Abstract: Resistance of tumor cells is a complex biological process involving multiple mechanisms and factors, in which anti-apoptosis is the most important cause of drug resistance. Previous studies have shown that the DNA binding activity of Runt related transcription factor 3 (RUNX3) increased prominently in Herceptin resistant gastric cancer cells (NCI N87R) while the relevance of which to drug resistance has not yet been confirmed. In this study, we employed CRISPR/Cas9 to establish RUNX3 knock-out cell line (△RUNX3/NCI N87R) to investigate the functions of RUNX3 in Herceptin resistance of NCI N87R cells and its potential mechanisms. We investigated proteomics profiling of △RUNX3/NCI N87R cells based on label free quantitative proteomics. Differentially expressed proteins were screened out according to fold change and significance level between △RUNX3/NCI N87R and NCI N87R cells. Pathway enrichment analysis was done using GeneAnalytics database, and gene ontology analysis was conducted by DAVID Bioinformatics Resources database. Protein-protein interaction networks were constructed based on STRING database. The results showed that △RUNX3/NCI N87R cells increased the sensitivity to Herceptin. Proteomic data demonstrated that the expression of 577 genes changed significantly in △RUNX3/NCI N87R cells, among which 191 genes were up-regulated while 386 ones down-regulated comparing with NCI N87R cells. Pathway analysis showed that autophagy, cell cycle, apoptosis, mitochondrial fatty acid β oxidation, neurogenic locus notch homolog protein 1 (NOTCH1), mammalian target of rapamycin (mTOR), Hedgehog and DNA damage response pathways exhibited notable changes based on pathway enrichment ratio and significance level (P<0.05). These results indicated that RUNX3 knock-out altered multiple signaling pathways of NCI N87R cells. Western blotting manifested that the expression of autophagy regulatory molecules autophagy-related protein (ATG) 13, 7 and BECN1 increased remarkably while cell cycle molecules serine/threonine-protein kinase Chk2 (CHEK2) and apoptosis regulator Bcl-2 (BCL2) decreased prominently in △RUNX3/NCI N87R cells. The p-AKT expression decreased significantly in △RUNX3/NCI N87R cells compared with NCI N87R cells (P<0.01) and was suppressed by Herceptin. These results indicated that RUNX3 knock-out altered cell cycle, increased inhibition to p-AKT by Herceptin, promoted autophagy and induced cell apoptosis of NCI N87R cells. These results suggested that RUNX3 may be a potential therapeutic target for reversing or reducing Herceptin resistance in gastric cancer cells.