GDI-1 phosphorylation switch at serine 96 induces RhoA activation and increased endothelial permeability

We identified the GDI-1-regulated mechanism of RhoA activation from the Rho-GDI-1 complex and its role in mediating increased endothelial permeability. Thrombin stimulation failed to induce RhoA activation and actin stress fiber formation in human pulmonary arterial endothelial cells transduced with full-length GDI-1. Expression of a GDI-1 mutant form (C-GDI) containing the C terminus (aa 69 to 204) also prevented RhoA activation, whereas further deletions failed to alter RhoA activation. We observed that protein kinase Calpha-mediated phosphorylation of the C terminus of GDI-1 at Ser96 reduced the affinity of GDI-1 for RhoA and thereby enabled RhoA activation. Rendering GDI-1 phosphodefective with a Ser96 --> Ala substitution rescued the inhibitory activity of GDI-1 toward RhoA but did not alter the thrombin-induced activation of other Rho GTPases, i.e., Rac1 and Cdc42. Phosphodefective mutant GDI-1 also suppressed myosin light chain phosphorylation, actin stress fiber formation, and the increased endothelial permeability induced by thrombin. In contrast, expressing the phospho-mimicking mutant S96D-GDI-1 protein induced RhoA activity and increased endothelial permeability independently of thrombin stimulation. These results demonstrate the crucial role of the phosphorylation of the C terminus of GDI-1 at S96 in selectively activating RhoA. Inhibiting GDI-1 phosphorylation at S96 is a potential therapeutic target for modulating RhoA activity and thus preventing the increase in endothelial permeability associated with vascular inflammation.

Structural characterization of the human carboxypeptidase D gene and its promoter

Human carboxypeptidase D (CPD) is a 180-kDa type I membrane protein with three tandem active site domains. CPD is a B-type (or kininase I-type) carboxypeptidase that cleaves C-terminal basic residues from proteins and peptides, such as Arg9 from bradykinin. The human carboxypeptidase D (CPD) gene was found to encompass approximately 88.3 kb of genomic sequence, containing 21 exons ranging in size from 65 to 1813 bp, and 21 introns ranging in size from 112 bp to 35.6 kb. Although CPD and CPM belong to the same metallocarboxypeptidase subfamily, their intron/exon structures differ significantly. Multiple transcription start sites were found in the CPD gene within a GC-rich sequence lacking the typical TATA box, but containing three GC boxes. Luciferase reporter assays with various size constructs containing the promoter region upstream of the start sites showed that it was active in three different cell lines, especially in the human hepatoma cell line HepG2 and the human monocytic cell line THP-1, which have high constitutive expression of CPD.

Structure of the human carboxypeptidase M gene. Identification of a proximal GC-rich promoter and a unique distal promoter that consists of repetitive elements

The human carboxypeptidase M (CPM) gene was found to encompass about 112.6 kb of genomic sequence, containing 11 exons of which eight (exons 2-9) are common to all transcripts and contain the entire coding region. We have cloned several alternative variants of CPM transcripts that result from differential promoter usage and alternative splicing. Although CPM belongs to the same metallocarboxypeptidase subfamily as CPE, their intron/exon structures differ significantly. Multiple transcription start sites were found in the CPM gene that cluster in two regions separated by about 30 kb and are flanked by two unique functional promoters. One ('proximal') is immediately upstream of the coding region and contains GC-rich sequences and a typical TATA box whereas the other ('distal') consists almost entirely of repetitive elements. Luciferase reporter assays with constructs of the promoter regions showed they were both quite active in several cell lines. However, the proximal promoter was much stronger than the distal one in two of the human cell lines tested (HepG2 and HEK293) whereas both promoters were highly and equally active in the human monocytic cell line THP-1, which has high constitutive expression of CPM.