COX-2 is not involved in thromboxane biosynthesis by activated human platelets

Area of Science:

• Biochemistry
• Pharmacology
• Cardiovascular Medicine

Background:

• Aspirin resistance, observed in patients with ischemic vascular syndromes, has been linked to the cyclooxygenase-2 (COX-2) enzyme.
• The cellular source of COX-2 implicated in aspirin resistance remains unclear, with recent reports suggesting its presence in human platelets.

Purpose of the Study:

• To investigate the potential role of cyclooxygenase-2 (COX-2) expression in platelet thromboxane (TX) biosynthesis and its contribution to aspirin resistance.
• To compare the inhibitory effects of selective COX inhibitors on prostanoid biosynthesis in platelets versus monocytes.

Main Methods:

• Compared the inhibitory effects of NS-398 (selective COX-2 inhibitor) and aspirin (selective COX-1 inhibitor) on thrombin-stimulated platelets and lipopolysaccharide (LPS)-stimulated monocytes.
• Utilized Western blot techniques to detect COX-1 and COX-2 expression in isolated platelets and monocytes.

Main Results:

• Aspirin effectively inhibited platelet thromboxane B2 (TXB2) production, while NS-398 showed minimal effect on platelet TXB2.
• Monocytes, known to express high levels of COX-2, were potently inhibited by NS-398, but not significantly affected by aspirin.
• Western blot analysis failed to detect COX-2 in platelets, though COX-1 was readily present.

Conclusions:

• The findings argue against the involvement of cyclooxygenase-2 (COX-2) in thromboxane biosynthesis by activated platelets.
• Platelet COX-2 expression is unlikely to be a significant mechanism underlying aspirin resistance in patients with ischemic vascular syndromes.