Transcription Factor ATF2 Cooperates with v-Jun To Promote Growth Factor-Independent Proliferation In Vitro and Tumor Formation In Vivo

ABSTRACT

ATF2 belongs to the bZIP family of transcription factors and controls gene expression via 8-bp ATF/CREB motifs either as a homodimer or as a heterodimer—for instance, with Jun—but has never been shown to be directly involved in oncogenesis. Experiments were designed to evaluate a possible role of ATF2 in oncogenesis in chick embryo fibroblasts (CEFs) in the presence or absence of v-Jun. We found that (i) forced expression of ATF2 cannot alone cause transformation, (ii) overexpression of ATF2 plus v-Jun specifically stimulates v-Jun-induced growth in medium with a reduced amount of serum, and (iii) the efficiency of low-serum growth correlates with the activity of a Jun-ATF2-dependent model promoter in stably transformed CEFs. Analysis of ATF2 and Jun dimerization mutants showed that the growth-stimulatory effect of ATF2 is likely to be mediated by v-Jun–ATF2 heterodimers since (i) v-Jun-m1, a mutant with enhanced affinity for ATF2, induces growth in low-serum medium much more efficiently than v-Jun, when expressed alone or in combination with ATF2; and (ii) ATF2/fos, a mutant that efficiently binds to v-Jun but is unable to form stable homodimers, shows enhanced oncogenic cooperation with v-Jun. In addition, we examined the role of ATF2 in tumor formation by subcutaneous injection of CEFs into chickens. In contrast to v-Jun, v-Jun-m1 gave rise to numerous fibrosarcomas while coexpression of ATF2 and v-Jun-m1 led to a dramatic development of fibrosarcomas visible within 1 week. Together these data demonstrate that overexpressed ATF2 potentiates the ability of v-Jun-transformed CEFs to grow in low-serum medium in vitro and contributes to the formation of tumors in vivo.

ACKNOWLEDGMENTS

This work was supported by fellowships from the Association pour la Recherche sur le Cancer (to S.H.) and from the Royal Netherlands Academy of Arts and Sciences (to H.V.D.). This work was also funded by grants from the Association pour la Recherche sur le Cancer, the Ligue contre le Cancer, and the Mutuelle Générale de l’Education Nationale (to M.C.), as well as from the Dutch Cancer Society and the Training and Mobility of Researchers Program of the European Community (to H.V.D.).

We warmly acknowledge Peter Herrlich, Peter Angel, and Alex van der Eb for their continuing interest and support of this work. We also thank Christophe Geourjon for advice in designing the chimeric protein ATF2/fos and Michael Rau and Edmund Derrington for critical reading of the manuscript. We are grateful to Suzy Markossian and Armelle Roisin for help with DNA sequencing and to Djamel Belgarbi for taking care of the animals.