The insulin-like growth factor I receptor protects tumor cells from apoptosis in vivo.

The role of the insulin-like growth factor I receptor (IGF-IR) in programmed cell death has been investigated in vivo in a biodiffusion chamber, where the extent of cell death could be determined quantitatively. We found that a decrease in the number of IGF-IRs causes massive apoptosis in vivo in several transplantable tumors, either from humans or rodents. Conversely, an overexpressed IGF-IR protects cells from apoptosis in vivo. We also show that the same conditions that in vitro cause only partial growth arrest with a minimum of cell death, induce in vivo almost complete cell death. We conclude that the IGF-IR activated by its ligands plays a very important protective role in programmed cell death, and that its protective action is even more striking in vivo than in vitro.