Iron deficiency downregulates ENPEP to promote angiogenesis in liver tumors

The abnormal iron metabolism in liver cancer leads to iron deficiency in tumor tissues. We previously found that iron deficiency promoted liver cancer metastasis, but the mechanisms were not fully understood. In the present study, we identified that the angiogenesis-associated glutamyl aminopeptidase (ENPEP) was consistently decreased in iron-deficient liver tissues, iron-deficient liver tumors, and iron-deprived liver cancer cells. Interestingly, the lower expression of ENPEP was correlated with the poor prognosis of liver cancer patients, while the biomarkers of angiogenesis, CD31 and CD34, were increased in tumor tissues. In vivo imaging of liver-orthotopically implanted and tail vein-injected liver cancer cells showed that iron deficiency increased the pulmonary metastasis of liver cancer. The angiogenesis in iron-deficient tumors was enhanced, and the expression of ENPEP was decreased. Silencing ENPEP expression increased the migration of liver cancer cells and the proliferation of cocultured HUVECs. By sequence analysis, we found that the transcription factor SP1 possessed abundant binding sites in the ENPEP promoter region. Its combination with ENPEP promoters was verified by chromatin immunoprecipitation. The inhibition of SP1 by mithramycin A effectively restored the expression of ENPEP, which was decreased by iron deficiency. In conclusion, these results revealed that iron deficiency in liver tumors decreased the expression of ENPEP by SP1 and increased the angiogenesis and metastasis of liver tumors, which further explained the mechanism by which iron deficiency promoted liver cancer metastasis.