Ischemia modified albumin in normal pregnancy and preeclampsia.

OBJECTIVE: Ischemia-modified albumin (IMA) has emerged as a new biomarker of myocardial ischemia. Currently, no information is available on maternal IMA levels during normal and complicated pregnancy. Preeclampsia is associated with ischemia and increased formation of free radicals in the placenta. We therefore hypothesized that production of IMA may occur in women with preeclampsia. METHODS: Serum IMA and albumin concentrations were assessed in 12 patients with preeclampsia, 12 normal pregnant controls, and 12 nonpregnant controls. IMA levels were compared between groups and corrected for albumin by multivariate regression analysis. RESULTS: Mean IMA levels were elevated in normal pregnant controls (107.3 U/mL; 95% CI, 102.5 to 112.01), compared with nonpregnant controls (94.5 U/mL; CI, 89.4 to 99.6; p = 0.015). In patients with preeclampsia, IMA levels were similar to those in normal pregnant controls (109.7 U/mL; CI, 102.2 to 117.2; p = 0.65). Also, no difference in IMA levels was observed between women with preeclampsia who delivered small-for-gestational-age (SGA) infants (99.0 U/mL; CI, 87.9 to 110.1; p = 0.13) and women with preeclampsia but without SGA. CONCLUSION: Serum IMA, which has been advocated as a clinical marker of cardiac ischemia, appears to be elevated during normal pregnancy. We found no significant relationship between IMA levels and preeclampsia, in women with or without SGA infants.

Effect of oxidation on the platelet-activating properties of low-density lipoprotein.

OBJECTIVE: Because of the large variation in oxidizing procedures and susceptibility to oxidation of low-density lipoprotein (LDL) and the lack in quantification of LDL oxidation, the role of oxidation in LDL-platelet contact has remained elusive. This study aims to compare platelet activation by native LDL (nLDL) and oxidized LDL (oxLDL). METHODS AND RESULTS: After isolation, nLDL was dialyzed against FeSO4 to obtain LDL oxidized to well-defined extents varying between 0% and >60%. The oxLDL preparations were characterized with respect to their platelet-activating properties. An increase in LDL oxidation enhances platelet activation via 2 independent pathways, 1 signaling via p38(MAPK) phosphorylation and 1 via Ca2+ mobilization. Between 0% and 15% oxidation, the p38(MAPK) route enhances fibrinogen binding induced by thrombin receptor (PAR-1)-activating peptide (TRAP), and signaling via Ca2+ is absent. At >30% oxidation, p38(MAPK) signaling increases further and is accompanied by Ca2+ mobilization and platelet aggregation in the absence of a second agonist. Despite the increase in p38(MAPK) signaling, synergism with TRAP disappears and oxLDL becomes an inhibitor of fibrinogen binding. Inhibition is accompanied by binding of oxLDL to the scavenger receptor CD36, which is associated with the fibrinogen receptor, alpha(IIb)beta3. CONCLUSIONS: At >30% oxidation, LDL interferes with ligand binding to integrin alpha(IIb)beta3, thereby attenuating platelet functions.

Platelet endothelial cell adhesion molecule-1 (PECAM-1) inhibits low density lipoprotein-induced signaling in platelets.

At physiological concentrations, low density lipoprotein (LDL) increases the sensitivity of platelets to aggregation- and secretion-inducing agents without acting as an independent activator of platelet functions. LDL sensitizes platelets by inducing a transient activation of p38MAPK, a Ser/Thr kinase that is activated by the simultaneous phosphorylation of Thr180 and Tyr182 and is an upstream regulator of cytosolic phospholipase A2 (cPLA2). A similar transient phosphorylation of p38MAPK is induced by a peptide mimicking amino acids 3359-3369 in apoB100 called the B-site. Here we report that the transient nature of p38MAPK activation is caused by platelet endothelial cell adhesion molecule 1 (PECAM-1), a receptor with an immunoreceptor tyrosine-based inhibitory motif. PECAM-1 activation by cross-linking induces tyrosine phosphorylation of PECAM-1 and a fall in phosphorylated p38MAPK and cPLA2. Interestingly, LDL and the B-site peptide also induce tyrosine phosphorylation of PECAM-1, and studies with immunoprecipitates indicate the involvement of c-Src. Inhibition of the Ser/Thr phosphatases PP1/PP2A (okadaic acid) makes the transient p38MAPK activation by LDL and the B-site peptide persistent. Inhibition of Tyr-phosphatases (vanadate) increases Tyr-phosphorylated PECAM-1 and blocks the activation of p38MAPK. Together, these findings suggest that, following a first phase in which LDL, through its B-site, phosphorylates and thereby activates p38MAPK, a second phase is initiated in which LDL activates PECAM-1 and induces dephosphorylation of p38MAPK via activation of the Ser/Thr phosphatases PP1/PP2A.

Site-specific phosphorylation of platelet focal adhesion kinase by low-density lipoprotein.

Focal adhesion kinase (FAK) is a non-receptor tyrosine kinase implicated in signalling pathways mediated by integrins and G-protein-coupled receptors (GPCRs). Upon stimulation FAK is phosphorylated on six tyrosine residues. Here we report the site-specific phosphorylation by low-density lipoprotein (LDL), which is known to induce integrin-independent FAK phosphorylation, and compare this with the effect of thrombin, which phosphorylates FAK via integrin alphaIIbbeta3. Stimulation with LDL reveals (i) a major role for Tyr-925 phosphorylation which surpasses the phosphorylation of the other residues, including Tyr-397, in rate and extent, (ii) alphaIIbbeta3-independent phosphorylation of Tyr-925 and Tyr-397, and (iii) complex formation between FAK and the Src-kinase Fgr but not with c-Src. These patterns differ profoundly from those induced by thrombin. LDL-induced phosphorylation of Tyr-925 and Tyr-397 was inhibited by 60-75% by receptor-associated protein, an inhibitor of members of the LDL receptor family. Thus these findings reveal a novel mechanism of FAK phosphorylation by signalling cascades involving a member of the LDL receptor family.