INVOLVEMENT OF RETINOIC ACID RECEPTOR-ALPHA IN THE STIMULATION OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR GENE-EXPRESSION IN HUMAN ENDOTHELIAL-CELLS

Retinoids stimulate tissue-type plasminogen-activator (t-PA) gene expr ession in human endothelial cells, and are likely to do so by binding to one or more nuclear retinoid receptors. The present study was initi ated to identify the retinoid receptor(s) involved in this process. Ex pression and regulation of retinoic acid receptors (RARs) and retinoid X receptors (RXRs) were analyzed by Northern-blot analysis of total o r poly(A)-rich RNA prepared from cultured human umbilical vein endothe lial cells (HUVEC). Prior to any exposure to retinoids, HUVEC express two transcripts for RAR-alpha (3.6 kb and 2.8 kb), and low levels of t ranscripts for RAR-beta (3.4 kb and 3.2 kb) and RAR-gamma (3.3 kb and 3.1 kb). Two RXR subtypes were identified, RXR-alpha (4.8 kb) and, at a much lower concentration, RXR-beta (2.4 kb); no evidence for the pre sence of RXR-gamma was found. Furthermore, HUVEC express cellular reti nol-binding protein I (CRBP-I) and cellular retinoic-acid-binding prot ein I (CRABP-I) mRNA. Exposure of HUVEC to 1 mu M retinoic acid or the retinobenzoic acid, Ch55, led to the induction of the two RAR-beta mR NAs, RXR-alpha mRNA and CRBP-I mRNA, whereas the expression of the oth er receptor and CRABP-I transcripts did not change appreciably. Using retinoid analogues that bind preferentially to one of the RAR or RXR s ubtypes, we found evidence that RAR-alpha is involved in the retinoid- induced t-PA expression in HUVEC. This conclusion was strengthened by experiments in which blocking of RAR-alpha with a specific RAR-alpha a ntagonist, Ro 41-5253, was demonstrated to suppress the induction of t -PA by retinoids.