Insulin utilizes the PI 3-kinase pathway to inhibit SP-A gene expression in lung epithelial cells.
BACKGROUND
It has been proposed that high insulin levels may cause delayed lung development in the fetuses of diabetic mothers. A key event in lung development is the production of adequate amounts of pulmonary surfactant. Insulin inhibits the expression of surfactant protein A (SP-A), the major surfactant-associated protein, in lung epithelial cells. In the present study, we investigated the signal transduction pathways involved in insulin inhibition of SP-A gene expression.


METHODS
H441 cells, a human lung adenocarcinoma cell line, or human fetal lung explants were incubated with or without insulin. Transcription run-on assays were used to determine SP-A gene transcription rates. Northern blot analysis was used to examine the effect of various signal transduction inhibitors on SP-A gene expression. Immunoblot analysis was used to evaluate the levels and phosphorylation states of signal transduction protein kinases.


RESULTS
Insulin decreased SP-A gene transcription in human lung epithelial cells within 1 hour. Insulin did not affect p44/42 mitogen-activated protein kinase (MAPK) phosphorylation and the insulin inhibition of SP-A mRNA levels was not affected by PD98059, an inhibitor of the p44/42 MAPK pathway. In contrast, insulin increased p70 S6 kinase Thr389 phosphorylation within 15 minutes. Wortmannin or LY294002, both inhibitors of phosphatidylinositol 3-kinase (PI 3-kinase), or rapamycin, an inhibitor of the activation of p70 S6 kinase, a downstream effector in the PI 3-kinase pathway, abolished or attenuated the insulin-induced inhibition of SP-A mRNA levels.


CONCLUSIONS
Insulin inhibition of SP-A gene expression in lung epithelial cells probably occurs via the rapamycin-sensitive PI 3-kinase signaling pathway.
Miakotina
Olga L
OL
Department of Anatomy and Cell Biology, College of Medicine, University of Iowa, Iowa City, Iowa 52242-1109, USA. olga-miakotina@uiowa.edu
Goss
Kelli L
KL
Snyder
Jeanne M
JM
D045744:Cell Line, Tumor; D002478:Cells, Cultured; D004847:Epithelial Cells / Q000187:drug effects / Q000378:metabolism; D005333:Fetus; D005786:Gene Expression Regulation / Q000187:drug effects / Q000502:physiology; D006801:Humans; D007328:Insulin / Q000494:pharmacology*; D008168:Lung / Q000166:cytology / Q000187:drug effects / Q000201:enzymology*; D019869:Phosphatidylinositol 3-Kinases / Q000235:genetics / Q000502:physiology*; D037662:Pulmonary Surfactant-Associated Protein A / Q000037:antagonists & inhibitors* / Q000096:biosynthesis* / Q000235:genetics; D020545:Respiratory Mucosa / Q000187:drug effects / Q000201:enzymology*; D015398:Signal Transduction / Q000187:drug effects / Q000502:physiology*
D007328:Insulin; D037662:Pulmonary Surfactant-Associated Protein A; D019869:Phosphatidylinositol 3-Kinases