Purpose. Besides being widely used in cosmetics, retinoids are potent therapeutic agents used topically and systemically as anti-acne agents. The aim of this study was to predict with the use of MetaSite the skin metabolism of selected retinoids employed in treatment of skin disorders and found in cosmeceuticals. The following compounds were studied: retinol, retinaldehyde, retinoic acid, retinyl acetate, retinyl palmitate, acitretin, etretinate, adapalene and bexarotene. Methods. MetaSite, Molecular Discovery Ltd. is a computational model that enables prediction of cytochrome P450-dependant metabolism. This software indicates atoms in the molecule structure that are mostly vulnerable to metabolic changes and predicts the metabolite structures. Results. MetaSite indicated that retinol and retinal metabolites were obtained through hydroxylation of the methyl group located in the position 3 of the aliphatic chain, whereas retinoic acid biotransformation would occur principally in the carbon atom situated in the position 4 in the cyclohexene ring. In acitretin molecule, carbon atom of the methoxy group attached to the benzene ring displayed the highest probability of biotransformation. In etretinate, metabolic reactions would occur principally on the carbon atom of the final ethyl group of the molecule. Conclusions. MetaSite metabolism predictions for retinoic acid, acitretin, etretinate, adapalene and bexarotene were in agreement with experimental findings. In case of compounds being converted by catalysts other than cytochrome P450 enzymes, the primary metabolites predicted by MetaSite differ from those reported previously. In conclusion, MetaSite is a useful tool that can aid identification of the major metabolites of compounds being administered topically.
Retinoic acid and transforming growth factor-β (TGF-β) affect differentiation, proliferation and carcinogenesis of epithelial cells. The effect of both compounds on the proliferation of cells of the hormone sensitive human breast cancer cell line (ER+) MCF-7 was assessed in the presence of estradiol and tamoxifen. The assay was based on [3H]thymidine incorporation and the proliferative activity of PCNA- and Ki 67-positive cells. The apoptotic index and expression of the Bcl-2 and p53 antigens in MCF-7 cells were also determined. Exogenous TGF-β1 added to the cell culture showed antiproliferative activity within the concentration range of 0.003-30 ng/ml. Irrespective of TGF-β1 concentrations, a marked reduction in the stimulatory action of estradiol (10-9 and 10-8 M) was observed whereas in combination with tamoxifen (10-7 and 10-6 M) only 30 ng/ml TGF-β1 caused a statistically significant reduction to aproximately 30% of the proliferative cells. In further experiments we examined the effect of exposure of breast cancer cells to retinoids in combination with TGF-β1. The incorporation of [3H]thymidine into MCF-7 cells was inhibited to 52 ± 19% (control =100%) by 3 ng/ml TGF-β1, and this dose was used throughout. It was found that addition of TGF-β1 and isotretinoin to the culture did not decrease proliferation, while TGF-β1 and tretinoin at low concentrations (3 × 10-8 and 3 × 10-7 M) reduced the percentage of proliferating cells by aproximately 30% (67±8% and 67±5%, P <0.05 compared to values in the tretinoin group). Both retinoids also led to a statistically significant decrease in the stimulatory effect of 10-9 M estradiol, attenuated by TGF-β1. In addition, the retinoids in combination with TGF-β1 and tamoxifen (10-6 M) caused a further reduction in the percentage of proliferating cells. Immunocytochemical analysis showed that all the examined compounds gave a statistically significant reduction in the percentage of cells with a positive reaction to PCNA and Ki 67 antigen. TGF-β1, isotretinoin and tretinoin added to the culture resulted in the lowest percentage of PCNA positive cells. However, the lowest fraction of Ki 67 positive cells was observed after addition of isotretinoin. The obtained results also confirm the fact that the well-known regulatory proteins Bcl-2 and p53 play an important role in the regulation of apoptosis in the MCF-7 cell line, with lowered Bcl-2 expression accompanying easier apoptotic induction. The majority of the examined compounds act via the p53 pathway although some bypass this important proapoptotic factor.
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