Regulation of nitric oxide production in macrophages

by Woo, Wai-hong

(Uncorrected OCR) Abstract of thesis entitled

Regulation of Nitric Oxide Production in Macrophages

Submitted by

Wai Hong Connie Woo

for the degree of Master of Philosophy at The University of Hong Kong

in May 2003

Hyperhomocysteinemia is an important risk factor for atherosclerosis. Inducible nitric

oxide synthase (iNOS) is mainly expressed in macrophages upon stimulation.

Overproduction of nitric oxide (NO) by iNOS can exacerbate the development of

atherosclerosis. Previous studies demonstrated that the extract of ginkgo biloba leaves

(EGb) inhibited the lipopolysaccharide (LPS)-stimulated iNOS expression in monocyte-

derived macrophage. Homocysteine was also shown to impair NO production in vascular

cells. The underlying mechanism by which homocysteine affected NOS expression was

unclear. The objectives of the present study were (1) to investigate the effect of

homocysteine on iNOS-mediated NO production in monocyte-derived macrophages and

the underlying mechanism, and (2) to investigate whether the extract of ginkgo biloba

leaves (EGb) could antagonize the effect of homocysteine on iNOS expression in

macrophages.

Human monocytic cell (THP-l)-derived macrophages were incubated with homocysteine

for various time periods. Homocysteine at concentrations of 0.05-0.1mM significantly

stimulated NO production and iNOS activity in macrophages by increasing the

expression of iNOS mRNA and protein. The increased iNOS expression was associated

with the activation of nuclear factor-kappa B (NFKB) arising from the reduced expression

of inhibitory protein-kappa B (IKBa) mRNA as well as the increased phosphorylation of

IKBa protein III homocysteine-treated cells. EGb and its terpenoids (ginkgolide A, ginkgolide Band bilobalide) could antagonize the effect of homocysteine on iNOS expression in macrophages by decreasing the oxidative stress resulting in attenuation of NFKB activation. Taken together, our results have demonstrated that homocysteine, at pathophysiological concentrations, stimulates iNOS-mediated NO production in macrophages. EGb and its terpenoids can reverse such stimulatory effect through the antioxidant effect and the capability of decreasing NFKB activation.

(265 words)