EXPLORING THE GUT MICROBIOME – IMMUNE AXIS IN CIGARETTE SMOKING-DRIVEN CANCER PROGRESSION

Background: Cigarette smoke is a major cancer risk factor, however, mechanisms governing this interaction are still unclear. Gut microbiome, which has emerged as a major determinant of cancer growth, is known to be altered by cigarette smoke. We investigated gut microbiome-targeting as a potential therapy against smoking-induced cancer progression.

Methodology: C57BL/6J mice were randomized to room air (controls)/cigarette smoke exposure (CSE) for 4 weeks. Gut microbiome and metabolome were analyzed using 16s rRNA sequencing and untargeted LC-MS metabolomics, respectively. To confirm the tumorigenic potential of CSE gut microbiome, fecal matter from control or CSE mice was transplanted (FMT) into C57BL/6J recipient mice followed by subcutaneous KPC pancreatic cancer cell implantation. In a separate experiment, C57BL/6J mice with or without gut microbiome (depleted using broad-spectrum non-absorbable oral antibiotics) underwent CSE followed by KPC implantation to assess effect of gut microbiome on smoking induced tumor growth. To evaluate the role of adaptive immune system, FMT experiment was repeated in Rag1 knockout mice. Finally, selective gut microbiome targeting with narrow spectrum antibiotics was performed in CSE mice implanted with KPC cancer cells to assess its effectiveness as a potential therapy.

Results: CSE significantly altered beta diversity and differential species enrichment of the gut microbiome, while the gut metabolome was also severely dysregulated, dominated by alterations in the arginine biosynthesis and arachidonic acid pathway metabolites. FMT from CSE mice was sufficient to drive increased KPC tumor growth in recipient mice when compared to FMT from control mice. CSE failed to induce KPC tumor growth in the absence of gut microbiome. Immunophenotyping of tumors revealed decreased CD8+ T cell as well as increased CD11b+Ly6G+ MDSC infiltration in CSE mice. CSE FMT was unable to promote tumor growth in Rag1 KO mice, suggesting that this effect is dependent on a functional adaptive immune system. Finally, narrow spectrum targeting with Neomycin (gram negative spectrum) achieved equivalent tumor growth reduction as compared to broad spectrum antibiotics.

Conclusion: Cigarette smoke induces severe alterations in the gut microbiome and metabolome, which can drive tumor progression. Gut microbiome can emerge as a novel biological target to ameliorate smoking-induced tumor growth.