Regulation of serotonin transport in human platelets by tyrosine kinase Syk.

Serotonin (5-hydroxytryptamine, 5-HT) is a neurotransmitter involved in the regulation of numerous neuro-physiological processes. The circulating level of 5-HT is regulated by the membrane transporter SERT present both in the presynaptic nerve terminals and blood platelets. 5-HT transport is a process tightly regulated by a variety of factors including protein phosphorylation. Aim of this study was to ascertain if also the SERT Tyr-phosphorylation mediated by Syk-kinase concurs to the regulation of SERT activity. Indeed we found that 5-HT uptake decreased upon platelet exposure to piceatannol or Syk-inhibitor II, two structurally unrelated inhibitors of the tyrosine-kinase Syk. Tyr-phosphorylation of anti-SERT-immuno-stained proteins in membrane extracts and in anti-SERT-immuno-precipitates, decreased upon platelet treatment with piceatannol, in parallel with a reduction of Syk-activity. Syk was immuno-revealed in the anti-SERT immuno-precipitates, which displayed a piceatannol-sensitive kinase activity towards SERT itself and the Syk-substrate α-sinuclein. Syk inhibitors also caused a decrease of the monensin-induced 5-HT-efflux from platelets and of imipramine binding to them. It is concluded that, in addition to the phosphorylation of SERT mediated by various other kinases, also that catalyzed by Syk might play an important role in the 5-HT transport, likely favoring the transporter conformation exposing the neurotransmitter binding sites.

Flavonoids diosmetin and hesperetin are potent inhibitors of cytochrome P450 2C9-mediated drug metabolism in vitro.

The aim of this study was to examine in vitro, by means of kinetic analysis and molecular docking simulations, the effects of the flavone diosmetin and its flavanone analog hesperetin on CYP (cytochrome P450) 2C9-mediated drug metabolism. To this purpose, the conversion of diclofenac to 4'-hydroxydiclofenac by human liver microsomes was used as a model assay for assessing the CYP2C9 inhibitory activity of these two flavonoids. Kinetic analyses showed that diosmetin and hesperetin were reversible, dead-end inhibitors of 4'-hydroxydiclofenac formation; their mean K(i) (inhibitor dissociation constant) values were 1.71 ± 0.58 and 21.50 ± 3.62 µM, respectively. Diosmetin behaved as a competitive inhibitor, since it increased markedly the K(m) (substrate concentration yielding 50% of V(max)) of the reaction without affecting the V(max) (maximum velocity of reaction). Hesperetin modified markedly K(m) and to a lesser extent also modified V(max), thus acting as a mixed competitive-noncompetitive inhibitor. The results of molecular docking simulations were consistent with those of kinetic analysis, since they showed that the putative binding sites of both diosmetin and hesperetin coincided with the CYP2C9 substrate binding site. The demonstration that diosmetin and hesperetin inhibit CYP2C9-mediated diclofenac metabolism at low micromolar concentrations is of potential clinical relevance because CYP2C9 is responsible for the biotransformation of various therapeutically important drugs that have narrow therapeutic indexes.

Serotonin (5-HT) transport in human platelets is modulated by Src-catalysed Tyr-phosphorylation of the plasma membrane transporter SERT.

BACKGROUND/AIM: platelets possess tightly regulated systems for serotonin (5-HT) transport. This study analysed whether the 5-HT transport mediated by the plasma-membrane transporter SERT is regulated by its Tyr-phosphorylation. METHODS: 5-HT transport was determined by filtration techniques, while immunoblotting procedures were adopted for detecting the Tyr-phosphorylation of SERT in human platelet fractions. RESULTS: 5-HT accumulation in platelets pre-treated with reserpine, which prevents the neurotransmitter transport into the dense granules, decreased upon cellular exposure to PP2 and SU6656, two structurally unrelated inhibitors of Src-kinases. By contrast, the protein Tyr-phosphatase inhibitor pervanadate increased the 5-HT accumulation. Anti-SERT immunostaining of the platelet fractions showed a major band displaying an apparent molecular mass of 50 kappaDa, indicating that, during the analytical procedure, SERT underwent proteolysis, which was counteracted by addition of 4 M urea in the cellular disrupting medium. The Tyr-phosphorylation degree of SERT immunoprecipitated from membrane extracts decreased by platelet treatment with SU6656 or PP2, and enhanced upon pervanadate treatment. The anti-SERT immunoprecipitates displayed anti-Src immunostaining and in vitro kinase activity towards a Src-specific peptide-substrate. Platelet treatment with PP2 or SU6656 also caused a decrease in the imipramine binding to platelets. It was concluded that the Src-mediated SERT Tyr-phosphorylation regulates the 5-HT transport by affecting the neurotransmitter binding sites.

Inhibitory effect by new monocyclic 4-alkyliden-beta-lactam compounds on human platelet activation.

In the present study some new beta-lactam compounds were screened for their ability to inhibit human platelet activation. In particular four compounds differing in the group on the nitrogen atom of the azetidinone ring were investigated. A beta-lactam having an ethyl 2-carboxyethanoate N-bound group was demonstrated to inhibit, in the micromolar range, both the Ca(2+) release from endoplasmic reticulum, induced either by thrombin or by the ATPase inhibitor thapsigargin, and the Ca(2+) entry in platelets driven by emptying the endoplasmic reticulum. The compound also inhibited the platelet aggregation induced by a variety of physiological agonists including ADP, collagen, thrombin and thrombin mimetic peptide TRAP. The beta-lactam reduced the phosphorylation of pleckstrin (apparent MW 47 kDa), elicited by thrombin but not by the protein kinase C activator phorbol ester. Accordingly it did not significantly affect the aggregation evoked by phorbol ester or Ca(2+) ionophore. It was concluded that the beta-lactam likely exerts its anti-platelet-activating action by hampering the agonist induced cellular Ca(2+) movements. The beta-lactam concentration, which significantly inhibited platelet activation, only negligibly affected the cellular viability. Even if it is still premature to draw definitive conclusions, the present results suggest that this new compound might constitute a tool of potential clinical interest and the starting-point for the synthesis of new more beneficial anti-thrombotic compounds.