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Hypoxic Induction of Vascular Endothelial Growth Factor (VEGF) and HIF-1 Protein in Retinal Pigment Epithelial Cells

Retinopathy of prematurity (ROP) is a neovascularizing disease that causes blindness in premature infants. ROP is initiated by an arrest of normal retinal blood vessel development. Subsequently, the non-perfused peripheral retina becomes hypoxic leading to retinal neovascularization. Vascular endothelial growth factor (VEGF) has been implicated in playing a role in the pathogenesis of ROP. HIF-1 is a heterodimeric DNA binding protein consisting of an α and β subunit. HIF-1 protein expression and DNA binding increases in response to hypoxia. HIF-1 binding sites are present in several hypoxia inducible genes and HIF-1 has been implicated in the activation of VEGF transcription in response to hypoxia. It is therefore possible that HIF-1 plays a role in the pathogenesis of ROP.

Erythropoietin (Epo) is a glycoprotein hormone that is the primary regulator of erythropoiesis. Transcription of the Epo gene increases in response to hypoxia or anemia. Epo is synthesized in the liver in fetal life and in the kidney later in gestation. Hypoxia inducible factor (HIF-1) is a DNA binding protein that binds to a hypoxia inducible enhancer in the 3′ region of the Epo gene. HIF-1 is a heterodimer consisting of a α andβ sub-unit. HIF-1 α and β mRNA is present in all tissues in adult mice. HIF-1β transcripts were detected in various tissues of 9.5 day fetal mice by in situ hybridization. Epo mRNA is present in only fetal mouse liver at day 14 and in kidney by day 19 of gestation. In the human fetus Epo mRNA is present in fetal kidney by 11 weeks gestation. In order to determine if there is a relationship between the expression of HIF-1 α and β sub-units with the shift in expression of the Epo gene from the liver to the kidney we analyzed the expression of these mRNA's in mouse and human fetuses. Total RNA was extracted from mouse brain, heart, kidney, liver and lung at day 15, 17 and 19 of gestation in the mouse and 12-22 weeks of gestation in the human fetus. RNA was analyzed by northern blot and dot blot hybridization using appropriate cDNA probes. HIF-1α and β were expressed in all tissues tested and at all stages of gestation in mouse and human. Expression of mouse HIF-1α in the heart and kidney was mildly in excess of other organs, and expression in the brain was minimal. Expression of human HIF-1α in the kidney and lung was several-fold higher than in other organs. In the human, increased expression of HIF-1β was present in the lung, and expression in the brain was minimal. Human Epo mRNA was expressed in the kidney and liver from 12-22 weeks of gestation. In summary, both HIF-1α and β sub-units are expressed ubiquitously at the various stages of gestation tested. Our studies indicate that either sub-unit of the transcription factor HIF-1 probably does not play a major role in either tissue-specific expression or in the switch of Epo gene expression from the fetal liver to the kidney. However, other unidentified members of this family of transcription factors may play a part in this mechanism.