Cell type-specific expression of beta-carotene 9',10'-monooxygenase in human tissues

Sequence characteristics, evolutionary history and expression pattern of BCO2 in Chinese mitten crab Eriocheir sinensis

β-carotene 9', 10'-oxygenase (BCO2) is a pivotal enzyme in the carotenoid cleavage. To fill the research gap of BCO2 in Chinese mitten crab Eriocheir sinensis, we first investigated ESIN_BCO2 gene from its genome, revealing its evolutionary history, gene structure, and expression patterns. The results showed that ESIN_BCO2 gene has a full-length open reading frame (ORF) of 1572 bp, encoding a protein of 523 amino acids. BCO2 was characterized by ten conserved motifs and an RPE65 domain, belonging to carotenoid cleavage oxygenase (CCO) family. Phylogenetic analysis revealed that BCO1 was the ancestral gene, from which BCO2 and NinaB diverged during evolution. Notably, the Ka/Ks ratios for BCO2 in Decapoda were approximately lower than BCO2 from other crustaceans (0.014 to 0.045 vs 0.112 to 0.185). ESIN_BCO2 was predominantly expressed in the hindgut, with significantly higher expression levels in females than in males. It was predominantly localized near the nuclei (N) of epithelial cells (epi) and basal cells (bc) in the hindgut. Moreover, dietary β-carotene supplementation significantly upregulated BCO2 expression in the female hindgut. These findings provide valuable insights into the evolution and function of BCO2 in E. sinensis as well as the other crustaceans, potentially shedding light on the conservation and divergence of carotenoid metabolism mechanisms across diverse crustaceans.

Recent advances in carotenoid absorption, distribution, and elimination

Carotenoids are a class of pigments with antioxidant properties synthesized by photosynthetic and heterotrophic organisms. Humans can store carotenoids in their intact form or cleave them enzymatically to apocarotenoids such as vitamin A, a hormone-like nutrient with crucial roles in gene expression and vision. Clinical and preclinical studies suggest that the consumption of diets rich in carotenoids attenuate cardiometabolic diseases, some types of cancer, neurodegenerative disorders, and inflammatory conditions. The bioactive properties of carotenoids depend, at least in part, on their accumulation in target tissues. However, the pathways that drive carotenoid absorption, delivery, and accumulation in tissues remain largely uncharacterized. This review provides a critical overview of the experimental models utilized to monitor carotenoid homeostasis in mammals. We also delve into recent findings concerning carotenoid intestinal uptake, bodily distribution, cellular uptake, and intracellular trafficking. Finally, we discuss the physiological relevance of a fecal carotenoid elimination pathway that operates independently of carotenoid enzymatic cleavage. Establishing the players governing carotenoid biodistribution and elimination is essential to maximize the bioactive properties of carotenoids in humans to prevent chronic diseases.

Unveiling BCO2 function in macular pigment metabolism: Mitochondrial processing and expression in the primate retina

BCO2 (β-carotene oxygenase 2) converts carotenoids into apocarotenoids by oxidative cleavage across double bonds and controls carotenoid homeostasis in vertebrate tissues. In this study, we examined BCO2's expression, localization, and activity in human cell lines and the retina. We generated peptide antibodies directed against primate BCO2 and validated their specificity using recombinant BCO1 (β-carotene oxygenase 1) and BCO2 proteins expressed in bacteria. The antibodies specifically detected human BCO2 by Western blot. In BCO2 expressing HepG2 cells, the antibodies recognized a 65 kDa mitochondrial protein that co-migrated with a recombinant truncated 522-amino-acid BCO2 variant, suggesting post-translational processing of the 579 amino acid long human BCO2 protein. Immunohistochemical analysis of macaque retina sections revealed BCO2 localization in the retinal pigment epithelium, photoreceptor inner segments, plexiform layer, and ganglion cell layer. Co-staining with COX IV indicated a mitochondrial localization of retinal BCO2 within photoreceptor inner segments. Western blot analysis of human donor retinas, separated into central and peripheral regions, identified higher BCO2 expression in the peripheral retina. Enzymatic activity assays demonstrated that BCO2 interacted with Aster proteins that transport carotenoids within cells. Our studies establish BCO2 as a mitochondrial protein expressed in the primate retina, where it likely plays a pivotal role in the metabolism of macular pigments and the maintenance of retinal health.

Functional Analysis of β-Carotene Oxygenase 2 (BCO2) Gene in Yesso Scallop (Patinopecten yessoensis)

Carotenoids are essential nutrients for humans and animals, and carotenoid coloration represents an important meat quality parameter for many farmed animals. Increasingly, studies have demonstrated that vertebrate carotenoid cleavage oxygenases (CCOs) are essential enzymes in carotenoid metabolism and are therefore potential candidate genes for improving carotenoid deposition. However, our understanding of carotenoid bioavailability and CCOs functions in invertebrates, particularly marine species, is currently quite limited. We previously identified that a CCO homolog, PyBCO-like 1, was the causal gene for carotenoid coloration in the 'Haida golden scallop', a variety of Yesso scallop (Patinopecten yessoensis) characterized by carotenoid enrichment. Here, we found that another CCO-encoding gene named PyBCO2 (β-carotene oxygenase 2) was widely expressed in P. yessoensis organs/tissues, with the highest expression in striated muscle. Inhibiting BCO2 expression in P. yessoensis through RNA interference led to increased carotenoid (pectenolone and pectenoxanthin) deposition in the striated muscle, and the color of the striated muscle changed from white to light orange. Our results indicate that PyBCO2 might be a candidate gene used for improving carotenoid content in normal Yesso scallops, and also in 'Haida golden scallops'.

Preventive Action of Beta-Carotene against the Indoxyl Sulfate-Induced Renal Dysfunction in Male Adult Zebrafish via Regulations of Mitochondrial Inflammatory and β-Carotene Oxygenase-2 Actions

Indoxyl sulfate (IS) is a metabolic byproduct of indole metabolism. IS readily interacts with the mitochondrial redox metabolism, leading to altered renal function. The β-carotene oxygenase-2 (BCO2) enzyme converts carotenoids to intermediate products. However, the role of β-carotene (BC) in IS-induced renal dysfunction in zebrafish and their modulatory action on BCO2 and mitochondrial inflammations have not been explored yet. Hence, the present study is designed to investigate the role of BC in the attenuation of IS-induced renal dysfunction via regulations of mitochondrial redox balance by BCO2 actions. Renal dysfunction was induced by exposure to IS (10 mg/L/hour/day) for 4 weeks. BC (50 and 100 mg/L/hour/day) and coenzyme Q10 (CoQ10; 20 mg/L/hour/day) were added before IS exposure. BC attenuated the IS-induced increase in blood urea nitrogen (BUN) and creatinine concentrations, adenosine triphosphate (ATP), and complex I activity levels, and the reduction of renal mitochondrial biomarkers, i.e., BCO2, superoxide dismutase-2 (SOD2), glutathione peroxidase-1 (GPX1), reduced and oxidized glutathione (GSH/GSSG) ratio, and carbonylated proteins. Moreover, renal histopathological changes were analyzed by the eosin and hematoxylin staining method. As a result, the administration of BC attenuated the IS-induced renal damage via the regulation of mitochondrial function.

Lycopene Accumulation in Transgenic Mice Lacking One or Both Carotenoid Cleaving Enzymes

BACKGROUND

β-carotene oxygenase 1 (BCO1) and β-carotene oxygenase 2 (BCO2) are responsible for the cleavage of carotenoids in mammals.

OBJECTIVE

The goals of this study were to (1) establish the relative contribution of each enzyme on lycopene accumulation in mice and (2) examine the role of lycopene on gene expression in the gut of wild type (WT) mice.

METHODS

We utilized male and female WT, Bco1-/-, Bco2-/-, and Bco1-/-Bco2-/- double knockout (DKO) mice. We gavaged the mice with either 1 mg of lycopene resuspended in cottonseed oil or vehicle as a control group daily for 2 wk. In a second study, we evaluated the effect of dietary vitamin A on lycopene absorption and intestinal gene expression by RT-PCR. We also quantified lycopene concentration isomer distribution by high performance liquid chromatography.

RESULTS

Of the 11 tissues measured, the liver accounted for 94 to 98% of the lycopene content across genotypes. We did not observe sex differences between genotypes, although hepatic lycopene levels in Bco1-/- mice were approximately half in comparison to the other genotypes; Bco1-/- verses Bco2-/- (P < 0.0001), DKO mice (P < 0.001), WT (ns). Analyses of mitochondrial lycopene content revealed a 3- to 5-fold enrichment compared with total hepatic content (P < 0.05) in all genotypes and sexes. In our second study, WT mice fed a vitamin A-deficient diet (VAD) accumulated greater amounts of lycopene in the liver than those fed a vitamin A-sufficient diet (VAS) (P < 0.01). These changes were accompanied by an upregulation of the vitamin A-responsive transcription factor intestine specific homeobox (ISX) in mice fed VAD + lycopene and VAS + lycopene diets compared with VAD control-fed mice (P < 0.05).

CONCLUSIONS

Our data suggest that BCO2 is the primary lycopene cleavage enzyme in mice. Lycopene concentration was enriched in the mitochondria of hepatocytes independently of genotype, and lycopene stimulated vitamin A signaling in WT mice.

Genetic deletion of Bco2 and Isx establishes a golden mouse model for carotenoid research

OBJECTIVE

Low plasma levels of carotenoids are associated with mortality and chronic disease states. Genetic studies in animals revealed that the tissue accumulation of these dietary pigments is associated with the genes encoding β-carotene oxygenase 2 (BCO2) and the scavenger receptor class B type 1 (SR-B1). Here we examined in mice how BCO2 and SR-B1 affect the metabolism of the model carotenoid zeaxanthin that serves as a macular pigment in the human retina.

METHODS

We used mice with a lacZ reporter gene knock-in to determine Bco2 expression patterns in the small intestine. By genetic dissection, we studied the contribution of BCO2 and SR-B1 to zeaxanthin uptake homeostasis and tissue accumulation under different supply conditions (50 mg/kg and 250 mg/kg). We determined the metabolic profiles of zeaxanthin and its metabolites in different tissues by LC-MS using standard and chiral columns. An albino Isx-/-/Bco2-/- mouse homozygous for Tyrc-2J was generated to study the effect of light on ocular zeaxanthin metabolites.

RESULTS

We demonstrate that BCO2 is highly expressed in enterocytes of the small intestine. Genetic deletion of Bco2 led to enhanced accumulation of zeaxanthin, indicating that the enzyme serves as a gatekeeper of zeaxanthin bioavailability. Relaxing the regulation of SR-B1 expression in enterocytes by genetic deletion of the transcription factor ISX further enhanced zeaxanthin accumulation in tissues. We observed that the absorption of zeaxanthin was dose-dependent and identified the jejunum as the major zeaxanthin-absorbing intestinal region. We further showed that zeaxanthin underwent oxidation to ε,ε-3,3'-carotene-dione in mouse tissues. We detected all three enantiomers of the zeaxanthin oxidation product whereas the parent zeaxanthin only existed as (3R, 3'R)-enantiomer in the diet. The ratio of oxidized to parent zeaxanthin varied between tissues and was dependent on the supplementation dose. We further showed in an albino Isx-/-/Bco2-/- mouse that supra-physiological supplementation doses (250 mg/kg) with zeaxanthin rapidly induced hypercarotenemia with a golden skin phenotype and that light stress increased the concentration of oxidized zeaxanthin in the eyes.

CONCLUSIONS

We established the biochemical basis of zeaxanthin metabolism in mice and showed that tissue factors and abiotic stress affect the metabolism and homeostasis of this dietary lipid.

Reproductive toxicity in marine medaka (Oryzias melastigma) due to embryonic exposure to PCB 28 or 4'-OH-PCB 65

Previous studies have shown that juvenile or adult exposure to polychlorinated biphenyls (PCBs) induces alterations in reproductive functions (e.g., reduced fertilization rate) and behavior (e.g., reduced nest maintenance) in fish. Embryonic exposures to other endocrine disrupting chemicals have been reported to induce long-term reproductive toxicity in fish. However, the effects of embryonic exposure to PCBs or their metabolites, OH-PCBs, on long-term reproductive function in fish are unknown. In the present study, we used the marine medaka fish (Oryzias melastigma) as a model to assess the reproductive endpoints in response to embryonic exposure to either PCB 28 or 4'-OH-PCB 65. Our results showed that the sex ratio of marine medaka was feminized by exposure to 4'-OH-PCB 65. Fecundity was decreased in the medaka treated with either PCB 28 or 4'-OH-PCB 65, whereas the medaka from embryonic exposure to 4'-OH-PCB 65 additionally exhibited reduced fertilization and a reduction in the hatching success rate of offspring, as well as decreased sperm motility. Serum 11-KT concentrations were reduced in the PCB 28-treated medaka, and serum estradiol (E2)/testosterone (T) and E2/11-ketotestosterone (11-KT) ratios were decreased in the 4'-OH-PCB 65-treated medaka. To explain these observations at the molecular level, transcriptomic analysis of the gonads was performed. Bioinformatic analysis using Gene Ontology and Ingenuity Pathway Analysis revealed that genes involved in various pathways potentially involved in reproductive functions (e.g., steroid metabolism and cholesterol homeostasis) were differentially expressed in the testes and ovaries of either PCB- or OH-PCB-treated medaka. Thus, the long-term reproductive toxicity in fish due to embryonic exposure to PCB or OH-PCB should be considered for environmental risk assessment.

The microbiome as a major function of the gastrointestinal tract and its implication in micronutrient metabolism and chronic diseases

The composition and function of microbes harbored in the human gastrointestinal lumen have been underestimated for centuries because of the underdevelopment of nucleotide sequencing techniques and the lack of humanized gnotobiotic models. Now, we appreciate that the gut microbiome is an integral part of the human body and exerts considerable roles in host health and diseases. Dietary factors can induce changes in the microbial community composition, metabolism, and function, thereby altering the host immune response, and consequently, may influence disease risks. An imbalance of gut microbiome homeostasis (i.e., dysbiosis) has been linked to several chronic diseases, such as inflammatory bowel diseases, obesity, and diabetes. Remarkable progress has recently been made in better understanding the extent to which the influence of the diet-microbiota interaction on host health outcomes in both animal models and human participants. However, the exact causality of the gut microbiome on the development of diseases is still controversial. In this review, we will briefly describe the general structure and function of the intestine and the process of nutrient absorption in humans. This is followed by a summarization of the recent updates on interactions between gut microbiota and individual micronutrients, including carotenoids, vitamin A, vitamin D, vitamin C, folate, iron, and zinc. In the opinion of the authors, these nutrients were identified as representative of vitamins and minerals with sufficient research on their roles in the microbiome. The host responses to the gut microbiome will also be discussed. Future direction in microbiome research, for example, precision microbiome, will be proposed.

Carotenoids and Their Health Benefits as Derived via Their Interactions with Gut Microbiota

Carotenoids have been related to a number of health benefits. Their dietary intake and circulating levels have been associated with a reduced incidence of obesity, diabetes, certain types of cancer, and even lower total mortality. Their potential interaction with the gut microbiota (GM) has been generally overlooked but may be of relevance, as carotenoids largely bypass absorption in the small intestine and are passed on to the colon, where they appear to be in part degraded into unknown metabolites. These may include apo-carotenoids that may have biological effects because of higher aqueous solubility and higher electrophilicity that could better target transcription factors, i.e., NF-κB, PPARγ, and RAR/RXRs. If absorbed in the colon, they could have both local and systemic effects. Certain microbes that may be supplemented were also reported to produce carotenoids in the colon. Although some bactericidal aspects of carotenoids have been shown in vitro, a few studies have also demonstrated a prebiotic-like effect, resulting in bacterial shifts with health-associated properties. Also, stimulation of IgA could play a role in this respect. Carotenoids may further contribute to mucosal and gut barrier health, such as stabilizing tight junctions. This review highlights potential gut-related health-beneficial effects of carotenoids and emphasizes the current research gaps regarding carotenoid-GM interactions.

Fat-soluble vitamin and phytochemical metabolites: Production, gastrointestinal absorption, and health effects

Consumption of diets rich in fruits and vegetables, which provide some fat-soluble vitamins and many phytochemicals, is associated with a lower risk of developing certain degenerative diseases. It is well accepted that not only the parent compounds, but also their derivatives formed upon enzymatic or nonenzymatic transformations, can produce protective biological effects. These derivatives can be formed during food storage, processing, or cooking. They can also be formed in the lumen of the upper digestive tract during digestion, or via metabolism by microbiota in the colon. This review compiles the known metabolites of fat-soluble vitamins and fat-soluble phytochemicals (FSV and FSP) that have been identified in food and in the human digestive tract, or could potentially be present based on the known reactivity of the parent compounds in normal or pathological conditions, or following surgical interventions of the digestive tract or consumption of xenobiotics known to impair lipid absorption. It also covers the very limited data available on the bioavailability (absorption, intestinal mucosa metabolism) and summarizes their effects on health. Notably, despite great interest in identifying bioactive derivatives of FSV and FSP, studying their absorption, and probing their putative health effects, much research remains to be conducted to understand and capitalize on the potential of these molecules to preserve health.

Proteins involved in fat-soluble vitamin and carotenoid transport across the intestinal cells: New insights from the past decade

It is now well established that vitamins D, E, and K and carotenoids are not absorbed solely through passive diffusion. Broad-specificity membrane transporters such as SR-BI (scavenger receptor class B type I), CD36 (CD36 molecule), NPC1L1 (Niemann Pick C1-like 1) or ABCA1 (ATP-binding cassette A1) are involved in the uptake of these micronutrients from the lumen to the enterocyte cytosol and in their secretion into the bloodstream. Recently, the existence of efflux pathways from the enterocyte back to the lumen or from the bloodstream to the lumen, involving ABCB1 (P-glycoprotein/MDR1) or the ABCG5/ABCG8 complex, has also been evidenced for vitamins D and K. Surprisingly, no membrane proteins have been involved in dietary vitamin A uptake so far. After an overview of the metabolism of fat-soluble vitamins and carotenoids along the gastrointestinal tract (from the mouth to the colon where interactions with microbiota may occur), a focus is placed on the identified and candidate proteins participating in the apical uptake, intracellular transport, basolateral secretion and efflux back to the lumen of fat-soluble vitamins and carotenoids in enterocytes. This review also highlights the mechanisms that remain to be identified to fully unravel the pathways involved in fat-soluble vitamin and carotenoid intestinal absorption.

Factors determining the oral absorption and systemic disposition of zeaxanthin in rats: in vitro, in situ, and in vivo evaluations

CONTEXT

Zeaxanthin is a yellow‑coloured dietary carotenoid widely recognized as an essential component of the macula. It exerts blue light filtering and antioxidant activities, offering eye health and vision benefits.

OBJECTIVE

This study explores the oral absorption and systemic disposition of zeaxanthin from biopharmaceutical and pharmacokinetic perspectives.

MATERIALS AND METHODS

In vivo intravenous (5 and 10 mg/kg) and intraportal (5 mg/kg) pharmacokinetic studies were performed to determine intrinsic tissue‑blood partition coefficient, elimination pathway, and hepatic clearance, of zeaxanthin in rats. Moreover, in vitro physicochemical property test, in situ closed loop study, in vivo oral pharmacokinetic study (20 and 100 mg/kg), and in vivo lymphatic absorption study (100 mg/kg) were conducted to investigate the gut absorption properties of zeaxanthin and assess the effects of several lipids on the lymphatic absorption of zeaxanthin in rats.

RESULTS

Zeaxanthin exhibited poor solubility (≤144 ng/mL) and stability (6.0-76.9% of the initial amount remained at 24 h) in simulated gut luminal fluids. Gut absorption of zeaxanthin occurred primarily in the duodenum, but the major fraction (≥84.7%) of the dose remained unabsorbed across the entire gut tract. Considerable fractions of intravenous zeaxanthin accumulated in the liver, lung, and spleen (21.3, 11.7, and 2.0%, respectively). It was found that the liver is the major eliminating organ of zeaxanthin, accounting for 53.5-90.1% of the total clearance process (hepatic extraction ratio of 0.623).

DISCUSSION AND CONCLUSIONS

To our knowledge, this is the first systematic study to report factors that determine the oral bioavailability and systemic clearance of zeaxanthin.

Carotenoid fates in plant foods: Chemical changes from farm to table and nutrition

Carotenoids in plant foods are sources of pro-vitamin A and nutrients with several health benefits, including antioxidant and anticancer activities. However, humans cannot synthesize carotenoids de novo and must obtain them from the diet, typically via plant foods. We review the chemical changes of carotenoids in plant foods from farm to table and nutrition, including nutrient release and degradation during processing and metabolism in vivo. We also describe the influencing factors and proposals corresponding to enhancing the release, retention and utilization of carotenoids, thus benefiting human health. Processing methods influence the release and degradation of carotenoids, and nonthermal processing may optimize processing effects. The carotenoid profile, food matrix, and body status influence the digestion, absorption, and biotransformation of carotenoids in vivo; food design (diet and carotenoid delivery systems) can increase the bioavailability levels of carotenoids in the human body. In this review, the dynamic fate of carotenoids in plant foods is summarized systematically and deeply, focusing on changes in their chemical structure; identifying critical control points and influencing factors to facilitate carotenoid regulation; and suggesting multi-dimensional strategies based on the current state of food processing industries to achieve health benefits for consumers.

Retinoic acid regulates pyruvate dehydrogenase kinase 4 (Pdk4) to modulate fuel utilization in the adult heart: Insights from wild-type and β-carotene 9',10' oxygenase knockout mice

Regulation of the pyruvate dehydrogenase (PDH) complex by the pyruvate dehydrogenase kinase PDK4 enables the heart to respond to fluctuations in energy demands and substrate availability. Retinoic acid, the transcriptionally active form of vitamin A, is known to be involved in the regulation of cardiac function and growth during embryogenesis as well as under pathological conditions. Whether retinoic acid also maintains cardiac health under physiological conditions is unknown. However, vitamin A status and intake of its carotenoid precursor β-carotene have been linked to the prevention of heart diseases. Here, we provide in vitro and in vivo evidence that retinoic acid regulates cardiac Pdk4 expression and thus PDH activity. Furthermore, we show that mice lacking β-carotene 9',10'-oxygenase (BCO2), the only enzyme of the adult heart that cleaves β-carotene to generate retinoids (vitamin A and its derivatives), displayed cardiac retinoic acid insufficiency and impaired metabolic flexibility linked to a compromised PDK4/PDH pathway. These findings provide novel insights into the functions of retinoic acid in regulating energy metabolism in adult tissues, especially the heart.

Molecular Properties of β-Carotene Oxygenases and Their Potential in Industrial Production of Vitamin A and Its Derivatives

β-Carotene 15,15'-oxygenase (BCO1) and β-carotene 9',10'-oxygenase (BCO2) are potential producers of vitamin A derivatives, since they can catalyze the oxidative cleavage of dietary provitamin A carotenoids to retinoids and derivative such as apocarotenal. Retinoids are a class of chemical compounds that are vitamers of vitamin A or are chemically related to it, and are essential nutrients for humans and highly valuable in the food and cosmetics industries. β-carotene oxygenases (BCOs) from various organisms have been overexpressed in heterogeneous bacteria, such as Escherichia coli, and their biochemical properties have been studied. For the industrial production of retinal, there is a need for increased production of a retinal producer and biosynthesis of retinal using biocatalyst systems improved by enzyme engineering. The current review aims to discuss BCOs from animal, plants, and bacteria, and to elaborate on the recent progress in our understanding of their functions, biochemical properties, substrate specificity, and enzyme activities with respect to the production of retinoids in whole-cell conditions. Moreover, we specifically propose ways to integrate BCOs into retinal biosynthetic bacterial systems to improve the performance of retinal production.

Mechanisms of Feedback Regulation of Vitamin A Metabolism

Vitamin A is an essential nutrient required throughout life. Through its various metabolites, vitamin A sustains fetal development, immunity, vision, and the maintenance, regulation, and repair of adult tissues. Abnormal tissue levels of the vitamin A metabolite, retinoic acid, can result in detrimental effects which can include congenital defects, immune deficiencies, proliferative defects, and toxicity. For this reason, intricate feedback mechanisms have evolved to allow tissues to generate appropriate levels of active retinoid metabolites despite variations in the level and format, or in the absorption and conversion efficiency of dietary vitamin A precursors. Here, we review basic mechanisms that govern vitamin A signaling and metabolism, and we focus on retinoic acid-controlled feedback mechanisms that contribute to vitamin A homeostasis. Several approaches to investigate mechanistic details of the vitamin A homeostatic regulation using genomic, gene editing, and chromatin capture technologies are also discussed.

De novo assembly transcriptome analysis reveals the genes associated with body color formation in the freshwater ornamental shrimps Neocaridina denticulate sinensis

The body color of Neocaridina denticulate sinensis is a compelling phenotypic trait, in which a cascade of carotenoid metabolic processes plays an important role. The study was conducted to compare the transcriptome of cephalothoraxes among three pigmentation phenotypes (red, blue, and chocolate) of N. denticulate sinensis. The purpose of this study was to explore the candidate genes associated with different colors of N. denticulate sinensis. Nine cDNA libraries in three groups were constructed from the cephalothoraxes of shrimps. After assembly, 75022 unigenes were obtained in total with an average length of 1026 bp and N50 length of 1876 bp. There were 45977, 25284, 23605, 21913 unigenes annotated in the Nr, Swissprot, KOG, and KEGG databases, respectively. Differential expression analysis revealed that there were 829, 554, and 3194 differentially expressed genes (DEGs) in RD vs BL, RD vs CH, and BL vs CH, respectively. These DEGs may play roles in the absorption, transport, and metabolism of carotenoids. We also emphasized that electron transfer across the inner mitochondrial membrane (IMM) was a key process in pigment metabolism. In addition, a total of 6328 simple sequence repeats (SSRs) were also detected in N. denticulate sinensis. The results laid a solid foundation for further research on the molecular mechanism of integument pigmentation in the crustacean and contributed to developing more attractive aquatic animals.

Overflow phenomenon in serum lutein after supplementation: a systematic review supported with SNPs analyses

Lutein, a type of carotenoids, is found to delay the onset and progression of age-related macular degeneration (AMD). Several lutein supplementation studies showed that after an initial increase, lutein serum levels demonstrated a subsequent decrease despite continuous supplementation. In this systematic literature review, this obscure phenomenon was tried to be explained. The subsequent drop in lutein levels was postulated due to down-regulation of lutein receptors scavenger receptor class B type I (SR-BI) in the gastrointestinal tract, upregulation of lutein degrading enzyme β-carotene dioxygenase (BCDO2), or perhaps a combination of both. Some single nucleotides polymorphisms (SNPs) that could have influence on the occurrence of this phenomenon. To date, an exact scientific explanation for this phenomenon has not been established. Further research is needed to investigate this phenomenon in depth to reach an irrefutable explanation, giving that lutein is proven to be effective in delaying the onset and progression of AMD and its metabolism in the human body becomes of equal importance.

Characterization of Chicken Skin Yellowness and Exploration of Genes Involved in Skin Yellowness Deposition in Chicken

Skin color is an important economic trait in meat-type chickens. A uniform bright skin color can increase the sales value of chicken. Chickens with bright yellow skin are more popular in China, especially in the broiler market of South China. However, the skin color of chickens can vary because of differences in breeds, diet, health, and individual genetics. To obtain greater insight into the genetic factors associated with the process of skin pigmentation in chickens, we used a colorimeter and high-resolution skin photographs to measure and analyze the skin color of chickens. By analyzing 534 chickens of the same breed, age, and feed condition, we found that the yellowness values of the chickens varied within this population. A significant positive correlation was found between the cloacal skin yellowness values before and after slaughter, and the cloacal skin yellowness value of live chickens was positively correlated with the overall body skin yellowness value. Additionally, chicken skin yellowness exhibited low heritability, ranging from 0.07 to 0.27. Through RNA sequencing, 882 genes were found to be differentially expressed between the skin with the highest and lowest yellowness values. Some of these differentially expressed genes may play an important role in yellow pigment deposition in chicken skin, which included TLR2B, IYD, SMOC1, ALDH1A3, CYP11A1, FHL2, TECRL, ACACB, TYR, PMEL, and GPR143. In addition, we found that the expression and variations of the BCO2 gene, which is referred to as the yellow skin gene, cannot be used to estimate the skin yellowness value of chickens in this population. These data will help to further our understanding of chicken skin yellowness and might contribute to the selection of specific chicken strains with consistent skin coloration.

The Structural and Biochemical Basis of Apocarotenoid Processing by β-Carotene Oxygenase-2

In mammals, carotenoids are converted by two carotenoid cleavage oxygenases into apocarotenoids, including vitamin A. Although knowledge about β-carotene oxygenase-1 (BCO1) and vitamin A metabolism has tremendously increased, the function of β-carotene oxygenase-2 (BCO2) remains less well-defined. We here studied the role of BCO2 in the metabolism of long chain β-apocarotenoids, which recently emerged as putative regulatory molecules in mammalian biology. We showed that recombinant murine BCO2 converted the alcohol, aldehyde, and carboxylic acid of a β-apocarotenoid substrate by oxidative cleavage at position C9,C10 into a β-ionone and a diapocarotenoid product. Chain length variation (C20 to C40) and ionone ring site modifications of the apocarotenoid substrate did not impede catalytic activity or alter the regioselectivity of the double bond cleavage by BCO2. Isotope labeling experiments revealed that the double bond cleavage of an apocarotenoid followed a dioxygenase reaction mechanism. Structural modeling and site directed mutagenesis identified amino acid residues in the substrate tunnel of BCO2 that are critical for apocarotenoid binding and catalytic processing. Mice deficient for BCO2 accumulated apocarotenoids in their livers, indicating that the enzyme engages in apocarotenoid metabolism. Together, our study provides novel structural and functional insights into BCO2 catalysis and establishes the enzyme as a key component of apocarotenoid homeostasis in mice.

Xanthophylls Modulate Palmitoylation of Mammalian β-Carotene Oxygenase 2

An extensive body of work has documented the antioxidant role of xanthophylls (lutein and zeaxanthin) in human health and specifically how they provide photoprotection in human vision. More recently, evidence is emerging for the transcriptional regulation of antioxidant response by lutein/lutein cleavage products, similar to the role of β-carotene cleavage products in the modulation of retinoic acid receptors. Supplementation with xanthophylls also provides additional benefits for the prevention of age-related macular degeneration (AMD) and attenuation of Alzheimer's disease symptoms. Mammalian β-carotene oxygenase 2 (BCO2) asymmetrically cleaves xanthophylls as well as β-carotene in vitro. We recently demonstrated that mouse BCO2 (mBCO2) is a functionally palmitoylated enzyme and that it loses palmitoylation when cells are treated with β-carotene. The mouse enzyme is the easiest model to study mammalian BCO2 because it has only one isoform, unlike human BCO2 with several major isoforms with various properties. Here, we used the same acyl-RAC methodology and confocal microscopy to elucidate palmitoylation and localization status of mBCO2 in the presence of xanthophylls. We created large unilamellar vesicle-based nanocarriers for the successful delivery of xanthophylls into cells. We demonstrate here that, upon treatment with low micromolar concentration of lutein (0.15 µM), mBCO2 is depalmitoylated and shows partial nuclear localization (38.00 ± 0.04%), while treatment with zeaxanthin (0.45 µM) and violaxanthin (0.6 µM) induces depalmitoylation and protein translocation from mitochondria to a lesser degree (20.00 ± 0.01% and 35.00 ± 0.02%, respectively). Such a difference in the behavior of mBCO2 toward various xanthophylls and its translocation into the nucleus in the presence of various xanthophylls suggests a possible mechanism for transport of lutein/lutein cleavage products to the nucleus to affect transcriptional regulation.

Ionone Is More than a Violet's Fragrance: A Review

The term ionone is derived from “iona” (Greek for violet) which refers to the violet scent and “ketone” due to its structure. Ionones can either be chemically synthesized or endogenously produced via asymmetric cleavage of β-carotene by β-carotene oxygenase 2 (BCO2). We recently proposed a possible metabolic pathway for the conversion of α-and β-pinene into α-and β-ionone. The differences between BCO1 and BCO2 suggest a unique physiological role of BCO2; implying that β-ionone (one of BCO2 products) is involved in a prospective biological function. This review focuses on the effects of ionones and the postulated mechanisms or signaling cascades involved mediating these effects. β-Ionone, whether of an endogenous or exogenous origin possesses a range of pharmacological effects including anticancer, chemopreventive, cancer promoting, melanogenesis, anti-inflammatory and antimicrobial actions. β-Ionone mediates these effects via activation of olfactory receptor (OR51E2) and regulation of the activity or expression of cell cycle regulatory proteins, pro-apoptotic and anti-apoptotic proteins, HMG-CoA reductase and pro-inflammatory mediators. α-Ionone and β-ionone derivatives exhibit anti-inflammatory, antimicrobial and anticancer effects, however the corresponding structure activity relationships are still inconclusive. Overall, data demonstrates that ionone is a promising scaffold for cancer, inflammation and infectious disease research and thus is more than simply a violet’s fragrance.

Carotenoid metabolism at the intestinal barrier

Carotenoids exert a rich variety of physiological functions in mammals and are beneficial for human health. These lipids are acquired from the diet and metabolized to apocarotenoids, including retinoids (vitamin A and its metabolites). The small intestine is a major site for their absorption and bioconversion. From here, carotenoids and their metabolites are distributed within the body in triacylglycerol-rich lipoproteins to support retinoid signaling in peripheral tissues and photoreceptor function in the eyes. In recent years, much progress has been made in identifying carotenoid metabolizing enzymes, transporters, and binding proteins. A diet-responsive regulatory network controls the activity of these components and adapts carotenoid absorption and bioconversion to the bodily requirements of these lipids. Genetic variability in the genes encoding these components alters carotenoid homeostasis and is associated with pathologies. We here summarize the advanced state of knowledge about intestinal carotenoid metabolism and its impact on carotenoid and retinoid homeostasis of other organ systems, including the eyes, liver, and immune system. The implication of the findings for science-based intake recommendations for these essential dietary lipids is discussed. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Engineering carotenoid production in mammalian cells for nutritionally enhanced cell-cultured foods

Metabolic engineering of mammalian cells has to-date focused primarily on biopharmaceutical protein production or the manipulation of native metabolic processes towards therapeutic aims. However, significant potential exists for expanding these techniques to diverse applications by looking across the taxonomic tree to bioactive metabolites not synthesized in animals. Namely, cross-taxa metabolic engineering of mammalian cells could offer value in applications ranging fromfood and nutrition to regenerative medicine and gene therapy. Towards the former, recent advances in meat production through cell culture suggest the potential to produce meat with fine cellular control, where tuning composition through cross-taxa metabolic engineering could enhance nutrition and food-functionality. Here we demonstrate this possibility by engineering primary bovine and immortalized murine muscle cells with prokaryotic enzymes to endogenously produce the antioxidant carotenoids phytoene, lycopene and β-carotene. These phytonutrients offer general nutritive value and protective effects against diseases associated with red and processed meat consumption, and so offer a promising proof-of-concept for nutritional engineering in cultured meat. We demonstrate the phenotypic integrity of engineered cells, the ability to tune carotenoid yields, and the antioxidant functionality of these compounds in vitro towards both nutrition and food-quality objectives. Our results demonstrate the potential for tailoring the nutritional profile of cultured meats. They further lay a foundation for heterologous metabolic engineering of mammalian cells for applications outside of the clinical realm.

Enzymology of vertebrate carotenoid oxygenases

Mammals and higher vertebrates including humans have only three members of the carotenoid cleavage dioxygenase family of enzymes. This review focuses on the two that function as carotenoid oxygenases. β-Carotene 15,15'-dioxygenase (BCO1) catalyzes the oxidative cleavage of the central 15,15' carbon-carbon double of β-carotene bond by addition of molecular oxygen. The product of the reaction is retinaldehyde (retinal or β-apo-15-carotenal). Thus, BCO1 is the enzyme responsible for the conversion of provitamin A carotenoids to vitamin A. It also cleaves the 15,15' bond of β-apocarotenals to yield retinal and of lycopene to yield apo-15-lycopenal. β-Carotene 9',10'-dioxygenase (BCO2) catalyzes the cleavage of the 9,10 and 9',10' double bonds of a wider variety of carotenoids, including both provitamin A and non-provitamin A carotenoids, as well as the xanthophylls, lutein and zeaxanthin. Indeed, the enzyme shows a marked preference for utilization of these xanthophylls and other substrates with hydroxylated terminal rings. Studies of the phenotypes of BCO1 null, BCO2 null, and BCO1/2 double knockout mice and of humans with polymorphisms in the enzymes, has clarified the role of these enzymes in whole body carotenoid and vitamin A homeostasis. These studies also demonstrate the relationship between enzyme expression and whole body lipid and energy metabolism and oxidative stress. In addition, relationships between BCO1 and BCO2 and the development or risk of metabolic diseases, eye diseases and cancer have been observed. While the precise roles of the enzymes in the pathophysiology of most of these diseases is not presently clear, these gaps in knowledge provide fertile ground for rigorous future investigations. This article is part of a Special Issue entitled Carotenoids: Recent Advances in Cell and Molecular Biology edited by Johannes von Lintig and Loredana Quadro.

The human mitochondrial enzyme BCO2 exhibits catalytic activity toward carotenoids and apocarotenoids

The enzyme β-carotene oxygenase 2 (BCO2) converts carotenoids into more polar metabolites. Studies in mammals, fish, and birds revealed that BCO2 controls carotenoid homeostasis and is involved in the pathway for vitamin A production. However, it is controversial whether BCO2 function is conserved in humans, because of a 4-amino acid long insertion caused by a splice acceptor site polymorphism. We here show that human BCO2 splice variants, BCO2a and BCO2b, are expressed as pre-proteins with mitochondrial targeting sequence (MTS). The MTS of BCO2a directed a green fluorescent reporter protein to the mitochondria when expressed in ARPE-19 cells. Removal of the MTS increased solubility of BCO2a when expressed in Escherichia coli and rendered the recombinant protein enzymatically active. The expression of the enzymatically active recombinant human BCO2a was further improved by codon optimization and its fusion with maltose-binding protein. Introduction of the 4-amino acid insertion into mouse Bco2 did not impede the chimeric enzyme's catalytic proficiency. We further showed that the chimeric BCO2 displayed broad substrate specificity and converted carotenoids into two ionones and a central C14-apocarotendial by oxidative cleavage reactions at C9,C10 and C9',C10'. Thus, our study demonstrates that human BCO2 is a catalytically competent enzyme. Consequently, information on BCO2 becomes broadly applicable in human biology with important implications for the physiology of the eyes and other tissues.

Carotenoid metabolism in mitochondrial function

Mitochondria are highly dynamic organelles that are found in most eukaryotic organisms. It is broadly accepted that mitochondria originally evolved from prokaryotic bacteria, e.g. proteobacteria. The mitochondrion has its independent genome that encodes 37 genes, including 13 genes for oxidative phosphorylation. Accumulative evidence demonstrates that mitochondria are not only the powerhouse of the cells by supplying adenosine triphosphate, but also exert roles as signalling organelles in the cell fate and function. Numerous factors can affect mitochondria structurally and functionally. Carotenoids are a large group of fat-soluble pigments commonly found in our diets. Recently, much attention has been paid in carotenoids as dietary bioactives in mitochondrial structure and function in human health and disease, though the mechanistic research is limited. Here, we update the recent progress in mitochondrial functioning as signalling organelles in human health and disease, summarize the potential roles of carotenoids in regulation of mitochondrial redox homeostasis, biogenesis, and mitophagy, and discuss the possible approaches for future research in carotenoid regulation of mitochondrial function.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

Apo-12'-lycopenal, a Lycopene Metabolite, Promotes Adipocyte Differentiation via Peroxisome Proliferator-Activated Receptor γ Activation

Apo-lycopenals, lycopene metabolites produced by an initial cleavage by β,β-carotene-9',10'-oxygenase, exhibit diverse biologically active effects. In this study, we investigated the effect of apo-lycopenals on the activation of nuclear receptors involved in glucose and lipid metabolism. Only apo-12'-lycopenal exhibited selective and dose-dependent transactivation activity for peroxisome proliferator-activated receptor γ (PPARγ), whereas neither apo-6'- nor apo-8'-lycopenals displayed this activity ((7.83 ± 0.66)-, (1.32 ± 0.10)-, and (1.31 ± 0.37)-fold higher activity relative to control, respectively). Additionally, apo-12'-lycopenal promoted adipocyte differentiation of 3T3-L1 cells and subsequently increased the mRNA levels of PPARγ (a (2.36 ± 0.07)-fold increase relative to control; p < 0.01) and its target genes, as well as enhanced adiponectin secretion (a (3.25 ± 0.27)-fold increase relative to control; p < 0.01) and insulin-stimulated glucose uptake (1486 ± 85 pmol/well; p < 0.001) in 3T3-L1 cells. Our results indicated that apo-12'-lycopenal promoted adipocyte differentiation by direct binding and activation of PPARγ.

β-apo-10'-carotenoids support normal embryonic development during vitamin A deficiency

Vitamin A deficiency is still a public health concern affecting millions of pregnant women and children. Retinoic acid, the active form of vitamin A, is critical for proper mammalian embryonic development. Embryos can generate retinoic acid from maternal circulating β-carotene upon oxidation of retinaldehyde produced via the symmetric cleavage enzyme β-carotene 15,15'-oxygenase (BCO1). Another cleavage enzyme, β-carotene 9',10'-oxygenase (BCO2), asymmetrically cleaves β-carotene in adult tissues to prevent its mitochondrial toxicity, generating β-apo-10'-carotenal, which can be converted to retinoids (vitamin A and its metabolites) by BCO1. However, the role of BCO2 during mammalian embryogenesis is unknown. We found that mice lacking BCO2 on a vitamin A deficiency-susceptible genetic background (Rbp4^-/-) generated severely malformed vitamin A-deficient embryos. Maternal β-carotene supplementation impaired fertility and did not restore normal embryonic development in the Bco2^-/-Rbp4^-/- mice, despite the expression of BCO1. These data demonstrate that BCO2 prevents β-carotene toxicity during embryogenesis under severe vitamin A deficiency. In contrast, β-apo-10'-carotenal dose-dependently restored normal embryonic development in Bco2^-/-Rbp4^-/- but not Bco1^-/-Bco2^-/-Rbp4^-/- mice, suggesting that β-apo-10'-carotenal facilitates embryogenesis as a substrate for BCO1-catalyzed retinoid formation. These findings provide a proof of principle for the important role of BCO2 in embryonic development and invite consideration of β-apo-10'-carotenal as a nutritional supplement to sustain normal embryonic development in vitamin A-deprived pregnant women.

Carotenoid Lutein Selectively Inhibits Breast Cancer Cell Growth and Potentiates the Effect of Chemotherapeutic Agents through ROS-Mediated Mechanisms

Increasing evidence suggests that dietary carotenoids may reduce the risk of breast cancer. However, anti-breast cancer effects of carotenoids have been controversial, albeit understudied. Here, we investigated the effects of specific carotenoids on a wide range of breast cancer cell lines, and found that among several carotenoids (including β-carotene, lutein, and astaxanthin), lutein significantly inhibits breast cancer cell growth by inducing cell-cycle arrest and caspase-independent cell death, but it has little effect on the growth of primary mammary epithelial cells (PmECs). Moreover, lutein-mediated growth inhibition of breast cancer cells is quantitatively similar to that induced by chemotherapeutic taxanes, paclitaxel and docetaxel, and exposure to lutein plus taxanes additively inhibits breast cancer cell growth. Analysis of mechanisms showed that lutein treatment significantly increases the intracellular reactive oxygen species (ROS) production in triple-negative breast cancer (TNBC) cells, but not in normal PmECs. Lutein-induced growth inhibition is also attenuated by the radical oxygen scavenger N-acetyl cysteine, suggesting a role for ROS generation in the growth inhibitory effect of lutein on TNBC cells. Additionally, we found that the p53 signaling pathway is activated and HSP60 levels are increased by lutein treatment, which may contribute partly to the induction of growth inhibition in TNBC cells. Our findings show that lutein promotes growth inhibition of breast cancer cells through increased cell type-specific ROS generation and alternation of several signaling pathways. Dietary lutein supplementation may be a promising alternative and/or adjunct therapeutic candidate against breast cancer.

Odorant Receptor 51E2 Agonist β-ionone Regulates RPE Cell Migration and Proliferation

The odorant receptor 51E2 (OR51E2), which is well-characterized in prostate cancer cells and epidermal pigment cells, was identified for the first time as the most highly expressed OR in human fetal and adult retinal pigment epithelial (RPE) cells. Immunofluorescence staining and Western blot analysis revealed OR51E2 localization throughout the cytosol and in the plasma membrane. Additionally, immunohistochemical staining of diverse layers of the eye showed that the expression of OR51E2 is restricted to the pigment cells of the RPE and choroid. The results of Ca²⁺-imaging experiments demonstrate that activation of OR51E2 triggers a Ca²⁺ dependent signal pathway in RPE cells. Downstream signaling of OR51E2 involves the activation of adenylyl cyclase, ERK1/2 and AKT. The activity of these protein kinases likely accounts for the demonstrated increase in the migration and proliferation of RPE cells upon stimulation with the OR51E2 ligand β-ionone. These findings suggest that OR51E2 is involved in the regulation of RPE cell growth. Thus, OR51E2 represents a potential target for the treatment of proliferative disorders.

Targeted Metabolomics Reveals Abnormal Hepatic Energy Metabolism by Depletion of β-Carotene Oxygenase 2 in Mice

β-carotene oxygenase 2 (BCO2) is a carotenoid cleavage enzyme located in the inner mitochondrial membrane. Ablation of BCO2 impairs mitochondrial function leading to oxidative stress. Herein, we performed a targeted metabolomics study using ultrahigh performance liquid chromatography-tandem mass spectroscopy and gas chromatography-mass spectroscopy to discriminate global metabolites profiles in liver samples from six-week-old male BCO2 systemic knockout (KO), heterozygous (Het), and wild type (WT) mice fed a chow diet. Principal components analysis revealed distinct differences in metabolites in the livers of KO mice, compared to WT and Het mice. However, no marked difference was found in the metabolites of the Het mouse liver compared to the WT. We then conducted random forest analysis to classify the potential biomarkers to further elucidate the different metabolomics profiles. We found that systemic ablation of BCO2 led to perturbations in mitochondrial function and metabolism in the TCA cycle, amino acids, carnitine, lipids, and bile acids. In conclusion, BCO2 is essential to macronutrient and mitochondrial metabolism in the livers of mice. The ablation of BCO2 causes dysfunctional mitochondria and altered energy metabolism, which further leads to systemic oxidative stress and inflammation. A single functional copy of BCO2 largely rescues the hepatic metabolic homeostasis in mice.

Host-related factors explaining interindividual variability of carotenoid bioavailability and tissue concentrations in humans

Carotenoid dietary intake and their endogenous levels have been associated with a decreased risk of several chronic diseases. There are indications that carotenoid bioavailability depends, in addition to the food matrix, on host factors. These include diseases (e.g. colitis), life-style habits (e.g. smoking), gender and age, as well as genetic variations including single nucleotide polymorphisms that govern carotenoid metabolism. These are expected to explain interindividual differences that contribute to carotenoid uptake, distribution, metabolism and excretion, and therefore possibly also their association with disease risk. For instance, digestion enzymes fostering micellization (PNLIP, CES), expression of uptake/efflux transporters (SR-BI, CD36, NPC1L1), cleavage enzymes (BCO1/2), intracellular transporters (FABP2), secretion into chylomicrons (APOB, MTTP), carotenoid metabolism in the blood and liver (LPL, APO C/E, LDLR), and distribution to target tissues such as adipose tissue or macula (GSTP1, StARD3) depend on the activity of these proteins. In addition, human microbiota, e.g. via altering bile-acid concentrations, may play a role in carotenoid bioavailability. In order to comprehend individual, variable responses to these compounds, an improved knowledge on intra-/interindividual factors determining carotenoid bioavailability, including tissue distribution, is required. Here, we highlight the current knowledge on factors that may explain such intra-/interindividual differences.

Ablation of β,β-carotene-9',10'-oxygenase 2 remodels the hypothalamic metabolome leading to metabolic disorders in mice

β,β-Carotene-9',10'-oxygenase 2 (BCO2) is a protein localized to the inner membrane of mitochondria. It was initially discovered as an enzyme that catalyzes the asymmetric cleavage of carotenoids. Systemic depletion of BCO2 causes increased food intake and impaired hepatic lipid metabolism in mice. The aim of this current study was to determine the extent to which BCO2 exerts its role in hypothalamic nutrient metabolism and feeding behavior through remodeling the hypothalamic metabolome in mice. Male BCO2 knockout (KO) and the isogenic wild-type 129S6 (WT) mice at 6 weeks of age were used for metabolic and cytokine and hypothalamic metabolomics and biochemical analysis. Compared to the WT, BCO2 KO mice exhibited widespread disruptions in metabolism and metabolite homeostasis, an increase in fasting blood glucose, a decrease in circulating glucagon and leptin, an elevation of plasma interleukin 1 beta and tumor necrosis factor alpha, and impaired AMP-activated protein kinase signaling. The global hypothalamic metabolomic results revealed that depletion of BCO2 resulted in striking metabolic changes, including suppression of long-chain fatty acids transport into mitochondria, inhibition of the metabolism of dipeptides and sulfur-containing amino acids, and stimulation of local oxidative stress and inflammation in the hypothalamus of BCO2 KO mice. These findings suggest that BCO2 regulates hypothalamic mitochondrial function, nutrient metabolism, and local oxidative stress and inflammation. Complex interplay between the hormone signaling and impaired lipid and glucose metabolism could account for initiation of oxidative stress, inflammation and eventual metabolic disorders in BCO2 KO mice.

Effect of supplemental β-carotene compared to retinyl palmitate on fatty acid profile and expression of mRNA from genes involved in vitamin A metabolism in beef feedlot cattle

To examine the effects of dietary β-carotene (βC) or retinyl palmitate (RP) on fatty acid (FA) profile and mRNA expression, samples were collected from 24 Angus-cross calves that were allotted to four treatments consisting of RP supplemented at 2200 IU/kg, and synthetic β-carotene (SβC) supplemented at one, five or 10 times RP. Longissimus muscle (LM) cis-9, trans-11 conjugated linoleic acid was greater in RP compared to SβC1X (P = 0.04). The polyunsaturated:saturated FA increased linearly (P = 0.04) in the LM as dietary SβC increased. Expression of βC oxygenase 2 (βCO2), an enzyme that cleaves β-carotene, was greater in the LM for SβC1X compared to RP and decreased linearly as SβC increased (P ≤ 0.02). Peroxisome proliferator activated receptor γ (PPARγ) expression in the LM increased in SβC1X compared to RP (P = 0.03); however, PPARγ and retinoic acid X receptor α (RXRα) expression decreased linearly (P = 0.02) in the LM with increasing SβC. Retinoic acid receptor α (RARα) expression tended (P = 0.10) to decrease linearly in the LM with increased SβC. In conclusion, SβC supplementation increased mRNA expression of some lipogenic genes in the LM, but increasing dietary SβC inhibited their expression and tended to increase polyunsaturated FA.

New insights and changing paradigms in the regulation of vitamin A metabolism in development

Vitamin A and its active metabolite retinoic acid are essential for embryonic development and adult homeostasis. Surprisingly, excess or deficiency of vitamin A and retinoic acid can cause similar developmental defects. Therefore, strict feedback and other mechanisms exist to regulate the levels of retinoic acid within a narrow physiological range. The oxidation of vitamin A to retinal has recently been established as a critical nodal point in the synthesis of retinoic acid, and over the past decade, RDH10 and DHRS3 have emerged as the predominant enzymes that regulate this reversible reaction. Together they form a codependent complex that facilitates negative feedback maintenance of retinoic acid levels and thus guard against the effects of dysregulated vitamin A metabolism and retinoic acid synthesis. This review focuses on advances in our understanding of the roles of Rdh10 and Dhrs3 and their impact on development and disease. WIREs Dev Biol 2017, 6:e264. doi: 10.1002/wdev.264 For further resources related to this article, please visit the WIREs website.

Lack of β, β-carotene-9', 10'-oxygenase 2 leads to hepatic mitochondrial dysfunction and cellular oxidative stress in mice

SCOPE

β,β-Carotene-9',10'-dioxygenase 2 (BCO2) is a carotenoid cleavage enzyme localized to the inner mitochondrial membrane in mammals. This study was aimed to assess the impact of genetic ablation of BCO2 on hepatic oxidative stress through mitochondrial function in mice.

METHODS AND RESULTS

Liver samples from 6-wk-old male BCO2^-/- knockout (KO) and isogenic wild-type (WT) mice were subjected to proteomics and functional activity assays. Compared to the WT, KO mice consumed more food (by 18%) yet displayed significantly lower body weight (by 12%). Mitochondrial proteomic results demonstrated that loss of BCO2 was associated with quantitative changes of the mitochondrial proteome mainly shown by suppressed expression of enzymes and/or proteins involved in fatty acid β-oxidation, the tricarboxylic acid cycle, and the electron transport chain. The mitochondrial basal respiratory rate, proton leak, and electron transport chain complex II capacity were significantly elevated in the livers of KO compared to WT mice. Moreover, elevated reactive oxygen species and increased mitochondrial protein carbonylation were also demonstrated in liver of KO mice.

CONCLUSIONS

Loss of BCO2 induces mitochondrial hyperactivation, mitochondrial stress, and changes of the mitochondrial proteome, leading to mitochondrial energy insufficiency. BCO2 appears to be critical for proper hepatic mitochondrial function.

Mechanisms of selective delivery of xanthophylls to retinal pigment epithelial cells by human lipoproteins

The xanthophylls, lutein and zeaxanthin, are dietary carotenoids that selectively accumulate in the macula of the eye providing protection against age-related macular degeneration. To reach the macula, carotenoids cross the retinal pigment epithelium (RPE). Xanthophylls and β-carotene mostly associate with HDL and LDL, respectively. HDL binds to cells via a scavenger receptor class B1 (SR-B1)-dependent mechanism, while LDL binds via the LDL receptor. Using an in-vitro, human RPE cell model (ARPE-19), we studied the mechanisms of carotenoid uptake into the RPE by evaluating kinetics of cell uptake when delivered in serum or isolated LDL or HDL. For lutein and β-carotene, LDL delivery resulted in the highest rates and extents of uptake. In contrast, HDL was more effective in delivering zeaxanthin and meso-zeaxanthin leading to the highest rates and extents of uptake of all four carotenoids. Inhibitors of SR-B1 suppressed zeaxanthin delivery via HDL. Results show a selective HDL-mediated uptake of zeaxanthin and meso-zeaxanthin via SR-B1 and a LDL-mediated uptake of lutein. This demonstrates a plausible mechanism for the selective accumulation of zeaxanthin greater than lutein and xanthophylls over β-carotene in the retina. We found no evidence of xanthophyll metabolism to apocarotenoids or lutein conversion to meso-zeaxanthin.

Mitochondrial β-Carotene 9',10' Oxygenase Modulates Prostate Cancer Growth via NF-κB Inhibition: A Lycopene-Independent Function

Despite numerous inquiries into protective roles of lycopene in prostate cancer prevention or therapy, little is known about mechanisms by which lycopene or its metabolites inhibit prostate cancer. The enzyme β-carotene 9',10'-oxygenase (BCO2), which catalyzes asymmetric cleavage of several carotenoids, is the principal regulator of lycopene metabolism, but the range of BCO2 biological functions is incompletely understood. This study investigated expression and functional roles of BCO2 in human prostate cancer. Expression of the bco2 gene is dramatically decreased in prostate cancer tissue and in a range of prostate cancer cell lines as compared with nonneoplastic prostate tissue and normal prostatic epithelial cells, respectively. Inhibition of DNA methyltransferase activity restored bco2 expression in prostate cancer cell lines tested. Treatment with lycopene or its metabolite, apo-10-lycopenal, also increased bco2 expression and reduced cell proliferation in androgen-sensitive cell lines, but lycopene neither altered bco2 expression nor cell growth in androgen-resistant cells. Notably, restoring bco2 expression in prostate cancer cells inhibited cell proliferation and colony formation, irrespective of lycopene exposure. Exogenous expression of either wild-type BCO2 or a mutant (enzymatically inactive) BCO2 in prostate cancer cells reduced NF-κB activity and decreased NF-κB nuclear translocation and DNA binding. Together, these results indicate epigenetic loss of BCO2 expression is associated with prostate cancer progression. Moreover, these findings describe previously unanticipated functions of BCO2 that are independent of its enzymatic role in lycopene metabolism.

IMPLICATIONS

This study identifies BCO2 as a tumor suppressor in prostate cancer. BCO2-mediated inhibition of NF-κB signaling implies BCO2 status is important in prostate cancer progression. Mol Cancer Res; 14(10); 966-75. ©2016 AACR.

Molecular aspects of β, β-carotene-9', 10'-oxygenase 2 in carotenoid metabolism and diseases

Carotenoids, the carotenes and xanthophylls, are essential components in human nutrition. β, β-carotene-9', 10'-oxygenase 2 (BCO2), also named as β, β-carotene-9', 10'-dioxygenase 2 (BCDO2) catalyzes the asymmetrical cleavage of carotenoids, whereas β, β-carotene-15, 15'-monooxygenase (BCMO1) conducts the symmetrical cleavage of pro-vitamin A carotenoids into retinoid. Unlike BCMO1, BCO2 has a broader substrate specificity and has been considered an alternative way to produce vitamin A. In contrast to BCMO1, a cytoplasmic protein, BCO2 is located in the inner mitochondrial membrane. The difference in cellular compartmentalization may reflect the different substrate specificity and physiological functions with respect to BCMO1 and BCO2. The BCO2 gene mutations are proven to be associated with yellow color of skin and fat tissue and milk in livestock. Mutation in intron 2 of BCO2 gene is also supposed to be related to the expression of IL-18, a pro-inflammatory cytokine associated with obesity, cardiovascular diseases, and type 2 diabetes. Further, BCO2 is associated with the development of mitochondrial oxidative stress, macular degeneration, anemia, and hepatic steatosis. This review of the literature will mostly address recent updates regarding the role of BCO2 in carotenoid metabolism, and discuss the potential impacts of BCO2 protein and the mutations in mammalian diseases.

Substrate Specificity of Purified Recombinant Chicken β-Carotene 9',10'-Oxygenase (BCO2)

Provitamin A carotenoids are oxidatively cleaved by β-carotene 15,15'-dioxygenase (BCO1) at the central 15-15' double bond to form retinal (vitamin A aldehyde). Another carotenoid oxygenase, β-carotene 9',10'-oxygenase (BCO2) catalyzes the oxidative cleavage of carotenoids at the 9'-10' bond to yield an ionone and an apo-10'-carotenoid. Previously published substrate specificity studies of BCO2 were conducted using crude lysates from bacteria or insect cells expressing recombinant BCO2. Our attempts to obtain active recombinant human BCO2 expressed in Escherichia coli were unsuccessful. We have expressed recombinant chicken BCO2 in the strain E. coli BL21-Gold (DE3) and purified the enzyme by cobalt ion affinity chromatography. Like BCO1, purified recombinant chicken BCO2 catalyzes the oxidative cleavage of the provitamin A carotenoids β-carotene, α-carotene, and β-cryptoxanthin. Its catalytic activity with β-carotene as substrate is at least 10-fold lower than that of BCO1. In further contrast to BCO1, purified recombinant chicken BCO2 also catalyzes the oxidative cleavage of 9-cis-β-carotene and the non-provitamin A carotenoids zeaxanthin and lutein, and is inactive with all-trans-lycopene and β-apocarotenoids. Apo-10'-carotenoids were detected as enzymatic products by HPLC, and the identities were confirmed by LC-MS. Small amounts of 3-hydroxy-β-apo-8'-carotenal were also consistently detected in BCO2-β-cryptoxanthin reaction mixtures. With the exception of this activity with β-cryptoxanthin, BCO2 cleaves specifically at the 9'-10' bond to produce apo-10'-carotenoids. BCO2 has been shown to function in preventing the excessive accumulation of carotenoids, and its broad substrate specificity is consistent with this.

Lutein, zeaxanthin, and meso-zeaxanthin: The basic and clinical science underlying carotenoid-based nutritional interventions against ocular disease

The human macula uniquely concentrates three carotenoids: lutein, zeaxanthin, and meso-zeaxanthin. Lutein and zeaxanthin must be obtained from dietary sources such as green leafy vegetables and orange and yellow fruits and vegetables, while meso-zeaxanthin is rarely found in diet and is believed to be formed at the macula by metabolic transformations of ingested carotenoids. Epidemiological studies and large-scale clinical trials such as AREDS2 have brought attention to the potential ocular health and functional benefits of these three xanthophyll carotenoids consumed through the diet or supplements, but the basic science and clinical research underlying recommendations for nutritional interventions against age-related macular degeneration and other eye diseases are underappreciated by clinicians and vision researchers alike. In this review article, we first examine the chemistry, biochemistry, biophysics, and physiology of these yellow pigments that are specifically concentrated in the macula lutea through the means of high-affinity binding proteins and specialized transport and metabolic proteins where they play important roles as short-wavelength (blue) light-absorbers and localized, efficient antioxidants in a region at high risk for light-induced oxidative stress. Next, we turn to clinical evidence supporting functional benefits of these carotenoids in normal eyes and for their potential protective actions against ocular disease from infancy to old age.

Vitamin A Absorption, Storage and Mobilization

It is well established that chylomicron remnant (dietary) vitamin A is taken up from the circulation by hepatocytes, but more than 80 % of the vitamin A in the liver is stored in hepatic stellate cells (HSC). It presently is not known how vitamin A is transferred from hepatocytes to HSCs for storage. Since retinol-binding protein 4 (RBP4), a protein that is required for mobilizing stored vitamin A, is synthesized solely by hepatocytes and not HSCs, it similarly is not known how vitamin A is transferred from HSCs to hepatocytes. Although it has long been thought that RBP4 is absolutely essential for delivering vitamin A to tissues, recent research has proven that this notion is incorrect since total RBP4-deficiency is not lethal. In addition to RBP4, vitamin A is also found in the circulation bound to lipoproteins and as retinoic acid bound to albumin. It is not known how these different circulating pools of vitamin A contribute to the vitamin A needs of different tissues. In our view, better insight into these three issues is required to better understand vitamin A absorption, storage and mobilization. Here, we provide an up to date synthesis of current knowledge regarding the intestinal uptake of dietary vitamin A, the storage of vitamin A within the liver, and the mobilization of hepatic vitamin A stores, and summarize areas where our understanding of these processes is incomplete.