Alterations in the enzyme activity and protein contents of protein disulfide isomerase in rat tissues during fasting and refeeding

Mechanisms by which the thiazolidinedione troglitazone protects against sucrose-induced hepatic fat accumulation and hyperinsulinaemia

BACKGROUND AND PURPOSE

Thiazolidinediones (TZD) are known to ameliorate fatty liver in type 2 diabetes. To date, the underlying mechanisms of their hepatic actions remain unclear.

EXPERIMENTAL APPROACH

Hepatic triglyceride content and export rates were assessed in 2 week high-sucrose-fed Wistar rats treated with troglitazone and compared with untreated high-sucrose rodent controls. Fractional de novo lipogenesis (DNL) contributions to hepatic triglyceride were quantified by analysis of triglyceride enrichment from deuterated water. Hepatic insulin clearance and NO status during a meal tolerance test were also evaluated.

KEY RESULTS

TZD significantly reduced hepatic triglyceride (P < 0.01) by 48%, decreased DNL contribution to hepatic triglyceride (P < 0.01) and increased postprandial non-esterified fatty acids clearance rates (P < 0.01) in comparison with the high-sucrose rodent control group. During a meal tolerance test, plasma insulin AUC was significantly lower (P < 0.01), while blood glucose and plasma C-peptide levels were not different. Insulin clearance was increased (P < 0.001) by 24% and was associated with a 22% augmentation of hepatic insulin-degrading enzyme activity (P < 0.05). Finally, hepatic NO was decreased by 24% (P < 0.05).

CONCLUSIONS

Overall, TZD show direct actions on liver by reducing hepatic DNL and increasing hepatic insulin clearance. The alterations in hepatic insulin clearance were associated with changes in insulin-degrading enzyme activity, with possible modulation of NO levels.

Dietary soy isoflavones increased hepatic protein disulfide isomerase content and suppressed its enzymatic activity in rats

Protein disulfide isomerase (PDI) is a multifunctional protein and plays important roles in protein folding, triglyceride transfer, insulin degradation, and thyroid hormone transportation. This study examined the modulation of PDI expression by soy consumption using rat as a model. Sprague-Dawley male and female rats at 50 days (d) of age were fed diets containing either 20% casein or alcohol-washed soy protein isolate (SPI, containing 50 mg isoflavones (ISFs)/kg diet) or SPI plus ISF (250 mg/kg diet) and mated at age of 120 d. The offspring (F1) were fed the same diets as their parents. Addition of ISF to SPI diet markedly increased PDI protein content in the liver and testis of the adult rats compared with the casein or SPI diet. PDI mRNA abundance in the liver and protein content in the brain, thyroid, heart, and uterus were unchanged by the diets. Two-dimensional Western blot showed that the rats fed diets containing SPI had a diminished hepatic PDI protein with an isoelectric point (pI) of 6.12, a dephosphorylated form, compared with the rats fed diets containing either casein or SPI with supplemental ISF. Soy ISF added into SPI diet remarkably suppressed hepatic PDI activity of the rats compared with the casein diet. Moreover, soy ISF dose-dependently increased PDI and thyroid hormone receptor (TR) β protein content, whereas reduced TR DNA binding ability in human hepatocytes. Overall, this study shows that soy ISF increased hepatic PDI protein content, but addition of ISF into SPI diet inhibited its enzymatic activities and this effect may be mediated through a post-transcriptional mechanism.

Channel catfish (Ictalurus punctatus) protein disulphide isomerase, PDIA6: molecular characterization and expression regulated by bacteria and virus inoculation

Protein disulfide isomerases (PDIs) are thought to aid protein folding and assembly by catalyzing formation and shuffling of cysteine disulfide bonds in the endoplasmic reticulum (ER). Currently, increasing evidence suggests PDIs play an important role in host cell invasion and they are relevant targets for the host immune response. However the roles of specific PDIs in teleosts are little known. Here, we characterized the Protein disulfide isomerase family A, member 6 (PDIA6) from channel catfish, Ictalurus punctatus (named as ccPDIA6). The catfish ccPDIA6 gene was homologous to those of other vertebrate species with 13 exons and 12 introns. The consensus full-length ccPDIA6 cDNA contained an ORF of 1320 bp encoding a putative protein of 439 amino acids. It had a 19 amino acid signal peptide and two active thioredoxin-like domains. Sequence of phylogenic analysis and multiple alignments showed that ccPDIA6 was conserved throughout vertebrate evolution. Southern blot analysis suggested the presence of one copy of the ccPDIA6 gene in the catfish genome. Tissue distribution shows that ccPDIA6 was expressed in all examined tissues at the mRNA level. When using the aquatic zoonotic pathogens such as Edwardsiella tara, Streptococcus iniae, and channel catfish reovirus （CCRV） to challenge channel catfish, ccPDIA6 expression was significant changed in immune-related tissues such as head kidney, intestine, liver and spleen. The results suggested that ccPDIA6 might play an important role in the immunity of channel catfish. This is the first report that the PDI gene may be involved in fish host defense against pathogen infection.

Proteomic Assessment of Tanshinone II A Sodium Sulfonate on Doxorubicin Induced Nephropathy

Although doxorubicin (DXR) is an important antineoplastic agent, the serious toxicity mediated by the production of reactive oxygen species has remained a considerable clinical problem. Our hypothesis is that tanshinone II A sodium sulfonate (TSNIIA-SS), which holds significant effects against oxidative stress, protects against DXR-induced nephropathy. Firstly, the antioxidative effects of TSNIIA-SS were confirmed using oxygen radicals absorbance capacities (ORAC) assay in vitro. Then, DXR nephropathy was induced by repeated DXR treatment and verified by kidney index (20.76 ± 3.04 mg/mm versus 14.76 ± 3.04 mg/mm, p < 0.001) and histochemical stain. The mice were randomized into three groups: Control group, DXR group and DXR-TSNIIA-SS group. TSNIIA-SS treatment not only improved DXR lesion identified by histochemical stain, but also regulated the expression of several proteins related with the cytoskeleton, oxidative stress and protein synthesis or degradation detected by two-dimensional electrophoresis (2-DE). These data have provided the evidence that TSNIIA-SS is a protective agent against DXR-induced nephropathy.

Downregulation of protein disulfide isomerase in sepsis and its role in tumor necrosis factor-alpha release

Introduction

Protein disulfide isomerase (PDI) is an important factor for the protein modification step in the post-translational event. PDI plays an essential role in cell survival under various stress conditions. It has been reported that PDI can serve as a negative regulator of nuclear factor-kappa-B (NF-κB) and that it can inhibit lipopolysaccharide (LPS)-induced proinflammatory cytokine production in macrophages. Thus, PDI may be an intracellular anti-inflammatory molecule. Although we have previously shown that Kupffer cell-derived proinflammatory cytokines cause liver injury in sepsis, the effect of sepsis on PDI expression as well as the effect of PDI inhibition on cytokine production have not been investigated. We therefore hypothesized that sepsis downregulates PDI expression and that the inhibition of PDI promotes proinflammatory cytokine production.

Method

Adult male rats were subjected to sepsis by cecal ligation and puncture (CLP) or endotoxemia (continuous infusion of 1 μg/kg body weight LPS by an osmotic pump) for 20 hours. Hepatic tissues were collected and PDI gene expression was determined. In additional experiments, cells from a macrophage-like cell line, RAW 264.7, were treated with 100 ng/mL LPS for 4 hours and protein expressions were measured. RAW 264.7 cells were also treated with bacitracin, a specific PDI inhibitor, for 24 hours, and tumor necrosis factor-alpha (TNF-α) gene and protein expression as well as its release in the cell supernatant were determined. To further confirm the beneficial effect of PDI in sepsis, RAW 264.7 cells were transfected with PDI short interfering RNA (siRNA) and PDI gene expression and TNF-α release were measured by quantitative polymerase chain reaction and enzyme-linked immunosorbent assay, respectively.

Results

PDI gene expression was significantly decreased by 28% and 69% at 20 hours after CLP or LPS infusion, respectively. LPS also decreased PDI protein expression by 33% in RAW 264.7 cells. Incubation of RAW 264.7 cells with bacitracin significantly increased TNF-α gene expression and TNF-α release as well as its cellular levels in a dose-dependent manner. Transfection of RAW 264.7 cells with PDI siRNA produced an average 36.8% inhibition of the PDI gene expression. This downregulation was correlated with a 3.19-fold increase in TNF-α release into the cell supernatant.

Conclusion

Taken together, these results suggest that downregulation of PDI by sepsis significantly increases proinflammatory cytokine production. Thus, prevention of PDI downregulation in sepsis may be a novel approach to attenuate hyperinflammation and to reduce tissue injury under such conditions.

Small Intestine and Liver Microsomal Triacylglycerol Transfer Protein in the Bovine and Rat: Effects of Dietary Coconut Oil

The bovine liver is characterized by a chronic low capacity to secrete triacylglycerols (TAG). In situations favoring their hepatic synthesis, such as coconut oil feeding, TAG accumulate, leading to a lipid infiltration in the liver of preruminant calves. To assess the possible role of the microsomal TAG transfer protein (MTP) in this phenomenon and to put into evidence a tissue-specific regulation in the bovine species, we compared by Western blot the content in both MTP subunits in the liver and in different portions of the small intestine in preruminant calves and in growing rats receiving coconut oil or beef tallow as the sole source of fat in the diet. The pattern of MTP distribution was similar between calf and rat tissues, the jejunum being the major site for both MTP expression and intestinal absorption of dietary lipid endproducts. Concentrations of the MTP large and small subunits were 10- to 20-fold lower and 2- to 3-fold lower, respectively, in calf than in rat tissues, including the liver. Coconut oil in the diets of calves and rats did not significantly affect the expression of MTP large subunits even though TAG content was strongly increased 12-fold in the calf liver. These results clearly indicated that calf liver handled fat metabolically in a manner different from rat liver. However, present experimental conditions did not allow proof that MTP was directly related to the accumulation of fat in calf liver.