The human retinoblastoma gene (RB1) encodes a protein (Rb) of 105 kilodaltons that can be phosphorylated. Analysis of Rb metabolism has shown that the protein has a half-life of more than 10 hours and is synthesized at all phases of the cell cycle. Newly synthesized Rb is not extensively phosphorylated (it is "underphosphorylated") in cells in the G0 and G1 phases but is phosphorylated at multiple sites at the G1/S boundary and in S phase. HL-60 cells that were induced to terminally differentiate by various chemicals lost their ability to phosphorylate newly synthesized Rb at multiple sites when cell growth was arrested. These findings suggest that underphosphorylated Rb may restrict cell proliferation.

A new source of stem cells has recently been isolated from amniotic fluid; these amniotic fluid stem cells have significant potential for regenerative medicine. These cells are multipotent, showing the ability to differentiate into cell types from each embryonic germ layer. We investigated the ability of human amniotic fluid stem cells (hAFSC) to integrate into murine lung and to differentiate into pulmonary lineages after injury. Using microinjection into cultured mouse embryonic lungs, hAFSC can integrate into the epithelium and express the early human differentiation marker thyroid transcription factor 1 (TTF1). In adult nude mice, following hyperoxia injury, tail vein-injected hAFSC localized in the distal lung and expressed both TTF1 and the type II pneumocyte marker surfactant protein C. Specific damage of Clara cells through naphthalene injury produced integration and differentiation of hAFSC at the bronchioalveolar and bronchial positions with expression of the specific Clara cell 10-kDa protein. These results illustrate the plasticity of hAFSC to respond in different ways to different types of lung damage by expressing specific alveolar versus bronchiolar epithelial cell lineage markers, depending on the type of injury to recipient lung.

Apoptosis is a genetically controlled cellular response to developmental stimuli and environmental insult that culminates in cell death. Sublethal hyperoxic injury in rodents is characterized by a complex but reproducible pattern of lung injury and repair during which the alveolar surface is damaged, denuded, and finally repopulated by type 2 alveolar epithelial cells (AEC2). Postulating that apoptosis might occur in AEC2 after hyperoxic injury, we looked for the hallmarks of apoptosis in AEC2 from hyperoxic rats. A pattern of increased DNA end labeling, DNA laddering, and induction of p53, p21, and Bax proteins, strongly suggestive of apoptosis, was seen in AEC2 cultured from hyperoxic rats when compared with control AEC2. In contrast, significant apoptosis was not detected in freshly isolated AEC2 from oxygen-treated rats. Thus the basal culture conditions appeared to be insufficient to ensure the ex vivo survival of AEC2 damaged in vivo. The oxygen-induced DNA strand breaks were blocked by the addition of 20 ng/ml of keratinocyte growth factor (KGF) to the culture medium from the time of plating and were partly inhibited by Matrigel or a soluble extract of Matrigel. KGF treatment resulted in a partial reduction in the expression of the p21, p53, and Bax proteins but had no effect on DNA laddering. We conclude that sublethal doses of oxygen in vivo cause damage to AEC2, resulting in apoptosis in ex vivo culture, and that KGF can reduce the oxygen-induced DNA damage. We speculate that KGF plays a role as a survival factor in AEC2 by limiting apoptosis in the lung after acute hyperoxic injury.

Alveolar epithelial type 2 cells (AEC2) isolated from hyperoxia-treated animals exhibit increases in both proliferation and DNA damage in response to culture. AEC2 express the zonula adherens proteins E-cadherin, -, - and -catenin, desmoglein, and pp120, as demonstrated by Western blotting. Immunohistochemical analysis of cultured AEC2 showed expression of E-cadherin on cytoplasmic membranes varying from strongly to weakly staining. When cultured AEC2 placed in suspension were labeled with fluorescent-tagged antibodies to E-cadherin, cells could be sorted into at least two subpopulations, either dim or brightly staining for this marker. With the use of antibody to E-cadherin bound to magnetic beads, cells were physically separated into E-cadherin-positive and -negative subpopulations, which were then analyzed for differences in proliferation and DNA damage. The E-cadherin-positive subpopulation contained the majority of damaged cells, was quiescent, and expressed low levels of telomerase activity, whereas the E-cadherin-negative subpopulation was undamaged, proliferative, and expressed high levels of telomerase activity.