Effect of dietary conjugated linoleic acid on vitamin A status of lactating rats and their offspring
The absorption and metabolism of the lipophilic vitamin A (retinol, retinaldehyde, retinoic acid) is linked to the absorption and metabolism of lipids (D'Ambrosio, Clugston, & Blaner, 2011). Substances which affect the lipid metabolism of animals thus could also affect retinoid metabolism and status. Dietary CLA, usually a mixture of the cis‐9, trans‐11 CLA and the trans‐10, cis‐12 CLA isomer, affects the lipid metabolism in a range of species including rats, pigs, poultry and ruminants and in various tissues, like liver, mammary gland and adipose tissue (Ramiah, Meng, & Ebrahimi, 2015; Ringseis, Saal, Muller, Steinhart, & Eder, 2004; Schlegel et al., 2012; Tous et al., 2012). For example, CLA reduces the expression of genes involved in lipid synthesis and uptakes and influences fatty acid (FA) composition in body tissues (Ringseis et al., 2004). In lactating dairy cows and ewes, it was recently found that dietary CLA increases vitamin A concentrations in the milk (Gessner et al., 2015; Zeitz, Most, & Eder, 2015). This observation suggests that dietary CLA influences vitamin A metabolism during lactation. However, potential changes of vitamin A metabolism in lactating animals on a molecular level have not yet been investigated. Moreover, as the milk is the only source of nutrient supply for suckling offspring, it is likely that potential alterations of the vitamin A concentration of the milk, caused by supplementation of the dam's diet with CLA, might influence the vitamin A status of the suckling offspring. The present study was performed to investigate the hypothesis that dietary CLA influences vitamin A metabolism in lactating rats, which were chosen as animal model, and influences tissue vitamin A concentrations in the newborn and suckling pups. While many studies dealing with the effects of CLA in rodents use rather high dietary concentrations of CLA (commonly in the range between 0.8 and 2%), we used a diet with moderate concentration of CLA (0.87% total CLA in the diet) in order to reflect the practical use of CLA as a supplement in livestock or human nutrition. In order to study the effects of CLA on vitamin A status, we determined vitamin A concentrations in various body tissues of the dams and of their pups at different ages. To study the effects of CLA on vitamin A metabolism in the dams, we determined expression of genes involved in retinol transport and tissue uptake, esterification, storage and degradation. Precisely, we investigated mRNA abundance of the cellular retinol‐binding protein 1 (CRBP1) and of the lecithin retinol acyltransferase (LRAT), which catalyses retinyl ester formation from retinol bound, for example, to CRBP1 for subsequent storage (D'Ambrosio et al., 2011; O'Byrne & Blaner, 2013). Also, we determined gene expression of CYP26A1, a cytochrome P450 enzyme important for retinoic acid (RA) degradation in the liver (Ross & Zolfaghari, 2011), and of the cellular RA binding protein 1 (CRABP1) which presents RA to metabolizing enzymes (Liu et al., 2015; Rhinn & Dolle, 2012). Additionally, mRNA abundance of transthyretin (TTR) and retinol‐binding protein 4 (RBP4), which accomplish plasma retinol transport as a retinol–RBP–TTR complex (D'Ambrosio et al., 2011), was analysed. Finally, we determined gene expression of STRA6, a transmembrane protein which binds the retinol–RBP complex and facilitates its uptake into cells (Kawaguchi et al., 2007), and of the low‐density lipoprotein receptor (LDLR) and the lipoprotein lipase (LPL) which are involved in the cellular uptake of vitamin A from chylomicrons and lipoproteins (D'Ambrosio et al., 2011).