Thromboxane A2 synthase activity in platelet free human monocytes

Area of Science:

• Immunology
• Biochemistry
• Molecular Biology

Background:

• Human monocytes produce thromboxane A2 (TXA2) upon activation by serum, with highest amounts detected soon after platelets.
• The precise enzymatic steps and regulatory mechanisms governing TXA2 synthesis in monocytes remain incompletely understood.

Purpose of the Study:

• To investigate whether the levels of prostaglandin H synthase (PGH synthase) or TXA2 synthase limit the conversion of arachidonate to TXA2 in human monocytes.
• To elucidate the expression and activity patterns of PGH synthase and TXA2 synthase during monocyte activation.

Main Methods:

• Utilized platelet-free human monocytes incubated with fetal calf serum (FCS) or recombinant colony-stimulating factor-1 (hr CSF-1).
• Employed transcriptional inhibitors, Western blotting, and enzyme assays using PGH2 as a substrate to assess PGH synthase and TXA2 synthase levels and activity.
• Investigated the role of de novo synthesis by using actinomycin D and cycloheximide.

Main Results:

• Fetal calf serum (FCS) induces PGH synthase in monocytes after 16 hours, an effect partially mimicked by hr CSF-1.
• TXA2 synthase is constitutively expressed in monocytes with high levels present upon isolation and after serum incubation.
• TXA2 synthesis is delayed (onset at 3 hours) and strictly dependent on the induction kinetics of PGH synthase, not TXA2 synthase levels.

Conclusions:

• The rate-limiting step for TXA2 synthesis in serum- or CSF-1-stimulated monocytes is the de novo synthesis of PGH synthase.
• TXA2 synthase is constitutively expressed and stable in monocytes, with its gene actively transcribed in resting cells.
• Monocyte activation by serum or CSF-1 leads to TXA2 production solely through the induction of PGH synthase.