Effects of Ferroportin-Mediated Iron Depletion in Cells Representative of Different Histological Subtypes of Prostate Cancer.

AIMS: Ferroportin (FPN) is an iron exporter that plays an important role in cellular and systemic iron metabolism. Our previous work has demonstrated that FPN is decreased in prostate tumors. We sought to identify the molecular pathways regulated by FPN in prostate cancer cells. RESULTS: We show that overexpression of FPN induces profound effects in cells representative of multiple histological subtypes of prostate cancer by activating different but converging pathways. Induction of FPN induces autophagy and activates the transcription factors tumor protein 53 (p53) and Kruppel-like factor 6 (KLF6) and their common downstream target, cyclin-dependent kinase inhibitor 1A (p21). FPN also induces cell cycle arrest and stress-induced DNA-damage genes. Effects of FPN are attributable to its effects on intracellular iron and can be reproduced with iron chelators. Importantly, expression of FPN not only inhibits proliferation of all prostate cancer cells studied but also reduces growth of tumors derived from castrate-resistant adenocarcinoma C4-2 cells in vivo. INNOVATION: We use a novel model of FPN expression to interrogate molecular pathways triggered by iron depletion in prostate cancer cells. Since prostate cancer encompasses different subtypes with a highly variable clinical course, we further explore how histopathological subtype influences the response to iron depletion. We demonstrate that prostate cancer cells that derive from different histopathological subtypes activate converging pathways in response to FPN-mediated iron depletion. Activation of these pathways is sufficient to significantly reduce the growth of treatment-refractory C4-2 prostate tumors in vivo. CONCLUSIONS: Our results may explain why FPN is dramatically suppressed in cancer cells, and they suggest that FPN agonists may be beneficial in the treatment of prostate cancer.

Iron and Cancer.

This review explores the multifaceted role that iron has in cancer biology. Epidemiological studies have demonstrated an association between excess iron and increased cancer incidence and risk, while experimental studies have implicated iron in cancer initiation, tumor growth, and metastasis. The roles of iron in proliferation, metabolism, and metastasis underpin the association of iron with tumor growth and progression. Cancer cells exhibit an iron-seeking phenotype achieved through dysregulation of iron metabolic proteins. These changes are mediated, at least in part, by oncogenes and tumor suppressors. The dependence of cancer cells on iron has implications in a number of cell death pathways, including ferroptosis, an iron-dependent form of cell death. Uniquely, both iron excess and iron depletion can be utilized in anticancer therapies. Investigating the efficacy of these therapeutic approaches is an area of active research that promises substantial clinical impact.

Iron-responsive element-binding protein 2 plays an essential role in regulating prostate cancer cell growth.

Iron-responsive element-binding proteins (IRPs) are master regulators of cellular iron homeostasis. Our previous work demonstrated that iron homeostasis is altered in prostate cancer and contributes to prostate cancer progression. Here we report that prostate cancer cells overexpress IRP2 and that overexpression of IRP2 drives the altered iron phenotype of prostate cancer cells. IRP2 knockdown in prostate cancer cell lines reduces intracellular iron and causes cell cycle inhibition and apoptosis. Cell cycle analysis demonstrates that IRP2-depleted prostate cancer cells accumulate in G0/G1 due to induction of p15, p21, and p27. Activation of these pathways is sufficient to significantly reduce the growth of PC3 prostate tumors in vivo. In contrast, IRP1 knockdown does not affect iron homeostasis and only modestly affects cell growth, likely through an iron-independent mechanism. These results demonstrate that upregulation of IRP2 in prostate cancer cells co-opts normal iron regulatory mechanisms to facilitate iron retention and drive enhanced tumor growth.

DCYTB is a predictor of outcome in breast cancer that functions via iron-independent mechanisms.

BACKGROUND: Duodenal cytochrome b (DCYTB) is a ferrireductase that functions together with divalent metal transporter 1 (DMT1) to mediate dietary iron reduction and uptake in the duodenum. DCYTB is also a member of a 16-gene iron regulatory gene signature (IRGS) that predicts metastasis-free survival in breast cancer patients. To better understand the relationship between DCYTB and breast cancer, we explored in detail the prognostic significance and molecular function of DCYTB in breast cancer. METHODS: The prognostic significance of DCYTB expression was evaluated using publicly available microarray data. Signaling Pathway Impact Analysis (SPIA) of microarray data was used to identify potential novel functions of DCYTB. The role of DCYTB was assessed using immunohistochemistry and measurements of iron uptake, iron metabolism, and FAK signaling. RESULTS: High DCYTB expression was associated with prolonged survival in two large independent cohorts, together totaling 1610 patients (cohort #1, p = 1.6e-11, n = 741; cohort #2, p = 1.2e-05, n = 869; log-rank test) as well as in the Gene expression-based Outcome for Breast cancer Online (GOBO) cohort (p < 1.0e-05, n = 1379). High DCYTB expression was also associated with increased survival in homogeneously treated groups of patients who received either tamoxifen or chemotherapy. Immunohistochemistry revealed that DCYTB is localized on the plasma membrane of breast epithelial cells, and that expression is dramatically reduced in high-grade tumors. Surprisingly, neither overexpression nor knockdown of DCYTB affected levels of ferritin H, transferrin receptor, labile iron or total cellular iron in breast cancer cells. Because SPIA pathway analysis of patient microarray data revealed an association between DCYTB and the focal adhesion pathway, we examined the influence of DCYTB on FAK activation in breast cancer cells. These experiments reveal that DCYTB reduces adhesion and activation of focal adhesion kinase (FAK) and its adapter protein paxillin. CONCLUSIONS: DCYTB is an important predictor of outcome and is associated with response to therapy in breast cancer patients. DCYTB does not affect intracellular iron in breast cancer cells. Instead, DCYTB may retard cancer progression by reducing activation of FAK, a kinase that plays a central role in tumor cell adhesion and metastasis.

Mitochondria and Iron: current questions.

INTRODUCTION: Mitochondria are cellular organelles that perform numerous bioenergetic, biosynthetic, and regulatory functions and play a central role in iron metabolism. Extracellular iron is taken up by cells and transported to the mitochondria, where it is utilized for synthesis of cofactors essential to the function of enzymes involved in oxidation-reduction reactions, DNA synthesis and repair, and a variety of other cellular processes. Areas covered: This article reviews the trafficking of iron to the mitochondria and normal mitochondrial iron metabolism, including heme synthesis and iron-sulfur cluster biogenesis. Much of our understanding of mitochondrial iron metabolism has been revealed by pathologies that disrupt normal iron metabolism. These conditions affect not only iron metabolism but mitochondrial function and systemic health. Therefore, this article also discusses these pathologies, including conditions of systemic and mitochondrial iron dysregulation as well as cancer. Literature covering these areas was identified via PubMed searches using keywords: Iron, mitochondria, Heme Synthesis, Iron-sulfur Cluster, and Cancer. References cited by publications retrieved using this search strategy were also consulted. Expert commentary: While much has been learned about mitochondrial and its iron, key questions remain. Developing a better understanding of mitochondrial iron and its regulation will be paramount in developing therapies for syndromes that affect mitochondrial iron.

Iron and cancer: recent insights.

Iron is an essential dietary element. However, the ability of iron to cycle between oxidized and reduced forms also renders it capable of contributing to free radical formation, which can have deleterious effects, including promutagenic effects that can potentiate tumor formation. Dysregulation of iron metabolism can increase cancer risk and promote tumor growth. Cancer cells exhibit an enhanced dependence on iron relative to their normal counterparts, a phenomenon we have termed iron addiction. Work conducted in the past few years has revealed new cellular processes and mechanisms that deepen our understanding of the link between iron and cancer. Control of iron efflux through the combined action of ferroportin, an iron efflux pump, and its regulator hepcidin appears to play an important role in tumorigenesis. Ferroptosis is a form of iron-dependent cell death involving the production of reactive oxygen species. Specific mechanisms involved in ferroptosis, including depletion of glutathione and inhibition of glutathione peroxidase 4, have been uncovered. Ferritinophagy is a newly identified mechanism for degradation of the iron storage protein ferritin. Perturbations of mechanisms that control transcripts encoding proteins that regulate iron have been observed in cancer cells, including differences in miRNA, methylation, and acetylation. These new insights may ultimately provide new therapeutic opportunities for treating cancer.

Modulation of hepcidin to treat iron deregulation: potential clinical applications.

The secreted peptide hormone hepcidin regulates systemic and local iron homeostasis through degradation of the iron exporter ferroportin. Dysregulation of hepcidin leads to altered iron homeostasis and development of pathological disorders including hemochromatosis, and iron loading and iron restrictive anemias. Therapeutic modulation of hepcidin is a promising method to ameliorate these conditions. Several approaches have been taken to enhance or reduce the effects of hepcidin in vitro and in vivo. Based on these approaches, hepcidin modulating drugs have been developed and are undergoing clinical evaluation. In this article we review the rationale for development of these drugs, the data concerning their safety and efficacy, their therapeutic uses, and potential future prospects.

Spatiotemporal changes to the subventricular zone stem cell pool through aging.

Through adulthood, the rodent subventricular zone (SVZ) stem cell niche generates new olfactory bulb interneurons. We had previously reported that the number of new neurons produced in the SVZ declines through aging; however, age-related changes attributable specifically to the SVZ neural stem cell (NSC) population have not been fully characterized. Here, we conducted a spatiotemporal evaluation of adult SVZ NSCs. We assessed ventricle-contacting NSCs, which together with ependymal cells form regenerative units (pinwheels) along the lateral wall of the lateral ventricle. Based on their apical GFAP-expressing process, individual NSCs were identified across the ventricle surface using serial reconstruction of the SVZ. We observed an 86% decline in total NSCs/mm² of intact ependyma in 2-year old versus 3-month-old mice, with fewer NSC processes within each aged pinwheel. This resulted in an associated 78% decline in total pinwheel units/mm². Regional analysis along the lateral ventricle surface revealed that the age-dependent decline of NSCs and pinwheels is spatially uniform and ultimately maintains the conserved ratio of olfactory bulb interneuron subtypes generated in young mice. However, the overall neurogenic output of the aged SVZ is reduced. Surprisingly, we found no significant change in the number of actively proliferating NSCs per mm² of ventricle surface. Instead, our data reveal that, although the total NSC number, pinwheel units and NSCs per pinwheel decline with age, the percentage of actively, mitotic NSCs increases, indicating that age-related declines in SVZ-mediated olfactory bulb neurogenesis occur downstream of NSC proliferation.