The Molecular Basis of Retinoid Absorption: A Genetic Dissection

Nuttaporn Wongsiriroj,Roseann Piantedosi,Krzysztof Palczewski,Ira J Goldberg,Thomas P Johnston,Ellen Li,William S Blaner

doi:10.1074/jbc.m800777200

Abstract

The intestine and other tissues are able to synthesize retinyl esters in an acyl-CoA-dependent manner involving an acyl-CoA:retinol acyltransferase (ARAT). However, the molecular identity of this ARAT has not been established. Recent studies of lecithin:retinol acyltransferase (LRAT)-deficient mice indicate that LRAT is responsible for the preponderance of retinyl ester synthesis in the body, aside from in the intestine and adipose tissue. Our present studies, employing a number of mutant mouse models, identify diacylglycerol acyltransferase 1 (DGAT1) as an important intestinal ARAT in vivo. The contribution that DGAT1 makes to intestinal retinyl ester synthesis becomes greater when a large pharmacologic dose of retinol is administered by gavage to mice. Moreover, when large retinol doses are administered another intestinal enzyme(s) with ARAT activity becomes apparent. Surprisingly, although DGAT1 is expressed in adipose tissue, DGAT1 does not catalyze retinyl ester synthesis in adipose tissue in vivo. Our data also establish that cellular retinol-binding protein, type II (CRBPII), which is expressed solely in the adult intestine, in vivo channels retinol to LRAT for retinyl ester synthesis. Contrary to what has been proposed in the literature based on in vitro studies, CRBPII does not directly prevent retinol from being acted upon by DGAT1 or other intestinal ARATs in vivo.

Highlights

The intestine and other tissues are able to synthesize retinyl esters in an acyl-CoA-dependent manner involving an acylCoA:retinol acyltransferase (ARAT)
The synthesis of retinyl ester by LratϪ/Ϫ mice in response to a gavage dose of a physiological amount of retinol was less than 10% that of wild type mice given the same dose
We investigated whether LratϪ/Ϫ/ Dgat1Ϫ/Ϫ mice were able to esterify retinol to retinyl esters in response to a physiological challenge of retinol (6 ␮g containing 1 ␮Ci of [3H]retinol; this is approximately the amount of retinol consumed by a mouse in 1 day when maintained on a standard rodent chow diet)

Summary

A GENETIC DISSECTION*

Yen et al [15], Orland et al [16], and O’Byrne et al [11] have shown through in vitro assays that the enzyme diacylglycerol acyltransferase 1 (DGAT1) is able to esterify retinol to retinyl esters in an acylCoA-dependent manner It is not clear whether DGAT1 has a physiological role in catalyzing retinyl ester formation in vivo. We used previously described LratϪ/Ϫ [21], CrbpIIϪ/Ϫ [22], and Dgat1Ϫ/Ϫ [23] single knock-out mice as well as LratϪ/Ϫ/CrbpIIϪ/Ϫ and LratϪ/Ϫ/Dgat1Ϫ/Ϫ double knock-out mice that we generated for these studies We have used these mouse models to gain new insight into several questions about the biochemical mechanisms that are important for mediating uptake and processing of dietary retinoid from the intestine. These questions include the following. 1) Is DGAT1 a physiologically and/or pharmacologically important intestinal ARAT? 2) Are there other physiologically important ARATs in the body? 3) In vivo, does CRBPII channel retinol to LRAT and/or directly prevent retinol from being acted upon by intestinal ARATs?

EXPERIMENTAL PROCEDURES

RESULTS

DISCUSSION