The ultraviolet region of sunlight causes a significant oxidative stress to human skin cells and modulates expression of a series of genes in dermal fibroblasts and other cell types. The human heme oxygenase 1 (HO-1) gene is strongly activated within the first hours that follow UVA irradiation of normal human dermal fibroblasts (FEK4) and this response is being used as a marker of oxidative stress in cells. It has been shown that the induction of this gene occurs via singlet oxygen (O-1(2)) produced upon interaction of UVA radiation with an as yet undefined cellular chromophore. Carotenoids, as the most potent singlet oxygen quenchers in nature, are expected to effectively suppress the UVA-induced HO-1 gene activation in human cells. In this study, we measured the suppression of UVA-induced levels of HO-1 mRNA after the addition of a series of six all-trans-beta-carotene concentrations (0.07, 0.2, 0.8, 2.3, 8.0, and 21 gM) to the culture medium of exponentially growing FEK4 cells. The corresponding levels of beta-carotene uptake and apo-carotenal formation were measured following HPLC separation. The results of this study show a concentrationdependent suppression of UVA- (250 kJ/m(2)) induced transcriptional activation of HO-1 in exponentially growing FEK4 cells by beta-carotene. Suppression occurred at concentrations that have been observed in human plasma after dietary supplementation with beta-carotene . (C) 2003 Elsevier Science Inc.