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. 2022 Mar 21;14(6):1312. doi: 10.3390/nu14061312

Figure 2.

Figure 2

The absorption and delivery of vitamin A. Vitamin A is absorbed from the lumen of the small intestine following hydrolysis of retinyl esters (RE) and re-esterification of retinol via cellular retinol binding protein 2 (CRBP2) and lecithin:retinol acyltransferase (LRAT). REs are secreted by enterocytes as part of chylomicrons and circulate via the lymphatic system (green) and enter the circulation (dashed red). Chylomicrons are hydrolyzed via lipoprotein lipase (LPL) to deliver retinol to target tissues such as the eye, adipose tissue and placenta and return to be cleared by the liver as remnants. Liver stores retinol as RE in HSC through the action of LRAT. When needed hepatic stellate cells (HSC) hydrolyze RE and secrete retinol bound to retinol binding protein 4 (RBP4) in association with transthyretin (TTR) in the circulation. RBP4 can both deliver as well as take up retinol from tissues that express its receptor stimulated by retinoic acid 6 (STRA6). RBP4 is reabsorbed from the proximal tubule of the kidney via lipoprotein receptor-related protein 2 (LRP-2 or megalin)–cubilin complex. A hepatic RBP4 receptor RBPR2 may also play a role in the uptake of RBP4 by the liver. During fasting the liver can also secrete RE in conjunction with VLDL as an alternate means to mobilize retinol (not shown). Created with BioRender.com (accessed on 16 February 2022).