Summary
Plasminogen activator inhibitor type-1 (PAI-1) is a physiologic modulator of the fibrinolytic
system. We have shown previously that PAI-1 biosynthesis in cultured cells depends
on several factors in serum. Because platelets are richly endowed with specific growth
factors and because the release reaction is an integral part of thrombosis, the present
study was performed to determine whether platelets augment PAI-1 production and if
so, to define mediators responsible. Hep G2 cells were used to determine whether platelet
lysates increased PAI-1 synthesis in a dose and time-dependent manner. In cells labeled
metabolically with 35S-methionine for 6 h, an increase in labeled PAI-1 was elicited indicative of de novo
synthesis as well as increased secretion of PAI-1 mediated by platelet lysates. Steady
state levels of both the 3.2 and 2.2 kb forms of PAI-1 mRNA increased after 2 h and
peaked in 3-5 h in a dose-dependent fashion as well. Incubation of Hep G2 cells with
collagen activated platelets resulted in a similar induction of PAI-1 mRNA. The increase
in PAI-1 mRNA occurred with exposure of the cells to platelet lysates for intervals
as brief as 15 min and was not inhibited by cycloheximide indicating its independence
of new protein synthesis. In order to identify the factors in platelets responsible
for the induction of PAI-1 synthesis in the Hep G2 cell model system, neutralizing
antibodies were used to inhibit specific platelet associated growth factors. Antibodies
to transforming growth factor-β (TGF-β) and to the epidermal growth factor (EGF)/transforming
growth factor alpha (TGF-α) receptor inhibited the platelet lysate-mediated increase
in PAI-1 protein by 77%. The results indicate that physiologic concentrations of platelet
derived TGF-β and EGF/TGF-α increase PAI-1 expression in Hep G2 cells and suggest
that the augmentation of PAI-1 synthesis induced by platelets is mediated by specific
platelet associated growth factors that may contribute to observed increases in circulating
PAI-1 levels in patients with disorders manifested by thrombosis.
