Calcitonin gene-related peptide in medullary thyroid carcinomas: characterization of molecular forms including the amidated C-terminus.

CGRP was extracted from three familial and four sporadic medullary thyroid carcinomas (MTC) and was measured by an assay specific for the amidated C-terminus. The antibody showed equal affinity for alpha- and beta-CGRP. All tumors contained high concentrations of CGRP (range: 63-7889 pmol/g) compared to spinal cord (86 pmol/g), thyroid gland (4 pmol/g), and two small-cell lung carcinomas (4 and 1 pmol/g, respectively). The concentration of calcitonin (CT) was determined with an assay specific for an epitope involving the midportion and C-terminal end of the molecule. In six of the seven tumors investigated, concentrations of CT were found to be higher than for CGRP. Gel chromatography showed heterogeneity with respect to CGRP immunoreactivity. Thus, in all seven extracts, three peaks were seen with Kd values 0.37, 0.63, and 0.80, respectively. This profile of immunoreactive CGRP was similar to that obtained from human medulla spinalis, thereby indicating normal posttranslational processing of pro-CGRP in MTC tumors. Further characterization of the three main peaks identified by gel chromatography was performed on pooled fractions from one of the tumors using HPLC, sequencing, and mass spectrometry. The immunoreactive peak with Kd 0.37 was identified as human beta-CGRP, the peak with Kd 0.63 as 19-37 beta-CGRP, and the peak with Kd 0.80 as 25-37 beta-CGRP. No alpha-CGRP was identified in this tumor. This indicates selective expression of beta-CGRP, at least in the tumor investigated.