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 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.