Nitric oxide (NO) is a key player in both the development and suppression of tumourigenesis depending on the source and concentration of NO. In this review, we discuss the mechanisms by which NO induces DNA damage, influences the DNA damage repair response, and subsequently modulates cell cycle arrest. In some circumstances, NO induces cell cycle arrest and apoptosis protecting against tumourigenesis. NO in other scenarios can cause a delay in cell cycle progression, allowing for aberrant DNA repair that promotes the accumulation of mutations and tumour heterogeneity. Within the tumour microenvironment, low to moderate levels of NO derived from tumour and endothelial cells can activate angiogenesis and epithelial-to-mesenchymal transition, promoting an aggressive phenotype. In contrast, high levels of NO derived from inducible nitric oxide synthase (iNOS) expressing M1 and Th1 polarised macrophages and lymphocytes may exert an anti-tumour effect protecting against cancer. It is important to note that the existing evidence on immunomodulation is mainly based on murine iNOS studies which produce higher fluxes of NO than human iNOS. Finally, we discuss different strategies to target NO related pathways therapeutically. Collectively, we present a picture of NO as a master regulator of cancer development and progression.

Mesenchymal stem cells (MSCs) are a heterogeneous population of multipotent cells that are capable of differentiating into osteocytes, chondrocytes and adipocytes. Recently, MSCs have been found to home to the tumour site and engraft in the tumour stroma. However, it is not yet known whether they have a tumour promoting or suppressive function. We investigated the interaction between prostate cancer cell lines 22Rv1, DU145 and PC3, and bone marrow-derived MSCs. MSCs were 'educated' for extended periods in prostate cancer cell conditioned media and PC3-educated MSCs were found to be the most responsive with a secretory profile rich in pro-inflammatory cytokines. PC3-educated MSCs secreted increased osteopontin (OPN), interleukin-8 (IL-8) and fibroblast growth factor-2 (FGF-2) and decreased soluble fms-like tyrosine kinase-1 (sFlt-1) compared to untreated MSCs. PC3-educated MSCs showed a reduced migration and proliferation capacity that was dependent on exposure to PC3-conditioned medium. Vimentin and α-smooth muscle actin (αSMA) expression was decreased in PC3-educated MSCs compared to untreated MSCs. PC3 and DU145 education of healthy donor and prostate cancer patient-derived MSCs led to a reduced proportion of FAP+ αSMA+ cells contrary to characteristics commonly associated with cancer associated fibroblasts (CAFs). The migration of PC3 cells was increased toward both PC3-educated and DU145-educated MSCs compared to untreated MSCs, while DU145 migration was only enhanced toward patient-derived MSCs. In summary, MSCs developed an altered phenotype in response to prostate cancer conditioned medium which resulted in increased secretion of pro-inflammatory cytokines, modified functional activity and the chemoattraction of prostate cancer cells.

Human endogenous retroviruses (HERV), ancient integrations of exogenous viruses, make up 8% of our genome. Long thought of as mere vestigial genetic elements, evidence is now accumulating to suggest a potential functional role in numerous pathologies including neurodegenerative diseases, autoimmune disorders, and multiple cancers. The youngest member of this group of transposable elements is HERV-K (HML-2). Like the majority of HERV sequences, significant post-insertional mutations have disarmed HERV-K (HML-2), preventing it from producing infectious viral particles. However, some insertions have retained limited coding capacity, and complete open reading frames for all its constituent proteins can be found throughout the genome. For this reason HERV-K (HML-2) has garnered more attention than its peers. The tight epigenetic control thought to suppress expression in healthy tissue is lost during carcinogenesis. Upregulation of HERV-K (HML-2) derived mRNA and protein has been reported in a variety of solid and liquid tumour types, and while causality has yet to be established, progressively more data are emerging to suggest this phenomenon may contribute to tumour growth and metastatic capacity. Herein we discuss its potential utility as a diagnostic tool and therapeutic target in light of the current in vitro, in vivo and clinical evidence linking HERV-K (HML-2) to tumour progression.

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Background: Human endogenous retrovirus K (HERV-K) belongs to a family of endogenous retroviruses that are present in our genome with similarities to present day exogenous retroviruses. HERV-K, like other endogenous retroviruses, is transmitted vertically in a Mendelian fashion through the human genome. This virus can express several proteins but our knowledge of HERV-K expression in human cancers is mainly limited to the envelope (Env) protein. Elevated HERV-K env protein expression has been shown in breast cancer both in in vitro and in vivo studies. Previous work from this laboratory has shown increased HERV-K expression in blood is predictive of a prostate cancer diagnosis. Aims & Experimental Approach: This project aimed to decipher whether individual HERV-K proteins (Env and Gag) display differential association with histological type, molecular subtype and patient outcomes in an Irish cohort of triple negative breast cancer patients (n=177). Additionally we aimed to assess the impact of HERV-K targeting shRNA mediated knockdown on TNBC characteristics including cell migration, invasion and proliferative capacity in 2D and 3D cell culture models. RNA seq and bioinformatics analysis was performed to identify key molecular processes regulated by HERV-K in TNBC. Results: HERV-K Env expression showed a trend towards an association with poor survival in TNBC (Log rank test, p=0.105), while HERV-K Gag was not found to be associated with survival. HERV-K Env positive metastatic patients had an increased involvement of the bone and lungs compared to HERV-K Env negative patients, while HERV-K gag positive patients showed reduced liver metastasis involvement. Knockdown of HERV-K in TNBC cell lines MDA-MB-231 and MDA-MB-468 reduced their proliferative, migratory and invasive potential. Preliminary screens using a cancer drug target array shows significantly reduced expression of HDAC4, a regulator of cell growth and the anti-apoptotic protein. GSEA analysis of RNAseq data from MDA-MB-231 HERV-K shRNA knockdown demonstrated enrichment for genes associated with both the Charafe downregulated in Luminal A versus Mesenchymal gene signature (p= 5.14x10-133), and the Charafe downregulated in Luminal A versus Basal gene signature (p= 7.03x10-110), indicating that HERV-K may play an important role in mesenchymal and basal phenotype TNBC. Additionally pathway analysis points toward a role in the regulation of many different pathways and cellular process, including focal adhesion (p= 9.77x10-7), cellular senescence (p= 7.63x10-7) and viral signalling pathways (p= 6.86x10-14). Discussion: Reduced rates of migration, invasion and proliferation in HERV-K knockdowns points towards the essential role of HERV-K in tumorigenesis and metastasis. HERV-K knockdown also modulated key gene expression signatures traditionally associated with the basal and mesenchymal phenotypes in breast cancer. Taken together, our findings indicate that HERV-K may be a useful molecular target for the treatment of TNBC. Citation Format: Sharon A Glynn, Dibyangana Bhattacharyya, Shauna Lambe, Emma Kerr, Simon McDade, Faizan H Khan, Eoin Dervan, Feng Wang-Johanning, Sean Hynes, Grace Callagy. Human endogenous retrovirus-K (HERV-K) is aberrantly expressed in triple negative breast cancer (TNBC) and associated with increased distant metastasis: Impact of HERV-K knockdown on gene expression patterns and invasive potential of mesenchymal TNBC [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 P3-06-05.