Dynorphin B Promotes Autophagy and Cytotoxicity in Thyroid Cancer Cells via the mTORC1-TFE3 Axis.

Autophagy can either support tumor survival or induce cell death in thyroid cancer. This process is a promising therapeutic target. This study aimed to investigate the effects and molecular mechanisms of Dynorphin B in TPC-1 thyroid cancer cells. Dynorphin B, an endogenous opioid peptide, triggers autophagic flux. It increases LC3-II/I ratios and p62 degradation. Autophagosome formation rises, confirmed by monodansylcadaverine staining. Dynorphin B also induces dose-dependent cytotoxicity. It disrupts mitochondrial membrane potential and ATP production. Oxidative stress increases, marked by elevated 8-OHdG levels and reduced SOD activity. Mechanistically, Dynorphin B inhibits mTORC1 activity. This reduces p-S6K phosphorylation levels. TFE3 dephosphorylation at Ser321 occurs. TFE3 translocates to the nucleus, activating transcription of autophagy genes ATG5 and LC3. Lysosomal genes LAMP1 and CTSD are also upregulated. TFE3 knockdown blocks these effects, confirming its essential role. Chromatin immunoprecipitation shows TFE3 binding to CLEAR motifs in ATG5 and p62 promoters. Luciferase assays validate CLEAR gene activation. Clinically, immunohistochemical evaluation of biopsy specimens demonstrated markedly lower KOR-1 expression in malignant thyroid tissues than in adjacent normal tissues. These findings reveal that Dynorphin B modulates autophagy and cytotoxicity through the mTORC1-TFE3 axis, indicating its potential to regulate thyroid cancer cell fate.