Abstract
Androgen receptor (AR) is commonly expressed in both the epithelium of normal mammary glands and in breast cancers. AR expression in breast cancers is independent of estrogen receptor alpha (ERα) status and is frequently associated with overexpression of the ERBB2 oncogene. AR signaling effects on breast cancer progression may depend on ERα and ERBB2 status. Up to 30% of human breast cancers are driven by overactive ERBB2 signaling and it is not clear whether AR expression affects any steps of tumor progression in this cohort of patients. To test this, we generated mammary specific Ar depleted mice (MARKO) by combining the floxed allele of Ar with the MMTV-cre transgene on an MMTV-NeuNT background and compared them to littermate MMTV-NeuNT, Arfl/+ control females. Heterozygous MARKO females displayed reduced levels of AR in mammary glands with mosaic AR expression in ductal epithelium. The loss of AR dramatically accelerated the onset of MMTV-NeuNT tumors in female MARKO mice. In this report we show that accelerated MMTV-NeuNT-dependent tumorigenesis is due specifically to the loss of AR, as hormonal levels, estrogen and progesterone receptors expression, and MMTV-NeuNT expression were similar between MARKO and control groups. MMTV-NeuNT induced tumors in both cohorts displayed distinct loss of AR in addition to ERα, PR, and the pioneer factor FOXA1. Erbb3 mRNA levels were significantly elevated in tumors in comparison to normal mammary glands. Thus the loss of AR in mouse mammary epithelium accelerates malignant transformation rather than the rate of tumorigenesis.
Highlights
The majority of breast cancers originate in epithelial cells that line the ducts of the mammary glands
MMTV-Cre induced recombination of the Arfl allele resulting in deletion of exon 2 was confirmed by PCR analysis of MARKO genomic DNA isolated from partially dissociated mammary glands (Figure 1B)
We counted Androgen receptor (AR) positive luminal epithelial cells (Figure 1C) and found that the percentage of cells positively stained for AR in MARCO (,32%) mammary glands was significantly lower than in controls (,68%) (p = 0.0019)
Summary
The majority of breast cancers originate in epithelial cells that line the ducts of the mammary glands. ERBB2 is part of a complex network comprised of four receptor tyrosine kinases (RTK), ERBB1-4 They can be bound by a variety of peptide hormones which cause receptors to homo- and/or heterodimerize and become active. In human breast cancer cell lines with amplified ERBB2 expression, depletion of ERBB3 reduces cell proliferation to the same extent as depletion of ERBB2, while loss of ERBB1 (epidermal growth factor receptor (EGFR)) does not affect proliferation [4]. The activated form of ERBB3 was detected in human breast cancers with amplified ERBB2 expression [4]. Multiple mouse models overexpressing ERBB2 in mammary glands have been established, all of which lead to development of mammary tumors [5,6,7]. As noted in human cancers, elevated expression of activated ERBB3 has been detected in transgenic mice overexpressing activated ERBB2 [10]
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