A novel mechanism for the inhibition of type 2 iodothyronine deiodinase by tumor necrosis factor α: involvement of proteasomal degradation

The effect of anti-TNF therapy on thyroid function in patients with inflammatory bowel disease

The aim of this study was to investigate for first time the thyroid function in patients with inflammatory bowel disease (IBD) and the potential effect of anti-TNF (tumor necrosis factor) therapy. We evaluated 41 patients with IBD (25M/16F, 36.5 ± 11.3 y, 27 with Crohn's disease and 14 with ulcerative colitis), without any known thyroid disorder. Eighteen patients (9M/9F, 33.6 ± 8.8 y) were on anti-TNF therapy, while 23 patients (16M/7F, 38.7 ± 12.5 y) were treated with Azathioprine and Mesalazine (Aza/Mes) for more than 1 year. Twelve patients from the second group were then treated with anti-TNF and studied 6 months later. We assessed thyroid function by measuring thyroid stimulating hormone (TSH), free thyroxine (FT4), triiodothyronine (T3), thyroid peroxidase autoantibodies (TPOAb) and thyroglobulin autoantibodies (TgAb) levels. One patient presented with overt and one with subclinical hyperthyroidism. Thyroid auto-antibodies were positive in 12.2%. Patients from the anti-TNF group had lower levels of FT4 (1.09 ± 0.15 vs. 1.38 ± 0.9 ng/dL, p = 0.042), while TSH and T3 were comparable. The percentage of patients with positive thyroid auto-antibodies was lower in the anti-TNF group (5.6% vs. 17.4%). In the subgroup of patients who changed to anti-TNF, we found statistically significant reduction in FT4 after 6 months (1.26 ± 0.24 vs. 1.08 ± 0.15 ng/dL, p = 0.044), without changes in TSH and T3 levels. There was no change regarding thyroid auto-antibodies. In conclusion, patients with IBD showed a quite high percentage of thyroid autoimmunity. After treatment with anti-TNF, FT4 levels were found to be reduced, while no changes in TSH, T3 levels and thyroid auto-antibodies were noted.

Glucagon-like peptide-1 stimulates type 3 iodothyronine deiodinase expression in a mouse insulinoma cell line

AIMS

The pathophysiological roles of thyroid hormones in glucose metabolism remain uncertain. Type 3 iodothyronine deiodinase (D3) converts thyroxine (T4) and 3,5,3'-triiodothyronine (T3) to 3,3',5'-triiodothyronine (rT3) and 3,3'-diiodothyronine (T2), respectively, inactivating thyroid hormones in a cell-specific fashion. In the present study, we identified D3 expression in MIN6 cells derived from a mouse insulinoma cell line and examined the mechanisms regulating D3 expression in these cells.

MAIN METHODS

We characterized D3 activity using HPLC analysis, and examined the effect of GLP-1 or exendin-4 on D3 expression and cAMP accumulation in MIN6 cells. We also measured insulin secretion from MIN6 cells exposed to GLP-1 and T3.

KEY FINDINGS

We identified enzyme activity that catalyzes the conversion of T3 to T2 in MIN6 cells, which showed characteristics compatible with those for D3. D3 mRNA was identified in these cells using RT-PCR analysis. Forskolin rapidly stimulated D3 mRNA and D3 activity. Glucagon-like peptide-1 (GLP-1) increased D3 expression in a dose-dependent manner, and this effect was inhibited by the protein kinase A (PKA) inhibitor H-89. Exendin-4, a GLP-1 receptor agonist, also stimulated D3 expression in MIN6 cells. These results suggest that a cAMP-PKA-mediated pathway participates in GLP-1-stimulated D3 expression in MIN6 cells. Furthermore, GLP-1 stimulated insulin secretion was suppressed by the addition of T3 in MIN6 cells.

SIGNIFICANCE

Our findings indicate that GLP-1 regulates intracellular T3 concentration in pancreatic β cells via a cAMP-PKA-D3-mediated pathway that may also regulate β-cell function.