Many basic aspects of the metabolism of the native forms of retinoids, as well as the various interactions among native and synthetic forms of these compounds, remain to be clarified. We have been studying the mechanisms involved in the normal physiological metabolism of native forms of vitamin A as well as how these mechanisms are affected by the administration of various retinoids used in cancer treatment and/or prevention. We have completed two comprehensive long-term vitamin A turnover studies in rats in which we investigated the effects of either N-(4-hydroxyphenyl)retinamide (4-HPR) or all-trans retinoic acid (RA) on normal vitamin A metabolism. Using the SAAM/CONSAM computer modeling programs, we are in the process of developing detailed compartmental models to describe whole body metabolism of vitamin A in these studies. With both treatments, plasma vitamin A levels were greatly reduced and plasma vitamin A kinetics altered. There were differential effects on the vitamin A tissue stores and kinetics depending on the treatment and the tissues examined. For example, the vitamin A content of the eyes of the 4-HPR treated animals decreased over time, suggesting altered compensatory mechanisms that may explain the visual impairment associated with long-term administration of 4-HPR in human trials. Studies in HepG2 cells in vitro showed that 4-HPR treatment is associated with decreased secretion of RBP from these cells. It is not clear whether or not a similar mechanism is responsible for the lower plasma vitamin A levels in human subjects treated with this retinoid. However, it would appear that this cell line might provide an in vitro system in which a number of our in vivo findings can be tested. In other in vitro studies, we have developed probes for several of the retinoid binding proteins including RBP, CRBP, and CRABP, and are using these to screen tissues collected from our in vivo studies for possible differences in expression of these proteins with different retinoi treatments. Finally, biochemical and in vivo physiological tests of the human RBP that we have expressed in E. coli support the notion that our recombinant protein is behaving in a manner similar to native material. Thus, we are exploring the possibility of conducting vitamin A metabolism in human subjects, similar to those we have done using a rodent model, using our recombinant RBP which has been combined with a stable isotope form of retinol.
