Interleukin-18 prevents apoptosis via PI3K/Akt pathway in normal human keratinocytes.

Interleukin-18 (IL-18) is a pleiotropic cytokine expressed in both immune and non-immune cells. In the present study, we demonstrate an anti-apoptotic role of IL-18 in normal human neonatal foreskin epidermal keratinocytes (NHEK-F). Cultured NHEK-F spontaneously produced the active form of IL-18. Treatment of NHEK-F cells with anti-IL-18 receptor alpha-chain neutralizing antibody increased apoptosis and caspase-3 activity. Exogenous IL-18 augmented phosphorylation of Akt and activation of NF-kappaB. The promotion of Akt phosphorylation by IL-18 was abolished by LY294002, a PI3K inhibitor, but not SN50, an NF-kappaB inhibitor, indicating that IL-18 functions via the PI3K/Akt pathway and independently of NF-kappaB. In addition, IL-18 was found to augment expression of anti-apoptotic proteins, Bcl-2, XIAP and glucose regulated protein78/BiP, while anti-IL-18 receptor alpha-chain neutralizing antibody suppressed expression of Bcl-2, XIAP, glucose regulated protein94 and protein disulfide isomerase. Taken together, these results indicate that IL-18 plays an important role in keratinocyte survival.

Inflammatory liver steatosis caused by IL-12 and IL-18.

Acute fatty degeneration in the liver is caused by various agents, such as aspirin, valproic acid, and ibuprofen, that directly inhibit mitochondrial beta-oxidation of fatty acid and oxidative phosphorylation. Endogenous molecules, such as cytokines and hormones, are also known to mediate microvesicular steatosis in liver failure. In this study, we examined how interleukin-12 (IL-12) and IL-18 cause steatosis in the liver. Administration of these cytokines in combination caused marked hepatosteatosis and weight loss in mice. There were marked increases in levels of interferon-gamma (IFN-gamma), nitrite (NO(2)/NO(3)), and fibrinogen in the circulation in these mice. On the other hand, the ATP concentration and blood flow in the liver were significantly reduced. These changes, except the production of IFN-gamma and NO, were partially inhibited by Z-VAD-fmk, a synthetic tripeptide inhibitor for NO-induced caspases. These results indicate that IL-12 and IL-18 may mediate inflammatory hepatosteatosis through impairment of the microcirculation, which leads to mitochondrial dysfunction in hepatocytes.

Cytokine-induced injury of the lacrimal and salivary glands.

Damages to the lacrimal and salivary glands that accompany various autoimmune diseases are categorized as secondary Sjögren syndrome. Cytokines and free radicals are thought to be responsible for the pathologic changes, but the precise mechanisms are not clear. We evaluated whether cytokines alone can cause the damages in these exocrine tissues, and whether gaseous molecules such as nitric oxide (NO) play a role in these injuries. Various knockout (KO) mice as well as wild-type mice were injected intraperitoneally (i.p.) with the proinflammatory cytokines, IL-12 and IL-18, singly or in combination. Concurrent administration of IL-12 and IL-18 to mice caused serious atrophy in the lacrimal and salivary glands, which was spared when each cytokine was singly administered. Microscopically, there were apparently no infiltrating cells; nonetheless, numerous apoptotic cells were observed in the epithelium, which was confirmed by DNA ladder formation on gel electrophoresis. Serum levels of IFN-gamma and NO2/NO3 were markedly elevated. Combined injections of IL-12 and IL-18 caused the same changes in Fas-deficient and Fas-ligand deficient mice, as well as in perforin-KO mice, but the same changes were not detected in inducible NO synthase-KO mice or IFN-gamma KO mice. Thus, the synergistic effect of IL-12 and IL-18 was dependent on production of IFN-gamma and NO, but independent of Fas/Fas ligand system and perforin-dependent cytotoxic T cells. IL-18 together with IL-12 caused destructive changes in the glandular tissues without apparent lymphocyte infiltration. It is suggested that these cytokines can mediate apoptosis in glandular epithelial cells and that the elevated NO production is responsible for the change.