Chronic TGFβ stimulation promotes the metastatic potential of lung cancer cells by Snail protein stabilization through integrin β3-Akt-GSK3β signaling.

Gab-Yong Bae,Ok-Seon Kwon,Soon-Ki Hong,Keun-Tae Kim,Jeong-Rak Park,Hyuk-Jin Cha,Ensel Oh,Jaehyung Koo

doi:10.18632/oncotarget.8295

Abstract

Chronic exposure to TGFβ, a frequent occurrence for tumor cells in the tumor microenvironment, confers more aggressive phenotypes on cancer cells by promoting their invasion and migration while at the same time increasing their resistance to the growth-inhibitory effect of TGFβ. In this study, a transdifferentiated (TD) A549 cell model, established by chronically exposing A549 cells to TGFβ, showed highly invasive phenotypes in conjunction with attenuation of Smad-dependent signaling. We show that Snail protein, the mRNA expression of which strongly correlates with a poor prognosis in lung cancer patients, was highly stable in TD cells after TGFβ stimulation. The increased protein stability of Snail in TD cells correlated with elevated inhibitory phosphorylation of GSK3β, resulting from the high Akt activity. Notably, integrin β3, whose expression was markedly increased upon sustained exposure to TGFβ, was responsible for the high Akt activity as well as the increased Snail protein stability in TD cells. Consistently, clinical database analysis on lung cancer patients revealed a negative correlation between overall survival and integrin β3 mRNA levels. Therefore, we suggest that the integrin β3-Akt-GSK3β signaling axis plays an important role in non-canonical TGFβ signaling, determining the invasive properties of tumor cells chronically exposed to TGFβ.

Highlights

TGFβ signaling plays an important role in tumor suppression as well as tumor promotion events such as initiation, progression and metastasis [1]
We show that Snail protein, the mRNA expression of which strongly correlates with a poor prognosis in lung cancer patients, was highly stable in TD cells after TGFβ stimulation
Chronic exposure of A549 tumor cells to TGFβ was sufficient by itself to induce epithelial-mesenchymal transition (EMT), which is characterized by reduced E-cadherin expression and increased N-cadherin expression and cytoskeletal reorganization

Summary

Introduction

TGFβ signaling plays an important role in tumor suppression as well as tumor promotion events such as initiation, progression and metastasis [1]. TGFβ, the prototype of the TGFβ superfamily, confers metastatic potential to tumor cells by inducing epithelial-mesenchymal transition (EMT), characterized by the acquisition of invasive phenotypes. Considering that tumor microenvironments are often enriched in secreted TGFβ [6], cancer cells are considered to be chronically exposed to TGFβ. Emerging reports demonstrate that after chronic TGFβ exposure, cancer cells acquire resistance to the growth-inhibitory effect of TGFβ and concurrently acquire malignancy or/ and invasive potential (e.g., EMT). Adaptation to chronic TGFβ exposure is achieved via alterations in the signaling networks of tumor cells that make them more prone to malignant progression [7,8,9]

Methods

Results

Conclusion