TP receptor activation and inhibition in atherothrombosis: the paradigm of diabetes mellitus

Patients with type 2 diabetes mellitus are characterized by increased incidence of cardiovascular events and enhanced thromboxane-dependent platelet activation. Urinary enzymatic TXA(2) metabolites (such as 11-dehydro-TXB2), reflecting the whole TXA(2) biosynthesis by platelet and extra-platelet sources, are significantly increased in diabetes with the absolute post-aspirin values of 11-dehydro-TXB2 in diabetics being comparable to non-aspirated controls and such residual TXA(2) biosynthesis despite low-dose aspirin treatment is predictive of vascular events in high-risk patients. Thus, elevated urinary 11-dehydro-TXB2 levels identify patients who are partially insensitive to aspirin and who may benefit from alternative antiplatelet therapies or treatments that more effectively block in vivo TXA(2) production or activity. Potential mechanisms relatively insensitive to aspirin include extraplatelet, nucleate sources of TXA(2) biosynthesis, possibly triggered by inflammatory stimuli, or lipid peroxidation with enhanced generation of F2-isoprostane (reflecting ongoing in vivo oxidative stress) than can activate platelets via the platelet TP receptor thus escaping inhibition by aspirin. In fact, aspirin does not inhibit isoprostane formation. Moreover, intraplatelet or extraplatelet thromboxane generation may be only partly inhibited by aspirin under certain pathological conditions, at least at the usual low doses given for cardiovascular protection. TXA(2) receptors (TP) are expressed on several cell types and exert antiatherosclerotic, antivasoconstrictive and antithrombotic effects, depending on the cellular target. Thus, targeting TP receptor, a common downstream pathway for both platelet and extraplatelet TXA(2) as well as for isoprostanes, may be an useful antithrombotic intervention in clinical settings, such as diabetes mellitus characterized by persistently enhanced thromboxane-dependent platelet activation.