Figure 4. Endothelial cells internalize angiogenin in culture. Rf/6a chorioretinal endothelial cells incubated with recombinant human angiogenin (A-C) and with the control protein, BSA (BSA; D-F). Cells exposed to angiogenin exhibit intense angiogenin labeling, indicating that they actively import the protein. Cells that were not exposed to angiogenin demonstrated only faint labeling in the cytosol, attributed to endogenous angiogenin. Filamentous actin is labeled with phalloidin (A, D) and nuclei are labeled with diamidino-2-phenylindole (DAPI; blue).