The exposure of ultraviolet radiation (UVR) to murine skin suppresses the development of allergic contact hypersensitivity (CHS) response which is considered to be a prototypic T-cell mediated immune response. UVR-induced suppression of immune system has been implicated in skin cancer risk. Honokiol has been shown to have anti-skin carcinogenic activity, and here we have assessed the effect of honokiol on UVR-induced immunosuppression and its possible mechanism of action. Our CHS experiments revealed that exposure of C3H/HeN mouse skin with UVB radiation (150 mJ/cm2) for 4 consecutive days resulted in significant suppression of CHS response (75%, P<0.001) to the skin contact sensitizer, 2,4-dinitrofluorobenzene (DNFB), which was associated with the enhancement in the levels of cyclooxygenase-2 (COX-2) expression and prostaglandin E2 (PGE2) production. Topical treatment of mice with honokiol (0.5 and 1.0 mg/cm2 skin area) in hydrophilic-cream based topical formulation or indomethacin (50 μg/mouse), a COX-2 inhibitor, significantly inhibits UV radiation-induced suppression of CHS response (P<0.01-0.005). The exposure of mice with UVB resulted in DNA hypermethylation, increase in DNA methyltransferase (Dnmt) activity, and elevated levels of Dnmt proteins in mouse skin, while treatment of mice with honokiol before each UVB exposure reduced the levels of DNA methylation as well as Dnmt activity compared with non-honokiol-treated control mice. The COX-2 deficient mice are resistant to UV-induced suppression of CHS response. Treatment of UVB exposed COX-2-deficient mice with PGE2 (50 μg/mouse/day) results in UV-induced suppression of CHS, increase in DNA methylation and Dnmt activity in the skin. Further, treatment of honokiol reversed the effect of PGE2 on UV-induced suppression of CHS response in COX-2-deficient mice. Treatment of C3H/HeN mice with 5-aza-2’-deoxycytidine (5-Aza-dc, 1.0 mg/kg body weight, i.p.), a DNA demethylating agent, after each UVB exposure restored the CHS response which was associated with a reduction in Dnmt activity and global DNA methylation levels compared to the UV-exposed mice which were not treated with 5-Aza-dc. We also compared the effect of honokiol with commercially available anti-cancer drugs, such as imiquimod and 5-fluorouracil, on CHS response in UV-exposed mice. Topical treatment of mice with equi-molar concentrations of honokiol and cancer drugs did not show significant difference in terms of inhibition of UVB induced immune suppression and thus suggests honokiol as a substitute for these drugs. These findings provide evidence that honokiol acts through inhibition of inflammatory mediators and subsequently DNA demethylation in UVR-exposed mouse skin.
Citation Format: Santosh K. Katiyar, Tripti Singh, Ram Prasad. Honokiol, a phytochemical from magnolia plant, prevents UV radiation-induced immune suppression in mice through inhibition of PGE2 and DNA methylation. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2607.