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Aggressive breast cancers secrete heme metabolites to alter macrophage immune suppression and function

Year: 2026
Type: article
Abstract: The heme catabolism pathway is often elevated in aggressive cancers; however, the impact of this pathway and some of its byproducts on the tumor microenvironment remain largely unknown. In human breast cancers, tumor expression of the heme catabolizing enzyme heme oxygenase-1 (HO-1/ HMOX1 ) is positively associated with macrophage abundance. In mouse mammary tumors, knockdown of Hmox1 significantly decreased tumor growth and lung metastasis. Analysis of mammary tumor interstitial fluid compared to matching plasma revealed that the heme metabolite bilirubin was elevated intratumorally, which could be partially reversed via Hmox1 knockdown. Further investigation revealed that bilirubin nearly ablates macrophage engulfment of dead tumor cells and significantly increases macrophage T cell suppression. Mammary tumors harboring Hmox1 knockdown had a significant decrease in tumor growth rate and number of pro-metastatic CD206 + macrophages upon treatment with αPD-1. Depletion of intratumoral bilirubin levels impacts pro-tumor macrophage populations, particularly in combination with immunotherapy, demonstrating that heme catabolism and bilirubin act as immunomodulators in cancer.
DOI: 10.1038/s42003-026-10212-0
Authors: Michelle M. Williams, Jessica L. Christenson, Nicole S. Spoelstra, Sabrina A. Hafeez, Li-Wei Kuo, Lyndsey S. Crump, Kathleen I. O’Neill, Alexis A. Hill, Angelica Phan, Lauren M. McCue, Thomas M. Hintelmann, Juanantonio Ruiz, Haley E. Sax, Jared Williams, Meher Preethi Boorgula, Andrew Goodspeed, Jill E. Slansky, Jennifer K. Richer
Institutions: University of Pittsburgh, UPMC Health System, UPMC Hillman Cancer Center, University of Pittsburgh Medical Center, University of Colorado Anschutz, University of Colorado Cancer Center
Language: English
Subfield: Immunology
Open Access status: gold
Estimated APC paid: $4,190
Funders: National Science Foundation, U.S. Department of Defense, Colorado State University, University of Pittsburgh, Cancer League of Colorado, National Institutes of Health, University of Colorado Boulder, Cancer Center, University of Colorado, National Cancer Institute
Awards: G1309041795, G1332742017, G1565180698, G323416294, G324030696, G3298087253, G3373745158, Targeting heme metabolism to initiate an immune response against breast cancer liver metastasis, G3767889688, G3816289806, G4119534169, G5029337574, G5424111711, G5850682332, G5919981984, G6966134840, CC* Compute: HPC Services for the Colorado State University System, G741073515, G7448966117, TARGETING METASTASIS BY INHIBITING BREAST CANCER METABOLISM AND IMMUNE-SUPPRESSION, G7724295162, G8067353062, G9000010196, G9000089931, G9000306397, University of Colorado Cancer Center Support Grant, Targeting heme metabolism to initiate an immune response against breast cancer liver metastasis, G9001225491
OpenAlex ID: W7160693642
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