Differential activity of conditional MYC and its variant MYC-S in human mortal fibroblasts.
We have explored the effects of the conditional MYC-estrogen receptor fusion protein, MYC-ERTM, in human mortal fibroblasts, WI38, on cell-cycle entry, apoptosis and gene expression. The results indicate that activation of MYC-ERTM in WI38 cells is sufficient to cause S phase entry of quiescent cells, which is preceded by phosphorylation of Rb and activation of the Cdk2-associated kinase. We also analysed the MYC protein variant, MYC-S, which lacks part of the transcriptional activation domain but includes the conserved MYC box II and 26 amino acids N-terminal to it. MYC-S was previously shown to promote proliferation and apoptosis of immortalized rodent cell lines. The results indicate that MYC-S has undetectable activity as an inducer of S phase or apoptosis of quiescent WI38 cells. However, Myc-S stimulates proliferation of WI38 cells in the presence of 10% fetal calf serum. Surprisingly, we found that MYC-S, previously considered solely a repressor of specific reporter genes, is instead a weak transactivator of endogenous target genes both in mortal and immortalized cells. In addition, MYC-S exhibit a weak repressor activity upon an endogenous target gene only in immortalized cells. MYC-S transcriptional properties suggest that MYC box II and the adjacent N-terminal amino acids, while not sufficient for full repression function, participate in transactivation of endogenous target genes.
Hirst
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Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, Washington, WA 98109, USA.
Grandori
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D042846:CDC2-CDC28 Kinases; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases / Q000378:metabolism; D005347:Fibroblasts / Q000166:cytology*; D005786:Gene Expression Regulation; D014644:Genetic Variation; D018257:Helix-Loop-Helix Motifs; D006801:Humans; D016350:Leucine Zippers; D017346:Protein Serine-Threonine Kinases / Q000378:metabolism; D016271:Proto-Oncogene Proteins c-myc / Q000235:genetics / Q000378:metabolism*; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D016160:Retinoblastoma Protein / Q000378:metabolism; D016196:S Phase; D015534:Trans-Activators / Q000235:genetics / Q000378:metabolism*
D016271:Proto-Oncogene Proteins c-myc; D011960:Receptors, Estrogen; D011993:Recombinant Fusion Proteins; D016160:Retinoblastoma Protein; D015534:Trans-Activators; D017346:Protein Serine-Threonine Kinases; D042846:CDC2-CDC28 Kinases; C495894:CDK2 protein, human; D051357:Cyclin-Dependent Kinase 2; D018844:Cyclin-Dependent Kinases