Abstract P5-08-17: Small G protein RALA is a driver and potential therapeutic target in triple negative breast cancer

Triple Negative Breast Cancer (TNBC) is the leading cause of cancer mortality in women, mostly due to the lack of targeted treatment for this subtype of breast cancer (BC). RALA and RALB are small GTPases implicated in tumor proliferation, survival, and metastasis in a variety of cancers. However, little is known of their roles in breast cancer. Utilizing 3D spheroid invasion assays, we identified that knockout (KO) of RALA greatly reduced the invasion of MDA-MB-231 spheroids in basement membrane extract (BME). Conversely, RALB-KO significantly increased 3D invasion of MDA-MB-231 cells. We further investigated roles for RALA and RALB in TNBC with cell viability assays, transwell assays, and 3D growth assays. Results indicate that KO or depletion of RALA in TNBC cell lines MDA-MB-231 and MDA-MB-468 reduces cell viability and cell migration capabilities in vitro. On the contrary, loss of RALB increased cell migration and viability. Treating TNBC cells with a small molecule inhibitor of both RAL isoforms (BQU57) reduced cell growth in vitro as well as tumor growth and metastasis in vivo. Furthermore, RALA expression, but not RALB expression, was predictive of response to chemotherapy in TNBC patients and RAL inhibitor sensitized TNBC cells to paclitaxel. Combined, these results highlight the importance of the RALs, particularly RALA, as a therapeutic targets in TNBC.

Citation Format: Dillon S. Richardson, Matthew W. Cole, Rachel E. Schafer, Jonathan M. Spehar, Sarah A. Steck, Manjusri Das, Arthur W. Lian, Alo Ray, Reena Shakya, Sue E. Knoblaugh, Cynthia D. Timmers, Gina M. Sizemore, Steven T. Sizemore. Small G protein RALA is a driver and potential therapeutic target in triple negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-08-17.

Abstract PD3-05: The paradoxical role of RalA and RalB in triple negative breast cancer

Background: Breast Cancer (BC) is the most common cancer and leading cause of cancer associated mortality in women worldwide. TNBC patients have the highest mortality mainly due to lack of receptors for targeted therapies. RalA and RalB are small GTPases that are known to regulate growth and metastasis in several cancers. However, roles for these GTPases in BC is poorly understood. The goal of this study was to investigate the contributions of RalA and RalB in TNBC. Methods: Control, RalA or RalB CRISPR knockout (KO) MDA-MB-231 cells were injected into the mammary gland of nod-skid-gamma (NSG) mice. Tumor growth was monitored and groups were taken as they met early removal criteria. Tumors and lungs were formalin-fixed and paraffin embedded. Tumors underwent immunohistochemical staining for Ki-67 and Cleaved caspase-3 and lungs were stained for hematoxylin and eosin and imaged on a Leica Aperio ScanScope XT to calculate lung metastasis. RalA or RalB were also depleted in MDA-MB-231 and MVT1 cells by shRNA. In addition, RalA depleted MDA-MB-231 cells were labeled with luciferase and injected into the tail vein of NSG mice and imaged on an IVIS spectrum to test seeding and lung colonization. Immunohistochemistry of patient TMAs was preformed on a Bond RX autostainer using RalA (Abcam, ab126627, 1:2000). Immunohistochemical stains were imaged on a PerkinElmer’s Vectra® Automatic Imaging System and quantified using inForm® Advanced Image Analysis software. Statistical significance of Kaplan-Meier survival curves were determined by log rank. Results: RalA knockout and depletion slowed primary orthotopic tumor growth in MDA-MB-231 and MVT1 cells. RalB KO had the opposite effect and increased growth rate compared to controls and RalA KO cells. Ki67 and cleaved caspase 3 IHC staining of tumors indicate KO of RalA decreased proliferation, whereas KO RalB increased proliferation with no change in apoptosis. RalA KO decreased the number and area of lung metastasis in both spontaneous and experimental metastasis assays. RalB KO or depletion caused an increase in the area and number of metastasis. Utilizing data from the METABRIC and TCGA BC datasets, elevated RALA, but not RALB, was prognostic of worse outcome in the overall BC populations and the TNBC populations specifically. RALA was shown to be more highly expressed in BC, particularly TNBC, relative to normal mammary tissue whereas RalB was decreased in BC and TNBC. IHC staining of a TMA comprised of all BC subtypes and a TMA of only TNBC samples confirmed RalA as a prognostic marker of patient outcome. Conclusions: RalA and RalB have important but paradoxical roles in TNBC.

Citation Format: Jonathan M. Spehar, Katie A. Thies, Matthew W. Cole, Rachel E. Schafer, Dillon S. Richardson, Sarah A. Steck, Manjusri Das, Arthur W. Lian, Alo Ray, Sue E. Knoblaugh, Cynthia D. Trimmers, Gina M. Sizemore, Steven T. Sizemore. The paradoxical role of RalA and RalB in triple negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD3-05.

The small G-protein RalA promotes progression and metastasis of triple-negative breast cancer

Background

Breast cancer (BC) is the most common cancer in women and the leading cause of cancer-associated mortality in women. In particular, triple-negative BC (TNBC) has the highest rate of mortality due in large part to the lack of targeted treatment options for this subtype. Thus, there is an urgent need to identify new molecular targets for TNBC treatment. RALA and RALB are small GTPases implicated in growth and metastasis of a variety of cancers, although little is known of their roles in BC.

Methods

The necessity of RALA and RALB for TNBC tumor growth and metastasis were evaluated in vivo using orthotopic and tail-vein models. In vitro, 2D and 3D cell culture methods were used to evaluate the contributions of RALA and RALB during TNBC cell migration, invasion, and viability. The association between TNBC patient outcome and RALA and RALB expression was examined using publicly available gene expression data and patient tissue microarrays. Finally, small molecule inhibition of RALA and RALB was evaluated as a potential treatment strategy for TNBC in cell line and patient-derived xenograft (PDX) models.

Results

Knockout or depletion of RALA inhibited orthotopic primary tumor growth, spontaneous metastasis, and experimental metastasis of TNBC cells in vivo. Conversely, knockout of RALB increased TNBC growth and metastasis. In vitro, RALA and RALB had antagonistic effects on TNBC migration, invasion, and viability with RALA generally supporting and RALB opposing these processes. In BC patient populations, elevated RALA but not RALB expression is significantly associated with poor outcome across all BC subtypes and specifically within TNBC patient cohorts. Immunohistochemical staining for RALA in patient cohorts confirmed the prognostic significance of RALA within the general BC population and the TNBC population specifically. BQU57, a small molecule inhibitor of RALA and RALB, decreased TNBC cell line viability, sensitized cells to paclitaxel in vitro and decreased tumor growth and metastasis in TNBC cell line and PDX models in vivo.

Conclusions

Together, these data demonstrate important but paradoxical roles for RALA and RALB in the pathogenesis of TNBC and advocate further investigation of RALA as a target for the precise treatment of metastatic TNBC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13058-021-01438-3.

Modeling Brain Metastases Through Intracranial Injection and Magnetic Resonance Imaging

Metastatic spread of cancer is an unfortunate consequence of disease progression, aggressive cancer subtypes, and/or late diagnosis. Brain metastases are particularly devastating, difficult to treat, and confer a poor prognosis. While the precise incidence of brain metastases in the United States remains hard to estimate, it is likely to increase as extracranial therapies continue to become more efficacious in treating cancer. Thus, it is necessary to identify and develop novel therapeutic approaches to treat metastasis at this site. To this end, intracranial injection of cancer cells has become a well-established method in which to model brain metastasis. Previously, the inability to directly measure tumor growth has been a technical hindrance to this model; however, increasing availability and quality of small animal imaging modalities, such as magnetic resonance imaging (MRI), are vastly improving the ability to monitor tumor growth over time and infer changes within the brain during the experimental period. Herein, intracranial injection of murine mammary tumor cells into immunocompetent mice followed by MRI is demonstrated. The presented injection approach utilizes isoflurane anesthesia and a stereotactic setup with a digitally controlled, automated drill and needle injection to enhance precision, and reduce technical error. MRI is measured over time using a 9.4 Tesla instrument in The Ohio State University James Comprehensive Cancer Center Small Animal Imaging Shared Resource. Tumor volume measurements are demonstrated at each time point through use of ImageJ. Overall, this intracranial injection approach allows for precise injection, day-to-day monitoring, and accurate tumor volume measurements, which combined greatly enhance the utility of this model system to test novel hypotheses on the drivers of brain metastases.

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

Platelet-derived growth factor receptor-beta (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-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 PDGFRβ (PDGFRβ^D849V) exerted proproliferative signals on adjacent mammary tumor cells. Stromal expression of PDGFRβ^D849V also promoted brain metastases of mammary tumor cells expressing high PDGFB when injected intravenously. In the brain, expression of PDGFRβ^D849V was observed within a subset of astrocytes, and aged mice expressing PDGFRβ^D849V 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 PDGFRβ^D849V 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-PDGFRβ signaling in women with breast cancer. SIGNIFICANCE: These studies reveal a previously unknown role for PDGFB-to-PDGFRβ 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.

Abstract P6-06-06: Platelet derived growth factor-b (PDGFB) promotes breast cancer progression

Objectives of the Study: Platelet-derived growth factor (PDGF) mediated signaling is pro-tumorigenic in many types of cancer, including breast cancer. However, the requirement of individual PDGF ligands in mediating breast cancer progression remains unclear. Our evaluation of publicly available breast cancer patient datasets shows that high primary tumor expression of PDGFB correlates with reduced overall survival (OS) and reduced metastasis free survival (MFS). This is in contrast to the lack of prognostic power of PDGFA and PDGFC and the association with improved OS and MFS for PDGFD. Based on our analysis of patient datasets, we set out to test the requirement of PDGFB in mammary tumor growth.

Methods Used: Gain-of-function and loss-of-function experiments were performed wherein PDGFB was either overexpressed or knocked-down in mammary tumor cells. Manipulated tumor cells were injected directly into the mammary fat pads of adult female mice, and tumor growth was monitored over time. PDGFB is produced by tumor cells whereas the corresponding receptor, PDGFRβ, is expressed by mesenchymal cells in the stroma. As another way to mimic oncogenic PDGFB-to-PDGFRβ signaling in the breast, we developed a mouse model of stroma-specific PDGFRβ activation using the Fsp-cre transgene. We evaluated changes mammary gland development upon mesenchymal specific PDGFRβ activation, and performed orthotopic injections with mammary tumor cells in these animals to test the functional role of receptor activation in mammary tumor growth.

Results and Conclusions: We evaluated expression of PDGF ligands in FVB/N murine mammary tumor cell lines and found that the PDGFB is dramatically higher in DB7 tumor cells compared to other syngeneic cell lines. In the high-PDGFB expressing DB7 cells, we used shRNA technology to knockdown the ligand. At the same time, we overexpressed the ligand in an isogenic cell line that expresses low levels of PDGFB. These cells were injected orthotopically into the mammary fat pads of adult female mice, and in both cases, expression of PDGFB dictated tumor growth. There was a significant reduction in tumor growth with shRNA-mediated knockdown of PDGFB whereas overexpression of the ligand accelerated tumor growth.

In our mouse model of mesenchymal-specific PDGFRβ activation, we reveal that activation of the receptor exerts pro-proliferative signals on adjacent mammary epithelial cells and accelerates orthotopic tumor growth of PDGFB-expressing cells. These findings indicate that PDGFB-to-PDGFRβ signaling is a viable therapeutic target for breast cancer. In fact, treatment with the PDGFR inhibitor, imatinib, impedes tumor cell proliferation when mouse mammary fibroblasts are co-injected orthotopically with DB7 cells.

Significance: The requirement of other PDGF ligands in breast cancer remains to be evaluated, but our data support a pro-tumorigenic role for PDGFB in the breast. Importantly, PDGFR inhibitors are being used in clinical trials for several cancer types. Our data advocate for (1) the potential utility of PDGFB as a prognostic biomarker and (2) the pre-clinical evaluation of PDGFR inhibitors in breast cancer models.

Citation Format: Katie A Thies, Anisha M Hammer, Sarah A Steck, Manjusri Das, Raleigh D Kladney, Steven T Sizemore, Michael C Ostrowski, Gina M Sizemore. Platelet derived growth factor-b (PDGFB) promotes breast cancer progression [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P6-06-06.

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.

Disruption of stromal hedgehog signaling initiates RNF5-mediated proteasomal degradation of PTEN and accelerates pancreatic tumor growth

Disrupting paracrine Hedgehog signaling in pancreatic cancer stroma through genetic deletion of fibroblast Smoothened leads to proteasomal degradation of fibroblast PTEN and accelerates tumor growth.

The contribution of the tumor microenvironment to pancreatic ductal adenocarcinoma (PDAC) development is currently unclear. We therefore examined the consequences of disrupting paracrine Hedgehog (HH) signaling in PDAC stroma. Herein, we show that ablation of the key HH signaling gene Smoothened (Smo) in stromal fibroblasts led to increased proliferation of pancreatic tumor cells. Furthermore, Smo deletion resulted in proteasomal degradation of the tumor suppressor PTEN and activation of oncogenic protein kinase B (AKT) in fibroblasts. An unbiased proteomic screen identified RNF5 as a novel E3 ubiquitin ligase responsible for degradation of phosphatase and tensin homolog (PTEN) in Smo-null fibroblasts. Ring Finger Protein 5 (Rnf5) knockdown or pharmacological inhibition of glycogen synthase kinase 3β (GSKβ), the kinase that marks PTEN for ubiquitination, rescued PTEN levels and reversed the oncogenic phenotype, identifying a new node of PTEN regulation. In PDAC patients, low stromal PTEN correlated with reduced overall survival. Mechanistically, PTEN loss decreased hydraulic permeability of the extracellular matrix, which was reversed by hyaluronidase treatment. These results define non-cell autonomous tumor-promoting mechanisms activated by disruption of the HH/PTEN axis and identifies new targets for restoring stromal tumor-suppressive functions.

Stromal PTEN determines mammary epithelial response to radiotherapy

The importance of the tumor-associated stroma in cancer progression is clear. However, it remains uncertain whether early events in the stroma are capable of initiating breast tumorigenesis. Here, we show that in the mammary glands of non-tumor bearing mice, stromal-specific phosphatase and tensin homolog (Pten) deletion invokes radiation-induced genomic instability in neighboring epithelium. In these animals, a single dose of whole-body radiation causes focal mammary lobuloalveolar hyperplasia through paracrine epidermal growth factor receptor (EGFR) activation, and EGFR inhibition abrogates these cellular changes. By analyzing human tissue, we discover that stromal PTEN is lost in a subset of normal breast samples obtained from reduction mammoplasty, and is predictive of recurrence in breast cancer patients. Combined, these data indicate that diagnostic or therapeutic chest radiation may predispose patients with decreased stromal PTEN expression to secondary breast cancer, and that prophylactic EGFR inhibition may reduce this risk.