Effect of autophagy on the shikonin induced apoptosis of human medullary thyroid carcinoma TT cells

Objective: This study aimed to investigate the role of autophagy in the shikonin (SKI) induced apoptosis in human medullary thyroid carcinoma TT cells and the potential molecular mechanism. Methods: Human medullary thyroid carcinoma TT cells were used in the present experiment. The proliferative capability was evaluated by MTT assay, the cell apoptosis rate was determined by flow cytometry, the ultrastructure of cells was observed by transmission electron microscopy, and Western blot assay was performed to examine the expression of autophagy proteins (LC3 and p62/SQSTM1) and apoptosis-related proteins (cleaved caspase3 and cleaved PARP). Results: As compared to blank control group, SKI was able to inhibit the growth of TT cells in a dose dependent manner; after treatment with 2 and 4 μg/mL SKI for 24 h, the apoptosis rate increased significantly (24.3%±6.3% and 50.2%±8.8%, respectively, P<0.05 vs 6.7%±3.2% in control group), the expression of cleaved caspase3 and cleaved PARP increased markedly; typical vacuoles in apoptosis were observed in SKI treated cells, and the LC3-II expression increased, but p62/SQSTM1 expression decreased; in the presence of autophagy inhibitor chloroquine, SKI significantly increased the apoptosis rate and the expression of cleaved caspase3 and cleaved PARP as compared to cells treated with SKI alone. Conclusion: SKI was able to induce the autophagy and apoptosis of TT cells, and to inhibit the autophagy of TT cells may accelerate their apoptosis, suggesting that protective autophagy may partially antagonize the SKI induced inhibition of TT cell proliferation.