Role Of Regulator Of G Protein Signaling Proteins Research Articles

Altered expression and function of regulator of G-protein signaling-17 (RGS17) in hepatocellular carcinoma

Guanine nucleotide regulatory proteins (G-proteins) are central to normal hepatocyte function and are implicated in hepatic disease initiation and progression. Regulators of G-protein signaling (RGS) are critical to defining G-protein-dependent signal fidelity, yet the role of RGS proteins in the liver is poorly defined. The aims of this study were to determine RGS17 expression in normal and transformed hepatic tissue and cells, and address the function of RGS17 in hepatic tumorgenicity. RGS17 expression was determined in human and rat HCC tissue and cell lines. Molecular approaches were used to alter RGS17 expression in HCC cells, effects on cell function measured, and RGS17 association with specific Gα-subunits determined. Using these approaches RGS17 mRNA, but not protein, was detectable in human and rat HCC tissue and cells. Conversely, RGS17 mRNA was not detected in normal tissue, isolated hepatocytes, or non-tumorigenic hepatic cells. Subsequent studies using transfected cells demonstrated that RGS17 proteins were not post-translationally modified in HCC cells, and RGS17 expression is governed by protein degradation and not via miRNAs. Notwithstanding inherently low RGS17 protein levels, altering RGS17 expression profoundly affected HCC cell mitogenesis and migration. Analysis of RGS17-G-protein interaction demonstrated RGS17 associates with both Giα- and Gqα-subunits in HCC cells of human and rat origin. In conclusion, these data demonstrate that, despite difficulties in measuring endogenous RGS protein expression, RGS17 is differentially expressed in HCC and plays a central role in regulating transformed hepatocyte tumorgenicity.

A role of RGS proteins in drug addiction

The diverse family of Regulators of G protein signaling (RGS) proteins are widely distributed proteins with multiple functions, including GAP activity for heterotrimeric G protein alpha subunits. Three members of the RGS family, RGS9-2, RGS4 and RGSz, have been shown to play an essential modulatory role in psychostimulant and opiate drug actions. Interestingly, these proteins show distinct structure, distribution pattern and cellular localization. In addition, each of these proteins is differentially regulated by drugs of abuse in particular brain networks and appears to modulate distinct signal transduction events. The striatal enriched RGS9 plays a prominent role in opiate and psychostimulant drug reward; RGS4 appears to modulate opiate dependence via actions in the locus coeruleus, whereas RGSz modulates analgesia via activation of the PKC pathway.

Regulator of G Protein Signaling 2 (RGS2) Inhibits Intracellular Ca2+ Mobilization and RhoA Activation: Role in Regulation of Endothelial Permeability

The microvascular endothelial monolayer actively regulates the tissue fluid balance. Edemagenic agonists such as thrombin binds to protease activated receptor-1 (PAR-1) that activates Gq, G12 and G13 proteins, which in turn stimulates small GTPase RhoA and increase intracellular Ca2+ mobilization leading to increased endothelial permeability. Regulators of G protein signaling (RGS) bind to G subunits of G proteins and stimulate their GTPase activity, which ‘switch off’ G-protein-mediated signaling events. However, the role of RGS proteins in regulating endothelial permeability in response to G protein coupled receptor-agonists remains unclear. We investigated the possible role of RGS2, a GTPase activating protein (GAP) for Gq, in the mechanism of thrombin-induced increase in endothelial permeability. We showed, using human endothelial cells, that overexpression of RGS2 attenuated thrombin-induced increase in intracellular Ca2+ transients and RhoA activity, reduced actin stress fiber formation and suppressed endothelial permeability-increase in response to thrombin. Importantly, down regulation of endogenous RGS2 by siRNA potentiated RhoA activity as well as the increase in endothelial permeability after stimulation with thrombin. Thus, RGS2 is a critical regulator of endothelial barrier function because its GAP activity antagonizes Gq-induced increase in intracellular Ca2+ as well as RhoA activity tapering off thrombin-induced increase in endothelial permeability. ααα

Analyses of RGS Protein Control of Agonist-Evoked Ca2+ Signaling

Analysis of the function of regulator of G-protein signaling (RGS) protein function and their selectivity of action in vivo is complicated by the expression of multiple RGS proteins in a single cell and requires precise control of cytosolic RGS protein concentration. This article describes two experimental systems using pancreatic acinar cells suitable for such analyses. The first is pancreatic acini permeabilized with streptolysin O, which retains agonist responsiveness while allowing RGS proteins and molecules with molecular masses of up to 25-30 kDa access to the cytosol. The second is a whole cell recording of the Ca(2+)-activated Cl- current of single pancreatic acinar cells as a reporter of [Ca2+]i. This system can be used to introduce to the cytosol any protein of interest, including recombinant RGS proteins and RGS protein-scavenging antibodies. The use of these systems to study the specificity of RGS proteins action, the function of their domains, and the role of RGS proteins in controlling Ca2+ oscillations is discussed.

The regulator of G protein signaling family.

Regulator of G protein signaling (RGS) proteins are responsible for the rapid turnoff of G protein-coupled receptor signaling pathways. The major mechanism whereby RGS proteins negatively regulate G proteins is via the GTPase activating protein activity of their RGS domain. Structural and mutational analyses have characterized the RGS/G alpha interaction in detail, explaining the molecular mechanisms of the GTPase activating protein activity of RGS proteins. More than 20 RGS proteins have been isolated, and there are indications that specific RGS proteins regulate specific G protein-coupled receptor pathways. This specificity is probably created by a combination of cell type-specific expression, tissue distribution, intracellular localization, posttranslational modifications, and domains other than the RGS domain that link them to other signaling pathways. In this review we discuss what has been learned so far about the role of RGS proteins in regulating G protein-coupled receptor signaling and point out areas that may be fruitful for future research.