The Role of Autophagy Flux in Chemoresistance of Medullary Thyroid Carcinoma and the Sensitizing Effect of Chloroquine Combined with Doxorubicin

　　Objective: To investigate the role of cellular autophagy in medullary thyroid carcinoma (MTC) in response to doxorubicin (DOX)-induced stress and chemoresistance, and to evaluate the synergistic antitumor effect of combining the autophagy inhibitor chloroquine (CQ) with DOX.
　　Methods: The human MTC cell line TT was used as an in vitro model and divided into Control, DOX, CQ, and DOX+CQ combination groups. Changes in autophagy flux were assessed by Western Blotting detecting LC3-II protein expression, and autophagic structures were observed via transmission electron microscopy (TEM). An MTC xenograft model in nude mice was established, and tumor-bearing mice were randomly grouped for corresponding treatments. Tumor growth volume and weight were monitored, and autophagy levels in tumor tissues were detected.
　　Results: In vitro experiments showed that DOX treatment significantly increased LC3-II levels, indicating autophagy induction. The combination of DOX and CQ led to further accumulation of LC3-II, and TEM revealed numerous undegraded autophagosomes and abnormal autolysosomes, confirming successful blockade of autophagic flux. In vivo, the DOX+CQ combination therapy inhibited tumor growth significantly more effectively than any single-agent group (Tumor inhibition rate: 60.0%, P < 0.01), without causing significant systemic toxicity. LC3-II also accumulated markedly in the combination group's tumor tissues, indicating effective inhibition of autophagic flux in vivo.
　　Conclusion: Functional autophagy flux is a key mechanism underlying doxorubicin chemoresistance in MTC. Chloroquine can effectively reverse this resistance by blocking autophagic flux, and its combination with doxorubicin exhibits a significant synergistic antitumor effect, providing a potential novel strategy for the treatment of advanced MTC.