The major objective of this research plan is to understand how exogenous retinoids, principally retinoic acid (RA) and 4-hydroxyphenyl retinamide (4-HPR), influence the hepatic metabolism of retinol. Hepatic retinol may potentially follow three main metabolic pathways: esterification with long-chain fatty acids to form retinyl esters, a reversible storage process; exit from the cell into plasma in association with retinol- binding protein (RBP); or oxidation leading to retinaldehyde and then RA, an irreversible activation or degradative process. The regulatory mechanisms that serve to partition retinol among these different pathways are largely unknown. New information has demonstrated the importance of the cellular retinoid-binding proteins in directing retinoids to specific enzymes. Retinol bound to the cellular retinol-binding protein, GRBP, is available for esterification by the microsomal enzyme lecithin: retinol acyltransferase (LRAT) and has also been shown to be the substrate of a microsomal retinol dehydrogenase. Recently, we have shown that liver LRAT activity is strongly regulated by vitamin A nutritional status. In the proposed research we will address 7 specific aims concerning hepatic retinol metabolism. In particular, we will examine how it is regulated by changes in vitamin A nutritional status and by exogenous retinoids with anti-cancer activity. One such retinoid, 4-HPR, has been shown to inhibit carcinogenesis and to be relatively non-toxic but it causes a marked suppression of plasma retinol and RBP. The mechanism of this decrease is not known. The first R aims examine the regulation of microsomal LRAT while Aims 5 and 6 address the synthesis of RBP and the oxidation of retinol, respectively.
In Aim 1 we will determine which retinoids besides RA can regulate LRAT activity. The goal of Aim 2 is to test the hypothesis that there is competition between LRAT and the RBP secretion pathway for retinol. We will examine whether induction of LRAT by retinoids such as 4-HPR reduces the availability of retinol for secretion on RBP.
In Aim 3 we will determine the cellular distribution of LRAT between parenchymal and nonparenchymal cells.
In Aim 4 we will use the technique of radiation inactivation to explore the properties of LRAT in liver and intestinal microsomes and extracts. The goal of Aim 5, which complements Aim 2, is to determine whether RA or 4-HPR acts as a signal to regulate the transcription or translation of RBP.
In Aim 6, we will test the hypothesis that oxidation of CRBP-bound retinol by microsomal retinol dehydrogenase is increased after treatment with RA or 4-HPR. The proposed research is relevant to understanding the normal feedback regulation of retinol metabolism by endogenously-produced RA as well as the influence of retinoids used in cancer chemoprevention, such as RA and 4-HPR,on the metabolism of retinol and the dietary requirement for vitamin A.
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