Macrophage-to-myofibroblast transition-derived itaconate promotes bone metastasis in lung cancer through targeting of HSPA8.

Lung cancer bone metastasis carries a poor prognosis, yet the metabolic determinants driving tumour-stroma crosstalk remain largely elusive. Despite extensive investigations into itaconate, a prominent immunometabolite implicated in macrophage polarization, the precise mechanisms by which it modulates bone metastasis remain unresolved. Integrating metabolomics and transcriptomics profiling, the molecular landscape of lung cancer bone metastasis is delineated and the mechanistic role of itaconate is uncovered. Further ubiquitination proteomics of tumour cells and CRISPR-Cas9-mediated knockout of the gene encoding immune-responsive gene 1 (IRG1) confirmed the results in an animal model of lung cancer bone metastasis. The macrophage-to-myofibroblast transition generated cancer-associated fibroblasts that secreted elevated levels of itaconate, significantly accelerating tumour growth. A drug affinity responsive target stability screening pinpointed heat shock protein family A member 8 (HSPA8) as a direct molecular target of itaconate. Mechanistically, itaconate promoted HSPA8 ubiquitination and subsequent proteasomal degradation, thereby releasing activated transcription factor 4 (ATF4) from cytosolic sequestration. Liberated ATF4 translocated to the nucleus, where it bound the promoter region of phosphoserine aminotransferase 1 (PSAT1) to upregulate its expression. In vivo validation demonstrated that administration of adeno-associated virus-delivered PSAT1 short hairpin RNA or of a cell-penetrating itaconate antagonist significantly reduced tumour burden and prolonged survival. Our findings elucidate an unappreciated metabolic reprogramming axis in lung cancer bone metastases: macrophage-to-myofibroblast transition-derived itaconate alleviated cytoplasmic sequestration of ATF4 via HSPA8 ubiquitination, thereby activating its transcriptional target PSAT1. This mechanism converts immunometabolic byproducts into pro-tumorigenic signals that enhance bone metastasis. Notably, the HSPA8-ATF4-PSAT1 axis was identified as a key regulatory pathway governing metabolic reprogramming, thereby establishing a translational framework for targeting immunometabolic crosstalk in bone metastasis therapy.