Fates of endocytosed somatostatin sst(2) receptors and associated agonists

Somatostatin agonists are rapidly and efficiently internalized with the som atostatin sst, receptor. The fate of internalized agonists and receptors is of critical importance because the rate of ligand recycling back to the ce ll surface can limit the amount of radioligand accumulated inside the cells , whereas receptor recycling might be of vital importance in providing the cell surface with dephosphorylated, resensitized receptors. Furthermore the accumulation of radioisotope-conjugated somatostatin agonists inside cance r cells resulting from receptor-mediated internalization has been used as a treatment for cancers that overexpress somatostatin receptors. In the pres ent study, radioiodinated agonists at the sst, somatostatin receptor were e mployed to allow quantitative analysis of the fate of endocytosed agonist. After endocytosis, recycling back to the cell surface was the main pathway for both I-125-labelled somatostatin-14 (SRIF-14) and the more stable agoni st I-125-labelled cyclo(N-Me-Ala-Tyr-D-Trp-Lys-Abu-Phe) (BIM-23027; Abu sta nds for aminobutyric acid), accounting for 75-85% of internalized ligand wh en re-endocytosis of radioligand was prevented. We have shown that there is a dynamic cycling of both somatostatin agonist ligands and receptors betwe en the cell surface and internal compartments both during agonist treatment and after surface-bound agonist has been removed, unless steps are taken t o prevent the re-activation of receptors by recycled agonist. Internalizati on leads to increased degradation of I-125-labelled SRIF-14 but not I-125-l abelled BIM-23027. The concentration of recycled agonist accumulating in th e extracellular medium was sufficient to re-activate the receptor, as measu red both by the inhibition of forskolin-stimulated adenylate cyclase and th e recovery of surface receptor number after internalization.