Lack of L-type amino acid transporter 2 in murine thyroid tissue induces autophagy

Abstract

Proteolytic cleavage of thyroglobulin (Tg) for thyroid hormone (TH) liberation is followed by TH release from thyroid follicles into the circulation, enabled by TH transporters. The existence of a functional link between Tg-processing cathepsin proteases and TH transporters has been shown to be independent of the hypothalamus-pituitary-thyroid axis. Thus, lack of cathepsin K, combined with genetic defects in the TH transporters Mct8 and Mct10, i.e. the Ctsk‑/-/Mct8-/y/Mct10-/- genotype, results in persistent Tg proteolysis due to autophagy induction. Because amino acid transport by L-type amino acid transporter 2 (Lat2) has been described to regulate autophagy, we asked whether Lat2 availability is affected in Ctsk‑/-/Mct8-/y/Mct10-/- thyroid glands. Our data revealed that while mRNA amounts and sub-cellular localization of Lat2 remained unaltered in thyroid tissue of Ctsk‑/-/Mct8-/y/Mct10-/- mice in comparison to wild-type (WT) controls, the Lat2 protein amounts were significantly reduced. These data suggest a direct link between Lat2 function and autophagy induction in Ctsk‑/-/Mct8-/y/Mct10‑/‑ mice. Indeed, thyroid tissue of Lat2-/- mice showed enhanced endo-lysosomal cathepsin activities, increased autophagosome formation, and enhanced autophagic flux. Collectively, these results suggest a mechanistic link between insufficient Lat2 protein function and autophagy induction in the thyroid gland of male mice.