No abstract

The tertiary structure for the region 1-63 of the 74 amino acid human complement protein C5a in solution was calculated from a large number of distance constraints derived from nuclear Overhauser effects with an angular distance geometry algorithm. The protein consists of four helices juxtaposed in an approximately antiparallel topology connected by peptide loops located at the surface of the molecule. The structures obtained for the helices are compatible with alpha-helical hydrogen-bonding patterns, which provides an explanation for the observed slow solvent exchange kinetics of the amide protons in these peptide regions. In contrast to the peptide region 1-63, no defined structure could be assigned to the C-terminal region 64-74, which increasingly acquires dynamic random coil characteristics as the end of the peptide chain is approached. An average root-mean-square deviation of 1.6 A was obtained for the alpha-carbons of the first 63 residues in the calculated ensemble of C5a structures, while the alpha-helices were determined with an average root-mean-square deviation of 0.8 A for the alpha-carbons. A comparison between the solution structure of C5a and the crystal structure of the functionally related C3a protein, as well as inferences for the interaction of C5a with its receptor on polymorphonuclear leukocytes, is discussed.

Engagement of the T cell antigen receptor (TcR)1 with the antigen-major histocompatibility complex on antigen-presenting cells triggers a complex TcR signaling cascade that leads to T cell activation and cytokine secretion (1). During this process, T cells express the autocrine growth factor interleukin 2 (IL-2), which promotes T cell proliferation by interacting with the IL-2 receptor, which is also up-regulated on activated T cells. The transcriptional regulation of the IL-2 gene has been extensively analyzed at the IL-2 promoter, a 275-bp region located upstream of the transcriptional start site of the gene (2, 3). Several transcription factors have been identified to bind elements within this regulatory region, including AP-1, NF-B, and the nuclear factor of activated T cells (NFAT) (2).The transcription factor NFAT plays an essential role in IL-2 expression. Binding sites for NFATs have also been found within the promoter regions of several other cytokine genes, including IL-3, IL-4, IL-5, IL-8, IL-13, tumor necrosis factor ␣, granulocyte-macrophage colony-stimulating factor, and ␥-IFN (4, 5). NFAT is a complex composed of a cytoplasmic subunit and an inducible nuclear component comprised of AP-1 (Fos/ Jun) family members. At least four structurally related NFAT cytoplasmic subunit members, NFATp/NFAT1, NFATc/ NFAT2, NFAT3, and NFATX/NFATc3/NFAT4, have been identified (5). NFAT proteins share a conserved domain located toward the C terminus (6) that binds DNA and also participates in cooperative protein-protein interactions with AP-1 transcription factors (7,8). Immediately N-terminal to the DNA-binding domain is a second conserved module of ϳ300 residues known as the NFAT homology (NFAT-h) region. The N terminus of NFAT, including the NFAT-h region, regulates nuclear/cytoplasm trafficking in response to changes in intracellular Ca 2ϩ concentrations. In resting T cells, the protein is retained in the cytoplasm and its NFAT-h domain is heavily phosphorylated. Engagement of the TcR or treatment of cells with the Ca 2ϩ ionophore activates the Ca 2ϩ /calmodulin-dependent Ser/Thr phosphatase, calcineurin. CaN dephosphorylates the NFAT-h domain, resulting in translocation of NFAT to the nucleus (9).

C5a is an inflammatory mediator potentially involved in a number of diseases. To help define which of its 74 residues are important for receptor binding and response triggering, changes in the amino acid sequence of C5a were introduced by site-directed mutagenesis. Synthetic C5a-encoding genes incorporating point mutations were expressed in Escherichia coli, and the mutant proteins were purified to homogeneity. Modifications of the C5a molecule causing parallel reductions in binding to polymorphonuclear leukocyte membranes and in stimulation of polymorphonuclear leukocyte locomotion (chemokinesis) suggest that carboxyl-terminal residues Lys-68, Leu-72, and Arg-74 interact with the receptor. Substitutions in the disulfide-linked core of C5a revealed involvement of Arg-40 or nearby residues, because potency losses were associated with only localized conformational changes as detected by NMR. Surprisingly, a substitution at core residue Ala-26, which did not alter C5a core structure, appeared from NMR results to reduce potency by causing a long-distance conformational change centered on residue His-15. Thus, at least three discontinuous regions of the C5a molecule appear to act in concert to achieve full potency.

A series of bis(trifluoromethyl)pyrazoles (BTPs) has been found to be a novel inhibitor of cytokine production. Identified initially as inhibitors of IL-2 synthesis, the BTPs have been optimized in this regard and even inhibit IL-2 production with a 10-fold enhancement over cyclosporine in an ex vivo assay. Additionally, the BTPs show inhibition of IL-4, IL-5, IL-8, and eotaxin production. Unlike the IL-2 inhibitors, cyclosporine and FK506, the BTPs do not directly inhibit the dephosphorylation of NFAT by calcineurin.

Rapamycin, a macrolide antibiotic known to prevent allograft rejection, is a potent inhibitor of cell proliferation. Therefore we studied the effects of orally administered rapamycin in a pig model of balloon injury in an attempt to reduce the cellular proliferation and neointimal formation thought to play a role in restenosis. Twenty Yucatan minipigs, divided into groups of 10 animals each, were subjected to balloon inflation of the carotid arteries. One group received the methylcellulose vehicle for rapamycin, whereas the second group was treated for a total of 31 days with 2.0 mg/kg of rapamycin administered daily by oral gavage. This dose and treatment regimen produced significant (p < 0.05) reductions in neointimal area (59%) and in the maximal thickness of the neointima (59%) when comparisons were made with vehicle-treated animals. These effects were accompanied by a significant increase in the lumen area in animals that received rapamycin (33%). Medial area was decreased by 18% in these animals. Blood samples from rapamycin-treated pigs indicated peak concentrations of 1.87 +/- 0.45 and 1.70 +/- 0.24 ng/ml at 2 and 4 weeks after balloon angioplasty, respectively. Significant increases in blood pressure of 21 mm Hg and decreases in heart rate of 25 beats/min also were observed in rapamycin-treated animals relative to those that received vehicle. These results indicate that the antiproliferative effect of rapamycin can be demonstrated after oral dosing in a pig vascular injury model, suggesting a possible therapeutic utility for rapamycin or its analogs in patients undergoing balloon angioplasty.

No abstract

Incubation of recombinant human C5a (rC5a) with the 7360 strain of group B streptococci (GBS) destroyed the ability of rC5a to stimulate chemotaxis or adherence of purified human polymorphonuclear leucocytes (PMNs). Treatment of 125I-labelled rC5a with GBS 7360 correspondingly decreased rC5a binding to human PMNs. This also resulted in an approx. 600 Da decrease in the molecular mass of rC5a as determined by SDS/PAGE. Incubation of rC5a with the GBS strain GW, which only minimally altered the ability of rC5a to activate human PMNs, did not affect rC5a binding to PMNs and did not alter the molecular mass of rC5a on SDS/PAGE. Plasma-desorption m.s. of rC5a inactivated by GBS 7360 showed that the GBS cleaved the rC5a between histidine-67 and lysine-68 near the C-terminus of rC5a. This mechanism of inactivation of C5a by proteolytic cleavage at the C-terminus of C5a is consistent with the known critical role of the C-terminus in C5a activation of human PMNs. This C5a-cleaving proteinase activity may contribute to the pathophysiology of GBS infections.