Biphasic glucocorticoid regulation of pulmonary SP-A: characterization of inhibitory process

Abstract

Pulmonary surfactant, which is necessary for normal lung function, is under both developmental and hormonal regulation. Glucocorticoids induce all components of surfactant and have a unique biphasic effect on surfactant protein A (SP-A), either stimulating or inhibiting accumulation in cultured fetal lung depending on dose and time of exposure. In this study we further characterized glucocorticoid inhibition of SP-A in cultured explants of human fetal lung. Decreased content of SP-A mRNA was the dominant response to dexamethasone added either early or later during culture. Inhibition occurred at < or = 1 nM dexamethasone on prolonged exposure, was blocked by RU 486, and was observed with other glucocorticoids but not sex steroids. When cortisol was removed from the culture medium, inhibition was rapidly reversed. The immediate inhibitory effect of 100 nM dexamethasone on SP-A mRNA content was completely blocked in the presence of cycloheximide. SP-A gene transcription, measured by nuclear elongation assay, was decreased by 60% after 4- to 8-h exposure to 100 nM dexamethasone. Stability of SP-A mRNA, determined both by addition of actinomycin D and by label-chase experiments, was transiently decreased immediately after adding dexamethasone (t1/2 approximately 3 h). In tissue treated with dexamethasone for > or = 8 h the stability of SP-A mRNA in control and treated explants was not different (t1/2 approximately 8 h). Our findings indicate that inhibition of SP-A is the dominant response to glucocorticoid. This effect is receptor mediated and apparently involves induction of a labile protein(s) that decreases gene transcription and transiently reduces mRNA stability.

Regulation of translation by upstream translation initiation codons of surfactant protein A1 splice variants

“…In fetal lung explants, high concentrations of dexamethasone decreased SP-A expression and the SP-A1/SP-A2 ratio (34,49). This is in accordance with the biphasic effects of dexamethasone as shown in fetal lung; this was shown to depend on the duration of treatment and concentration (30,33,51). Similarly to our results, dexamethasone has been shown to upregulate the mRNA of p27 (Kip1) (11,22).…”

Glucocorticoid inhibition of human SP-A1 promoter activity in NCI-H441 cells

“…Dexamethasone has been shown to decrease total SP-A transcription in both human fetal lung explants [13] and NCI-H441 cells [21]. In order to identify cis-acting elements that may mediate the transcriptional effects of dexamethasone, PCR fragments representing various deletions of the SP-A1 (6A% allele) promoter were cloned into the SalI site of the pCATenhancer vector.…”

“…In human fetal lung explants, the synthetic glucocorticoid dexamethasone increases SP-A mRNA levels at low concentrations ( 10 nM), but decreases SP-A mRNA and protein levels at higher concentrations (100 nM) [10][11][12][13][14][15]. Moreover, the degree of inhibition of SP-A1 and SP-A2 expression varies in explant cultures [16][17][18][19].…”

Glucocorticoid inhibition ofSP-Agene expression in lung type II cells is mediated via the TTF-1-binding element

“…The regulation of SP-A gene expression by glucocorticoids is complex and multifaceted, involves transcriptional and posttranscriptional effects (11,12,24,26), and may be mediated by direct and indirect actions of the steroid on type II cells. In type II cell transfection studies using rabbit (1) or human SP-A:hGH fusion gene constructs, we consistently observed an inhibitory effect of Dex on cAMP stimulation of SP-A promoter activity.…”