INDUCTION OF SMOOTH-MUSCLE CELL NITRIC-OXIDE SYNTHASE BY HUMAN LEUKEMIA INHIBITORY FACTOR - EFFECTS IN-VITRO AND IN-VIVO

We have previously shown that human leukaemia inhibitory factor (hLIF) inhibits perivascular cuff-induced neointimal formation in the rabbit carotid artery. Since nitric oxide (NO) is a known inhibitor of smoot h muscle growth, NO synthase (NOS) activity in the presence of hLIF wa s examined in vivo and in vitro. In rabbit aortic smooth muscle cell ( SMC) culture, significant NOS activity was observed at 50 pg/ml hLIF, with maximal activity at 5 ng/ml. In the presence of the NOS inhibitor L-NAME, hLIF-induced activation of NOS was greatly decreased, however it was still 63-fold higher than in control (p < 0.05). SMC-DNA synth esis was significantly reduced (-47%) following incubation with hLIF p lus L-arginine, the substrate required for NO production (p < 0.05), w ith no effect observed in the absence of L-arginine. Silastic cuff pla cement over the right carotid artery of rabbits resulted in a neointim a 19.3 +/- 5.4% of total wall cross-sectional area, which was increase d in the presence of L-NAME (27.0 +/- 2.0%; p < 0.05) and reduced in t he presence of L-arginine (11.3 +/- 2.0%; p < 0.05). The effect of L-a rginine was ameliorated by co-administration of L-NAME (16.4 +/- 1.5%) . However, administration of L-NAME with hLIF had no effect on the pot ent inhibition of neointimal formation by hLIF (3.2 +/- 2.5 vs. 2.1 +/ - 5.4%, respectively). Similarly, with hLIF administration, NOS activi ty in the cuffed carotid increased to 269.0 +/- 14.0% of saline-treate d controls and remained significantly higher with coadministration of L-NAME (188.5 +/- 14.7%). These results indicate that hLIF causes supe rinduction of NO by SMC, and that it is, either partially or wholly, t hrough this mechanism that hLIF is a potent inhibitor of neointimal fo rmation in vivo and of smooth muscle proliferation in vitro.