INTRODUCTION
It is well-known that continuous exposure to ultraviolet irradiation (UVR) leads to various of biological effects, including cutaneous erythema, edema, sunburn, immunosuppression, photoaging, as well as skin carcinogenesis.1 One of the mechanisms is that UVR increases the cellular levels of reactive oxygen species (ROS), which damage lipids, proteins, and nucleic acids in both epidermal and dermal cells, and stimulates the inflammatory process in the skin.2
Ultraviolet A (UVA), in a wavelength of 320 to 400 nm, contributes up to 95% of total UV exposure and is a significant source of oxidative stress in human skin.3,4 Experimental findings show that cumulative UVA doses are capable of inducing cellular DNA damage, which may lead to photoaging of the skin, altered expression of oncogenes and tumor suppressor genes, and skin carcinoma.5-7 Ultraviolet B (UVB), in a wavelength of 290 to 320 nm, critically damages cellular macromolecules and induces the formation of ROS. Newly formed ROS, such as hydroxyl radical and superoxide anion, can activate genes, damage DNA, oxidize cell lipids and proteins, and ultimately lead to apoptotic or necrotic cell death.8-10 To protect cells from UVR-induced damage, the skin has an elaborate antioxidant defense system consisting of enzymatic and nonenzymatic components to quench reactive oxygen intermediates.11 However, with the ongoing exposure to UVR, the body’s endogenous antioxidative system will be overwhelmed and the production of ROS increases. Supporting the skin’s antioxidant defense system is a necessary strategy to combat oxidative damage induced by UVR.
Vitamin C (ascorbic acid) and vitamin E (mainly α-tocopherol) are the 2 elemental nutrients that could quench free radicals and prevent ROS-induced cellular damage. Vitamin C can directly scavenge free radicals in the water phase and can protect cellular membranes.12 In contrast, vitamin E is mainly a free radical scavenger in the lipid phase by means of inhibiting lipid peroxidation.13 When vitamin C and vitamin E coexist, they demonstrate synergistic effects.14 Ferulic acid is a potent phenolic antioxidant found ubiquitously and at high concentrations in plants.15 It has been demonstrated that ferulic acid can protect membranes from lipid peroxidation, neutralize alkoxyl radicals, and prevent cells from damage induced by hydroxyl radicals, nitric oxide, and superoxide radical.16 When ferulic acid was incorporated into a formulation of vitamin C and/or vitamin E, the topical delivery of
