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Open Access Research article

Mammary gland specific expression of Brk/PTK6 promotes delayed involution and tumor formation associated with activation of p38 MAPK

Kristopher A Lofgren123, Julie H Ostrander13, Daniel Housa134, Gregory K Hubbard13, Alessia Locatelli13, Robin L Bliss3, Kathryn L Schwertfeger235 and Carol A Lange1236*

Author Affiliations

1 Department of Medicine (Division of Hematology, Oncology, and Transplantation), University of Minnesota, 420 Delaware St. SE, MMC 806, Minneapolis, MN 55455, USA

2 Microbiology, Immunology and Cancer Biology Graduate Program, University of Minnesota, 420 Delaware St. SE, MMC 196, Minneapolis, MN 55455, USA

3 Masonic Cancer Center, University of Minnesota, 425 E. River Road, Minneapolis, MN 55455, USA

4 Department of Pathology, Third Faculty of Medicine, Ruska 87, 100 00 Praha 10, Czech Republic

5 Department of Lab Medicine and Pathology, University of Minnesota, 420 Delaware St SE, MMC 609, Minneapolis, MN 55455, USA

6 Department of Pharmacology, University of Minnesota, 321 Church St SE, Minneapolis, MN, 55455, USA

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Breast Cancer Research 2011, 13:R89  doi:10.1186/bcr2946

Published: 17 September 2011

Abstract

Introduction

Protein tyrosine kinases (PTKs) are frequently overexpressed and/or activated in human malignancies, and regulate cancer cell proliferation, cellular survival, and migration. As such, they have become promising molecular targets for new therapies. The non-receptor PTK termed breast tumor kinase (Brk/PTK6) is overexpressed in approximately 86% of human breast tumors. The role of Brk in breast pathology is unclear.

Methods

We expressed a WAP-driven Brk/PTK6 transgene in FVB/n mice, and analyzed mammary glands from wild-type (wt) and transgenic mice after forced weaning. Western blotting and immunohistochemistry (IHC) studies were conducted to visualize markers of mammary gland involution, cell proliferation and apoptosis, as well as Brk, STAT3, and activated p38 mitogen-activated protein kinase (MAPK) in mammary tissues and tumors from WAP-Brk mice. Human (HMEC) or mouse (HC11) mammary epithelial cells were stably or transiently transfected with Brk cDNA to assay p38 MAPK signaling and cell survival in suspension or in response to chemotherapeutic agents.

Results

Brk-transgenic dams exhibited delayed mammary gland involution and aged mice developed infrequent tumors with reduced latency relative to wt mice. Consistent with delayed involution, mammary glands of transgenic animals displayed decreased STAT3 phosphorylation, a marker of early-stage involution. Notably, p38 MAPK, a pro-survival signaling mediator downstream of Brk, was activated in mammary glands of Brk transgenic relative to wt mice. Brk-dependent signaling to p38 MAPK was recapitulated by Brk overexpression in the HC11 murine mammary epithelial cell (MEC) line and human MEC, while Brk knock-down in breast cancer cells blocked EGF-stimulated p38 signaling. Additionally, human or mouse MECs expressing Brk exhibited increased anchorage-independent survival and resistance to doxorubicin. Finally, breast tumor biopsies were subjected to IHC analysis for co-expression of Brk and phospho-p38 MAPK; ductal and lobular carcinomas expressing Brk were significantly more likely to express elevated phospho-p38 MAPK.

Conclusions

These studies illustrate that forced expression of Brk/PTK6 in non-transformed mammary epithelial cells mediates p38 MAPK phosphorylation and promotes increased cellular survival, delayed involution, and latent tumor formation. Brk expression in human breast tumors may contribute to progression by inducing p38-driven pro-survival signaling pathways.