Downregulation of PHLPP induced by endoplasmic reticulum stress promotes eIF2\u03b1 phosphorylation and chemoresistance in colon cancer

Bianqin Guo,Austin T Li,Sumati Hasani,Ashley T Skaggs,Xiaopeng Xiong,Yang-An Wen,Tianyan Gao,Rebecca Martinez

doi:10.1038/s41419-021-04251-0

Abstract

Aberrant activation of endoplasmic reticulum (ER) stress by extrinsic and intrinsic factors contributes to tumorigenesis and resistance to chemotherapies in various cancer types. Our previous studies have shown that the downregulation of PHLPP, a novel family of Ser/Thr protein phosphatases, promotes tumor initiation, and progression. Here we investigated the functional interaction between the ER stress and PHLPP expression in colon cancer. We found that induction of ER stress significantly decreased the expression of PHLPP proteins through a proteasome-dependent mechanism. Knockdown of PHLPP increased the phosphorylation of eIF2α as well as the expression of autophagy-associated genes downstream of the eIF2α/ATF4 signaling pathway. In addition, results from immunoprecipitation experiments showed that PHLPP interacted with eIF2α and this interaction was enhanced by ER stress. Functionally, knockdown of PHLPP improved cell survival under ER stress conditions, whereas overexpression of a degradation-resistant mutant PHLPP1 had the opposite effect. Taken together, our studies identified ER stress as a novel mechanism that triggers PHLPP downregulation; and PHLPP-loss promotes chemoresistance by upregulating the eIF2α/ATF4 signaling axis in colon cancer cells.

Highlights

The PHLPP family of phosphatases, consisting of PHLPP1 and PHLPP2 isoforms, belongs to the PPM superfamily of Ser/Thr protein phosphatases [1, 2]
Similar downregulation of PHLPP proteins was observed in HCT116 and SW480 cells treated with oxaliplatin, another commonly used chemotherapy drug, as endoplasmic reticulum (ER) stress was induced in these cells (Supplemental Fig. S1)
Both tumor intrinsic factors and conditions presented by the tumor microenvironment can lead to the activation of ER stress that controls the balance of apoptosis and cell survival

Summary

Introduction

The PHLPP family of phosphatases, consisting of PHLPP1 and PHLPP2 isoforms, belongs to the PPM superfamily of Ser/Thr protein phosphatases [1, 2]. Studies on determining molecular mechanisms underlying PHLPP downregulation have identified the proteasome-dependent protein degradation as a major pathway leading to decreased PHLPP expression under cellular stress conditions, such as inflammation, hypoxia, and nutrient deprivation [15,16,17]. PHLPP-mediated regulation of AKT signaling has often been cited as a major underlying mechanism, how PHLPP-loss provides a survival advantage to cancer cells, especially under stress conditions, remains elusive. The phosphorylation of eIF2α reduces global translation but preferentially increases ATF4 translation, a master regulator controlling the transcription of key genes involved in the integrated stress response (ISR) [20]. A better understanding of molecular mechanisms that regulate ER stress and ISR is needed to improve the anticancer efficacy of standard chemotherapy drugs

Methods

Results

Conclusion