Small cell lung cancer (SCLC) is the most aggressive histologic subtype of lung cancer, with a strong predilection for early brain metastases. Despite efforts and advances in new therapeutics for SCLC, the prognosis of patients with SCLC with brain metastases is consistently poor. Therefore, a better understanding of the mechanisms of SCLC brain metastasis is important in improving current treatments. In this study, elevated S100A16 levels were associated with SCLC brain metastases, which was a possible secondary event arising from the brain metastatic microenvironment. Using an in vitro cell coculture system, we found that the coculturing of SCLC cells with human brain microvascular endothelial cells (HBMECs) led to an increased expression of S100A16 in SCLC cells. Conversely, treatment of HBMECs with GW4869, an inhibitor of exosome release, significantly blocked this effect in the cocultured SCLC cells. Alternatively, the results from Western blot analyses and immunofluorescence indicated that the HBMEC exosomes purified by ultracentrifugation also induced the elevation and translocation from the cytoplasm to the nucleus of S100A16 in the recipient SCLC cells. The inhibition experiments demonstrated that elevated S100A16 contributed a benefit of HBMEC exosomes for the survival of the recipient SCLC cells under stress. Moreover, the elevation of S100A16 in SCLC cells prevented the loss of mitochondrial membrane potential (Δψm) and enhanced resistance to apoptosis under stressful conditions, which were determined by Annexin V/propidium iodide and JC-1 assay. Further results showed that the S100A16-mediated protective effect was caused by the presence of an important element in Δψm, prohibitin (PHB)-1, a protein in the mitochondrial inner membrane. Conversely, the delivery of PHB-1 siRNAs into S100A16 overexpressing SCLC cells weakened these protective effects. Our findings suggest that elevated S100A16 plays an active role in facilitating the survival of SCLC cells through modulating the mitochondrial function, identifying S100A16 as an important potential target in SCLC brain metastasis.-Xu, Z.-H., Miao, Z.-W., Jiang, Q.-Z., Gan, D.-X., Wei, X.-G., Xue, X.-Z., Li, J.-Q., Zheng, F., Qin, X.-X., Fang, W.-G., Chen, Y.-H., Li. B. Brain microvascular endothelial cell exosome-mediated S100A16 up-regulation confers small cell lung cancer cell survival in brain.
Keywords: brain metastasis; mitochondrial membrane potential; prohibitin.