Retinoic acid can be produced from excentric cleavage of beta-carotene in human intestinal mucosa

Identification of Enzymatic Cleavage Products of -Carotene-Rich Extracts of Kale and Biofortified Maize

Provitamin A carotenoids in plant foods provide more than 80% of vitamin A intake for people in developing countries. Therefore, the conversion efficiency of β-carotene to vitamin A is important, as it determines the effectiveness of plant foods as sources of vitamin A in humans. The objective of this study was to determine the effect of plant food antioxidants such as α-tocopherol, γ-tocopherol, α-tocotrienol, γ-tocotrienol and total γ-oryzanol on the cleavage of β-carotene in vitro. Rat intestinal mucosa post mitochondrial fractions were incubated with β-carotene-rich extracts of kale and biofortified maize for an hour at 37°C. Rat intestinal mucosa post mitochondrial fractions were also incubated with β-carotene in the presence of either α-tocopherol, γ-tocopherol, α-tocotrienol, γ-tocotrienol or γ-oryzanol for 60 min at 37°C. The β-carotene cleavage products were extracted and analyzed by an HPLC equipped with a C18 column at 340nm and 450nm. When β-carotene alone was incubated without intestinal mucosa homogenate (control), no cleavage products were detected. When β-carotene alone was incubated with intestinal mucosa homogenate, β-apo-13-carotenone, β-apo-14-carotenal, retinal, retinol and retinoic acid were formed. However, incubation of β-carotene with either α-tocopherol, γ-tocopherol or α-tocotrienol resulted in a 10 fold inhibition of β-apo-14-carotenal and β-apo-13-carotenone formation. Antioxidant rich biofortified maize extract incubated with postmitochondrial fraction produced less β-apo-13-carotenone compared to the kale extract. These results suggest that antioxidants inhibit the cleavage of β-carotene and the formation of excentric cleavage products (β-apo-13-carotenone, β-apo-14-carotenal).

Biomarkers of carotenoid bioavailability

The use of biomarkers constitutes an essential tool to assess the bioavailability of carotenoids in humans. The present article aims to review several methodological, host-related and modulating factors relevant on assessing and interpreting carotenoid bioavailability. Markers for carotenoid bioavailability can be broadly divided into direct, biochemical or "analytical" markers and indirect, physiological or "functional" indicators. Analytical markers usually refer to biochemical indicators of intake and/or status (short and long term exposure) while functional measures may be interpreted in terms of cumulative exposure, biological effect (bioactivity) or modification of risk factors. Both types of markers display advantages and limitations but, in general, a relationship exists among the type of marker, the biological specimen needed and the time required for a change. Humans may absorb a wide range of carotenes and xanthophylls and many of them may be found in serum and tissues. However, under physiological conditions, the several classes of dietary carotenoids may behave unequally leading to a different systemic profile and, moreover, they can be selectively accumulated at target tissues. In addition, some carotenoids may be chemically and enzymatically modified generating different oxidative metabolites and apocarotenoids. Quantitatively, the biological response upon carotenoid intervention (assessed by analytical and functional markers) is highly variable but the use of large doses and long-term protocols may lead to saturation effects and the loss of linearity in the response. Also, despite carotenoid exposition is considered to be safe, markers of overexposure include clinical signs (i.e. carotenodermia, corneal rings and retinopathy) and biochemical indicators (hypercarotenemia, xanthophyll esters). Overall, both host-related and methodological factors may influence analytical and functional markers to assess carotenoid bioavailability although the different subclasses of carotenoids may not be equally affected.

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.

Transcriptional Factors Mediating Retinoic Acid Signals in the Control of Energy Metabolism

Retinoic acid (RA), an active metabolite of vitamin A (VA), is important for many physiological processes including energy metabolism. This is mainly achieved through RA-regulated gene expression in metabolically active cells. RA regulates gene expression mainly through the activation of two subfamilies in the nuclear receptor superfamily, retinoic acid receptors (RARs) and retinoid X receptors (RXRs). RAR/RXR heterodimers or RXR/RXR homodimers bind to RA response element in the promoters of RA target genes and regulate their expressions upon ligand binding. The development of metabolic diseases such as obesity and type 2 diabetes is often associated with profound changes in the expressions of genes involved in glucose and lipid metabolism in metabolically active cells. RA regulates some of these gene expressions. Recently, in vivo and in vitro studies have demonstrated that status and metabolism of VA regulate macronutrient metabolism. Some studies have shown that, in addition to RARs and RXRs, hepatocyte nuclear factor 4α, chicken ovalbumin upstream promoter-transcription factor II, and peroxisome proliferator activated receptor β/δ may function as transcriptional factors mediating RA response. Herein, we summarize current progresses regarding the VA metabolism and the role of nuclear receptors in mediating RA signals, with an emphasis on their implication in energy metabolism.

Evidence for compartmentalization of mammalian carotenoid metabolism

The critical role of retinoids (vitamin A and its derivatives) for vision, reproduction, and survival has been well established. Vitamin A is produced from dietary carotenoids such as β-carotene by centric cleavage via the enzyme BCO1. The biochemical and molecular identification of a second structurally related β-carotene metabolizing enzyme, BCO2, has led to a prolonged debate about its relevance in vitamin A biology. While BCO1 cleaves provitamin A carotenoids, BCO2 is more promiscuous and also metabolizes nonprovitamin A carotenoids such as zeaxanthin into long-chain apo-carotenoids. Herein we demonstrate, in cell lines, that human BCO2 is associated with the inner mitochondrial membrane. Different human BCO2 isoforms possess cleavable N-terminal leader sequences critical for mitochondrial import. Subfractionation of murine hepatic mitochondria confirmed the localization of BCO2 to the inner mitochondrial membrane. Studies in BCO2-knockout mice revealed that zeaxanthin accumulates in the inner mitochondrial membrane; in contrast, β-carotene is retained predominantly in the cytoplasm. Thus, we provide evidence for a compartmentalization of carotenoid metabolism that prevents competition between BCO1 and BCO2 for the provitamin and the production of noncanonical β-carotene metabolites.

Maternal-fetal transfer and metabolism of vitamin A and its precursor β-carotene in the developing tissues

The requirement of the developing mammalian embryo for retinoic acid is well established. Retinoic acid, the active form of vitamin A, can be generated from retinol and retinyl ester obtained from food of animal origin, and from carotenoids, mainly β-carotene, from vegetables and fruits. The mammalian embryo relies on retinol, retinyl ester and β-carotene circulating in the maternal bloodstream for its supply of vitamin A. The maternal-fetal transfer of retinoids and carotenoids, as well as the metabolism of these compounds in the developing tissues are still poorly understood. The existing knowledge in this field has been summarized in this review in reference to our basic understanding of the transport and metabolism of retinoids and carotenoids in adult tissues. The need for future research on the metabolism of these essential lipophilic nutrients during development is highlighted. This article is part of a Special Issue entitled: Retinoid and Lipid Metabolism.

Importance of β,β-carotene 15,15'-monooxygenase 1 (BCMO1) and β,β-carotene 9',10'-dioxygenase 2 (BCDO2) in nutrition and health

In humans, varying amounts of absorbed β-carotene are oxidatively cleaved by the enzyme β,β-carotene 15,15'-monooxygenase 1 (BCMO1) into two molecules of all-trans-retinal. The other carotenoid cleavage enzyme β,β-carotene 9',10'-dioxygenase (BCDO2) cleaves β-carotene at the 9',10' double bond forming β-apo-10'-carotenal and β-ionone. Although the contribution of BCDO2 to vitamin A formation has long been debated, BCMO1 is currently considered the key enzyme for retinoid metabolism. Furthermore, BCMO1 has limited enzyme activity towards carotenoids other than provitamin A carotenoids, whereas BCDO2 exhibits a broader specificity. Both enzymes are located at different sites within the cell, with BCMO1 being a cytosolic protein and BCDO2 being located in the mitochondria. Expression of BCMO1 in tissues other than the intestine has recently revealed its function for tissue-specific retinoid metabolism with importance in embryogenesis and lipid metabolism. On the other hand, biological activity of BCDO2 metabolites has been shown to be important in protecting against carotenoid-induced mitochondrial dysfunction. Single-nucleotide polymorphisms (SNPs) such as R267S and A379V in BCMO1 can partly explain inter-individual variations observed in carotenoid metabolism. Advancing knowledge about the physiological role of these two enzymes will contribute to understanding the importance of carotenoids in health and disease.

Animal models in carotenoids research and lung cancer prevention

Numerous epidemiological studies have consistently demonstrated that individuals who eat more fruits and vegetables (which are rich in carotenoids) and who have higher serum β-carotene levels have a lower risk of cancer, especially lung cancer. However, two human intervention trials conducted in Finland and in the United States have reported contrasting results with high doses of β-carotene supplementation increasing the risk of lung cancer among smokers. The failure of these trials to demonstrate actual efficacy has resulted in the initiation of animal studies to reproduce the findings of these two studies and to elucidate the mechanisms responsible for the harmful or protective effects of carotenoids in lung carcinogenesis. Although these studies have been limited by a lack of animal models that appropriately represent human lung cancer induced by cigarette smoke, ferrets and A/J mice are currently the most widely used models for these types of studies. There are several proposed mechanisms for the protective effects of carotenoids on cigarette smoke-induced lung carcinogenesis, and these include antioxidant/prooxidant effects, modulation of retinoic acid signaling pathway and metabolism, induction of cytochrome P450, and molecular signaling involved in cell proliferation and/or apoptosis. The technical challenges associated with animal models include strain-specific and diet-specific effects, differences in the absorption and distribution of carotenoids, and differences in the interactions of carotenoids with other antioxidants. Despite the problems associated with extrapolating from animal models to humans, the understanding and development of various animal models may provide useful information regarding the protective effects of carotenoids against lung carcinogenesis.

β-Carotene and its cleavage enzyme β-carotene-15,15'-oxygenase (CMOI) affect retinoid metabolism in developing tissues

The mammalian embryo relies on maternal circulating retinoids (vitamin A derivatives) for development. β-Carotene is the major human dietary provitamin A. β-Carotene-15,15'-oxygenase (CMOI) has been proposed as the main enzyme generating retinoid from β-carotene in vivo. CMOI is expressed in embryonic tissues, suggesting that β-carotene provides retinoids locally during development. We performed loss of CMOI function studies in mice lacking retinol-binding protein (RBP), an established model of embryonic vitamin A deficiency (VAD). We show that, unexpectedly, lack of CMOI in the developing tissues further exacerbates the severity of VAD and thus the embryonic malformations of RBP(-/-) mice. Since β-carotene was not present in any of the mouse diets, we unveiled a novel action of CMOI independent from its β-carotene cleavage activity. We also show for the first time that CMOI exerts an additional function on retinoid metabolism by influencing retinyl ester formation via modulation of lecithin:retinol acyltransferase (LRAT) activity, at least in developing tissues. Finally, we demonstrate unequivocally that β-carotene can serve as an alternative vitamin A source for the in situ synthesis of retinoids in developing tissues by the action of CMOI.

Enzymatic formation of apo-carotenoids from the xanthophyll carotenoids lutein, zeaxanthin and β-cryptoxanthin by ferret carotene-9',10'-monooxygenase

Xanthophyll carotenoids, such as lutein, zeaxanthin and β-cryptoxanthin, may provide potential health benefits against chronic and degenerative diseases. Investigating pathways of xanthophyll metabolism are important to understanding their biological functions. Carotene-15,15'-monooxygenase (CMO1) has been shown to be involved in vitamin A formation, while recent studies suggest that carotene-9',10'-monooxygenase (CMO2) may have a broader substrate specificity than previously recognized. In this in vitro study, we investigated baculovirus-generated recombinant ferret CMO2 cleavage activity towards the carotenoid substrates zeaxanthin, lutein and β-cryptoxanthin. Utilizing HPLC, LC-MS and GC-MS, we identified both volatile and non-volatile apo-carotenoid products including 3-OH-β-ionone, 3-OH-α-ionone, β-ionone, 3-OH-α-apo-10'-carotenal, 3-OH-β-apo-10'-carotenal, and β-apo-10'-carotenal, indicating cleavage at both the 9,10 and 9',10' carbon-carbon double bond. Enzyme kinetic analysis indicated the xanthophylls zeaxanthin and lutein are preferentially cleaved over β-cryptoxanthin, indicating a key role of CMO2 in non-provitamin A carotenoid metabolism. Furthermore, incubation of 3-OH-β-apo-10'-carotenal with CMO2 lysate resulted in the formation of 3-OH-β-ionone. In the presence of NAD(+), in vitro incubation of 3-OH-β-apo-10'-carotenal with ferret hepatic homogenates formed 3-OH-β-apo-10'-carotenoic acid. Since apo-carotenoids serve as important signaling molecules in a variety of biological processes, enzymatic cleavage of xanthophylls by mammalian CMO2 represents a new avenue of research regarding vertebrate carotenoid metabolism and biological function.

Relationship of serum antioxidant micronutrients and sociodemographic factors to cervical neoplasia: a case-control study

BACKGROUND

Although there have been some epidemiological studies on the effects of diet and nutritional status on cervical carcinogenesis, evidence for a protective effect of antioxidant micronutrients against cervical neoplasia is insufficient. The relationship between serum antioxidant micronutrients and sociodemographic factors and the risk of cervical neoplasia was investigated in this multi-center, case-control study.

METHODS

The study population included women with histopathological diagnosis of cervical intraepithelial neoplasia (CIN) 1 (n=147), CIN 2/3 (n=177), cervical cancer (n=160), and a control group (n=378). Epidemiological data were collected and the serum concentrations of beta-carotene, lycopene, zeaxanthin plus lutein, retinol, alpha-tocopherol, and gamma-tocopherol were measured using reverse-phase, gradient high-pressure liquid chromatography.

RESULTS

Cervical cancer was found to be associated with older age, increased body mass index, and lower socioeconomic status as measured by education level and income. The mean serum concentrations of beta-carotene, lycopene, zeaxanthin plus lutein, retinol, alpha-tocopherol, and gamma-tocopherol of cervical cancer patients were significantly lower than those of control subjects. Odds ratio adjusted for age, smoking status, alcohol consumption, and human papillomavirus infection status revealed a significant gradient of decreasing risk of CIN 1, CIN 2/3, and cervical cancer with increasing serum concentrations of most antioxidant micronutrients.

CONCLUSIONS

The results of this study show an inverse association between serum antioxidant micronutrient concentrations and the risk of cervical neoplasia. These results suggest that antioxidant micronutrients play a role in the prevention of cervical carcinogenesis.

Application of a key events dose-response analysis to nutrients: a case study with vitamin A (retinol)

The methodology used to establish tolerable upper intake levels (UL) for nutrients borrows heavily from risk assessment methods used by toxicologists. Empirical data are used to identify intake levels associated with adverse effects, and Uncertainty Factors (UF) are applied to establish ULs, which in turn inform public health decisions and standards. Use of UFs reflects lack of knowledge regarding the biological events that underlie response to the intake of a given nutrient, and also regarding the sources of variability in that response. In this paper, the Key Events Dose-Response Framework (KEDRF) is used to systematically consider the major biological steps that lead from the intake of the preformed vitamin A to excess systemic levels, and subsequently to increased risk of adverse effects. Each step is examined with regard to factors that influence whether there is progression toward the adverse effect of concern. The role of homeostatic mechanisms is discussed, along with the types of research needed to improve understanding of dose-response for vitamin A. This initial analysis illustrates the potential of the KEDRF as a useful analytical tool for integrating current knowledge regarding dose-response, generating questions that will focus future research efforts, and clarifying how improved knowledge and data could be used to reduce reliance on UFs.

beta-Carotene conversion products and their effects on adipose tissue

Recent epidemiological data suggest that beta-carotene may be protective against metabolic diseases in which adipose tissue plays a key role. Adipose tissue constitutes the major beta-carotene storage tissue and its functions have been shown to be modulated in response to beta-carotene breakdown products, especially retinal produced after cleavage by beta-carotene 15,15'-monooxygenase (BCMO1), and retinoic acid arising from oxidation of retinal. However, the possibility exists that beta-carotene in its intact form can also affect adipocyte function. Development of a knock out model and identification of a loss-of-function mutation have pointed out BCMO1 as being probably the sole enzyme responsible for provitamin A conversion into retinal in mammals. The utilisation of BCMO1(-/-)mice should provide insights on beta-carotene effect on its own in the future. In humans, intervention studies have highlighted the huge interindividual variation of beta-carotene conversion efficiency, possibly due to genetic polymorphisms, which might impact on response to beta-carotene. This brief review discusses the processes involved in beta-carotene conversion and the effect of cleavage products on body fat and adipose tissue function.

Lycopene biodistribution is altered in 15,15'-carotenoid monooxygenase knockout mice

15,15'-carotenoid monooxygenase (CMO I) is generally recognized as the central carotenoid cleavage enzyme responsible for converting provitamin A carotenoids to vitamin A, while having little affinity for nonprovitamin A carotenoids, such as lycopene. To investigate the role of CMO I in carotenoid metabolism, approximately 90-d-old C57BL/6 x 129/SvJ [CMO I wild-type (WT)] and B6;129S6-Bcmo1tm1Dnp [CMO I knockout (KO)] mice were fed a high-fat, moderate vitamin A, cholesterol-containing diet supplemented with 150 mg/kg diet of beta-carotene, lycopene, or placebo beadlets for 60 d (n = 12-14). CMO I KO mice fed lycopene (Lyc-KO) exhibited significant decreases in hepatic, spleen, and thymus lycopene concentrations and significant increases in prostate, seminal vesicles, testes, and brain lycopene concentrations compared with WT mice fed lycopene (Lyc-WT). Furthermore, in the serum and all tissues analyzed, excluding the testes, there was a significant increase in the percent lycopene cis isomers in Lyc-KO mice compared with Lyc-WT mice. CMO I KO mice fed beta-carotene (betaC-KO) had significantly lower hepatic vitamin A concentrations (17% of WT mice fed beta-carotene [betaC-WT]). Concordantly, betaC-KO mice had higher serum and tissue beta-carotene concentrations than betaC-WT mice. In addition, phenotypically CMO I KO mice had significantly higher final body weights and CMO I KO female mice had significantly lower uterus weights than CMO I WT mice. In conclusion, CMO I KO mice fed low levels of vitamin A have altered lycopene biodistribution and isomer patterns and do not cleave beta-carotene to vitamin A at appreciable levels.

Doxorubicin as an antioxidant: maintenance of myocardial levels of lycopene under doxorubicin treatment

The mechanism of doxorubicin-induced cardiotoxicity remains controversial. Wistar rats (n=96) were randomly assigned to a control (C), lycopene (L), doxorubicin (D), or doxorubicin+lycopene (DL) group. The L and DL groups received lycopene (5 mg/kg body wt/day by gavage) for 7 weeks. The D and DL groups received doxorubicin (4 mg/kg body wt intraperitoneally) at 3, 4, 5, and 6 weeks and were killed at 7 weeks for analyses. Myocardial tissue lycopene levels and total antioxidant performance (TAP) were analyzed by HPLC and fluorometry, respectively. Lycopene metabolism was determined by incubating (2)H(10)-lycopene with intestinal mucosa postmitochondrial fraction and lipoxygenase and analyzed with HPLC and APCI mass spectroscopy. Myocardial tissue lycopene levels in DL and L were similar. TAP adjusted for tissue protein were higher in myocardium of D than those of C (P=0.002). Lycopene metabolism study identified a lower oxidative cleavage of lycopene in D as compared to those of C. Our results showed that lycopene was not depleted in myocardium of lycopene-supplemented rats treated with doxorubicin and that higher antioxidant capacity in myocardium and less oxidative cleavage of lycopene in intestinal mucosa of doxorubicin-treated rats suggest an antioxidant role of doxorubicin rather than acting as a prooxidant.

Excentral cleavage of beta-carotene in vivo in a healthy man

BACKGROUND

Excentral cleavage of beta-carotene to retinoids and apocarotenoids occurs in vitro and in animal models. Whether it occurs in humans is unclear.

OBJECTIVE

We tested the hypothesis of whether humans can cleave beta-carotene excentrally.

DESIGN

A healthy man was given an oral dose of all-trans [10,10',11,11'-(14)C]-beta-carotene (1.01 nmol; 100 nCi). Its fate and that of its metabolites were measured in serial plasma samples. Its fate in feces and urine was also measured over time. Selected plasma samples were spiked with reference standards of retinol, beta-apo-12'-carotenal, beta-apo-8'-carotenal, 13-cis-retinoic acid, all-trans-retinoic acid, beta-carotene-5,6-epoxide, all-trans-beta-carotene, and retinyl palmitate and subjected to reverse-phase HPLC fractionation. The plasma, plasma fractions, urine, and feces were measured for (14)C with the use of accelerator mass spectrometry.

RESULTS

Sixty-five percent of administered (14)C was absorbed, and 15.7% was eliminated in urine during the first 21 d after dosing. (14)C-beta-carotene and (14)C-retinyl palmitate appeared in plasma 0.25 d after the dose. (14)C-beta-carotene and (14)C-retinol both appeared at 0.5 d only. On day 3 after the dose, 2 large (14)C peaks appeared in plasma: one matched the retention time of beta-apo-8'-carotenal, and the other did not match any of the reference standards used. The delayed appearance of (14)C-beta-apo-8'-carotenal in plasma suggests that the excentral cleavage occurred after the (14)C-beta-apo-8'-carotene was absorbed into the body.

CONCLUSION

These data suggest that excentral cleavage of ingested beta-carotene occurs in vivo in humans. Confirmation of that possibility and further study to identify and characterize additional metabolites are needed.

The acute and chronic toxic effects of vitamin A

The acute and chronic effects of vitamin A toxicity are well documented in the literature. Emerging evidence suggests that subtoxicity without clinical signs of toxicity may be a growing concern, because intake from preformed sources of vitamin A often exceeds the recommended dietary allowances (RDA) for adults, especially in developed countries. Osteoporosis and hip fracture are associated with preformed vitamin A intakes that are only twice the current RDA. Assessing vitamin A status in persons with subtoxicity or toxicity is complicated because serum retinol concentrations are nonsensitive indicators in this range of liver vitamin A reserves. The metabolism in well-nourished persons of preformed vitamin A, provided by either liver or supplements, has been studied by several research groups. To control vitamin A deficiency, large therapeutic doses are administered in developing countries to women and children, who often are undernourished. Nevertheless, little attention has been given to the short-term kinetics (ie, after absorption but before storage) of a large dose of vitamin A or to the short- and long-term effects of such a dose given to lactating women on serum and breast-milk concentrations of retinol and its metabolites. Moreover, appropriate dosing regimens have not been systematically evaluated to ascertain the quantitative improvement in vitamin A status of the women and children who receive these supplements. The known acute and chronic effects of vitamin A toxicity have been reported previously. However, further research is needed to ascertain the areas of the world in which subclinical toxicity exists and to evaluate its effects on overall health and well-being.

Human intestinal and lung cell lines exposed to β-carotene show a large variation in intracellular levels of β-carotene and its metabolites

Although in vitro models are often used in beta-carotene research, knowledge about the uptake and metabolism of beta-carotene in cell lines is lacking. We measured by HPLC the intracellular levels of beta-carotene and its metabolites in 9 human intestinal and lung cell lines after exposure to 1 microM beta-carotene during 2, 6, 30, 54 h, and 3 weeks. In three colorectal carcinoma cell lines only low levels of beta-carotene could be detected and an apparent linear increase in intracellular beta-carotene was observed during the whole exposure period of 3 weeks. The remaining cell lines (an SV40 transformed colon cell line, a small intestinal carcinoma cell line and several lung cell lines) had medium or high intracellular beta-carotene levels. In these cell lines intracellular beta-carotene quickly increased during the first 54 h of exposure and after 3 weeks no further increase was observed, suggesting a stable level of beta-carotene after 54 h. Estimated intracellular concentrations at steady-state levels varied between 2 and 5 microM (low) or 9 and 55 microM (medium/high). Our results seem to indicate that an active uptake mechanism of beta-carotene exists in at least a subset of cell lines. Seven different beta-carotene metabolites were detected in the various cell lines (cis-carotene, retinol, three epoxy-carotenes, and two retinyl esters). Metabolite levels were the highest in cells with medium or high beta-carotene levels. Each cell line appeared to have a distinct metabolite profile. No intestinal or lung specific pattern could be found, but two epoxy-carotene metabolites were not detectable in the colon cell lines.

Retinoic acid imprints gut-homing specificity on T cells

For a preferential homing of T cells to the gut, expression of the integrin alpha4beta7 and the chemokine receptor CCR9 is essential and is induced by antigenic stimulation with dendritic cells from the gut-associated lymphoid organs. Here, we show that the vitamin A (retinol) metabolite, retinoic acid, enhances the expression of alpha4beta7 and CCR9 on T cells upon activation and imprints them with the gut tropism. Dendritic cells from the gut-associated lymphoid organs produced retinoic acid from retinol. The enhanced alpha4beta7 expression on T cells by antigenic stimulation with these dendritic cells was suppressed by the retinal dehydrogenase inhibitor citral and the retinoic acid receptor antagonist LE135. Accordingly, vitamin A deficiency caused a reduction in alpha4beta7(+) memory/activated T cells in lymphoid organs and a depletion of T cells from the intestinal lamina propria. These findings revealed a novel role for retinoic acid in the imprinting of gut-homing specificity on T cells.

Enzymatic and oxidative metabolites of lycopene

Using the post-mitochondrial fraction of rat intestinal mucosa, we have investigated lycopene metabolism. The incubation media was composed of NAD+, KCI, and DTT with or without added lipoxygenase. The addition of lipoxygenase into the incubation significantly increased the production of lycopene metabolites. The enzymatic incubation products of 2H10 lycopene were separated using high-performance liquid chromatography and analyzed by UV/Vis spectrophotometer and atmospheric pressure chemical ionization-mass spectroscopy. We have identified two types of products: cleavage products and oxidation products. The cleavage products are likely: (1) 3-keto-apo-13-lycopenone (C18H24O2 or 6,10,14-trimethyl-12-one-3,5,7,9,13-pentadecapentaen-2-one) with lambdamax = 365 nm and m/z =272 and (2) 3,4-dehydro-5,6-dihydro-15-apo-lycopenal (C20H28O or 3,7,11,15-tetramethyl-2,4,6,8,12,14-hexadecahexaen-l-al) with lambdamax= 380 nm and m/z = 284. The oxidative metabolites are likely: (3) 2-ene-5,8-lycopenal-furanoxide (C37H50O) with lambdamax = 415 nm, 435 nm, and 470 nm, and m/z = 510; (4) lycopene-5, 6, 5', 6'-diepoxide (C40H56O2) with lambdamax = 415 nm, 440 nm, and 470 nm, and m/z =568; (5) lycopene-5,8-furanoxide isomer (I) (C40H56O2) with lambdamax = 410 nm, 440 nm, and 470 nm, and m/z = 552; (6) lycopene-5,8-epoxide isomer (II) (C40H56O) with lambdamax = 410, 440, 470 nm, and m/z = 552; and (7) 3-keto-lycopene-5',8'-furanoxide (C40H54O2) with lambdamax = 400 nm, 420 nm, and 450 nm, and m/z = 566. These results demonstrate that both central and excentric cleavage of lycopene occurs in the rat intestinal mucosa in the presence of soy lipoxygenase.

Beta-carotene and beta-apo-14'-carotenoic acid prevent the reduction of retinoic acid receptor beta in benzo[a]pyrene-treated normal human bronchial epithelial cells

Low-dose beta-carotene (BC) supplementation, such as would be provided by daily consumption of approximately 5-9 servings of fruits and vegetables, has no apparent detrimental effects, but rather appears to have a protective effect against cigarette smoke-induced lung lesions in ferrets. In the present study, we investigated the effects of BC, beta-apo-14'-carotenoic acid (14'CA), or benzo[a]pyrene (BP; a primary lung carcinogen from cigarette smoke) treatments, either alone or in combination, on cell growth and expression of the retinoic acid receptor (RAR) of normal human bronchial epithelial (NHBE) cells. We found that both BC and 14'CA inhibited the growth of NHBE cells (P < 0.05) with or without BP. The level of RARbeta, a tumor suppressor, but not RARalpha or RARgamma, was reduced by 50% in the NHBE cells treated with BP. However, treatment with either BC or 14'CA significantly induced the expression of RARbeta in the NHBE cells, and prevented the reduction of RARbeta by BP. Furthermore, 14'CA transactivated the RARbeta promoter primarily via its conversion to retinoic acid (RA). In the presence of 3-mercaptopropionic acid, an inhibitor of fatty acid oxidation, both RA formation and transactivation activity from 14'CA were decreased. These observations indicate that the growth inhibitory effects of BC and beta-apo-carotenoic acid are through their conversion to RA and upregulation of RARbeta.

The enigma of beta-carotene in carcinogenesis: what can be learned from animal studies

Beta-carotene and other carotenoids have been thought to have anti-cancer activity, either because of antioxidant activity or because of their ability to be converted to vitamin A. Nevertheless, two large scale intervention studies in humans using high doses of beta-carotene found that beta-carotene supplementation resulted in more lung cancer rather than less lung cancer among smoking and asbestos exposed populations. Studies conducted in the ferret have elucidated molecular mechanisms behind this observation, in that high-dose beta-carotene and smoke exposure in these animals leads to squamous metaplasia, a pre-cancerous lesion in the lung. High dose beta-carotene in the smoke exposed animals was found to give rise to a number of transient oxidative metabolites, which include P450 enzymes that result in the destruction of retinoic acid, and diminished retinoid signaling, and enhanced cell proliferation. In addition, eccentric cleavage beta-carotene metabolites facilitate the binding of smoke derived carcinogens to DNA. In other ferret studies low dose beta-carotene smoke exposure provided mild protection against squamous metaplasia. Thus, it appears that the explanation of the apparent paradoxical effects of beta-carotene on lung cancer is related to dose. The metabolism and breakdown of natural products should be thoroughly investigated in animal models before embarking on large scale intervention trials, particularly when using unusually high doses that greatly exceed normal dietary levels.

?-Carotene storage, conversion to retinoic acid, and induction of the lipocyte phenotype in hepatic stellate cells

Hepatic stellate cells (HSCs) are the major site of retinol (ROH) metabolism and storage. GRX is a permanent murine myofibroblastic cell line, derived from HSCs, which can be induced to display the fat-storing phenotype by treatment with retinoids. Little is known about hepatic or serum homeostasis of beta-carotene and retinoic acid (RA), although the direct biogenesis of RA from beta-carotene has been described in enterocytes. The aim of this study was to identify the uptake, metabolism, storage, and release of beta-carotene in HSCs. GRX cells were plated in 25 cm(2) tissue culture flasks, treated during 10 days with 3 micromol/L beta-carotene and subsequently transferred into the standard culture medium. beta-Carotene induced a full cell conversion into the fat-storing phenotype after 10 days. The total cell extracts, cell fractions, and culture medium were analyzed by reverse phase high-performance liquid chromatography for beta-carotene and retinoids. Cells accumulated 27.48 +/- 6.5 pmol/L beta-carotene/10(6) cells, but could not convert it to ROH nor produced retinyl esters (RE). beta-Carotene was directly converted to RA, which was found in total cell extracts and in the nuclear fraction (10.15 +/- 1.23 pmol/L/10(6) cells), promoting the phenotype conversion. After 24-h chase, cells contained 20.15 +/- 1.12 pmol/L beta-carotene/10(6) cells and steadily released beta-carotene into the medium (6.69 +/- 1.75 pmol/ml). We conclude that HSC are the site of the liver beta-carotene storage and release, which can be used for RA production as well as for maintenance of the homeostasis of circulating carotenoids in periods of low dietary uptake.

Carotene oxygenases: a new family of double bond cleavage enzymes

Beta,beta-carotene 15,15'-monooxygensae (betaCMOOX) is the key enzyme involved in the metabolism of provitamin A carotenoids to retinal. Although the enzyme has been known for >40 y, it has been only within the last 2 y that the cloning and the molecular characterization of the betaCMOOX from several species was reported in literature. New clones of the carotene metabolizing enzyme have emerged, all belonging to the family of double bond cleavage enzymes, suggesting common ancestry. BetaCMOOX cleaves beta,beta-carotene to retinal in an in vitro activity assay; no apo-carotenals were identified. The second enzyme involved in carotenoid metabolism, beta,beta-carotene 9',10'-dioxygenase, is responsible for the excentric cleavage pathway of carotenoids, cleaving beta,beta-carotene to 10'-apo-carotenal and beta-ionone. In an expression overview, the betaCMOOX was detected in duodenum, liver, kidney and in the lungs of chickens. In mice, the mRNA for the central cleavage enzyme was highly expressed in liver, testes, small intestine, and kidney. betaCMOOX expression was highest in epithelial and endothelial structures in both species. These results suggest that the source of vitamin A originates from carotenoids in the corresponding tissues, in addition to retinol supplied from liver stores.

Enzymatic and oxidative metabolites of lycopene

Using the post-mitochondrial fraction of rat intestinal mucosa, we have investigated lycopene metabolism. The incubation media was composed of NAD(+), KCl, and DTT with or without added lipoxygenase. The addition of lipoxygenase into the incubation significantly increased the production of lycopene metabolites. The enzymatic incubation products of (2)H(10) lycopene were separated using high-performance liquid chromatography and analyzed by UV/Vis spectrophotometer and atmospheric pressure chemical ionization-mass spectroscopy. We have identified two types of products: cleavage products and oxidation products. The cleavage products are likely: (1) 3-keto-apo-13-lycopenone (C(18)H(24)O(2) or 6,10,14-trimethyl-12-one-3,5,7,9,13-pentadecapentaen-2-one) with lambdamax = 365 nm and m/z = 272 and (2) 3,4-dehydro-5,6-dihydro-15,15'-apo-lycopenal (C(20)H(28)O or 3,7,11,15-tetramethyl-2,4,6,8,12,14-hexadecahexaen-1-al) with lambdamax = 380 nm and m/z = 284. The oxidative metabolites are likely: (3) 2-apo-5,8-lycopenal-furanoxide (C(37)H(50)O) with lambdamax = 415 nm, 435 nm, and 470 nm, and m/z = 510; (4) lycopene-5, 6, 5', 6'-diepoxide (C(40)H(56)O(2)) with lambdamax = 415 nm, 440 nm, and 470 nm, and m/z = 568; (5) lycopene-5,8-furanoxide isomer (I) (C(40)H(56)O) with lambdamax = 410 nm, 440nm, and 470 nm, and m/z = 552; (6) lycopene-5,8-epoxide isomer (II) (C(40)H(56)O) with lambdamax = 410, 440, 470 nm, and m/z = 552; and (7) 3-keto-lycopene-5',8'-furanoxide (C(40)H(54)O(2)) with lambdamax = 400 nm, 420 nm, and 450 nm, and m/z = 566. These results demonstrate that both central and excentric cleavage of lycopene occurs in the rat intestinal mucosa in the presence of soy lipoxygenase.

Vitamin A and infancy. Biochemical, functional, and clinical aspects

Vitamin A is a very intriguing natural compound. The molecule not only has a complex array of physiological functions, but also represents the precursor of promising and powerful new pharmacological agents. Although several aspects of human retinol metabolism, including absorption and tissue delivery, have been clarified, the type and amounts of vitamin A derivatives that are intracellularly produced remain quite elusive. In addition, their precise function and targets still need to be identified. Retinoic acids, undoubtedly, play a major role in explaining activities of retinol, but, recently, a large number of physiological functions have been attributed to different retinoids and to vitamin A itself. One of the primary roles this vitamin plays is in embryogenesis. Almost all steps in organogenesis are controlled by retinoic acids, thus suggesting that retinol is necessary for proper development of embryonic tissues. These considerations point to the dramatic importance of a sufficient intake of vitamin A and explain the consequences if intake of retinol is deficient. However, hypervitaminosis A also has a number of remarkable negative consequences, which, in same cases, could be fatal. Thus, the use of large doses of retinol in the treatment of some human diseases and the use of megavitamin therapy for certain chronic disorders as well as the growing tendency toward vitamin faddism should alert physicians to the possibility of vitamin overdose.

Absorption and retinol equivalence of beta-carotene in humans is influenced by dietary vitamin A intake

The effect of vitamin A supplements on metabolic behavior of an oral tracer dose of [14C]beta-carotene was investigated in a longitudinal test-retest design in two adults. For the test, each subject ingested 1 nmol of [14C]beta-carotene (100 nCi) in an emulsified olive oil-banana drink. Total urine and stool were collected for up to 30 days; concentration-time patterns of [14C]beta-carotene, [14C]retinyl esters, and [14C]retinol were determined for 46 days. On Day 53, the subjects were placed on a daily vitamin A supplement (10000 IU/day), and a second dose of [14C]beta-carotene (retest) was given on Day 74. All 14C determinations were made using accelerator mass spectrometry. In both subjects, the vitamin A supplementation was associated with three main effects: 1). increased apparent absorption: test versus retest values rose from 57% to 74% (Subject 1) and from 52% to 75% (Subject 2); 2). an approximately 10-fold reduction in urinary excretion; and 3). a lower ratio of labeled retinyl ester/beta-carotene concentrations in the absorptive phase. The molar vitamin A value of the dose for the test was 0.62 mol (Subject 1) and 0.54 mol (Subject 2) vitamin A to 1 mol beta-carotene. Respective values for the retest were 0.85 and 0.74. These results show that while less cleavage of beta-carotene occurred due to vitamin A supplementation, higher absorption resulted in larger molar vitamin A values.

Breast cancer, serum antioxidant vitamins, and p53 protein overexpression

Although several studies have suggested that the relationship between the various risk factors and breast cancer may differ according to the breast cancer subtypes, it is not clear whether the p53 protein expression status of breast cancer represents an etiologically distinct form of the disease with different risk factor profiles. This study was carried out to investigate whether the relationship between breast cancer and serum antioxidant vitamins would differ according to p53 expression status (p53-positive and p53-negative). Breast cancer cases (n = 92) and controls (n = 122) were recruited between January 1993 and April 1994 at the Asan Medical Center. p53 overexpression in tissue sections from 92 women with breast cancer was determined using immunohistochemistry. The serum concentrations of the carotenoids and alpha-tocopherol were measured using liquid chromatography. For serum antioxidant vitamins, odds ratios with respect to the common control group for breast cancers of different p53 protein overexpression status were compared using multiple polytomous logistic regression. Serum concentrations of beta-carotene and zeaxanthin + lutein were significantly associated with the risk of breast cancer in p53-positive and p53-negative cancers. The adjusted odds ratios for the highest compared with the lowest quartile were 0.19 [95% confidence interval (CI) = 0.01-2.51] for beta-carotene and 0.34 (95% CI = 0.06-1.87) for zeaxanthin + lutein in p53-positive cancer cases and 0.05 (95% CI = 0.00-0.57) for beta-carotene and 0.06 (95% CI = 0.01-0.40) for zeaxanthin + lutein in p53-negative cancer cases. However, none of these associations differed significantly between p53-positive and p53-negative cancers. The results of this study showed that the relationship of antioxidant vitamins with breast cancer does not differ according to the presence or absence of the p53 mutation.

Molecular analysis of vitamin A formation: cloning and characterization of beta-carotene 15,15'-dioxygenases

Beta-carotene 15,15'-dioxygenase cleaves beta-carotene into two molecules of retinal and is the key enzyme in the metabolism of carotene to vitamin A. Although the enzyme has been known for more than 40 years, all attempts to purify the protein to homogeneity or to clone its gene have failed until recently, when the successful cloning and sequencing of cDNAs encoding enzymes with beta-carotene 15,15'-dioxygenase activity from Drosophila (J. von Lintig and K. Vogt, 2000, J. Biol. Chem. 275, 11915-11920) and chicken (A. Wyss et al., 2000, Biochem. Biophys. Res. Commun. 271, 334-336) were reported. Very soon it became clear, that we have cloned two members of a new family of carotenoid cleaving enzymes. Overall homologies are very high, certain amino acid stretches almost identical. Thus, beta-carotene 15,15'-dioxygenase can be considered as evolutionarily well conserved. These findings open up wide perspectives for further analysis of this important biosynthetic pathway, concerning basic and medical research as well as biotechnological aspects related to vitamin A supply, which are discussed here.

Expression pattern and localization of beta,beta-carotene 15,15'-dioxygenase in different tissues

Beta,beta-carotene 15,15'-dioxygenase cleaves beta,beta-carotene into two molecules of retinal, and is the key enzyme in the metabolism of beta,beta-carotene to vitamin A. The enzyme has been known for more than 40 years, yet all attempts to purify the protein to homogeneity have failed. Recently, the successful cloning and sequencing of an enzyme with beta,beta-carotene 15,15'-dioxygenase activity from chicken, as well as from Drosophila, has been reported. Here, we describe in detail our attempt to enrich the chicken beta,beta-carotene 15,15'-dioxygenase to such an extent as to allow determination of partial amino acid sequences, which were then used to design degenerate oligonucleotides. Screening of a chicken duodenal expression library yielded a full-length clone containing a coding sequence of 1578 bp. Functional expression in Escherichia coli and in eukaryotic cell lines confirmed that we had cloned the first vertebrate dioxygenase that cleaves beta,beta-carotene at the central 15,15'-double bond. By performing a sequence homology search, the cDNA sequence of the mouse homologue was found as an expressed sequence tag (EST) in the gene bank. At the amino-acid level, the degree of homology between the chicken and mouse sequences is 81%. Thus beta,beta-carotene 15,15'-dioxygenase can be considered as being an enzyme that is evolutionarily rather well conserved. We established the expression pattern of beta,beta-carotene 15,15'-dioxygenase in chicken and mouse tissues with a combination of Northern blots and in situ hybridization. The mRNA for beta,beta-carotene 15,15'-dioxygenase was localized primarily in duodenal villi, as well as in liver and in tubular structures of lung and kidney. These new findings demonstrate that beta,beta-carotene 15,15'-dioxygenase is also expressed in epithelial structures, where it serves to provide the tissue-specific vitamin A supply.

Pulse radiolytic oxidation of beta-carotene with halogenated alkylperoxyl radicals in a quaternary microemulsion: formation of retinol

Pulse radiolytically generated halogenated alkylperoxyl radicals, namely CCl3OO*, CBr3OO*, CHCl2OO*, CHBr2OO*, etc. have been employed for a study of the oxidation of beta-carotene in a quaternary microemulsion. All these halogenated alkylperoxyl radicals produce a radical cation (absorption maximum at 840 nm), either via the initial formation of an adduct (absorption maximum at 740 nm), or by direct reaction. There was a considerable blue shift of both absorption peaks in the present system as compared to those earlier reported in non-polar as well as in micellar media. An additional intense absorption peak at approximately 345 nm was noted at a longer time scale and was stable up to 5 ms. On irradiation with a large number of electron pulses, a stable product with an absorption maximum at 315 nm appeared. On excitation at 315 nm, this product gave a fluorescence spectrum with an emission maximum at 525 nm. The absorption and fluorescence spectra of the stable product compared with those of retinol; this was further confirmed by HPLC analysis. A suitable mechanism has been proposed.

Cloning and expression of beta,beta-carotene 15,15'-dioxygenase

beta,beta-Carotene 15,15'-dioxygenase cleaves beta-carotene into two molecules of retinal and is therefore the key enzyme in beta-carotene metabolism to vitamin A. In the present study, it was possible to enrich the chicken beta,beta-carotene 15,15'-dioxygenase to such an extent that partial amino acid sequence information could be obtained to design degenerate oligonucleotides. With RT-PCR a cDNA fragment could be obtained and used subsequently in a radioactive screening of a chicken duodenal expression library. We cloned the first eukaryotic beta,beta-carotene 15,15'-dioxygenase which symmetrically cleaves beta-carotene at the 15,15'-double bond.

The vitamin A spectrum: from deficiency to toxicity

Dark adaptation has been used as a tool for identifying patients with subclinical vitamin A deficiency. With this functional test it was shown that tissue vitamin A deficiency occurs over a wide range of serum vitamin A concentrations. However, serum vitamin A concentrations >1.4 micromol/L predict normal dark adaptation 95% of the time. Other causes of abnormal dark adaptation include zinc and protein deficiencies. Stable isotopes of vitamin A and isotope-dilution techniques were used recently to evaluate body stores of vitamin A and the efficacy of vitamin A intervention programs in field settings and are being used to determine the vitamin A equivalences of dietary carotenoids. Vitamin A toxicity was described in patients taking large doses of vitamin A and in patients with type I hyperlipidemias and alcoholic liver disease. Conversely, tissue retinoic acid deficiency was described in alcoholic rats as a result of hepatic vitamin A mobilization, impaired oxidation of retinaldehyde, and increased destruction of retinoic acid by P450 enzymes. Abnormal oxidation products of carotenoids can cause toxicity in animal models and may have caused the increased incidence of lung cancer seen in 2 epidemiologic studies of the effects of high-dose beta-carotene supplementation. Major issues that remain to be studied include the efficiency of conversion of carotenoids in whole foods to vitamin A by using a variety of foods in various field settings and whether intraluminal factors (eg, parasitism) and vitamin A status affect this conversion. In addition, the biological activity of carotenoid metabolites should be better understood, particularly their effects on retinoid signaling.

Biological activities of Apo-canthaxanthinoic acids related to gap junctional communication

Apo-carotenoids with different numbers of conjugated double bonds are formed upon eccentric cleavage of carotenoids. These compounds may exhibit biological activities similar to those of the parent carotenoids or their central cleavage products, the retinoids. 11-Apo-canthaxanthin-11-oic acid, 13-apo-canthaxanthin-13-oic acid, and 14'-apo-canthaxanthin-14'-oic acid, carrying 2, 3, or 5 conjugated double bonds in the polyene chain, respectively, were tested for their effects on gap junctional communication (GJC), on stabilization of connexin43 mRNA, and on the activation of the retinoic acid-beta2 receptor (RAR-beta2 receptor); the effects were compared to those of retinoic acid and 4-oxo-retinoic acid, known to stimulate GJC and to activate the RAR-beta2 receptor. The effects of 4-oxo-retinoic acid were comparable to those of retinoic acid. 4-Oxo-retinoic acid, like retinoic acid, influences the stability of connexin 43 mRNA via elements located in the 3'-UTR. No effects were observed with the short-chain apo-canthaxanthinoic acids. A small but statistically significant induction of GJC and transactivation activity towards the RARbeta2 was found with 14'-apo-canthaxanthin-14'-oic acid. This might be due to biological effects of the compound itself or to biologically active breakdown products. The data suggest that the major biological effects of canthaxanthin on retinoid signaling pathways are related to activities mediated by the products of the central cleavage.

Antiproliferative effect of carotenoids on human colon cancer cells without conversion to retinoic acid

The present study employed two human colon cancer cell lines, DLD-1 and Colo 320DM, to investigate whether the provitamin A activity of carotenoids is necessary for their antitumor effect on colon cancer. Carotenoids, including alpha- and beta-carotene and canthaxanthin, significantly suppressed cell viability [measured by tetrazolium (MTT) assay], DNA synthesis (measured by [3H]thymidine uptake), and cell proliferation (measured by cell counting) and thus showed growth-inhibitory effects on both cancer cell lines. Because canthaxanthin does not have provitamin A activity, these results suggest that the carotenoid directly inhibited the cell growth. Moreover, the effective dose of retinoic acid, an active metabolite of vitamin A, was much higher than that of alpha- or beta-carotene. A retinoic acid-inducible gene, retinoic acid receptor-beta, was not enhanced in either type of cancer cell by treatment with alpha- or beta-carotene. Therefore, like canthaxanthin, alpha- and beta-carotene might also exert their tumor-suppressing effects without being converted to retinoids. These results suggest that a certain antitumor activity of carotenoids may not necessarily require their provitamin A activity.

Correlation between carotenoid concentrations in serum and normal breast adipose tissue of women with benign breast tumor or breast cancer

To evaluate the relationship between carotenoid concentrations in serum and breast tissue, we measured serum carotenoid concentrations and endogenous carotenoid levels in breast adipose tissue of women with benign breast tumor (n = 46) or breast cancer (n = 44). Before extraction, serum was digested with lipase and cholesterol esterase, and breast adipose tissue was saponified. Serum and tissue carotenoids were extracted with ether/hexane and measured by using HPLC with a C30 column. Serum retinoic acid was extracted with chloroform/methanol and measured using HPLC with a C18 column. There were no significant differences in serum carotenoids [lutein, zeaxanthin, cryptoxanthin (both alpha- and beta-), alpha-carotene, all-trans beta-carotene, 13-cis beta-carotene and lycopene], retinoids (retinol, all-trans and 13-cis retinoic acids), and alpha- and gamma- tocopherol concentrations between benign breast tumor patients and breast cancer patients. A substantial amount of 9-cis beta-carotene was present in adipose tissue and was the only carotenoid that had a significantly lower level in benign breast tumor patients than in breast cancer patients. Correlations between carotenoid concentrations in serum and in breast adipose tissue were determined by combining the data of the two groups. Concentrations of the major serum carotenoids except cryptoxanthin showed significant correlations with breast adipose tissue carotenoid levels. When the concentrations of serum carotenoids were adjusted for serum triglycerides or LDL, correlations between serum carotenoid concentrations and breast adipose tissue carotenoid levels markedly increased, including that of cryptoxanthin (P <0. 001). The strong correlation between serum carotenoid concentrations and endogenous breast adipose tissue carotenoid levels indicate that dietary intake influences adipose tissue carotenoid levels as well as serum concentrations, and that adipose tissue is a dynamic reservoir of fat-soluble nutrients.

In vitro and in vivo inhibition of beta-carotene dioxygenase activity by canthaxanthin in rat intestine

beta-Carotene dioxygenase catalyzes the conversion of provitamin A carotenoids to vitamin A in mammalian tissues. Whether the enzyme can also cleave non-provitamin A carotenoids to retinoid analogs with biological activities is still unclear. We investigated (i) substrate specificities of beta-carotene dioxygenase toward provitamin A and non-provitamin A carotenoids and (ii) potential antagonistic effects of non-provitamin A carotenoids on beta-carotene conversion to vitamin A. Provitamin A substrates were 8 to 23% as active as beta-carotene. No polar metabolites were detected with canthaxanthin or zeaxanthin as substrates; these compounds efficiently inhibited the beta-carotene cleavage reaction by 71 and 40%, respectively. Kinetic studies indicated mixed inhibition for canthaxanthin (Ki = 1.6 microM) and non-competitive for zeaxanthin (Ki = 7.8 microM), suggesting that both compounds do not interact significantly with the active site of the enzyme. In vivo, dietary combinations of canthaxanthin and beta-carotene resulted in lower liver levels of both carotenoids and vitamin A and in a higher beta-carotene/vitamin A ratio as compared to groups supplemented with the compounds separately. This supports the view that canthaxanthin at high doses competes with beta-carotene for intestinal absorption and inhibits the conversion of beta-carotene to vitamin A. Thus, we suggest that although canthaxanthin is not a substrate for beta-carotene dioxygenase, it is likely to affect the activity of provitamin A carotenoids by direct interaction with the enzyme.

Specific oxidative cleavage of carotenoids by VP14 of maize

The plant growth regulator abscisic acid (ABA) is formed by the oxidative cleavage of an epoxy-carotenoid. The synthesis of other apocarotenoids, such as vitamin A in animals, may occur by a similar mechanism. In ABA biosynthesis, oxidative cleavage is the first committed reaction and is believed to be the key regulatory step. A new ABA-deficient mutant of maize has been identified and the corresponding gene, Vp14, has been cloned. The recombinant VP14 protein catalyzes the cleavage of 9-cis-epoxy-carotenoids to form C25 apo-aldehydes and xanthoxin, a precursor of ABA in higher plants.

Retinoic acid synthesis in normal and Alzheimer diseased brain and human neural cells

Retinoids play fundamental roles in CNS development, but their distribution, metabolism, and function within the mature human CNS are unknown. In these studies, extracts of autopsy tissues recovered from histopathologically confirmed control and Alzheimer diseased brains were tested for their ability to synthesize retinoic acid. Retinaldehyde dehydrogenase (RLDH), the enzyme that forms retinoic acid from retinaldehyde, was present in hippocampus, frontal cortex, and parietal cortex. The RLDH activity of hippocampus and parietal cortex from Alzheimer diseased brains was 1.5- to 2-fold higher (p < 0.05) compared to the controls. In contrast, the RLDH activity of frontal cortex was the same for both Alzheimer diseased and control groups. A cultured human glioblastoma (U251) and neuroblastoma (LA-N-5) cell line synthesized retinoic acid from retinaldehyde or retinol, suggesting that a variety of neural cell types possess this activity. LA-N-5 cells grown in vitamin A-depleted medium had higher (p < 0.05) RLDH activity (0.35 +/- 0.04 nmol/mg/h) than LA-N-5 cells grown in vitamin A-replete media (0.15 +/- 0.02 nmol/mg/h). This difference was lost when retinol was added back to the medium, confirming that a reduction in vitamin A supply can induce RLDH activity in neural cells. However, this feedback mechanism does not appear to explain the higher RLDH activity of Alzheimer diseased hippocampus and parietal cortex, because the overall vitamin A status as indicated by serum retinol and carotenoid levels and by hippocampal retinoid content was similar for the Alzheimer diseased and control groups. These studies establish the presence of retinoids and RLDH activity in human brain tissues, and indicate that retinoic acid synthesis is modulated in some regions of Alzheimer diseased brain.