Human medullary thyroid carcinoma: characterization of the serotonergic and neuronal properties of a neurectodermally derived cell line.

Parafollicular (PF) cells have been found to be a good model system for the study of serotonergic cellular mechanisms relevant to neurons. PF cells are derived from the same region of the neural crest that gives rise to the neurons of the gut and are capable of extending neurofilament-bearing neuritic processes. PF cells also synthesize 5-hydroxytryptamine (5-HT) and costore 5-HT in the same vesicles as the specific 5-HT-binding protein, 45 kDa SBP. A hypothesis has been advanced that PF cells and enteric neurons share a common developmental precursor. The present investigation was undertaken in order to determine whether a human medullary thyroid carcinoma (MTC) cell line, which is derived from PF cells, sufficiently mimics PF cells that it can be substituted for them in investigations of serotonergic cellular biology. In contrast to PF cells, MTC cells can be propagated in vitro to provide adequate amounts of material for biochemical studies. MTC cells were found to contain neuropeptides, including calcitonin, calcitonin gene-related peptide, and somatostatin, which have also been reported to be present in PF cells and enteric neurons. MTC cells also were observed to store endogenous 5-HT, to be able to synthesize 3H-5-HT from 3H-L-tryptophan, and to take up 3H-5-HT from the ambient medium by a carrier-mediated mechanism very similar to that of serotonergic neurons. In addition, the longterm accumulation of 3H-5-HT in MTC cells was antagonized by reserpine, suggesting that the cells contain 5-HT storage vesicles that, like the synaptic vesicles of serotonergic neurons, are characterized by a reserpine-sensitive transporter of biogenic amines. MTC cells also contain type A, but not type B, monoamine oxidase. Finally, MTC cells were found to contain both 45 and 56 kDa SBP. MTC cells thus retain a great many of the properties of PF cells, and, like PF cells, they are serotonergic cells with characteristics similar to serotonergic neurons. Substantial differences were found in the content of immunoreactive 5-HT and neuropeptides in individual MTC cells. Moreover, the release of newly synthesized 5-HT to the medium exceeded the ability of the cells to store the amine. Studies of the ultrastructure of the MTC cells revealed a limited and highly variable number of secretory granules, probably accounting for their limited 5-HT storage capacity and for the heterogeneity of immunostaining with antisera to 5-HT or neuropeptides.(ABSTRACT TRUNCATED AT 400 WORDS)