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. 2020 May 29;4(10):nzaa096. doi: 10.1093/cdn/nzaa096

FIGURE 2.

FIGURE 2

Schematic of intestinal β-carotene metabolism and its feedback regulation by retinoic acid. An enterocyte is illustrated. Newer information since the time of the 2001 report on vitamin A has identified brush border-associated transporters involved in carotene uptake, SCARB1; a mechanism for the enzymatic cleavage of β-carotene by BCO1; and a feedback mechanism involving the intestine-specific transcription factor, ISX, which is activated by RA, an active downstream product of vitamin A and β-carotene metabolism. When the RA concentration rises and the expression of the ISX gene is induced, then ISX protein serves to repress the transcription of the SCARB1 and BCO1 genes, thereby reducing the efficiency of cleavage of β-carotene. See refs (38–43) and text for discussion. Abbreviations: BCO1, ß-carotene oxygenase 1; CM, chylomicron; ISX, intestine-specific transcription factor; RA, retinoic acid; RARE, retinoic acid receptor response element; SCARB1, scavenger receptor class B type 1.