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- PublicationRap1GAP2 is a new GTPase-activating protein of Rap1 expressed in human plateletsThe Ras-like guanine-nucleotide-binding protein Rap1 controls integrin alpha(IIb)beta3 activity and platelet aggregation. Recently, we have found that Rap1 activation can be blocked by the nitric oxide/cyclic guanosine monophosphate (NO/cGMP) signaling pathway by type 1 cGMP-dependent protein kinase (cGKI). In search of possible targets of NO/cGMP/cGKI, we studied the expression of Rap1-specific GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs) in platelets. We could detect mRNAs for a new protein most closely related to Rap1GAP and for postsynaptic density-95 discs-large and zona occludens protein 1 (PDZ)-GEF1 and CalDAG-GEFs I and III. Using 5'-rapid amplification of cDNA ends (RACE), we isolated the complete cDNA of the new GAP encoding a 715-amino acid protein, which we have termed Rap1GAP2. Rap1GAP2 is expressed in at least 3 splice variants, 2 of which are detectable in platelets. Endogenous Rap1GAP2 protein partially colocalizes with Rap1 in human platelets. In transfected cells, we show that Rap1GAP2 exhibits strong GTPase-stimulating activity toward Rap1. Rap1GAP2 is highly phosphorylated, and we have identified cGKI as a Rap1GAP2 kinase. cGKI phosphorylates Rap1GAP2 exclusively on serine 7, a residue present only in the platelet splice variants of Rap1GAP2. Phosphorylation of Rap1GAP2 by cGKI might mediate inhibitory effects of NO/cGMP on Rap1. Rap1GAP2 is the first GTPase-activating protein of Rap1 found in platelets and is likely to have an important regulatory role in platelet aggregation.
345Scopus© Citations 73
- PublicationQuantitative analysis of the cardiac fibroblast transcriptome-implications for NO/cGMP signalingCardiac fibroblasts regulate tissue repair and remodeling in the heart. To quantify transcript levels in these cells we performed a comprehensive gene expression study using serial analysis of gene expression (SAGE). Among 110,169 sequenced tags we could identify 30,507 unique transcripts. A comparison of SAGE data from cardiac fibroblasts with data derived from total mouse heart revealed a number of fibroblast-specific genes. Cardiac fibroblasts expressed a specific collection of collagens, matrix proteins and metalloproteinases, growth factors, and components of signaling pathways. The NO/cGMP signaling pathway was represented by the mRNAs for α1 and β1 subunits of guanylyl cyclase, cGMP-dependent protein kinase type I (cGK I), and, interestingly, the G-kinase-anchoring protein GKAP42. The expression of cGK I was verified by RT-PCR and Western blot. To establish a functional role for cGK I in cardiac fibroblasts we studied its effect on cell proliferation. Selective activation of cGK I with a cGMP analog inhibited the proliferation of serum-stimulated cardiac fibroblasts, which express cGK I, but not higher passage fibroblasts, which contain no detectable cGK I. Currently, our data suggest that cGK I mediates the inhibitory effects of the NO/cGMP pathway on cardiac fibroblast growth. Furthermore the SAGE library of transcripts expressed in cardiac fibroblasts provides a basis for future investigations into the pathological regulatory mechanisms underlying cardiac fibrosis.
327Scopus© Citations 18
- PublicationThe NO/cGMP pathway inhibits Rap 1 activation in human platelets via cGMP-dependent protein kinase IThe NO/cGMP signalling pathway strongly inhibits agonist-induced platelet aggregation. However, the molecular mechanisms involved are not completely defined. We have studied NO/cGMP effects on the activity of Rap 1, an abundant guanine-nucleotidebinding protein in platelets. Rap 1-GTP levels were reduced by NO-donors and activators of NO-sensitive soluble guanylyl cyclase. Four lines of evidence suggest that NO/cGMP effects are mediated by cGMP-dependent protein kinase (cGKI): (i) Rap 1 inhibition correlated with cGKI activity as measured by the phosphorylation state of VASP, an established substrate of cGKI, (ii) 8-pCPT-cGMP, a membrane permeable cGMP-analog and activator of cGKI, completely blocked Rap1 activation, (iii) Rp-8pCPT-cGMPS, a cGKI inhibitor, reversed NO effects and (iv) expression of cGKI in cGKI-deficient megakaryocytes inhibited Rap1 activation. NO/cGMP/cGKI effects were independent of the type of stimulus used for Rap1 activation. Thrombin-,ADP- and collagen-induced formation of Rap 1-GTP in platelets as well as turbulence-induced Rap 1 activation in megakaryocytes were inhibited. Furthermore, cGKI inhibited ADP-induced Rap 1 activation induced by the Galpha(i)-coupled P2Y12 receptor alone, i.e. independently of effects on Ca2+-signalling. From these studies we conclude that NO/cGMP inhibit Rap 1 activation in human platelets and that this effect is mediated by cGKI. Since Rap1 controls the function of integrin alpha(IIb)beta3, we propose that Rap 1 inhibition might play a central role in the anti-aggregatory actions of NO/cGMP.
335Scopus© Citations 49