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

Abstract

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.

Pre-Clinical Insights into the Iron and Breast Cancer Hypothesis

“…Consistent with other preclinical reports, effects were modest and not statistically significant [ 47 ]. More importantly, the data were consistent with three population studies published between 2019 and 2021 [ 6 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 ]. Specifically, we evaluated the data ( Table 5 ) in a manner that paralleled the common approach in epidemiology, contrasting low versus high iron status, i.e., by comparing groups 1–3 vs. 4–7: the cancer incidence was 63.3 vs. 70.0 (RR = 1.10 (0.95, 1.20; p = 0.098)).…”

“…Recently reported results from population studies, systematic reviews, and meta-analyses revealed no relationship between iron status and breast cancer, a weak positive association, or a small protective effect of low iron status [ 6 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 ]. However, in most of those studies, the authors concluded that further investigation was merited.…”

“…The prevailing issues underlying those investigations included the impact of iron deficiency in limiting processes related to tumor development and the effects of excess iron-induced oxidative stress on the carcinogenic process. Papers published between 2017 and 2021 [ 6 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 ] indicated that this cycle of interest is once again repeating. The prevailing hypotheses include not only those that were prominent in the 2008–2013 timeframe but also those that are centered on iron metabolism as regulated by hepcidin, with extension to how the gut microbiome and immune system might be involved in effect mediation.…”

Macrophages and Iron: A Special Relationship

“…Apoptotic tumor cells induced the polarization of macrophages toward the alternative M2 phenotype and stimulated expression of iron-bound Lcn-2 [ 45 ]. Notably, it has been shown that Lcn-2 was sufficient to provide iron for breast cancer progression [ 46 ].…”

“…On the other hand, the iron export activity of M2 macrophages is probably functional to the growth of adjacent cells in the microenvironment. This Janus-like function may favor efficient tissue repair [ 53 ], but also the growth of tumor cells [ 5 , 46 ]. The differences in iron metabolism existing in the various macrophage subsets can be relevant also for immunometabolism, as iron may be involved in the regulation of energy production and amino acid catabolism [ 54 ].…”