Regulation of plasminogen activator inhibitor mRNA levels in lipopolysaccharide-stimulated human monocytes. Correlation with production of the protein.

Abstract

Peripheral blood monocytes are essential participants in processes that require pericellular plasminogen activation, a regulated proteolytic pathway that is greatly influenced by the relative concentrations of urokinase-type plasminogen activator (profibrinolytic) and plasminogen activator inhibitor type 2 (PAI-2) (anti-fibrinolytic). Monocyte synthesis of these molecules is inducible by bacterial lipopolysaccharide (LPS) although PAI-2 production is regulated over a much wider concentration range than is urokinase-type PA. The PAI-2 response of LPS-stimulated monocytes was investigated and found to be biphasic, with a peak of mRNA at 4-6 h after stimulation, a decrement in mRNA levels at 8-10 h, and a secondary increase at 16 h. The primary (early phase) response was studied in detail wherein PAI-2 protein production was found to depend on the levels of PAI-2 mRNA. The profiles of steady-state PAI-2 mRNA levels and PAI-2 protein production were parallel with respect to LPS concentration, time of exposure to LPS, and persistence of the response. PAI-2 mRNA accumulation was inducible by cycloheximide but prevented by actinomycin D. The increase in steady-state PAI-2 mRNA was mediated both by an increase in gene transcription and by stabilization of the mRNA once formed. Therefore, the initial phase of PAI-2 production by LPS-stimulated monocytes is determined by the amount of PAI-2 mRNA in these cells; levels of PAI-2 mRNA are controlled by several mechanisms, allowing for rapid variations in production of this molecule.