Regulation of t-pa expression by retinoic acid in cultured human endothelial cells in vitro and in rats in vivo

We and others have previously shown that retinoids (vitamin A derivatives) regulate t-PA synthesis in human endothelial cells in vitro and in rats in vivo. Retinoids exert their effect on t-PA expression via specific nuclear receptors, retinoic acid receptors (RARs). of which several subtypes and isoforms exist. Using subtype-specific retinoids and an antagonist with high preference for RARa, we could identify RARa to be involved in the stimulation of t-PA synthesis by retinoids in cultured human endothelial cells (Eur J Biochem 232:425-432, 1995). However, the rather slow onset of the induction of t-PA by retinoic acid (RA) and the need for ongoing protein synthesis suggested a mechanism in which the induction of t-PA is a secondary response to activation of RARa. On the basis of the following observations we conclude that RARB2 is the ultimate mediator of t-PA induction by RA: (1) the induction of RAR by RA exactly precedes that of t-PA; (2) cultured human endothelial cells with elevated RAR mRNA levels show an undelayed t-PA induction upon stimulation with RA; this response can be almost completely inhibited with a RAR antagonist; (3) an antisense oligodeoxynucleotide directed against the translation initiation site of RAR2 mRNA greatly reduces the t-PA induction by RA. (4) Bulens et al.(J Biol Chem 270:7167-7175. 1995) showed the presence of a functional retinoic acid response element in the t-PA promoter to which RARs can bind. The same mechanism (induction of RAR, followed by induction of t-PA) may he relevant in vivo, as deduced from experiments in which RA was administered to vitamin A-deficient rats. In summary, RA stimulates t-PA synthesis in endothelial cells via a two step mechanism, in which first RAR2 is induced by RARa and next RARB2 induces t-PA synthesis.