A sequence within the cytoplasmic tail of GpIIb independently activates platelet aggregation and thromboxane synthesis

Chemoselective acylation of fully deprotected hydrazino acetyl peptides. Application to the synthesis of lipopetides

Fully deprotected N-terminal alpha-hydrazino acetyl peptides were synthesized and chemoselectively acylated on the hydrazine moiety with various fatty acid succinimidyl esters or N-(cholesterylcarbonyloxy) succinimide to give lipopeptides of high purity. The buffer and pH were adjusted in order to minimize the oxidation of the hydrazine moiety and to achieve the best conversion and selectivity. The acylation was performed in a citrate-phosphate buffer/2-methylpropan-2-ol mixture of pH 5.1. The pKa of the alpha-hydrazino acetyl group on our model peptide was found to be 6.45, i.e., about 2 units lower than the pKa of a glycyl residue. The reaction was subsequently applied to the synthesis of a 38AA peptide derivatized by a palmitoyl group.

The platelet integrin alpha IIbbeta 3 has an endogenous thiol isomerase activity

Integrins are cysteine-rich heterodimeric cell-surface adhesion molecules that alter their affinity for ligands in response to cellular activation. The molecular mechanisms involved in this activation of integrins are not understood. Treatment with the thiol-reducing agent, dithiothreitol, can induce an activation-like state in many integrins suggesting that cysteine-cysteine dithiol bonds are important for the receptor's tertiary structure and may be involved in activation-induced conformational changes. Here we demonstrate that the platelet-specific integrin, alpha(IIb)beta(3), contains an endogenous thiol isomerase activity, predicted from the presence of the tetrapeptide motif, CXXC, in each of the cysteine-rich repeats of the beta(3) polypeptide. This motif comprises the active site in enzymes involved in disulfide exchange reactions, including protein-disulfide isomerase (EC ) and thioredoxin. Intrinsic thiol isomerase activity is also observed in the related integrin, alpha(v)beta(3), which shares a common beta-subunit. Thiol isomerase activity within alpha(IIb)beta(3) is time-dependent and saturable, and is inhibited by the protein-disulfide isomerase inhibitor, bacitracin. Furthermore, this activity is calcium-sensitive and is regulated in the EDTA-stabilized conformation of the integrin. This novel demonstration of an enzymatic activity associated with an integrin subunit suggests that altered thiol bonding within the integrin or its substrates may be locally modified during alpha(IIb)beta(3) activation.

Evidence of platelet activation during treatment with a GPIIb/IIIa antagonist in patients presenting with acute coronary syndromes

OBJECTIVES

The study was done to determine the role of partial agonist activity in the lack of effectiveness of the oral GPIIb/IIIa antagonist orbofiban.

BACKGROUND

Orbofiban, an oral GPIIb/IIIa antagonist, was found to increase the mortality of patients with acute coronary syndromes (ACS) in the OPUS-TIMI-16 trial, despite the fact that it is a very potent anti-platelet agent and that IV agents have proven very effective.

METHODS

Patients (n = 520) with ACS were randomized to orbofiban 30 mg, 40 mg or 50 mg twice daily or 50 mg once daily or placebo. Platelet activity was assessed in 175 patients by examining GPIIb/IIIa receptor conformation, expression of CD63 antigen, and platelet aggregation.

RESULTS

Plasma concentrations of orbofiban at the highest dose (74 +/- 6 ng/ml peak, 61 +/- 5 ng/ml trough) exceeded the IC50 for platelet aggregation to adenosine diphosphate (ADP) (29 +/- 6 ng/ml) and thrombin-activating peptide (61 +/- 18 ng/ml). Orbofiban induced a conformational change in GPIIb/IIIa detected as the displacement of the monoclonal antibody mAb2; such conformational changes have been linked to partial agonist activity. Consistent with this, platelet expression of CD63 ex vivo was significantly increased at five time points during the study. In vitro, orbofiban increased platelet aggregation to a submaximal concentration of epinephrine (67 +/- 19% vs. 27 +/- 9%, n = 5) and increased thromboxane formation when the platelet GPIIb/IIIa were clustered using monoclonal antibodies to the receptor.

CONCLUSIONS

Orbofiban is both an antagonist and a partial agonist of platelet GPIIb/IIIa. At low concentrations of the drug, this partial agonist activity may enhance platelet aggregation. Along with suboptimal plasma drug levels, these findings may help explain the lack of efficacy seen with orbofiban in patients with ACS.

A structural basis for integrin activation by the cytoplasmic tail of the alpha IIb-subunit

A key step in the activation of heterodimeric integrin adhesion receptors is the transmission of an agonist-induced cellular signal from the short alpha- and/or beta-cytoplasmic tails to the extracellular domains of the receptor. The structural details of how the cytoplasmic tails mediate such an inside-out signaling process remain unclear. We report herein the NMR structures of a membrane-anchored cytoplasmic tail of the alpha(IIb)-subunit and of a mutant alpha(IIb)-cytoplasmic tail that renders platelet integrin alpha(IIb)beta(3) constitutively active. The structure of the wild-type alpha(IIb)-cytoplasmic tail reveals a "closed" conformation where the highly conserved N-terminal membrane-proximal region forms an alpha-helix followed by a turn, and the acidic C-terminal loop interacts with the N-terminal helix. The structure of the active mutant is significantly different, having an "open" conformation where the interactions between the N-terminal helix and C-terminal region are abolished. Consistent with these structural differences, the two peptides differ in function: the wild-type peptide suppressed alpha(IIb)beta(3) activation, whereas the mutant peptide did not. These results provide an atomic explanation for extensive biochemical/mutational data and support a conformation-based "on/off switch" model for integrin activation.

Extension of HLA-A*0201-restricted minimal epitope by N epsilon-palmitoyl-lysine increases the life span of functional presentation to cytotoxic T cells

The delineation of the minimal requirements for efficient delivery of functional cytotoxic epitopes into APC could be a step toward the definition of "minimal length" lipopeptides for the modulation of CTL activity. Several analogues of the HLA-A*0201-restricted HIV-1 polymerase (pol476-484) minimal cytotoxic epitope were obtained by modifying P0, P1, or P10 positions by a single N epsilon-palmitoyl-lysine residue. The use of fluorescent derivatives confirmed the cell-permeating activities and suggested that a P0- and a P1-modified lipopeptide possessing ionizable extremities fulfills the structural requirements for MHC loading. The expressions of HLA-peptide complexes at the surface of TAP-deficient cells incubated with the parent epitope or lipopeptide derivatives were compared, in terms of intensity and stability. Both lipopeptides induced a considerably prolonged expression of conformationally correct complexes, which were dependent on the integrity of the exocytosis pathway, suggesting a dynamic mechanism of formation or reloading of the complexes from an intracellular pool. The agonistic activities of the different HLA-peptide complexes were evaluated using two independent T cell lines from HIV-infected donors. We report that a lipodecapeptide obtained by N-terminal addition of a N epsilon-palmitoyl-lysine to the pol476-484 epitope was able to increase the life span of functional presentation to cytotoxic T cells specific for the parent peptide.

Integrin alpha(IIb)beta(3) signaling in platelet adhesion and aggregation

Intracellular signals are received and generated by the alpha(IIb)beta(3) integrin on platelets. Recent advances have been made in the areas of agonist receptors that initiate platelet activation, downstream signaling molecules (e.g. small G-proteins and kinases) and changes in ligand-occupied alpha(IIb)beta(3) that cause further signaling and clot retraction.