Protective Autophagy in Inducing Stemness and
Therapeutic Resistance in Oral Squamous Cell Carcinoma

Abstract

Oral squamous cell carcinoma (OSCC) is associated with chemo-radio-resistance due to the existence of a small subset of therapy tolerant intra-tumoral cells called as cancer stem cells (CSCs). Interestingly, we found that stemness surface marker protein CD44, drug resistance marker protein ABCB1, and invasion associated protein ADAM17 are highly co-expressed and their expression is significantly co-related with the tumor progression. This finding implicates the candidature of CD44+ ABCB1+ ADAM17+ cells as a putative CSC compartment in the clinical OSCC specimen and the likeliness of finding a putative stem cell in OSCC is associated with poor differentiation status and high grade of tumors. Further, resistant oral cancer cells were found to have greater stem-like properties when compared to the parental cells as evident from their high sphere forming capacity, possession of higher CD44+ population and increased expression of self-renewal protein β-catenin. The chemoresistant cells had higher autophagic flux and the blockade of autophagy could lead to the down regulation of β-catenin and reduced sphere formation suggesting that autophagy plays a key role in promoting stemness in oral cancer. Interestingly, functional mitophagy was higher in the resistant oral cancer cells suggesting the probable involvement of mitophagy in chemoresistance. Again, we investigated the expression of heat shock chaperone secretory clusterin (sCLU) in OSCC and its association with autophagy. As expected, sCLU is found to be positively correlated with progression of OSCC. Concomitant to the sCLU expression, ATG14 and ULK1 expression increases grade wise in OSCC. Furthermore, our study showed significant interaction of sCLU and ULK1 suggesting the potential role of sCLU in autophagy process. It was shown that the normal human keratinocyte cell HaCaT has low expression of sCLU and LC3-B-II accumulation as compared to the various oral cancer cell lines. Intriguingly, sCLU overexpression could potentially induce autophagy at basal level. Similarly, the expression of mitophagy associated protein PARKIN is positively associated with tumor progression in OSCC. It was unraveled that the stress responder sCLU has a propensity to induce the mitophagy process in the serum starvation condition in oral cancer cells which might have a crucial role in cytoprotection and chemoresistance. Moreover, the meta-analysis study showed that CLU is significantly associated with therapeutic resistance among cancer. Moreover, sCLU-mediated autophagy and/or mitophagy is found to induce cell survival and therapeutic resistance under nutritional and therapeutic stress in oral cancer cells which might have a crucial role in cytoprotection and chemoresistance. In conclusion, this study unravels the role of autophagic and/or mitophagic regulation of acquisition of stem-like properties and chemoresistance in OSCC.