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

Abstract

Cell culture experiments using hematoporphyrin photosensitized bovine hoof fibroblasts and longwave UV-irradiation revealed 2 distinct and separable patterns of lethal photosensitization according to 2 different sensitization procedures. Cell membranes were photosensitized by short exposure (5 min) to hematoporphyrin and cytoplasmic photosensitized by a 2 h exposure of cells to hematoporphyrin. Cell membrane photosensitization was reversible by incubation of cells in serum which removed surface bound hematoporphyrin; cytoplasmic photosensitization was irreversible. .beta.-carotene was tested in these 2 systems. Preincubation of bovine hoof fibroblasts in .beta.-carotene protects from lethal hematoporphyrin photosensitization. Protection with .beta.-carotene is acheived against both types of photosensitization, and the protective effect of .beta. carotene depends upon the duration of pretreatment, reaching a maximum after 7 days. .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. Hematoporphyrin 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 hematoporphyrin photosensitization results in a pronounced reduction red fluorescence. .beta.-carotene may act, at least in cell membrane photosensitization, at the level of the cell membrane into which it appears to be incorporated.