Immunocytochemical Localization of Prohormone Convertase 1/3 and 2 in Thyroid C-Cells and Medullary Thyroid Carcinomas.

Prohormone convertases (PCs) are the key enzymes in the regulated pathways for the post-translational processing of peptide hormones and are involved in converting larger prohormones to smaller, biologically active hormones. Immunolocalization of PC1/3 and PC2 was performed with thyroid C-cells from normal thyroid glands and medullary thyroid carcinomas (MTCs); thyroid C-cells were not consistently positive for PCs, whereas MTCs were consistently positive for PCs. Positive staining for PCs included tumor cell nests adjacent to the main MTCs in cases of multiple endocrine neoplasia type 2. Procalcitonin was originally detected in MTCs and a large amount of procalcitonin was present together with abundant PCs in MTCs. Thus, immunocytochemical staining for PCs may be another characteristic of MTCs.