BASAL EXPRESSION AND INSULIN-MEDIATED INDUCTION OF PAI-1 MESSENGER-RNA IN HEP G2 CELLS

We have explored the effect of insulin on the modulation of mRNA and antigen levels of components of the fibrinolytic enzyme system in human hepatoma (Hep G2) and fibrosarcoma (HT-1080) cells. Treatment of Hep G2 cells with a physiological concentration of insulin provokes a 2-fold increase in plasminogen activator inhibitor (PAI)-1 antigen. A similar increase is observed for both the 3.2 and 2.3kb transcripts of PAI-1 mRNA. Cycloheximide when added alone to cells preferentially suppresses constitutive expression of the 3.2kb PAI-1 mRNA and blocks insulin-mediated induction of both 3.2 and 2.3kb PAI-1 mRNA. There was no detectable expression of urokinase (u-PA) or tissue-type plasminogen activator (t-PA) antigen or mRNA in these cells under constitutive conditions or after treatment with insulin. In HT-1080 cells, basal expression of PAI-1, t-PA and u-PA mRNA and antigen was not modulated by insulin treatment. The results from these experiments indicate firstly, that induction of PAI-1 by insulin requires on-going protein biosynthesis, and secondly, that the two species of PAI-1 mRNA have a different requirement for protein biosynthesis.