Oncogenic co-operation in beta-cell tumorigenesis

Pancreatic islet neoplasms are rare endocrine tumours. The most common type is of beta-cell origin and is known as insulinoma, which can be either benign or malignant. The majority of insulinomas arise sporadically, but a small proportion develop as part of the hereditary multiple endocrine neoplasia type 1 (MEN1) syndrome. As for many human tumours, the genetic events that occur during the initiation and progression of insulinoma are poorly known. The men1 gene product, menin, is deficient in most hereditary cases, but is not obviously affected in the majority of sporadic tumours. Activation of the proto-oncogenes c-myc and ras has been observed during malignant progression, but their role in tumour initiation remains unproven. To address these questions, transgenic mouse models have been increasingly used to explore molecular and genetic events that might also precipitate human neoplasia. Transgenic mice expressing SV40 large T-antigen (Tag) oncogene in beta-cells develop tumours in a multi-stage progression from hyperplasia, angiogenesis, to solid encapsulated tumours. However, Tag, which inactivates the key tumour suppressors p53 and Rb, is not known to be involved in the pathogenesis of human insulinoma. The proto-oncogene, c-myc is implicated in beta-cell growth in both diabetes and tumorigenesis. Activation of Myc appears to be an early event in progression of human insulinoma. The effect of deregulated Myc expression on adult beta-cells in vivo has recently been investigated by developing transgenic mouse models in which the activity of Myc can be regulated ectopically. Although Myc activation initially promotes both proliferation and apoptosis in pancreatic beta-cells, apoptosis is the predominant outcome, giving rise to islet involution and diabetes. Importantly, inhibiting Myc-induced apoptosis (by co-expression of Bcl-x(L)) leads to significantly enlarged islets, many becoming highly vascularized, hyperplastic and invasive. These results suggest that, in the pancreatic beta-cells, early suppression of apoptosis is essential for the survival of Myc-activated beta-cells and islet neoplasia.