Protective action of beta-carotene against lethal photosensitization of fibroblasts in vitro.

Cell culture experiments using haematoporphyrin photosensitized bovine hoof fibroblasts and long-wave uv-irradiation revealed two distinct and separable patterns of lethal photosensitization according to two different sensitization procedures: (1) Photosensitization of cell membranes by short exposure (5 min) of cells to haematoporphyrin. (2) Cytoplasmic photosensitization elicited by a 2 h exposure of cells to haematoporphyrin. Cell membrane photosensitization was reversible by incubation of cells in serum which removed surface bound haematoporphyrin; cytoplasmic photosensitization was irreversible. Beta-carotene was tested in these two systems and the following results were obtained: (1) Preincubation of bovine hoof fibroblasts in beta-carotene protects from lethal haematoporphyrin photosensitization. (2) Protection with beta-carotene is achieved against both types of photosensitization. (3) The protective effect of beta-carotene depends upon the duration of pretreatment, reaching a maximum after 7 days. (4) Beta-carotene protection is maintained even after trypsinization of bovine hoof fibroblasts and withdrawal of beta-carotene from the medium for 24 h or more. (5) Haematoporphyrin sensitized bovine hoof fibroblasts show a distinct pattern of red fluorescence for each type of photosensitization. Incubation of bovine hoof fibroblasts in beta-carotene prior to haematoporphyrin photosensitization results in a pronounced reduction of red fluorescence. Some of these data indicate that beta-carotene acts, at least in cell membrane photosensitization, at the level of the cell membrane into which it appears to be incorporated.