EXPRESSION OF CYCLIN D1 MESSENGER-RNA IS NOT UP-REGULATED BY MYC IN RAT FIBROBLASTS

Conflicting results have been published regarding the regulation of cy clin D1 mRNA in rat fibroblasts expressing a hormone-regulated Myc pro tein, MycER. We confirm that activation of MycER with oestrogen rapidl y induces cyclin D1 mRNA, even in the presence of cycloheximide. Howev er, we show that this is an artefact resulting from an oestrogen-activ ated transcriptional activation domain in the oestrogen receptor part of the MycER chimaera. First, addition of 4-hydroxy-tamoxifen (4OHT), which does not activate this domain, allows association of MycER with Max and induces cell proliferation in serum-starved Rat-1-MycER cells without affecting cyclin D1 mRNA levels or the activity of D1 promoter -luciferase constructs. Second, Rat-1 cells expressing a mutant MycER with a hormone-binding domain that still binds 4OHT but no longer bind s oestrogen, are driven into the cell cycle in response to 4OHT but fa il to up-regulate cyclin D1 mRNA. Finally, Rat-1 cells in which wild-t ype human c-Myc expression can be induced, also progress into the cell cycle without increased D1 mRNA expression.