Smooth muscle cell phenotypic transition associated with calcification: upregulation of Cbfa1 and downregulation of smooth muscle lineage markers.

Bovine aortic smooth muscle cell (BASMC) cultures undergo mineralization on addition of the organic phosphate donor, beta-glycerophosphate (betaGP). Mineralization is characterized by apatite deposition on collagen fibrils and the presence of matrix vesicles, as has been described in calcified vascular lesions in vivo as well as in bone and teeth. In the present study, we used this model to investigate the molecular mechanisms driving vascular calcification. We found that BASMCs lost their lineage markers, SM22alpha and smooth muscle alpha-actin, within 10 days of being placed under calcifying conditions. Conversely, the cells gained an osteogenic phenotype as indicated by an increase in expression and DNA-binding activity of the transcription factor, core binding factor alpha1 (Cbfa1). Moreover, genes containing the Cbfa1 binding site, OSE2, including osteopontin, osteocalcin, and alkaline phosphatase were elevated. The relevance of these in vitro findings to vascular calcification in vivo was further studied in matrix GLA protein null (MGP(-/-)) mice whose arteries spontaneously calcify. We found that arterial calcification was associated with a similar loss in smooth muscle markers and a gain of osteopontin and Cbfa1 expression. These data demonstrate a novel association of vascular calcification with smooth muscle cell phenotypic transition, in which several osteogenic proteins including osteopontin, osteocalcin, and the bone determining factor Cbfa1 are gained. The findings suggest a positive role for SMCs in promoting vascular calcification.

Mapping of MCP-1 functional domains by peptide analysis and site-directed mutagenesis.

Monocyte chemoattractant protein-1 (MCP-1) is a member of the beta chemokine family which acts through specific seven transmembrane receptors to recruit monocytes, basophils, and T lymphocytes to sites of inflammation. To identify regions of the human MCP-1 protein which are important for its biological activity, we have synthesized domain-specific peptides and tested their ability to antagonize MCP-1 binding and chemotaxis in THP-1 cells. We have found that an intercysteine first loop peptide encompassing amino acids 13-35 inhibits MCP-1 binding and chemotactic activity, while peptides representing the amino-terminus (amino acids 1-10), second loop (amino acids 37-51), and carboxy-terminus (amino acids 56-71) of MCP-1 have no effect. In addition, we have found that cyclization of the first loop peptide by disulfide linkage and blocking the C-terminus of the peptide by amidation increases the activity of this peptide to block MCP-1 binding and chemotaxis. In order to specifically identify amino acid residues within the first loop that are crucial for MCP-1 functional activity, we have substituted alanine for tyrosine (Y13A) or arginine (R18A) in MCP-1 recombinant proteins. While baculovirus produced wild type and R18A MCP-1 proteins are indistinguishable in their ability to induce THP-1 chemotaxis and show modest effects in binding activity compared to commercially available recombinant MCP-1 protein, the Y13A point mutation causes a dramatic loss in function. The identification of functional domains of MCP-1 will assist in the design of MCP-1 receptor antagonists which may be clinically beneficial in a number of inflammatory diseases.

Wnt-mediated relocalization of dishevelled proteins.

The Wnt family of proto-oncogenes encodes secreted signaling proteins that are required for mouse development. The Drosophila Wnt homolog, the wingless (Wg) segment polarity gene, mediates a signal transduction pathway in which the downstream elements appear to be conserved through evolution. One such element, the dishevelled gene product, becomes hyperphosphorylated and translocates to the plasma membrane in response to Wg (Yanagawa et al., 1995). We report here that the mouse Dishevelled-1 (Dvl-1) and Dishevelled-2 genes encode proteins that are differentially localized in Wnt-overexpressing PC12 cell lines (PC12/Wnt). Whereas Dvl-1 and Dvl-2 proteins are limited to the soluble fraction of parental PC12 cells, PC12/Wnt cells display a subset of Dvl-1 protein associated with the membrane and Dvl-2 protein with the cytoskeletal fraction. These results suggest a conserved role for Dvl in Wnt/wg signal transduction.

Mammalian prenylcysteine carboxyl methyltransferase is in the endoplasmic reticulum.

Prenylcysteine carboxyl methyltransferase (pcCMT) is the third of three enzymes that posttranslationally modify C-terminal CAAX motifs and thereby target CAAX proteins to the plasma membrane. Here we report the molecular characterization and subcellular localization of the first mammalian (human myeloid) pcCMT. The deduced amino acid sequence of mammalian pcCMT predicts a multiple membrane-spanning protein with homologies to the yeast pcCMT, STE14, and the mammalian band 3 anion transporter. The human gene complemented a ste14 mutant. pcCMT mRNAs were ubiquitously expressed in human tissues. An anti-pcCMT antiserum detected a 33-kDa protein in myeloid cell membranes. Ectopically expressed recombinant pcCMT had enzymatic activity identical to that observed in neutrophil membranes. Mammalian pcCMT was not expressed at the plasma membrane but rather restricted to the endoplasmic reticulum. Thus, the final enzyme in the sequence that modifies CAAX motifs is located in membranes topologically removed from the CAAX protein target membrane.