Downregulation of Caveolin-1 and Upregulation of Deiodinase 3, Associated with Hypoxia-Inducible Factor-1α Increase, Are Involved in the Oxidative Stress of Graves' Orbital Adipocytes

Background: Even though the clinical features of Graves' orbitopathy (GO) are well known, its exact pathogenesis remains controversial. The imbalance of redox homeostasis in the connective tissue could play a crucial role leading to an inflammatory state and edema of soft orbital tissues, thus contributing to orbital hypoxia and increase in hypoxia-inducible factor (HIF)-1α. This oxidative stress appears to target the orbital cells such as fibroblasts and also adipocytes. This study aims to explore which pathways can lead to the aforementioned oxidative stress in GO adipose cells and therefore offers new plausible therapeutic targets. Methods: Orbital fat samples were obtained from patients with GO (Western blot [WB]: n = 8, immunohistochemistry [IHC]: n = 8) and from control patients (WB: n = 5, IHC: n = 3-5). They were processed for WB analysis and IHC of the antioxidants (catalase, superoxide dismutase 1) and for HIF-1α. The expression of caveolin-1 (Cav-1) and deiodinase 3 (DIO3), known to be regulated by HIF-1α, was also analyzed by WB and IHC, as well as the targets of Cav-1: glucose transporter type 4 (Glut-4), NADPH oxidase (NOX)-2, and endothelial nitric oxide synthase (eNOS). Triiodothyronine (T3) expression was also analyzed by IHC. Results: In GO adipocytes, the expression of catalase was reduced, whereas that of HIF-1α was strongly increased. A decreased local T3 supply was associated with DIO3 upregulation. The low expression of Cav-1 in GO adipocytes was associated not only with low expression of Glut-4 but also with an increased expression of NOX-2 and active eNOS phosphorylated on serine 1177. Conclusions: Cav-1 and DIO3, both sensitive to hypoxia and to the increase of HIF-1α, play a pivotal role in the oxidative stress in GO adipocytes. DIO3 regulates the cellular supply of T3, which is essential for the cell homeostasis. Cav-1 determines the cellular glucose supply through Glut-4 and regulates the activity of NOX-2 generating superoxide anions and that of eNOS generating nitric oxide (NO).

Protein tyrosine phosphatase 1B as a therapeutic target for Graves' orbitopathy in an in vitro model

Graves' orbitopathy (GO) is characterised in early stages by orbital fibroblast inflammation, which can be aggravated by oxidative stress and often leads to fibrosis. Protein tyrosine protein 1B (PTP1B) is a regulator of inflammation and a therapeutic target in diabetes. We investigated the role of PTP1B in the GO mechanism using orbital fibroblasts from GO and healthy non-GO subjects. After 24 hours of transfection with PTPN1 siRNA, the fibroblasts were exposed to interleukin (IL)-1β, cigarette smoke extract (CSE), H2O2, and transforming growth factor (TGF)-β stimulations. Inflammatory cytokines and fibrosis-related proteins were analysed using western blotting and/or enzyme-linked immunosorbent assay (ELISA). Reactive oxygen species (ROS) release was detected using an oxidant-sensitive fluorescent probe. IL-1β, tumor necrosis factor (TNF)-α, bovine thyroid stimulating hormone (bTSH), high-affinity human stimulatory monoclonal antibody of TSH receptor (M22), and insulin-like growth factor-1 (IGF-1) significantly increased PTP1B protein production in GO and non-GO fibroblasts. PTPN1 silencing significantly blocked IL-1β-induced inflammatory cytokine production, CSE- and H2O2-induced ROS synthesis, and TGF-β-induced expression of collagen Iα, α-smooth muscle actin (SMA), and fibronectin in GO fibroblasts. Silencing PTPN1 also decreased phosphorylation levels of Akt, p38, and c-Jun N-terminal kinase (JNK) and endoplasmic reticulum (ER)-stress response proteins in GO cells. PTP1B may be a potential therapeutic target of anti-inflammatory, anti-oxidant and anti-fibrotic treatment of GO.

Hyperthyroidism in Patients with Graves' Ophthalmopathy, and Thyroidal, Skeletal and Eye Muscle Specific Type 2 Deiodinase Enzyme Activities

Graves' ophthalmopathy is characterized by hyperthyroidism, which is associated with higher serum T3 levels than T4 due to deiodinase enzymes.The effect of Graves' patient's sera (n=52) with elevated thyroid hormone and TSH receptor or thyroid peroxidase antibody (anti-TPO) levels was investigated on thyroidal, skeletal and eye muscle type 2 deiodinase enzyme (DII) activities. DII activities were measured with 125I-T4 substrate, while thyroid hormone and antibody levels with immunoassays.In Graves' ophthalmopathy, sera with elevated FT4 or FT3 levels reduced DII activites remarkably in all tissue fractions. Thyroidal DII activities were lower than those using eye muscle fraction (0.6±0.22 vs 1.14±0.43 pmol/mg/min, P<0.006). Effect of sera with increased FT3 levels demonstrated also reduced DII activities in patients with Graves' ophthalmopathy after methimazole therapy compared to those who had no ophthalmopathy (2.88±2 vs 20.42±11.82 pmol/mg/min, P<0.006 for thyroidal fraction, 4.07±2.72 vs 29.22±15.46 pmol/mg/min, P<0.004 for skeletal muscle, 5.3±3.47 vs 37.87±18.82 pmol/mg/min, P<0.003 for eye muscle). Hyperthyroid sera with TSH receptor antibodies resulted in increased DII activities, while sera with anti-TPO antibodies were connected to lower DII activities in Graves' ophthalmopathy.In summary, the actions of hyperthyroid sera derived from patients with Graves' disease were tested on tissue-specific DII activities. Elevated FT4 level-induced DII inactivation is present in Graves' ophthalmopathy, which seems to be also present at the beginning of methimazole therapy. Stimulating TSH receptor antibiodies increased DII activities via their nongenomic effects using sera of hyperthyroid Graves' ophthalmopathy, but anti-TPO antibodies could influence DII activities via altering FT4 levels.

Expression of cyclooxygenase-2 in orbital fibroadipose connective tissues of Graves' ophthalmopathy patients

OBJECTIVE

The aim of this study is to evaluate the expression of cycloocygenase-2 (COX-2) in orbital fibroadipose connective tissue in Graves' ophthalmopathy (GO) patients, and investigate the associations between COX-2 expression and GO characteristics.

METHODS

The orbital fibroadipose connective tissues of 23 cases demonstrating moderate or severe GO, and eight control subjects without any history of thyroid or autoimmune disease were analyzed for COX-2 mRNA expression. Real-time relative quantitative PCR was performed to assess transcripts of COX-2 using the LightCycler. The disease activity was evaluated by the clinical activity score (CAS). The clinical features of GO were evaluated by total eye score (TES) and the cases were divided into two groups; type 1 cases included higher degrees of proptosis with orbital fat volume increase, and type 2 cases included cases with compressive neuropathy and limited extraocular muscle functions.

RESULTS

The mean +/- s.d. disease duration was 5.7 +/- 7.1 years. The mean +/- s.d. CAS and TES of cases were 1.60 +/- 1.04 and 7.5 +/- 1.8 respectively. The mean +/- s.d. expression of COX-2 was 0.023 +/- 0.013 and 0.010 +/- 0.002 in GO cases and controls (P = 0.008), and 0.015 +/- 0.073 and 0.029 +/- 0.135 in type 1 and type 2 cases respectively (P = 0.007). COX-2 expression showed a statistically significant positive correlation with TES (r = 0.634, P = 0.001), and a negative correlation with the disease duration (r = -0.621, P = 0.002).

CONCLUSIONS

COX-2 is expressed at higher levels in orbital fibroadipose tissues of GO cases. This showed a positive correlation with increasing severity of orbital disease suggesting possible relation with COX-2 expression and orbital inflammation in GO.