Figure 2.
Cellular pathway, uptake and transport of orally given vitamin A. After metabolization in the intestinal lumen, penetration into enterocytes and its association to chylomicrons (ChM), retinyl esters (RE) and β-carotene are secreted into the lymphatic system. Later, they reach the blood (systemic circulation) and are subsequently delivered to the liver, which functions as the main retinoid storage organ in the body or target tissues/cells. The dashed line represents the portion of retinol, which is not metabolized in the intestinal cells into retinyl esters and is secreted directly into the bloodstream, where it can bind to retinol-binding protein (RBP). From the liver, retinoids can be directly secreted into the blood in association with RBP or bind later to other transport proteins (e.g., albumin) found in the blood. Transport to target tissues is enabled via the RBP-receptor (RBPR). Once they enter the target cells, retinyl esters or retinol (ROH) are further oxidized into all-trans-retinoic acid (ATRA), which is responsible for the genetic functions of vitamin A in the body (other abbreviations: RAL—retinal; RChM-RE—remnant chylomicrons-retinyl esters; TTR—transthyretin; LR—lipoprotein receptor; NRs—nuclear receptors; CRBP—cellular retinol-binding protein).