Background Much progress has been made in hepatocellular carcinoma (HCC) treatment, however, HCC is still the fifth most common cause of cancer-related death worldwide. Therapies still have limited effectiveness and a range of side effects. Understanding of the pathogenesis and underlying molecular mechanisms is of pivotal importance for the development of novel treatment strategies. The family of p53 proteins, (p53, p63, p73 and their isoforms) play an important role in defending cells against toxic stress by transcriptionally regulating specific sets of target genes. We previously identified IGFBP2 (Insulin like growth factor binding protein 2) as a p73 target gene in HCC. IGFBPs control IGF availability and limit its proliferative effects. Aim of this study was to investigate the impact and physiologic relevance of IGFBP2 induction in HCC cells.
Methods IGF secretion and surface levels of IGF receptors were analyzed in Hep3B cells. The impact of p73 overexpression on IGFBP2 was studied in rAd-TAp73-transfected cells. Effects on p73 and IGFBP2 expression were evaluated after stimulation with HCC therapeutics bleomycin, doxorubicin and regorafenib. Transcriptional regulation was determined by qPCR, protein levels by Western Blot and IGFBP2 secretion by ELISA. Overexpression of IGFBP2 or treatment with recombinant IGFBP2 was performed to analyze the impact of high doses of IGFBP2.
Results Hep3B cells displayed physiologic IGF secretion and surface expression of insulin receptor and IGF receptor 1. p73 overexpression induced IGFBP2 production and secretion. Treatment of Hep3B cells with HCC-therapeutics resulted in an induction of TP73 and IGFBP2 and also led to increased cell death rates. However, only the two chemotherapeutics led to a relevant secretion of IGFBP2 in the range of 500 pg/ml. In contrast, high IGFBP2 concentrations increased cell viability, proliferation and migration and resulted in reduced drug-mediated cell death.
Conclusion Depending on the amount, IGFBP2 has divergent effects in HCC cells: p73 activity induces endogenous IGFBP2 production and secretion in low amounts, which exerts potentially anti-proliferative effects. Contrariwise, high levels of (exogenous) IGFBP2 triggers the proliferative arm of the IGF axis. Thus, fine-tuning of the p73- and IGF-pathways is essential for homeostasis and a detailed understanding might offer new therapeutic options in HCC.
