Hormonal regulation and cellular localization of fatty acid synthase in human fetal lung

Abstract

Fatty acid synthase (FAS; EC 2.3.1.85 ) supplies de novo fatty acids for pulmonary surfactant synthesis, and FAS gene expression is both developmentally and hormonally regulated in the fetal lung. To further examine hormonal regulation of FAS mRNA and to determine the cellular localization of FAS gene expression, we cultured human fetal lungs (18–22 wk gestation) as explants for 1–4 days in the absence (control) or presence of glucocorticoid [dexamethasone (Dex), 10 nM] and/or cAMP agents (8-bromo-cAMP, 0.1 mM and IBMX, 0.1 mM). FAS protein content and activity increased similarly in the presence of Dex (109 and 83%, respectively) or cAMP (87 and 111%, respectively), and responses were additive in the presence of both hormones (230 and 203%, respectively). With a rabbit anti-rat FAS antibody, FAS immunoreactivity was not detected in preculture lung specimens but appeared in epithelial cells lining the tubules with time in culture. Dex and/or cAMP markedly increased staining of epithelial cells, identified as type II cells, whereas staining of mesenchymal fibroblasts was very low under all conditions. With in situ hybridization, FAS mRNA was found to be enriched in epithelial cells lining the alveolar spaces, and the reaction product increased in these cells when the explants were cultured with the hormones. The increased FAS mRNA content in the presence of Dex and/or cAMP is primarily due to increased stabilization of mRNA, although Dex alone increased the transcription rate by ∼30%. We conclude that hormonal treatment of cultured human fetal lungs increases FAS gene expression primarily by increasing stability of the message. The induction of FAS during explant culture and by hormones occurs selectively in type II epithelial cells, consistent with the regulatory role of this enzyme in de novo synthesis of fatty acid substrate for surfactant synthesis in perinatal lungs.

Differentiation of human pulmonary type II cells in vitro by glucocorticoid plus cAMP

“…Our findings that dexamethasone alone strongly induces SPs in lung explants (3), but not in isolated epithelial cells, highlight the critical role of cAMP. Previous studies with fetal lung explants found stimulatory effects of cAMP analogs (5) and synergism with glucocorticoids on phospholipid synthesis (27), formation of lamellar bodies (46), and induction of SPs (39,40) and fatty acid synthase (61). Our present studies in cultured cells indicate a more extensive role of cAMP than previously surmised from explant studies.…”

Up-Regulation of the Cardiac Lipid Metabolism at the Onset of Heart Failure

“…It is well established that catecholamines exert major metabolic effects, which could finally culminate in enhanced lipogenesis and lipid overload: Catecholamines induce inappropriate FFA use by triggering (i) an increase in serum levels of FFAs, (ii) wasteful cycling of FFAs through enhanced intramyocardial lipolysis, and (iii) FFA-mediated suppression of glucose metabolism [41]. Concomitantly, catecholamines increase the RNA stability of FASN [42], and trigger UCP1 gene expression via β-adrenergic receptor-mediated stimulation of cAMP [43]. Apart from over-activation of the adrenergic system, the angiotensin II AT1 receptor, another key player in the development of heart failure, also up-regulates the cardiac fat metabolism by increasing the expression of FASN [44, 45].…”

Identification of LBM180, a Lamellar Body Limiting Membrane Protein of Alveolar Type II Cells, as the ABC Transporter Protein ABCA3

“…Fetal lung parenchyma was minced into 1-mm 3 pieces and placed in organ culture, as described previously (40). Briefly, tissue pieces were distributed in two parallel strips on 60-mm culture dishes placed on a platform that rocks (3 oscillations/min) to expose the explants alternately to serum-free Waymouth medium (2 ml/dish) or an atmosphere of 95% air, 5% CO 2 .…”