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The human surface antigen CD24 is over-expressed in small-cell lung cancer. Here we describe the isolation, sequencing and functional characterization of the 5'-flanking region of the human CD24 gene. A sequence (accession number: Y14692) of 3.4 kb regulates the activity of a luciferase reporter gene in CD24-expressing small-cell-lung-cancer cell lines up to 1.6-fold more than the control SV40 promoter and enhancer. In contrast, little or no luciferase activity was detected in 4 non-small-cell-lung-cancer cell lines. A deletion fragment of 269 bp had maximal activity in small-cell-lung-cancer cell lines (15- to 20-fold higher than control), while activity remained 2- to 10-fold below control in non-small-cell-lung-cancer cell lines. We conclude that the CD24 promoter has a strong and cell-type-specific activity, and propose its exploitation to drive the expression of therapeutic genes in small-cell lung cancer.

Human CD24 is a highly glycosylated glycosylphosphatidylinositol-linked (GPI-linked) cell surface protein. GPI-linked proteins are involved in signal transduction mediated by members of the protein tyrosine kinase (PTK) family. Therefore we studied associated molecules providing the signaling capacity of CD24. Lysates of SW2 and K562 cells were analysed for expression of PTK of the c-src family by Western blotting. We identified c-fgr in SW2 lysates and c-fgr and also lyn in K562 lysates. To study a putative association of these PTK with CD24 we performed immunoprecipitations with the mAb SWA11 directed against CD24. Western analysis of the precipitates showed an association of c-fgr with CD24 in SW2 cells and lyn in K562 cells. We conclude that either c-fgr or lyn is physically associated with CD24 in a cell-type depending manner. An involvement of these complexes in signaling phenomenons of CD24 in small cell lung cancer and in leukaemias is discussed.