Vascular Endothelial Growth Factor (VEGF), VEGF Receptor (FLK-1) and Hypoxia Inducible Factor - Like Factor (HLF) mRNA Abundance Are Increased by Dexamethasone in Postnatal Mouse Lung

Abstract 275 Poster Session IV, Tuesday, 5/4 (poster 300)

Although the endothelial cell is the most abundant cell type in the lung, little is known about regulation of lung developmental vasculogenesis. VEGF is an endothelial cell mitogen and angiogenic factor that is expressed by alveolar epithelial cells. Actions of VEGF are mediated by two tyrosine kinase receptors KDR/FLK-1 and FLT-1. VEGF has four isoforms (VEGF₁₈₉, VEGF₁₆₅, VEGF₁₂₁ and VEGF₂₀₁) produced by alternative splicing of the primary RNA. HLF is a transcription factor that increases gene transcription of VEGF. Dexamethasone augments lung maturation and microvascular development in postnatal animals. However, in vitro studies suggest that dexamethasone blocks induction of VEGF. We hypothesized that dexamethasone will downregulate VEGF mRNA abundance in newborn mouse lung. We also hypothesized that the dexamethasone effect on VEGF may be mediated through HLF. We measured the effects of dexamethasone in vivo on VEGF, FLK-1 and HLF mRNA abundance in the lungs of newborn mice. Swiss-Webster mice pups received dexamethasone (0.1- 5mg/kg, DEX group) or sterile water (CON group) intraperitoneally from day 6 to 9. They were sacrificed 24 hours after the last injection. Lung, heart and kidney were analyzed. Compared to CON, the DEX group (5 mg/kg) had a significantly lower mean weight gain (p<0.002). In addition, the ratio of mean lung weight/mean body weight was significantly lower in the DEX group (p<0.005). Northern blot analysis showed a dose dependent maximal 2 fold increase in VEGF mRNA abundance in lung (p<0.005). There was no effect on heart or kidney VEGF message. In lung, FLK-1 mRNA abundance increased 2 fold (p<0.04) and HLF mRNA abundance increased 3 fold (p<0.02) in the DEX group. In situ hybridization showed that VEGF was expressed predominantly by alveolar epithelial cells. This pattern was not altered in the DEX group. ELISA of lung homogenates showed increased (60%) VEGF protein in the DEX group. The molar ratios of VEGF mRNA splice variants in CON lung (44% VEGF₁₈₉, 32% VEGF₁₆₅ and 24% VEGF₁₂₁) were not changed by dexamethasone. We conclude that dexamethasone significantly increases VEGF, FLK-1 and HLF mRNA abundance in newborn mouse lung. We speculate that dexamethasone may modulate postnatal vasculerization by inducing VEGF and VEGF receptor mRNA. The mechanism of the dexamethasone effect may be by inducing HLF mRNA.

Supported by NIH HL 54632