Upregulation of mitochondrial peripheral benzodiazepine receptor expression by cytokine-induced damage of human pancreatic islets

Cytokines produced by immune system cells infiltrating pancreatic islets are candidate mediators of islet beta-cell destruction in autoimmune insulin-dependent diabetes mellitus. After 72 h exposure of human pancreatic islets to a cytotoxic cytokine combination of interleukin 1 beta (50 U/ml), tumor necrosis factor alpha (1,000 U/ml), and interferon gamma (1,000 U/ml), an increase of cell death vs. control islets was demonstrated by TUNEL and cell death detection ELISA method. Islet death was associated with apoptosis and mitochondrial swelling as evidenced by electron microscopy. This effect was correlated with a marked decrease of Bcl-2 mRNA expression (without any major change of Bax mRNA) and a marked increase of inducible nitric oxide synthase mRNA. Since peripheral benzodiazepine receptors constitute the aspecific mitochondrial permeability transition pore, and that it has been suggested to be involved in cytokine-induced cell death, we evaluated the effects of the cytotoxic cytokines on PBR density and mRNA expression. We demonstrated that cytokine treatment of human islets induced an increase of maximum density of (3)H1-(2-chlorophenyl-N-methyl-1-methylpropyl)-3- isoquinolinecarboxamide binding sites, (5,110+/-193 vs. 3,421+/-336 fmol/mg proteins, P<0.05) with no significant change in the affinity constant value (9.45+/-0.869 vs. 8.7+/-1.159 nM). Moreover, an increase of the expression of peripheral benzodiazepine receptor mRNA was observed, suggesting an increased transcription from the coding gene. These results suggest a possible role of peripheral benzodiazepine receptors in the organism response to tissue damage associated with inflammatory mediator production.