Tumor-expressed GPNMB orchestrates Siglec-9 <sup>+</sup> TAM polarization and EMT to promote metastasis in triple-negative breast cancer
Ting-Lan Cao, Kunio Kawanishi, Sachie Hashimoto, Kowit Hengphasatporn, Chiaki Nagai‐Okatani, Takaharu Kimura, Mohammed Abdelaziz, Rie Shiratani, Thanasis Poullikkas, Nuriza Ulul Azmi, Masaki Baba, Yukari Okita, Yukihide Watanabe, Hiroko Bando, Satoshi Yamazaki, Yasuteru Shigeta, Atsushi Kuno, Mitsuyasu Kato
Abstract
Metastasis remains the leading cause of cancer-related mortality, driven by complex interactions within the tumor microenvironment (TME). Tumor-associated macrophages (TAMs) play a pivotal role in metastatic progression, yet their molecular diversity and upstream regulators remain poorly defined. Glycoprotein nonmetastatic melanoma protein B (GPNMB), overexpressed in subsets of tumors including triple-negative breast cancer (TNBC), is implicated in epithelial–mesenchymal transition (EMT) and cancer stemness. Recent single-cell RNA sequencing (scRNA-seq) identified GPNMB as a marker of immunosuppressive TAMs associated with poor prognosis, but its mechanistic role in TNBC has remained unclear. Coculturing monocytic cells with three-dimensional TNBC spheres induced GPNMB + TAMs expressing sialic acid-binding immunoglobulin-like lectin 9 (Siglec-9). Tumor-expressed GPNMB promotes monocyte-to-TAM polarization by inducing secondary GPNMB expression in monocytes, establishing a feed-forward amplification loop. GPNMB knockdown in TNBC cells inhibited immunosuppressive TAM subsets, including Siglec-9 + and EMT-associated populations, as determined by deconvolution of bulk RNA-seq data using a custom TAM signature matrix derived from publicly available TNBC scRNA-seq datasets. TNBC-derived GPNMB carried α2,3-sialylation, whereas macrophage-derived GPNMB carried α2,6-sialylation, enabling differential Siglec-9 recognition. Elevated GPNMB and Siglec-9 correlated with poor prognosis in TNBCcohorts. Importantly, dual inhibition of Siglec-E (murine Siglec-9 ortholog) and PD-1 reduced tumor stemness, suppressed IL-6-dependent EMT, and limited lung metastasis in vivo. The GPNMB–Siglec-9 axis thus represents a critical glyco-immunological checkpoint driving TAM-mediated metastasis, providing a promising therapeutic target in TNBC.