Propionibacterium acnes overabundance in gastric cancer promote M2 polarization of macrophages via a TLR4/PI3K/Akt signaling

Eradication of Helicobacter pylori (H. pylori) could not completely prevent the progression of gastric cancer (GC), suggesting that non-H. pylori bacteria may participate in the carcinogenesis of GC. The dysbiosis of microbiota in the stomach of GC has gradually been investigated, while the detailed mechanism that promotes GC in this process has not been elucidated. We aimed to identify a non-H. pylori bacteria that contribute to GC. GC tissues and adjacent normal tissues were collected to identify bacteria that significantly increased in GC tissues by 16S rRNA gene sequencing and fluorescence in situ hybridization (FISH) analysis. CCK8, wound healing assay, and trans-well assay were performed to analyze the tumor-promoting effect of this bacteria. Next, we detailed the mechanism for tumor-promoting effect of the bacteria by immunofluorescence, RT-qPCR, and western-blotting analysis. Comparing the microbial community from GC tissues and adjacent normal tissues, we found that Propionibacterium acnes (P. acnes) significantly increased in GC tissues, especially in H. pylori-negative tissues. We further found that the abundance of P. acnes correlated with TNM stages of GC patients. Interestingly, condition medium (CM) from P. acnes-primed macrophages promoted migration of GC cells, while P. acnes only could not. We next proved that P. acnes triggers M2 polarization of macrophages via TLR4/PI3K/Akt signaling. Together, our finding identified that P. acnes could be a possible agent for the progression of GC besides H. pylori. M2 polarization of macrophages could be promoted by P. acnes via TLR4/PI3K/Akt signaling, thus triggers the progression of GC.