UVA irradiation promotes ROS-mediated formation of the common deletion in mitochondrial DNA
Ultraviolet (UV) radiation from the sun causes adverse skin changes such as premature aging. UVA radiation is the primary factor for photoaging due to its deep penetration into the dermis, and UV-induced mitochondrial DNA (mtDNA) alterations, including deletions, contribute to photoaging and cellular dysfunction. The most frequent mtDNA rearrangement is the common deletion (CD), characterized by the loss of nearly one-third of the genome, 4,977 base pairs. UV radiation exposure leads to the formation of the CD, however, a distinct characterization of UV-induced CD and the underlying molecular mechanisms driving its initiation remains unexplored. In this study, we showed that increasing doses of UV radiation led to an increase in the CD in human skin fibroblasts. We found that UVA induce the formation of the CD by increasing the cellular reactive oxygen species (ROS) and oxidized bases content in the mtDNA. Preconditioning cells with antioxidants prevented the accumulation of the UVA-induced CD, suggesting that this mutational mechanism is ROS-dependent. In stark contrast, UVB did not alter cellular ROS levels but increased the formation of cyclobutane pyrimidine dimers (CPD), leading to CD generation though a ROS-independent mechanism. We corroborated our findings by using a 3D human full-thickness skin equivalent model, where we detected UVA-dependent CD formation in both the epidermal and dermal layers of the skin. By analyzing bulk RNA from UVA-exposed human skin fibroblasts by RNA-Seq, we found that UVA led to the upregulation of genes encoding mitochondrial DNA replication proteins and to the downregulation of genes involved genes encoding mtDNA repair factors. Taken together, our findings provide insight into how UVA and UVB differ in their detrimental effects on mtDNA, with UVA impacting mtDNA maintenance and transcription via a ROS-dependent mechanism. Our findings also established the mtDNA CD as a novel potential biomarker for monitoring UVA-induced oxidative stress and photoaging in skin cells in vitro and in vivo.