Effects of Thyrotropin and Thyrotropin-Receptor-Stimulating Graves' Disease Immunoglobulin G on Cyclic Adenosine Monophosphate and Hyaluronan Production in Nondifferentiated Orbital Fibroblasts of Graves' Ophthalmopathy Patients

Background: Orbital fibroblasts are involved in the pathogenesis of Graves' ophthalmopathy (GO) by producing hyaluronan (HA), synthesized by three types of hyaluronan synthases (HAS1, HAS2, and HAS3). Thyrotropin receptors (TSHR) expressed in orbital fibroblasts activate the cyclic adenosine monophosphate (cAMP) pathway. Only sparse data are available at present supporting a role for TSHR activation in the regulation of HA in GO orbital fibroblasts. We hypothesize that TSHR activation, via cAMP signaling, results in induction of HAS1–3 mRNA expression and HA production by nondifferentiated GO orbital fibroblasts. Methods: Cultured nondifferentiated orbital fibroblasts obtained during orbital decompression surgery from 15 GO patients were stimulated with recombinant human TSH (rhTSH), TSHR-stimulating Graves' disease immunoglobulin G (GD-IgG) or forskolin (FSK), or interleukin-1β (IL-1β). Results: FSK significantly stimulated cAMP production, HAS1 and HAS3 mRNA expression, and HA secretion in orbital fibroblasts. IL-1β slightly induced cAMP production, but induced HAS mRNA expression of all three isoforms and HA secretion. In contrast, the effects of rhTSH and GD-IgG on cAMP were modest and absent, respectively, and on HAS mRNA and HA synthesis were completely absent. Conclusions: The strong increase in cAMP synthesis by FSK in nondifferentiated GO orbital fibroblasts results in increased HA synthesis, but TSHR activation by rhTSH or GD-IgG does not result in altered HA synthesis. Our results do not support a predominant role for GD-IgGs in the accumulation of orbital glycosaminoglycans; cytokines like IL-1β seem largely responsible for excessive glycosaminoglycan production by nondifferentiated orbital fibroblasts in early immunopathogenesis of GO.