FK506 inhibits tumour necrosis factor-alpha secretion in human keratinocytes via regulation of nuclear factor-kappaB

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Abstract
Tacrolimus (FK506) ointment has been used for treatment of inflammatory dermatoses with remarkable success. Our previous studies have indicated that direct modulation of tacrolimus on keratinocytes (KCs) may have an impact on its therapeutic effect. The use of monoclonal antibody specific for tumour necrosis factor (TNF)-alpha has shown efficacy in treating both psoriasis and Crohn disease. Topical tacrolimus has also been shown to be effective for treating cutaneous manifestations of both diseases. To explore the effects of FK506 on human KCs in terms of TNF-alpha secretion and to investigate the regulatory pathway involved. Ultraviolet (UV) B-irradiated cultured KCs were treated with various concentrations of FK506. At indicated time points after UVB irradiation we determined: (i) the TNF-alpha concentrations present in the culture supernatants; (ii) the activation and translocation of nuclear factor (NF)-kappaB in the cell nucleus; and (iii) the protein expressions of IkappaB kinase (IKK) and IkappaB in the cell lysates. In addition, a mouse model was used to corroborate our in vitro findings in vivo. More specifically, topical tacrolimus was applied on to mouse skin unilaterally after UVB irradiation. The effects of FK506 on nuclear NF-kappaB expression of UVB-irradiated mouse skin were determined. Our results showed that FK506 dose-dependently downregulated the secretion of TNF-alpha from UVB-irradiated KCs. The activation and translocation of NF-kappaB in UVB-irradiated KCs were also dose-dependently suppressed by FK506. The degradation of IkappaB induced by UVB was also inhibited by FK506, while no change in IKK expression was noted regardless of UVB and FK506 treatment. Murine skin biopsies showed that nuclear NF-kappaB expression induced by UVB was inhibited by topical tacrolimus treatment. Our results indicate that FK506 inhibits TNF-alpha secretion in human KCs via direct regulation of NF-kappaB. This modulatory effect of FK506 on KCs offers a possible mechanism for how topical tacrolimus regulates cutaneous inflammatory conditions.

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