Endocrine Abstracts

Thyrotropin receptor (TSHR, signals via CREB) expression increases during adipogenesis, a process occurring in Graves’ orbits. CREB activation has been reported ‘necessary and sufficient’ to induce adipogenesis. However, we previously demonstrated that expression of constitutively active TSHR in orbital preadipocytes stimulated early but inhibited later differentiation stages, even when PPARγ agonist induced. Overproduction of proteoglycans is also a feature of Graves’ and we have investigated whether TSHR activation is implicated in this.

Retroviral vectors for WT or constitutively active mutant TSHR (TSHR*) were introduced into human preadipocytes from orbital (n=3), omental (n=2) and sub-cutaneous (n=4) fat. Experiments were performed in complete (CM) or differentiation medium (DM), containing a PPARγ agonist. Adipogenesis was assessed by oil red O staining and QPCR measurement of differentiation markers. This method was used for hyaluronan synthases (HAS1 and HAS2) transcripts and metabolic labelling plus size-exclusion chromatography provided a measure of proteoglycans production (secreted and cell-associated).

In CM, there was no spontaneous adipogenesis but a significant increase in PPARγ and LPL transcripts in all 9 fat samples expressing TSHR* compared with non-modified populations. Transcripts for HAS1 (n=9) and HAS2 (sub-cutaneous only) were also induced or significantly increased and accompanied by at least 2-fold increase in dermatan, chondroitin and heparan sulphate proteoglycans. In DM, non-modified cells underwent adipogenesis with upregulation of differentiation markers; HAS1/HAS2 transcripts decreased. This also occurred in the TSHR* expressing orbital preadipocytes, but differentiation was significantly inhibited. In contrast, adipogenesis in the TSHR* sub-cutaneous and omental populations was greatly increased and accompanied by 3–6 fold increases in HAS1 transcripts to achieve levels >10 higher than the equivalent non-modified cells at the end of differentiation.

The results suggest that TSHR activation stimulates overproduction of proteoglycans, which may potentiate adipogenesis, in some fat depots.