ObjectiveTo investigate the effects of pipecolic acid oxidase (PIPOX) on the proliferation, apoptosis, migration and invasion of primary liver cancer cells. MethodsImmunohistochemical staining and analysis of The Cancer Genome Atlas (TCGA) database were used to examine the PIPOX expression levels in liver cancer tissues and paired adjacent normal tissues, and studied their relationship with patient prognosis. Liver cancer cell lines stably overexpressing or knocking out PIPOX were constructed to explore PIPOX’s impact on liver cancer cell proliferation, apoptosis, migration and invasion by conducting in vitro functional experiments such as CCK-8, EdU, apoptosis detection, and Transwell assays. In vivo, nude mice subcutaneous tumor models and lung metastasis models were used to verify PIPOX’s effect on liver cancer growth and metastasis. Real-time quantitative polymerase chain reaction (RT-qPCR) and western blot were both employed to detect the expression of epithelial-mesenchymal transition (EMT) markers in liver cancer cells. ResultsImmunohistochemical staining and TCGA database analysis revealed that PIPOX expression was significantly lower in liver cancer tissues compared to paired adjacent normal tissues (P<0.05). Prognostic analysis indicated shorter overall survival and disease-free survival in PIPOX low expression group (P<0.05). In vitro gain- and loss-of-function experiments showed that PIPOX significantly inhibited liver cancer cell migration and invasion (P<0.05), while having no significant effects on their proliferation and apoptosis (P>0.05). Animal experiments also confirmed that PIPOX significantly inhibited liver cancer lung metastasis (P<0.05), but had no significant effects on tumor growth (P>0.05). Finally, RT-qPCR and western blot results revealed that PIPOX promoted the expression of the epithelial marker E-cadherin (P<0.05) and inhibited the expression of mesenchymal markers (N-cadherin, vimentin, Snail) (P<0.05). ConclusionsPIPOX significantly inhibits liver cancer cell migration and invasion, potentially via suppressing the EMT process. However, PIPOX does not significantly affect liver cancer cell proliferation and apoptosis.