Tocopherol transfer protein sensitizes prostate cancer cells to vitamin E

The tocopherol transfer protein TTP mediates Vitamin Vitamin E trafficking between cerebellar astrocytes and neurons

Alpha-tocopherol (vitamin E) is an essential nutrient that functions as a major lipid-soluble antioxidant in humans. The tocopherol transfer protein (TTP) binds α-tocopherol with high affinity and selectivity and regulates whole-body distribution of the vitamin. Heritable mutations in the TTPA gene result in familial vitamin E deficiency, elevated indices of oxidative stress, and progressive neurodegeneration that manifest primarily in spinocerebellar ataxia. Although the essential role of vitamin E in neurological health has been recognized for over 50 years, the mechanisms by which this essential nutrient is transported in the central nervous system are poorly understood. Here we found that, in the murine cerebellum, TTP is selectively expressed in GFAP-positive astrocytes, where it facilitates efflux of vitamin E to neighboring neurons. We also show that induction of oxidative stress enhances the transcription of the TtpA gene in cultured cerebellar astrocytes. Furthermore, secretion of vitamin E from astrocytes is mediated by an ABC-type transporter, and uptake of the vitamin into neurons involves the low-density lipoprotein receptor-related protein 1 (LRP1) receptor. Taken together, our data indicate that TTP-expressing astrocytes control the delivery of vitamin E from astrocytes to neurons, and that this process is homeostatically responsive to oxidative stress. These are the first observations that address the detailed molecular mechanisms of vitamin E transport in the central nervous system, and these results have important implications for understanding the molecular underpinnings of oxidative stress-related neurodegenerative diseases.

Microalgal Co-Cultivation Prospecting to Modulate Vitamin and Bioactive Compounds Production

Microalgal biotechnology is gaining importance. However, key issues in the pipeline from species selection towards large biomass production still require improvements to maximize the yield and lower the microalgal production costs. This study explores a co-cultivation strategy to improve the bioactive compounds richness of the harvested microalgal biomass. Based on their biotechnological potential, two diatoms (Skeletonema marinoi, Cyclotella cryptica) and one eustigmatophyte (Nannochloropsis oceanica) were grown alone or in combination. Concentrations of ten vitamins (A, B₁, B₂, B₆, B₁₂, C, D₂, D₃, E and H), carotenoids and polyphenols, together with total flavonoids, sterols, lipids, proteins and carbohydrates, were compared. Moreover, antioxidant capacity and chemopreventive potential in terms inhibiting four human tumor-derived and normal cell lines proliferation were evaluated. Co-cultivation can engender biomass with emergent properties regarding bioactivity or bioactive chemical profile, depending on the combined species. The high vitamin content of C. cryptica or N. oceanica further enhanced (until 10% more) when co-cultivated, explaining the two-fold increase of the antioxidant capacity of the combined C. cryptica and N. oceanica biomass. Differently, the chemopreventive activity was valuably enhanced when coupling the two diatoms C. cryptica and S. marinoi. The results obtained in this pilot study promote microalgal co-cultivation as a valuable strategy aiming to boost their application in eco-sustainable biotechnology.

Organoids Increase the Predictive Value of in vitro Cancer Chemoprevention Studies for in vivo Outcome

Epidemiological and preclinical data suggest that antioxidants are protective against prostate cancer whose pathogenesis has been linked to oxidative stress. However, the selenium and vitamin E Cancer Prevention Trial (SELECT), found no efficacy for selenium in reducing prostate cancer incidence while vitamin E was associated with an increased risk of the disease. These results have called in to question the models used in preclinical chemoprevention efficacy studies and their ability to predict in vivo outcomes. Chemoprevention agents have traditionally been tested on two dimensional monolayer cultures of cell lines derived from advanced prostate cancers. But as SELECT demonstrates, results from advanced disease models were not predictive of the outcome of a primary chemoprevention trial. Additionally, lack of cell-matrix interactions in two dimensional cultures results in loss of biochemical and mechanical cues relevant for native tissue architecture. We use recent findings in three dimensional organoid cultures that recapitulated the SELECT trial results to argue that the organoid model could increase the predictive value of in vitro studies for in vivo outcomes.

Could nanotechnology make vitamin E therapeutically effective?

Vitamin E (VitE) has important antioxidant and anti-inflammatory effects and is necessary for normal physiological function. α-Tocopherol (α-T), the predominant form of VitE in human tissues, has been extensively studied. Other VitE forms, particularly γ-tocopherol (γ-T), are also potent bioactive molecules. The effects are complex, involving both reactive oxygen and nitrogen species, but trials of VitE have been generally negative. We propose that a nanoparticle approach to delivery of VitE might provide effective delivery and therapeutic effect.

α-Tocopherol transfer protein mediates protective hypercapnia in murine ventilator-induced lung injury

RATIONALE

Hypercapnia is common in mechanically ventilated patients. Experimentally, 'therapeutic hypercapnia' can protect, but it can also cause harm, depending on the mechanism of injury. Hypercapnia suppresses multiple signalling pathways. Previous investigations have examined mechanisms that were known a priori, but only a limited number of pathways, each suppressed by CO₂, have been reported.

OBJECTIVE

Because of the complexity and interdependence of processes in acute lung injury, this study sought to fill in knowledge gaps using an unbiased screen, aiming to identify a specifically upregulated pathway.

METHODS AND RESULTS

Using genome-wide gene expression analysis in a mouse model of ventilator-induced lung injury, we discovered a previously unsuspected mechanism by which CO₂ can protect against injury: induction of the transporter protein for α-tocopherol, α-tocopherol transfer protein (αTTP). Pulmonary αTTP was induced by inspired CO₂ in two in vivo murine models of ventilator-induced lung injury; the level of αTTP expression correlated with degree of lung protection; and, absence of the αTTP gene significantly reduced the protective effects of CO₂. α-Tocopherol is a potent antioxidant and hypercapnia increased lung α-tocopherol in wild-type mice, but this did not alter superoxide generation or expression of NRF2-dependent antioxidant response genes in wild-type or in αTTP^-/- mice. In concordance with a regulatory role for α-tocopherol in lipid mediator synthesis, hypercapnia attenuated 5-lipoxygenase activity and this was dependent on the presence of αTTP.

CONCLUSIONS

Inspired CO₂ upregulates αTTP which increases lung α-tocopherol levels and inhibits synthesis of a pathogenic chemoattractant.

Vitamin E and Phosphoinositides Regulate the Intracellular Localization of the Hepatic α-Tocopherol Transfer Protein

α-Tocopherol (vitamin E) is an essential nutrient for all vertebrates. From the eight naturally occurring members of the vitamin E family, α-tocopherol is the most biologically active species and is selectively retained in tissues. The hepatic α-tocopherol transfer protein (TTP) preferentially selects dietary α-tocopherol and facilitates its transport through the hepatocyte and its secretion to the circulation. In doing so, TTP regulates body-wide levels of α-tocopherol. The mechanisms by which TTP facilitates α-tocopherol trafficking in hepatocytes are poorly understood. We found that the intracellular localization of TTP in hepatocytes is dynamic and responds to the presence of α-tocopherol. In the absence of the vitamin, TTP is localized to perinuclear vesicles that harbor CD71, transferrin, and Rab8, markers of the recycling endosomes. Upon treatment with α-tocopherol, TTP- and α-tocopherol-containing vesicles translocate to the plasma membrane, prior to secretion of the vitamin to the exterior of the cells. The change in TTP localization is specific to α-tocopherol and is time- and dose-dependent. The aberrant intracellular localization patterns of lipid binding-defective TTP mutants highlight the importance of protein-lipid interaction in the transport of α-tocopherol. These findings provide the basis for a proposed mechanistic model that describes TTP-facilitated trafficking of α-tocopherol through hepatocytes.

α-Tocopherol transfer protein is not required for the discrimination against γ-tocopherol in vivo but protects it from side-chain degradation in vitro

SCOPE

The mechanisms underlying the preferential retention of a single compound (α-tocopherol (αT)) of the eight vitamin E compounds in the body are incompletely understood. We hypothesized that vitamin E metabolism and not the hepatic α-tocopherol transfer protein (TTP) is responsible for the discrimination against non-αT congeners.

METHODS AND RESULTS

TTP knockout and wild-type mice (n = 12/group) were fed equimolar concentrations of αT and γ-tocopherol (γT; 50 mg/kg diet each) alone or together with sesamin (2 g/kg diet) for 6 wk. Inhibition of vitamin E metabolism with sesamin, but not TTP knockout, increased γT tissue concentrations. TTP-expressing and TTP-free cells were incubated with equimolar concentrations of αT and γT (25 μmol/L each) with or without sesamin (2 μmol/L). The preferential degradation of γT independently of TTP expression was confirmed and a decrease in the production of the metabolite γ-carboxyethyl hydroxychromanol (CEHC) with increasing TTP expression revealed. Displacing γT from TTP in these cells by incubation with increasing αT concentrations enhanced the secretion of γ-CEHC in TTP-transfected cells, suggesting that TTP might protect γT from β-oxidation.

CONCLUSIONS

We conclude that vitamin E metabolism and not TTP controls γT concentrations in vivo and observed an interaction of TTP with vitamin E metabolism that results in reduced production of the metabolite γ-CEHC.

Vitamin E: a dark horse at the crossroad of cancer management

It appears that the story on vitamin E and its role in human health remains incomplete. It is apparent that vitamin E supplementation involves many variables, some of which include its uptake from the intestine, the preference for α-tocopherol, transport by tocopherol specific proteins and lipid transporters and the differential metabolism of different vitamin E isoforms. The fundamental differences within population genetics can have significant implications for the effect that dietary supplementation might have on human health. When evaluating the efficacy of vitamin E prophylactic or therapeutic use in previous and future studies, it is critical to consider dosage to be administered, form of vitamin E and source (such as whether from synthetic or purified from natural sources). Further studies are needed to determine the effects of all vitamin E isoforms on cell growth, tumorigenicity, to clarify its possible use as an adjuvant to existing chemotherapeutics. The Alpha-Tocopherol, Beta Carotene (ATBC) Cancer Prevention Study Group and Selenium and Vitamin E Cancer Prevention Trial (SELECT) studies along with the numerous studies of vitamin E should help guide the next chapter of vitamin E research.

Expression of the α-tocopherol transfer protein gene is regulated by oxidative stress and common single-nucleotide polymorphisms

Vitamin E (α-tocopherol) is the major lipid-soluble antioxidant in most animal species. By controlling the secretion of vitamin E from the liver, the α-tocopherol transfer protein regulates whole-body distribution and levels of this vital nutrient. However, the mechanism(s) that regulates the expression of this protein is poorly understood. Here we report that transcription of the TTPA gene in immortalized human hepatocytes is induced by oxidative stress and by hypoxia, by agonists of the nuclear receptors PPARα and RXR, and by increased cAMP levels. The data show further that induction of TTPA transcription by oxidative stress is mediated by an already-present transcription factor and does not require de novo protein synthesis. Silencing of the cAMP response element-binding (CREB) transcription factor attenuated transcriptional responses of the TTPA gene to added peroxide, suggesting that CREB mediates responses of this gene to oxidative stress. Using a 1.9-kb proximal segment of the human TTPA promoter together with a site-directed mutagenesis approach, we found that single-nucleotide polymorphisms that are commonly found in healthy humans dramatically affect promoter activity. These observations suggest that oxidative stress and individual genetic makeup contribute to vitamin E homeostasis in humans. These findings may explain the variable responses to vitamin E supplementation observed in human clinical trials.

Vitamin E protects against lipid peroxidation and rescues tumorigenic phenotypes in cowden/cowden-like patient-derived lymphoblast cells with germline SDHx variants

PURPOSE

Cowden syndrome (CS), a Mendelian autosomal-dominant disorder, predisposes to breast, thyroid, and other cancers. Germline variations in succinate dehydrogenase genes (SDHx) occur in approximately 10% PTEN mutation-negative CS and CS-like (CSL) individuals (SDH(var+)). We previously showed that SDHx variants result in elevated reactive oxygen species (ROS), disruption of nicotinamide adenine dinucleotide (NAD) equilibrium, and destabilization of p53 hence apoptosis resistance in CS/CSL patient-derived lymphoblastoid cells. In the present study, we sought to address the tumorigenic impacts of increased ROS and the potential of protecting SDH(var+) cells with antioxidants.

EXPERIMENTAL DESIGN

We measured the lipid peroxidation levels in patient-derived SDH(var+) lymphoblastoid cells and sequenced 74 controls or SDH(var+) germline DNA samples for mitochondrial hypervariable region II (HVRII) polymorphisms. SDH(var+) lymphoblastoid cells were treated with various antioxidants to check p53 expression and sub-G(1) cell population with cell-cycle analysis.

RESULTS

We showed that elevated ROS results in higher lipid peroxidation in SDH(var+) cells. Accumulation of polymorphisms in mitochondrial HVRII was observed in SDH(var+) samples. Interestingly, α-tocopherol (vitamin E) treatment, but not other antioxidants, rescued SDH(var+) cells from apoptosis resistance and protected SDH(var+) cells from oxidative damage such as decreased lipid peroxidation as well as partially recovered p53 expression and NAD/NADH levels.

CONCLUSIONS

We conclude that disruption of complex II because of SDHx variants leads to increased ROS generation, specifically accompanied by lipid peroxidation. The lipid soluble antioxidant α-tocopherol can selectively protect SDH(var+) cells from oxidative damage, apoptosis resistance, and rebalance redox metabolites NAD/NADH.

The contribution of surface residues to membrane binding and ligand transfer by the α-tocopherol transfer protein (α-TTP)

Previous work has shown that the α-tocopherol transfer protein (α-TTP) can bind to vesicular or immobilized phospholipid membranes. Revealing the molecular mechanisms by which α-TTP associates with membranes is thought to be critical to understanding its function and role in the secretion of tocopherol from hepatocytes into the circulation. Calculations presented in the Orientations of Proteins in Membranes database have provided a testable model for the spatial arrangement of α-TTP and other CRAL-TRIO family proteins with respect to the lipid bilayer. These calculations predicted that a hydrophobic surface mediates the interaction of α-TTP with lipid membranes. To test the validity of these predictions, we used site-directed mutagenesis and examined the substituted mutants with regard to intermembrane ligand transfer, association with lipid layers and biological activity in cultured hepatocytes. Substitution of residues in helices A8 (F165A and F169A) and A10 (I202A, V206A and M209A) decreased the rate of intermembrane ligand transfer as well as protein adsorption to phospholipid bilayers. The largest impairment was observed upon mutation of residues that are predicted to be fully immersed in the lipid bilayer in both apo (open) and holo (closed) conformations such as Phe165 and Phe169. Mutation F169A, and especially F169D, significantly impaired α-TTP-assisted secretion of α-tocopherol outside cultured hepatocytes. Mutation of selected basic residues (R192H, K211A, and K217A) had little effect on transfer rates, indicating no significant involvement of nonspecific electrostatic interactions with membranes.