Screening GABRP from human CRC chemoresistant and sensitive patients' samples
In order to screen potential DEGs in chemo-resistant and chemo-sensitive colorectal patients, GSE28691, GSE77932, and GSE81006 datasets had been intersected. Firstly, GABRP stood out among 3981 overlapping genes from both GSE28691 and GSE81006 datasets with the selection condition of the adjusted p-value<0.01 presented by the Venn diagram (Fig 1a). Secondly, GABA family molecules had been enriched when crossing the datasets of GSE28691 and GSE77932, with the threshold setting as |log2FC|>0.5 (Fig 1b), we found GABARAPL1, GABRB1, GABRA3, GABRQ becoming the overlap. Collectively, GABRP involved in the GABA family seems a promising differentiated gene between colorectal sensitive and resistant cells, therefore, we further decided to validate GABRP in two colorectal chemo-resistant and chemo-sensitive cancer cells.
KEGG pathway analysis and GO annotation
We imported 3981 genes into the DAVID database for GO and KEGG enrichment analysis. The results showed that the DEGs mainly participated in the biological process based on FDR ranking: such as signal transduction, positive regulation of transcription from RNA polymerase II promoter, inflammatory response, response to drugs, positive regulation of cell migration, cell-cell signaling, intracellular signal transduction, axon guidance, chemical synaptic transmission, ion transport (Fig 2a). Our candidate gene GABRP was enriched in three biological processes: signal transduction, chemical synaptic transmission, and ion transport. Regarding cellular components, the DEGs were mainly concentrated in extracellular exosome, plasma membrane, extracellular region, integral component of plasma membrane, cell surface, membrane, cytosol, extracellular space, cytoplasm, and neuron projection, etc (Fig 2b). The gene of our interest GABRP was enriched in plasma membrane, integral component of plasma membrane, membrane, and neuron projection. KEGG pathway analysis enriched in phagosome, rheumatoid arthritis, Th17 cell differentiation, relaxin signaling pathway, calcium signaling pathway, human papillomavirus infection, axon guidance, PI3K-Akt signaling pathway, GABAergic synapse, and neuroactive ligand-receptor interaction (Fig 2d). Of note, the last two pathways listed in our representative bar graph contained candidate gene GABRP.
Validation of GABRP from the transcriptional level between chemoresistant and chemosenstive CRC cells
GABRP mRNA expression increased more than two folds in oxaliplatin-resistant SW480 cells compared to its corresponding oxaliplatin-sensitive cells (Fig 3a). Interestingly, 5-Fu resistant HCT-8 colorectal cells also detected a relatively higher level, nearly one and a half fold of GABRP at the transcriptional perspective. β-actin served as the reference gene (Fig 3b). These results demonstrated that GABRP was positively associated with resistant colorectal cancer cells.
Clinical relevance of GABRP in CRC patients
In order to further validate our findings in cancer patients in the clinical setting, we firstly compared GABRP expression among several cancer types with different cancer patients, such as breast cancer, colon cancer, gastric cancer, and pancreatic cancer. It was obvious that GABRP was overexpressed in colon cancer compared to the other three cancer types (Fig 4a). Therefore, we switched gear to colorectal cancer patients. We found most colon and rectal cancer patients detected high GABRP expression (Fig 4b). Moreover, GABRP expression was upregulated at least two folds in colorectal adenocarcinoma than in normal colorectal tissues (Fig 4c). High expression of GABRP had a relatively lower survival rate or disease-free survival rate than those GABRP low expression group in colorectal cancer patients. Collectively, consistent with our findings in cell line models, clinical data supported the oncogenic role of GABRP in colorectal cancer patients.
Prediction of protein-protein interaction
In order to find interactive proteins with our candidate gene GABRP, we created its PPI network using the STRING database (Fig 5a,5b). GABRP was radiated as the hub gene in a plain PPI network with 10 circles and 17 lines connecting to it (Fig 5a). There are ten proteins predicted to interact with GABRP: Ankyrin Repeat Domain 66(ANKRD66), Clathrin Interactor 1(CLINT1), Huntingtin Associated Protein 1(HAP1), Phospholipase C Like 1 (Inactive)(PLCL1), GABA Type A Receptor-Associated Protein(GABARAP), GABA Type A Receptor Associated Protein Like 1(GABARAPL1), N-Ethylmaleimide Sensitive Factor, Vesicle Fusing ATPase(NSF), GABA Type A Receptor Associated Protein Like 2(GABARAPL2), Trafficking Kinesin Protein 2(TRAK2), and Chloride Intracellular Channel 3(CLIC3)(Fig 5a). In this network, the average node degree was 3.09, the average local clustering coefficient was 0.896, and the PPI enrichment P-value was 0.0389. Moreover, we found that GABRP has a relatively stronger interaction with GABARAP, GABARAPL1, ANKRD66, CLINT1, and CLIC3 than the above-mentioned molecules due to the indication of more line numbers (Fig 5b). Therefore, we enriched highly interactive proteins in Fig 5b. The average node degree was 2, the average local clustering coefficient was 0.85.