Respiratory failure due to differentiation arrest and expansion of alveolar cells following lung-specific loss of the transcription factor C/EBPalpha in mice.

The leucine zipper family transcription factor CCAAT enhancer binding protein alpha (C/EBPalpha) inhibits proliferation and promotes differentiation in various cell types. In this study, we show, using a lung-specific conditional mouse model of C/EBPalpha deletion, that loss of C/EBPalpha in the respiratory epithelium leads to respiratory failure at birth due to an arrest in the type II alveolar cell differentiation program. This differentiation arrest results in the lack of type I alveolar cells and differentiated surfactant-secreting type II alveolar cells. In addition to showing a block in type II cell differentiation, the neonatal lungs display increased numbers of proliferating cells and decreased numbers of apoptotic cells, leading to epithelial expansion and loss of airspace. Consistent with the phenotype observed, genes associated with alveolar maturation, survival, and proliferation were differentially expressed. Taken together, these results identify C/EBPalpha as a master regulator of airway epithelial maturation and suggest that the loss of C/EBPalpha could also be an important event in the multistep process of lung tumorigenesis. Furthermore, this study indicates that exploring the C/EBPalpha pathway might have therapeutic benefits for patients with respiratory distress syndromes.

Metabolic response of mice to a postnatal ablation of CCAAT/enhancer-binding protein alpha.

Although CCAAT/enhancer-binding protein alpha (C/EBPalpha) is essential for initiating or sustaining several metabolic processes during the perinatal period, the consequences of total ablation of C/EBPalpha during postnatal development have not been investigated. We have created a conditional knock-out model in which the administration of poly(I:C) caused a virtually total deletion of c/ebpalpha (C/EBPalpha(Delta/-) mice) in the liver, spleen, white and brown adipose tissues, pancreas, lung, and kidney of the mice. C/EBPalpha itself was completely ablated in the liver by day 4 after the injection of poly(I:C). There was no noticeable change in phenotype during the first 15 days after the injection. The mice maintained a normal level of fasting blood glucose and responded to the diabetogenic action of streptozotocin. From day 16 onward, the mice developed hypophagia, exhibited severe weight loss, lost triglyceride in white but not brown adipose tissue, became hypoglycemic and hypoinsulinemic, depleted their hepatic glycogen, and developed fatty liver. They also exhibited lowered plasma levels of free fatty acid, triglyceride, and cholesterol, as well as marked changes in hepatic mRNA for C/EBPdelta, peroxisome proliferator-activated receptor alpha, sterol regulatory element-binding protein 1, hydroxymethylglutaryl-coenzyme A reductase, and apolipoproteins. Although basal levels of hepatic mRNA for the cytosolic isoform of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase were reduced, transcription of the genes for these enzymes was inducible by dibutyryl cyclic AMP in C/EBPalpha(Delta/-) mice. The animals died about 1 month after the injection of poly(I:C). These findings demonstrate that C/EBPalpha is essential for the survival of animals during postnatal life and that its ablation leads to distinct biphasic change in metabolic processes.

Distinctive and indispensable roles of PU.1 in maintenance of hematopoietic stem cells and their differentiation.

The PU.1 transcription factor is a key regulator of hematopoietic development, but its role at each hematopoietic stage remains unclear. In particular, the expression of PU.1 in hematopoietic stem cells (HSCs) could simply represent "priming" of genes related to downstream myelolymphoid lineages. By using a conditional PU.1 knock-out model, we here show that HSCs express PU.1, and its constitutive expression is necessary for maintenance of the HSC pool in the bone marrow. Bone marrow HSCs disrupted with PU.1 in situ could not maintain hematopoiesis and were outcompeted by normal HSCs. PU.1-deficient HSCs also failed to generate the earliest myeloid and lymphoid progenitors. PU.1 disruption in granulocyte/monocyte-committed progenitors blocked their maturation but not proliferation, resulting in myeloblast colony formation. PU.1 disruption in common lymphoid progenitors, however, did not prevent their B-cell maturation. In vivo disruption of PU.1 in mature B cells by the CD19-Cre locus did not affect B-cell maturation, and PU.1-deficient mature B cells displayed normal proliferation in response to mitogenic signals including the cross-linking of surface immunoglobulin M (IgM). Thus, PU.1 plays indispensable and distinct roles in hematopoietic development through supporting HSC self-renewal as well as commitment and maturation of myeloid and lymphoid lineages.

Enhancement of hematopoietic stem cell repopulating capacity and self-renewal in the absence of the transcription factor C/EBP alpha.

The transcription factor C/EBP alpha is required for granulopoiesis and frequently disrupted in human acute myeloid leukemia (AML). Here, we show disruption of C/EBP alpha blocks the transition from the common myeloid to the granulocyte/monocyte progenitor but is not required beyond this stage for terminal granulocyte maturation. C/EBP alpha-deficient hematopoietic stem cells (HSCs) have increased expression of Bmi-1 and enhanced competitive repopulating activity. Bone marrow in adult C/EBP alpha-deficient mice was filled with myeloblasts, similar to human AML, supporting the notion that disruption of C/EBP alpha cooperates with other events in the development of leukemia. Therefore, C/EBP alpha is not only essential for granulocyte development but, in addition, is a regulator of hematopoietic stem cell activity.