Abstract PD9-11: Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis

PDGFRβ 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 PDGFRβ and PDGFB has been noted in patient breast tumors for decades, how PDGFB-to-PDGFRβ tumor-stromal signaling mediates breast cancer initiation, progression, and metastasis remains unclear. To test this important research question, we developed a mouse model of mesenchymal-specific PDGFRβ hyper-activation. PDGFRβ mutant mammary glands exhibit increased tertiary side-branching and epithelial proliferation confirming a stromal-specific PDGFRβ effect on neighboring epithelium during normal development. To test the effect of hyper-active mesenchymal PDGFRβ on disease progression, experimental tail vein metastasis assays were performed where we observed prominent brain metastases in 50% of the PDGFRβ mutantmice (n=5/10) with no brain lesions seen in controls (n=0/19). There was no difference in the incidence of lung or liver metastases in the mutant mice suggesting a pro-metastatic function for PDGFRβ in the brain metastatic niche. To rule out dysfunction of the blood brain barrier contributing to the observed metastatic spread, we then intracranially injected mammary tumor cells, and as expected based on our metastasis assay, found that larger tumors formed in the brains of PDGFRβ mutant mice versus controls. To our knowledge, these combined findings are the first example where genetic manipulation of the stroma increases breast cancer associated brain metastases (BCBM). Given that these pre-clinical data suggest that primary breast tumors expressing high PDGFB could preferentially metastasize to the brain, we analyzed PDGFB protein expression in a tissue microarray comprised of HER2-positive and triple negative breast cancer (TNBC) primary tumors (total n=425). While high PDGFB did not correlate with site-independent metastatic recurrence, it was prognostic of brain metastasis, mirroring our mouse data. Evaluation of PDGFB in a small cohort of matched primary breast tumors with associated brain (n=5) and lung metastases (n=2) revealed intense PDGFB staining in 100% of the brain metastases, but only 50% of the lung metastases. These findings further suggest that high primary tumor PDGFBexpression defines a subset of breast cancer patients predisposed to brain metastases and that these patients may benefit from therapeutic inhibition of PDGFRβ signaling. To test this pre-clinically, we treated mice harboring intracranial tumors with the PDGFR specific inhibitor crenolanib. Excitingly, crenolanib treatment significantly inhibited the brain tumor burden in these mice. Combined, our findings to date (1) advocate that primary tumor expression of PDGFB is a novel prognostic biomarker for the development of BCBM and (2) support clinical trial evaluation of PDGFR inhibitors for the prevention and treatment of BCBM. Ongoing studies are evaluating how the PDGFRβ-expressing mesenchymal cells within the brain promote a pro-metastatic niche.

Citation Format: Sizemore GM, Hammer AM, Thies KA, Hildreth BE, Russell LO, Sizemore ST, Trimboli AJ, Kladney RD, Steck SA, Das M, Bolyard CM, Pilarski R, Schoenfield L, Otero J, Chakravarti A, Ringel M, Kaur B, Leone G, Ostrowski MC. Platelet derived growth factor receptor-β signaling: A novel therapeutic target for breast cancer associated brain metastasis [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr PD9-11.

Abstract 4337: PDGFR-α induced stiffness abrogates mammary ductal development and enhances tumorigenesis in vivo

Breast cancer is a leading cause of mortality in women worldwide, in part due to the tumor microenvironment which increases tumor heterogeneity and abets tumor growth. Fibroblasts are cells of mesenchymal origin that are an important component of normal and tumor stroma. Genetic alterations in these cells were shown by several groups including our own to cause fibroblast activation and fuel tumor progression giving rise to more aggressive disease. Platelet-Derived Growth Factor Receptor (PDGFR) alpha is a receptor tyrosine kinase that is chiefly expressed in mesenchymal cells such as fibroblasts. Ligand binding (PDGFAA) activates this receptor. PDGFRα signaling plays critical roles in development and aberrant signaling is seen in several types of cancer, such as lung, pancreas, GI and brain. The central goal of this study was to elucidate the role of stromal PDGFRα in breast cancer development and metastasis, where its role remains largely unknown. To address this goal, we developed a genetic mouse model of stromal activation of PDGFRα in the mammary gland by crossing an auto-activating Pdgfra mutant allele with a mesenchymal specific Cre recombinase. We found that stromal PDGFRα activation completely abrogated postnatal mammary gland ductal formation, with significantly reduced terminal end bud formation. PDGFRα activation also led to progressive fibrosis in the mouse mammary fat pad. As early as four weeks of age, mammary collagen (trichrome staining; second harmonic generation) and hyaluronan deposition (Alcian Blue) was greatly increased in vivo. In fact, this increase in collagen and hyaluronan deposition in mutant animals is believed to be responsible for the observed increased in stiffness of mutant mammary tissue (atomic force microscopy {AFM}). pFAK, which can be activated due to mechanical stress, was increased in mammary epithelia of the mutant mice in vivo corroborating the AFM results. Further, when tumor cells were injected into the mammary glands of the PDGFRα mutants, tumors grew faster as compared to controls. Importantly, we found that mRNA expression of PDGFRA correlates with worsened patient outcomes in HER2+ disease, while expression of both the ligand (PDGFA) and the receptor were found to correlate with increased incidence of lung metastases. We further discovered that in HER2+ patients, PDGFRA levels correlate with breast density. Breast density is the third strongest risk factor for breast cancer, and is directly related to collagen deposition and breast stiffness, thus suggesting a novel predictive role of PDGFRA as a molecular readout of stiffness and density. Studies are underway to utilize mouse models of HER2+ breast cancer to study both primary tumor growth and metastases. Taken together, our mouse studies and paralleling human data analyses suggest that the stromal PDGFRα signaling provides a novel theranostic window in breast cancer treatment and prognosis.

Citation Format: Anisha Mathur Hammer, Gina M. Sizemore, Vasudha Shukla, Steven T. Sizemore, Maria Cuitino, Cynthia J. Timmers, Quinn Verfurth, Arnab Chakravarti, Gustavo W. Leone, Samir N. Ghadiali, Michael C. Ostrowski. PDGFR-α induced stiffness abrogates mammary ductal development and enhances tumorigenesis in vivo [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4337. doi:10.1158/1538-7445.AM2017-4337

Stromal PTEN inhibits the expansion of mammary epithelial stem cells through Jagged-1

Fibroblasts within the mammary tumor microenvironment are active participants in carcinogenesis mediating both tumor initiation and progression. Our group has previously demonstrated that genetic loss of PTEN in mammary fibroblasts induces an oncogenic secretome that remodels the extracellular milieu accelerating ErbB2-driven mammary tumor progression. While these prior studies highlighted a tumor suppressive role for stromal PTEN, how the adjacent normal epithelium transforms in response to PTEN loss was not previously addressed. To identify these early events, we have evaluated both phenotypic and genetic changes within the pre-neoplastic mammary epithelium of mice with and without stromal PTEN expression. We report that fibroblast-specific PTEN deletion greatly restricts mammary ductal elongation and induces aberrant alveolar side-branching. These mice concomitantly exhibit an expansion of the mammary epithelial stem cell (MaSC) enriched basal/myoepithelial population and an increase in in vitro stem cell activity. Further analysis revealed that NOTCH signaling, specifically through NOTCH3, is diminished in these cells. Mechanistically, JAGGED-1, a transmembrane ligand for the NOTCH receptor, is downregulated in the PTEN-null fibroblasts leading to a loss in the paracrine activation of NOTCH signaling from the surrounding stroma. Reintroduction of JAGGED-1 expression within the PTEN-null fibroblasts was sufficient to abrogate the observed increase in colony forming activity implying a direct role for stromal JAGGED-1 in regulation of mammary stem cell properties. Importantly, breast cancer patients whose tumors express both low stromal JAG1 and low stromal PTEN exhibit a shorter time to recurrence than those whose tumors express low levels of either alone suggesting similar stromal signaling in advanced disease. Combined, these results unveil a novel stromal PTEN-to-JAGGED-1 axis in maintaining the mammary epithelial stem cell niche, and subsequently inhibiting breast cancer initiation and disease progression.

Phase III randomized double-blind study to evaluate the efficacy of a polycarbophil-based vaginal moisturizer in women with breast cancer

PURPOSE

Vaginal dryness and dyspareunia are significant estrogen-depletion symptoms that affect many breast cancer survivors. The present trial was developed to evaluate the nonhormonal vaginal lubricating preparation, Replens, for alleviating these symptoms.

MATERIALS AND METHODS

A double-blind, crossover, randomized clinical trial was developed. Patients received 4 weeks of Replens (Columbia Research Laboratories, Rockville Centre, NY) followed by a 1-week washout period followed by 4 weeks of a placebo lubricating product, or vice versa. Weekly patient-completed diaries were used for measuring efficacies and toxicities of therapy.

RESULTS

The 45 assessable patients provided well-balanced treatment groups. During the first 4 weeks, average vaginal dryness decreased by 62% and 64% in the placebo and Replens groups, respectively (P = .3). Average dyspareunia scores also improved by 41% and 60%, respectively (P = .05). Crossover analysis indicated that the bulk of the beneficial effects appeared within the first 2 weeks of the first treatment and remained constant thereafter. Both treatments were relatively well tolerated.

CONCLUSION

Both Replens and the placebo appear to substantially ameliorate vaginal dryness and dyspareunia in breast cancer survivors.