Molecular and cellular characterization of alveolar epithelium

UnrestrictedAlveolar epithelium is comprised of two morphologically distinct cell types, type I (AT1) and type II (AT2) cells, each expressing specific differentiation-related genes (e.g., aquaporin-5 (AQP5) and surfactant protein C (SP-C)). The mechanisms involved in transcriptional regulation of these cell-specific genes were investigated in vivo and in vitro. In the first study, hypertonic induction of AQP5 was demonstrated to be transcriptionally regulated and mediated by hypoxia-inducible factor (HIF 1), indicating a novel role for HIF-1 in modulating cellular adaptive responses to osmotic stress. In the second study, GATA-6 activation of AQP5 transcription was shown to be mediated through cooperative interactions with Sp1. Increased expression of GATA-6 concurrent with upregulation of AQP5 and transition to an AT1-like phenotype in AEC primary culture suggest that temporal and spatial regulation of GATA-6 contributes to activation of AQP5 transcription through protein-protein interaction with Sp1.; In the third study, Foxp2 was shown to inhibit transcriptional activation of SP-C by Nkx2.1 by preventing interaction of Nkx2.1 with its cognate DNA binding site. However, binding of FoxP2 to its cognate site was also demonstrated by EMSA and chromatin immunoprecipitation. Together with reciprocal changes in expression of FoxP2 and Nkx2.1 concurrent with downregulation of SP-C during transition of AT2 cells to AT1-like cells, these studies indicate that repression of SP-C by FoxP2 during AEC transdifferentiation occurs both in a DNA-dependent and independent fashion. In the last study, transgenic animals were generated harboring a 4.3-kb AQP5 promoter linked to an EGFP reporter. High levels of transgene expression were detected in acinar cells of salivary gland, but only low levels were present in AT1 cells in lung, indicating that 1) AQP5 transcription is differentially regulated in salivary gland and lung, and 2) additional upstream or intronic cis-active elements are likely required for high level expression in lung. In summary, these studies suggest that temporal and spatial regulation of transcription factors and their recruitment to the proximal promoter in a particular combination may be an important mechanism mediating transcriptional regulation of cell-specific gene expression in AT1 cells