Sunscreens prevent erythema and premalignant skin lesions in humans, and are considered to be a major defense against photoageing and skin cancers. Lack of direct evidence for the efficacy of sunscreens for the latter purpose and reports of systemic absorption of sunscreens prompted this study, the aim of which was to determine whether sunscreens had any direct effect on human skin cells.
Uptake of sunscreen active ingredients or their UV-absorbing metabolites was investigated in human melanoma cells (MM96L) by a spectrophotometric method. After 8 h of treatment, uptake of ethylhexyl p-methoxycinnamate (EHMC) and octyl p- dimethylaminobenzoate (o-PABA) or their metabolites could be detected and this continued over 24 h. No accumulation of oxybenzone (OB) was found. EHMC and o- PABA were possibly trapped in the cells by hydrolysis to the UV-absorbing carboxylic acid.
Treatment of cultured human cells showed that the above 3 sunscreens inhibited cell growth, caused rapid but incomplete inhibition of DNA synthesis and delayed cells in Gl phase. The sensitivity of the growth of CI80-13S ovarian cancer cells to EHMC and o-PABA indicated that a target related to mitochondrial function was involved.
Simultaneous treatment of a mitochondrially-competent cell line with sunscreen and the mitochondrial inhibitor ethidium bromide produced very marked toxicity. This combination of agents was also associated with greatly increased uptake of ethidium, suggesting that as found previously for other lipids, the sunscreens bound the hydrophobic ethidium cation and facilitated its uptake. The fact that OB, itself not selectively toxic to CI80-13S cells, was synergistically toxic with ethidium in MM96L cells suggests that the enhanced uptake of ethidium was alone responsible for the increase in toxicity of the combination rather than the antimitochondrial activity of the sunscreen. Such results indicated that sunscreens might act as liposomes to greatly increase the cellular uptake of other molecules even if the latter are present at low levels.
Pretreatment with a non-toxic level of o-PABA highly sensitized cells to killing by short wavelength UVA or long wavelength UVB, the latter simulating the solar UV spectrum penetrating to the basal layer of the epidermis. EHMC did not show such an effect. Moreover, o-PABA itself rather than its hydrolysis product, 4- dimethylaminobenzoic acid, was sensitized by UVR. Sensitization was found in all cell types tested except normal keratinocytes, which have stronger antioxidant defenses than other cell types. Photosensitization could be prevented by certain antioxidants particularly pyruvate and mannitol, suggesting that hydroxyl radicals were involved.
A nutrient component, β-carotene, was shown to interact with sunscreens. The degradation products and the Vitamin A-related metabolites of β-carotene were toxic alone and synergised the toxicity of sunscreens to cultured cells. A pilot expression profiling study showed that the polyamine pathway, the isoprene/sterol synthesis pathway, and some other genes including Cyr61, ATF4, thrombospondins and metallothionein genes were modified at the transcriptional level.
Sunscreens are known to protect against p53 gene mutations and p53 protein induced by acute UV. The skin biopsies taken for one of the studies nested within the Nambour Skin Cancer Prevention Trial were used in this project to assess the ability of sunscreen to prevent p53 expression in chronically sun-exposed skin. P53 expression was decreased though not at a statistically significant level by daily sunscreen application, in agreement with the epidemiological finding of the Nambour trial that some s e c could be prevented by routine sunscreen application.
P53 expression was increased with purely β-carotene supplementation, in agreement with the epidemiological finding of the trial that there was a small non-significant increase in incidence of SCC with β-carotene supplementation alone. Moreover, p53 expression was further increased in β-carotene group within the subgroups of current-smoker and solar keratosis. A working hypothesis that links p53 and the polyamine pathway as well as metallothionein is offered to explain the discordant effects of P-carotene intake on cancer prevention and carcinogenesis.
From this combined in vitro and in vivo approach, the following overall conclusions were drawn: (1) Sunscreens are not biologically inert, causing a range of cellular responses relevant to skin carcinogenesis including specific changes in gene expression. (2) Sunscreens are probably effective for prevention of skin cancer. (3) Retinoids rather than β-carotene itself may play important roles in carcinogenesis, where excess amounts of β-carotene may disturb the balance of retinoids.