Multiple pathways ensure retinoid delivery to milk: studies in genetically modified mice

Abstract

We studied the delivery of retinoids to milk, availing of mouse models modified for proteins thought to be essential for this process. Milk retinyl esters were markedly altered in mice lacking the enzyme lecithin:retinol acyltransferase (Lrat Ϫ/Ϫ ), indicating that this enzyme is normally responsible for the majority of retinyl esters incorporated into milk and not an acyl-CoA dependent enzyme, as proposed in the literature. Unlike wild-type milk, much of the retinoid in Lrat Ϫ/Ϫ milk is unesterified retinol, not retinyl ester. The composition of the residual retinyl ester present in Lrat Ϫ/Ϫ milk was altered from predominantly retinyl palmitate and stearate to retinyl oleate and medium chain retinyl esters. This was accompanied by increased palmitate and decreased oleate in Lrat Ϫ/Ϫ milk triglycerides. In other studies, we investigated the role of retinol-binding protein in retinoid delivery for milk formation. We found that Rbp Ϫ/Ϫ mice maintain milk retinoid concentrations similar to those in matched wild-type mice. This appears to arise due to greater postprandial delivery of retinoid, a lipoprotein lipase (LPL)-dependent pathway. Importantly, LPL also acts to assure delivery of long-chain fatty acids (LCFA) to milk. The fatty acid transporter CD36 also facilitated LCFA but not retinoid incorporation into milk. Our data show that compensatory pathways for the delivery of retinoids ensure their optimal delivery and that LRAT is the most important enzyme for milk retinyl ester formation. fatty acid; lactation; vitamin A RETINOIDS (vitamin A and its natural and synthetic analogs) are required for maintaining many essential physiological processes within the body (30,40). This is especially true for processes important to the normal growth and development of infants, including the development of the immune system, the brain, and other organ systems (30,40). Retinoids act as potent transcriptional regulators; more than 500 genes may be responsive to these compounds (2,8,15). The transcriptional effects of retinoids are mediated primarily by all-trans-and 9-cisretinoic acid acting through six distinct ligand-dependent transcription factors: the three retinoic acid receptors (RAR␣, RAR␤, and RAR␥) and the three retinoid X receptors (RXR␣, RXR␤, and RXR␥) (2,8,15).Mammals are incapable of synthesizing retinoids de novo. Consequently, all retinoids must be acquired from the diet either as preformed retinoid (primarily as dietary retinol or retinyl ester) or as proretinoid carotenoids (primarily as carotenes) (6,32,40). Different dietary retinoid forms are processed to retinol within the intestinal enterocytes and packaged into nascent chylomicrons as retinyl esters (6,32,40). The majority of retinyl esters are formed from retinol through the actions of lecithin:retinol acyltransferase (LRAT) (3,33,55). Approximately 66 -75% of dietary retinoid is taken up and stored as retinyl ester in the liver, primarily in the nonparenchymal hepatic stellate cells (also called Ito cells, lipocytes, or fat-storing cells) (6,17,19)....

Short communication: Effect of conjugated linoleic acid on concentrations of fat-soluble vitamins in milk of lactating ewes

“…Mammary gland uptake of RBP-bound retinol could be an explanation for the observation of the present study that milk retinol concentrations have been maintained in CLA-fed ewes despite the reduction in milk fat percentage. For example, it has been shown that mice lacking LPL were still able to incorporate retinol into milk via delivery of RBP-bound retinol and were able to maintain milk retinol concentrations (O'Byrne et al, 2010). In conclusion, we show that milk concentrations of fatsoluble vitamins were maintained or slightly enhanced in CLA-fed lactating ewes and were markedly enhanced when related to milk fat.…”

“…Additionally, feeding the trans-10,cis-12-CLA isomer to mice increased liver retinol storage, liver retinol secretion, serum retinol and RBP levels, and adipose tissue retinyl esters (Ortiz et al, 2009). The mammary gland can incorporate retinoids from chylomicrons (via LPL) and from RBPbound retinol via specific receptors (O'Byrne et al, 2010;Alapatt et al, 2013;Li et al, 2014). Mammary gland uptake of RBP-bound retinol could be an explanation for the observation of the present study that milk retinol concentrations have been maintained in CLA-fed ewes despite the reduction in milk fat percentage.…”

Chylomicron- and VLDL-derived Lipids Enter the Heart through Different Pathways

“…The reduced CM-RE uptake into the heart was not reflected in the cardiac retinoid levels or in downstream genes modulated by tissue retinoids. Because retinoids are essential for normal cellular function, lack of one retinoid uptake pathway leads to compensatory mechanisms required for maintenance of function as we have found in other locations such as the eye (56) and milk (57).…”

Molecular Basis for Vitamin A Uptake and Storage in Vertebrates

“…In addition to the essential role in vitamin A homeostasis, LRAT contributes to the retinoid composition of milk secreted from lactating mammary glands [127]. At birth, newborns do not possess sufficient retinoid storage to support normal growth and development and are thus dependent on their mother’s milk for acquiring enough retinoids.…”