Understanding how aroma terpenes prevent sunburn and/or skin cancer in mice could lead to more effective and safer ways of blocking sun damage to human skin. In our previous study, β-damascenone, an aroma terpene, protected against sunburn by activating both keratinocyte and sebaceous gland pathways that fortified and thickened the cornified envelope and sebum layers of mouse epidermis. In the current study, the sunburn preventive activity of d-limonene was examined to confirm that these related compounds work similarly. In addition to sunburn, immunohistochemical analyses of proliferation (PCNA) and a damage-response gene N-Myc Down Regulated Gene 1 (NDRG1) were investigated in control versus d-limonene-treated Skh-1 mice. The mice were treated either topically on the dorsum or orally with different concentrations of d-limonene in liponate (100%, 10%, 1% and 0%) followed by exposure of the dorsum to solar simulation ultraviolet radiation (UVR) of 1.5 kJ/m2 from fluorescent ‘sunlamps’. Either 4 topical 5 μL doses or a single dose of 20 μL (0.95 μg/g body weight) of 100% d-limonene provided complete protection from UVR-induced sunburn. The protection was less but still significant at 10% d-limonene but was absent at 1% d-limonene in comparison to UVR alone. PCNA-labeling showed a marked proliferation increase of epidermal basal cells, outer root sheath cells of hair follicles and sebaceous gland cells by separate exposures to d-limonene and UVR relative to controls. The combined treatment of d-limonene followed by UVR produced fewer PCNA-positive cells in epithelial tissue relative to UVR-only mice. The NDRG1 protein (an indirect measure of DNA damage) was overwhelmingly (∼85% of keratinocytes) induced by UVR indicating heavy DNA damage. The UVR-induced NDRG1 index remained at ∼85% at 1% d -limonene, but was significantly reduced at 10% d-limonene to ∼24% and to less than 1% at 100% d -limonene indicating nearly complete elimination of cellular damage as a pure compound; a finding that is consistent with complete sunburn prevention. The thickness of the cornified envelope plus sebum layers nearly doubled within 5 days after administration of the d-limonene corresponding to the time of UVR application. Overall the results indicate that d-limonene protected against sunburn by activating keratinocyte/sebaceous gland-based pathways that fortified and thickened the cornified envelope plus sebum layers on the skin surface by releasing elevated levels of UVR-absorbing proteins that decreased UVR dose to underlying cutaneous tissues.
Citation Format: Ahmed N. Uddin, Ivica Labuda, Feng Wu, KamMeng Tchou-Wong, Fredric J. Burns. Aroma terpenes prevent UVB-induced sunburn in Skh1 mouse skin. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 4864. doi:10.1158/1538-7445.AM2013-4864