No abstract

Tumor necrosis factor-alpha receptor knockout (TNF-alphaRKO) mice have homozygous deletions of the genes that code for both the 55- and 75-kD receptors. The mice are protected from the fibrogenic effects of bleomycin, silica, and inhaled asbestos. The asbestos-exposed animals exhibit reduced expression of other peptide growth factors such as transforming growth factor (TGF)-alpha, platelet-derived growth factors, and TGF-beta. In normal animals, these and other cytokines are elaborated at high levels during the development of fibroproliferative lung disease, but there is little information available that has allowed investigators to establish the role of the individual growth factors in disease pathogenesis. Here, we show that overexpression of TGF-beta(1) by means of a replication-deficient adenovirus vector induces fibrogenesis in the lungs of the fibrogenic-resistant TNF-alphaRKO mice. The fibrogenic lesions developed in both the KO and background controls within 7 d, and both types of animals exhibited similar incorporation of bromodeoxyuridine. Interestingly, airway epithelial cell proliferation appeared to be suppressed, perhaps due to the presence of the TGF-beta(1), a well-known inhibitor of epithelial mitogenesis. Before these experiments, there was no information available that would provide a basis for predicting whether or not TGF-beta(1) expression induces fibroproliferative lung disease in fibrogenic-resistant TNF-alphaRKO mice, an increasingly popular animal model.

Cell cycle proteins are important regulators of diverse cell fate decisions, and in this capacity have pivotal roles in neurogenesis and brain development. The mechanisms by which cell cycle regulation is integrated with cell fate control in the brain and other tissues are poorly understood, and an outstanding question is whether the cell cycle machinery regulates fate decisions directly or instead as a secondary consequence of proliferative control. Identification of the genes targeted by E2 promoter binding factor (E2f) transcription factors, effectors of the pRb/E2f cell cycle pathway, will provide essential insights into these mechanisms. We identified the promoter regions bound by three neurogenic E2f factors in neural precursor cells in a genome-wide manner. Through bioinformatic analyses and integration of published genomic data sets we uncovered hundreds of transcriptionally active E2f-bound promoters corresponding to genes that control cell fate processes, including key transcriptional regulators and members of the Notch, fibroblast growth factor, Wnt and Tgf-β signaling pathways. We also demonstrate a striking enrichment of the CCCTC binding factor transcription factor (Ctcf) at E2f3-bound nervous system-related genes, suggesting a potential regulatory co-factor for E2f3 in controlling differentiation. Finally, we provide the first demonstration of extensive tissue specificity among E2f target genes in mammalian cells, whereby E2f3 promoter binding is well conserved between neural and muscle precursors at genes associated with cell cycle processes, but is tissue-specific at differentiation-associated genes. Our findings implicate the cell cycle pathway as a widespread regulator of cell fate genes, and suggest that E2f3 proteins control cell typespecific differentiation programs by regulating unique sets of target genes. This work significantly enhances our understanding of how the cell cycle machinery impacts cell fate and differentiation, and will importantly drive further discovery regarding the mechanisms of cell fate control and transcriptional regulation in the brain, as well as in other tissues.