Photosynthesis without ß-carotene.

Carotenoids are essential in oxygenic photosynthesis: they stabilize the pigment-protein complexes, are active in harvesting sunlight and in photoprotection. In plants, they are present as carotenes and their oxygenated derivatives, xanthophylls. While mutant plants lacking xanthophylls are capable of photoautotrophic growth, no plants without carotenes in their photosystems have been reported so far, which has led to the common opinion that carotenes are essential for photosynthesis. Here, we report the first plant that grows photoautotrophically in the absence of carotenes: a tobacco plant containing only the xanthophyll astaxanthin. Surprisingly, both photosystems are fully functional despite their carotenoid-binding sites being occupied by astaxanthin instead of ß-carotene or remaining empty (i.e. are not occupied by carotenoids). These plants display non-photochemical quenching, despite the absence of both zeaxanthin and lutein and show that tobacco can regulate the ratio between the two photosystems in a very large dynamic range to optimize electron transport.

Most life on Earth depends on photosynthesis, the process used by plants and many other organisms to store energy from sunlight and produce oxygen. The first steps of photosynthesis, the capture and conversion of sunlight into chemical energy, happen in large assemblies of proteins containing many pigment molecules called photosystems. In plants, the pigments involved in photosynthesis are green chlorophylls and carotenoids. In addition to harvesting light, carotenoids have an important role in preventing damage caused by overexposure to sunlight There are over one thousand different carotenoids in living beings, but only one, ß-carotene, is present in every organism that performs the type of photosynthesis in which oxygen is released, and is thought to be essential for the process. However, this could never be proved because it is impossible to remove ß-carotene from cells using typical genetic approaches without affecting all other carotenoids. Xu et al. used genetic engineering to create tobacco plants that produced a pigment called astaxanthin in place of ß-carotene. Astaxanthin is a carotenoid from salmon and shrimp, not normally found in plants. These plants are the first living things known to perform photosynthesis without ß-carotene and demonstrate that this pigment is not essential for photosynthesis as long as other carotenoids are present. Xu et al. also show that the photosystems can adapt to using different carotenoids, and can even operate with a reduced number of them. Xu et al's findings show the high flexibility of photosynthesis in plants, which are able to incorporate non-native elements to the process. These results are also important in the context of increasing the photosynthetic efficiency, and thus the productivity of crops, since they show that a radical redesign of the photosynthetic machinery is feasible.

ß-carotene suppresses Porphyromonas gingivalis lipopolysaccharide-mediated cytokine production in THP-1 monocytes cultured with high glucose condition.

Periodontitis is associated with development of diabetes mellitus. Although lipopolysaccharide (LPS) of Porphyromonas gingivalis (Pg), a major pathogen of periodontitis, may lead the progression of diabetes complications, the precise mechanisms are unclear. We, therefore, investigated the effects of ß-carotene on production of Pg LPS-induced inflammatory cytokines in human monocytes cultured high glucose (HG) condition. THP-1 cells were cultured under 5.5 mM or 25 mM glucose conditions, and cells were stimulated with Pg LPS. To investigate the productivity of TNF-α, IL-6, and MCP-1, cell supernatants were collected for ELISA. To examine the effects of NF-kB signals on cytokine production, Bay11-7082 was used. HG enhanced Pg LPS-induced production of TNF-α, IL-6, and MCP-1 via NF-kB signals in THP-1. ß-carotene suppressed the enhancement of the Pg LPS-induced cytokine production in THP-1 via NF-κB inactivation. Our results suggest that ß-carotene might be a potential anti-inflammatory nutrient for circulating Pg LPS-mediated cytokine production in diabetic patients with periodontitis.

Meeting the Vitamin A Requirement: The Efficacy and Importance of ß-Carotene in Animal Species.

Vitamin A is essential for life in all vertebrate animals. Vitamin A requirement can be met from dietary preformed vitamin A or provitamin A carotenoids, the most important of which is ß-carotene. The metabolism of ß-carotene, including its intestinal absorption, accumulation in tissues, and conversion to vitamin A, varies widely across animal species and determines the role that ß-carotene plays in meeting vitamin A requirement. This review begins with a brief discussion of vitamin A, with an emphasis on species differences in metabolism. A more detailed discussion of ß-carotene follows, with a focus on factors impacting bioavailability and its conversion to vitamin A. Finally, the literature on how animals utilize ß-carotene is reviewed individually for several species and classes of animals. We conclude that ß-carotene conversion to vitamin A is variable and dependent on a number of factors, which are important to consider in the formulation and assessment of diets. Omnivores and herbivores are more efficient at converting ß-carotene to vitamin A than carnivores. Absorption and accumulation of ß-carotene in tissues vary with species and are poorly understood. More comparative and mechanistic studies are required in this area to improve the understanding of ß-carotene metabolism.

ß-carotene regulates expression of ß-carotene 15,15'-monoxygenase in human alveolar epithelial cells.

ß-Carotene 15,15'-monooxygenase (CMO1, BCMO1) converts ß-carotene to retinaldehyde (retinal) and is a key enzyme in vitamin A metabolism. CMO1 activity is robust in the intestine and liver, where cmo1 gene transcription may be subject to negative feedback by accumulation of its metabolic products. Evidence from CMO1 null animals also indicates that non-gastrointestinal CMO1 may be required for tissue-specific conversion of ß-carotene into vitamin A. The aim of this study was to investigate the effects of the enzymatic substrate, ß-carotene, on regulation of CMO1 in a cell model of human alveolar pneumocytes. We demonstrate that CMO1 is expressed in human alveolar epithelial (A549) cells and converts ß-carotene into retinal and biologically active retinoic acids (RA). Exposure to ß-carotene suppresses CMO1 expression at both mRNA and protein levels. ß-Carotene, but not all-trans RA, decreases CMO1 promoter activity in a time- and dosage-dependent manner. This ß-carotene-mediated inhibition of CMO1 expression results from decreased binding of peroxisome proliferator-activated receptor γ (PPARγ) and retinoid X receptor α (RXRα) in the CMO1 promoter. ß-Carotene treatment also antagonizes PPARγ activity in HEK293 cells that stably express CMO1 wild-type, but not in cells that express the CMO1 mutant or vector alone. These findings have implications for local vitamin A synthesis in the lung, especially during systemic vitamin A insufficiency and may also help to explain, in part, the mechanism underlying the increased lung cancer risk upon ß-carotene supplementation in smokers.

Morphology, ultrastructure and life cycle of Vitrella brassicaformis n. sp., n. gen., a novel chromerid from the Great Barrier Reef.

Chromerida are photoautotrophic alveolates so far only isolated from corals in Australia. It has been shown that these secondary plastid-containing algae are closely related to apicomplexan parasites and share various morphological and molecular characters with both Apicomplexa and Dinophyta. So far, the only known representative of the phylum was Chromera velia. Here we provide a formal description of another chromerid, Vitrella brassicaformis gen. et sp. nov., complemented with a detailed study on its ultrastructure, allowing insight into its life cycle. The novel alga differs significantly from the related chromerid C. velia in life cycle, morphology as well as the plastid genome. Analysis of photosynthetic pigments on the other hand demonstrate that both chromerids lack chlorophyll c, the hallmark of phototrophic chromalveolates. Based on the relatively high divergence between C. velia and V. brassicaformis, we propose their classification into distinct families Chromeraceae and Vitrellaceae. Moreover, we predict a hidden and unexplored diversity of the chromerid algae.

Increased vitamin plasma levels in Swedish military personnel treated with nutrients prior to automatic weapon training.

Noise-induced hearing loss (NIHL) is a significant clinical, social, and economic issue. The development of novel therapeutic agents to reduce NIHL will potentially benefit multiple very large noise-exposed populations. Oxidative stress has been identified as a significant contributor to noise-induced sensory cell death and NIHL, and several antioxidant strategies have now been suggested for potential translation to human subjects. One such strategy is a combination of beta-carotene, vitamins C and E, and magnesium, which has shown promise for protection against NIHL in rodent models, and is being evaluated in a series of international human clinical trials using temporary (military gunfire, audio player use) and permanent (stamping factory, military airbase) threshold shift models (NCT00808470). The noise exposures used in the recently completed Swedish military gunfire study described in this report did not, on average, result in measurable changes in auditory function using conventional pure-tone thresholds and distortion product otoacoustic emission (DPOAE) amplitudes as metrics. However, analysis of the plasma samples confirmed significant elevations in the bloodstream 2 hours after oral consumption of active clinical supplies, indicating the dose is realistic. The plasma outcomes are encouraging, but clinical acceptance of any novel therapeutic critically depends on demonstration that the agent reduces noise-induced threshold shift in randomized, placebo-controlled, prospective human clinical trials. Although this noise insult did not induce hearing loss, the trial design and study protocol can be applied to other populations exposed to different noise insults.

Hypoxia-induced ß-catenin downregulation involves p53-dependent activation of Siah-1.

Solid tumors contain extensive hypoxic areas and it is of considerable importance to decipher the potential role of hypoxia in signaling pathway regulation. In the present study, we examined the impact of hypoxia on ß-catenin status and the mechanisms involved. Hypoxia significantly decreased ß-catenin protein, but had no effect on glycogen synthase kinase (GSK)-3ß or adenomatous polyposis coli (APC) levels. However, hypoxia-induced ß-catenin downregulation seemed to require APC but not GSK-3ß. Further investigation revealed that hypoxia significantly upregulated Siah-1, the human homolog of Drosophila seven in absentia. In addition, hypoxia augmented the interaction between ß-catenin and SIP and Skp1. Silencing of Siah-1, as well as the use of a dominant negative Siah-1 mutant, attenuated these responses to hypoxia and rescued ß-catenin transactivation. The Siah-1-mediated degradation of ß-catenin during hypoxia may involve p53, but not hypoxia-inducible factor-1, activation. Together, the results suggest that hypoxia downregulates ß-catenin by increasing Siah-1 expression in a p53-dependent manner.

Factors influencing the chemical stability of carotenoids in foods.

In recent years, a number of studies have produced evidence to suggest that consuming carotenoids may provide a variety of health benefits including a reduced incidence of a number of cancers, reduced risk of cardiovascular disease, and improved eye health. Evolving evidence on the health benefits of several carotenoids has sparked interest in incorporating more carotenoids into functional food products. Unfortunately, the same structural attributes of carotenoids that are thought to impart health benefits also make these compounds highly susceptible to oxidation. Given the susceptibility of carotenoids to degradation, particularly once they have been extracted from biological tissues, it is important to understand the major mechanisms of oxidation in order to design delivery systems that protect these compounds when they are used as functional food ingredients. This article reviews current understanding of the oxidation mechanisms by which carotenoids are degraded, including pathways induced by heat, light, oxygen, acid, transition metal, or interactions with radical species. In addition, several carotenoid delivery systems are evaluated for their potential to decrease carotenoid degradation in functional food products.

Downregulation of Fzd6 and Cthrc1 and upregulation of olfactory receptors and protocadherins by dietary beta-carotene in lungs of Bcmo1-/- mice.

An ongoing controversy exists on beneficial versus harmful effects of high beta-carotene (BC) intake, especially for the lung. To elucidate potential mechanisms, we studied effects of BC on lung gene expression. We used a beta-carotene 15,15'-monooxygenase 1 (Bcmo1) knockout mouse (Bcmo1(-/-)) model, unable to convert BC to retinoids, and wild-type mice (Bcmo1(+/+)) mice to dissect the effects of intact BC from effects of BC metabolites. As expected, BC supplementation resulted in a higher BC accumulation in lungs of Bcmo1(-/-) mice than in lungs of Bcmo1(+/+) mice. Whole mouse genome transcriptome analysis on lung tissue revealed that more genes were regulated in Bcmo1(-/-) mice than Bcmo1(+/+) mice upon BC supplementation. Frizzled homolog 6 (Fzd6) and collagen triple helix repeat containing 1 (Cthrc1) were significantly downregulated (fold changes -2.99 and -2.60, respectively, false discovery rate < 0.05) by BC in Bcmo1(-/-). Moreover, many olfactory receptors and many members of the protocadherin family were upregulated. Since both olfactory receptors and protocadherins have an important function in sensory nerves and Fzd6 and Cthrc1 are important in stem cell development, we hypothesize that BC might have an effect on the highly innervated pulmonary neuroendocrine cell (PNEC) cluster. PNECs are highly associated with sensory nerves and are important cells in the control of stem cells. A role for BC in the innervated PNEC cluster might be of particular importance in smoke-induced carcinogenesis since PNEC-derived lung cancer is highly associated with tobacco smoke.

[Assessment of the structure and quality of nutrition as a factor that influences thyroid functional activity].

The purpose of the study was to determine the provision of schoolchildren with vitamin A, beta-carotene, and zinc, by taking into account their mutual effect on thyroid functional activity. The study was retrospective and performed by the Adygei Branch of the Kuban State Medical University, Federal Health Care and Social Development Agency (Maikop). The nutrition pattern was studied in 103 pupils living in the Republic of Adygea. The amount of food used with- in 3 days (with one day-off being compulsorily included) was estimated to determine the actual dietary levels of micronutrients in the schoolchildren. Their provision with vitamin A and beta-carotene was determined by the dark adaptation test (Kravkov-Purkinje test). The data were processed by parametric statistical methods, by using the computer program "Statistica 5.0". Questionnaire analysis has indicated that the diet is unbalanced in the majority of the examined schoolchildren. The study revealed the insufficient dietary intake of iodine, zinc, vitamin A, and beta-carotene. About half the parents of the schoolchildren participating in the study indicated the use of iodinated salt for cooking. Insufficient vitamin A provision was revealed in 77.8% of the pupils. For prevention of deficiencies of vitamin A and traces, 37% of the schoolchildren were found to take multivitamin preparations. Vitamin A deficiency is one of the factors contributing to the development of iodine deficiencies. Along with this, by affecting vitamin A metabolism, inadequate dietary intake of zinc adversely affects thyroid functional activity. Monitoring of adequate dietary intake of vitamin A and zinc should be one of the stages in preventing iodine deficiencies and reducing the number of risk groups in an area with low environmental iodine levels.

Beta-carotene and arachidonic acid induced DNA methylation and the regulation of pro-chemotactic activity of endothelial cells and its progenitors.

DNA methylation is one of the important mechanisms regulating gene expression. Since beta-carotene (BC) was shown to have pro-chemotactic activity and stimulates expression of pro-angiogenic genes, this study was undertaken to define the possible changes in DNA methylation in endothelial cell and its progenitors in the presence of BC. The culture medium for human umbilical vein endothelial cells (HUVEC) and endothelial progenitor cells (EPC) was supplemented with BC (1 - 10 microM) with the presence of arachidonic acid (AA) (3 microM). Global DNA methylation tended to be lower in both endothelial cell lines, after incubation with BC and AA. HUVEC incubated with AA demonstrated the lowest DNA methylation. The decrease of DNA methylation in EPC, induced by BC, was concentration-dependent. The microarray study revealed, that the angiogenesis and homing-related genes were mostly influenced by BC and AA in investigated cells. Our results indicate that BC and AA-induced DNA hypomethylation in EPC and HUVEC, might be a mechanism which may alter gene expression in endothelial cells what in certain conditions may be connected with the suggested pro-malignant effect of this compounds.

Effects of beta-carotene supplementation on chick growth, immune status and behaviour in the grey partridge, Perdix perdix.

Carotenoids are important for various functions during chick development. Since these pigments cannot be synthesized, they can be considered limited resources that the mother optimally allocates between herself and her offspring (maternal effect). Some studies have examined the effects of carotenoids on growth and immune function but little is known about their role in behaviour. In this study of the grey partridge, we conducted two supplementation experiments: (1) laying females were fed with beta-carotene enriched or impoverished diets; (2) chicks were fed directly with beta-carotene enriched or impoverished diets. We then evaluated the effects of this carotenoid on chick growth, immunocompetence and anti-predator behaviour (reactions to a raptor model). In the first experiment, the beta-carotene enriched diet given to mothers did not cause any difference in chick physiology. In the second experiment, beta-carotene supplementation of chicks had a significant beneficial effect on their growth and immune response, although their behavioural reactions did not differ in relation to the diet. Therefore, beta-carotene supplementation had beneficial effects on growth and immunocompetence only when directly supplied to chicks. The beneficial effect reported in other species for begging or pecking behaviours was not confirmed for the anti-predator behaviour of grey partridge chicks.

Beta-carotene breakdown products enhance genotoxic effects of oxidative stress in primary rat hepatocytes.

Since it has to be expected that individuals exposed to oxidative stress who take supplements of beta-carotene are simultaneously exposed to both beta-carotene cleavage products (CPs) and oxidative stress, and both exposures have been demonstrated to cause genotoxic effects in primary rat hepatocytes, cyto- and genotoxic effects on primary rat hepatocytes after supplementation of the medium with increasing concentrations of a CP mixture during exposure to oxidative stress by treatment with either DMNQ (2,3-dimethoxy-1,4-naphthoquinone) or hypoxia/reoxygenation (Hy/Reox) was investigated. The cytological endpoints analysed were the mitotic indices, the percentages of apoptotic and necrotic cells, the percentages of micronucleated (MN) cells and the number of chromosomal aberrations (CAs) and sister chromatid exchanges (SCE). The results obtained clearly demonstrate that the CP mixture enhances the genotoxic effects of oxidative stress exposure, whereas it had no effect at all on the endpoints of cytotoxicity studied. These results further support the hypothesis that CP might be responsible for the reported carcinogenic response in the beta-CArotene and Retinol Efficacy Trial (CARET) and Alpha-Tocopherol Beta-carotene Cancer prevention (ATBC) chemoprevention trials.

Can beta-carotene regulate cell growth by a redox mechanism? An answer from cultured cells.

Many studies suggest a protective role of beta-carotene against cancer. However, the ATBC and the CARET trials have shown that beta-carotene increases the incidence of lung cancer in heavy smokers and asbestos workers. To explain this paradox, it can be hypothesized that beta-carotene modulates intracellular redox status and through this mechanism, it affects redox-sensitive molecular pathways involved in the regulation of cell cycle progression and apoptosis. Studies conducted in cultured cells seem to confirm such a hypothesis. At low concentrations, the carotenoid may serve as an antioxidant, inhibiting free radical production, while at relatively high concentrations and/or in the presence of a chronic oxidative stress (i.e. smoke), it may behave as a prooxidant, propagating free radical-induced reactions, consuming endogenous antioxidants and inducing DNA oxidative damage. In this context, it may regulate cell growth and death by the modulation of redox-sensitive genes and transcription factors.

The macular xanthophylls.

The macular pigments are predominantly composed of three carotenoids: lutein, zeaxanthin, and meso-zeaxanthin. These carotenoids are concentrated and distributed in a selective manner. The properties of these pigments are further explored along with their methods of uptake, stabilization, and storage. The dual nature of these pigments as filters and antioxidants are elaborated upon in relation to their protective effects upon the macula, specifically in age-related macular degeneration. Evidence suggests that increased levels of macular pigment are correlated with a decreased risk of age-related macular degeneration. Many have sought to exploit this therapeutic relation. Studies reveal that oral supplementation with lutein and zeaxanthin can increase the levels of macular pigments in the retina and plasma. The effects of such supplementation on actual ocular function have yet to be fully addressed. New and standardized methods of assessing macular pigment density are discussed and future areas of research to further our understanding of macular xanthophylls as they pertain to age-related macular degeneration are highlighted.

In vivo and in vitro evidences that carotenoids could modulate the neutrophil respiratory burst during dietary manipulation.

BACKGROUND: The primary role of polymorphonuclear neutrophils (PMNs) is to destroy pathogenic microorganisms after phagocytosis by producing reactive oxygen species (ROS) and toxic molecules. However, PMNs produce sufficient amounts of ROS during an oxidative burst to be autotoxic and detrimental to their own functions and to possibly cause DNA damage, protein and lipid oxidation and cell membrane destructuration. OBJECTIVE: The aim of this study was to investigate in vivo the role of the antioxidant capacities of carotenoids in modulating ROS content in PMNs during oxidative burst. Moreover to investigate the direct or indirect effect of carotenoids, the modification of PMN ROS content was explored after in vitro supplementation with beta-carotene or lycopene, chosen taking account of their vitamin A and no vitamin A precursor effect, respectively. DESIGN: In vivo study: Venous blood was collected from 10 healthy male volunteers and ROS production from phorbol myristate acetate (PMA)-stimulated PMNs was determined, by flow cytometry using the fluorescent dye dihydrorhodamine 123, at baseline, after 3 weeks of carotenoid depletion (carotenoid intake limited to 25% of usual intake) and after 5 weeks of carotenoid repletion (30 mg beta-carotene, 15 mg lycopene and 9 mg lutein per day). In vitro study: ROS content in PMA-stimulated PMNs isolated from carotenoid depleted subjects and controls was quantified after an in vitro enrichment with beta-carotene (1 micromol/L) or lycopene (0.3 micromol/L). RESULTS: In vivo carotenoid depletion increased PMN H2O2 content after PMA activation by 38% (p < 0.05 vs baseline),while supplementation for 5 weeks restored basal H2O2 generation (p < 0.05 vs depletion). Although H2O2 measurement in PMNs from non-depleted subjects was not affected by an in vitro supply with beta-carotene or lycopene, a significant decrease in H2O2 content by 78.9 % and 81.2%, respectively, was observed in PMNs from carotenoid depleted subjects (p < 0.01 vs depleted control subjects). CONCLUSIONS: The carotenoid ROS quenching capacities control both in vivo and in vitro the PMNs ROS generation and probably protect these cells against DNA, membrane lipid and protein damages during oxidative burst. Moreover, these effects appear independent from the metabolic conversion of carotenoids to vitamin A.

Macular carotenoids: lutein and zeaxanthin.

The yellow color of the macula lutea is due to the presence of the carotenoid pigments lutein and zeaxanthin. In contrast to human blood and tissues, no other major carotenoids including Beta-carotene or lycopene are found in this tissue. The macular carotenoids are suggested to play a role in the protection of the retina against light-induced damage. Epidemiological studies provide some evidence that an increased consumption of lutein and zeaxanthin with the diet is associated with a lowered risk for age-related macular degeneration, a disease with increasing incidence in the elderly. Protecting ocular tissue against photooxidative damage carotenoids may act in two ways: first as filters for damaging blue light, and second as antioxidants quenching excited triplet state molecules or singlet molecular oxygen and scavenge further reactive oxygen species like lipid peroxides or the superoxide radical anion.

A potential role for beta-carotene in avian embryonic development.

Vitamin A is essential for vertebrate embryonic development; dietary carotenoids are the primary source of vitamin A since animals cannot synthesize it de novo. To study the role of beta-carotene during embryonic development, we analyzed in chick embryos the expression of beta,beta-carotene 15,15'-oxygenase (beta-oxy) which cleaves beta-carotene to produce two molecules of retinal. Beta-oxy transcripts were detected in one-and-a-half- to five-day-old embryo homogenates and in situ hybridization in five-day-old embryos, revealing their presence in tissues including the central nervous system, lungs, limbs, and cardiovascular system. Moreover, we detected beta-oxy enzymatic activity in extracts from five-day-old embryos as well as small amounts of beta-carotene in the egg yolk. These results indicate that beta-oxy is present during early developmental stages, raising the possibility that yolk-stored beta-carotene is utilized as a source of vitamin A. Thus, our results suggest that beta-carotene could play an important role in early avian embryonic development as a local source of vitamin A in specific tissues.

beta-carotene and oxidative desaturation of fatty acids: a plausible explanation of the conflicting responses of coronary heart disease to beta-carotene?

Studies of the desaturation of saturated fatty acids in animals may help explain conflicting reports of the response of coronary heart disease (CHD) to beta-carotene in humans. A negative relationship exists between desaturation and adipose beta-carotene in cattle when they consume different quantities of beta-carotene. Opposing this finding, however, is a positive relationship between desaturation and adipose beta-carotene when cattle are fed the same quantity of beta-carotene. The reason for this apparent contradiction appears to be due to differences in consumption, or variability in the metabolism of beta-carotene. Animals that efficiently metabolize beta-carotene to vitamin A have low desaturation but high antioxidant potential. These results in animals show some similarity between the consumption of the antioxidant beta-carotene and the risk of coronary heart disease where the oxidation of low-density lipoproteins (LDL) is believed to play a role in the development of atherosclerotic plaque. Genetic differences in carotenoid metabolism in humans, similar to those in animals, would assist in explaining differences in lipoprotein oxidation in humans and variation in the risk of coronary heart disease.

beta-Carotene exacerbates DNA oxidative damage and modifies p53-related pathways of cell proliferation and apoptosis in cultured cells exposed to tobacco smoke condensate.

Human intervention trials have suggested that supplemental beta-carotene resulted in more cancer in smokers, whereas it was protective in non-smokers. However, the mechanisms underlying these effects are still unknown. The aim of this study was to evaluate the effects of an association of cigarette smoke condensate (tar) and beta-carotene on DNA oxidative damage and molecular pathways involved in cell cycle progression and apoptosis in cultured cells. In RAT-1 fibroblasts, tar caused increased levels of 8-hydroxyl-2'-deoxyguanosine (8-OHdG) and this effect was enhanced by the concomitant presence of beta-carotene (0.5-4.0 microM) in a dose- and time-dependent manner. In contrast, beta-carotene alone did not significantly modify it. Fibroblasts treated with tar alone decreased their cell growth with respect to control cells through an arrest of cell cycle progression in the G0/G1 phase and an induction of apoptosis. These effects were accompanied by an increased expression of p53, p21 and Bax and by a decreased expression of cyclin D1. In contrast, fibroblasts treated with tar and beta-carotene, after an initial arrest of cell growth at 12 h, re-entered in cell cycle and were unable to undergo apoptosis at 36 h. Concomitantly, their p53 expression, after an increase at 12 h, progressively returned at basal levels at 36 h by a mechanism independent of Mdm2. Such a decrease was followed by a decrease in p21 and Bax expression and by an increase in cyclin D1 expression. Moreover, the presence of the carotenoid remarkably enhanced cyclooxygenase-2 expression induced by tar. During tar treatment, a depletion of beta-carotene was observed in fibroblasts. The effects of tar and beta-carotene on 8-OHdG levels, cell growth and apoptosis were also observed in Mv1Lu lung, MCF-7 mammary, Hep-2 larynx and LS-174 colon cancer cells. This study supports the evidence for potential detrimental effects of an association between beta-carotene and cigarette smoke condensate.