From Molecular Mechanisms of UVA-Induced Skin Photooxidative Stress to Experimental Therapeutic Interventions Targeting Skin Cancer

Abstract

Ultraviolet A is a major spectral component of solar electromagnetic energy reaching the surface of the earth. Excessive exposure to solar UVA is a known contributor to skin photoaging and photocarcinogenesis, associated with increased incidence rates and a significant health burden imposed by skin cancer worldwide. However, the molecular mechanisms underlying UVA-induced skin photodamage remain largely undefined. UVA radiation has been shown to cause cutaneous oxidative stress and photosensitization reactions involving the light-driven photochemistry of specific skin chromophores upstream of reactive oxygen species formation, recognized as key players in skin photooxidative damage. Consequently, there has been significant interest in the identification of endogenous compounds that facilitate these reactions serving as endogenous photosensitizers. In my graduate research, we assessed the potential of selected endogenous chromophores, pyridoxal and 6-formylindolo[3,2-b]carbazole (FICZ), to elicit UVA-induced photo- and genotoxicity in relevant models of human skin, and further identified the underlying molecular mechanisms involved. We demonstrated for the first time that the B6-vitamer pyridoxal, previously shown to contain the phototoxic 3-hydroxypyridine moiety, is a micromolar sensitizer of UVA-induced genotoxicity in human primary keratinocytes and human epidermal reconstructs, which may be relevant to human skin exposed to high concentrations of B6-vitamers. Additionally, we have demonstrated that FICZ, a tryptophan photoproduct and endogenous high-affinity aryl hydrocarbon receptor (AhR) agonist, as the most potent endogenous UVA- and visible light-activated photosensitizer identified as of today. FICZ potentiates photooxidative stress, an effect that occurs independent of AhR ligand activity. Given the extraordinary photodynamic potency of FICZ, which surpasses that of any known endogenous photosensitizer, including protoporphyrin IX, and its rapid metabolic turnover, we tested the feasibility of using FICZ for the photodynamic elimination of malignant skin cancer cells in vitro and in vivo. Indeed, light photoactivation of FICZ-induced phototoxidative damage and cytotoxicity in a panel of cultured human malignant skin cells, and furthermore suppressed post-UVB tumorigenic progression in high-risk SKH-1 mice. Based on these pilot studies, follow up experiments will further optimize FICZ-based photodynamic interventions targeting human skin malignancies in relevant model systems. In pursuit of minimizing the need for invasive therapeutic methods, exploitation of stress response pathways has become a topic of interest for interventions aimed at the eradication of skin cancer at early or late progressional stages. Therefore, we tested feasibility of harnessing the cellular metal stress response for the elimination of skin cancer cells using the zinc-ionophore and FDA-approved microbicidal agent zinc pyrithione (ZnPT). Indeed, in a panel of cultured malignant skin cancer cells it was observed that ZnPT treatment caused rapid intracellular zinc overload and redox dysregulation, followed by a loss of genomic integrity and induction of caspase-independent cell death. In a murine photocarcinogenesis model, chronic topical ZnPT-administration post-UV caused epidermal zinc-overload and stress response gene expression with pronounced blockade of tumorigenic progression. These data suggest the feasibility of repurposing a topical OTC-drug for zinc-directed photochemoprevention of solar UV-induced nonmelanoma skin cancer. In summary, these studies contribute to our mechanistic understanding of photosensitizer- and zinc-induced stress responses in human skin, and furthermore provide the molecular basis for innovative therapeutic strategies aimed at the elimination of skin cancer cells.