A. Picon et al., Analysis of the human proopiomelanocortin gene promoter in a small cell lung carcinoma cell line reveals an unusual role for E2F transcription factors, ONCOGENE, 18(16), 1999, pp. 2627-2633
The small cell lung carcinoma (SCLC) cell line DMS-79 has been used as a mo
del for studying the molecular mechanism underlying the ectopic ACTH syndro
me, We previously showed that two domains of the human Proopiomelanocortin
(POMC) gene promoter were specifically active in DMS-79 cells. The present
study focuses on the more distal one, Domain IV (-376/-417), DNaseI footpri
nting experiments identified a single binding site for DMS-79 cell proteins
in this domain, Gel-shift and sequence analysis indicated that E2F protein
s 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 antib
odies directed against E2F proteins. Further, we show that the rat POMC dis
tal promoter domain contains a homologous sequence which constitutes a natu
ral 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 t
he 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 rol
e of E2F factors in POMC gene transcription in SCLC cells, but our results
have identified mechanisms different from those in pituitary corticotroph c
ells that are used by these SCLC tumor cells.