Role of hypoxia-inducible transcription factors (HIFs) in oxygen-dependent modulation of IGF binding protein–1 biosynthesis in primary rat hepatocytes

Abstract

Listen

Translate article

Higher levels of IGFBP–1 mRNA are expressed in the less aerobic perivenous zone of the liver. Since gradients in oxygen tension (pO2) may contribute to zonated gene expression, the influence of arterial and venous pO2 on IGFBP–1 biosynthesis was studied in primary cultures of rat hepatocytes. Maximal IGFBP–1 mRNA and protein levels were observed under venous pO2 whereas less than 30% of maximal levels were observed under arterial pO2. In contrast, the expression of IGFBP–4 was greatest under arterial pO2, indicating that this effect of hypoxia on IGFBP–1 gene expression is specific. The response to hypoxia appeares to involve reactive oxygen species, since treatment with H2O2 results in a dose-dependent decrease of IGFBP–1 mRNA levels under venous pO2, whereas IGFBP–1 mRNA expression under arterial pO2 was not affected. Inhibition of the hypoxia-dependent IGFBP–1 mRNA induction by actinomycin D indicates that this effect is mediated at the level of gene transcription, inhibition of IGFBP–1 mRNA by the iron chelator desferrioxamine under both venous and arterial pO2 suggested an involvement of hypoxia-inducible transcription factors (HIF). Transfection experiments demonstrated that especially HIF–3α and HIF–2α, and to a lesser extent HIF–1α, contribute to the induction of IGFBP–1 mRNA expression in isolated hepatocytes, while experiments with vectors for the HIF prolyl hydroxylases (PHD) indicated a major role of PHD–2 in destabilization of HIFs, attenuating the induction of IGFBP–1 under venous pO2. Reporter gene studies indicate that hypoxia stimulates IGFBP–1 expression through a putative HIF response element located ~250 bp upstream from the transcription initiation site. Together, these results support the concept that iron, radical oxygen species, the HIF–2 and –3 as well as the PHD pathways play important roles in mediating effects of hypoxia on IGFBP–1 gene expression in the liver.