Three different endocytoses contribute to ATP internalization. Three different endocytoses contribute to ATP internalization. A549 and H1299 cells were incubated with ATP, NHF-ATP, and HMWFD or LMWFD, in the presence or absence of different endocytosis inhibitors, for 30 minutes. After incubation, cells were fluorescence microscopically examined for compound internalization or measured for their intracellular ATP levels. A, colocalization of NHF-ATP with HMWFD or LMWFD in H1299 cells. B and C, intracellular ATP level elevation was reduced by IPA-3 or nystatin, an inhibitor to caveolae-mediated endocytosis, or chlorpromazine, an inhibitor to clathrin-mediated endocytosis in A549 (B) and H1299 cells (C). *, P < 0.05; **, P < 0.01; ***, P < 0.001. D, fluorescent microscopy of internalization of NHF-ATP or LMWFD in A549 cells in the presence or absence of endocytosis inhibitors. Quantifications of the internalizations are shown in Supplementary Fig. S4. Yanrong Qian et al. Mol Cancer Res 2016;14:1087-1096 ©2016 by American Association for Cancer Research