VITAMIN A METABOLISM IN THE NEONATAL LUNG: STUDIES IN A RAT MODEL

Open Access
Author:
Wu, Lili
Graduate Program:
Nutrition
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 27, 2011
Committee Members:
  • A. Catharine Ross, Committee Chair
  • Okhee Han, Committee Member
  • Katarzyna Kordas, Committee Member
  • Pamela J. Mitchell, Committee Member
  • Jeffery M. Dodds, Committee Member
Keywords:
  • Vitamin A
  • neonatal lung
  • metabolism
  • retinoic acid
  • retinoid homeostatic genes
  • development
Abstract:
Vitamin A (VA) is an essential nutrient for differentiation and maturation of the lungs. Direct evidence has been presented that keratinizing squamous metaplasia of the bronchopulmonary tree can be caused by VA deprivation in the lungs of VA deficient animals and this morphologic change can be reversed by refeeding the animals with VA. Other biochemical and molecular genetic evidence revealed that specific retinoids-binding proteins and nuclear retinoic acid receptors are contained in the lungs and retinoids can affect lung cells differentiation by influencing lung gene expression. In rats, significant storage of VA in the lungs starts in late gestation just before the onset of alveologenesis and surfactant synthesis, and then quickly is depleted during late pregnancy and postnatal life as the lungs are still developing, suggesting a high and acute demand of VA for postnatal lung development. However, the VA status in the lung of human and other mammals is known to be low at birth and postnatal lungs are very sensitive to dietary VA deprivation. VA deficiency associated with premature infants or infants with very-low-body-weight (VLBW) can interrupt normal development and compromise the respiratory function of the lung, thereby putting this population at high risk to develop various respiratory diseases. Therefore, it is important to improve lung VA status at the early time of the postnatal life. Although supplementation with VA to neonates is an effective way to prevent VA deficiency, it shows limited effectiveness in improving lung VA status. Previously we have shown that retinol combined with retinoic acid (RA), a biologically active metabolite of VA (VARA), is able to increase lung retinyl ester (RE) formation synergistically and RA redirects more of the VA given as a supplement into the neonatal lung. Our work investigating the molecular mechanism of VARA synergy revealed that RA is able to affect lung VA metabolism by upregulating several important retinoid homeostatic genes: LRAT, lecithin:retinol acyltransferase, an enzyme converting retinol to its storage form; CYP26, a cytochrome P450, an enzyme metabolizing RA to inactive polar metabolites; and STRA6, stimulated by retinoic acid gene 6, a transmembrane receptor for the retinol-RBP complex that mediate cellular retinol uptake. However, these findings are based on a single dose study, and the activity of RA on gene induction appears to be transient. In the present study, we tested the effects of repeated supplementation with VARA in increasing lung RE contents. We also examined whether inflammation state and reduced RA concentration could affect the capability of RA in promoting RE formation. At the same time, we speculated the spatial expression pattern of LRAT, CYP26B1 and STRA6 to further understand VA metabolism in the lungs of the neonates. We carried out several studies to 1) investigate how multiple treatments of RA during the period of lung septation affect RE accumulation and the expression pattern of lung retinoid homeostatic genes, or genes required for normal lung function; 2) compare a reduced amount of RA in the VARA dose to test the potential of RA in elevating lung RE; 3) examine how lipopolysaccharide (LPS)-induced inflammation state affects VA homeostasis in neonatal lung; 4) determine the localization of retinoid homeostasis proteins in the lung. The results of our studies have shown that repeated treatments of VARA dramatically increase neonatal lung RE store in a cumulative and synergistic way. Diluted RA in VARA still promotes higher RE formation in neonatal lung more than VA alone after a single dose, but not after multiple doses. LPS-induced inflammation doesn’t significantly impact lung RE formation promoted by RA. The localization study suggested the expression of LRAT in lipofibroblasts, STRA6 in endothelial cells, and CYP26B1 in bronchiolar epithelium. Overall, these studies have shown the great ability of RA in promoting lung RE formation, even when given in much diluted concentration. Compared with a single dose, multiple treatments of VARA produced a cumulative effect on RE storage. The synergistic effect of VARA was not significantly affected by inflammation. These results together with our findings of the localization of retinoid homeostatic proteins provide a better understanding of retinoid uptake, accumulation and metabolism in the neonatal lung. Our findings also suggest a promising therapeutic approach in clinical use for a rapid restoration of lung VA in preterm or VLBW infants to promote normal lung maturation and prevent these infants from developing respiratory diseases.