Inadequately repaired post-UV DNA damage results in skin cancers. DNA repair requires energy but skin cells have limited capacity to produce energy after UV insult. We examined whether energy supply is important for DNA repair after UV exposure, in the presence of 1α,25-dihydroxyvitamin D (1,25(OH)D), which reduces UV-induced DNA damage and photocarcinogenesis in a variety of models. After UV exposure of primary human keratinocytes, the addition of 1,25(OH)D increased unscheduled DNA synthesis, a measure of DNA repair. Oxidative phosphorylation was depleted in UV-irradiated keratinocytes to undetectable levels within an hour of UV irradiation. Treatment with 1,25(OH)D but not vehicle increased glycolysis after UV. 2-Deoxyglucose-dependent inhibition of glycolysis abolished the reduction in cyclobutane pyrimidine dimers by 1,25(OH)D, whereas inhibition of oxidative phosphorylation had no effect. 1,25(OH)D increased autophagy and modulated PINK1/Parkin consistent with enhanced mitophagy. These data confirm that energy availability is limited in keratinocytes after exposure to UV. In the presence of 1,25(OH)D, glycolysis is enhanced along with energy-conserving processes such as autophagy and mitophagy, resulting in increased repair of cyclobutane pyrimidine dimers and decreased oxidative DNA damage. Increased energy availability in the presence of 1,25(OH)D is an important contributor to DNA repair in skin after UV exposure.

The vitamin D hormone, 1,25dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), and related compounds derived from vitamin D 3 or lumisterol as a result of metabolism via the enzyme CYP11A1, have been shown, when applied 24 hours before or immediately after UV irradiation, to protect human skin cells and skin from DNA damage due to UV exposure, by reducing both cyclobutane pyrimidine dimers (CPD) and oxidative damage in the form of 8-oxo-7,8-dihydro-2 0 -deoxyguanosine (8-OHdG). We now report that knockdown of either the vitamin D receptor or the endoplasmic reticulum protein ERp57 by small, interfering RNA (siRNA) abolished the reductions in UVinduced DNA damage with 20-hydroxyvitamin D 3 or 24-hydroxylumisterol 3, as previously shown for 1,25(OH) 2 D 3 . Treatment with 1,25(OH) 2 D 3 reduced oxygen consumption rates in UV-exposed and sham-exposed human keratinocytes and reduced phosphorylation of cyclic AMP response binding element protein (CREB). Both these actions have been shown to inhibit skin carcinogenesis after chronic UV exposure, consistent with the anticarcinogenic activity of 1,25(OH) 2 D 3 . The requirement for a vitamin D receptor for the photoprotective actions of 1,25(OH) 2 D 3 and of naturally occurring CYP11A1-derived vitamin D-related compounds may explain why mice lacking the vitamin D receptor in skin are more susceptible to UV-induced skin cancers, whereas mice lacking the 1αhydroxylase and thus unable to make 1,25(OH) 2 D 3 are not more susceptible.

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The epidermis maintains a cellular calcium gradient that supports keratinocyte differentiation from its basal layers (low) to outer layers (high) leading to the development of the stratum corneum, which resists penetration of UV radiation. The calcium-sensing receptor (CaSR) expressed in keratinocytes responds to the calcium gradient with signals that promote differentiation. In this study, we investigated whether the CaSR is involved more directly in protection from UV damage in studies of human keratinocytes in primary culture and in mouse skin studied in vivo. siRNA-directed reductions in CaSR protein levels in human keratinocytes significantly reduced UV-induced direct cyclobutane pyrimidine dimers (CPD) by ~80% and oxidative DNA damage (8-OHdG) by ~65% compared with control transfected cells. Similarly, in untransfected cells, the CaSR negative modulator, NPS-2143 (500 nM), reduced UV-induced CPD and 8-OHdG by ~70%. NPS-2143 also enhanced DNA repair and reduced reactive oxygen species (ROS) by ~35% in UV-exposed keratinocytes, consistent with reduced DNA damage after UV exposure. Topical application of NPS-2143 also protected hairless Skh: hr1 mice from UV-induced CPD, oxidative DNA damage and inflammation, similar to the reductions observed in response to the well-known photoprotection agent 1,25(OH) 2 D 3 (calcitriol). Thus, negative modulators of the CaSR offer a new approach to reducing UV-induced skin damage.