Lipopolysaccharide initiates a bypass feedback loop of epidermal growth factor receptor signaling by HPS70-induced COX-2 in H22 hepatocarcinoma cells

Abstract

LPS can induce TACE upregulation via signaling from TLR4-derived EGFR activation in tumor cells. The regulation and activity of TACE have been investigated with the observation that gene expression is upregulated in response to LPS followed by EGFR activation, however, the process remains poorly understood. In this study, we examined the effects of LPS on H22 hepatocarcinoma cells that displayed constitutively active TLR4 expression. Upon TLR4 shRNA transfection into H22 cells, HSP70 expression significantly increased. However, LPS induced early phosphorylation of EGFR in H22 cells, which reached maximum levels within 30 min. Inhibition of TLR4 in H22 cells resulted in a significant rise in both EGFR phosphorylation and TACE upregulation 24 h after exposure to LPS. Exogenous HSP70 also induced rapid phosphorylation of EGFR, upregulated the expression of COX-2 via a signaling pathway that involved TACE-dependent TGF-α release. Furthermore, inhibition of EGFR activation and reduction of COX-2 expression by COX-2 inhibitor prevented HSP70-induced cell invasion in vitro. These findings demonstrate that the biological importance of HSP70/COX-2 is crucial to the second, but not the first, phase of EGFR phosphorylation in tumor cells. The growth of tumor cells by inserting shRNA plasmid TLR4 combination with COX-2 inhibitor could be effectively reduced in LPS stimulation. We concluded that LPS triggered a bypass feedback loop of EGFR activation and involved HSP70/COX-2 in H22 cells by inhibition of TLR4 and that EGFR phosphorylation is implicated in tumor growth by LPS stimulation.