Autophagy induces transforming growth factor-β-dependent epithelial-mesenchymal transition in hepatocarcinoma cells through cAMP response element binding signalling.

Shaobo Hu,Xi Zhang,Yongzhong Wu,Jing Yang,Liyu Wang,Jun Li

doi:10.1111/jcmm.13825

Abstract

Autophagy promotes invasion of hepatocarcinoma cells through transforming growth factor (TGF)‐β‐dependent epithelial‐mesenchymal transition (EMT). This study investigated the mechanism by which autophagy induces TGF‐β‐triggered EMT and invasion of hepatocarcinoma cells. Autophagy was induced in HepG2 and BEL7402 cells by starvation in Hank's balanced salt solution. Induction of autophagy degraded phosphodiesterase (PDE) 4A and increased intracellular cAMP, PKA activity and PKA phosphorylation, resulting in increased cAMP response element binding (CREB) phosphorylation in hepatocarcinoma cells. Autophagy‐induced activation of cAMP/PKA/CREB signalling further enhanced TGF‐β1 expression, downregulated the expression of epithelial markers and upregulated the expression of mesenchymal markers, accelerating invasion of hepatocarcinoma cells. Inhibition of autophagy by Atg3 and Atg7 knockdown or by chloroquine treatment prevented degradation of PDE4A and activation of cAMP/PKA/CREB signalling, suppressing TGF‐β1 expression, EMT and invasion in hepatocarcinoma cells. In addition, inhibition of cAMP/PKA/CREB signalling also blocked autophagy‐induced TGF‐β1 expression and prevented EMT and invasion of hepatocarcinoma cells under starvation. Furthermore, exogenous inhibition of PDE4A or activation of cAMP/PKA/CREB signalling rescued TGF‐β1 expression, EMT and invasion in autophagy‐deficient hepatocarcinoma cells. These findings suggest that autophagy induces TGF‐β1 expression and EMT in hepatocarcinoma cells via cAMP/PKA/CREB signalling, which is activated by autophagy‐dependent PDE4A degradation.

Highlights

Invasion and metastasis of hepatocarcinoma cells are the leading cause of refractory cancer and poor prognosis in the clinic
Since activation of cAMP/ PKA/cAMP response element binding (CREB) signalling was significantly promoted by autophagy (Figure 2), and autophagy induced transforming growth factor (TGF)‐β1 expression in hepatocarcinoma cells under starvation,[7] we further investigated the role of cAMP/PKA/CREB signalling in autophagy‐induced TGF‐β1 expression in HepG2 and BEL7402 cells
We describe a further mechanism by which autophagy induces TGF‐β1 expression, epithelial‐mesenchymal transition (EMT) and invasion in hepatocarcinoma cells

Summary

| INTRODUCTION

Invasion and metastasis of hepatocarcinoma cells are the leading cause of refractory cancer and poor prognosis in the clinic. Deficiencies in angiogenesis frequently result in starvation and/or hypoxia in solid tumour cells during tumour growth, which may induce autophagy of these cells.[3,4]. Activation of the CREB signalling pathway and phosphorylation of CREB were shown to fulfill numerous cellular functions ranging from cell proliferation and the cell cycle to cell differentiation and cytokine production by binding of phosphorylated‐ CREB to the cAMP response element (CRE) in target genes and promoting their transcription.[11-14]. Autophagy is a lysosome‐dependent protein and organelle degradation mechanism that maintains the homoeostasis of the cellular metabolic pool This process modulates multiple cellular signalling pathways and physiological functions by degrading redundant proteins or enzymes under stress, such as starvation or hypoxia.[22]. This study investigated the role of autophagy in PDE4 degradation and cAMP/PKA/CREB signalling modulation as a mechanism to induce TGF‐β‐triggered EMT and invasion of hepatocarcinoma cells under starvation

| MATERIALS AND METHODS

| RESULTS

Findings

| DISCUSSION