Transcription Factor Klf4, Induced in the Lung by Oxygen at Birth, Regulates Perinatal Fibroblast and Myofibroblast Differentiation

Abstract

The fluid-filled lung exists in relative hypoxia in utero (∼25 mm Hg), but at birth fills with ambient air where the partial pressure of oxygen is ∼150 mm Hg. The impact of this change was studied in mouse lung with microarrays to analyze gene expression one day before, and 2, 6, 12 and 24 hours after birth into room air or 10% O2. The expression levels of >150 genes, representing transcriptional regulation, structure, apoptosis and antioxidants were altered 2 hrs after birth in room air but blunted or absent with birth in 10% O2. Kruppel-like factor 4 (Klf4), a regulator of cell growth arrest and differentiation, was the most significantly altered lung gene at birth. Its protein product was expressed in fibroblasts and airway epithelial cells. Klf4 mRNA was induced in lung fibroblasts exposed to hyperoxia and constitutive expression of Klf4 mRNA in Klf4-null fibroblasts induced mRNAs for p21cip1/Waf1, smooth muscle actin, type 1 collagen, fibronectin and tenascin C. In Klf4 perinatal null lung, p21cip1/Waf1mRNA expression was deficient prior to birth and associated with ongoing cell proliferation after birth; connective tissue gene expression was deficient around birth and smooth muscle actin protein expression was absent from myofibroblasts at tips of developing alveoli; p53, p21cip1/Waf1 and caspase-3 protein expression were widespread at birth suggesting excess apoptosis compared to normal lung. We propose that the changing oxygen environment at birth acts as a physiologic signal to induce lung Klf4 mRNA expression, which then regulates proliferation and apoptosis in fibroblasts and airway epithelial cells, and connective tissue gene expression and myofibroblast differentiation at the tips of developing alveoli.

Perinatal Lung Development: The Lung at Birth

“…It was as if birth into an environment of hypoxia lacked any change in gene expression at all and retained a pattern like that of the fetal lung. The transient rise and fall in glutathione disulfide in the newborn lung in room air, despite the induction of several antioxidants in the late fetal lung and the lung at birth which buffered some but not all such stress [56][57][58][59], supports our hypothesis that a measured degree of redox stress, a birth shock, may serve a function in the lung at birth which we surmise is signalling expression of lung genes required for the onset of postnatal lung development [55].…”

“…With this background in mind, our finding that lung Klf4 mRNA was dramatically upregulated with birth in a normal oxygen environment and attenuation of this induction by birth in a more hypoxic environment led us to hypothesize that Klf4 was regulated in a redoxresponsive fashion in lung mesenchymal cells and that its target genes regulated fibroblast proliferation and differentiation events associated with postnatal lung development [55].…”

Sialyltransferase7A, a Klf4-responsive gene, promotes cardiomyocyte apoptosis during myocardial infarction

“…In the hypoxic lung or endothelial cell, Klf4 expression is decreased [10,17]. Conversely, hypoxic stimulus increases Klf4 expression in human embryonic stem cells and astrocytes [30,35].…”

KLF4 regulates adult lung tumor-initiating cells and represses K-Ras-mediated lung cancer

“…6,11,12 In normal lung tissue, KLF4 is expressed in fibroblasts and airway epithelial cells, and was found to be the most significantly altered lung gene at birth. 13 KLF4 is downregulated in gastrointestinal cancers and has been identified as a tumor suppressor in many types of cancer. [14][15][16] As one of the four factors that induce pluripotent stem cells, KLF4 has a role in cell fate reprogramming and selfrenewal of embryonic stem cells.…”