Thrombomodulin (TM) is a surface protein on endothelial cells, and represents one of the most valuable regulatory factors in the anticoagulant system. In this paper, we demonstrate that retinoic acid (RA) causes an increase in TM antigen on human umbilical vein endothelial cells (HUVECs) in vitro. The effect of RA on the surface TM level of HUVECs was dose-dependent in the range from 0.01 to 10 microM-RA. Antigen levels began to increase 3 h after addition of 10 microM-RA, and plateaued at a maximum level of approx. 2.5 times that of the untreated control at 24 h. TM levels remained at a maximum for a further 12 h, and then gradually decreased. The effects of RA on cell surface TM activity and antigen levels were parallel in all experiments. TM expression was also increased by treatment with 10 microM-retinal or 10 microM-retinol for 24 h, though the increases were approx. 70% and 30% respectively of that produced by 10 microM-RA. Pretreatment of HUVECs with cycloheximide inhibited the effect of RA. When HUVECs were incubated with both 10 microM-RA and 5 mM-8-bromo cyclic AMP (or 1 mM-3-isobutyl-1-methylxanthine, a phosphodiesterase inhibitor), the increase in TM antigen was greater than that observed with either compound alone. Northern blot analysis showed that treatment of HUVECs with 8-bromo cyclic AMP, RA or RA plus 8-bromo cyclic AMP increased TM mRNA levels by 2.2-, 4.5- and 5.5-fold respectively compared with the untreated control. Furthermore, no significant difference in cellular cyclic AMP levels was observed between RA-treated and control cells. These results indicate that the expression of TM is not only controlled by the intracellular cyclic AMP level but is also affected by RA, and suggest that RA-induced up-regulation of TM on HUVECs is independent of cyclic AMP regulation.

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