Protein tyrosine kinase 6 regulates mammary gland tumorigenesis in mouse models

Abstract

Protein tyrosine kinase 6 (PTK6, also called BRK) is an intracellular tyrosine kinase expressed in the majority of human breast tumors and breast cancer cell lines, but its expression has not been reported in normal mammary gland. To study functions of PTK6 in vivo, we generated and characterized several transgenic mouse lines with expression of human PTK6 under control of the mouse mammary tumor virus (MMTV) long terminal repeat. Ectopic active PTK6 was detected in luminal epithelial cells of mature transgenic mammary glands. Lines expressing the MMTV-PTK6 transgene exhibited more than a two-fold increase in mammary gland tumor formation compared with nontransgenic control animals. PTK6 activates signal transducer and activator of transcription 3 (STAT3), and active STAT3 was detected in PTK6-positive mammary gland epithelial cells. Endogenous mouse PTK6 was not detected in the normal mouse mammary gland, but it was induced in mouse mammary gland tumors of different origin, including spontaneous tumors that developed in control mice, and tumors that formed in PTK6, H-Ras, ERBB2 and PyMT transgenic models. MMTV-PTK6 and MMTV-ERBB2 transgenic mice were crossed to explore crosstalk between PTK6 and ERBB2 signaling in vivo. We found no significant increase in tumor incidence, size or metastasis in ERBB2/PTK6 double transgenic mice. Although we detected increased proliferation in ERBB2/PTK6 double transgenic tumors, an increase in apoptosis was also observed. MMTV-PTK6 clearly promotes mammary gland tumorigenesis in vivo, but its impact may be underrepresented in our transgenic models because of induction of endogenous PTK6 expression.

PTK6/BRK is expressed in the normal mammary gland and activated at the plasma membrane in breast tumors

“…Recently, PTK6 was shown to be regulated by hypoxia-inducible factors 1α/2α and proposed to be a mediator of hypoxia-induced breast cancer progression [39]. Similar to findings in human breast cancer, we recently demonstrated that PTK6 is induced in mouse mammary gland tumors of different origins, including spontaneous tumors and ERBB2 (HER2) induced tumors [23]. …”

“…Without enrichment for epithelial cells, the epithelium may not have been well represented in surgical discard tissues that would also have included adipose, endothelial and connective tissue. While we have not detected Ptk6 protein or transcripts in the normal mouse mammary gland [23, 35], there may be distinctions in expression between species, related to lifespan and exposure to stress. PTK6 can be induced by stress and DNA damage in the intestinal epithelium and nontransformed cells where it promotes apoptosis [36-38].…”

“…In addition, PTK6 confers cancer cell resistance to anoikis [10, 21] and knockdown of PTK6 increases sensitivity of cancer cells to different therapeutic agents [13, 22]. Transgene expression of PTK6 in the mouse mammary gland enhances mammary gland tumorigenesis in vivo [23, 24]. …”

Cancer-Associated Mutations in Breast Tumor Kinase/PTK6 Differentially Affect Enzyme Activity and Substrate Recognition

“…Expression of Brk increases cell proliferation and promotes anchorage independent growth, 14 a measure of transforming potential. Brk also promotes tumorigenesis in the mouse mammary gland, 35 and decreases tumor latency. 21 Knock-down of Brk inhibits cell growth.…”

PTK6 (protein tyrosine kinase 6)

“…PTK6 protein expression has prognostic significance in breast cancer (Born et al, 2005;Aubele et al, 2009), and active PTK6 protein was detected in human breast tumors but not normal human mammary gland (Peng et al, 2014). Overexpression of PTK6 promotes mammary gland cancer tumorigenesis in mouse models (Lofgren et al, 2011;Peng et al, 2013). PTK6 sustains EGFR signaling via transactivation as well as by inhibition of EGFR degradation (Kang et al, 2010;Li et al, 2012).…”