Type II CGMP-dependent Protein Kinase Inhibits EGF-triggered Signal Transduction of the MAPK/ERK-mediated Pathway in Gastric Cancer Cells

Our previous study found that Type II cGMP-dependent protein kinase (PKG II) is expressed at lower levels in human gastric cancer tissues and cell lines and increasing the expression and activity of PKG II inhibited the proliferation of cancer cell line BGC-823. However, the mechanism through which PKG II inhibits proliferation of gastric cancer cells is still not clear. Herein, we show that PKG II can inhibit EGF-induced MAPK signal transduction. In the gastric cancer cell line BGC-823, the expression and activity of PKG II were increased by infecting the cells with adenoviral construct encoding PKG II cDNA and treating the cells with the cGMP analogue 8-pCPT-cGMP. We found that PKG II inhibited the EGF-induced dual phosphorylation of ERK, a key component of the MAPK signal transduction pathway. Upstream of ERK, PKG II inhibited the phosphorylation of MEK1/2, the phosphorylation/activation of Raf-1, the activation of Ras, and the binding between adaptor protein Grb2 and GTP exchange factor Sos1 induced by EGF. Of note, PKG II inhibited the tyrosine phosphorylation of EGFR induced by EGF. Downstream of ERK, the EGF-induced nuclear translocation of phospho-ERK was also inhibited by PKG II. The results suggest that PKG II inhibits the proliferation of gastric cancer cells through blocking EGF-triggered MAPK signal transduction and the key blocking point is the tyrosine phosphorylation of the EGF receptor.