EFFECT OF BETA-CAROTENE ON CELL-CYCLE PROGRESSION OF HUMAN FIBROBLASTS

The uptake of beta-carotene (BC) and its effect on the cell cycle prog ression of normal human fibroblasts in primary culture were investigat ed by using two different delivery methods: exposure to BC solubilized in the organic solvent tetrahydrofuran (THF) or to BC incorporated in to dipalmitoylphosphatidylcholine (DPPC) liposomes. Cell cycle progres sion was evaluated by inmunofluorescence detection and flow cytometric analysis of the proliferating cell nuclear antigen (PCNA). In contras t to THF, which induced a marked reduction in the number of cells in S phase and in the extent of PcNA immunolabeling, DPPC liposomes proved to be an effective delivery system that does not interfere with cell proliferation, Cellular uptake of 0.23 nmol/10(6) cells was found afte r 24 h incubation in BC-containing DPPC liposomes. This value increase d to 1.2 nmol/10(6) cells after 72 h. After the first day of incubatio n, the number of cells in S phase was reduced by similar to 50%, with a consequent accumulation of cells in G(1) phase. This effect was main tained up to 3 days incubation, with no detectable effects on cell via bility. This cell cycle delay was found to be reversible, returning th e percentage of cells in S phase to the control value 24 h after remov al of sc from the medium. In order to determine whether the activity o f sc could be attributed to the molecule itself or to its conversion i nto retinoids, the production of sc metabolites was assessed. Analysis of cellular levels of retinoids failed to demonstrate the presence of retinal, retinol, retinoic acid or retinyl esters during an incubatio n period of 6 days. These results suggest that in normal human fibrobl asts, BC induces a cell cycle delay in the G(1) phase and that this ef fect is independent of conversion to known retinoids.