Stefania D'Angelo

Transport mechanism and metabolism of olive oil hydroxytyrosol in Caco-2 cells

3,4‐Dihydroxyphenylethanol (hydroxytyrosol; DPE) is the major phenolic antioxidant present in extra virgin olive oil, either in a free or esterified form. Despite its relevant biological effects, no data are available on its bioavailability and metabolism. The aim of the present study is to examine the molecular mechanism of DPE intestinal transport, using differentiated Caco‐2 cell monolayers as the model system. The kinetic data demonstrate that [14C]DPE transport occurs via a passive diffusion mechanism and is bidirectional; the calculated apparent permeability coefficient indicates that the molecule is quantitatively absorbed at the intestinal level. The only labelled DPE metabolite detectable in the culture medium by HPLC (10% conversion) is 3‐hydroxy‐4‐methoxyphenylethanol, the product of catechol‐O‐methyltransferase; when DPE is assayed in vitro with the purified enzyme a K m value of 40 μM has been calculated.

Protein Isoaspartate Methyltransferase Prevents Apoptosis Induced by Oxidative Stress in Endothelial Cells: Role of Bcl-Xl Deamidation and Methylation

Background Natural proteins undergo in vivo spontaneous post-biosynthetic deamidation of specific asparagine residues with isoaspartyl formation. Deamidated-isomerized molecules are both structurally and functionally altered. The enzyme isoaspartyl protein carboxyl-O-methyltransferase (PCMT; EC 2.1.1.77) has peculiar substrate specificity towards these deamidated proteins. It catalyzes methyl esterification of the free α-carboxyl group at the isoaspartyl site, thus initiating the repair of these abnormal proteins through the conversion of the isopeptide bond into a normal α-peptide bond. Deamidation occurs slowly during cellular and molecular aging, being accelerated by physical-chemical stresses brought to the living cells. Previous evidence supports a role of protein deamidation in the acquisition of susceptibility to apoptosis. Aim of this work was to shed a light on the role of PCMT in apoptosis clarifying the relevant mechanism(s). Methodology/Principal Findings Endothelial cells transiently transfected with various constructs of PCMT, i.e. overexpressing wild type PCMT or negative dominants, were used to investigate the role of protein methylation during apoptosis induced by oxidative stress (H2O2; 0.1–0.5 mM range). Results show that A) Cells overexpressing “wild type” human PCMT were resistant to apoptosis, whereas overexpression of antisense PCMT induces high sensitivity to apoptosis even at low H2O2 concentrations. B) PCMT protective effect is specifically due to its methyltransferase activity rather than to any other non-enzymatic interactions. In fact negative dominants, overexpressing PCMT mutants devoid of catalytic activity do not prevent apoptosis. C) Cells transfected with antisense PCMT, or overexpressing a PCMT mutant, accumulate isoaspartyl-containing damaged proteins upon H2O2 treatment. Proteomics allowed the identification of proteins, which are both PCMT substrates and apoptosis effectors, whose deamidation occurs under oxidative stress conditions leading to programmed cell death. These proteins, including Hsp70, Hsp90, actin, and Bcl-xL, are recognized and methylated by PCMT, according to the general repair mechanism of this methyltransferase. Conclusion/Significance Apoptosis can be modulated by “on/off” switch partitioning the amount of specific protein effectors, which are either in their active (native) or inactive (deamidated) molecular forms. Deamidated proteins can also be functionally restored through methylation. Bcl-xL provides a case for the role of PCMT in the maintenance of functional stability of this antiapoptotic protein.

UVA irradiation induces L-isoaspartyl formation in melanoma cell proteins.

It has been reported that UVA effects are partly mediated by production of reactive oxygen species. Moreover, oxidative stress increases protein damage, involving the occurrence of isoaspartyl residues, a product of protein deamidation/isomerization reactions. This work was undertaken in order to study the effects of UVA irradiation, mediated by oxidation, on sensitive protein targets. Melanoma cells exposed to UVA rays have been chosen as a model for monitoring the occurrence of L-isoaspartyl sites. A dramatic increase of these abnormal residues, specifically recognized and methylated by the enzyme L-isoaspartate(D-aspartate) O-methyltransferase (PCMT; EC 2.1.1.77), can be detected after exposure of M14 cells to raising doses of UVA. The effect of UVA on NO and TBARS accumulation, as well as on DNA fragmentation, has also been investigated. NO formation parallels the increase in isoaspartyl formation, while lipid peroxidation occurs only at the highest UVA doses. No DNA fragmentation has been detected under the employed experimental conditions. These results, as a whole, indicate that protein damages are one of the early events on UVA-induced cell injury. The endogenous activity of PCMT remains remarkably stable under UVA treatment, suggesting that this enzyme might play a crucial role in the repair and/or disposal of damaged proteins in UVA-irradiated cells.

Accumulation of altered aspartyl residues in erythrocyte proteins from patients with Down's syndrome

Spontaneous protein deamidation of labile Asn residues, generating l‐isoaspartates and d‐aspartates, is associated with cell aging and is enhanced by an oxidative microenvironment; to minimize the damage, the isoaspartate residues can be ‘repaired’ by a specific l‐isoaspartate (d‐aspartate) protein O‐methyltransferase (PIMT). As both premature aging and chronic oxidative stress are typical features of Downs syndrome (DS), we tested the hypothesis that deamidated proteins may build up in trisomic patients. Blood samples were obtained from children with karyotypically confirmed full trisomy 21 and from age‐matched healthy controls. Using recombinant PIMT as a probe, we demonstrated a dramatic rise of l‐isoaspartates in erythrocyte membrane proteins from DS patients. The content of d‐aspartate was also significantly increased. The integrity of the repair system was checked by evaluating methionine transport, PIMT specific activity, and intracellular concentrations of adenosylmethionine and adenosylhomocysteine. The overall methylation pathway was directly monitored by incubating fresh red blood cells with methyl‐labeled methionine; a three‐fold increase of protein methyl esters was detected in trisomic children. Deamidated species include ankyrin, band 4.1, band 4.2 and the integral membrane protein band 3; ankyrin and band 4.1 were significantly hypermethylated in DS. When DS red blood cells were subjected to oxidative treatment in vitro, the increase of protein deamidation paralleled lipid peroxidation and free radical generation. We observed a similar pattern in Epstein–Barr virus B‐lymphocytes from trisomic patients. In conclusion, our findings support the hypothesis that protein instability at asparagine sites is a biochemical feature of DS, presumably depending upon the oxidative microenvironment. The possible pathophysiological implications are discussed.

Effect of Annurca apple polyphenols on human HaCaT keratinocytes proliferation.

Polyphenols have been demonstrated to have clear antioxidant activities in vitro. However, in complex biological systems, they exhibit additional properties, which are yet poorly understood. The apple is among the most consumed fruits worldwide, and several studies suggest that apple polyphenols could play a role in the prevention of degenerative diseases. The present study aimed at evaluating the Annurca apple polyphenol extract (APE) effects both proliferation and apoptosis on HaCaT cells. The data indicate that apple polyphenolic compounds had significant antiproliferative action on HaCaT cells. The fluorescence-activated cell-sorting analysis showed that APE induced cell apoptosis in a dose-dependent manner. Moreover, apple polyphenols induced apoptosis in epithelial cells by triggering a death receptor-associated extrinsic pathway p53-independent. APE was also capable of inducing morphological changes as evidenced by nuclear condensation. The cellular, morphological, and molecular data unequivocally demonstrated that induction of cellular apoptosis was mainly responsible for the previously observed antiproliferation-induced APE on HaCaT keratinocytes. Our experimental results suggest that apple polyphenols are a promising source from which a natural-based topical agent could be developed for skin diseases treatment.