DMS-79 is a human cell line derived from a small cell lung carcinoma (SCLC), which expresses the pro-opiomelanocortin (POMC) gene. We took it as a model in which to study the mechanism of POMC gene expression in these tumors: precursor processing is altered and gene expression is essentially unresponsive to glucocorticoids. POMC gene structure appeared normal by Southern blot analysis, indicating that gene rearrangement was not responsible for its expression in DMS-79. Indeed, using transient expression of human POMC-luciferase fusion genes in DMS-79, we showed that (1) the normal human POMC promoter was functional in DMS-79, and (2) the same proximal promoter region (-417; + 21) produced the full transcriptional activity in DMS-79 and in the mouse pituitary cell line AtT-20. Progressive 5' deletion analysis revealed differences between AtT-20 and DMS-79: region (-611; -376) was active in AtT-20 and not in DMS-79, whereas region (-95; -161) was active in both cell lines and (-376; -417) was only active in DMS-79. The latter partially overlaps a motif homologous to the DE-2 rat element which confers the tissue-specific expression of POMC in AtT-20 cells; however, this motif had no effect in DMS-79. These data suggest that POMC gene transcription is achieved through a different set of transacting factors in DMS-79 and AtT-20. Altogether, our results provide evidence that DMS-79 is a valid model of tumors responsible for the ectopic ACTH syndrome and that the mechanism of POMC gene expression in these SCLC cells is different from that in pituitary cells.

The small cell lung carcinoma (SCLC) cell line DMS-79 has been used as a model for studying the molecular mechanism underlying the ectopic ACTH syndrome. We previously showed that two domains of the human Proopiomelanocortin (POMC) gene promoter were speci®cally active in DMS-79 cells. The present study focuses on the more distal one, Domain IV (7376/ 7417). DNaseI footprinting experiments identi®ed a single binding site for DMS-79 cell proteins in this domain. Gel-shift and sequence analysis indicated that E2F proteins might bind this site. Indeed, proteins from DMS-79 cells which bind this site (i) have in vitro DNA binding properties indistinguishable from those of E2F proteins (ii) form, like E2F proteins, multiprotein complexes which can be dissociated by sodium deoxycholate and (iii) are recognized by antibodies directed against E2F proteins. Further, we show that the rat POMC distal promoter domain contains a homologous sequence which constitutes a natural mutant of the human POMC E2F binding site, since it does not bind E2F. We show by transient transfection that this natural mutant, in the context of the rat POMC promoter, is not active in DMS-79 cells by contrast to the human POMC E2F binding site. We conclude that E2F binding is required for the activity of Domain IV in DMS-79 cells and contributes to the expression of the POMC gene in SCLC. Further studies are required to elucidate the role of E2F factors in POMC gene transcription in SCLC cells, but our results have identi®ed mechanisms dierent from those in pituitary corticotroph cells that are used by these SCLC tumor cells.