Orosomucoid has a cAMP-dependent effect on human endothelial cells and inhibits the action of histamine

The plasma protein orosomucoid toll-acid glycoprotein) has previously been shown to constitute a critical component of the capillary barrier. The prot ein has also been suggested to act as an anti-inflammatory mediator in a di versity of experimental situations. Recently we reported that orosomucoid i s synthesized by the microvascular endothelial cells per se. In the present study, the effects of orosomucoid on primary cultures of human umbilical v ein endothelial cells (HUVEC) were studied using the Cytosensor microphysio meter. We found that I)orosomucoid (0.01 g/l) increased the metabolic activ ity of HUVEC as reflected by the increased acidification rate of + 14 +/- 1 %; 2) pretreatment with 0.5 mM 8-bromo-cAMP for 20 min markedly and reversi bly inhibited the effect of orosomucoid, whereas 8-bromo-cGMP did not; 3) h istamine elicited a dose-dependent response that was abolished by pretreatm ent with either cAMP or cGMP; and finally 4) pretreatment of HUVEC for 6 mi n with orosomucoid (0.01 g/l) inhibited the action of histamine. In summary , this is the first report demonstrating that orosomucoid affects human end othelial cells and that it does so by using cAMP as a second messenger. Thi s provides an explanation for previous findings of anti-inflammatory effect s of the protein and shows that orosomucoid affects the endothelium during both normal and pathophysiological conditions.