Mitochondrial tolerance to single and repeat exposure to simulated sunlight in human epidermal and dermal skin cells

•Previous studies using the Q-sun solar simulator were carried using the maximum output of 0.68 W/m2.•Q-sun solar simulator was set to 0.55 W/m2 at 340 nm, equal to 1 standard erythemal dose (SED)/minute.•A dramatic decrease in cell viability after repeat exposure 7 days apart was observed.•More pronounced changes in mitochondrial mass and MMP in epidermal cells compared to dermal cells•This study highlights a cell type specific response to sunlight.

BackgroundSunlight represents the primary threat to mitochondrial integrity in skin given the unique nature of the mitochondrial genome and its proximity to the electron transport chain. The accumulation of mitochondrial DNA (mtDNA) mutations is a key factor in many human pathologies and this is linked to key roles of mitochondrial function in terms of energy production and cell regulation.ObjectiveThe main objective of this study was to evaluate solar radiation induced changes in mitochondrial integrity, function and dynamics in human skin cells using a Q-Sun solar simulator to deliver a close match to the intensity of summer sunlight.MethodsSpontaneously immortalised human skin epidermal keratinocytes (HaCaT) and Human Dermal Fibroblasts (HDFn) were divided into two groups. Group A were irradiated once and Group B twice 7 days apart and evaluated using cell survival, viability and mitochondrial membrane potential (MMP) and mass at 1, 4 and 7 days post one exposure for Group A and 1, 4, 7 and 14 days post second exposure for Group B.ResultsViability and survival of HaCaT and HDFn cells decreased after repeat exposure to Simulated Sunlight Irradiation (SSI) with no recovery. HDFn cells showed no loss in MMP after one or two exposures to SSI compared to HaCaT cells which showed a periodic loss of MMP after one exposure with a repeat exposure causing a dramatic decrease from which cells did not recover. Mitochondrial Mass in exposed HDFn cells was consistent with control after one or two exposures to SSI; however mitochondrial mass was significantly decreased in HaCaT cells.ConclusionData presented here suggests that mitochondria in epidermal cells are more sensitive to sunlight damage compared to mitochondria in dermal cells, despite their origin, confirming a skin layer specific sensitivity to sunlight, but not as expected.