Effect of follicular cells on calcitonin gene expression in thyroid parafollicular cells in cell culture

Expression of calcitonin (CT) gene in thyroid parafollicular cells involves alternate formation of CT mRNA or CGRP mRNA. High amounts of CT mRNA are f ormed only in thyroid gland and formation of CGRP mRNA dominates in the rem aining organs. Apart from paracrine and endocrine factors, mRNA formation o n the CT gene seems to be affected also by direct contacts with other cells present in the thyroid gland, in which parafollicular cells are located ne xt to follicular cells. The present study aimed at examining whether thyroid follicular cells affec t formation of mRNAs for CT and CGRP in parafollicular cells. The studies w ere performed in cell cultures. A parafollicular cell line (TT cells) and a follicular cell line (F6BTY cells) served as the experimental model. For c omparison, co-cultures with fibroblasts, 3T3 cells, and malignant melanoma, MM cells, were also examined. CT gene expression was examined at the level of mRNAs (in situ hybridization and morphometric studies) and at the level of hormones (immunocytochemistry, morphometric studies and radioimmunologi cal estimation of hormone levels in the medium). The immunocytochemical and hybridocytochemical studies, in line with morpho metry studies, demonstrated that F6BTY and 3T3 cells were capable of affect ing mRNA production for CT and CGRP and that they changed the ratio of CGRP /CT secretion by TT cells, as a sequel of contact between the two cell type s and due to mediation of secreted substances. On the other hand, the malig nant melanoma MM cells showed no effect on the secretion ratio. Our study seems to indicate that control of mRNA formation from CT gene may involve not only humoral factors but also direct contacts with other cells , which may explain differences in expression of the gene between cells loc alized in different organs.