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J. Biol. Chem., Vol. 281, Issue 27, 18668-18676, July 7, 2006

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A Requirement for Dimerization of HP1^Hs in Suppression of Breast Cancer Invasion^*

Laura E. Norwood¹, Timothy J. Moss¹, Naira V. Margaryan, Sara L. Cook, Lindsay Wright, Elisabeth A. Seftor, Mary J. C. Hendrix, Dawn A. Kirschmann, and Lori L. Wallrath²

From the Department of Biochemistry, University of Iowa, Iowa City, Iowa 52242 and the Children's Memorial Research Center, Robert H. Lurie Comprehensive Cancer Center, Feinberg School of Medicine at Northwestern University, Chicago, Illinois 60611

The development and progression of cancer is controlled by gene expression, often regulated through chromatin packaging. Heterochromatin protein 1^Hs (HP1^Hs), one of three human HP1 family members, participates in heterochromatin formation and gene regulation. HP1^Hs possesses an amino-terminal chromodomain, which binds methylated lysine 9 of histone H3 (meK9 H3), and a carboxyl-terminal chromoshadow domain (CSD) that is required for dimerization and interaction with partner proteins. HP1^Hs is down-regulated in invasive metastatic breast cancer cells compared with poorly invasive nonmetastatic breast cancer cells. Expression of EGFP-HP1^Hs in highly invasive MDA-MB-231 cells causes a reduction in in vitro invasion, without affecting cell growth. Conversely, knock-down of HP1^Hs levels in the poorly invasive breast cancer cell line MCF-7 increased invasion, without affecting cell growth. To determine whether functions of the CSD were required for the regulation of invasion, mutant forms of HP1^Hs were expressed in MDA-MB-231 cells. A W174A mutation that disrupts interactions between HP1^Hs and PXVXL-containing partner proteins reduced invasion similar to that of the wild type protein. In contrast, an I165E mutation that disrupts dimerization of HP1^Hs did not decrease invasion. No gross changes in localization and abundance of HP1^Hs, HP1^Hs, and meK9 H3 were observed upon expression of wild type and mutant forms of HP1^Hs in MDA-MB-231 cells. Taken together, these data demonstrate that modulation of HP1^Hs alters the invasive potential of breast cancer cells through mechanisms requiring HP1 dimerization, but not interactions with PXVXL-containing proteins.

Received for publication, November 21, 2005 , and in revised form, April 20, 2006.

^* This work was supported by Grant DAMD17-02-1-0424 from the Dept. of Defense Breast Cancer Research Program (to L. L. W. and D. A. K.), Susan G. Komen Dissertation Research Award DISS0403121 (to L. E. N.), and the Order of the Eastern Star Breast Cancer Research Fund (to M. J. C. H.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ These authors contributed equally to this work.

² To whom correspondence should be addressed: Dept. of Biochemistry, The University of Iowa, 3136 Medical Education Research Facility (MERF), Iowa City, IA 52242. Tel.: 319-335-7920; Fax: 319-384-4770; E-mail: lori-wallrath{at}uiowa.edu.

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