Animal models of pheochromocytoma.

Pheochromocytomas are neuroendocrine tumors of adrenal chromaffin cells. They are rare in all species except rats but occur with increased frequency in several human familial tumor syndromes. Concurrence of pheochromocytoma with other tumors sometimes parallels these human syndromes in rats, bovines, horses and dogs but a shared genetic basis for human and spontaneously occurring animal pheochromocytomas has thus far not been established. Pheochromocytomas are inducible in rats by a variety of non-genotoxic substances that may act indirectly by stimulating chromaffin cell proliferation. They are not known to be similarly inducible in other species but arise with increased frequency in transgenic and knockout mice that to varying degrees recapitulate human tumor syndromes. Preliminary evidence suggests that homologous somatic genetic changes might contribute to pheochromocytoma development in humans and some mouse models. The nerve growth factor-responsive PC12 cell line, established from a rat pheochromocytoma, has for almost 30 years served as a research tool for many aspects of neurobiology involving normal and neoplastic conditions. Recently developed pheochromocytoma cell lines from neurofibromatosis knockout mice supplement the PC12 line and have generated additional applications. Advantages of the mouse lines include expression of substantial levels of the epinephrine-synthesizing enzyme, phenylethanolamine N-methyltransferase and expression of high levels of the receptor tyrosine kinase, Ret, which is characteristic of sporadic and familial human pheochromocytomas but not of PC12 cells. Disadvantages include an apparently less stable phenotype. It is difficult to establish pheochromocytoma cell lines from any species, although the tumor cells persist in culture for many months. Understanding of factors that permit pheochromocytoma cells to proliferate might itself provide important insights for tumor biology.