Expansion of the orbital contents, by adipogenesis and hyaluronan (HA) overproduction, causes Graves orbitopathy (GO). Orbital HA is generated predominantly by hyaluronan synthase 2 (HAS2) whose transcription is positively regulated by pAkt in cell lines. GO is most frequently associated with Graves disease (GD) in which thyroid stimulating antibodies bind the TSH receptor (TSHR) increasing cAMP. Previous studies highlight a role for TSAB and neutral TSHR antibodies in GO, suggesting the involvement of additional TSHR or other signalling cascades e.g. IGF1 via pAkt. Furthermore the effects of TSHR activation may vary with its expression level and orbital fat (neural crest derived) may exhibit depot specific differences relevant to GO.
We have investigated using in vitro models to assess adipogenesis and HA production in preadipocyte/fibroblasts from orbital (n=10) and subcutaneous (n=10) adipose tissue in response to TSHR (activating mutation) and IGF1R activation (IGF1 addition).
In orbital preadipocytes adipogenesis significantly increased (4060%) HAS2 transcripts (but decreased HAS1 and HAS3) and secreted HA (P<0.002). In contrast, subcutaneous preadipocyte adipogenesis significantly decreased (5075%) all three HAS isoforms and secreted HA (P<0.03). Combining TSHR and IGF1R activation did not induce spontaneous adipogenesis but produced a synergistic effect on HAS2 transcription and HA generation in both depots. Surprisingly IGF1 treatment alone, either chronic or acute, had no effect on HAS2 transcription, despite significantly increasing the phospho:total Akt ratio; a pAkt inhibitor increased orbital HAS2 transcription. A stimulating effect of IGF1 on HAS2 transcription can be revealed by rapamycin in subcutaneous but a MEK inhibitor in orbital preadipocytes.
We conclude that a single physiological process (adipogenesis) in the orbit produces both mechanisms causing GO. Depot specific variations in HAS2 transcriptional control (feedback inhibition by mTOR or MEK in subcutaneous and orbit respectively) provide an explanation for these differences and the pathogenesis of GO.