Analysis of characteristic metabolites significantly responsible for distinguishing cell lines between the two groups. (A) OPLS-DA score plot and OPLS-DA loading plot. The low-grade group includes CHG5 and SHG44; while the high-grade group includes U87,U251 and U118. The loading profile corresponds to tp1. The color of the loading plot can be used to identify the significant variables in the class separation. The red color indicates that the variables are very significant (|r| > 0.463 and VIP > 1); orange indicates that the variables are significant (0.362 < |r| < 0.463 and VIP > 1); and blue indicates that the variables are not significant (NS). (B) VIP rank-score of quantified metabolites. The overall VIP score from PLS-DA shown in the figure was the average across all the selected components. (C) The comparison of relative intensities of characteristic metabolites between the low-grade and the high-grade groups. (Student’s t-test was used, *p < 0.05; ***p < 0.001).