Regulation of expression of the gene encoding the major surfactant protein (SP-A) in human fetal lung in vitro. Disparate effects of glucocorticoids on transcription and on mRNA stability.
We previously observed that dexamethasone had a biphasic effect on the levels of mRNA encoding the major surfactant protein (SP-A) in human fetal lung in vitro; at concentrations of 10(-10) and 10(-9) M, dexamethasone caused an increase in the levels of SP-A mRNA, whereas at concentrations of greater than 10(-8) M, the steroid had a pronounced inhibitory effect on SP-A mRNA levels. It was also found that dexamethasone antagonized the stimulatory effect of dibutyryl cyclic AMP (Bt2cAMP) on SP-A mRNA levels in human fetal lung in vitro. It was our objective, in the present study, to characterize further the effects of dexamethasone and Bt2cAMP on SP-A mRNA levels in human fetal lung tissue and to determine whether such effects are associated with comparable changes in the transcriptional activity of the SP-A gene. We found that the action of dexamethasone (10(-7) M) to reduce the levels of SP-A mRNA in control and Bt2cAMP-treated fetal lung explants was evident within 2 h of its addition to the culture medium; SP-A mRNA was reduced to barely detectable levels in control and in Bt2cAMP-treated tissues after 24 h of dexamethasone treatment. The action of dexamethasone to reduce SP-A mRNA levels was not prevented by co-incubation with either actinomycin D or cycloheximide. In contrast to its dose-related biphasic effects on the levels of SP-A mRNA, we found that dexamethasone caused a dose-dependent stimulation of SP-A gene transcription. Bt2cAMP also increased the transcriptional activity of the SP-A gene in the human fetal lung in vitro. In fetal lung explants incubated in the presence of dexamethasone plus Bt2cAMP, a synergistic induction of SP-A gene transcription was observed at concentrations of dexamethasone of 10(-9)-10(-7) M. Our findings are indicative that the stimulatory effects of dexamethasone (10(-10)-10(-9) M) on SP-A mRNA levels are reflective of a stimulatory effect of the steroid on SP-A gene transcription, whereas the inhibitory effects of dexamethasone (10(-7) M) on SP-A mRNA levels are the result of a dominant effect of the steroid in elevated concentrations to reduce SP-A mRNA stability.
Boggaram
V
V
Department of Biochemistry, Cecil H. and Ida Green Center for Reproductive Biology Sciences, University of Texas Southwestern Medical Center, Dallas 75235.
Smith
M E
ME
Mendelson
C R
CR
D003994:Bucladesine / Q000494:pharmacology; D003513:Cycloheximide / Q000494:pharmacology; D003609:Dactinomycin / Q000494:pharmacology; D003907:Dexamethasone / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004357:Drug Synergism; D005786:Gene Expression Regulation / Q000187:drug effects*; D006023:Glycoproteins / Q000235:genetics; D006801:Humans; D007700:Kinetics; D008168:Lung / Q000196:embryology* / Q000378:metabolism; D011510:Proteolipids / Q000235:genetics*; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism*; D014158:Transcription, Genetic / Q000187:drug effects*
D006023:Glycoproteins; D011510:Proteolipids; D037662:Pulmonary Surfactant-Associated Protein A; D037661:Pulmonary Surfactant-Associated Proteins; D011663:Pulmonary Surfactants; D012333:RNA, Messenger; D003609:Dactinomycin; D003994:Bucladesine; D003907:Dexamethasone; D003513:Cycloheximide