Immunocytochemical characterization of two thyroid medullary carcinoma cell lines in vitro

The immunocytochemical characterization of cell lines originating from thyroid medullary carcinoma, i.e. human TT cells and rat rMTC 6-23 cells, was undertaken. The immunocytochemical studies were supplemented by ultrastructural studies, including ultrastructural immunocytochemistry, and by radioimmunological estimation of calcitonin secretion to the medium. In rMTC 6-23 cells (subcultures 24 to 30), no hormone presence was demonstrated immunocytochemically, which corresponded to the absence of secretory granules at the ultrastructural level. Of various proteins sought, only neuron-specific enolase could be demonstrated. Nevertheless, the cells secreted calcitonin into the medium. TT cells (passages 145 to 160) produced secretory granules. The granules contained calcitonin, calcitonin gene-related peptide, somatostatin, neurotensin, met-enkephalin, leu-enkephalin, gastrin releasing peptide, parathyroid hormone-related protein, functional proteins of the chromogranin group and synaptophysin. Other functional proteins found in the cytosol of TT cells included non-specific enolase, calbindin and tyrosine hydroxylase. Receptor for calcitriol was localized in the cell nucleus. Marker proteins were localized in the cytosol (carcinoembryonic antigen) and in the cell skeleton (alpha-tubulin, cytokeratin). Following changes in ionized calcium levels in the medium, changes in calcitonin secretion and in immunocytochemical detectability of some hormones and functional proteins were observed. TT cells demonstrated the expression of numerous hormones and functional proteins associated with calcitonin secretion. Further, the cells in their ultrastructure, immunocytochemical and secretory characteristics, resemble more closely normal parafollicular cells of the thyroid and, in our opinion, represent a more appropriate model for functional studies.