TSH signaling pathways that regulate MCP-1 in human differentiated adipocytes

OBJECTIVE

Adipose tissue is an extra-thyroidal thyroid-stimulating hormone (TSH) target. Increases in lipolysis and in expression and release of interleukin-6 (IL-6) occur in TSH-stimulated adipocytes, and levels of circulating free fatty acids and IL-6 rise following TSH administration to patients with previous thyroidectomy and radioablation for thyroid cancer. Our first objective was to compare how TSH stimulates protein kinase A (PKA) and inhibitor of κB (IκB) kinase (IKK)-β. Our second objective was to investigate whether TSH induces other cytokines besides IL-6.

METHODS

TSH stimulation of either CHO cells expressing human TSH receptor or human abdominal subcutaneous differentiated adipocytes.

RESULTS

Signaling studies showed TSH increased NADPH oxidase activity, and either diphenyleneiodonium (oxidase inhibitor) or N-acetyl cysteine (scavenger of reactive oxygen species) reduced IKKβ phosphorylation. Phosphorylation of protein kinase C-δ, an upstream regulator of NADPH oxidase, was increased by TSH, and rottlerin (PKCδ inhibitor) reduced TSH-stimulated IKKβ phosphorylation. TSH upregulated monocyte chemoattractant protein-1 (MCP-1) mRNA expression and the release of MCP-1 protein in human abdominal differentiated adipocytes. H89 (PKA inhibitor) and sc-514 (IKKβ inhibitor) each blocked TSH-stimulated MCP-1 mRNA expression and protein release, suggesting PKA and IKKβ participate in this pathway.

CONCLUSIONS

These data provide new information about TSH signaling in human differentiated adipocytes, and add to the evidence that TSH is a pro-inflammatory stimulus of adipocytes.

Thyroid-stimulating hormone induces interleukin-6 release from human adipocytes through activation of the nuclear factor-kappaB pathway

Our objective was to identify the signaling pathway activated by TSH that induces IL-6 secretion from human abdominal sc differentiated adipocytes. Human abdominal sc preadipocytes in culture were differentiated into adipocytes. IL-6 release stimulated by TSH was inhibited by 35% (P < 0.05) with SN50, an inhibitor of nuclear factor-kappaB (NF-kappaB) nuclear translocation, and 60% (P < 0.01) with sc-514, an inhibitor of inhibitory-kappaB (IkappaB) kinase (IKK)-beta. Phosphorylation of IKKbeta increased upon TSH treatment (10.3-fold, P < 0.01), and IkappaBalpha levels were reduced by 78% (P < 0.01). TSH activated NF-kappaB (23-fold, P < 0.001), a process that was inhibited (60%, P < 0.01) by SN50. Inhibition of protein kinase A by H89 did not affect TSH-stimulated IKKbeta phosphorylation or IkappaBalpha degradation. TSH-mediated NF-kappaB activation and IL-6 induction also specifically occurred in Chinese hamster ovarian cells expressing the human TSH receptor, resulting in a 5.9-fold (P < 0.001) increase in IKKbeta phosphorylation and a 9.5-fold increase in IL-6 mRNA expression. Our data demonstrate that the IKKbeta/NF-kappaB pathway is a novel TSH target that is required for TSH-induced IL-6 release from human adipocytes.