Inhibition of transferrin receptor 1 transcription by a cell density response element.
TfR1 (transferrin receptor 1) mediates the uptake of transferrin-bound iron and thereby plays a critical role in cellular iron metabolism. Its expression is coupled to cell proliferation/differentiation and controlled in response to iron levels and other signals by transcriptional and post-transcriptional mechanisms. It is well established that TfR1 levels decline when cultured cells reach a high density and in the present study we have investigated the underlying mechanisms. Consistent with previous findings, we demonstrate that TfR1 expression is attenuated in a cell-density-dependent manner in human lung cancer H1299 cells and in murine B6 fibroblasts as the result of a marked decrease in mRNA content. This response is not associated with alterations in the RNA-binding activity of iron regulatory proteins that are indicative of a transcriptional mechanism. Reporter assays reveal that the human TfR1 promoters contains sequences mediating cell-density-dependent transcriptional inhibition. Mapping of the human and mouse TfR1 promoters identified a conserved hexa-nucleotide 5'-GAGGGC-3' motif with notable sequence similarity to a previously described element within the IGF-2 (insulin-like growth factor-2) promoter. We show that this motif is necessary for the formation of specific complexes with nuclear extracts and for cell-density-dependent regulation in reporter gene assays. Thus the TfR1 promoter contains a functional 'cell density response element' (CDRE).
Wang
Jian
J
Lady Davis Institute for Medical Research, Sir Mortimer B. Davis Jewish General Hospital, 3755 Cote-Ste-Catherine Road, Montreal, Quebec, Canada H3T 1E2.
Chen
Guohua
G
Pantopoulos
Kostas
K
D000818:Animals; D015703:Antigens, CD / Q000235:genetics*; D001483:Base Sequence; D002452:Cell Count; D002460:Cell Line; D004247:DNA / Q000235:genetics / Q000378:metabolism; D005347:Fibroblasts / Q000166:cytology; D005786:Gene Expression Regulation / Q000235:genetics*; D006801:Humans; D035925:Iron-Regulatory Proteins / Q000378:metabolism; D008175:Lung Neoplasms / Q000473:pathology; D011401:Promoter Regions, Genetic / Q000235:genetics; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D011990:Receptors, Transferrin / Q000235:genetics*; D020218:Response Elements / Q000235:genetics*; D014158:Transcription, Genetic
D015703:Antigens, CD; C068126:CD71 antigen; D035925:Iron-Regulatory Proteins; D012333:RNA, Messenger; D011990:Receptors, Transferrin; C480029:Tfrc protein, mouse; D004247:DNA