Biological aspects of neuroendocrine gastro-enteropancreatic tumours.

Carcinoid tumours may develop from enterochromaffin cells in the gastrointestinal tract. Benign insulin-producing tumours may develop from islet cells, whereas other islet cell tumours might derive from multipotent stem cells in the pancreatic ducts. The idea that multiple endocrine neoplasia type 1 (MEN-1) tumours in the pancreas originate from multipotent stem cells is supported by our demonstration that CD44 is expressed in exocrine cells, in gastrin-producing endocrine cells only and in some non-functioning islet cell tumours; there are no gastrin-producing cells in the adult pancreas. We have identified phospholipase C beta 3 (PLC beta 3) as the gene implicated in MEN-1. It appears to be a tumour suppressor gene since it is expressed in endocrine pancreatic tumours, some lung carcinoids, and medullary thyroid carcinomas. So neuroendocrine tumours might have a dual growth-regulating system, involving both traditional growth factors through the tyrosine kinase system and also G-protein-mediated growth signals. Deletion of PLC beta 3, which is an important enzyme in the signal transduction pathway of G-protein-mediated signals, might be important in the growth regulation of neuroendocrine tumours. It is proposed that its deletion causes dysregulation of growth control in neuroendocrine cells, with possible distortion of the apoptotic process. In the last stage of the disease, tumour biology is altered and becomes more aggressive. Further, chromogranin A may be both a tumour marker for neuroendocrine tumours and a growth-promoting agent for neuroendocrine tumour cells; it is a very good marker of tumour mass but is also related to poor prognosis of survival. Mutation analyses of PLC beta 3 and studies of the growth-promoting effect of chromogranin are ongoing and should lead to more effective therapies.