FK506 independently upregulates transforming growth factor β and downregulates inducible nitric oxide synthase in cultured human keratinocytes: possible mechanisms of how tacrolimus ointment interacts with atopic skin

Summary Background  Tacrolimus ointment (FK506) has been used in recent years for the treatment of atopic dermatitis (AD), with favourable results. Most of the therapeutic efficacy of FK506 in AD has been attributed to its immunomodulatory effects on different immune cell types, but its effects on keratinocytes (KCs) have rarely been discussed. Studies have shown that low expression of transforming growth factor (TGF)‐β and high expression of nitric oxide synthase (NOS) are implicated in the pathogenesis of AD. Objectives  To investigate the direct effects of FK506 on KCs in terms of TGF‐β and inducible NOS (iNOS), and to explore the interactions between TGF‐β and iNOS in the KC system. Methods  Cultured human KCs treated with different concentrations of FK506 were used for investigation. The changes in the KC system induced by FK506 were documented in terms of TGF‐β and iNOS using enzyme‐linked immunosorbent assay and Western blotting techniques, respectively. The gene expression of both TGF‐β and iNOS was also determined. A certain amount of tumour necrosis factor (TNF)‐α was introduced to mimic atopic skin in vivo . Results  Our results showed that the release of TGF‐β was upregulated in FK506‐treated KCs, particularly in the presence of TNF‐α, while the expression of iNOS was downregulated. The gene expression of iNOS was also downregulated, as shown by reverse transcriptase–polymerase chain reaction analysis. However, the addition of TNF‐α did not further downregulate the expression of iNOS protein, suggesting that FK506 may regulate TGF‐β and iNOS through different pathways. Conclusions  Our findings indicate that the direct effects of FK506 on KCs probably contribute to its therapeutic efficacy in the treatment of AD.