Unfractionated and Low-Molecular-Weight Heparin and the Phosphodiesterase Inhibitors, IBMX and Cilostazol, Block Ex Vivo Equid Herpesvirus Type-1-Induced Platelet Activation

Equid herpes virus type-1 (EHV-1) is a major pathogen of horses, causing abortion storms and outbreaks of herpes virus myeloencephalopathy. These clinical syndromes are partly attributed to ischemic injury from thrombosis in placental and spinal vessels. The mechanism of thrombosis in affected horses is unknown. We have previously shown that EHV-1 activates platelets through virus-associated tissue factor-initiated thrombin generation. Activated platelets participate in thrombus formation by providing a surface to localize coagulation factor complexes that amplify and propagate thrombin generation. We hypothesized that coagulation inhibitors that suppress thrombin generation (heparins) or platelet inhibitors that impede post-receptor thrombin signaling [phosphodiesterase (PDE) antagonists] would inhibit EHV-1-induced platelet activation ex vivo. We exposed platelet-rich plasma (PRP) collected from healthy horses to the RacL11 abortigenic and Ab4 neuropathogenic strains of EHV-1 at 1 plaque-forming unit/cell in the presence or absence of unfractionated heparin (UFH), low-molecular-weight heparin (LMWH) or the PDE inhibitors, 3-isobutyl-1methylxanthine (IBMX), and cilostazol. We assessed platelet activation status in flow cytometric assays by measuring P-selectin expression. We found that all of the inhibitors blocked EHV-1- and thrombin-induced platelet activation in a dose-dependent manner. Platelet activation in PRP was maximally inhibited at concentrations of 0.05 U/mL UFH and 2.5 μg/mL LMWH. These concentrations represented 0.1–0.2 U/mL anti-factor Xa activity measured in chromogenic assays. Both IBMX and cilostazol showed maximal inhibition of platelet activation at the highest tested concentration of 50 μM, but inhibition was lower than that seen with UFH and LMWH. Our results indicate that heparin anticoagulants and strong non-selective (IBMX) or isoenzyme-3 selective (cilostazol) PDE antagonists inhibit ex vivo EHV-1-induced platelet activation. These drugs have potential as adjunctive therapy to reduce the serious complications associated with EHV-1-induced thrombosis. Treatment trials are warranted to determine whether these drugs yield clinical benefit when administered to horses infected with EHV-1.

Phosphodiesterases as targets for intermittent claudication

Intermittent claudication (IC) is one of the most frequent forms of lower extremity peripheral arterial disease (PAD) and is most commonly caused by arterial atherosclerosis. Its clinical manifestation includes fatigue, discomfort, or pain occurring in limb muscles due to exercise-induced ischemia, thus limiting the ability of IC patients to walk and exercise. In addition to lifestyle changes (diet, exercise, and smoking cessation), pharmacological treatments are needed. Pathologically, atherosclerotic lesions cause a mismatch in oxygen supply and metabolic demand in the leg muscles during walking/exercise. This subjects the muscles to repeated ischemia and reperfusion injury that can alter structure and oxidative metabolism, resulting in insufficient utilization of oxygen supply. Despite extensive research efforts, cilostazol and pentoxifylline are the only drugs indicated for relieving the symptoms of IC, with cilostazol demonstrating significant improvement in walking distance and quality of life in these patients. Originally developed as a PDE3 inhibitor, cilostazol was later found to have several other pharmacological actions, and its success has been attributed to its multifactorial actions on platelets, endothelium, smooth muscle, and lipid profiles. Using cilostazol as an example, we discuss the rationales and pitfalls of targeting PDEs in IC, and potential strategies for the development of new and more effective pharmacological treatments.

CXCL8 attenuates chemoattractant-induced equine neutrophil migration

The chemokine, CXCL8, is a potent chemoattractant but it has also been shown to attenuate the migratory response of human neutrophils to the bacterial peptide, FMLP; this could lead to retention of cells in infected tissue and, potentially, to enhanced clearance of bacteria. This study has examined the effect of CXCL8 on equine neutrophil migration and adherence in response to PAF and LTB(4), chemoattractants that may play a role in non-infectious inflammatory conditions of the horse associated with neutrophil recruitment to the target tissue. The effects of CXCL8 on PAF- and LTB(4)-induced responses were determined using a ChemoTx plate migration assay and by measuring adhesion to protein-coated plastic. The CXCR1/2 antagonist, SB225002, was used to investigate whether the observed effects were receptor mediated and the role of cAMP was examined by measuring intracellular cAMP following exposure to agonists alone and in combination and by establishing the effect of dibutyryl cAMP on neutrophil migration. CXCL8, LTB(4) and PAF each induced migration and adhesion. Exposure of neutrophils to a combination of CXCL8 and PAF reduced the magnitude of the responses to that of unstimulated cells. In contrast, although the effect was less than additive, the response to co-stimulation with CXCL8 and LTB(4) were not nearly as pronounced. CXCL8 acted in a receptor mediated manner, the attenuation of PAF-induced responses being reversed by SB225002 at a concentration that blocks CXCR2. CXCL8, PAF and LTB(4) alone increased intracellular cAMP. In co-incubation studies, combination of CXCL8 with PAF led to an additive increase in cAMP whereas no increase above that obtained in response to LTB(4) alone was seen. Dibutyryl cAMP significantly reduced neutrophil migration in response to either CXCL8 or PAF alone. These results demonstrate that CXCL8, in addition to being a potent chemoattractant and pro-adhesive molecule for equine neutrophils, is able to attenuate responses to PAF and, to a much lesser extent, LTB(4). This effect, which appears to be CXCR2-mediated and cAMP dependent, could lead in vivo to trapping of cells at sites of inflammation resulting potentially in either enhanced clearance of injurious stimuli or increased local tissue damage by activated cells.

Regulation of platelet activating factor-induced equine platelet activation by intracellular kinases

Lipopolysaccharide (LPS) can activate equine platelets directly or indirectly, via leukocyte-derived platelet activating factor (PAF). Thromboxane (Tx) production by LPS-stimulated equine platelets requires p38 MAPK and this kinase has been suggested as a therapeutic target in endotoxaemia. The present study has utilised selective inhibitors to investigate the role of p38 MAPK and two other kinases, phosphatidylinositol-3 kinase (PI3K) and protein kinase C (PKC), in regulating PAF-induced Tx production, aggregation and 5-HT release in equine platelets, and the modification of these responses by LPS. LPS enhanced PAF-induced 5-HT release, an effect that was reduced by the p38 MAPK inhibitor, SB203580 (60 +/- 8% reduction; n = 6). SB203580 did not affect responses to PAF alone; whereas inhibition of PKC reduced PAF-induced 5-HT release, Tx production and aggregation (maximal inhibition by the PKCdelta inhibitor, rottlerin: 69 +/- 13%, 63 +/- 14% and 97 +/- 1%, respectively; n = 6). Wortmannin and LY249002, which inhibit PI3K, also caused significant inhibition of PAF-induced aggregation (maximal inhibition 78 +/- 3% and 88 +/- 2%, respectively; n = 6). These data suggest that inhibition of platelet p38 MAPK may be of benefit in equine endotoxaemia by counteracting some of the effects of LPS. However, detrimental effects of platelet activation mediated by PAF and not enhanced by LPS are unlikely to be markedly affected.