Protocatechuic Acid Prevents oxLDL-Induced Apoptosis by Activating JNK/Nrf2 Survival Signals in Macrophages.

Protocatechuic acid (PCA), one of the main metabolites of complex polyphenols, exerts numerous biological activities including antiapoptotic, anti-inflammatory, and antiatherosclerotic effects. Oxidised LDL have atherogenic properties by damaging arterial wall cells and inducing p53-dependent apoptosis in macrophages. This study was aimed at defining the molecular mechanism responsible for the protective effects of PCA against oxidative and proapoptotic damage exerted by oxLDL in J774 A.1 macrophages. We found that the presence of PCA in cells treated with oxLDL completely inhibited the p53-dependent apoptosis induced by oxLDL. PCA decreased oxLDL-induced ROS overproduction and in particular prevented the early increase of ROS. This decrease seemed to be the main signal responsible for maintaining the intracellular redox homeostasis hindering the activation of p53 induced by ROS, p38MAPK, and PKCδ. Consequently the overexpression of the proapoptotic p53-target genes such as p66Shc protein did not occur. Finally, we demonstrated that PCA induced the activation of JNK, which, in turn, determined the increase of nuclear Nrf2, leading to inhibition of the early ROS overproduction. We concluded that the antiapoptotic mechanism of PCA was most likely related to the activation of the JNK-mediated survival signals that strengthen the cellular antioxidant defences rather than to the PCA antioxidant power.

Protocatechuic acid activates key components of insulin signaling pathway mimicking insulin activity.

SCOPE: Insulin resistance represents an independent risk factor for metabolic and cardiovascular diseases. Researchers have been interested in identifying active harmless compounds, as many insulin-sensitizing drugs have shown unwanted side-effects. It has been demonstrated that anthocyanins and one of their representative metabolites, protocatechuic acid (PCA), ameliorate hyperglycemia, and insulin sensitivity. This study investigated the mechanism of action of PCA responsible for the glucose uptake upregulation. METHODS AND RESULTS: In human visceral adipocytes, PCA stimulated insulin receptor substrate-1 (IRS-1) tyrosine phosphorylation (+40% with respect to untreated cells) and the downstream events, i.e. phosphoinositide 3-kinase binding to IRS-1 and Akt phosphorylation (+100%, +180%, respectively, with respect to untreated cells). The insulin-like activity of PCA seemed to be mediated by insulin receptor since by inhibiting its autophosphorylation, the PCA effects were completely abolished. Furthermore, PCA was able to activate adenosine monophosphate-activated protein kinase, a serine/threonine kinase whose activation elicits insulin-sensitizing effects. CONCLUSION: This study showed that PCA stimulates the insulin signaling pathway in human adipocytes increasing GLUT4 translocation and glucose uptake. Decreasing insulin resistance is a most desirable aim to be reached for an effective therapeutic/preventive action against metabolic syndrome and type 2 diabetes. Identifying specific food/food components able to improve glucose metabolism can offer an attractive, novel, and economical strategy.

Protocatechuic acid inhibits human dendritic cell functional activation: role of PPARγ up-modulation.

Polyphenols have been shown to exhibit anti-inflammatory, anti-oxidant and immunomodulatory activities. However, the effects of anthocyanins, flavonoids of great nutritional interest, in particular of their metabolite protocatechuic acid (PCA) on the phenotypic and functional maturation of human dendritic cells (DCs) are still largely unknown. In this study, we report that PCA is efficiently taken up and accumulated in human monocyte-derived DCs (MD-DCs). PCA exposure of MD-DCs markedly impaired the production of proinflammatory cytokines and chemokines (i.e. IL-6, IL-8 and CCL2) in response to bacterial endotoxin and leptin, and down-regulated the lipopolysaccharide (LPS)-induced migratory response of MD-DCs to CCL19. Conversely, the phenotypic profile induced by LPS-mediated activation as well as IL-12 production was not affected. Interestingly, we found that PPARγ is a main factor in the PCA-induced effects as blocking its activity abolish PCA capacity to down-regulate IL-6 and IL-8, but not CCL2, secretion and to inhibit MD-DC migration. In keeping with this observation, cytosol to nucleus translocation and PPARγ activity were found to be directly stimulated by PCA exposure of MD-DCs. These novel findings provide new insight into the immunoregulatory effects of polyphenol metabolites in DCs opening new perspectives on their potential application in the prevention of acute and chronic inflammatory diseases.

Management of reproduction and pregnancy complications in maternal obesity: which role for dietary polyphenols?

Obesity is a global and dramatic public health problem; maternal obesity represents one of the main risk factors of infertility and pregnancy complications as it is associated with adverse maternal and offspring outcomes. In the last few years, adipose tissue dysfunction associated with altered adipocytokine secretion has been suggested to play a critical role in all the phases of reproductive process. Obesity is a nutrition-related disorder. In this regard, dietary intervention strategies, such as high intake of fruit and vegetables, have shown significant effects in both preserving health and counteracting obesity-associated diseases. Evidence has been provided that polyphenols, important constituents of plant-derived food, can influence developmental program of oocyte and embryo, as well as pregnancy progression by modulating several cellular pathways. This review will examine the controversial results so far obtained on adipocytokine involvement in fertility impairment and pregnancy complications. Furthermore, the different effects exerted by polyphenols on oocyte, embryo, and pregnancy development will be also taken in account.

Biomarkers of subclinical atherosclerosis in patients with autoimmune disorders.

Atherosclerosis is accelerated in rheumatoid arthritis (RA) and psoriatic arthritis (PsA). We investigated a possible association of oxidized low-density lipoproteins (ox-LDLs), nitric oxide (NO), 3-nitrotyrosine, vitamin A, vitamin E, and ß-carotene serum levels with subclinical atherosclerosis in RA and PsA. By the use of ELISA, we observed higher ox-LDL levels in patients with intima-media thickness (IMT) > 1 than in patients with IMT ≤ 1 and a negative correlation between NO levels and IMT values. By the use of high-performance liquid chromatography, we determined higher levels of vitamin A in patients with PsA and IMT ≤ 1 than in controls and lower levels of ß-carotene in patients with RA and PsA than in controls. ß-carotene concentrations were negatively correlated to the duration of disease in RA. Our study confirms that ox-LDLs and NO may be markers of accelerated atherosclerosis in RA and PsA whereas vitamins seem to be associated only to the presence of the autoimmune disorders.

Predominant role of obesity/insulin resistance in oxidative stress development.

BACKGROUND: Hyperlipidaemia, hyperglycaemia and hyperinsulinaemia, hallmarks of the postprandial state, have been also associated with increased oxidative stress and lipoprotein oxidation contributing to vascular injury and atherosclerosis. However, the specific links among metabolic disorders, postprandial state, insulin resistance and oxidative stress are still to be clarified. This study aimed at investigating the individual role played by obesity, insulin resistance and type 2 diabetes in the occurrence of fasting and postprandial oxidative stress. DESIGN: Biomarkers of oxidative stress [F2-isoprostanes and circulating oxidized low-density lipoproteins (LDL)], LDL oxidability (conjugated diene kinetic, thiobarbituric acid reactive substances (TBARs) formation and electronegativity increase) and antioxidant vitamins (ß-carotene, α-tocopherol and retinol) were evaluated at fasting and 6 h after a standard fat-rich meal in 10 obese diabetic (ObD), 11 obese and 11 normal-weight control men. Insulin sensitivity was evaluated by euglycaemic hyperinsulinaemic clamp. RESULTS: ObD and obese subjects, characterized by a similar level of adiposity and insulin resistance, showed higher urinary F2-isoprostanes and circulating oxidized LDL, an increased susceptibility to oxidation of plasma LDL (lower lag phase, higher TBARs formation, and higher relative electrophoretic mobility), and lower plasma content of ß-carotene and retinol than control subjects, both at fasting and after the test meal. CONCLUSIONS: Obesity and insulin resistance, more than type 2 diabetes, play the most relevant role in oxidative stress development. The correction of obesity and insulin resistance might be a useful strategy in counteracting systemic oxidative stress.

Cyanidin-3-O-ß-glucoside and protocatechuic acid exert insulin-like effects by upregulating PPARγ activity in human omental adipocytes.

OBJECTIVE: Insulin resistance (IR) represents an independent risk factor for metabolic, cardiovascular, and neoplastic disorders. Preventing/attenuating IR is a major objective to be reached to preserve population health. Because many insulin-sensitizing drugs have shown unwanted side effects, active harmless compounds are sought after. Dietary anthocyanins have been demonstrated to ameliorate hyperglycemia and insulin sensitivity. This study aimed at investigating whether cyanidin-3-O-ß-glucoside (C3G) and its metabolite protocatechuic acid (PCA) might have a role in glucose transport activation in human omental adipocytes and 3T3-L1 cells. RESEARCH DESIGN AND METHODS: In cells treated with 50 µmol/L C3G and 100 µmol/L PCA, [(3)H]-2-deoxyglucose uptake, GLUT4 translocation by immunoblotting, adiponectin secretion, and peroxisome proliferator-activated receptor-γ (PPARγ) activation by enzyme-linked immunosorbent assay kits were evaluated. Parallel experiments were carried out in murine adipocyte 3T3-L1. To define the role of PPARγ in modulating polyphenol effects, small interfering RNA technique and PPARγ antagonist were used to inhibit transcription factor activity. RESULTS: C3G and PCA increased adipocyte glucose uptake (P < 0.05) and GLUT4 membrane translocation (P < 0.01). Significant increases (P < 0.05) in nuclear PPARγ activity, as well as in adiponectin and GLUT4 expressions (P < 0.01), were also shown. It is interesting that PPARγ inhibition counteracted the polyphenol-induced adiponectin and GLUT4 upregulations, suggesting a direct involvement of PPARγ in this process. CONCLUSIONS: Our study provides evidence that C3G and PCA might exert insulin-like activities by PPARγ activation, evidencing a causal relationship between this transcription factor and adiponectin and GLUT4 upregulation. Dietary polyphenols could be included in the preventive/therapeutic armory against pathological conditions associated with IR.

CCAAT/enhancer-binding protein-ß participates in oxidized LDL-enhanced proliferation in 3T3-L1 cells.

Increased circulating oxidized LDL (oxLDL) have been found in obese subjects. Obesity is characterized by an excess of fat mass resulting from an increase in adipocyte number and size. The generation of new adipocytes is a tightly controlled process where multiple factors acting in a signaling cascade follow a precise temporal expression pattern; oxLDL appear to have a role in the impairment of this process. The purpose of this study was to examine the effects of oxLDL on the mechanisms involved in the proliferative stage of the differentiation process in 3T3-L1 cells. After hormonal induction, 3T3-L1 cells undergo approximately two rounds of mitotic clonal expansion (MCE), a process required for adipogenesis. CCAAT/enhancer-binding protein ß (C/EBPß) is immediately expressed after induction, and plays a crucial role in MCE, but its expression must decrease to allow preadipocytes to mature into adipocytes. We found that, in the presence of stimuli to differentiate, oxLDL induced a higher proliferation rate in this cell line, associated with a sustained up-regulation of C/EBPß, which remained activated inside the nucleus for several days. RNAi-mediated knockdown of C/EBPß 24 h after oxLDL treatment counteracted the increase in proliferation rate. Both C/EBPß expression and proliferation processes appear to be influenced by cAMP/protein kinase A (PKA) and extracellular signal-regulated kinases1/2 (ERK1/2) pathways. OxLDL treatment led to increased levels of cAMP, and to a strong, prolonged phosphorylation of ERK1/2 and C/EBPß. The addition of cAMP and PKA inhibitors, SQ22536 and H-89, respectively, reduced proliferation only in oxLDL-treated cells, whereas the addition of ERK1/2 inhibitor U0126 blocked proliferation in both control and oxLDL-treated cells. C/EBPß nuclear expression and DNA-binding activity were reduced by U0126, under all tested conditions. These findings show that the altered expression pattern of C/EBPß is involved in the increase in the number of proliferating cells induced by oxLDL, in hormone-stimulated 3T3-L1 cells.

Protocatechuic acid induces antioxidant/detoxifying enzyme expression through JNK-mediated Nrf2 activation in murine macrophages.

Protocatechuic acid (PCA) is a main metabolite of anthocyanins, whose daily intake is much higher than that of other polyphenols. PCA has biological effects, e.g., it induces the antioxidant/detoxifying enzyme gene expression. This study was aimed at defining the molecular mechanism responsible for PCA-induced over-expression of glutathione (GSH) peroxidase (GPx) and GSH reductase (GR) in J774 A.1 macrophages. New evidence is provided that PCA increases GPx and GR expression by inducing C-JUN NH(2)-terminal kinase (JNK)-mediated phosphorylation of Nuclear factor erythroid 2 (NF-E2)-related factor 2 (Nrf2). RNA and proteins were extracted from cells treated with PCA (25 µM) for different time points. Quantitative real-time polymerase chain reaction and immunoblotting analyses showed a rapid increase in mRNA (>60%) and protein (>50%) for both the enzymes. This was preceded by the up-regulation of Nrf2, in terms of mRNA and protein, and by its significant activation as assessed by increased Nrf2 phosphorylation and nuclear translocation (+60%). By using specific kinase inhibitors and detecting the activated form, we showed that JNK was the main upstream kinase responsible for Nrf2 activation. Convincing evidence is provided of a causal link between PCA-induced Nrf2 activation and increased enzyme expression. By silencing Nrf2 and using a JNK inhibitor, enzyme enhancement was counteracted. Finally, with the ChIP assay, we demonstrated that PCA-activated Nrf2 specifically bound ARE sequences in enzyme gene promoters. Our study demonstrates for the first time that PCA improves the macrophage endogenous antioxidant potential by a mechanism in which JNK-mediated Nrf2 activation plays an essential role. This knowledge could contribute to novel diet-based approaches aimed at counteracting oxidative injury by reinforcing endogenous defences.

Bioavailability of the polyphenols: status and controversies.

The current interest in polyphenols has been driven primarily by epidemiological studies. However, to establish conclusive evidence for the effectiveness of dietary polyphenols in disease prevention, it is useful to better define the bioavailability of the polyphenols, so that their biological activity can be evaluated. The bioavailability appears to differ greatly among the various phenolic compounds, and the most abundant ones in our diet are not necessarily those that have the best bioavailability profile. In the present review, we focus on the factors influencing the bioavailability of the polyphenols. Moreover, a critical overview on the difficulties and the controversies of the studies on the bioavailability is discussed.

Oxidized LDL impair adipocyte response to insulin by activating serine/threonine kinases.

Oxidized LDL (oxLDL) increase in patients affected by type-2 diabetes, obesity, and metabolic syndrome. Likewise, insulin resistance, an impaired responsiveness of target tissues to insulin, is associated with those pathological conditions. To investigate a possible causal relationship between oxLDL and the onset of insulin resistance, we evaluated the response to insulin of 3T3-L1 adipocytes treated with oxLDL. We observed that oxLDL inhibited glucose uptake (-40%) through reduced glucose transporter 4 (GLUT4) recruitment to the plasma membrane (-70%), without affecting GLUT4 gene expression. These findings were associated to the impairment of insulin signaling. Specifically, in oxLDL-treated cells insulin receptor (IR) substrate-1 (IRS-1) was highly degraded likely because of the enhanced Ser(307)phosphorylation. This process was largely mediated by the activation of the inhibitor of kappaB-kinase beta (IKKbeta) and the c-Jun NH(2)-terminal kinase (JNK). Moreover, the activation of IKKbeta positively regulated the nuclear content of nuclear factor kappaB (NF-kappaB), by inactivating the inhibitor of NF-kappaB (IkappaBalpha). The activated NF-kappaB further impaired per se GLUT4 functionality. Specific inhibitors of IKKbeta, JNK, and NF-kappaB restored insulin sensitivity in adipocytes treated with oxLDL. These data provide the first evidence that oxLDL, by activating serine/threonine kinases, impaired adipocyte response to insulin affecting pathways involved in the recruitment of GLUT4 to plasma membranes (PM). This suggests that oxLDL might participate in the development of insulin resistance.

Effect of high intakes of fruit and vegetables on redox status in type 2 onset diabetes: a pilot study.

Evidence has accumulated indicating that oxidative stress may play a key role in the etiology of diabetic complications and the protective effects of antioxidant nutrients are a topic of intense research. The purpose of this study was both to obtain preliminary data on the effect of a diet high in fruit and vegetables on metabolic control and the oxidative status of patients with type 2 onset diabetes, and to identify the most useful biochemical parameters for future research. At the beginning of the study all subjects were asked to follow their usual diet and keep a seven-day food diary. Diabetic patients then received a dietary treatment designed to ensure a daily intake of 700-1000 g of fruit and vegetables; no dietary advice was given to controls. Dietary antioxidants, redox status markers, and parameters of metabolic control were measured in plasma and erythrocytes before and after the diet. Before following the diet, diabetic patients had lower levels of ascorbic acid, beta-carotene, and alpha-tocopherol/cholesterol ratio than controls. After the diet these parameters increased and there was also a reduction in total antioxidant capacity, uric acid, and malondialdehyde and a rise in reduced glutathione accompanied by a reduction in body mass index and cholesterol. In conclusion, a high consumption of fruit and vegetables by diabetic patients not receiving pharmacological treatment, seems to produce an improvement in some redox status parameters.