Postpartum involution is the process by which the lactating mammary gland returns to the pre-pregnant state after weaning. Expression of tumor-promotional collagen, upregulation of matrix metalloproteinases, infiltration of M2 macrophages, and remodeling of blood and lymphatic vasculature are all characteristics shared by the involuting mammary gland and breast tumor microenvironment. The tumor promotional nature of the involuting mammary gland is perhaps best evidenced by cases of postpartum breast cancer (PPBC), or those cases diagnosed within 10 years of most recent childbirth. Women with PPBC experience more aggressive disease and higher risk of metastasis than nulliparous patients and those diagnosed outside the postpartum window. Semaphorin 7a (SEMA7A), cyclooxygenase-2 (COX-2), and collagen are all expressed in the involuting mammary gland and, together, predict for decreased metastasis free survival in breast cancer. Studies investigating the role of these proteins in involution have been important for understanding their contributions to PPBC. Postpartum involution thus represents a valuable model for the identification of novel molecular drivers of PPBC and classical cancer hallmarks. In this review, we will highlight the similarities between involution and cancer in the mammary gland, and further define the contribution of SEMA7A/COX-2/collagen interplay to postpartum involution and breast tumor progression and metastasis.

Young women diagnosed with breast cancer (BC) have poor prognosis due to increased rates of metastasis. Additionally, women within 10 years of most recent childbirth at diagnosis are ~3 times more likely to develop metastasis than age and stage matched nulliparous women. We define these cases as postpartum BC (PPBC) and propose that the unique biology of the postpartum mammary gland drives tumor progression. Our published results revealed roles for SEMA7A in breast tumor cell growth, motility, invasion, and tumor associated-lymphangiogenesis, all of which are also increased in pre-clinical models of PPBC. However, whether SEMA7A drives progression in PPBC remains largely unexplored. Our results presented herein show that silencing of SEMA7A decreases tumor growth in a model of PPBC while overexpression is sufficient to increase growth in nulliparous hosts. Further, we show that SEMA7A promotes multiple known drivers of PPBC progression including tumor associated COX-2 expression and fibroblast-mediated collagen deposition in the tumor microenvironment. Additionally, we show for the first time that SEMA7A expressing cells deposit fibronectin to promote tumor cell survival. Finally, we show that co-expression of SEMA7A/COX-2/FN predicts for poor prognosis in breast cancer patient cohorts. These studies suggest SEMA7A as a key mediator of BC progression and that targeting SEMA7A may open avenues for novel therapeutic strategies.

Highlights Clinical –The studies are the first to identify CSPG4 as an oncoantigen which is associated with poor outcome in patients with ovarian cancer. CSPG4 expression by tumor cells significantly enhances tumor expansion in vivo and tumor invasion, cisplatin resistance and spheroid formation. CSPG expression stimulates cell adhesion related pathways (e.g. Focal Adhesion Kinase) which stimulates expression of ZEB1 which stimulates invasion and spheroid formation. Antibody targeting a specific domain of CSPG4 limits invasion and promotes apoptosis of cells within spheroids. The studies identify CSPG4 as a targetable oncoantigen that could improve ovarian cancer patient management by limiting metastasis and improving standard of care chemotherapy response.

Breast cancer invasion and metastasis result from a complex interplay between tumor cells and the tumor microenvironment (TME). Key oncogenic changes in the TME include aberrant metabolism and subsequent signaling of hyaluronan (HA). Hyaluronan Mediated Motility Receptor (RHAMM, HMMR) is a HA receptor that enables tumor cells to sense and respond to the TME during breast cancer progression. Focused gene expression analysis of an internal breast cancer patient cohort demonstrates increased RHAMM expression correlates with aggressive clinicopathological features. We also develop a 27-gene RHAMM-dependent signature (RDS) by intersecting differentially expressed genes in lymph node positive cases with the transcriptome of a RHAMM-dependent model of cell transformation, which we validate in an independent cohort. We demonstrate RDS predicts for poor survival and associates with invasive pathways. Further analyses using CRISPR/Cas9 generated RHAMM -/- breast cancer cells provide direct evidence that RHAMM promotes invasion in vitro and in vivo. Additional immunohistochemistry studies highlight heterogeneous RHAMM expression, and spatial transcriptomics confirms the RDS emanates from RHAMM-high invasive niches. We conclude RHAMM upregulation leads to the formation of ‘invasive niches’, which are enriched in RDS-related pathways that drive invasion and could be targeted to limit invasive progression and improve patient outcomes.

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Breast cancer (BC) remains the second leading cause of cancer related deaths in women in the US --mainly due to metastatic disease. Thus, understanding how BC progresses is of the utmost importance to developing new strategies to block metastasis. Young women diagnosed with BC generally have poor prognosis due to increased rates of metastasis. Additionally, women who are within 5 years of most recent child-birth at diagnosis are ~3 times more likely to develop metastasis than age and stage matched nulliparous women. We define these cases as postpartum breast cancers (PPBC) and propose that the unique biology of the postpartum mammary gland drives tumor progression and metastasis. Semaphorin 7a (SEMA7A) is a unique member of the Semaphorin family as it is the only GPI-anchored member that can be shed into the extracellular environment. Our published results show that SEMA7A expression is associated with decreased overall survival in BC patient cohorts and revealed roles for SEMA7A in breast tumor cell growth, motility, invasion, and tumor associated-lymphangiogenesis all of which are also increased in pre-clinical models of PPBC. However, whether primary SEMA7A driver PPBC progression remains largely unexplored. We utilized a pre-clinical of PPBC with breast cancer cells where we silenced or overexpressed SEMA7A. Our results show that silencing of SEMA7A decreases tumor growth in postpartum hosts while overexpression increases growth in nulliparous hosts to rates closer to those observed in postpartum hosts. Further, we show that SEMA7A effects multiple pro-metastatic tumor attributes such as cell survival, tumor associated COX-2 expression, fibronectin deposition, and fibroblast-mediated collagen deposition in the tumor microenvironment regardless of whether the host is nulliparous or postpartum. Finally, we show that co-expression of SEMA7A/Collagen/COX2/FN mRNA predicts for increased metastasis in breast and ovarian cancer cohorts. These studies suggest SEMA7A as a key mediator of general BC progression and that targeting SEMA7A alone or in combination may open avenues for novel therapeutic strategies to prevent BC progression to metastasis. INTRODUCTION:Postpartum breast cancers (PPBC), or breast cancers diagnosed within 5-10 years of last childbirth, are nearly three times as likely to become metastatic and result in death from breast cancer 1-3 . Specifically, PPBC patients diagnosed within five years of last competed pregnancy exhibit 70% distant metastasis free five-year survival (DMFS) rates 1 that are further decreased to 50% after ten years 2 . Using this definition, PPBC may account over half of breast cancers diagnosed in women aged <45 in two independent cohorts. Using a pre-clinical model of PPBC we have shown that noninvasive breast tumor cells, MCF10DCIS, become invasive and metastatic if they are implanted into mouse mammary tissues during the normal physiological process of postpartum mammary gland involution 4 . The MCF10DCIS model closely resembles ductal carcinoma in situ (DCIS) and progresse...

Our overall goal is to understand drivers of chemoresistance in triple-negative breast cancer (TNBC) and the mechanisms that contribute to metastatic spread in young women. TNBC is more frequently diagnosed in women aged &lt;40 and confers worse overall and distant-metastatic-free survival. Our lab identified a ß1 integrin-ligand, Semaphorin 7A (SEMA7A), that is increased in TNBC and young women's breast cancer (BC). Our central hypothesis is that SEMA7A is increased in TNBC and signals via ß1-integrin to activate epithelial-to-mesenchymal plasticity (EMP) and chemoresistance. We have incorporated genetically manipulated TNBC cell lines and ex vivo tumor samples to define how SEMA7A expression impacts metastasis and chemoresistance. We have also incorporated our Young Women’s BC Cohort to analyze SEMA7A protein expression in human tumors and mammary epithelial tissue. In this cohort, we found that the normal mammary epithelium and tumors exhibit higher SEMA7A expression in women with postpartum BC, defined as BC diagnosed within 10 years postpartum, who are between 30&39-years-old. This sub-population also has increased tumor-associated lymphatic vessel density, as well as increased lymphovascular invasion and LN involvement. Using a 9-month-old mouse model, which is equivalent to a 35-year-old woman, we have shown that mammary tumor growth and metastasis are increased compared to young 8-week-old mice, which can be mimicked by SEMA7A OE tumors in the young mice; these results suggest that the aged microenvironment can replicate the tumor promotional effects of SEMA7A. Similarly, in orthotopic models, SEMA7A OE cells metastasize more frequently to lymph nodes (LNs) and lungs compared to controls. Additionally, via a tail vein injection, we observe that SEMA7A OE cells survive in circulation, seed the lung, and outgrow faster compared to controls. In vitro, SEMA7A promotes migration and invasion via ß1-integrin signaling. Finally, ex vivo metastatic tumor cells, when compared to parental cells, exhibit evidence of EMP. Namely, prior to dissemination, SEMA7A OE cells are mostly mesenchymal-like. Ex vivo, SEMA7A OE cells from LN metastasis express both epithelial/mesenchymal markers, suggesting a hybrid phenotype and maintenance of an invasive phenotype. Whereas those from the lung are more epithelial, which can be more proliferative. EMP is also known to promote chemoresistance and we have found that SEMA7A OE cells are resistant to caspase-mediated cell death in the presence of paclitaxel, a known chemotherapeutic utilized for TNBC, and have increased activation of pAkt, which can mediate pro-survival phenotypes. Overall, our results suggest that the aged mammary gland and SEMA7A expression can promote metastatic spread and chemoresistance and that these patient populations should be considered for direct and indirect targeting of SEMA7A by novel therapeutics. Citation Format: Kelsey Kines, Sarah Tarullo, Virginia Borges, Traci Lyons. The effect of Semaphorin 7A on breast cancer metastases and chemoresistance in young women [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 1284.

Breast cancer invasion and metastasis result from a complex interplay between tumor cells and the tumor microenvironment (TME). Key oncogenic changes in the TME include aberrant synthesis, processing, and signaling of hyaluronan (HA). Hyaluronan‐mediated motility receptor (RHAMM, CD168; HMMR) is an HA receptor enabling tumor cells to sense and respond to this aberrant TME during breast cancer progression. Previous studies have associated RHAMM expression with breast tumor progression; however, cause and effect mechanisms are incompletely established. Focused gene expression analysis of an internal breast cancer patient cohort confirmed that increased RHAMM expression correlates with aggressive clinicopathological features. To probe mechanisms, we developed a novel 27‐gene RHAMM‐related signature (RRS) by intersecting differentially expressed genes in lymph node (LN)‐positive patient cases with the transcriptome of a RHAMM‐dependent model of cell transformation, which we validated in an independent cohort. We demonstrate that the RRS predicts for poor survival and is enriched for cell cycle and TME‐interaction pathways. Further analyses using CRISPR/Cas9‐generated RHAMM−/− breast cancer cells provided direct evidence that RHAMM promotes invasion in vitro and in vivo. Immunohistochemistry studies highlighted heterogeneous RHAMM protein expression, and spatial transcriptomics associated the RRS with RHAMM‐high microanatomic foci. We conclude that RHAMM upregulation leads to the formation of ‘invasive niches’, which are enriched in RRS‐related pathways that drive invasion and could be targeted to limit invasive progression and improve patient outcomes. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.