Circulating oxidized low-density lipoprotein and common carotid artery intima-media thickness in a random sample of middle-aged men

Circulating oxidized low-density lipoprotein (oxLDL) has been suggested to play an important role in atherosclerosis development. According to previous observations, oxLDL correlates with clinically manifest coronary and carotid artery disease. We investigated the association between the oxLDL concentration measured directly in plasma and common carotid artery intima-media thickness (IMT) in a population-based, case-control study in middle-aged men from Southern Finland. oxLDL was determined in 214 men by a commercially available sandwich ELISA test (Mercodia). Carotid artery IMT was measured at 12 standardized segments by B-mode ultrasonography (at the near and far wall of the left and right common carotid arteries, bifurcations and internal carotid arteries), and the overall mean maximum IMT (MMaxIMT) was calculated. The MMaxIMT of the carotid arteries was significantly associated with circulating oxLDL (r_s=0.16, p=0.018). In a stepwise multiple regression model with MMaxIMT as dependent variable and systolic blood pressure, smoking, oxLDL, HDL cholesterol and apolipoprotein B as covariates, systolic blood pressure (=0.22, p<0.001), oxLDL (=0.15, p=0.022) and smoking (=0.17, p=0.014) showed an independent association with IMT (R²=0.10, p<0.001). Our results show that oxLDL measured directly from plasma is independently associated with subclinical carotid artery atherosclerosis in middle-aged men.     

Beacon interacts with cdc2/cdc28-like kinases

Previously we found elevated beacon gene expression in the hypothalamus of obese Psammomys obesus. Beacon administration into the lateral ventricle of P. obesus stimulated food intake and body weight gain. In the current study we used yeast two-hybrid technology to screen for proteins in the human brain that interact with beacon. CLK4, an isoform of cdc2/cdc28-like kinase family of proteins, was identified as a strong interacting partner for beacon. Using active recombinant proteins and a surface plasmon resonance based detection technique, we demonstrated that the three members of this subfamily of kinases (CLK1, 2, and 4) all interact with beacon. Based on the known sequence and functional properties of beacon and CLKs, we speculate that beacon could either modulate the function of key regulatory molecules such as PTP1B or control the expression patterns of specific genes involved in the central regulation of energy metabolism.     

Structure of homo- and hetero-oligomeric meprin metalloproteases. Dimers,tetramers, and high molecular mass multimers

Meprin A and B, metalloproteases consisting of evolutionarily related alpha and/or beta subunits, are membrane-bound and secreted enzymes expressed by kidney and intestinal epithelial cells, leukocytes, and cancer cells. Previous work established that the multidomain meprin subunits (each approximately 80 kDa) form disulfide-bridged homo- and heterodimers, and differ in substrate and peptide bond specificities. The work herein clearly demonstrates that meprin dimers differ markedly in their ability to oligomerize. Electrophoresis, light scattering, size exclusion chromatography, and electron microscopy were used to characterize quaternary structures of recombinant rat meprins. Meprin B, consisting of meprin beta subunits only, was dimeric under a wide range of conditions. By contrast, meprin alpha homodimers formed heterogeneous multimers (ring-, circle-, spiral-, and tube-like structures) containing up to 100 subunits, with molecular masses at protein peaks ranging from approximately 1.0 to 6.0 MDa. The size of the meprin alpha homo-oligomers was dependent on protein concentration, ionic strength, and activation state. Meprin alphabeta heterodimers tended to form tetramers but not higher oligomers. Thus, the presence of meprin beta, which has a transmembrane domain in vivo, restricts the oligomerization potential of meprin molecules and localizes meprins to the plasma membrane. By contrast, the propensity of secreted meprin alpha homodimers to self-associate concentrates proteolytic potential into high molecular mass multimers and thus allows for autocompartmentalization. The work indicates that different mechanisms exist to localize and concentrate the proteolytic activity of membrane-bound and secreted meprin metalloproteinases.     

Probing the active sites and mechanisms of rat metalloproteases meprin A and B

Meprin A and B are highly regulated, secreted and cell-surface homo- and hetero-oligomeric enzymes. Meprins are abundantly expressed in kidney and intestine. The multidomain alpha and beta subunits have high sequence identity, however they have very different substrate specificities, oligomerization potentials and are differentially regulated. Here we describe that meprin subunit activities are modulated differently by physico-chemical factors. Homo-oligomeric meprin B had an acidic pH optimum. The low pH protonation indicated the existence of at least two ionizable groups. An additional ionizable group generated a shoulder in the basic pH range. Homo-oligomeric meprin A had a neutral pH optimum and the activity curve revealed that two ionizable groups might be protonated at acidic pH similar to meprin B. Increasing the concentration of salt generally inhibited meprin B activity. Meprin A was inhibited at low salt concentrations but activated as salt was increased. This work has important implications in the elucidation of the catalytic mechanisms of meprins and other metalloproteases. In addition, the activity of meprin oligomers that arise in tissues will be affected by variations in pH and NaCl. This could have profound implications because meprins are exposed to a range of conditions in the extracellular milieu of renal and intestinal tissues and in inflammation and cancer.     

Zinc ligands in an astacin family metalloprotease meprin A

A conserved tyrosine residue in the 'astacin family' of metalloproteases is one of five ligands proposed to coordinate zinc at the active site. Site-directed mutagenesis of the conserved Tyr (Y226) of recombinant mouse meprin alpha was used to test the hypothesis that this residue is essential for zinc binding and enzymatic activity. In addition, another proposed zinc binding ligand, H167, in the conserved (HEXXH) zinc binding motif of the meprin alpha protease domain was replaced by an alanine residue. Both mutants were expressed and secreted with the same subunit mass as wild type (90 kDa). The Y226F mutant retained the capacity to oligomerize to higher covalently and noncovalently-linked oligomers as the wild type, whereas H167A was predominantly a monomer. The kcat/Km for Y226F against a fluorgenic bradykinin substrate analog was approximately 15% of the wild type, while the H167A mutant had no detectable activity. Both Y226F and H167A were more susceptible to extensive degradation by trypsin compared with the wild-type protein. The zinc content in the wild-type and Y226F mutant proteins were similar, one molecule of zinc per subunit. The results indicate that Y226 is not essential for zinc binding, but Y226 and H167 are essential for full enzymatic activity and stability of the metalloproteinase.     

Molecular organization of amyloid protofilament-like assembly of betabellin 15D: helical array of beta-sandwiches

Betabellin is a 32-residue peptide engineered to fold into a four-stranded antiparallel beta-sheet protein. Upon air oxidation, the betabellin peptides can fold and assemble into a disulfide-bridged homodimer, or beta-sandwich, of 64 residues. Recent biophysical and ultrastructural studies indicate that betabellin 15D (B15D) (a homodimer of HSLTAKIpkLTFSIAphTYTCAVpkYTAKVSH, where p = DPro, k = DLys, and h = DHis) forms unbranched, 35-A wide assemblies that resemble the protofilaments of amyloid fibers. In the present study, we have analyzed in detail the X-ray diffraction patterns of B15D prepared from acetonitrile. The fiber diffraction analysis indicated that the B15D fibril was composed of a double helix defined by the selection rule l = n + 7m (where l is even, and n and m are any integers), and having a 199-A period and pitch, 28-A rise per unit, and 10-A radius. This helical model is equivalent to a reverse-handed, single helix with half the period and defined by the selection rule l = -3n + 7m (where l is any integer). The asymmetric unit is the single B15D beta-sandwich molecule. These results suggest that the betabellin assembly that models the protofilaments of amyloid fibers is made up of discrete subunits on a helical array. Multiple intersheet hydrogen bonds in the axial direction and intersandwich polar interactions in the lateral direction stabilize the array.     

Meprin proteolytic complexes at the cell surface and in extracellular spaces

Meprins are metalloproteinases of the astacin family and metzincin superfamily that are composed of evolutionarily related alpha and beta subunits, which exist as homo- and hetero-oligomeric complexes. These complexes are abundant at the brush border membranes of kidney proximal tubule cells and epithelial cells of the intestine, and are also expressed in certain leucocytes and cancer cells. Meprins cleave bioactive peptides such as gastrin, cholecystokinin and parathyroid hormone, cytokines such as osteopontin and monocyte chemotactic peptide-1, as well as proteins such as gelatin, collagen IV, fibronectin and casein. Database predictions and initial data indicate that meprins are also capable of shedding proteins, including itself, from the cell surface. Membrane-bound meprin subunits are composed of dimeric meprin beta subunits or tetrameric hetero-oligomeric alpha beta complexes of approx. 200-400 kDa, and can be activated at the cell surface; secreted forms of homo-oligomeric meprin alpha are zymogens that form high-molecular-mass complexes of 1-6 MDa. These are among the largest extracellular proteases identified thus far. The latent (self-associating) homo-oligomeric complexes can move through extracellular spaces in a non-destructive manner, and deliver a concentrated form of the metalloproteinase to sites that have activating proteases, such as sites of inflammation, infection or cancerous growth. Meprins provide examples of novel ways of concentrating proteolytic activity at the cell surface and in the extracellular milieu, which may be critical to proteolytic function.     

A novel function for tissue inhibitor of metalloproteinases-3 (TIMP3): inhibition of angiogenesis by blockage of VEGF binding to VEGF receptor-2

Tissue inhibitor of metalloproteinases-3 (TIMP3) is one of four members of a family of proteins that were originally classified according to their ability to inhibit matrix metalloproteinases (MMP). TIMP3, which encodes a potent angiogenesis inhibitor, is mutated in Sorsby fundus dystrophy, a macular degenerative disease with submacular choroidal neovascularization. In this study we demonstrate the ability of TIMP3 to inhibit vascular endothelial factor (VEGF)-mediated angiogenesis and identify the potential mechanism by which this occurs: TIMP3 blocks the binding of VEGF to VEGF receptor-2 and inhibits downstream signaling and angiogenesis. This property seems to be independent of its MMP-inhibitory activity, indicating a new function for this molecule.