ABSTRACT
Glucocorticoids have been shown to impair lung growth by altering development of the alveolar structure. To characterize the mechanisms involved in this process, we examined the effects of dexamethasone on proliferation of the stem cells of the alveolar epithelium, the type 2 cells. Treatment of type 2 cells with dexamethasone rapidly decreased DNA synthesis, and this effect was observed for concentrations less than 10(-8)M. Inhibition of cell proliferation by glucocorticoids was associated with a marked accumulation of insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) in the culture medium. Studies of the mechanisms involved in this accumulation indicated that it was associated with an enhanced production of IGFBP-2 and with a similar increase in the level of IGFBP-2 messenger RNA expression without any changes in its stability, as evaluated by actinomycin D experiments. Furthermore, transfection studies using plasmids conveying expression of luciferase gene transcribed from the fragment of rat IBFBP-2 promoter extending from +12 bp relative to the start of transcription plus 1.4 kilobases of the 5'-flanking sequence showed a stimulation of luciferase activity in cells treated with dexamethasone that was similar to the increase in IGFBP-2 messenger RNA and protein. Study of the other components of the IGF system also revealed induction of IGF-II expression upon treatment with dexamethasone. Together with other previously reported results using various modulators of type 2 cell proliferation, the present study strongly suggests that IGFBP-2 is likely to play an important role in the control of alveolar epithelial cell proliferation.
Subject(s)
Dexamethasone/pharmacology , Insulin-Like Growth Factor Binding Protein 2/biosynthesis , Pulmonary Alveoli/cytology , Pulmonary Alveoli/metabolism , Animals , Cell Cycle/drug effects , Dexamethasone/antagonists & inhibitors , Epithelial Cells , Epithelium/metabolism , Fibroblasts/metabolism , Insulin-Like Growth Factor Binding Protein 2/genetics , Insulin-Like Growth Factor II/metabolism , Mifepristone/pharmacology , RNA, Messenger/metabolism , Rats , Receptors, Somatomedin/metabolism , Thymidine/pharmacokinetics , Transcription, Genetic/drug effectsABSTRACT
Pulmonary alveolar type 2 cells act as a reservoir of stem cells which can be induced to proliferate during periods of lung growth and repair after lung injury. Despite the importance of this process, the mechanisms that regulate type 2 cell proliferation have not been well characterized. We show in this study that insulin-like growth factor (IGF)-binding protein-2 (IGFBP-2) accumulates to high levels in culture medium of growth-arrested type 2 cells. This is associated with an increased expression of IGFBP-2 messenger RNA (mRNA). Study of the other components of the IGF system also reveals induction of IGF-II and type 2 IGF receptor mRNA during the process of type 2 cell block of proliferation. When growth-arrested cells are allowed to resume proliferation by the addition of serum, the level of expression of IGFBP-2, type 2 IGF receptor, and IGF-II rapidly decreased. Despite the similarities in the timing of induction, it is likely that these components are not necessarily linked to mediate effects through a single pathway. Indeed, we show that the addition of conditioned medium from growth-arrested cells on proliferative cells results in down-regulation of IGFBP-2 and increased expression of IGF-II and type 2 IGF receptor mRNA. Treatment of the cells with various concentrations of IGF-II affects only the level of expression of type 2 IGF receptor, whereas IGF-I and insulin appear to influence only the expression of IGFBP-2. From the results presented in this study, it can be suggested that IGFBP-2, IGF-II, and type 2 IGF receptor play an important role in the transition of lung alveolar epithelial cells in and out of the cell cycle.
