Rescue of thyroid cells from antibody induced cell death via induction of autophagy.

Background: Graves' disease (GD) is associated with thyroid stimulating hormone (TSH) receptor (TSHR) antibodies of variable bioactivity. We have previously characterized "neutral" TSHR antibodies (N-TSHR-Abs) that bind to the hinge region of the TSHR ectodomain. We showed that an N-TSHR monoclonal antibody (mAb) failed to induce any G proteins to sustain survival signaling and lead to excessive stress and apoptosis. Furthermore, the addition of TSH, or the antioxidant N-acetyl-l-cysteine (NAC), rescued N-TSHR-mAb-induced apoptotic death. However, the detailed mechanisms of this rescue remained unclear.

Methods: Autophagy is activated in response to diverse stress related stimuli so we have, therefore, studied the autophagy response in rat thyroid cells (FRTL-5) during N-TSHR-mAb induced thyrocyte stress and apoptosis using the In Cell Western technique for quantitation along with immunocytochemistry.

Results: Under starvation conditions with N-TSHR-mAb the addition of TSH or NAC prevented thyroid cell death by enhancing autophagy. This was evidenced by elevated levels of autophagy related proteins including beclin 1, LC3A, LC3B, ULK1, p62, and also activated pink and perkin mitophagy related proteins. The phenomenon was further confirmed by image analyses using Cyto-ID and Mito-ID autophagy detection systems. We also found that either TSH or NAC enhanced PKA, Akt, mTORC, AMPK, Sirtuins, PGC1α, NRF-2, mitofusin-2, TFAM and catalase in the N-TSHR-mAb stressed cells. Thus TSH or NAC restored cell survival signaling which reduced cell stress and enhanced mitochondrial biogenesis. The N-TSHR-mAb also activated cytochrome-C, Bax, caspase-9, caspase-3A, and had less effect on FADD or caspase-8 indicating activation of the intrinsic pathway for apoptosis.

Conclusions: These findings indicated that TSH or antioxidant can rescue thyroid cells from N-TSHR-mAb induced apoptosis via enhanced autophagy. These observations signify that N-TSHR-mAb in GD under low TSH conditions caused by the hyperthyroidism could be detrimental for thyrocyte survival which would be another factor able to precipitate ongoing autoinflammation.