DEPTOR promotes survival of cervical squamous cell carcinoma cells and its silencing induces apoptosis through downregulating PI3K/AKT and by up-regulating p38 MAP kinase.

Kalanghad Puthankalam Srinivas,Subhadra Lakshmi,Thankayyan R Santhoshkumar,Rajappan Prakash,Remadevi Viji,Madhavan Radhakrishna Pillai,Indira Sukumaran Sajitha,Vipin Mohan Dan,Puthan Valappil Rahul

doi:10.18632/oncotarget.8131

Abstract

DEPTOR is an endogenous inhibitor of mTOR complexes, de-regulated in cancers. The present study reveals a vital role for DEPTOR in survival of cervical squamous cell carcinoma (SCC). DEPTOR was found to be overexpressed in both cervical SCC cells and tissues and it's silencing in cervical SCC cells induced apoptosis, mainly by up-regulation of p38 MAPK and by inhibiting PI3K/AKT pathway via a feed-back inhibition from mTORC1-S6K. DEPTOR silencing resulted in reduced expression of the nitric oxide synthases iNOS and eNOS, as well as increased activation of ERK1/2 and p38 MAP kinases. Activation of AKT signaling by overexpression of constitutively active-AKT (CA-AKT) failed to overcome the apoptosis caused by DEPTOR silencing. Similarly pharmacological inhibition of ERK also failed to control apoptosis. However pharmacological inhibition of p38 MAPK rescued the cells from apoptosis, indicating the major role of p38 MAPK in cell death induced by DEPTOR silencing. DEPTOR was also found to regulate ERK1/2 in an AKT dependent manner. DEPTOR knockdown induced cell death in SiHa cells overexpressing the anti-apoptotic Bcl-2 and Bcl-xL, indicating strong survival role of DEPTOR in these cells. DEPTOR overexpression activated PI3K/AKT by relieving the negative feed-back inhibition from mTORC1-S6K. DEPTOR regulation was also observed to be independent of HPV E6/E7 oncoproteins, but it might be a molecular co-factor contributing to cervical carcinogenesis. In summary, DEPTOR is found to promote survival of cervical SCC cells and its reduction induced apoptosis via differential effects on PI3K/AKT and p38 MAPK and can be a potential target in cervical SCC.

Highlights

The mammalian target of rapamycin is an evolutionarily conserved serine/threonine kinase integrating intracellular and extracellular signals
(DEP domain containing mammalian target of rapamycin (mTOR) inhibitor) (DEPTOR) silencing induces apoptosis in cervical squamous cell carcinoma cells To address the role of DEPTOR in cervical cancer cells, we knocked down DEPTOR in SiHa, ME-180 and HeLa cells (Figure 1A)
In annexin binding assay for quantification of apoptosis by FACS, the DEPTORsilenced SiHa and ME-180 cells showed approximately ten-fold annexin positive population when compared to the scramble Small interfering RNA (siRNA) transfected cells (Figure 1B) and this is far stronger in comparison to the cells treated with reported mTOR inhibitors rapamycin and Torin2 (Figure 1B)

Summary

Introduction

The mammalian target of rapamycin (mTOR) is an evolutionarily conserved serine/threonine kinase integrating intracellular and extracellular signals. It serves as a master regulator of several metabolic processes including growth, proliferation, survival and autophagy. High levels of DEPTOR was reported to be essential for the survival of various cancer cells [4, 7, 8]. DEPTOR expression has frequently been reported to be essential for the survival and proliferation of tumor cells in multiple myeloma, thyroid cancer, paclitaxel resistant ovarian cancer and hepatocellular carcinoma [4, 7, 9,10,11]

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