Regulation of Protein Kinases (Syk and PKC zeta) in platelets

Abstract

Platelets are crucial components of the hemostatic machinery of the body. When the endothelial continuity is disrupted due to injury or atherosclerotic plaque rupture, one of the earliest responses to arrest the bleeding is the adhesion of circulating platelets to the exposed subendothelial collagen matrix. Subsequent intracellular signaling mediated downstream of various receptor systems leads to alpha IIb beta 3 activation, thromboxane generation, ADP release, etc., culminating in platelet clot or thrombus formation. The protein kinase family of enzymes mediates a significant number of these intracellular signaling events that culminate in platelet activation. These enzymes can be broadly classified into two classes- tyrosine kinases and serine/threonine kinases. Syk (spleen tyrosine kinase) is an important non-receptor tyrosine kinase present in platelets and plays an important role downstream of GPVI-FcR gamma chain receptor complex activation. We studied the effects of curcumin (diferuloylmethane), which is the active ingredient found in the herbal remedy and food spice turmeric, on the GPVI-mediated platelet activation. We have found that it significantly inhibits the kinase activity of Syk without affecting its phosphorylation. Pre-incubating the platelets with curcumin for only a minute resulted in a concentration-dependent inhibition of aggregation and secretion, with approximately 75% inhibition observed at 50 mM curcumin. Additionally, the activation-dependent phosphorylation of tyrosines 753/759 on PLC gamma2 and phosphorylation of tyrosine 191 on the transmembrane scaffold protein LAT, were inhibited (p<0.05). However, the phosphorylation of the activation loop tyrosines 525/526 on Syk and of the tyrosine 145 on intracellular adaptor molecule SLP-76 were not significantly affected. Furthermore, the inhibitory action of curcumin on the catalytic activity of Syk was independent of any of its effects on the thromboxane generation because all our studies were performed using aspirin-treated platelets. PKC zeta is an atypical member of the PKC family of serine/threonine kinases. In this study, we have confirmed that it is expressed in human platelets and is constitutively phosphorylated at the activation loop threonine 410 as well as the turn motif threonine 560, which is an autophosphorylation site. Phosphorylation at these two residues has been shown to be important for its kinase activity. Furthermore, agonist-mediated platelet aggregation under stirring condition results in dephosphorylation of the Thr410 residue, which can be prevented by blocking integrin alpha IIb beta 3 by its antagonist SC-57101 (p<0.01). The dephosphorylation of Thr410 can also be prevented by okadaic acid, a Ser/Thr protein phosphatase inhibitor, at concentrations above 100 nM. However, in PP1c gamma null mice, we did not observe any effect on the dephosphorylation, suggesting that other isoforms of PP1 or other classes of the phosphatases could be responsible for this phenomenon, at least in these knockout mice. The basal phosphorylation of Thr560, however, remained unaffected by agonist stimulation, integrin activation, integrin blockade, okadaic acid treatment and in the PP1c gamma null mice. It can be speculated that PKC zeta may be constitutively active under basal resting conditions and acts as a negative regulator of platelet activation or functional responses. The Thr560 autophosphorylation signal alone may not be sufficient to sustain its full enzymatic activity.