Control of plasminogen‐activator inhibitor type 2 gene expression in the differentiation of monocytic cells

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Abstract
Plasminogen‐activator inhibitor type 2 (PAI‐2) is a potent and primary inhibitor of urokinase‐type plasminogen activator. Its production in monocytic cells is thought to play an important role in the control of localized proteolysis at sites of invasion as occurs in the control of inflammatory processes, tumor invasion and cellular differentiation. Therefore, we have investigated the mechanisms responsible for the regulation of PAI‐2 gene expression in differentiating monocytic cells using the human promyelocytic cell line, HL‐60, as a model. These cells are induced to differentiate to a macrophage‐like phenotype in response to phorbol ester [4‐phorbol‐12‐myristate 13‐acetate (PMA)]. The levels of PAI‐2 mRNA are barely detectable in undifferentiated cells, however, activation with PMA is associated with a rapid induction of PAI‐2 transcripts, reaching a maximum of 25‐fold in 4 h. Nuclear run on assays demonstrate that this induction is related primarily to an enhanced rate of gene transcription. Inhibition of de novo protein synthesis by cycloheximide increases PAI‐2 mRNA levels in both resting (sevenfold) and PMA‐treated cells (fivefold) after 4 h, but has no detectable effect on the rate of PAI‐2 gene transcription. The initial apparent half‐life of the induced PAI‐2 mRNA, determined by actinomycin‐d‐decay experiments, is very short, 32 min, suggesting rapid turnover. Furthermore, the PAI‐2 mRNA transcript is stabilized in the presence of cycloheximide, with a fourfold increase in the observed half‐life. The results demonstrate that PAI‐2 gene expression is regulated through post‐transcriptional mechanisms in undifferentiated cells, while both transcriptional and post‐transcriptional events govern the level of PAI‐2 transcripts in cells differentiated along the monocytic pathway. Destabilization of the PAI‐2 transcript may be associated with (A + U)‐rich sequences found in the 3′‐untranslated region of PAI‐2 mRNA. The short half life and rapid, strong induction of PAI‐2 point to an important, perhaps crucial, role in the differentiation of monocyte cells.