Regulation of vascular endothelial growth factor expression by insulin-like growth factor-II in human keratinocytes, differential involvement of mitogen-activated protein kinases and feedback inhibition of protein kinase C

BACKGROUND: Vascular endothelial growth factor (VEGF) is overexpressed in hyperproliferative diseases such as psoriasis and cancer, which are characterized by an increased angiogenesis. It was reported that insulin-like growth factor (IGF)-II is highly expressed during hepatocarcinogenesis and is increased in psoriatic lesions. The increase in IGF-II is believed to be associated with the pathogenesis of these diseases by increasing angiogenesis. OBJECTIVES: In order to investigate the relationship between IGF-II and angiogenesis-related VEGF, VEGF expression in the IGF-II-treated human keratinocytes was monitored and the IGF-II signalling pathways were examined with respect to VEGF expression. METHODS: Northern blot analysis for the VEGF mRNA levels and an enzyme-linked immunosorbent assay for the VEGF protein were performed to determine if IGF-II (100 ng mL(-1)) can increase the VEGF expression levels with or without a pretreatment with protein inhibitors in primary normal human keratinocytes and HaCaT cells. RESULTS: The mRNA and protein levels of VEGF by IGF-II were increased in a time-dependent manner and reached the maximum level 2 h and 8 h after the IGF-II treatment, respectively. However, this increase was abrogated by pretreatment with an extracellular signal-regulated kinase (ERK) inhibitor but not by a p38 inhibitor. The IGF-II-mediated VEGF induction was also effectively inhibited by a pretreatment with the tyrosine kinase inhibitor and Src inhibitor. The PI3-kinase inhibitor also inhibited the expression of VEGF by IGF-II. However, the phospholipase C (PLC) and protein kinase C (PKC) inhibitors did not block the increases of VEGF mRNA level and its protein level by IGF-II, and the PKC inhibitor instead increased VEGF expression by IGF-II. CONCLUSIONS: These results suggest that the tyrosine kinase-Src-ERK1/2 pathway and the PI3-kinase pathway are involved in IGF-II-mediated VEGF expression, but PKC is negatively associated in the IGF-II-mediated VEGF expression.