Decreased tumorigenicity of c-Myc-transformed fibroblasts expressing active USF2.
USF is a small family of basic helix-loop-helix leucine zipper (bHLH-zip) transcription factors with DNA binding specificities similar to that of the c-Myc oncoprotein. Evidence for a role of USF in growth control includes inhibition of c-Myc-dependent cellular transformation in vitro and loss of USF transcriptional activity in many cancer cell lines. However, a direct effect of USF on the tumorigenicity of an established cell line has never been demonstrated. Here, cell lines derived from rat embryo fibroblasts transformed by c-Ha-Ras and either c-Myc or E1A were used as model system to investigate the tumor suppression ability of USF. Overexpression of USF2 stimulated transcription and inhibited colony formation in c-Myc-transformed, but not E1A-transformed, fibroblasts. Stable clones expressing high USF2 levels were constructed from c-Myc-transformed fibroblasts. In two of these clones, overexpressed USF2 did not activate transcription, and there was no significant change in the transformed phenotype. In contrast, a clone that expressed transcriptionally active USF2 exhibited altered morphology and a strongly decreased ability to proliferate in semisolid medium. The ability of these cells to form tumors in nude mice was also decreased by a factor of more than 30 as compared to the parental cell line or cells overexpressing transcriptionally inactive USF2. Cotransfection assays with USF- or Myc-specific dominant-negative mutants indicated that active USF2 inhibited cellular transformation by preventing transcriptional repression by c-Myc.
Choe
Chungyoul
C
Department of Molecular Genetics, The University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA.
Chen
Nanyue
N
Sawadogo
Michèle
M
D000818:Animals; D002455:Cell Division / Q000235:genetics; D002461:Cell Line, Transformed; D002471:Cell Transformation, Neoplastic / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000096:biosynthesis* / Q000235:genetics; D015536:Down-Regulation / Q000235:genetics*; D005260:Female; D005347:Fibroblasts / Q000166:cytology / Q000378:metabolism*; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics; D005809:Genes, Regulator / Q000235:genetics; D017930:Genes, Reporter / Q000235:genetics; D051379:Mice; D008819:Mice, Nude; D010641:Phenotype; D011401:Promoter Regions, Genetic / Q000235:genetics; D017434:Protein Structure, Tertiary / Q000235:genetics; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D051381:Rats; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D014157:Transcription Factors / Q000096:biosynthesis* / Q000235:genetics; D014162:Transfection; D025521:Tumor Suppressor Proteins / Q000096:biosynthesis* / Q000235:genetics; D051720:Upstream Stimulatory Factors
D004268:DNA-Binding Proteins; D016271:Proto-Oncogene Proteins c-myc; D012097:Repressor Proteins; D014157:Transcription Factors; D025521:Tumor Suppressor Proteins; D051720:Upstream Stimulatory Factors; C497183:Usf1 protein, mouse; C497182:Usf1 protein, rat; C497186:Usf2 protein, mouse; C497187:Usf2 protein, rat