Abstract 1512: Nicotine promotes brain metastasis of lung cancer by inducing M1/M2 conversion and suppressing the innate immune response of microglia

Approximately 50% of patients with metastatic lung cancer develop brain metastasis which is almost always fatal, and the median survival time of these patients is only a few months. Smoking is the major risk factor for lung cancer, and tobacco smoke contains a toxic mix of more than 7,000 chemical. Among those chemicals, nicotine has been shown to promote cancer progression and metastasis. However, the exact mechanism by which nicotine derives this process remain poorly understood. The communications between neoplastic cells and microglia, especially M2 type pro-tumor microglia, profoundly affect the brain metastasis progression. Therefore, we hypothesize that nicotine skews microglial polarization to M2 phenotype which promotes brain metastasis of lung cancer. We found that the brain metastasis-free survival rate was significantly decreased in current smokers compared to non-smokers. Abundant CD206+ M2 microglia were observed in brain metastatic lesions in such patients. Consistent with these results, when we transplanted the H2030 brain metastasis of lung cancer cell line (H2030BrM) into nude mice by intra-cardiac injection followed by with or without nicotine treatment, the brain metastasis-free survival was decreased in nicotine treatment group. The number of CD206+cells were also increased in the mouse brain after nicotine treatment. To test the effect of nicotine-driven microglia on tumor progression, H2030BrM cells were treated with conditioned medium (CM) generated from human microglia that were treated with or without nicotine. We found that the CM significantly increased the population of ESA+/CD44+ cancer stem cells and promoted their sphere forming abilities. The expression of SOX2 and Nanog were up-regulated when the H2030BrM cells were cultured with the CM of nicotine-treated microglia. Blocking nicotine-driven M2 microglial polarization by a STAT3 inhibitor suppressed the M2 polarization, resulting in inhibition of tumor growth. To examine the mechanism by which nicotine suppresses anti-tumor function of microglia, we measured the expression of “do not eat me” signal, SIRP on microglia and CD47 on tumor cells. H2030BrM and human microglial cells were treated with nicotine. We found that the mRNA and protein expression of SIRP were up-regulated on human microglia, while the expression of CD47 on H2030BrM were elevated by nicotine treatment. We used FACS to detect the phagocytic ability of microglial by co-culturing H2030BrM with microglial cells. We found that the anti-tumor phagocytic activities of microglia cells were significantly decreased upon nicotine treatment. In conclusion, our results suggest that nicotine promotes lung cancer brain metastasis by enhancing both tumor progression and stemness by skewing microglial polarization and inhibiting its anti-tumor phagocytic ability. Citation Format: Shih-Ying Wu, Fei Xing, Sambad Sharma, Abhishek Tyagi, Kerui Wu, Yin Liu, Dan Zhao, Ravindra Deshpande, Yusuke Shiozawa, Kounosuke Watabe. Nicotine promotes brain metastasis of lung cancer by inducing M1/M2 conversion and suppressing the innate immune response of microglia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 1512.