Temporospatial expression of specific regulators of angiogenesis during alveolization in rats

Normal rat alveolization occurs in the first two weeks of life. Concomitant steroid administration impedes normal alveolar septation. We hypothesized that normal angiogenesis during alveolization would require appropriate temporospatial expression of soluble factors, cell:cell and cell:matrix adhesion molecules, and that steroid treatment would disrupt this normal pattern. Since Vascular Endothelial Growth Factor (VEGF, soluble factor), Platelet-Endothelial Cell Adhesion Molecule (PECAM)-1 (cell:cell) and hyaluronan (HA, matrix) have previously been implicated in angiogenesis, we examined the expression of these molecules and the Receptor for HA-Mediated Motility (RHAMM) during normal and steroid-retarded alveolization in rats. Litter sizes were normalized to 10 pups/litter to equalize somatic growth. Untreated pups were compared to those receiving either 0.1 μg dexamethasone or an equivalent volume of saline daily subcutaneously. Immunostaining of normal lungs demonstrated the expression of VEGF (in epithelial cells) and PECAM-1 (on endothelium) at birth. These molecules became almost undetectable on days 5 and 10, and reappeared by day 15. In contrast, RHAMM and HA expression was restricted to epithelial and subepithelial areas of bronchioles at birth, increased in alveolar walls and developing septae on days 5 and 10, and returned to a more restricted pattern of expression by day 15 during normal development. Steroid treatment resulted in a delay in these changes as well as decreased septation at all timepoints examined. All changes in expression were confirmed by northern blot analysis of VEGF, PECAM-1 and RHAMM, and by HA content measured by radiometric assay. We conclude that steroid-induced changes in the normal temporospatial expression of angiogenic factors is associated with altered septation. We speculate that angiogenesis may be critical for alveolization.