Expression and processing of procholecystokinin in a rat medullary thyroid carcinoma cell line

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Abstract
A rat medullary thyroid carcinoma cell line, CA-77, was shown to express the cholecystokinin (CCK) gene. Measurements using a library of sequence-specific radioimmunoassays before and after enzymic treatment of extracts and chromatographic fractions showed that the cells contained 1.0 pmol of .alpha.-carboxyamidated cholecystokinins/106 cells, 0.4 pmol of glycine-extended intermediates/106 cells and 1.0 pmol of further C-terminal-extended pro-CCK/106 cells. Gel chromatography and reverse-phase h.p.l.c. revealed both sulphated and nonsulphated CCK-8 in the cells. The growth medium contained in addition .alpha.-amidated CCK-33, glycine-extended CCK-8 and pro-CCK. Exposure to 0.1 .mu.M-dexamethasone for 6 days increased the cellular content and secretion of all of the described CCK peptides by 2-3 fold. The increase was first noted after 3 days of treatment. Monensin inhibited the synthesis of .alpha.-carboxyamidated CCK and the secretion of all of the CCK forms measured. Colchicine at a low concentration (0.2 .mu.mol/l) apparently increased the synthesis and secretion of .alpha.-carboxyamidated CCK, whereas higher concentrations inhibited CCK synthesis. Finally, chloroquine inhibited the .alpha.-carboxyamidation of CCK. We conclude that the CA-77 cell line is a useful tool for studies of the expression and post-translational processing of pro-CCK.