Differential Regulation of Glucocorticoid Receptor Expression by Ligand in Fetal Rat Lung Cells

Abstract

The glucocorticoid receptor (GR) mediates glucocorticoid stimulation of surfactant production by fetal mammalian lung. In many other tissues, glucocorticoids decrease expression of GR, thereby reducing responsiveness to these hormones. We therefore determined whether there is a similar effect of exogenous glucocorticoids on GR in fetal rat whole lung, and in the principal cell types involved in the stimulation of surfactant, the fibroblasts and the epithelial cells. The ontogeny of GR in late gestation lung differed between the two cell types, with maximal levels occurring in fibroblasts on gestational d 19, and on d 20 in epithelial cells. Administration of dexamethasone (1 mg/kg) to the mother on gestational d 18 or 19 (term = 22 d) increased specific GR binding activity in whole lung 24 h later. Furthermore, in vitro, incubation of cultured fibroblasts of gestational d 20 with 10K7 M cortisol increased GR immunoreactive protein and binding activity in a dose-and time-dependent manner, without affecting cellular levels of GR mRNA. However, identical treatment of d 20 distal airway epithelial cells was followed by decreased GR protein without significant change in cellular GR mRNA. Surfactant protein-A protein levels, taken as assessments of lung maturation, were increased in response to the same treatment. Our findings suggest that hormonal regulation of GR in fetal lung cells occurs at a posttranscriptional level, and is cell-specific. In the context of substantial increases in circulating glucocorticoid concentrations during late gestation, these findings may be of physiologic importance to the biochemical maturation of the antenatal lung. (Pediatr Res 38: [506][507][508][509][510][511][512] 1995) Abbreviations GR, glucocorticoid receptor FBS, fetal bovine serum FPF, fibroblast-pneumocyte factor MEM, minimal essential medium SP-A, surfactant protein-A Glucocorticoids accelerate the onset of mature levels of surfactant synthesis by fetal mammalian lung (1-3) via the classic GR mechanism (1, 2, 4-9). However, in many tissues, glucocorticoids decrease GR expression, thereby decreasing tissue responsiveness to glucocorticoid stimulation. The effect of glucocorticoids on GR in late gestation fetal lung around the time of the onset of augmented surfactant production is incompletely understood.Administration of glucocorticoids decreases GR mRNA (10) and GR protein (1 1) in several adult rat organs, including lung. In various cell lines, glucocorticoids lower GR number by decreasing GR transcription rates without altering affinity for ligand (12,13). However, it is recognized that the response of GR mRNA steady-state levels to glucocorticoid treatment can

Mechanical strain and dexamethasone selectively increase surfactant protein C and tropoelastin gene expression

“…To assess the molecular mechanism by which physical force and glucocorticoids control tropoelastin and SP-C mRNA expression, we first inhibited transcription with actinomysin D. Because this treatment blocked mechanical strain-induced SP-C mRNA expression as well as mechanical strain-and/or dexamethasone-induced tropoelastin mRNA expression, we examined transcriptional regulation by determining the tropoelastin or SP-C hnRNA levels. Although this strategy does not directly measure the transcriptional activity, hnRNA levels reflect the rate of active, ongoing transcription (25). Compared with the commonly used nuclear runoff assays, the hnRNA approach is more sensitive and requires less material.…”

“…Thus it appears that the upregulation in SP-C and tropoelastin mRNA expression by mechanical strain and/or dexamethasone is controlled primarily at the level of tran- scription. To confirm that mechanical strain and/or dexamethasone treatment triggers transcriptional activation of the SP-C and tropoelastin genes, hnRNAs of SP-C and tropoelastin were analyzed by RT-PCR followed by Southern hybridization (25). In principle, hnRNA (or preprocessed mRNA) is a complete copy of a DNA template, which contains both introns and exons.…”

Antenatal steroids to prevent respiratory distress syndrome,multiple gestation as an effect modifier

“…Animal experiments using dose ranges of 0.1 mg/ kg to 2 mg/kg of betamethasone have shown a dose response curve in fetal sheep with no response at the low dose but equal mean responses from 0.5 mg/kg (23). Indeed Sweezey et al have suggested that a critical dose exists at which the glucocorticoid receptor mechanism is activated in the fetal lung (24). Our efforts should be directed at achieving that level in our extreme preterm population and multiple gestation infants.…”

Relationships between IGF-I, cortisol, and osteocalcin in peripheral plasma of growing pigs

“…The striking similarity between the cortisol pattern and the other two parameters may be due to their suggested role for differentiation of cells and thus the expression of mature cell function in several tissues (e.g. lung: Sweezey et al, 1995;small intestine: Sangild et al, 1995). A role of cortisol for osteoblast differentiation has been demonstrated in vitro (Pirskanen et al, 1993) and is reflected in our in vivo study by the osteocalcin pattern.…”