Pioglitazone normalizes insulin signaling in the diabetic rat retina through reduction in tumor necrosis factor α and suppressor of cytokine signaling 3.

Dysfunctional insulin signaling is a key component of type 2 diabetes. Little is understood of the effects of systemic diabetes on retinal insulin signaling. A number of agents are used to treat patients with type 2 diabetes to normalize glucose levels and improve insulin signaling; however, little has been done to investigate the effects of these agents on retinal insulin signal transduction. We hypothesized that pioglitazone, a peroxisome proliferator-activated receptor γ (PPARγ) agonist, would normalize retinal insulin signal transduction through reduced tumor necrosis factor α (TNFα) and suppressor of cytokine signaling 3 (SOCS3) activities in whole retina and retinal endothelial cells (REC) and Müller cells. To test this hypothesis, we used the BBZDR/Wor type 2 diabetic rat model, as well as REC and Müller cells cultured in normoglycemia and hyperglycemic conditions, to investigate the effects of pioglitazone on TNFα, SOCS3, and downstream insulin signal transduction proteins. We also evaluated pioglitazone's effects on retinal function using electroretinogram and markers of apoptosis. Data demonstrate that 2 months of pioglitazone significantly increased electroretinogram amplitudes in type 2 diabetic obese rats, which was associated with improved insulin receptor activation. These changes occurred in both REC and Müller cells treated with pioglitazone, suggesting that these two cell types are key to insulin resistance in the retina. Taken together, these data provide evidence of impaired insulin signaling in type 2 diabetes rats, which was improved by increasing PPARγ activity. Further investigations of PPARγ actions in the retina may provide improved treatment options.

Nuclear corepressors mediate the repression of phospholipase A2 group IIa gene transcription by thyroid hormone.

Secretory phospholipase A2 group IIa (PLA2g2a) is associated with inflammation, hyperlipidemia, and atherogenesis. Transcription of the PLA2g2a gene is induced by multiple cytokines. Here, we report the surprising observation that thyroid hormone (T3) inhibited PLA2g2a gene expression in human and rat hepatocytes as well as in rat liver. Moreover, T3 reduced the cytokine-mediated induction of PLA2g2a, suggesting that the thyroid status may modulate aspects of the inflammatory response. In an effort to dissect the mechanism of repression by T3, we cloned the PLA2g2a gene and identified a negative T3 response element in the promoter. This T3 receptor (TRß)-binding site differed considerably from consensus T3 stimulatory elements. Using in vitro and in vivo binding assays, we found that TRß bound directly to the PLA2g2a promoter as a heterodimer with the retinoid X receptor. Knockdown of nuclear corepressor or silencing mediator for retinoid and thyroid receptors by siRNA blocked the T3 inhibition of PLA2g2a. Using chromatin immunoprecipitation assays, we showed that nuclear corepressor and silencing mediator for retinoid and thyroid receptors were associated with the PLA2g2a gene in the presence of T3. In contrast with the established role of T3 to promote coactivator association with TRß, our experiments demonstrate a novel inverse recruitment mechanism in which liganded TRß recruits corepressors to inhibit PLA2g2a expression.

Role of sirtuin 1 in the regulation of hepatic gene expression by thyroid hormone.

Sirtuin 1 (SIRT1) is a nuclear deacetylase that modulates lipid metabolism and enhances mitochondrial activity. SIRT1 targets multiple transcription factors and coactivators. Thyroid hormone (T(3)) stimulates the expression of hepatic genes involved in mitochondrial fatty acid oxidation and gluconeogenesis. We reported that T(3) induces genes for carnitine palmitoyltransferase (cpt1a), pyruvate dehydrogenase kinase 4 (pdk4), and phosphoenolpyruvate carboxykinase (pepck). SIRT1 increases the expression of these genes via the activation of several factors, including peroxisome proliferator-activated receptor α, estrogen-related receptor α, and peroxisome proliferator-activated receptor γ coactivator (PGC-1α). Previously, we reported that PGC-1α participates in the T(3) induction of cpt1a and pdk4 in the liver. Given the overlapping targets of T(3) and SIRT1, we investigated whether SIRT1 participated in the T(3) regulation of these genes. Resveratrol is a small phenolic compound whose actions include the activation of SIRT1. Addition of resveratrol increased the T(3) induction of the pdk4 and cpt1a genes in hepatocytes. Furthermore, expression of SIRT1 in hepatocytes mimicked resveratrol in the regulation of gene expression by T(3). The deacetylase activity of SIRT1 was required and PGC-1α was deacetylated following addition of T(3). We found that SIRT1 interacted directly with T(3) receptor (TRß). Knockdown of SIRT1 decreased the T(3) induction of cpt1a and pdk4 and reduced the T(3) inhibition of sterol response element binding protein (srebp-1c) both in isolated hepatocytes and in rat liver. Our results indicate that SIRT1 contributes to the T(3) regulation of hepatic genes.

Mechanisms related to NO-induced motility in differentiated rat aortic smooth muscle cells.

Nitric oxide (NO) is thought to play an important role as an inhibitor of vascular cell proliferation, motility, and neointima formation. This effect is mediated, in part, via the upregulation of protein tyrosine phosphatase (PTP)1B. Conversely, studies have reported that in presumably hyperinsulinemic mice fed a high-fat diet, NO enhances vascular remodeling, whereas a deficit of NO attenuates vascular remodeling. We have reported that in differentiated cultured smooth muscle cells treated with insulin, NO induces a motogenic effect that is dependent on Src homology-2 domain PTP 2 (SHP2) upregulation. In the present study, we describe novel mechanisms relevant to the motogenic effect of NO. Treatment of cultured cells with the selective angiontensin type 1 receptor antagonist losartan, but not with the selective angiotensin type 2 receptor antagonist PD-123319, blocked the comotogenic capacity of NO and insulin. Insulin and NO increased the secretion of ANG II into the culture media by 2- and 2.5-fold (P < 0.05), respectively, whereas treatment of cells with ANG II uncovered the motogenic effect of NO (1.4-fold above control, P < 0.05) and decreased the levels of PTP1B to 45% of control (P < 0.05). Suppression of PTP1B function was sufficient to uncover the motogenic effect of NO. The capacity of insulin to suppress PTP1B activity was blocked by losartan, implicating ANG II function in mediating this effect. Both insulin and ANG II induced the upregulation of phosphatidyl inositol 3-kinase (PI3K)-δ by two- to threefold (P < 0.05), and this effect was both necessary and sufficient to uncover NO-induced motogenesis. Finally, suppression of PTP1B function potentiated, whereas overexpression of PTP1B inhibited, SHP2-induced motogenesis. These results support the hypothesis that the comotogenic effect of insulin and NO occurs via an ANG II-mediated effect involving the suppression of PTP1B and upregulation of PI3K-δ and SHP2.

Suppression of PKG by PDGF or nitric oxide in differentiated aortic smooth muscle cells: obligatory role of protein tyrosine phosphatase 1B.

Treatment of aortic smooth muscle cells with PDGF induces the upregulation of protein tyrosine phosphatase 1B (PTP1B). PTP1B, in turn, decreases the function of several growth factor receptors, thus completing a negative feedback loop. Studies have reported that PDGF induces the downregulation of PKG as part of a repertoire of dedifferentiation of vascular smooth muscle cells. Other studies have reported that chronic nitric oxide (NO) treatment also induces the downregulation of PKG. In the present study, we tested the hypothesis that the downregulation of PKG by PDGF or NO in differentiated rat aortic smooth muscle cells can be attributed to the upregulation of PTP1B. We found that treatment with PDGF or NO induced an upregulation of PTP1B levels. Overexpression of PTP1B induced a marked downregulation of PKG mRNA and protein levels, whereas the expression of dominant negative PTP1B or short interfering RNA directed against PTP1B blocked the capacity of PDGF or NO to decrease PKG levels. We conclude that the upregulation of PTP1B by PDGF or NO is both necessary and sufficient to induce the downregulation of PKG via an effect on PKG mRNA levels.

Sodium salicylate reduced insulin resistance in the retina of a type 2 diabetic rat model.

Sodium salicylate has been reported to reduce markers of diabetic retinopathy in a type 1 rat model. Because rates of type 2 diabetes are on the rise, we wanted to determine whether salicylate could improve insulin resistance in a type 2 rat model, as well as improve retinal function. We treated lean and obese BBZDR/Wor type 2 diabetic rats with salicylate in their chow for 2 months. Prior to salicylate treatment, rats underwent an electroretinogram to measure retinal function. After 2 months of treatment, rats underwent an additional electroretinogram prior to sacrifice. In addition to the animal model, we also treated retinal endothelial cells (REC) and rat Müller cells with salicylate and performed the same analyses as done for the rat retinal lysates. To investigate the role of salicylate in insulin signaling, we measured TNFα and caspase 3 levels by ELISA, as well as performed Western blotting for insulin receptor substrate 1, insulin receptor, SOCS3, and pro- and anti-apoptotic markers. Data demonstrated that salicylate significantly improved retinal function, as well as reduced TNFα and SOCS3-induced insulin resistance in all samples. Overall, results suggest that salicylate is effective in reducing insulin resistance in the retina of type 2 diabetic rat models.

Pioglitazone restores IGFBP-3 levels through DNA PK in retinal endothelial cells cultured in hyperglycemic conditions.

PURPOSE: Previously, we reported that pioglitazone prevented insulin resistance and cell death in type 2 diabetic retina by reducing TNFα and suppressor of cytokine signaling 3 (SOCS3) levels. Numerous reports suggest prominent vasoprotective effects of insulin growth factor binding protein-3 (IGFBP-3) in diabetic retinopathy. We hypothesized that pioglitazone protects against retinal cell apoptosis by regulating IGFBP-3 levels, in addition to reducing TNFα. The current study explored potential IGFBP-3 regulatory pathways by pioglitazone in retinal endothelial cells cultured in high glucose. METHODS: Primary human retinal endothelial cells (REC) were grown in normal (5 mM) and high glucose (25 mM) and treated with pioglitazone for 24 hours. Cell lysates were processed for Western blotting and ELISA analysis to evaluate IGFBP-3, TNFα, and cleaved caspase 3 protein levels. RESULTS: Our results show that treatment with pioglitazone restored the high glucose-induced decrease in IGFBP-3 levels. This regulation was independent of TNFα actions, as reducing TNFα levels with siRNA did not prevent pioglitazone from increasing IGFBP-3 levels. Pioglitazone required protein kinase A (PKA) and DNA-dependent protein kinase (DNA PK) activity to regulate IGFBP-3, as specific inhibitors for each protein prevented pioglitazone-mediated normalization of IGFBP-3 in high glucose. Insulin growth factor binding protein-3 activity was increased and apoptosis decreased by pioglitazone, which was eliminated when serine site 156 of IGFBP-3 was mutated suggesting a key role of this phosphorylation site in pioglitazone actions. CONCLUSIONS: Our findings suggest that pioglitazone mediates regulation of IGFBP-3 via activation of PKA/DNA PK pathway in hyperglycemic retinal endothelial cells.

Identification of Francisella tularensis lipoproteins that stimulate the toll-like receptor (TLR) 2/TLR1 heterodimer.

The innate immune response to Francisella tularensis is primarily mediated by TLR2, though the bacterial products that stimulate this receptor remain unknown. Here we report the identification of two Francisella lipoproteins, TUL4 and FTT1103, which activate TLR2. We demonstrate that TUL4 and FTT1103 stimulate chemokine production in human and mouse cells in a TLR2-dependent way. Using an assay that relies on chimeric TLR proteins, we show that TUL4 and FTT1103 stimulate exclusively the TLR2/TLR1 heterodimer. Our results also show that yet unidentified Francisella proteins, possibly unlipi-dated, have the ability to stimulate the TLR2/TLR6 heterodimer. Through domain-exchange analysis, we determined that an extended region that comprises LRR 9-17 in the extra-cellular portion of TLR1 mediates response to Francisella lipoproteins and triacylated lipopeptide. Substitution of the corresponding LRR of TLR6 with the LRR derived from TLR1 enables TLR6 to recognize TUL4, FTT1103, and triacylated lipopeptide. This study identifies for the first time specific Fran-cisella products capable of stimulating a proinflammatory response and the cellular receptors they trigger.

Oral administration of orthovanadate and Trigonella foenum graecum seed power restore the activities of mitochondrial enzymes in tissues of alloxan-induced diabetic rats.

The effect of oral administration of sodium orthovanadate (SOV) and Trigonella foenum graecum seed powder (TSP), a medicinal plant used extensively in Asia, on the mitochondrial metabolism in the alloxan diabetic rats has been investigated. Rats were injected with alloxan monohydrate (20 mg/100 g body wt) or vehicle (Na-acetate buffer), the former were treated with either 2 IU insulin i.p., 0.6 mg/ml SOV ad libitum, 5% TSP ad libitum, and a combination of 0.2% SOV and 5% TSP ad libitum for 21 days. Selected rate-limiting enzymes of the tricarboxylic acid cycle, hydrogen shuttle system, ketone body metabolism, amino acid metabolism and urea cycle were measured in the mitochondrial and cytosolic fractions of liver, kidney and brain tissues of the experimental rats. Majority of the mitochondrial enzymes in the tissues of the diabetic rats had significantly higher activities compared to the control rats. Similarly, the activities of mitochondrial and cytosolic aminotransferases and arginase were significantly higher in liver and kidney tissues of the diabetic rats. The separate administrations of SOV and TSP to diabetic rats were able to restore the activities of these enzymes to control values. The lower dose of SOV (0.2%) administered in combination with TSP to diabetic rats lowered the enzyme activities more significantly than when given in a higher dose (0.6%) separately. This is the first report of the effective combined action of oral SOV and TSP in ameliorating the altered mitochondrial enzyme activities during experimental type-1 diabetes. Our novel combined oral administration of SOV and TSP to diabetic rats thus conclusively proves as a possible method to minimize potential vanadate toxicity without compromising its positive effects in the therapy of experimental type-1 diabetes.

Trigonella foenum graecum seed powder protects against histopathological abnormalities in tissues of diabetic rats.

Trigonella foenum graecum is a well-known hypoglycemic agent used in traditional Indian medicines. It was previously reported that oral administration of its seed powder for 3 weeks to alloxan diabetic rats stabilized glucose homeostasis and free radical metabolism in liver and kidney. In the present study, we further investigated the effects of 3 weeks alloxan induced diabetes on the histological structure and function of liver and kidney and the protective effect of T. foenum graecum seed powder (TSP) oral administration to the diabetic rats utilizing enzyme analysis and light and transmission electron microscopy. The activity of the enzyme, glutamate dehydrogenase was significantly higher whereas the activity of D-beta-hydroxybutyrate dehydrogenase enzyme was significantly lower in liver and kidney of alloxan-induced diabetic rats. Histopathological studies showed liver degenerative and early nephropathic changes in diabetic rats. Ultrastructure of the diabetic liver revealed a reduction in the rough endoplasmic reticulum and swelling of mitochondria in the hepatocytes. TSP treatment to the diabetic rats effectively prevented the alteration in the activities of the two enzymes and partially prevented the structural abnormalities thus suggesting a protective effect of TSP on the liver and kidney of the diabetic rats. The role of TSP in reversing the diabetic state at the cellular level besides the metabolic normalization further proves its potential as an antidiabetic agent.