Ultraviolet Radiation Effects on Human Keratinocyte ICAM-1 Expression: UV-Induced Inhibition of Cytokine-Induced ICAM-1 mRNA Expression Is Transient, Differentially Restored for IFNγ Versus TNFα, and Followed by ICAM-1 Induction Via a TNFα-Like Pathway

Human keratinocytes (KC) during the course of inflammatory dermatoses strongly express the surface molecule ICAM-1, which plays an important role in the generation of the epidermal inflammatory infiltrate by mediating leukocyte-keratinocyte interactions. Accordingly, KC ICAM-1 expression is known to be induced in vivo and in vitro by cytokines either via the TNFα/TNFβ or via the IFNγ-mediated pathway. In contrast, ultraviolet (UV) radiation previously has been found to potently inhibit cytokine-induced KC ICAM-1 surface expression by a sublethal mechanism. In order to further define this novel immunosuppressive effect of UV light, the effects of in vitro UV radiation on ICAM-1 mRNA expression in transformed human KC (KB cells) were examined. Accordingly, UV light (0-100 J/m2) inhibited IFNγ-as well as TNFα-induced ICAM-1 mRNA expression, if KC were cytokine stimulated immediately after irradiation. After a 12-h incubation period, however, TFNγ responsiveness was found to be restored in irradiated cells, whereas restoration of responsiveness to TNFα required at least a 24-h recovery phase. Moreover, UV light alone did not alter ICAM-1 mRNA levels after 4, 12, or 24 h. After 48 h, however, a significant increase in ICAM-1 mRNA and surface expression in UV-irradiated KC could be observed. In addition, this increase could be superinduced by stimulation of irradiated KC with IFNγ, but not with TNFα. UV-induced upregulation of ICAM-1 expression could be mimicked by stimulating unirradiated cells with supernatants derived from UV-irradiated cells. Addition of biologically active anti-TNFα antibodies to UV-irradiated cells or to supernatants derived from UV-irradiated KC, however, did not even partially abolish this ICAM-1 — inducing activity. UV light thus seems to affect KC ICAM-1 mRNA expression in a biphasic manner: an early period of inhibition of cytokine-induced ICAM-1 expression is transient and followed by restoration of responsiveness to ICAM-1-inducing cytokines. Moreover, UV itself is able to induce ICAM-1 mRNA expression at this later time point via a TNFα-like pathway. These studies identify UV irradiation as a potent modulator of cytokine regulated ICAM-1 gene transcription with the capacity to induce both inhibitory as well as enhancing effects.