Expression studies on PPARγ in pancreatic neuroendocrine tumours

Abstract

Pancreatic NETs occur with an annual incidence of around 5 per 1,000,000 population per year, with survival rates of between 30 – 97% at 5 years depending on the tumour subtype. The PPARs (peroxisomal proliferator-activated receptors) are members of the nuclear receptor superfamily that includes receptors for thyroid, steroid and retinoid hormones. PPARγ protein is also thought to be expressed in human pancreatic islet cells and has been shown to be a negative regulator of islet β cell mass both in vivo and in vitro. Its emerging function in controlling cell proliferation, differentiation and apoptosis, both in vivo and in vitro, has suggested a putative role as a tumour suppressor gene. I postulated that PPARγ is expressed in pancreatic neuroendocrine tumours and that agonism with a thiazolidinedione will cause an anti-proliferative effect. Three different types of tissue/cells were available to me: frozen human pancreatic neuroendocrine tumours following surgical resection, paraffin-embedded samples held in the histopathology archives, and human neuroendocrine tumour cell lines CM, BON and QGP1 (insulinoma, carcinoid and somatostatinoma respectively). PPARγ RNA was shown to be present in the majority of frozen surgical samples. Immunohistochemistry for PPARγ protein on the paraffin-embedded samples, however, revealed a lack of positive staining. These samples were then subjected to further immunohistochemistry for detection of other potentially important proteins involved with cellular proliferation including p27, phospho-p27, JAB1, PTEN and phospho- AKT. In the tumour cell models, PPARγ RNA and protein was present in both BON and QGP1. Proliferation studies following treatment doses of PPARγ agonist 3 rosiglitazone show a significant anti-proliferative effect. Recovery of cells was shown following removal of treatment. However, inhibition of the effect was not achieved with the use of PPARγ antagonists raising the possibility that the anti-proliferative effects of thiazolidinediones may be independent of PPARγ.