DNA Topoisomerase I Is a Cofactor for c-Jun in the Regulation of Epidermal Growth Factor Receptor Expression and Cancer Cell Proliferation

Abstract

DNA topoisomerase I (Topo I) is a molecular target for the anticancer agent topotecan in the treatment of small cell lung cancer and ovarian carcinomas. However, the molecular mechanisms by which topotecan treatment inhibits cancer cell proliferation are unclear. We describe here the identification of Topo I as a novel endogenous interaction partner for transcription factor c-Jun. Reciprocal coimmunoprecipitation analysis showed that Topo I and c-Jun interact in transformed human cells in a manner that is dependent on JNK activity. c-Jun target gene epidermal growth factor receptor (EGFR) was identified as a novel gene whose expression was specifically inhibited by topotecan. Moreover, Topo I overexpression supported c-Jun-mediated reporter gene activation and both genetic and chemical inhibition of c-Jun converted cells resistant to topotecan-elicited EGFR downregulation. Topotecan-elicited suppression of proliferation was rescued by exogenously expressed EGFR. Furthermore, we demonstrate the cooperation of the JNK-c-Jun pathway, Topo I, and EGFR in the positive regulation of HT-1080 cell proliferation. Together, these results have identified transcriptional coactivator Topo I as a first endogenous cofactor for c-Jun in the regulation of cell proliferation. In addition, the results of the present study strongly suggest that inhibition of EGFR expression is a novel mechanism by which topotecan inhibits cell proliferation in cancer therapy.

ACKNOWLEDGMENTS

Dirk Bohmann, Johanna Ivaska, Panu Jaakkola, and Carsten Weiss are acknowledged for valuable comments on the manuscript. We also thank D. Bohmann for his contributions in the original identification of c-Jun complex. We thank P. D'Arpa, F. Barr, Y. Daaka, V.-M. Kähäri, S. Leppä, and P. Jaakkola for reagents. We thank also N. E. Fusenig and V.-M. Kähäri for the HaCat and NSF cell lines.

This project was supported by funding from the Academy of Finland (projects 53692 and 8212695), the Sigrid Juselius Foundation, the Turku University Foundation, and the Finnish Cancer Research Society.