Secretory clusterin promotes oral cancer cell survival via inhibiting apoptosis by activation of autophagy in AMPK/mTOR/ULK1 dependent pathway.

Secretory clusterin (sCLU) plays an important role in tumor development and cancer progression. However, the molecular mechanisms and physiological functions of sCLU in oral cancer is unclear. We examined the impact of sCLU-mediated autophagy in cell survival and apoptosis inhibition in oral cancer. Immunohistochemical analysis was performed to analyze protein expression in patient samples. Autophagy and mitophagy was studied by immunofluorescence microscopy and Western blot. The gain and loss of function was studied by overexpression of plasmid and siRNA approaches respectively. Cellular protection against nutrient starvation and therapeutic stress by sCLU was studied by cell viability, caspase assay and meta-analysis. The data from oral cancer patients showed that the expression levels of sCLU, ATG14, ULK1, and PARKIN increased in grade-wise manners. Interestingly, sCLU overexpression promoted autophagy through AMPK/Akt/mTOR signaling pathway leading to cell survival and protection from long exposure serum starvation induced-apoptosis. Additionally, sCLU was demonstrated to interact with ULK1 and inhibition of ULK1 activity by SBI206965 was found to abolish sCLU-induced autophagy indicating critical role of ULK1 in induction of autophagy. Furthermore, sCLU was observed to promote expression of mitophagy-associated proteins in serum starvation conditions to protect cells from nutrient deprivation. The meta-analysis elucidated that high CLU expression is associated with therapy resistance in cancer and we demonstrated that sCLU-mediated mitophagy was revealed to inhibit cell death by cisplatin. The present investigation has highlighted the probable implications of the clusterin-induced autophagy in cell survival and inhibition of apoptosis in oral cancer.