The iron load of lipocalin-2 (LCN-2) defines its pro-tumour function in clear-cell renal cell carcinoma

Bioinformatics and experimental approach identify lipocalin 2 as a diagnostic and prognostic indicator for lung adenocarcinoma

BACKGROUND

lipocalin 2 (LCN2) is a secreted glycoprotein that plays key roles in tumorigenesis and progression. Interestingly, LCN2 appears to have a contradictory function in developing lung adenocarcinoma (LUAD). Thus, we intend to explore the role of LCN2 in LUAD through bioinformatics and experimental validation.

METHODS

LCN2 expression of LUAD was investigated in the TCGA, TIMER and HPA databases. The relationship between LCN2 and prognosis was investigated by KM plotter, TCGA and GEO databases. GO, KEGG and protein-protein interactions network analysis were conducted to investigate the potential mechanism of LCN2. The relevance of LCN2 to cancer-immune infiltrates was investigated in the TCGA and TIMER databases. Quantitative reverse transcription PCR, western blot and enzyme-linked immunosorbent assay were performed to identify the expression level of LCN2 in cells and serum samples. The CCK-8, wound healing and transwell assay were used to confirm the effect of LCN2 on cell proliferation, migration and invasion in LUAD. The receiver operating characteristic curve was utilized to assess the diagnostic efficiency of LCN2 further.

RESULTS

LCN2 expression was significantly upregulated in LUAD (P < 0.05), and was correlated with the clinical stage, tumor size, lymph node metastasis and distant metastasis (P < 0.05). There was a high correlation between high LCN2 and worse prognosis in LUAD. Functional network analysis suggested that LCN2 was associated with multiple signal pathways in cancers, such as JAK-STAT, TNF, NF-κB, HIF-1 and PI3K-Akt signal pathways. In addition, the knockdown of LCN2 significantly inhibited the ability of cell proliferation, migration and invasion. Immune infiltration analysis indicated that LCN2 is associated with multiple immune cell infiltration. Notably, LCN2 demonstrated high diagnostic efficiency for LUAD (AUC = 0.818, P < 0.05), especially for stage III-IV patients could reach 0.895.

CONCLUSIONS

LCN2 as an oncogenic glycoprotein promotes the cancer progression related to immune infiltrates, which might be a potential diagnostic and prognostic marker in LUAD.

Neutrophil Gelatinase-Associated Lipocalin for the Differentiation of Mucinous Pancreatic Cystic Lesions

Undetermined pancreatic cystic lesion (PCL) differentiation benefits from endoscopic ultrasound (EUS) based on morphology and cyst fluid analysis, but room for new biomarkers exists. Our aim was to assess the intracystic and serum diagnostic value of neutrophil gelatinase-associated lipocalin (Ngal) and interleukin 1 beta (IL-1β) for differentiation of PCLs. This prospective study included patients from one tertiary hospital, evaluated between April 2018 and May 2020. EUS fine-needle aspiration or pancreatic pseudocysts drainage was the source of PCL intracystic liquid. The final diagnosis was based on surgery or EUS results (morphology, cytology, glucose, and CEA-carcinoembryogenic antigen). The intracystic samples were tested for Ngal, IL-1β, glucose, and CEA, and serum for Ngal and IL-1β. We evaluated 63 cysts, 33 pseudocysts, and 30 non-inflammatory cysts. The diagnostic sensitivity and specificity for mucinous PCL was 70.8% and 92.3% for intracystic Ngal (cut-off: 500-800 ng/dL), without correlation with serum Ngal, no matter the inclusion of infected pseudocysts. After exclusion of infected pseudocysts, the sensitivity and specificity for glucose were 87% and 75%, respectively, and for CEA, they were 87.1%, and 96.8%, respectively. Intracystic Ngal shows promise in differentiating mucinous PCLs, but researchers need to conduct further studies to confirm its effectiveness. Intracystic IL-1β and serum Ngal made no diagnostic contribution.

A biomimetic liver cancer on-a-chip reveals a critical role of LIPOCALIN-2 in promoting hepatocellular carcinoma progression

Hepatic stellate cells (HSCs) represent a significant component of hepatocellular carcinoma (HCC) microenvironments which play a critical role in tumor progression and drug resistance. Tumor-on-a-chip technology has provided a powerful in vitro platform to investigate the crosstalk between activated HSCs and HCC cells by mimicking physiological architecture with precise spatiotemporal control. Here we developed a tri-cell culture microfluidic chip to evaluate the impact of HSCs on HCC progression. On-chip analysis revealed activated HSCs contributed to endothelial invasion, HCC drug resistance and natural killer (NK) cell exhaustion. Cytokine array and RNA sequencing analysis were combined to indicate the iron-binding protein LIPOCALIN-2 (LCN-2) as a key factor in remodeling tumor microenvironments in the HCC-on-a-chip. LCN-2 targeted therapy demonstrated robust anti-tumor effects both in vitro 3D biomimetic chip and in vivo mouse model, including angiogenesis inhibition, sorafenib sensitivity promotion and NK-cell cytotoxicity enhancement. Taken together, the microfluidic platform exhibited obvious advantages in mimicking functional characteristics of tumor microenvironments and developing targeted therapies.

Role of the SLC22A17/lipocalin-2 receptor in renal endocytosis of proteins/metalloproteins: a focus on iron- and cadmium-binding proteins

The transmembrane protein SLC22A17 [or the neutrophil gelatinase-associated lipocalin/lipocalin-2 (LCN2)/24p3 receptor] is an atypical member of the SLC22 family of organic anion and cation transporters: it does not carry typical substrates of SLC22 transporters but mediates receptor-mediated endocytosis (RME) of LCN2. One important task of the kidney is the prevention of urinary loss of proteins filtered by the glomerulus by bulk reabsorption of multiple ligands via megalin:cubilin:amnionless-mediated endocytosis in the proximal tubule (PT). Accordingly, overflow, glomerular, or PT damage, as in Fanconi syndrome, results in proteinuria. Strikingly, up to 20% of filtered proteins escape the PT under physiological conditions and are reabsorbed by the distal nephron. The renal distal tubule and collecting duct express SLC22A17, which mediates RME of filtered proteins that evade the PT but with limited capacity to prevent proteinuria under pathological conditions. The kidney also prevents excretion of filtered essential and nonessential transition metals, such as iron or cadmium, respectively, that are largely bound to proteins with high affinity, e.g., LCN2, transferrin, or metallothionein, or low affinity, e.g., microglobulins or albumin. Hence, increased uptake of transition metals may cause nephrotoxicity. Here, we assess the literature on SLC22A17 structure, topology, tissue distribution, regulation, and assumed functions, emphasizing renal SLC22A17, which has relevance for physiology, pathology, and nephrotoxicity due to the accumulation of proteins complexed with transition metals, e.g., cadmium or iron. Other putative renal functions of SLC22A17, such as its contribution to osmotic stress adaptation, protection against urinary tract infection, or renal carcinogenesis, are discussed.

Lipocalin-2: A Nurturer of Tumor Progression and a Novel Candidate for Targeted Cancer Therapy

Within the tumor microenvironment (TME) exists a complex signaling network between cancer cells and stromal cells, which determines the fate of tumor progression. Hence, interfering with this signaling network forms the basis for cancer therapy. Yet, many types of cancer, in particular, solid tumors, are refractory to the currently used treatments, so there is an urgent need for novel molecular targets that could improve current anti-cancer therapeutic strategies. Lipocalin-2 (Lcn-2), a secreted siderophore-binding glycoprotein that regulates iron homeostasis, is highly upregulated in various cancer types. Due to its pleiotropic role in the crosstalk between cancer cells and stromal cells, favoring tumor progression, it could be considered as a novel biomarker for prognostic and therapeutic purposes. However, the exact signaling route by which Lcn-2 promotes tumorigenesis remains unknown, and Lcn-2-targeting moieties are largely uninvestigated. This review will (i) provide an overview on the role of Lcn-2 in orchestrating the TME at the level of iron homeostasis, macrophage polarization, extracellular matrix remodeling, and cell migration and survival, and (ii) discuss the potential of Lcn-2 as a promising novel drug target that should be pursued in future translational research.

The emerging roles of MAPK-AMPK in ferroptosis regulatory network

Ferroptosis, a newform of programmed cell death, driven by peroxidative damages of polyunsaturated-fatty-acid-containing phospholipids in cellular membranes and is extremely dependent on iron ions, which is differs characteristics from traditional cell death has attracted greater attention. Based on the curiosity of this new form of regulated cell death, there has a tremendous progress in the field of mechanistic understanding of ferroptosis recent years. Ferroptosis is closely associated with the development of many diseases and involved in many diseases related signaling pathways. Not only a variety of oncoproteins and tumor suppressors can regulate ferroptosis, but multiple oncogenic signaling pathways can also have a regulatory effect on ferroptosis. Ferroptosis results in the accumulation of large amounts of lipid peroxides thus involving the onset of oxidative stress and energy stress responses. The MAPK pathway plays a critical role in oxidative stress and AMPK acts as a sensor of cellular energy and is involved in the regulation of the energy stress response. Moreover, activation of AMPK can induce the occurrence of autophagy-dependent ferroptosis and p53-activated ferroptosis. In recent years, there have been new advances in the study of molecular mechanisms related to the regulation of ferroptosis by both pathways. In this review, we will summarize the molecular mechanisms by which the MAPK-AMPK signaling pathway regulates ferroptosis. Meanwhile, we sorted out the mysterious relationship between MAPK and AMPK, described the crosstalk among ferroptosis and MAPK-AMPK signaling pathways, and summarized the relevant ferroptosis inducers targeting this regulatory network. This will provide a new field for future research on ferroptosis mechanisms and provide a new vision for cancer treatment strategies. Video Abstract.

The von Hippel-Lindau Tumor Suppressor Gene Mutations Modulate Lipocalin-2 Expression in Ferroptotic-Inflammatory Pathways

A previous study of an animal model with tumor suppressor gene von Hippel-Lindau (VHL) conditional knockdown suggested that tissue inflammation and fibrosis play important roles in the development of clear-cell renal cell carcinoma (ccRCC), which is consistent with the epidemiological evidence linking inflammatory kidney disease and renal cancer. Ferroptosis and inflammation have been linked in a recent study, but the exact mechanism remains unclear. This study is aimed at investigating the mechanism of lipocalin-2- (LCN-2-) mediated ferroptosis and inflammation in vhl-mutated HK-2 cells and mouse primary proximal tubule cells (mRTCs) and the polarization of macrophage RAW 264.7 cells. Based on the levels of lipid reactive oxygen species (ROS) and the expression of glutathione peroxidase 4 (GPX4) in HK-2 cells, we observed that a VHL mutation increased ROS production and depressed GPX4 expression, whereas LCN-2 knockdown reversed these effects. Accordingly, VHL appears to affect ferroptosis in an LCN-2-dependent manner. We also revealed that LCN-2 sensitizes HK-2 cells to inflammation and macrophage RAW 264.7 cells to M1-like polarization. This study provides novel insights into the potential therapeutic target and strategy for attenuating the progression of ccRCC by revealing the role of VHL in regulating chronic inflammation within the LCN-2-ferroptosis pathway.

Croix CM, Sinha D. Increased LCN2 (lipocalin 2) in the RPE decreases autophagy and activates inflammasome-ferroptosis processes in a mouse model of dry AMD

In dry age-related macular degeneration (AMD), LCN2 (lipocalin 2) is upregulated. Whereas LCN2 has been implicated in AMD pathogenesis, the mechanism remains unknown. Here, we report that in retinal pigmented epithelial (RPE) cells, LCN2 regulates macroautophagy/autophagy, in addition to maintaining iron homeostasis. LCN2 binds to ATG4B to form an LCN2-ATG4B-LC3-II complex, thereby regulating ATG4B activity and LC3-II lipidation. Thus, increased LCN2 reduced autophagy flux. Moreover, RPE cells from cryba1 KO, as well as sting1 KO and Sting1Gt mutant mice (models with abnormal iron chelation), showed decreased autophagy flux and increased LCN2, indicative of CGAS- and STING1-mediated inflammasome activation. Live cell imaging of RPE cells with elevated LCN2 also showed a correlation between inflammasome activation and increased fluorescence intensity of the Liperfluo dye, indicative of oxidative stress-induced ferroptosis. Interestingly, both in human AMD patients and in mouse models with a dry AMD-like phenotype (cryba1 cKO and KO), the LCN2 homodimer variant is increased significantly compared to the monomer. Sub-retinal injection of the LCN2 homodimer secreted by RPE cells into NOD-SCID mice leads to retinal degeneration. In addition, we generated an LCN2 monoclonal antibody that neutralizes both the monomer and homodimer variants and rescued autophagy and ferroptosis activities in cryba1 cKO mice. Furthermore, the antibody rescued retinal function in cryba1 cKO mice as assessed by electroretinography. Here, we identify a molecular pathway whereby increased LCN2 elicits pathophysiology in the RPE, cells known to drive dry AMD pathology, thus providing a possible therapeutic strategy for a disease with no current treatment options.Abbreviations: ACTB: actin, beta; Ad-GFP: adenovirus-green fluorescent protein; Ad-LCN2: adenovirus-lipocalin 2; Ad-LCN2-GFP: adenovirus-LCN2-green fluorescent protein; LCN2AKT2: AKT serine/threonine kinase 2; AMBRA1: autophagy and beclin 1 regulator 1; AMD: age-related macular degeneration; ARPE19: adult retinal pigment epithelial cell line-19; Asp278: aspartate 278; ATG4B: autophagy related 4B cysteine peptidase; ATG4C: autophagy related 4C cysteine peptidase; ATG7: autophagy related 7; ATG9B: autophagy related 9B; BLOC-1: biogenesis of lysosomal organelles complex 1; BLOC1S1: biogenesis of lysosomal organelles complex 1 subunit 1; C57BL/6J: C57 black 6J; CGAS: cyclic GMP-AMP synthase; ChQ: chloroquine; cKO: conditional knockout; Cys74: cysteine 74; Dab2: DAB adaptor protein 2; Def: deferoxamine; DHE: dihydroethidium; DMSO: dimethyl sulfoxide; ERG: electroretinography; FAC: ferric ammonium citrate; Fe2+: ferrous; FTH1: ferritin heavy chain 1; GPX: glutathione peroxidase; GST: glutathione S-transferase; H2O2: hydrogen peroxide; His280: histidine 280; IFNL/IFNλ: interferon lambda; IL1B/IL-1β: interleukin 1 beta; IS: Inner segment; ITGB1/integrin β1: integrin subunit beta 1; KO: knockout; LC3-GST: microtubule associated protein 1 light chain 3-GST; C-terminal fusion; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; LCN2: lipocalin 2; mAb: monoclonal antibody; MDA: malondialdehyde; MMP9: matrix metallopeptidase 9; NLRP3: NLR family pyrin domain containing 3; NOD-SCID: nonobese diabetic-severe combined immunodeficiency; OS: outer segment; PBS: phosphate-buffered saline; PMEL/PMEL17: premelanosome protein; RFP: red fluorescent protein; rLCN2: recombinant LCN2; ROS: reactive oxygen species; RPE SM: retinal pigmented epithelium spent medium; RPE: retinal pigment epithelium; RSL3: RAS-selective lethal; scRNAseq: single-cell ribonucleic acid sequencing; SD-OCT: spectral domain optical coherence tomography; shRNA: small hairpin ribonucleic acid; SM: spent medium; SOD1: superoxide dismutase 1; SQSTM1/p62: sequestosome 1; STAT1: signal transducer and activator of transcription 1; STING1: stimulator of interferon response cGAMP interactor 1; TYR: tyrosinase; VCL: vinculin; WT: wild type.

Puzzling Out Iron Complications in Cancer Drug Resistance

Iron metabolism are frequently disrupted in cancer. Patients with cancer are prone to anemia and receive transfusions frequently; the condition which results in iron overload, contributing to serious therapeutic complications. Iron is introduced as a carcinogen that may increase tumor growth. However, investigations regarding its impact on response to chemotherapy, particularly the induction of drug resistance are still limited. Here, iron contribution to cell signaling and various molecular mechanisms underlying iron-mediated drug resistance are described. A dual role of this vital element in cancer treatment is also addressed. On one hand, the need to administer iron chelators to surmount iron overload and improve the sensitivity of tumor cells to chemotherapy is discussed. On the other hand, the necessary application of iron as a therapeutic option by iron-oxide nanoparticles or ferroptosis inducers is explained. Authors hope that this paper can help unravel the clinical complications related to iron in cancer therapy.

Understanding the Potential and Risk of Bacterial Siderophores in Cancer

Siderophores are iron chelating molecules produced by nearly all organisms, most notably by bacteria, to efficiently sequester the limited iron that is available in the environment. Siderophores are an essential component of mammalian iron homeostasis and the ongoing interspecies competition for iron. Bacteria produce a broad repertoire of siderophores with a canonical role in iron chelation and the capacity to perform versatile functions such as interacting with other microbes and the host immune system. Siderophores are a vast area of untapped potential in the field of cancer research because cancer cells demand increased iron concentrations to sustain rapid proliferation. Studies investigating siderophores as therapeutics in cancer generally focused on the role of a few siderophores as iron chelators; however, these studies are limited and some show conflicting results. Moreover, siderophores are biologically conserved, structurally diverse molecules that perform additional functions related to iron chelation. Siderophores also have a role in inflammation due to their iron acquisition and chelation properties. These diverse functions may contribute to both risks and benefits as therapeutic agents in cancer. The potential of siderophore-mediated iron and bacterial modulation to be used in the treatment of cancer warrants further investigation. This review discusses the wide range of bacterial siderophore functions and their utilization in cancer treatment to further expand their functional relevance in cancer detection and treatment.

Relation-Aware Shared Representation Learning for Cancer Prognosis Analysis With Auxiliary Clinical Variables and Incomplete Multi-Modality Data

The integrative analysis of complementary phenotype information contained in multi-modality data (e.g., histopathological images and genomic data) has advanced the prognostic evaluation of cancers. However, multi-modality based prognosis analysis confronts two challenges: (1) how to explore underlying relations inherent in different modalities data for learning compact and discriminative multi-modality representations; (2) how to take full consideration of incomplete multi-modality data for constructing accurate and robust prognostic model, since a host of complete multi-modality data are not always available. Additionally, many existing multi-modality based prognostic methods commonly ignore relevant clinical variables (e.g., grade and stage), which, however, may provide supplemental information to promote the performance of model. In this paper, we propose a relation-aware shared representation learning method for prognosis analysis of cancers, which makes full use of clinical information and incomplete multi-modality data. The proposed method learns multi-modal shared space tailored for prognostic model via a dual mapping. Within the shared space, it equips with relational regularizers to explore the potential relations (i.e., feature-label and feature-feature relations) among multi-modality data for inducing discriminatory representations and simultaneously obtaining extra sparsity for alleviating overfitting. Moreover, it regresses and incorporates multiple auxiliary clinical attributes with dynamic coefficients to meliorate performance. Furthermore, in training stage, a partial mapping strategy is employed to extend and train a more reliable model with incomplete multi-modality data. We have evaluated our method on three public datasets derived from The Cancer Genome Atlas (TCGA) project, and the experimental results demonstrate the superior performance of the proposed method.

NGAL as a Potential Target in Tumor Microenvironment

The signaling network between cancer and stromal cells plays a crucial role in tumor microenvironment. The fate of tumor progression mainly depends on the huge amount of information that these cell populations exchange from the onset of neoplastic transformation. Interfering with such signaling has been producing exciting results in cancer therapy: just think of anti-PD-1/anti-PD-L1/anti-CTLA-4 antibodies that, acting as immune checkpoint inhibitors, interrupt the inhibitory signaling exerted by cancer cells on immune cells or the CAR-T technology that fosters the reactivation of anti-tumoral immunity in a restricted group of leukemias and lymphomas. Nevertheless, many types of cancers, in particular solid tumors, are still refractory to these treatments, so the identification of novel molecular targets in tumor secretome would benefit from implementation of current anti-cancer therapeutical strategies. Neutrophil Gelatinase-Associated Lipocalin (NGAL) is a secreted protein abundantly expressed in the secretome of various human tumors. It represents a promising target for the multiple roles that are played inside cancer and stromal cells, and also overall in their cross-talk. The review focuses on the different roles of NGAL in tumor microenvironment and in cancer senescence-associated secretory phenotype (SASP), highlighting the most crucial functions that could be eventually targetable in cancer therapy.

NGAL as a Potential Target in Tumor Microenvironment

The signaling network between cancer and stromal cells plays a crucial role in tumor microenvironment. The fate of tumor progression mainly depends on the huge amount of information that these cell populations exchange from the onset of neoplastic transformation. Interfering with such signaling has been producing exciting results in cancer therapy: just think of anti-PD-1/anti-PD-L1/anti-CTLA-4 antibodies that, acting as immune checkpoint inhibitors, interrupt the inhibitory signaling exerted by cancer cells on immune cells or the CAR-T technology that fosters the reactivation of anti-tumoral immunity in a restricted group of leukemias and lymphomas. Nevertheless, many types of cancers, in particular solid tumors, are still refractory to these treatments, so the identification of novel molecular targets in tumor secretome would benefit from implementation of current anti-cancer therapeutical strategies. Neutrophil Gelatinase-Associated Lipocalin (NGAL) is a secreted protein abundantly expressed in the secretome of various human tumors. It represents a promising target for the multiple roles that are played inside cancer and stromal cells, and also overall in their cross-talk. The review focuses on the different roles of NGAL in tumor microenvironment and in cancer senescence-associated secretory phenotype (SASP), highlighting the most crucial functions that could be eventually targetable in cancer therapy.

NGAL as a Potential Target in Tumor Microenvironment

The signaling network between cancer and stromal cells plays a crucial role in tumor microenvironment. The fate of tumor progression mainly depends on the huge amount of information that these cell populations exchange from the onset of neoplastic transformation. Interfering with such signaling has been producing exciting results in cancer therapy: just think of anti-PD-1/anti-PD-L1/anti-CTLA-4 antibodies that, acting as immune checkpoint inhibitors, interrupt the inhibitory signaling exerted by cancer cells on immune cells or the CAR-T technology that fosters the reactivation of anti-tumoral immunity in a restricted group of leukemias and lymphomas. Nevertheless, many types of cancers, in particular solid tumors, are still refractory to these treatments, so the identification of novel molecular targets in tumor secretome would benefit from implementation of current anti-cancer therapeutical strategies. Neutrophil Gelatinase-Associated Lipocalin (NGAL) is a secreted protein abundantly expressed in the secretome of various human tumors. It represents a promising target for the multiple roles that are played inside cancer and stromal cells, and also overall in their cross-talk. The review focuses on the different roles of NGAL in tumor microenvironment and in cancer senescence-associated secretory phenotype (SASP), highlighting the most crucial functions that could be eventually targetable in cancer therapy.

Lipocalin 2 as a link between ageing, risk factor conditions and age-related brain diseases

Chronic (neuro)inflammation plays an important role in many age-related central nervous system (CNS) diseases, including Alzheimer's disease, Parkinson's disease and vascular dementia. Inflammation also characterizes many conditions that form a risk factor for these CNS disorders, such as physical inactivity, obesity and cardiovascular disease. Lipocalin 2 (Lcn2) is an inflammatory protein shown to be involved in different age-related CNS diseases, as well as risk factor conditions thereof. Lcn2 expression is increased in the periphery and the brain in different age-related CNS diseases and also their risk factor conditions. Experimental studies indicate that Lcn2 contributes to various neuropathophysiological processes of age-related CNS diseases, including exacerbated neuroinflammation, cell death and iron dysregulation, which may negatively impact cognitive function. We hypothesize that increased Lcn2 levels as a result of age-related risk factor conditions may sensitize the brain and increase the risk to develop age-related CNS diseases. In this review we first provide a comprehensive overview of the known functions of Lcn2, and its effects in the CNS. Subsequently, this review explores Lcn2 as a potential (neuro)inflammatory link between different risk factor conditions and the development of age-related CNS disorders. Altogether, evidence convincingly indicates Lcn2 as a key constituent in ageing and age-related brain diseases.

Iron-Bound Lipocalin-2 Protects Renal Cell Carcinoma from Ferroptosis

While the importance of the iron-load of lipocalin-2 (Lcn-2) in promoting tumor progression is widely appreciated, underlying molecular mechanisms largely remain elusive. Considering its role as an iron-transporter, we aimed at clarifying iron-loaded, holo-Lcn-2 (hLcn-2)-dependent signaling pathways in affecting renal cancer cell viability. Applying RNA sequencing analysis in renal CAKI1 tumor cells to explore highly upregulated molecular signatures in response to hLcn-2, we identified a cluster of genes (SLC7A11, GCLM, GLS), which are implicated in regulating ferroptosis. Indeed, hLcn-2-stimulated cells are protected from erastin-induced ferroptosis. We also noticed a rapid increase in reactive oxygen species (ROS) with subsequent activation of the antioxidant Nrf2 pathway. However, knocking down Nrf2 by siRNA was not sufficient to induce erastin-dependent ferroptotic cell death in hLcn-2-stimulated tumor cells. In contrast, preventing oxidative stress through N-acetyl-l-cysteine (NAC) supplementation was still able to induce erastin-dependent ferroptotic cell death in hLcn-2-stimulated tumor cells. Besides an oxidative stress response, we noticed activation of the integrated stress response (ISR), shown by enhanced phosphorylation of eIF-2α and induction of ATF4 after hLcn-2 addition. ATF4 knockdown as well as inhibition of the ISR sensitized hLcn-2-treated renal tumor cells to ferroptosis, thus linking the ISR to pro-tumor characteristics of hLcn-2. Our study provides mechanistic details to better understand tumor pro-survival pathways initiated by iron-loaded Lcn-2.

Role of iron in cancer

Iron is an essential metal for cellular metabolism. The reduced form of iron is a cofactor in numerous redox reactions in the cell and is therefore required for many vital physiological functions. Since iron is an oxidatively active metal, its homeostasis is tightly regulated in healthy cell. Most of iron exists in a protein-bound form, in erythrocytes as the heme compound hemoglobin, and in storage proteins such as ferritin, hemosiderin and myoglobin. Iron also is bound to proteins and non-heme enzymes involved in oxidation-reduction reactions and the transfer of electrons. There is no free iron inside the cell, however a small fraction of loosely bound iron is found in the cytoplasm. This poorly defined pool of ferrous iron is called labile iron pool. Under pathological conditions iron homeostasis may be disrupted at different levels including absorption, systemic transportation, and cellular uptake and storage. Cancer cells display dysregulated iron homeostasis and, for reasons yet poorly understood, require more iron for their metabolism and growth. As a result, in cancer cells labile iron pool is increased, and loosely bound iron catalyzes Fenton reaction and perhaps other reactions that generate reactive oxygen species. Oxygen-derived free radicals produce DNA mutations, damage proteins and lipids resulting in either cell death or cell transformation.

A novel immune-related prognostic signature in epithelial ovarian carcinoma

The immune response is associated with the progression and prognosis of epithelial ovarian cancer (EOC). However, the roles of infiltrated immune cells and immune-related genes (IRGs) in EOC have not been reported comprehensively. In the current study, the differentially expressed genes (DEGs) were filtered based on the integrated gene expression data acquired from The University of California at Santa Cruz (UCSC) Genome Browser. Then, IRGs and transcriptional factors (TFs) were screened based on the ImmPort database and Cistrome database. A total of 501 differentially expressed IRGs, and 76 TFs were detected. A TF-mediated network was constructed by univariate Cox analysis to reveal the potential regulatory mechanisms of IRGs. Next, a nine immune-based prognostic risk model using nine IRGs (PI3, CXCL10, CXCL11, LCN6, CCL17, CCL25, MIF, CX3CR1, and CSPG5) was established. Based on the risk score worked out from the signature, the EOC patients could be classified into low-risk and high-risk groups. Furthermore, the immune landscapes, elevated by the cell-type identification by estimating relative subsets of RNA transcripts (CIBERSORT) algorithm and the Tumor Immune Estimation Resource (TIMER) database, effectuated different patterns in two groups. Thus, an immune-based prognostic risk model of EOC elucidates the immune status in the tumor microenvironment, and hence, could be used for prognosis.

TAM-ing the CIA-Tumor-Associated Macrophages and Their Potential Role in Unintended Side Effects of Therapeutics for Cancer-Induced Anemia

Cancer-induced anemia (CIA) is a common consequence of neoplasia and has a multifactorial pathophysiology. The immune response and tumor treatment, both intended to primarily target malignant cells, also affect erythropoiesis in the bone marrow. In parallel, immune activation inevitably induces the iron-regulatory hormone hepcidin to direct iron fluxes away from erythroid progenitors and into compartments of the mononuclear phagocyte system. Moreover, many inflammatory mediators inhibit the synthesis of erythropoietin, which is essential for stimulation and differentiation of erythroid progenitor cells to mature cells ready for release into the blood stream. These pathophysiological hallmarks of CIA imply that the bone marrow is not only deprived of iron as nutrient but also of erythropoietin as central growth factor for erythropoiesis. Tumor-associated macrophages (TAM) are present in the tumor microenvironment and display altered immune and iron phenotypes. On the one hand, their functions are altered by adjacent tumor cells so that they promote rather than inhibit the growth of malignant cells. As consequences, TAM may deliver iron to tumor cells and produce reduced amounts of cytotoxic mediators. Furthermore, their ability to stimulate adaptive anti-tumor immune responses is severely compromised. On the other hand, TAM are potential off-targets of therapeutic interventions against CIA. Red blood cell transfusions, intravenous iron preparations, erythropoiesis-stimulating agents and novel treatment options for CIA may interfere with TAM function and thus exhibit secondary effects on the underlying malignancy. In this Hypothesis and Theory, we summarize the pathophysiological hallmarks, clinical implications and treatment strategies for CIA. Focusing on TAM, we speculate on the potential intended and unintended effects that therapeutic options for CIA may have on the innate immune response and, consequently, on the course of the underlying malignancy.

Iron-Bound Lipocalin-2 from Tumor-Associated Macrophages Drives Breast Cancer Progression Independent of Ferroportin

Macrophages supply iron to the breast tumor microenvironment by enforced secretion of lipocalin-2 (Lcn-2)-bound iron as well as the increased expression of the iron exporter ferroportin (FPN). We aimed at identifying the contribution of each pathway in supplying iron for the growing tumor, thereby fostering tumor progression. Analyzing the expression profiles of Lcn-2 and FPN using the spontaneous polyoma-middle-T oncogene (PyMT) breast cancer model as well as mining publicly available TCGA (The Cancer Genome Atlas) and GEO Series(GSE) datasets from the Gene Expression Omnibus database (GEO), we found no association between tumor parameters and Lcn-2 or FPN. However, stromal/macrophage-expression of Lcn-2 correlated with tumor onset, lung metastases, and recurrence, whereas FPN did not. While the total iron amount in wildtype and Lcn-2^-/- PyMT tumors showed no difference, we observed that tumor-associated macrophages from Lcn-2^-/- compared to wildtype tumors stored more iron. In contrast, Lcn-2^-/- tumor cells accumulated less iron than their wildtype counterparts, translating into a low migratory and proliferative capacity of Lcn-2^-/- tumor cells in a 3D tumor spheroid model in vitro. Our data suggest a pivotal role of Lcn-2 in tumor iron-management, affecting tumor growth. This study underscores the role of iron for tumor progression and the need for a better understanding of iron-targeted therapy approaches.

A Scoping Review on Lipocalin-2 and Its Role in Non-Alcoholic Steatohepatitis and Hepatocellular Carcinoma

Excess calorie intake and a sedentary lifestyle have made non-alcoholic fatty liver disease (NAFLD) one of the fastest growing forms of liver disease of the modern world. It is characterized by abnormal accumulation of fat in the liver and can range from simple steatosis and non-alcoholic steatohepatitis (NASH) to cirrhosis as well as development of hepatocellular carcinoma (HCC). Biopsy is the golden standard for the diagnosis and differentiation of all NAFLD stages, but its invasiveness poses a risk for patients, which is why new, non-invasive ways of diagnostics ought to be discovered. Lipocalin-2 (LCN2), which is a part of the lipocalin transport protein family, is a protein formally known for its role in iron transport and in inflammatory response. However, in recent years, its implication in the pathogenesis of NAFLD has become apparent. LCN2 shows significant upregulation in several benign and malignant liver diseases, making it a good candidate for the NAFLD biomarker or even a therapeutic target. What makes LCN2 more interesting to study is the fact that it is overexpressed in HCC development induced by chronic NASH, which is one of the primary causes of cancer-related deaths. However, to this day, neither its role as a biomarker for NAFLD nor the molecular mechanisms of its implication in NAFLD pathogenesis have been completely elucidated. This review aims to gather and closely dissect the current knowledge about, sometimes conflicting, evidence on LCN2 as a biomarker for NAFLD, its involvement in NAFLD, and NAFLD-HCC related pathogenesis, while comparing it to the findings in similar pathologies.

Molecular and Metabolic Subtypes in Sporadic and Inherited Clear Cell Renal Cell Carcinoma

The promise of personalized medicine is a therapeutic advance where tumor signatures obtained from different omics platforms, such as genomics, transcriptomics, proteomics, and metabolomics, in addition to environmental factors including metals and metalloids, are used to guide the treatments. Clear cell renal carcinoma (ccRCC), the most common type of kidney cancer, can be sporadic (frequently) or genetic (rare), both characterized by loss of the von Hippel-Lindau (VHL) gene that controls hypoxia inducible factors. Recently, several genomic subtypes were identified with different prognoses. Transcriptomics, proteomics, metabolomics and metallomic data converge on altered metabolism as the principal feature of the disease. However, in view of multiple biochemical alterations and high level of tumor heterogeneity, identification of clearly defined subtypes is necessary for further improvement of treatments. In the future, single-cell combined multi-omics approaches will be the next generation of analyses gaining deeper insights into ccRCC progression and allowing for design of specific signatures, with better prognostic/predictive clinical applications.

Iron Metabolism in the Tumor Microenvironment: Contributions of Innate Immune Cells

Cells of the innate immune system are a major component of the tumor microenvironment. They play complex and multifaceted roles in the regulation of cancer initiation, growth, metastasis and responses to therapeutics. Innate immune cells like neutrophils and macrophages are recruited to cancerous tissues by chemotactic molecules released by cancer cells and cancer-associated stromal cells. Once they reach the tumor, they can be instructed by a network of proteins, nucleic acids and metabolites to exert protumoral or antitumoral functions. Altered iron metabolism is a feature of cancer. Epidemiological studies suggest that increased presence of iron and/or iron binding proteins is associated with increased risks of cancer development. It has been shown that iron metabolism is involved in shaping the immune landscapes in inflammatory/infectious diseases and cancer-associated inflammation. In this article, we will dissect the contribution of macrophages and neutrophils to dysregulated iron metabolism in malignant cells and its impact on cancer growth and metastasis. The mechanisms involved in regulating the actions of macrophages and neutrophils will also be discussed. Moreover, we will examine the effects of iron metabolism on the phenotypes of innate immune cells. Both iron chelating and overloading agents are being explored in cancer treatment. This review highlights alternative strategies for management of iron content in cancer cells by targeting the iron donation and modulation properties of macrophages and neutrophils in the tumor microenvironment.

Role of neutrophil gelatinase-associated lipocalin in renal cell carcinoma

Human neutrophil gelatinase-associated lipocalin (NGAL) is a glycoprotein present in a wide variety of tissues and cell types. It exists as a monomer of 25 kDa, a homodimer of 45 kDa or a heterodimer of 135 kDa (disulfide bound to latent matrix metalloproteinase-9). NGAL is considered the biochemical gold standard for the early diagnosis of acute kidney injury and has attracted much attention as a diagnostic biomarker. NGAL has controversial (i.e. both beneficial and detrimental) effects on cellular processes associated with tumor development, such as cell proliferation, survival, migration, invasion and drug resistance. Therefore, the present review aimed at clarifying the role of NGAL in renal cell carcinoma (RCC). Relevant studies of NGAL and RCC were searched in PubMed and relevant information about the structure, expression, function and mechanism of NGAL in RCC were summarized. Finally, the following conclusions could be drawn from the literature: i) NGAL can be detected in cancer tissues, serum and urine of patients with RCC; ii) NGAL is not a suitable diagnostic marker for early screening of RCC; iii) NGAL expression may be used to predict the prognosis of patients with RCC; and iv) Further research on NGAL may be helpful to decrease sunitinib resistance and find new treatment strategies for RCC.

Iron Dysregulation in Human Cancer: Altered Metabolism, Biomarkers for Diagnosis, Prognosis, Monitoring and Rationale for Therapy

Simple Summary

Iron is the more abundant metal ion in humans. It is essential for life as it has a role in various cellular processes involved, for instance, in cell metabolism and DNA synthesis. These functions are crucial for cell proliferation, and it is therefore not surprising that iron is accumulated in tumors. In this review, we describe normal and altered iron homeostasis mechanisms. We also provide a vision of iron-related proteins with altered expression in cancers and discuss their potential as diagnostic and/or prognostic biomarkers. Finally, we give an overview of therapeutic strategies acting on iron metabolism to fight against cancers.

Abstract

Iron (Fe) is a trace element that plays essential roles in various biological processes such as DNA synthesis and repair, as well as cellular energy production and oxygen transport, and it is currently widely recognized that iron homeostasis is dysregulated in many cancers. Indeed, several iron homeostasis proteins may be responsible for malignant tumor initiation, proliferation, and for the metastatic spread of tumors. A large number of studies demonstrated the potential clinical value of utilizing these deregulated proteins as prognostic and/or predictive biomarkers of malignancy and/or response to anticancer treatments. Additionally, the iron present in cancer cells and the importance of iron in ferroptosis cell death signaling pathways prompted the development of therapeutic strategies against advanced stage or resistant cancers. In this review, we select relevant and promising studies in the field of iron metabolism in cancer research and clinical oncology. Besides this, we discuss some co-existing discrepant findings. We also present and discuss the latest lines of research related to targeting iron, or its regulatory pathways, as potential promising anticancer strategies for human therapy. Iron chelators, such as deferoxamine or iron-oxide-based nanoparticles, which are already tested in clinical trials, alone or in combination with chemotherapy, are also reported.

Iron and Cancer: 2020 Vision

New and provocative insights into the relationships between iron and cancer have been uncovered in recent years. These include delineation of connections that link cellular iron to DNA repair, genomic integrity, and oncogenic signaling as well as the discovery of ferroptosis, a novel iron-dependent form of cell death. In parallel, new molecules and pathways that regulate iron influx, intracellular iron trafficking, and egress in normal cells, and their perturbations in cancer have been discovered. In addition, insights into the unique properties of iron handling in tumor-initiating cells (cancer stem cells), novel contributions of the tumor microenvironment to the uptake and regulation of iron in cancer cells, and new therapeutic modalities that leverage the iron dependence of cancer have emerged.

Iron: The cancer connection

Iron plays an essential role in normal biological processes: The generation of cellular energy, oxygen transport, DNA synthesis and repair are all processes that require iron-coordinated proteins, either as elemental iron, heme or iron-sulfur clusters. As a transition metal with two major biological oxidation states, iron is also a critical intermediate in the generation of reactive oxygen species that can damage cellular structures and contribute to both aging and cancer. In this review, we focus on experimental and epidemiologic evidence that links iron and cancer, as well as strategies that have been proposed to either reduce or increase cellular iron for cancer therapy.

Altered Iron Metabolism and Impact in Cancer Biology, Metastasis, and Immunology

Iron is an essential nutrient that plays a complex role in cancer biology. Iron metabolism must be tightly controlled within cells. Whilst fundamental to many cellular processes and required for cell survival, excess labile iron is toxic to cells. Increased iron metabolism is associated with malignant transformation, cancer progression, drug resistance and immune evasion. Depleting intracellular iron stores, either with the use of iron chelating agents or mimicking endogenous regulation mechanisms, such as microRNAs, present attractive therapeutic opportunities, some of which are currently under clinical investigation. Alternatively, iron overload can result in a form of regulated cell death, ferroptosis, which can be activated in cancer cells presenting an alternative anti-cancer strategy. This review focuses on alterations in iron metabolism that enable cancer cells to meet metabolic demands required during different stages of tumorigenesis in relation to metastasis and immune response. The strength of current evidence is considered, gaps in knowledge are highlighted and controversies relating to the role of iron and therapeutic targeting potential are discussed. The key question we address within this review is whether iron modulation represents a useful approach for treating metastatic disease and whether it could be employed in combination with existing targeted drugs and immune-based therapies to enhance their efficacy.

Macrophage-secreted Lipocalin-2 Promotes Regeneration of Injured Primary Murine Renal Tubular Epithelial Cells

Lipocalin-2 (Lcn-2) is rapidly upregulated in macrophages after renal tubular injury and acts as renoprotective and pro-regenerative agent. Lcn-2 possesses the ability to bind and transport iron with high affinity. Therefore, the present study focuses on the decisive role of the Lcn-2 iron-load for its pro-regenerative function. Primary mouse tubular epithelial cells were isolated from kidney tissue of wildtype mice and incubated with 5μM Cisplatin for 24h to induce injury. Bone marrow-derived macrophages of wildtype and Lcn-2^-/- mice were isolated and polarized with IL-10 towards an anti-inflammatory, iron-release phenotype. Their supernatants as well as recombinant iron-loaded holo-Lcn-2 was used for stimulation of Cisplatin-injured tubular epithelial cells. Incubation of tubular epithelial cells with wildtype supernatants resulted in less damage and induced cellular proliferation, whereas in absence of Lcn-2 no protective effect was observed. Epithelial integrity as well as cellular proliferation showed a clear protection upon rescue experiments applying holo-Lcn-2. Notably, we detected a positive correlation between total iron amounts in tubular epithelial cells and cellular proliferation, which, in turn, reinforced the assumed link between availability of Lcn-2-bound iron and recovery. We hypothesize that macrophage-released Lcn-2-bound iron is provided to tubular epithelial cells during toxic cell damage, whereby injury is limited and recovery is favored.