Considerations for in vitro retinoid experiments: importance of protein interaction

Retinoids, natural and synthetic substances structurally related to vitamin A, are important modulators of cell proliferation and differentiation, and have proven activity in cancer therapy. Experiments to reveal the mechanis m of action of retinoids are routinely performed in in vitro models. As ret inoids are relatively hydrophobic and unstable, we hypothesized that the co mposition of culture media is of critical importance for the stability and bioavailability of these compounds. Various culture media were incubated wi th all-trans-, 13-cis- and 9-cis-retinoic acid (RA). Without fetal calf ser um (FCS) or bovine serum albumin (BSA) in the medium, the concentration of these retinoids was found to decrease to considerably low levels, This exce ssive loss of retinoids was due to absorption to culture plates, reaction t ubes and pipet tips. Binding of retinoids to BSA was demonstrated to have a ttenuating effects on uptake and metabolism of all-trans-RA, as studied in oral keratinocytes and head and neck cancer cells, indicating that a balanc e exists between the bioavailability and the aspecific loss of retinoids. I n this study we demonstrate that the type of culture medium and especially the presence of protein in the medium is of paramount importance to perform reproducible experiments with retinoids. (C) 1999 Elsevier Science B.V. Al l rights reserved.