Deciphering The Role Of Stonin 2 In Autophagy and Autophagic Lysosome Reformation to Control Cell Survival in Oral Squamous Cell Carcinoma

Abstract

Stonin 2 is an endocytic protein encoded by the STON2 gene, also known as STN2 and STNB. This gene is found on human chromosome 14q. It is an endocytic protein that is coupled with clathrin and contributes to the process of sorting endocytic complexes. Here, I report that STON2 expression increases with an increase in the grade of the oral cancer tissue. Further, STON2 overexpressed cells possess a higher proliferation and migration rate in oral cancer cells. STON2 helps maintain lysosomal functions by preserving the lysosomal membrane integrity. It activates the Akt-mTOR axis and retains the mTOR on the membrane of the lysosomes. Trifluoperazine dihydrochloride (TFP), a STON2 inhibitor, disrupts the Akt-mTOR pathway and permeabilizes the lysosomal axis. TFP targets STON2 to reveal its anti-cancer effects in oral cancer cells. Further, I have demonstrated that STON2 is essential for Beclin 1-dependent autophagy induction in oral cancer cells. In CAL33 and FADU cells, STON2 overexpression fails to degrade the autophagic adaptor p62 and increases LC3 puncta accumulation, indicating STON2 alters autophagic flux in oral cancer cells. In addition, STON2 accumulates LC3-LAMP1 dual-positive autolysosomes. Autophagic lysosome reformation (ALR) regenerates lysosomes from autolysosomes. In basal and starved CAL33 cells, STON2 induces tubulation events of ALR. In contrast, STON2 depletion does not trigger ALR under 10 h starvation, proving its vital role in ALR. To determine the role of STON2 in the late stages of ALR, I examined tubulation events in the presence of dynasore, a dynamin 2 inhibitor. The generation of larger tubules in dynasore-treated cells confirmed the role of STON2 in early ALR events. Further, STON2 reactivates mTOR under basal and deprived conditions to replenish the nutrient status of the oral cancer cells, and I found that STON2 does not trigger ALR in the presence of Rapamycin. Moreover, STON2 fails to initiate ALR in clathrin-depleted cells, confirming that clathrin is essential for STON2-induced ALR progression. The crucial mechanism of STON2-dependent endocytosis/autophagy in maintaining lysosomal homeostasis must be explored in detail. In future, STON2 could function as a biomarker for the early detection and diagnosis of oral cancer progression.