Cellular localization of β-carotene 15,15' oxygenase-1 (BCO1) and β-carotene 9',10' oxygenase-2 (BCO2) in rat liver and intestine

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.