Type II cGMP-dependent protein kinase directly inhibits HER2 activation of gastric cancer cells.

Our previous study demonstrated that type II cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG II) inhibited epidermal growth factor (EGF)-induced phosphorylation/activation of epidermal growth factor receptor (EGFR). Since human epidermal growth factor receptor 2 (HER2) has a similar molecular structure to EGFR, the present study was designed to investigate whether PKG II also inhibits HER2 activation. The human gastric cancer cell line HGC‑27 was infected with an adenoviral construct encoding cDNA of PKG II (Ad‑PKG II) to increase the expression of PKG II and treated with 8‑(4‑chlorophenylthio)guanosine‑3',5'‑cyclic monophosphate (8‑pCPT‑cGMP) to activate the kinase. Western blotting was performed to detect the tyrosine and serine/threonine phosphorylation of HER2. Co‑immunoprecipitation was performed in order to determine the binding between PKG II and HER2. In addition, a QuikChange Lightning Site‑Directed Mutagenesis kit was used to mutate threonine 686 of HER2 to glutamic acid or alanine. The results demonstrated that EGF treatment increased the tyrosine phosphorylation (activation) of HER2. Increasing the PKG II activity of HGC‑27 cells through infection with Ad‑PKG II and stimulation with 8‑pCPT‑cGMP inhibited the EGF‑induced tyrosine phosphorylation/activation of HER2. PKG II bound directly with HER2 and caused phosphorylation of threonine 686. When threonine 686 of HER2 was mutated to alanine, which could not be phosphorylated by PKG II, the inhibitory effect of PKG II on the activation of HER2 was eradicated. When threonine 686 of HER2 was mutated to glutamic acid, which mimicked the phosphorylation of this site, treatment with EGF had no stimulating effect on tyrosine phosphorylation/activation of the mutant HER2. The results suggested that PKG II inhibits EGF‑induced activation of HER2 through binding with and causing threonine 686 phosphorylation of this oncogenic protein.