Effect of acidic tumor microenvironment on invasion and migration and its mechanism in glioma cells

Objective To investigate the effect of acidic tumor microenvironment on invasion and migration and its mechanism in glioma cells. Methods (1) The pH value of the medium was adjusted by acid-base titration. Human glioma cells U87 and U251 were cultured in the acid group and the normal group with pH values of 6.4 and 7.4, respectively; and 3 d after cultivation, the expressions of hypoxia-inducible factor-2α (HIF-2α) and CD44 were detected by Western blotting; Transwell assay was used to examine the invasion and migration of U87 and U251 cells; immunofluorescence was employed to examine the CD44 expression. (2) The U87 and U251 cells were divided into small interfering RNA (siRNA)-nonsense sequence group and siRNA-CD44-1 group, and the siRNA nonsense sequences and siRNA-CD44-1 interfering fragments were transfected by lipofectin-3000, respectively; three d after transfection, the migration and invasion abilities of cells from the two groups were detected by Transwell assay. (3) U87 and U251 cells were divided into acid group (cultured with a pH value of 6.4), blank control group, siRNA nonsense sequence group, siRNA-CD44-1 group, and siRNA-CD44-2 group; and cells from the later four groups were cultured with a pH value of 7.4; after culture for 4 d, the siRNA-nonsense sequence group, siRNA-CD44-1 group and siRNA-CD44-2 group were transfected with siRNA-nonsense sequences, siRNA-cd44-1 interfering fragments and siRNA-CD44-2 interfering fragments, respectively; three d after transfection, the expressions of CD44, N-Ca, Vimentin, and matrix metalloproteinase (MMP)-2 proteins in these 5 groups were detected by Western blotting. Results (1) As compared with the normal group, the expression levels of HIF-2α and CD44 in U87 and U251 cells of the acid group were significantly increased; both Transwell and invasion experiments showed that the number of transmembrane cells in the acid group was significantly larger than that in the normal group (P<0.05); immunofluorescence staining showed that the CD44 expression in acid group was significantly higher than that in normal group (P<0.05). (2) Both Transwell and invasion experiments showed that the number of transmembrane cells in the siRNA-CD44-1 group was significantly smaller than that in the siRNA nonsense sequence group (P<0.05). (3) Western blotting showed that the expression levels of CD44, N-Ca, Vimentin and MMP-2 in U87 and U251 cells of the blank control group, siRNA nonsense sequence group, siRNA-CD44-1 group, and siRNA-CD44-2 group were obviously decreased as compared with those in the acid group; the expression levels of CD44, N-Ca, Vimentin and MMP-2 in U87 and U251 cells of the siRNA-CD44-1 group and siRNA-CD44-2 group were obviously lower than those in the siRNA nonsense sequence group. Conclusion Acidic tumor microenvironment enhances the capabilities of invasion and migration of glioma cells through increasing CD44 expression. Key words: Glioma; Acidic tumor microenvironment; CD44; Invasion; Migration