Stimulation of tissue-type plasminogen activator expression by retinoic acid in human endothelial cells requires retinoic acid receptor β2 induction

Abstract

We previously showed the involvement of retinoic acid receptor α (RARα) in the induction of tissue-type plasminogen activator (t-PA) synthesis by RA in human umbilical vein endothelial cells (HUVECs). However, the rather slow onset of this induction of t-PA synthesis suggested an indirect role of RARα. Here, we show that the protein synthesis inhibitor, cycloheximide completely blocks the induction of t-PA by RA, which points to the need of an intermediary protein in t-PA stimulation. This intermediary protein is likely to be RARβ2 on the basis of the following findings: (1) the induction of RARβ by RA exactly precedes that of t-PA; (2) HUVECs with elevated RARβ mRNA levels show an undelayed t-PA induction on stimulation with RA, and this response can be almost completely inhibited with an RAR antagonist; and (3) an antisense oligodeoxynucleotide against the translation initiation site of RARβ2 mRNA greatly reduces the t-PA induction by RA. Thus, induction of t-PA by RA in HUVECs involves a 2-step mechanism requiring induction of RARβ2 via RARα, followed by induction of t-PA synthesis via RARβ2. Each of these steps is shown to have a different activation profile with RA and 9-cis RA.