Proinflammatory macrophage secretome enhances temozolomide sensitivity in glioblastoma via pSTAT3-mediated downregulation of DNA repair enzymes

The mechanisms by which tumor-associated macrophages, key components of the glioblastoma (GBM) microenvironment, impair chemotherapy efficacy remain poorly understood. Resistance to temozolomide (TMZ), the standard chemotherapeutic agent for GBM, is associated with poor prognosis due to efficient DNA repair mechanisms. While low expression of the DNA repair enzyme O6-methylguanine-DNA methyltransferase (MGMT) has been linked to improved TMZ response, our previous findings suggest that N-methylpurine-DNA glycosylase (MPG) may also contribute to chemoresistance in GBM. Here, we report for the first time that conditioned medium from pro-inflammatory macrophages (CM-M1) enhances TMZ cytotoxicity by suppressing STAT3 phosphorylation, resulting in decreased MGMT and MPG expression in GBM cells. Proteomic profiling of CM-M1 revealed a unique, cytokine-rich secretome that may promote STAT1 activation, thereby inhibiting pSTAT3 and reducing DNA repair enzymes levels. Clinically, elevated MGMT and MPG protein levels were associated with increased pSTAT3 in our GBM patient cohort, and analysis of the TCGA database further showed that their combined overexpression correlates with significantly reduced progression-free survival. Gene silencing experiments confirmed the contribution of both enzymes to TMZ resistance, with dual knockdown producing a synergistic sensitizing effect. These findings uncover a novel mechanism of macrophage secretome-mediated chemoresistance and support the development of M1-based strategies to improve TMZ efficacy in GBM.