Inhibition of VEGF expression through blockade of Hif1α and STAT3 signalling mediates the anti-angiogenic effect of melatonin in HepG2 liver cancer cells

Background: Hepatocellular carcinoma (HCC) growth relies on angiogenesis via vascular endothelial growth factor (VEGF) release. Hypoxia within tumour environment leads to intracellular stabilisation of hypoxia inducible factor 1 alpha (Hif1 alpha) and signal transducer and activator of transcription (STAT3). Melatonin induces apoptosis in HCC, and shows anti-angiogenic features in several tumours. In this study, we used human HepG2 liver cancer cells as an in vitro model to investigate the anti-angiogenic effects of melatonin. Methods: HepG2 cells were treated with melatonin under normoxic or CoCl2-induced hypoxia. Gene expression was analysed by RT-qPCR and western blot. Melatonin-induced anti-angiogenic activity was confirmed by in vivo human umbilical vein endothelial cells (HUVECs) tube formation assay. Secreted VEGF was measured by ELISA. Immunofluorescence was performed to analyse Hif1 alpha cellular localisation. Physical interaction between Hif1 alpha and its co-activators was analysed by immunoprecipitation and chromatin immunoprecipitation (ChIP). Results: Melatonin at a pharmacological concentration (1 mM) decreases cellular and secreted VEGF levels, and prevents HUVECs tube formation under hypoxia, associated with a reduction in Hif1 alpha protein expression, nuclear localisation, and transcriptional activity. While hypoxia increases phospho-STAT3, Hif1 alpha, and CBP/p300 recruitment as a transcriptional complex within the VEGF promoter, melatonin 1 mM decreases their physical interaction. Melatonin and the selective STAT3 inhibitor Stattic show a synergic effect on Hif1 alpha, STAT3, and VEGF expression. Conclusion: Melatonin exerts an anti-angiogenic activity in HepG2 cells by interfering with the transcriptional activation of VEGF, via Hif1 alpha and STAT3. Our results provide evidence to consider this indole as a powerful anti-angiogenic agent for HCC treatment.