Proteome-wide identification and functional analysis of ubiquitinated proteins in Hepa1-6 cells by knockdown of E3 ubiquitin ligase SIAH1

Background: Hepatocellular carcinoma (HCC) is an aggressive malignancy that poses a serious threat to human life. The conventional therapies for HCC cannot substantially improve overall survival (OS), disease duration, and prognosis. Therefore, it is important to study the underlying mechanism of HCC and seek better methods for HCC prevention and treatment. Ubiquitination is a post-translational modification that modulates great cellular function by cooperating with E1, E2, and E3 ligases. Yet, the ubiquitination and lysine residues in HCC are still elusive. Seven in absentia homolog 1 (SIAH1), as an important E3 ubiquitin ligase, regulates ubiquitin-mediated proteolysis to function as a tumor suppressor in HCC. In the present study, we downregulated SIAH1 in the mouse HCC cell line Hepa1-6 and studied its function by using proteome-wide identification. Methods: SIAH1 was knocked down by SIAH1 short hairpin RNA (shRNA) in mouse HCC cell line Hepa1-6 cells, and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis was conducted to analyze the ubiquitinated proteins. Functional analysis was performed using Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) enrichment. Results: The systematic profiling showed a total of 550 differently expressed proteins (DEPs), including 263 upregulated DEPs and 287 downregulated DEPs. Considering the amino acid sequences around the modified lysine residues, seven proteins were identified as conserved ubiquitination motifs in the peptides. The ubiquitinated proteins were mainly distributed in the cytoplasm, nucleus, and plasma membrane. Functional analysis suggested that the ubiquitinated proteins were mostly enriched in the nucleus, cytoplasm, and extracellular space; in addition, the ubiquitinated proteins were mostly attributed to the protein binding, and disease. The ubiquitinated proteins modulate HCC by mapping lysine modification sites. Conclusions: The use of high-throughput characterization to identify novel and specific targets associated with SIAH1 is of great significance in terms of functional weight. The results obtained in this paper from the analysis of proteomic data provided novel insights into ubiquitination regulation in HCC, which pave the way for further research and mechanism discovery of HCC.