Induction of c-fos expression in rat vascular smooth muscle reporter cells by selective activation of the thrombin receptor

Receptor-selective effects of endogenous RGS3 and RGS5 to regulate mitogen-activated protein kinase activation in rat vascular smooth muscle cells

Regulators of G protein signaling (RGS) proteins compose a highly diverse protein family best known for inhibition of G protein signaling by enhancing GTP hydrolysis by Galpha subunits. Little is known about the function of endogenous RGS proteins. In this study, we used synthetic ribozymes targeted to RGS2, RGS3, RGS5, and RGS7 to assess their function. After demonstrating the specificity of in vitro cleavage by the RGS ribozymes, rat aorta smooth muscle cells were used for transient transfection with the RGS-specific ribozymes. RGS3 and RGS5 ribozymes differentially enhanced carbachol- and angiotensin II-induced MAP kinase activity, respectively, whereas RGS2 and RGS7 ribozymes had no effect. This enhancement was pertussis toxin-insensitive. Thus RGS3 is a negative modulator of muscarinic m3 receptor signaling, and RGS5 is a negative modulator of angiotensin AT1a receptor signaling through G(q/11). Also, RGS5 ribozyme enhanced angiotensin-stimulated inositol phosphate release. These results indicate the feasibility of using the ribozyme technology to determine the functional role of endogenous RGS proteins in signaling pathways and to define novel receptor-selective roles of endogenous RGS3 and RGS5 in modulating MAP kinase responses to either carbachol or angiotensin.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Characterization of gonadotropin-releasing hormone analogs based on a sensitive cellular luciferase reporter gene assay

A novel cellular assay for the functional characterization of agonistic and antagonistic analogs of gonadotropin-releasing hormone (GnRH) was developed. This assay is based on a fusion of the c-fos immediate-early gene promoter to Photinus pyralis luciferase (Luc) as a reporter gene, stably transfected in a recombinant cell line expressing the human GnRH receptor. Transcription of endogenous c-fos and fos-Luc fusion gene are transiently induced quite similar by fetal calf serum or the superagonistic analog [D-Trp6] GnRH in a selected cell line. The reporter gene was therefore used to monitor agonist-induced signaling via the human GnRH receptor. Whereas Luc activity was induced in a dose-dependent manner by GnRH or [D-Trp6] GnRH, different antagonistic peptides completely inhibited this stimulation. The antagonistic potency (IC50) of various peptides with Cetrorelix and Antarelix as lead compounds in general correlated well with the binding affinity (KD) as determined from ligand binding experiments. The specificity of an inhibitory effect was confirmed by GnRH receptor-independent stimulation with the phorbol ester 12-O-tetradecanoylphorbol 13-acetate or basic fibroblast growth factor. Since this new reporter gene assay is sensitive and simple and can be performed in a microtiter plate, it will significantly facilitate screening and functional characterization of GnRH analogs.