In vertebrates, symmetric versus asymmetric cleavage of ␤-carotene in the biosynthesis of vitamin A and its derivatives has been controversially discussed. Recently we have been able to identify a cDNA encoding a metazoan ␤,␤-carotene-15,15-dioxygenase from the fruit fly Drosophila melanogaster. This enzyme catalyzes the key step in vitamin A biosynthesis, symmetrically cleaving ␤-carotene to give two molecules of retinal. Mutations in the corresponding gene are known to lead to a blind, vitamin A-deficient phenotype. Orthologs of this enzyme have very recently been found also in vertebrates and molecularly characterized. Here we report the identification of a cDNA from mouse encoding a second type of carotene dioxygenase catalyzing exclusively the asymmetric oxidative cleavage of ␤-carotene at the 9,10 double bond of ␤-carotene and resulting in the formation of ␤-apo-10-carotenal and ␤-ionone, a substance known as a floral scent from roses, for example. Besides ␤-carotene, lycopene is also oxidatively cleaved by the enzyme. The deduced amino acid sequence shares significant sequence identity with the ␤,␤-carotene-15,15-dioxygenases, and the two enzyme types have several conserved motifs. To establish its occurrence in different vertebrates, we then attempted and succeeded in cloning cDNAs encoding this new type of carotene dioxygenase from human and zebrafish as well. As regards their possible role, the apocarotenals formed by this enzyme may be the precursors for the biosynthesis of retinoic acid or exert unknown physiological effects. Thus, in contrast to Drosophila, in vertebrates both symmetric and asymmetric cleavage pathways exist for carotenes, revealing a greater complexity of carotene metabolism.

Carotenoids are currently investigated regarding their potential to lower the risk of chronic disease and to combat vitamin A deficiency in humans. These plant-derived compounds must be cleaved and metabolically converted by intrinsic carotenoid oxygenases to support the panoply of vitamin A-dependent physiological processes. Two different carotenoid-cleaving enzymes were identified in mammals, the classical carotenoid-15,15-oxygenase (CMO1) and a putative carotenoid-9,10-oxygenase (CMO2). To analyze the role of CMO1 in mammalian physiology, here we disrupted the corresponding gene by targeted homologous recombination in mice. On a diet providing ␤-carotene as major vitamin A precursor, vitamin A levels fell dramatically in several tissues examined. Instead, this mouse mutant accumulated the provitamin in large quantities (e.g. as seen by an orange coloring of adipose tissues). Besides impairments in ␤-carotene metabolism, CMO1 deficiency more generally interfered with lipid homeostasis. Even on a vitamin A-sufficient chow, CMO1 ؊/؊ mice developed a fatty liver and displayed altered serum lipid levels with elevated serum unesterified fatty acids. Additionally, this mouse mutant was more susceptible to high fat diet-induced impairments in fatty acid metabolism. Quantitative reverse transcription-PCR analysis revealed that the expression of peroxisome proliferator-activated receptor ␥-regulated marker genes related to adipogenesis was elevated in visceral adipose tissues. Thus, our study identifies CMO1 as the key enzyme for vitamin A production and provides evidence for a role of carotenoids as more general regulators of lipid metabolism.Dietary lipids are precursors for signaling molecules that control many facets in cell physiology. As the classic example, fat-soluble vitamin A (all-trans-retinol) is essential for processes ranging from development to vision and cell proliferation (1-3). Retinol is the precursor for at least two critical metabolites, 11-cis-retinal, the chromophore of visual G-protein-coupled receptors (4), and retinoic acid (RA).5 Alltrans-RA and 9-cis-RA regulate gene expression via heterodimeric nuclear receptors consisting of an RA receptor and a retinoid X receptor (RXR) (5, 6). Both are ligand-dependent transcription factors belonging to the superfamily of nuclear hormone receptors (7). Additionally, RXRs form heterodimers with other members of the nuclear receptor family (8), including the peroxisome proliferator-activated receptors (PPARs).Because animals, including humans, are unable to synthesize vitamin A de novo, all retinoids (vitamin A and its derivatives) derive from the oxidative cleavage of dietary provitamin A carotenoids, mainly ␤-carotene (9 -11). How this conversion of ␤-carotene occurs (centric and/or eccentric cleavage) is still a matter of debate (12)(13)(14). Recently, two different carotenoidmonooxygenases, CMO1 and CMO2, were molecularly identified in animals, including humans (15). Both belong to a family of structurally related nonheme iron oxygenases, common to all...

The uptake of dietary lipids from the small intestine is a complex process that depends on the activities of specific membrane receptors with yet unknown regulatory mechanisms. Using both mouse models and human cell lines, we show here that intestinal lipid absorption by the scavenger receptor class B type 1 (SR-BI) is subject to control by retinoid signaling. Retinoic acid via retinoic acid receptors induced expression of the intestinal transcription factor ISX. ISX then repressed the expression of SR-B1 and the carotenoid-15,15'-oxygenase Bcmo1. BCMO1 acts downstream of SR-BI and converts absorbed beta,beta-carotene to the retinoic acid precursor, retinaldehyde. Using BCMO1-knockout mice, we demonstrated increased intestinal SR-BI expression and systemic beta,beta-carotene accumulation. SR-BI-dependent accumulation of beta,beta-carotene was prevented by dietary retinoids that induced ISX expression. Thus, our study revealed a diet-responsive regulatory network that controls beta,beta-carotene absorption and vitamin A production by negative feedback regulation. The role of SR-BI in the intestinal absorption of other dietary lipids, including cholesterol, fatty acids, and tocopherols, implicates retinoid signaling in the regulation of lipid absorption more generally and has clinical implications for diseases associated with dyslipidemia.

Background: Mammalian genomes encode two carotenoid oxygenases, but their contributions to vitamin A homeostasis remain undefined. Results: Mammals employ symmetric and eccentric cleaving carotenoid oxygenases to convert different provitamin A carotenoids to vitamin A. Conclusion: Both carotenoid oxygenases contribute to vitamin A production. Significance: Carotenoids are the major source for vitamin A in the human diet.