Stromal Platelet-Derived Growth Factor Receptor-ß Signaling Promotes Breast Cancer Metastasis in the Brain.

Platelet-derived growth factor receptor-beta (PDGFRß) is a receptor tyrosine kinase found in cells of mesenchymal origin such as fibroblasts and pericytes. Activation of this receptor is dependent on paracrine ligand induction, and its preferred ligand PDGFB is released by neighboring epithelial and endothelial cells. While expression of both PDGFRß and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRß tumor-stroma signaling mediates breast cancer initiation, progression, and metastasis remains unclear. Here we demonstrate this paracrine signaling pathway that mediates both primary tumor growth and metastasis, specifically, metastasis to the brain. Elevated levels of PDGFB accelerated orthotopic tumor growth and intracranial growth of mammary tumor cells, while mesenchymal-specific expression of an activating mutant PDGFRß (PDGFRßD849V) exerted proproliferative signals on adjacent mammary tumor cells. Stromal expression of PDGFRßD849V also promoted brain metastases of mammary tumor cells expressing high PDGFB when injected intravenously. In the brain, expression of PDGFRßD849V was observed within a subset of astrocytes, and aged mice expressing PDGFRßD849V exhibited reactive gliosis. Importantly, the PDGFR-specific inhibitor crenolanib significantly reduced intracranial growth of mammary tumor cells. In a tissue microarray comprised of 363 primary human breast tumors, high PDGFB protein expression was prognostic for brain metastases, but not metastases to other sites. Our results advocate the use of mice expressing PDGFRßD849V in their stromal cells as a preclinical model of breast cancer-associated brain metastases and support continued investigation into the clinical prognostic and therapeutic use of PDGFB-to-PDGFRß signaling in women with breast cancer. SIGNIFICANCE: These studies reveal a previously unknown role for PDGFB-to-PDGFRß paracrine signaling in the promotion of breast cancer brain metastases and support the prognostic and therapeutic clinical utility of this pathway for patients.See related article by Wyss and colleagues, p. 594.

Ablation of the Brca1-Palb2 Interaction Phenocopies Fanconi Anemia in Mice.

Heterozygous mutations in the BRCA1 gene predispose women to breast and ovarian cancer, while biallelic BRCA1 mutations are a cause of Fanconi anemia (FA), a rare genetic disorder characterized by developmental abnormalities, early-onset bone marrow failure, increased risk of cancers, and hypersensitivity to DNA-crosslinking agents. BRCA1 is critical for homologous recombination of DNA double-strand breaks (DSB). Through its coiled-coil domain, BRCA1 interacts with an essential partner, PALB2, recruiting BRCA2 and RAD51 to sites of DNA damage. Missense mutations within the coiled-coil domain of BRCA1 (e.g., L1407P) that affect the interaction with PALB2 have been reported in familial breast cancer. We hypothesized that if PALB2 regulates or mediates BRCA1 tumor suppressor function, ablation of the BRCA1-PALB2 interaction may also elicit genomic instability and tumor susceptibility. We generated mice defective for the Brca1-Palb2 interaction (Brca1 L1363P in mice) and established MEF cells from these mice. Brca1 L1363P/L1363P MEF exhibited hypersensitivity to DNA-damaging agents and failed to recruit Rad51 to DSB. Brca1 L1363P/L1363P mice were viable but exhibited various FA symptoms including growth retardation, hyperpigmentation, skeletal abnormalities, and male/female infertility. Furthermore, all Brca1 L1363P/L1363P mice exhibited macrocytosis and died due to bone marrow failure or lymphoblastic lymphoma/leukemia with activating Notch1 mutations. These phenotypes closely recapitulate clinical features observed in patients with FA. Collectively, this model effectively demonstrates the significance of the BRCA1-PALB2 interaction in genome integrity and provides an FA model to investigate hematopoietic stem cells for mechanisms underlying progressive failure of hematopoiesis and associated development of leukemia/lymphoma, and other FA phenotypes. SIGNIFICANCE: A new Brca1 mouse model for Fanconi anemia (FA) complementation group S provides a system in which to study phenotypes observed in human FA patients including bone marrow failure.See related commentary by Her and Bunting, p. 4044.

Evaluating the efficacy of enzalutamide and the development of resistance in a preclinical mouse model of type-I endometrial carcinoma.

Androgen Receptor (AR) signaling is a critical driver of hormone-dependent prostate cancer and has also been proposed to have biological activity in female hormone-dependent cancers, including type I endometrial carcinoma (EMC). In this study, we evaluated the preclinical efficacy of a third-generation AR antagonist, enzalutamide, in a genetic mouse model of EMC, Sprr2f-Cre;Ptenfl/fl. In this model, ablation of Pten in the uterine epithelium leads to localized and distant malignant disease as observed in human EMC. We hypothesized that administering enzalutamide through the diet would temporarily decrease the incidence of invasive and metastatic carcinoma, while prolonged administration would result in development of resistance and loss of efficacy. Short-term treatment with enzalutamide reduced overall tumor burden through increased apoptosis but failed to prevent progression of invasive and metastatic disease. These results suggest that AR signaling may have biphasic, oncogenic and tumor suppressive roles in EMC that are dependent on disease stage. Enzalutamide treatment increased Progesterone Receptor (PR) expression within both stromal and tumor cell compartments. Prolonged administration of enzalutamide decreased apoptosis, increased tumor burden and resulted in the clonal expansion of tumor cells expressing high levels of p53 protein, suggestive of acquired Trp53 mutations. In conclusion, we show that enzalutamide induces apoptosis in EMC but has limited efficacy overall as a single agent. Induction of PR, a negative regulator of endometrial proliferation, suggests that adding progestin therapy to enzalutamide administration may further decrease tumor burden and result in a prolonged response.