Purpose: Thyrotropin receptor (TSHR) expression is upregulated in the orbits of patients with Graves ophthalmopathy (GO), most of whom have TSHR-stimulating antibodies. The authors investigated the biological effects of TSHR activation in vitro in adipose tissue, the site of orbital TSHR expression.
Methods: Activating mutant TSHR (TSHR*) or wild-type (WT) was introduced into human orbital preadipocytes using retroviral vectors. Their proliferation (Coulter counting), basal cAMP accumulation (radioimmunoassay), and spontaneous and peroxisome proliferator-activated receptor (PPARgamma)-induced adipogenesis (quantitative oil red O staining) were assessed and compared with those of nonmodified cells. QRT-PCR was used to measure transcripts of CCAT/enhancer binding protein (C/EBP)beta, PPARgamma, and lipoprotein lipase (LPL; early, intermediate, and late markers of adipogenesis) and for uncoupling protein (UCP)-1 (brown adipose tissue [BAT]).
Results: Expression of TSHR* significantly inhibited the proliferation of preadipocytes and produced an increase in unstimulated cAMP of 200% to 600%. Basal lipid levels were significantly increased in TSHR* (127%-275%) compared with nonmodified (100%) or WT-expressing (104%-187%) cells. This was accompanied by 2- to 10-fold increases in early-intermediate markers and UCP-1 transcripts (2- to 8-fold); LPL was at the limit of detection. In nonmodified cells, adipogenesis produced significant increases in transcripts of all markers, including LPL (approximately 30-fold). This was not the case in TSHR*-expressing cells, which also displayed 67% to 84% reductions in lipid levels.
Conclusions: TSHR activation stimulates early differentiation (favoring BAT formation?) but renders preadipocytes refractory to PPARgamma-induced adipogenesis. In neither case did lipid-containing vacuoles accumulate, suggesting that terminal stages of differentiation were inhibited.