Expression and functional activity of PPARγ in pancreatic β cells

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Abstract
Rosiglitazone is an agonist of peroxisome proliferator activated receptor‐γ (PPARγ) and ameliorates insulin resistance in type II diabetes. In addition, it may also promote increased pancreatic β‐cell viability, although it is not known whether this effect is mediated by a direct action on the β cell. We have investigated this possibility. Semiquantitative real‐time reverse transcription–polymerase chain reaction analysis (Taqman®) revealed that freshly isolated rat islets and the clonal β‐cell line, BRIN‐BD11, express PPARγ, as well as PPARα and PPARδ. The levels of expression of PPARγ were estimated by reference to adipose tissue and were found to represent approximately 60% (islets) and 30% (BRIN‐BD11) of that found in freshly isolated visceral adipose tissue. Western blotting confirmed the presence of immunoreactive PPARγ in rat (and human) islets and in BRIN‐BD11 cells. Transfection of BRIN‐BD11 cells with a PPARγ‐sensitive luciferase reporter construct was used to evaluate the functional competence of the endogenous PPARγ. Luciferase activity was modestly increased by the putative endogenous ligand, 15‐deoxy‐Δ12,14 prostaglandin J2 (15dPGJ2). Rosiglitazone also caused activation of the luciferase reporter construct but this effect required concentrations of the drug (50–100 μM) that are beyond the expected therapeutic range. This suggests that PPARγ is relatively insensitive to activation by rosiglitazone in BRIN‐BD11 cells. Exposure of BRIN‐BD11 cells to the lipotoxic effector, palmitate, caused a marked loss of viability. This was attenuated by treatment of the cells with either actinomycin D or cycloheximide suggesting that a pathway of programmed cell death was involved. Rosiglitazone failed to protect BRIN‐BD11 cells from the toxic actions of palmitate at concentrations up to 50 μM. Similar results were obtained with a range of other PPARγ agonists. Taken together, the present data suggest that, at least under in vitro conditions, thiazolidinediones do not exert direct protective effects against fatty acid‐mediated cytotoxicity in pancreatic β cells. British Journal of Pharmacology (2004) 142, 1162–1170. doi:10.1038/sj.bjp.0705844