NADPH Oxidase NOX1 Controls Autocrine Growth of Liver Tumor Cells through Up-regulation of the Epidermal Growth Factor Receptor Pathway

Abstract

FaO rat hepatoma cells proliferate in the absence of serum through a mechanism that requires activation of the epidermal growth factor receptor (EGFR) pathway. The aim of this work was to analyze the molecular mechanisms that control EGFR activation in these and other liver tumor cells. Reactive oxygen species production is observed a short time after serum withdrawal in FaO cells, coincident with up-regulation of the NADPH oxidase NOX1. NOX1-targeted knockdown, the use of antioxidants, or pharmacological inhibition of NADPH oxidase attenuates autocrine growth, coincident with lower mRNA levels of EGFR and its ligand transforming growth factor-alpha (TGF-alpha) and a decrease in phosphorylation of EGFR. EGFR-targeted knockdown induces similar effects on cell growth and downstream signals to those observed in NOX1-depleted cells. Early NOX1 activation induces both a feedback-positive loop via an Src-ERK pathway that up-regulates its own levels, and a parallel signaling pathway through p38 MAPK and AKT resulting in EGFR and TGF-alpha up-regulation. Human hepatocellular carcinoma cell lines, but not non-tumoral hepatocytes, show autocrine growth upon serum withdrawal, which is also coincident with NOX1 up-regulation that mediates EGFR and TGF-alpha expression. The use of antioxidants, or pharmacological inhibition of NADPH oxidase, effectively attenuates autocrine growth in hepatocellular carcinoma cell lines. In summary, results presented in this study indicate that NOX1 might control autocrine cell growth of liver tumor cells through regulation of the EGFR pathway.