Objective: Peripheral benzodiazepine receptors (PBRs) are part of the mitochondrial permeability transition pore, and their activation may induce cell death. PBRs are expressed in human pancreatic islets, and cytokine-induced damage is accompanied by changes in their properties. We hypothesized that PBRs can have a role in human islet physiopathology, and evaluated the effects of prolonged exposure to two specific PBR ligands, PK11195 and Ro5-4864 on the function and survival of isolated human islets.
Design: Isolated human islets were prepared from the pancreas of 25 multiorgan cadaveric donors and incubated for 12 h in the presence of PK11195 or Ro5-4864. Insulin secretion studies and apoptosis experiments were then performed, together with assessment of intracellular pathways involved in islet cell function and survival.
Methods: Islets were prepared by enzymatic digestion and density gradient purification. Insulin secretion was assessed by the batch incubation method, and glucose oxidation was evaluated by the use of D-[U-(14)C]glucose. Apoptosis was studied using the TUNEL technique, ELISA methods, and electron microscopy evaluation. PCR experiments were performed by the use of specific primers.
Results: Glucose-stimulated insulin release was significantly lower after exposure to PK11195 than after exposure to Ro5-4864. This was accompanied by reduced glucose oxidation and no major change of insulin or GLUT-1 mRNA expression. Apoptosis was higher in PK11195-exposed islets, and electron microscopy demonstrated the involvement of beta-cells. The apoptotic effects were prevented by bongkrekic acid and low-dose cyclosporin A, which stabilize the mitochondrial membrane, and were associated with no evident change of inducible nitric oxide synthase (iNOS), B-cell leukemia/lymphoma-2 (Bcl-2) or Bcl-2-associated X protein (Bax) expression. Caspase inhibition markedly reduced the amount of apoptosis, and the role of these proteases was confirmed by the increased activity of caspase-3 and caspase-9.
Conclusions: Prolonged binding to PBRs may cause human beta-cells functional damage and apoptosis, a phenomenon which is prevented by stabilizing the mitochondrial membrane; occurs without changes of iNOS, Bax and Bcl-2 mRNA expression; and involves caspase activation. These results suggest an involvement of PBRs in human pancreatic beta-cell function and survival.