Interestingly, activation of NRG1 could compromise the inhibitory impact and rescue cell survival; whereas, transfection of siYAP1 sensitized trastuzumab-treated cells. expression of YAP1, a vital downstream interacted target of HER4, decreased when HER4 was knocked down. Interestingly, activation of NRG1 could compromise the inhibitory impact and rescue cell survival; whereas, transfection of siYAP1 sensitized trastuzumab-treated cells. Expression analysis of the proteins in patient-derived xenograft model (PDX) mice showed that HER4, p-HER4, YAP1, and Vimentin were clearly upregulated in the trastuzumab-resistant mice compared to mice without trastuzumab resistance. However, HER2 and E-cadherin were downregulated in response to continuous treatment with trastuzumab. These findings elucidated that this central role of the HER4-YAP1 axis in trastuzumab resistance of HER2-positive gastric malignancy cells through induction GKLF of EMT. Hence, regulating the HER4-YAP1 axis might be a encouraging strategy for clinical interventions in patients with HER2-positive gastric malignancy. Introduction Gastric malignancy is DPH the fifth most commonly diagnosed malignancy and the third leading cause of cancer death worldwide [1]. Adjuvant chemotherapy following radical surgical resection would specifically benefit patients with advanced gastric malignancy [2], but the high heterogeneity of the disease prospects to metastasis and recurrence, with a significantly poorer prognosis that is 1-12 months median survival and a 5-12 months survival rate 7% [3, 4]. Among the various genomic events, abnormal DPH expression of HER2, a prognostic factor for patients, is involved in as many as 7C34% of gastric cancers [5C7]. HER2 functions as a co-receptor that modulates signals after ligands bind to other receptors in the epithelial growth factor receptor (EGFR) family. By interacting independently with extracellular ligands or by heterodimerizing with other members of the ErbB family, downstream signaling pathways, including the MAPK and PI3K/AKT/mTOR pathways, are activated to facilitate uncontrolled cell growth and tumorigenesis [8]. Trastuzumab, the only target agent approved by the DPH FDA as a first-line treatment of metastatic gastric adenocarcinoma, substantially enhances the outcome for patients with HER2-positive gastric malignancy [9, 10]. However, most patients become refractory to the trastuzumab-based treatment within 1 year [11]. Although new agents have been explored to delay the emergence of resistance, acquired resistance still limits the duration of the response to trastuzumab [12]. Thus, a characterization of the resistance mechanism of trastuzumab is usually urgently needed to offer an alternative option for patients who would suffer from inevitable resistance. Currently, a large proportion of investigations about the molecular mechanisms of trastuzumab resistance stem from breast cancer, and the generally acknowledged processes include hyperactivation of the phosphatidylinositol-3-kinase (PI3K) pathway by PI3K alterations or PTEN loss [13C15]. Scaltriti et al.[16] demonstrated that p95HER2, a mutated form of HER2, did not bind to trastuzumab and retained its tyrosine DPH kinase activity, which partly accounted for trastuzumab resistance. Recent studies also uncovered that growth factors act as ligands of receptor tyrosine kinases, as well as upregulation of other growth factors, such as NRG1 and HGF, which also bind to HER2, can confer resistance to anti-HER2 drugs [17, 18]. Piro et al.[11] found that expression of fibroblast growth factor receptor 3 (FGFR3) could stimulate epithelial-to-mesenchymal transition (EMT) after resistance developed DPH in gastric malignancy, with the FGFR3/AKT axis as the presumed escape pathway responsible for trastuzumab resistance. However, the exact mechanisms of trastuzumab resistance in HER2-positive gastric malignancy still remained unknown. As a member of the ErbB family, HER3 can dimerize with HER2, transcriptional and posttranslational upregulation of HER3 were suggested to promote the phosphorylation of a residue on HER2, thus maintaining activation of the PI3K pathway; an anti-HER3 agent in combination with inhibitors of HER2 and the PI3K pathway were recommended to achieve effective treatment [19]. In HER2-positive gastric malignancy, dual inhibition of EGFR and HER2 displayed a satisfactory killing ability toward trastuzumab-resistant cells [20, 21]. HER4, another member of the ErbB family, also reportedly impacts HER2-positive malignancy cell survival after cells become resistant to trastuzumab, and nuclear localization of HER2 indicateed a poor prognosis in breast malignancy [22, 23]. The presence of HER4 sensitizes HER2-positive cells to trastuzumab and.