In this study, we investigated the role of tumor-associated macrophages (TAMs) in the progression of pancreatic ductal adenocarcinoma (PDAC). PDAC patients with higher levels of CD68+ TAMs exhibited shorter overall survival. In Transwell assays, PDAC cells incubated with TAMs or conditioned media from TAM cells (TAM-CM) showed higher migration and invasion rates than controls. PET/CT scan analysis of orthotopic PDAC model mice revealed greater primary tumor growth and liver metastasis in the TAM-CM treatment group than the controls. H&E staining of liver tissues showed significantly higher numbers of metastatic nodules in the TAM-CM treatment group. Heat inactivation of TAM-CM significantly reduced Transwell migration by PDAC cells, suggesting the involvement of one or more secreted proteins in PDAC progression. Transcriptome sequencing analysis of PDAC cells treated with TAM-CM revealed significant enrichment of transforming growth factor-β (TGF-β) signaling pathway genes. Western blot and qRT-PCR analysis showed that TAM-CM enhanced PDAC migration cells by inducing epithelial-to-mesenchymal transition through the TGF-β-Smad2/3/4-Snail signaling axis. The pro-tumorigenic effects of TAMs or TAM-CM were abolished by TGF-β signaling pathway inhibitors and neutralizing TGF-β antibody. These results demonstrate that TAMs promote PDAC progression through the TGF-β signaling pathway.
Keywords: TGF-β signaling pathway; epithelial-mesenchymal transition; metastasis; pancreatic ductal adenocarcinoma; tumor-associated macrophages.