Postnatal rat lung retinoic acid receptor (RAR) mRNA expression and effects of dexamethasone on RAR β mRNA

Abstract

Retinoids exert multiple effects upon lung differentiation and growth. Although the mechanisms involved are presently poorly understood, increasing evidence points to a central role of nuclear retinoic acid receptors (RAR). The purpose of this study was to determine RAR mRNA expression profile during postnatal alveolarization, compared with the expression in prenatal and adult rat lung, and to describe the effects of dexamethasone (DEX) and oxygen on postnatal lung RAR gene expression. Total RNA was isolated from lungs of Sprague-Dawley rats on prenatal day 19, on postnatal days 1, 3, 7, 10, and 14 of life, and from adults. One subgroup of littermate pups was treated with DEX daily for 3 or 7 days. In a second experiment, rats were exposed to room air or to 95% oxygen for 72 hours, and received either DEX or saline. Northern hybridization showed that the levels of all RAR subtypes in fetal lung were 45% or less of levels at postnatal day 1. The 3.7 kb RAR alpha transcript levels were lower than day 1 on days 10 and 14 (relative to day 1, day 10 = 0.54 +/- 0.05; day 14 = 0.54 +/- 0.08), but there was no change in a 2.7 kb RAR alpha transcript over this time period. By contrast, RAR beta mRNA levels were significantly higher at days 3, 10, and 14 compared with day 1 (day 3 = 1.79 +/- 0.19; day 10 = 1.41 +/- 0.14; day 14 = 1.53 +/- 0.05). Similarly, RAR gamma mRNA expression levels were higher on day 10 (1.45 +/- 0.09), but by day 14 there was no difference from day 1. Adult lung 3.7 kb RAR alpha, 2.7 kb RAR alpha, and RAR gamma were lower than day 1, but RAR beta was significantly greater (3.7 alpha = 0.52 +/- 0.05; 2.7 alpha = 0.49 +/- 0.26; gamma = 0.74 +/- 0.06; beta = 1.63 +/- 0.22). Treatment with DEX prevented the rise in RAR beta mRNA occurring on day 3 and significantly lowered (0.65 +/- 0.06) the amount of RAR beta mRNA in day 7 lung. Exposure of rat pups to oxygen caused an increase in RAR beta mRNA (1.21 +/- 0.03). DEX treatment again decreased RAR beta mRNA in both control (0.55 +/- 0.06) and oxygen-exposed pups (0.67 +/- 0.12). In addition, 2.7 kb RAR alpha mRNA was decreased with the combination of DEX and oxygen exposure (0.63 +/- 0.06). The differential gene expression profiles and the response to DEX and oxygen of the various members of the RAR family suggest that each subtype may have a specific role during the period of alveolarization in rat lung.