REGULATION OF CCSP (PCB-BP UTEROGLOBIN) EXPRESSION IN PRIMARY CULTURES OF LUNG-CELLS - INVOLVEMENT OF C/EBP/

The Clara-cell secretory protein (CCSP) is a cell-specific differentia tion marker for the bronchiolar Clara cell. Isolated rat Clara and alv eolar type 2 cells kept in primary culture proliferate and dedifferent iate, providing the opportunity to study differentiation-dependent mec hanisms. In freshly isolated Clara cells, high levels of CCSP and the corresponding mRNA were detected. During culture in vitro, these level s decreased. In the type 2 cell fraction, low levels of CCSP were dete cted, which decreased further during culture. A promoter fragment of t he rat CCSP gene encompassing the sequence from -188 to +53 was able t o drive high-level expression of reporter genes in transfected Clara c ells. Reporter gene expression in transfected type 2 cells was markedl y lower, and no expression could be detected in alveolar macrophages. Expression of transcription factors previously described to stimulate CCSP expression appeared not to parallel CCSP levels in the primary Cl ara cells. However, expression of the transcription factor C/EBP alpha correlated with the CCSP expression pattern. In electrophoretic mobil ity shift assays, we were able to demonstrate binding of C/EBP alpha f rom rat Clara cell nuclear extracts to an element located 85 bp upstre am of the start site of transcription. Overexpression of C/EBP alpha i ncreased expression from the CCSP -188 promoter fragment up to fivefol d in NCI-H441-cells and 30-fold in A549-cells, establishing the functi onal importance of C/EBP alpha. Our results show that primary cultures of Clara cells constitute a useful model for investigating terminal a irway differentiation and suggest a role for C/EBP-factor(s) in this p rocess.