Solar ultraviolet radiation (UVR), and in particular UVB spectrum (290-320 nm), are considered as a complete carcinogen for cutaneous malignancies. The over-exposure of UVR suppresses the development of allergic contact hypersensitivity (CHS) response in both laboratory animals as well as in humans. CHS response is considered to be a prototypic T-cell mediated immune response. UVR-induced suppression of immune sensitivity has been implicated in skin cancer risk. Therefore, the treatment options which can inhibit UVB-induced suppression of immune sensitivity may be useful in the management of skin cancers, including both melanoma and non-melanoma. Previously, we have shown that topical treatment of honokiol inhibits UVB induced immune suppression, which was associated with the reduction in UVB-induced inflammatory mediators in the mouse skin. Here, we have further determined the underlying mechanism of action of honokiol on UVR-induced immunosuppression including its effect on epigenetic regulators and their relationship with immune sensitivity in mouse skin. Topical treatment of C3H/HeN mice with honokiol (0.5 and 1.0 mg/cm2 skin area) in hydrophilic-cream based topical formulation significantly inhibits UVB-induced suppression of CHS response (P<0.01-0.005), which was associated with reduced levels of DNA methylation as well as Dnmt activity in the mouse skin compared with non-honokiol-treated and UVB-exposed control mice. To characterize the cell population responsible for the honokiol mediated inhibition of UVB-induced immunosuppression, we used an adoptive transfer approach. DNA-methylated (most of cells) dendritic cells (DCs), isolated from the draining lymph nodes of donor mice that had been UVB-exposed and sensitized to 2,4,-dinitrofluorobenzene (DNFB) with and without honokiol (0.5 and 1.0 mg/cm2 skin area) treatment, were transferred into naïve recipient mice. The CHS response of the recipient mice to DNFB was then measured. Honokiol treatment of UVB-exposed donor mice relieved this suppression of the CHS response in naïve mice, while the recipient mice, obtained DNA-methylated DCs from UVB-exposed donor mice that were not treated with honokiol, showed significant suppression in the CHS response. The restoration of CHS response in mice receiving DCs from honokiol treated, UV-exposed donor mice was also associated with enhanced secretion of Th1-type cytokines compared with the Th1-type cytokines from the DCs obtained from non-honokiol-treated and UVB-exposed donor mice. These data suggest that honokiol mediated inhibition of UVB induced suppression of CHS response is associated with functional activation of dendritic cells and that is dependent on DNA demethylation in DCs.
Note: This abstract was not presented at the meeting.
Citation Format: Santosh K. Katiyar, Tripti Singh, Harish C. Pal, Ram Prasad. Honokiol stimulates immune reactivity in UV-irradiated skin through DNA demethylation-dependent functional activation of dendritic cells in mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 2221. doi:10.1158/1538-7445.AM2017-2221