Understanding The Role Of Sirtuin1- Activating Compounds In Autophagy And Associated Signalling To Induce Cell Death In Oral Cancer

Abstract

The role SIRT1 in cancer during tumorigenesis is debatable since it could exhibit both tumor suppressor and oncogenic nature by transcriptional activation of various physiological processes, including apoptosis and autophagy. This study aimed to identify small molecule activators of SIRT1 as chemotherapeutic drugs via modulation of autophagic cell death pathways. Our study demonstrated that Terminalia bellirica fruit extract containing gallic acid (GA), a bioactive SIRT1 activator, induced apoptosis and autophagy by regulating the mitochondrial ROS in oral cancer cells. Further, gamma-irradiation-induced electrochemical changes in GA enhanced its anticancer activity via the induction of apoptosis. Downregulation of NFE2L2-antioxidant signalling paired with autophagic flux inhibition generated lipophagosome accumulation and aided apoptosis induction in oral cancer cells. Moreover, in synergism with anticancer drugs, gamma-irradiated GA chemosensitizes the cancer cells to apoptosis. In another study, we demonstrated that cisplatin-induced apoptosis inhibition caused SIRT1 downregulation, subsequently leading to mitochondrial hyperfusion associated mito-bulb formation as a resistance mechanism to apoptosis. Overexpression of SIRT1 or priming the cancer cells with GA inhibited the cisplatin-induced mitochondrial hyperfusion and resensitized the cells to apoptosis. Intriguingly, we established the role of SIRT1-regulated NFE2L2-antioxidant signalling in the induction of apoptosis after inhibiting mitochondrial hyperfusion in oral cancer and oral cancer-derived polyploid giant cancer cells (PGCCs). Moreover, our study established the critical involvement of GA-induced SIRT1 activation for mitochondrial asymmetric fission with mitophagic flux inhibition to generate mitochondrial superoxide. The mitochondrial superoxide elicits BAX-dependent lysosomal membrane permeabilization followed by TFEB-mediated lysosomal biogenesis inhibition and non-recruitment of ESCRT to orchestrate LMP-driven apoptosis via downregulation of NFE2L2-antioxidant signalling. In apoptosis-deficient and resistant PGCCs, GA-induced LMP provokes NLRP3-mediated pyroptotic cell death. In this setting, we have synthesized a small molecule activator of SIRT1, S36, facilitating nuclear translocation of SIRT1 to commence bulk autophagy. S36-induced bulk autophagy depleted the cellular lysosomal pool and inhibited TFEB-mediated lysosomal biogenesis, which havocked cellular homeostasis. In addition, S36 impaired the recruitment of clathrin, stonin 2 and PI(4)P to the autolysosomal perturbation site as a precursor of autophagic lysosome reformation, eventually leading to autophagy dependent cell death. In conclusion, our work established a novel therapeutic aspect of targeting autophagy via small molecule activators of SIRT1 to commence autophagy-mediated and autophagy-dependent cell death for oral cancer therapeutics.