Summary
We have investigated the intracellular mechanisms involved in microtubular remodelling
by thrombin and its possible involvement in platelet aggregation and secretion. Platelet
stimulation with thrombin induces a time- and concentration-dependent regulation of
the microtubular content, which was found to be maximally effective at the concentration
0.1 U/ml. Thrombin (0.1 U/ml) evoked an initial decrease in the microtubule content
detectable at 5 seconds (sec) and reached a minimum 10 sec after stimulation.The microtubular
content then increased, exceeding basal levels again approximately 30 sec after stimulation.
Inhibition of tyrosine phosphatases using vanadate abolished thrombin-induced microtubular
depolymerisation while inhibition of tyrosine kinases by methyl-2,5-dihydroxycinnamate
prevented microtubule polymerisation.Thrombin activates the cytosolic Brutons tyrosine
kinase (Btk) and Src proteins. Inhibition of Btk or Src by LFM-A13 or PP1, respectively,
abolished thrombin-induced microtubular polymerisation, while maintaining intact its
ability to induce initial depolymerisation. Microtubular disruption by colchicine
significantly reduced thrombin induced platelet aggregation and ATP secretion. Similar
results were observed after inhibition of microtubular disassembly by paclitaxel.These
findings indicate that thrombin induces microtubular remodelling by modifying the
balance between protein tyrosine phosphorylation and dephosphorylation. The former
seems to be required for microtubular polymerisation, while tyrosine dephosphorylation
is required for microtubular depolymerisation. Both, initial microtubular disassembly
and subsequent polymerisation are required for thrombin-induced platelet aggregation
and secretion in human platelets.
Keywords
Thrombin - microtubules - aggregation - platelets - pp60
src
- Btk - protein kinases - protein phosphatases - ATP