Regulatory effects of HDL on smooth muscle cell prostacyclin release

One mechanism by which high density lipoproteins (HDLs) exert their protect ive effect against coronary artery disease could be related to the inductio n of prostacyclin (PGI(2)) release in the vessel wall. We have recently sho wn that HDL increases PGI(2) production in rabbit smooth muscle cells (RSMC s) and that this increase is dependent on cyclooxygenase-2 (Cox-2). Here we analyze the mechanism by which rabbit HDL induces PGI(2) release in RSMCs. Our results show that although HDL2 and HDL3 share a similar capacity to i nduce Cox-2 protein levels, HDL3 stimulates a higher PGI(2) release than do es HDL2, probably because of their relative arachidonate contents. Acetylsa licylic acid pretreatment (300 mu mol/L, 30 minutes) significantly reduced the HDL-induced PGI(2) release, suggesting that both preexisting and induce d Cox-2 activities were involved in the HDL effect. Ca2+-dependent cytosoli c phospholipase A(2) (cPLA(2)) and Cox-1 protein levels were not altered by HDL. Dexamethasone (2 mu mol/L), which also inhibited the HDL-induced PGI( 2) release, reduced significantly both Cox-2 mRNA and protein levels withou t affecting cPLA(2) and Cox-1 protein levels. In addition, methylarachidony l fluorophosphonate, a potent inhibitor of cPLA(2), did not produce any eff ect on HDL-induced PGI(2) release. In the presence of cycloheximide, Cox-2 mRNA levels were induced by HDL and inhibited by dexamethasone, suggesting that HDL and dexamethasone work in the absence of de novo protein synthesis . These results indicate an early effect of HDL on PGI(2) biosynthesis, spe cifically increasing Cox-2. PD98059, an inhibitor of mitogen-activated prot ein kinase kinase, completely inhibited HDL-induced PGI(2) release, whereas GF109203X, a protein kinase C inhibitor, had no effect. Thus, HDL induces PGI(2) synthesis by a mechanism dependent on the mitogen-activated protein kinase pathway but independent of protein kinase C.