Transgenic glucocorticoid receptor expression driven by the SP-C promoter reduces neonatal lung cellularity and midkine expression in GRhypo mice

Glucocorticoid Signaling in Health and Disease: Insights From Tissue-Specific GR Knockout Mice

Glucocorticoids are adrenally produced hormones critically involved in development, general physiology, and control of inflammation. Since their discovery, glucocorticoids have been widely used to treat a variety of inflammatory conditions. However, high doses or prolonged use leads to a number of side effects throughout the body, which preclude their clinical utility. The primary actions of glucocorticoids are mediated by the glucocorticoid receptor (GR), a transcription factor that regulates many complex signaling pathways. Although GR is nearly ubiquitous throughout the body, glucocorticoids exhibit cell- and tissue-specific effects. For example, glucocorticoids stimulate glucose production in the liver, reduce glucose uptake in the skeletal muscle, and decrease insulin secretion from the pancreatic β-cells. Mouse models represent an important approach to understanding the dynamic functions of GR signaling in normal physiology, disease, and resistance. In the absence of a viable GR null model, gene-targeting techniques utilizing promoter-driven recombination have provided an opportunity to characterize the tissue-specific actions of GR. The aim of the present review is to describe the organ systems in which GR has been conditionally deleted and summarize the functions ascribed to glucocorticoid action in those tissues.

Glucocorticoid regulation of lung development: lessons learned from conditional GR knockout mice

Glucocorticoid (GC) steroid hormones have well-characterized roles in the regulation of systemic homeostasis, yet less understood is their known role in utero to mature the developing respiratory system in preparation for birth. During late gestation, endogenously produced GCs thin the interstitial tissue of the lung, causing the vasculature and future airspaces to come into close alignment, allowing for efficient gas exchange at birth. More potent synthetic GCs are also used worldwide to reduce the severity of respiratory distress suffered by preterm infants; however, their clinical benefits are somewhat offset by potential detrimental long-term effects on health and development. Here, we review the recent literature studying both global and conditional gene-targeted respiratory mouse models of either GC deficiency or glucocorticoid receptor ablation. Although some discrepancies exist between these transgenic mouse strains, these models have revealed specific roles for GCs in particular tissue compartments of the developing lung and identify the mesenchyme as the critical site for glucocorticoid receptor-mediated lung maturation, particularly for the inhibition of cell proliferation and epithelial cell differentiation. Specific mesenchymal and epithelial cell-expressed gene targets that may potentially mediate the effect of GCs have also been identified in these studies and imply a GC-regulated system of cross talk between compartments during lung development. A better understanding of the specific roles of GCs in specific cell types and compartments of the fetal lung will allow the development of a new generation of selective GC ligands, enabling better therapeutic treatments with fewer side effects for lung immaturity at birth in preterm infants.

Reduced viability of mice with lung epithelial-specific knockout of glucocorticoid receptor

Glucocorticoid (GC)-responsive epithelial-mesenchymal interactions regulate lung development. The GC receptor (GR) mediates GC signaling. Mice lacking GR in all tissues die at birth of respiratory failure. To determine the specific need for epithelial GR in lung development, we bred triple transgenic mice that carry SPC/rtTA, tet-O-Cre, and floxed, but not wild-type, GR genes. When exposed to doxycycline in utero, triple transgenic (GRepi⁻) mice exhibit a Cre-mediated recombination event that inactivates the floxed GR gene in airway epithelial cells. Immunofluorescence confirmed the elimination of GR in Cre-positive airway epithelial cells of late gestation GRepi⁻ mice. Embryonic Day 18.5 pups had a relatively immature appearance with increased lung cellularity and increased pools of glycogen in the epithelium. Postnatal Day 0.5 pups had decreased viability. We used quantitative RT-PCR to demonstrate that specific elimination of epithelial immunoreactive GR in GRepi⁻ mice is associated with reduced mRNA expression for surfactant proteins (SPs) A, B, C, and D; β- and γ-ENaC; T1α; the 10-kD Clara cell protein (CCSP); and aquaporin 5 (AQP5). Western blots confirmed reduced levels of AQP5 protein. No reduction in the levels of the GR transport protein importin (IPO)-13 was observed. Our findings demonstrate a requirement for lung epithelial cell GR in normal lung development. We speculate that impaired epithelial differentiation, leading to decreased SPs, transepithelial Na, and liquid absorption at birth, may contribute to the reduced survival of newborn mice with suppressed lung epithelial GR.

Identification of glucocorticoid-regulated genes that control cell proliferation during murine respiratory development

Glucocorticoids play a vital role in fetal respiratory development and act via the intracellular glucocorticoid receptor (GR) to regulate transcription of key target genes. GR-null mice die at birth due to respiratory dysfunction associated with hypercellularity and atelectasis. To identify events associated with this lung phenotype we examined perinatal cellular proliferation rates and apoptotic indices. We demonstrate that compared to wild-type controls, day 18.5 postcoitum (p.c.) GR-null mouse lungs display significantly increased cell proliferation rates (1.8-fold P < 0.05) and no change in apoptosis. To examine underlying molecular mechanisms, we compared whole genome expression profiles by microarray analysis at 18.5 days p.c. Pathways relating to cell proliferation, division and cell cycle were significantly down-regulated while pathways relating to carbohydrate metabolism, kinase activities and immune responses were significantly up-regulated. Differential levels of gene expression were verified by quantitative-RT-PCR and/or Northern analysis. Key regulators of proliferation differentially expressed in the lung of 18.5 p.c. GR-null lungs included p21 CIP1 (decreased 2.9-fold, P < 0.05), a negative regulator of the cell cycle, and Mdk (increased 6.0-fold, P < 0.05), a lung growth factor. The more under-expressed genes in 18.5 p.c. GR-null lungs included Chi3l3 (11-fold, P < 0.05), a macrophage inflammatory response gene and Ela1 (9.4-fold, P < 0.05), an extracellular matrix remodeling enzyme. Our results demonstrate that GR affects the transcriptional status of a number of regulatory processes during late fetal lung development. Amongst these processes is cell proliferation whereby GR induces expression of cell cycle repressors while suppressing induction of a well characterized cell cycle stimulator.