Macrophage Resistance to Ionizing Radiation Exposure Is Accompanied by Decreased Cathepsin D and Increased Transferrin Receptor 1 Expression

SIMPLE SUMMARY: Resistance to treatment, particularly to radiotherapy, is still a major clinical problem in cancer management. Macrophages are abundant immune cells at the tumor microenvironment, being exposed to ionizing radiation during cancer radiotherapy. Considering the role of macrophages in tumor progression and therapy outcome, it is crucial to investigate their response to clinically relevant ionizing radiation doses for the design of new strategies to overcome tumor radio resistance. In this work, we have used a proteomic approach to evaluate the expression profile of irradiated versus non-irradiated macrophages. This analysis, supported by validation using cell-based assays, led to the identification of two main deregulated targets, cathepsin D and transferrin receptor 1, in irradiated macrophages. Investigating macrophage response to ionizing radiation could lead to the identification of deregulated pathways and molecular players that can be targeted to overcome tumor radio resistance. ABSTRACT: Purpose: To identify a molecular signature of macrophages exposed to clinically relevant ionizing radiation (IR) doses, mirroring radiotherapy sessions. Methods: Human monocyte-derived macrophages were exposed to 2 Gy/ fraction/ day for 5 days, mimicking one week of cancer patient’s radiotherapy. Protein expression profile by proteomics was performed. Results: A gene ontology analysis revealed that radiation-induced protein changes are associated with metabolic alterations, which were further supported by a reduction of both cellular ATP levels and glucose uptake. Most of the radiation-induced deregulated targets exhibited a decreased expression, as was the case of cathepsin D, a lysosomal protease associated with cell death, which was validated by Western blot. We also found that irradiated macrophages exhibited an increased expression of the transferrin receptor 1 (TfR1), which is responsible for the uptake of transferrin-bound iron. TfR1 upregulation was also found in tumor-associated mouse macrophages upon tumor irradiation. In vitro irradiated macrophages also presented a trend for increased divalent metal transporter 1 (DMT1), which transports iron from the endosome to the cytosol, and a significant increase in iron release. Conclusions: Irradiated macrophages present lower ATP levels and glucose uptake, and exhibit decreased cathepsin D expression, while increasing TfR1 expression and altering iron metabolism.