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Lymphatic dissemination from the primary tumor is a major mechanism by which breast cancer cells access the systemic circulation, resulting in distant metastasis and mortality. Numerous studies link activation of hypoxia-inducible factor 1 (HIF-1) with tumor angiogenesis, metastasis, and patient mortality. However, the role of HIF-1 in lymphatic dissemination is poorly understood. In this study, we show that HIF-1 promotes lymphatic metastasis of breast cancer by direct transactivation of the gene encoding platelet-derived growth factor B (PDGF-B), which has proliferative and chemotactic effects on lymphatic endothelial cells. Lymphangiogenesis and lymphatic metastasis in mice bearing human breast cancer orthografts were blocked by administration of the HIF-1 inhibitor digoxin or the tyrosine kinase inhibitor imatinib. Immunohistochemical analysis of human breast cancer biopsies demonstrated colocalization of HIF-1α and PDGF-B, which were correlated with lymphatic vessel area and histological grade. Taken together, these data provide experimental support for breast cancer clinical trials targeting HIF-1 and PDGF-B.

Conventional maximum‐tolerated dose (MTD) chemotherapy relies on periodic, massive cancer cell ablation events followed by treatment‐free intermissions, stereotypically resulting in resistance, relapse, and mortality. Furthermore, MTD chemotherapy can promote metastatic dissemination via activation of a transcriptional program dependent on hypoxia‐inducible factor (HIF)‐1α and (HIF)‐2α (hereafter referred to as HIFα). Instead, frequent low‐dose metronomic (LDM) chemotherapy displays less adverse effects while preserving significant pre‐clinical anticancer activity. Consequently, we hereby compared the effect of MTD or LDM chemotherapy upon HIFα in models of advanced, metastatic colon and breast cancer. Our results revealed that LDM chemotherapy could offset paralog‐specific, MTD‐dependent HIFα induction in colon cancers disseminating to the liver and lungs, while limiting HIFα and hypoxia in breast cancer lung metastases. Moreover, we assessed the translational significance of HIFα activity in colorectal and breast TCGA/microarray data, by developing two compact, 11‐gene transcriptomic signatures allowing the stratification/identification of patients likely to benefit from LDM and/or HIFα‐targeting therapies. Altogether, these results suggest LDM chemotherapy as a potential maintenance strategy to stave off HIFα induction within the intra‐metastatic tumor microenvironment. Synopsis Low‐dose metronomic (LDM) chemotherapy offsets intra‐metastatic tumoral hypoxia, hypoxia‐inducible factor (HIF)‐α induction, and angiogenic responses elicited by conventional maximum‐tolerated dose (MTD) therapies in models of advanced, metastatic colon and breast cancer. LDM chemotherapy offsets HIF‐1α induction caused by MTD chemotherapy in large liver and lung metastases from colon and breast cancers, respectively; this effect is counterbalanced by HIF‐2α in colon cancer. HIF‐1α and ‐2α induction caused by MTD chemotherapy regimens is independent of metastatic tumor size. LDM chemotherapy decreases intra‐metastatic tumor hypoxic fractions, whilst leaving lung parenchymal perfusion unaffected. Intra‐metastatic tumor hypoxia parallels both microvessel density and HIFα expression. HIFα paralog expression depends on metastatic size and microenvironmental factors within the target organ. Graphical Abstract Low‐dose metronomic (LDM) chemotherapy offsets intra‐metastatic tumoral hypoxia, hypoxia‐inducible factor (HIF)‐α induction, and angiogenic responses elicited by conventional maximum‐tolerated dose (MTD) therapies in models of advanced, metastatic colon and breast cancer.

The role of tumor cells in synthesizing pro‐inflammatory molecules is still controversial. Here we report that hypoxic treatment of the MCF‐7 human mammary adenocarcinoma cell line induced activation of hypoxia‐inducible factor 1α (HIF‐1α) and nuclear factor‐kappa B (NF‐κB). Importantly, hypoxia regulated expression of alarmin receptors such as the receptor for advanced glycation end products (RAGE) and the purinoreceptor (P2X7R), and up‐regulated inflammatory response (IR) genes such as the inducible enzymes nitric oxide synthase (NOS2), cycloxygenase (COX2), and the acute‐phase protein pentraxin‐3 (PTX3). Hypoxia also stimulated chemokine (C‐X‐C motif) receptor 4 (CXCR4) mRNA synthesis. In fact, the CXCR4 ligand stromal‐derived factor‐1α (SDF‐1α) increased invasion and migration of hypoxic MCF‐7 cells. Inhibition of HIF‐1α by chetomin and NF‐κB by parthenolide reduced mRNA and protein expression of the studied molecules and prevented invasion of hypoxic MCF‐7 cells. Moreover, solid invasive mammary tumor microenvironment was analyzed after laser‐capture microdissection (LCMD) comparing tumor versus host normal tissue. Nuclear translocation of HIF‐1α and NF‐κB and up‐regulation of IR, CXCR4, estrogen receptor α (ERα), and epithelial growth factor receptor (EGFR) was observed in tumor but not in host normal tissue in the absence of a local inflammatory leukocyte infiltrate. We conclude that under hypoxic conditions MCF‐7 cells acquire a pro‐inflammatory phenotype, and that solid human mammary carcinoma evidenced a similar activation of HIF‐1α, NF‐κB, and IR genes in malignant tumor cells as compared to the normal host tissues. We suggest a role for IR activation in the malignant progression of transformed cells. (Cancer Sci 2010; 101: 1014–1023)

Metastasis involves critical interactions between cancer and stromal cells. Intratumoral hypoxia promotes metastasis through activation of hypoxia-inducible factors (HIFs). We demonstrate that HIFs mediate paracrine signaling between breast cancer cells (BCCs) and mesenchymal stem cells (MSCs) to promote metastasis. In a mouse orthotopic implantation model, MSCs were recruited to primary breast tumors and promoted BCC metastasis to LNs and lungs in a HIF-dependent manner. Coculture of MSCs with BCCs augmented HIF activity in BCCs. Additionally, coculture induced expression of the chemokine CXCL10 in MSCs and the cognate receptor CXCR3 in BCCs, which was augmented by hypoxia. CXCR3 expression was blocked in cocultures treated with neutralizing antibody against CXCL10. Conversely, CXCL10 expression was blocked in MSCs cocultured with BCCs that did not express CXCR3 or HIFs. MSC coculture did not enhance the metastasis of HIF-deficient BCCs. BCCs and MSCs expressed placental growth factor (PGF) and its cognate receptor VEGFR1, respectively, in a HIF-dependent manner, and CXCL10 expression by MSCs was dependent on PGF expression by BCCs. PGF promoted metastasis of BCCs and also facilitated homing of MSCs to tumors. Thus, HIFs mediate complex and bidirectional paracrine signaling between BCCs and MSCs that stimulates breast cancer metastasis.

HIF1α and NFkB are two transcription factors very frequently activated in tumors and involved in tumor growth, progression, and resistance to chemotherapy. In fact, HIF1α and NFkB together regulate transcription of over a thousand genes that, in turn, control vital cellular processes such as adaptation to the hypoxia, metabolic reprograming, inflammatory reparative response, extracellular matrix digestion, migration and invasion, adhesion, etc. Because of this wide involvement they could control in an integrated manner the origin of the malignant phenotype. Interestingly, hypoxia and inflammation have been sequentially bridged in tumors by the discovery that alarmin receptors genes such as RAGE, P2X7, and some TLRs, are activated by HIF1α; and that, in turn, alarmin receptors strongly activate NFkB and proinflammatory gene expression, evidencing all the hallmarks of the malignant phenotype. Recently, a large number of drugs have been identified that inhibit one or both transcription factors with promising results in terms of controlling tumor progression. In addition, many of these molecules are natural compounds or off-label drugs already used to cure other pathologies. Some of them are undergoing clinical trials and soon they will be used alone or in combination with standard anti-tumoral agents to achieve a better treatment of tumors with reduction of metastasis formation and, more importantly, with a net increase in survival. This review highlights the central role of HIF1α activated in hypoxic regions of the tumor, of NFkB activation and proinflammatory gene expression in transformed cells to understand their progression toward malignancy. Different molecules and strategies to inhibit these transcription factors will be reviewed. Finally, the central role of a new class of deacetylases called Sirtuins in regulating HIF1α and NFkB activity will be outlined.

Lymphatic dissemination from the primary tumor is a major mechanism by which breast cancer cells access the systemic circulation, resulting in distant metastasis and mortality. Numerous studies link activation of hypoxia-inducible factor 1 (HIF-1) with tumor angiogenesis, metastasis, and patient mortality. However, the role of HIF-1 in lymphatic dissemination is poorly understood. In this study, we show that HIF-1 promotes lymphatic metastasis of breast cancer by direct transactivation of the gene encoding platelet-derived growth factor B (PDGF-B), which has proliferative and chemotactic effects on lymphatic endothelial cells. Lymphangiogenesis and lymphatic metastasis in mice bearing human breast cancer orthografts were blocked by administration of the HIF-1 inhibitor digoxin or the tyrosine kinase inhibitor imatinib. Immunohistochemical analysis of human breast cancer biopsies demonstrated colocalization of HIF-1α and PDGF-B, which were correlated with lymphatic vessel area and histological grade. Taken together, these data provide experimental support for breast cancer clinical trials targeting HIF-1 and PDGF-B. [PUBLICATION ABSTRACT]

Hypoxia (low O2) signals into the nucleus of cancer cells through hypoxia-inducible factor (HIF)-1α-dependent transcription triggering proliferative, metabolic and vascular adaptations linked to therapy resistance and mortality due to overt metastasis. In contrast with the wealth of molecular data on primary intra-tumoral hypoxia, there is a dearth of statistical modelling studies addressing the mechanisms of intra-metastatic hypoxia. In this study, we used path analysis to model intra-metastatic hypoxia (Hx), HIF-1α expression and microvascular area (MVA) as functions of metastatic cross-sectional area (MCSA) in an advanced mouse breast cancer model; in this context, we tested the effect of conventional, maximum-tolerated dose (MTD) or low-dose metronomic (LDM) chemotherapy. Iterative analysis of 34 non-isomorphic paths yielded four well-fitting, configuration-invariant models [χ2(6,171) ≥ 6.12; P ≥ 0.328; CFI ≥ 0.998; RMSEA ≤ 0.07]. All four models contained HIF-1α as a mediating variable within the MCSA↔Hx↔MVA path, as well as significant Hx↔MVA interactions. LDM disrupted the HIF-1α↔MCSA→Hx and HIF-1α↔MVA paths; furthermore, all LDM and MTD combinations impaired Hx↔HIF-1α. These results confirmed well-established hypoxic interactions, whilst uncovering possible differential effects of chemotherapeutic modalities upon metastatic size, hypoxia, HIF-1α and vascularisation. Our data indicate that MVA can act as a downstream readout, rather than an adaptive angiogenic mechanism alleviating intra-metastatic hypoxia. Moreover, well-fitting path models locate HIF-1α activity either upstream or downstream of hypoxia, thereby allowing us to posit the existence of bi-directional feedback loops driving vascularisation and growth in metastatic tumours, of relevance for targeted therapies.