The Six1 homeoprotein stimulates tumorigenesis by reactivation of cyclin A1

doi:10.1073/pnas.0401139101

The Six1 homeoprotein stimulates tumorigenesis by reactivation of cyclin A1

Departments of ^*Obstetrics and Gynecology and ^‡Biochemistry and Molecular Genetics, University Colorado Health Sciences Center, Denver, CO 80262; ^†Discipline of Pathology, University of Campinas Dental School, 13414-018 Piracicaba-SP, Brazil; ^§Howard Hughes Medical Institute and Whitehead Institute/Massachusetts Institute of Technology Center for Genome Research, One Kendall Square, Cambridge, MA 02139; ^¶Department of Cancer Biology and ^††Pediatric Oncology, Dana–Farber Cancer Institute and Harvard Medical School, Boston, MA 02115; ^**Department of Medicine A, Hematology and Oncology, University of Münster, 48129 Münster, Germany; and ^‡‡Division of Biology, Center for Molecular Medicine, Jichi Medical School, Tochigi 329-0498, Japan

Edited by Arthur B. Pardee, Dana–Farber Cancer Institute, Boston, MA (received for review February 17, 2004)

Abstract

Homeobox genes constitute a large family of transcription factors that are essential during normal development and are often dysregulated in cancer. However, the molecular mechanisms by which homeobox genes influence cancer remain largely unknown. Here we show that the tissue-restricted cyclin A1 is a transcriptional target of the Six1 homeoprotein. Both genes are expressed in the embryonic but not the terminally differentiated mammary gland, and Six1-knockout mice show a dramatic reduction of cyclin A1 in the embryonic mammary gland. In addition, both genes are reexpressed in breast cancers. Six1 overexpression increases cyclin A1 mRNA levels and activity, cell proliferation, and tumor volume, whereas Six1 down-regulation decreases cyclin A1 mRNA levels and proliferation. Overexpression of Six1 in wild-type mouse embryonic fibroblasts, but not in knockout variants lacking the cyclin A1 gene, induces cell proliferation. Furthermore, inhibition of cyclin A1 in Six1-overexpressing mammary carcinoma cells decreases proliferation. Together these results demonstrate that cyclin A1 is required for the proliferative effect of Six1. We conclude that Six1 overexpression reinstates an embryonic pathway of proliferation in breast cancer by up-regulating cyclin A1.

Footnotes

↵ §§ To whom correspondence should be addressed. E-mail: heide.ford{at}uchsc.edu.
↵ ∥ Present address: Dyax Corp., 300 Technology Square, Cambridge, MA 01239.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: Ad-X, adenovirus containing gene for protein X; MEF, mouse embryonic fibroblast; siRNA, short interfering RNA; qRT-PCR, quantitative real-time RT-PCR; ChIP, chromatin immunoprecipitation.