Depletion of intracellular GTP results in nuclear factor-kappaB activation and intercellular adhesion molecule-1...

The expression of the intercellular adhesion molecule 1 (ICAM-1) on the surface of endothelial cells plays an important role in immune-mediated processes. The induction by the proinflammatory cytokine interleukin (IL)-1beta is regulated by nuclear transcription factor kappaB (NF-kappaB). We studied the effect of an inosine-5'-monophosphate dehydrogenase (IMPDH) inhibitor, mycophenolic acid (MPA), on constitutive and IL-1beta-induced expression of ICAM-1 in human umbilical vein endothelial cells (HUVECs). Unexpectedly, pretreatment with MPA enhanced the constitutive expression and potentiated the induction of ICAM-1 by IL-1beta, as detected by flow cytometry. Northern blot analysis revealed an increase in ICAM-1 mRNA levels in cells treated with MPA. This was associated with an increase in phosphorylation of IkappaB-alpha (an inhibitor of NF-kappaB), nuclear translocation of the NF-kappaB subunits p50 and p65 and their binding to DNA as detected by Western blotting, confocal microscopy, and electrophoretic mobility shift assay. The up-regulation of ICAM-1 by MPA was prevented by high doses (100 microM) of guanine or guanosine but not by physiological doses (0.1 microM), indicating that guanylates are involved in endothelial responses to IL-1beta. Cultivation of HUVECs in the absence of guanine enhanced further ICAM-1 expression during IMPDH inhibition. These results demonstrate that cytokine-mediated endothelial ICAM-1 expression can be modulated by IMPDH inhibition. We believe this represents a novel interaction between endothelial guanylate metabolism, NF-kappaB activation, and adhesion molecule expression.