Chronic Vitamin A Intake Affects the Expression of mRNA for Apolipoprotein A-I, but Not for Nuclear Retinoid Receptors, in Liver of Young and Aging Lewis Rats

We have investigated the effects of chronic retinoid status and age on the steady-state levels in rat liver of the mRNAs for the retinoid-responsive gene apolipoprotein A-I (apo A-I), and for three nuclear retinoid receptors, RAR-α, RAR-β, and RXR-α. Weanling male Lewis rats (five per age–diet group) were fed a semisynthetic diet with vitamin A at marginal, control, or supplemented levels until they were 2 or 8 months of age. The relative abundance of apo A-I mRNA (compared to β-actin which did not differ among treatments) was about 50% lower in aging versus young rats, regardless of their vitamin A status. For both ages, apo A-I mRNA was approximately 40–50% higher (P< 0.05) for the vitamin A-marginal groups and 20–30% lower (P > 0.05) for the vitamin A-supplemented groups than the controls. Neither chronic diet nor age affected the levels of mRNA for RAR-α or RXR-α, two major retinoid receptors in liver, or for RAR-β, a receptor known to be autoregulated by RA. Although vitamin A did not have a significant effect on the mRNA levels of the receptors examined in this study, they are still likely to be involved in regulating apo A-I gene expression because in vitro studies have shown that both RAR-α and RXR-α bind to the regulatory region of the apo A-I gene.