Plant Secondary Metabolites against Skin Photodamage: Mexican Plants, a Potential Source of UV-Radiation Protectant Molecules

Abstract

Human skin works as a barrier against the adverse effects of environmental agents, including ultraviolet radiation (UVR). Exposure to UVR is associated with a variety of harmful effects on the skin, and it is one of the most common health concerns. Solar UVR constitutes the major etiological factor in the development of cutaneous malignancy. However, more than 90% of skin cancer cases could be avoided with appropriate preventive measures such as regular sunscreen use. Plants, constantly irradiated by sunlight, are able to synthesize specialized molecules to fight against UVR damage. Phenolic compounds, alkaloids and carotenoids constitute the major plant secondary metabolism compounds with relevant UVR protection activities. Hence, plants are an important source of molecules used to avoid UVR damage, reduce photoaging and prevent skin cancers and related illnesses. Due to its significance, we reviewed the main plant secondary metabolites related to UVR protection and its reported mechanisms. In addition, we summarized the research in Mexican plants related to UV protection. We presented the most studied Mexican plants and the photoprotective molecules found in them. Additionally, we analyzed the studies conducted to elucidate the mechanism of photoprotection of those molecules and their potential use as ingredients in sunscreen formulas.

Keywords: Mexican plants; UV protection; UVR-damage; antioxidant activity; plant secondary metabolites.

Figure 1

Solar ultraviolet radiation (UVR) and the mechanisms of its main biological effects on the skin. UVC (100–280 nm) cannot penetrate the ozone layer; 2–5% of UVB (280–315 nm) crosses the ozone layer and penetrates the epidermis causing DNA damage and an increase in oxidative stress via ROS formation [1,17]. Approximately 95–98% of UVA (315–400 nm) reaches the skin and penetrates deeper into the epidermis and dermis. This causes diverse harmful effects on the skin, mainly through ROS. (1) Skin inflammation via induction of tumor necrosis factor alfa (TNF-α) and activation of pro-inflammatory cytokines as interleukin-1 (IL-1) and interleukin-6 (IL-6). (2) Skin aging by degradation of collagen and elastin through increasing matrix metalloproteinases (MMPs) induced by the nuclear factor-kB (NF-κB) and the activation protein-1 (AP-1). These last two proteins are also transcription factors for pro-inflammatory cytokines. Cytokines can also amplify AP-1 and NF-kB pathways, enhancing the response to UV radiation. (3) Immunosuppression via TNF-α and depletion of Langerhans cells. (4) Oxidative damage and apoptosis (5) Skin cancer by pyrimidine dimers formation [1,18,20]. All these harmful effects can be prevented by phytochemicals with photoprotection activity.

Figure 2

Plant secondary metabolites against skin photodamage. Mechanisms of action and skin-related bioactivities. Matrix metalloproteinases (MMPs) activation protein-1 (AP-1); Nuclear factor-kB (NF-κB), ultraviolet radiation (UVR); reactive oxygen species (ROS), reactive nitrogen species (RNS).