaction of beta-arrestins with two functional intracellular domains, the third intracellular loop (I3L) and the carboxyl terminus (CT), of delta-, mu-, and kappa-opioid receptors (DOR, MOR, and KOR, respectively). Results from the pull-down assay using glutathione S-transferase fusion proteins demonstrated that beta-arrestins (1 and 2) were able to bind to the I3L of DOR and to the CT of DOR and KOR. Surface plasmon resonance measurement gave similar results with typical dissociation equilibrium constant (K(D)) values in the micromolar range. The site-directed mutagenesis experiment further revealed that certain specific serine/threonine residues in these receptor domains play a critical role in their interaction with beta-arrestins. Taken together, our data clearly indicated that beta-arrestins interact differentially with the functional domains of different opioid receptors; this may provide a possible molecular basis for differential regulation of opioid receptors by beta-arrestins.

Direct binding of beta-arrestins to two distinct intracellular domains of the delta opioid receptor

beta-Arrestins regulate opioid receptor-mediated signal transduction and play an important role in opiate-induced analgesia and tolerance/dependence. This study was carried out to measure the direct interaction between beta-arrestins and opioid receptor. Immunoprecipitation experiments demonstrated that beta-arrestin 1 physically interacts with delta opioid receptor (DOR) co-expressed in human embryonic kidney 293 cells in an agonist-enhanced manner and truncation of the carboxyl terminus of DOR partially impairs the interaction. In vitro data from glutathione-S-transferase pull-down assay showed that the carboxyl terminus (CT) and the third intracellular loop (I3L) of DOR are both capable of and either domain is sufficient for binding to beta-arrestin 1 and 2. Surface plasmon resonance determination further revealed that binding of CT and I3L of DOR to beta-arrestin is additive, suggesting these two domains bind at distinctly different sites on beta-arrestin without considerable spatial hindrance. This study demonstrated for the first time the direct binding of beta-arrestins to the two distinct domains, the carboxyl terminus and the third intracellular loop, of DOR.

beta-arrestin differentially regulates the chemokine receptor CXCR4-mediated signaling and receptor internalization, and this implicates multiple interaction sites between beta-arrestin and CXCR4

The chemokine receptor CXCR4 has recently been shown to be a co-receptor involved in the entry of human immunodeficiency virus type 1 into target cells. This study shows that coexpression of beta-arrestin with CXCR4 in human embryonic kidney 293 cells attenuated chemokine-stimulated G protein activation and inhibition of cAMP production. Truncation of the C-terminal 34 amino acids of CXCR4 (CXCR4-T) abolished the effects of beta-arrestin on CXCR4/G protein signaling, indicating the functional interaction of the receptor C terminus with beta-arrestin. On the other hand, receptor internalization and the subsequent activation of extracellular signal-regulated kinases were significantly promoted by coexpression of beta-arrestin with CXCR4, whereas the C-terminal truncation of CXCR4 did not affect this regulation of beta-arrestin, suggesting that beta-arrestin can functionally interact with CXCR4 with or without the C terminus. Moreover, beta(2)V54D, the dominant inhibitory mutant of beta-arrestin 2, exerted no effects on CXCR4/G protein signaling, but strongly influenced receptor internalization and extracellular signal-regulated kinase activation. Further cross-linking experiments demonstrated that beta-arrestin as well as beta(2)V54D could physically contact both CXCR4 and CXCR4-T. Glutathione S-transferase pull-down assay showed that beta-arrestin was able to bind efficiently in vitro to both the third intracellular loop and the 34-amino acid C terminus of CXCR4. Taken together, our data clearly establish that beta-arrestin can effectively regulate different functions of CXCR4 and that this is mediated through its distinct interactions with the C terminus and other regions including the third loop of CXCR4.

Decreased expression and activity of G-protein-coupled receptor kinases in peripheral blood mononuclear cells of patients with rheumatoid arthritis

Beta2-Adrenergic and chemokine receptor antagonists delay the onset and reduce the severity of joint injury in rheumatoid arthritis. beta2-Adrenergic and chemokine receptors belong to the G-protein-coupled receptor family whose responsiveness is turned off by the G-protein-coupled receptor kinase family (GRK-1 to 6). GRKs phosphorylate receptors in an agonist-dependent manner resulting in receptor/G-protein uncoupling via subsequent binding of arrestin proteins. We assessed the activity of GRKs in lymphocytes of rheumatoid arthritis (RA) patients by rhodopsin phosphorylation. We found a significant decrease in GRK activity in RA subjects that is mirrored by a decrease in GRK-2 protein expression. Moreover, GRK-6 protein expression is reduced in RA patients whereas GRK-5 protein levels were unchanged. In search of an underlying mechanism, we demonstrated that proinflammatory cytokines induce a decrease in GRK-2 protein levels in leukocytes from healthy donors. Since proinflammatory cytokines are abundantly expressed in RA, it may provide an explanation for the decrease in GRK-2 expression and activity in patients. No changes in beta2-adrenergic receptor number and Kd were detected. However, RA patients showed a significantly increased cAMP production and inhibition of TNF-alpha production by beta2-adrenergic stimulation, suggesting that reduced GRK activity is associated with increased sensitivity to beta2-adrenergic activation.