The intestine and liver are crucial organs for vitamin A uptake and storage. Liver accounts for 70% of total body retinoid stores. Vitamin A deficiency (VAD) is a major micronutrient deficiency around the world. The provitamin A carotenoid, β-carotene, is a significant source of vitamin A in the diet. β-Carotene 15,15′ oxygenase-1 (BCO1) and β-carotene 9′,10′ oxygenase-2 (BCO2) are the two known carotenoid cleavage enzymes in humans. BCO1 and BCO2 are highly expressed in liver and intestine. Hepatocytes and hepatic stellate cells are two main cell types involved in the hepatic metabolism of retinoids. Stellate-like cells in the intestine also show ability to store vitamin A. Liver is also known to accumulate carotenoids, however, their uptake, retention and metabolism in specific liver and intestinal cell types is still unknown. Hence, we studied the cellular and subcellular expression and localization of BCO1 and BCO2 proteins in rat liver and intestine. We demonstrate that both BCO1 and BCO2 proteins are localized in hepatocytes and mucosal epithelium. We also show that BCO1 is also highly expressed in hepatic stellate cells (HSC) and portal endothelial cells in liver. At the subcellular level in liver, BCO1 is found in cytosol, while BCO2 is found in mitochondria. In intestine, immunohistochemistry showed strong BCO1 immunoreactivity in the duodenum, particularly in Brunner’s glands. Both BCO1 and BCO2 showed diffuse presence along epithelia with strong immunoreactivity in endothelial cells and in certain epithelial cells which warrant further investigation as possible intestinal retinoid storage cells.

β-Carotene oxygenase 2 cleaves β-carotene asymmetrically at non-central double bonds of the polyene chain, yielding apocarotenal molecules. The hypothesis tested was that apocarotenoids are able to stimulate transcription by activating retinoic acid receptors (RARs). The effects of long- and short-chain apocarotenals and apocarotenoic acids on the activation of RARα and RARβ transfected into monkey kidney fibroblast cells (CV-1) were investigated. We synthesized or purified β-apo-8′-carotenoic acid (apo-8′-CA), β-apo-14′-carotenoic acid (apo-14′-CA), β-cyclocitral (BCL), β-cyclogernanic acid (BCA), β-ionone (BI), β-ionylideneacetaldehyde (BIA) β-ionylideneacetic acid (BIAA) and a C13 ketone, β-apo-13-carotenone (C13). None of the apocarotenoids tested showed significant transactivation activity for the RARs when compared with all-trans retinoic acid (RA). The results suggest that biological effects of these apocarotenoids are through mechanisms other than activation of RARα and β.

β‐Carotene oxygenase 2 catalyzes the cleavage of β‐carotene at non‐central double bonds of the polyene chain, yielding apo‐carotenals. The biological roles and the potential regulation of retinoic acid receptors (RARs) by apo‐carotenoids are not clear. The hypothesis we seek to test is that apo‐carotenoids are able to stimulate transcription by activation of RARs. We will test the effects of all of the long and short chain apo‐carotenals arising from eccentric cleavage of β‐carotene (and the corresponding apo‐carotenoic acids) on the activation of RARα or RARβ transfected into monkey kidney cells along with RARE‐ luciferase reporter constructs. So far, we have obtained or synthesized apo‐8′‐carotenoic acid, apo‐14′‐carotenoic acid, β‐cyclocitral, β‐cyclogernaic acid, β‐ionylideneacetaldehyde, β‐ionylideneacetic acid, β‐ionone, and a C13 ketone. None of these apo‐carotenoids showed significant transactivation activity for RARs when compared to all‐trans retinoic acid (RA). The results suggest that the biological effects of these apo‐carotenoids are through mechanisms other than activation of RARα and β. Supported by NIH grants DK044498 and HL049879.