SPP1⁺ macrophage influence on differentiation of radioiodine-refractory thyroid cancer through secretion of FABP4.

239

Background: Radioiodine-refractory papillary thyroid carcinoma (RR-PTC) is a dedifferentiated subtype of thyroid cancer characterized by reduced iodine uptake and insensitivity to radioactive iodine (^131I) therapy. However, the mechanisms underlying tumor dedifferentiation and radioiodine refractoriness remain incompletely understood. Methods: Based on our previous single-cell RNA sequencing analyses of RR-PTC samples, we characterized the immune microenvironment and tumor cell states associated with dedifferentiation. Functional studies were conducted using a murine papillary thyroid cancer cell line (MPTC) derived from a BRAFV600E-mutant mouse model. The effects of targeting SPP1⁺ macrophages and macrophage-derived FABP4 on tumor growth were evaluated. Mechanistic investigations focused on FABP4-associated metabolic pathways, and clinical specimens were analyzed to assess the relationship between SPP1/FABP4 expression, tumor differentiation status, and patient prognosis. Results: Single-cell transcriptomic analysis revealed a marked increase in myeloid cell infiltration, particularly SPP1⁺ macrophages, in RR-PTC tumors, accompanied by pronounced tumor cell dedifferentiation. In the BRAFV600E-driven MPTC model, antagonizing SPP1⁺ macrophages or their secreted protein FABP4 effectively suppressed tumor growth. Further mechanistic studies demonstrated that FABP4 forms a hormone-like complex, termed Fabkin, with adenosine kinase (ADK) and nucleoside diphosphate kinase (NDPK). This complex induces metabolic reprogramming in thyroid cancer cells, thereby promoting dedifferentiation and progressive loss of iodine uptake capacity. Analysis of clinical samples showed that SPP1 and FABP4 expression levels increased in parallel with tumor dedifferentiation and were significantly associated with poor prognosis. Conclusions: This study proposes that SPP1⁺ macrophages and macrophage-derived FABP4 play a critical role in shaping the immune microenvironment and driving metabolic reprogramming–mediated dedifferentiation in RR-PTC. Elucidating this mechanism may provide novel therapeutic strategies to prevent or reverse the progression of differentiated thyroid cancer toward radioiodine-refractory disease.