¹Institute of Cancer Research, University of Vienna, Borschkegasse 8a, A-1090 Wien, Austria

²Microbiology and Tumor Biology Center, Karolinska Instituet, Box 280, SE-171 77 Stockholm, Sweden

³Department of Plant Biology, Uppsala Genetic Center, Box 7080, Swedish University of Agricultural Sciences, SE-750 07 Uppsala, Sweden

⁴Abteilung für Biochemie und Molekularbiologie, Institut für Biochemie, RWTH, Pauwelstrasse 30, D-52074 Aachen, Germany

Correspondence to: C Cerni, Institute of Cancer Research, University of Vienna, Borschkegasse 8a, A-1090 Wien, Austria, E-mail: Christa.Cerni@univie.ac.at or M Henriksson, Microbiology and Tumor Biology Center, Karolinska Instituet, Box 280, SE-171 77 Stockholm, Sweden, E-mail: Marie.Henriksson@mtc.ki.se

The Myc/Max/Mad network of transcriptional regulatory proteins plays an essential role in cell proliferation, growth, apoptosis, and differentiation. Whereas Myc proteins affect cell cycle progression positively, Mad proteins are negative regulators of cell proliferation. It has been shown in several in vitro systems that Mad proteins antagonize c-Myc functions. In this report we describe the inhibition of tumor cell outgrowth in vivo by Mad1 expression. Transformed cell lines were generated by co-transfection of c-myc, c-H-ras, and a chimeric mad1ER construct into primary rat embryo cells (MRMad1ER cells). Activation of Mad1 by 4-Hydroxy-Tamoxifen (OHT) resulted in abrogation of telomerase activity, reduced cloning efficiency, and decreased proportion of cells in S phase. Injection of MRMad1ER cells into syngenic rats induced aggressively growing tumors after a short latency period. This tumor growth was inhibited by OHT-treatment of animals, with the extent of inhibition correlating with the amount of OHT injected. No effect of OHT on tumor growth was observed with similarly transformed Myc/Ras cell lines which did not express Mad1ER. These data demonstrate that Mad1 is able to suppress Myc/Ras-mediated transformation under in vivo conditions.

Oncogene (2002) 21, 447-459 DOI: 10.1038/sj/onc/1205107

Mad1ER; Myc; cell cycle; gene repression; telomerase activity; tumor growth