Figure 5.

Comparison of localization of internalized sst₂ receptor with the lysosomal marker protein cathepsin D in tumor samples of octreotide-treated patients (top row, patient 5b; middle row, patient 3b) compared with [Tyr³] octreotide-treated, sst₂-expressing HEK293 cells (bottom row, HEK-sst₂) by confocal double-labeling immunofluorescence imaging. The left column (green) shows the internalized sst₂ receptor visualized by the R2-88 antiserum, and the middle column (red) shows the lysosomal staining of cathepsin D. The merged images are shown in the right column. In both tissue samples and in the HEK-sst₂ cells, the sst₂ receptor shows only a minimal overlap with cathepsin D in the merged images (yellow structures), suggesting that upon agonist-stimulated sst₂ receptor internalization, only few sst₂ receptors are localized in lysosomal structures.