Journal of Neuroscience, Vol 10, 3743-3753, Copyright © 1990 by Society for Neuroscience
Multiple signal transduction mechanisms leading to the secretion of 5- hydroxytryptamine by MTC cells, a neurectodermally derived cell line
H Tamir, KP Liu, SC Hsiung, M Adlersberg, EA Nunez and MD Gershon
Division of Neuroscience, New York State Psychiatric Institute, New York 10032.
Parafollicular (PF) cells of the thyroid gland are neural crest
derivatives. These cells remain plastic even in adult animals and can be
induced to exhibit neural properties when exposed to NGF in vitro. A human
cell line derived from PF cells, medullary thyroid carcinoma (MTC), has
previously been shown to synthesize and store 5-HT, a serotonin-binding
protein (SBP), and several neuropeptides; moreover, when grown in
impoverished media, MTC cells display neural properties. The purpose of the
current study was to utilize MTC cells as a neurally relevant model system
to investigate factors involved in mediating 5-HT secretion. Electron
microscopic immunocytochemistry revealed that secretory vesicles of MTC
cells costore immunoreactive 5-HT with SBP and calcitonin. The cAMP
derivative, N6-2'-O-dibutyryl-adenosine 3',5'- cyclic monophosphate
(dibutyryl-cAMP; 1.0 mM) increased the concentration of 5-HT in MTC cells
and almost doubled the rate of synthesis of 5-HT from L-tryptophan.
Dibutyryl-cAMP also significantly increased the secretion of 5-HT.
Cycloheximide (20 micrograms/ml) and anisomycin (20 microM) inhibited the
dibutyryl-cAMP-induced increase of 5-HT release, suggesting that this
action of dibutyryl-cAMP requires protein synthesis. Cholera toxin (1.0
microgram/ml) and forskolin (0.05 mM) in the presence of the
phosphodiesterase inhibitor 3-isobutyl-1- methylxanthine (1.0 mM) both
increased 5-HT biosynthesis and secretion. Attempts were made to identify a
ligand that stimulates cAMP-mediated secretion of 5-HT. Both
thyroid-stimulating hormone (TSH: 50 mU/ml) and elevated [Ca2+]e (7.0 mM),
each of which acts as a secretogogue for PF cells, stimulated the secretion
of 5-HT. The effect of TSH was Ca2(+)- dependent. Immunocytochemistry with
monoclonal antibodies to the TSH receptor confirmed that these receptors
are present on MTC cells. Neither TSH nor elevated [Ca2+]e elevated cAMP
levels. Measurements of Fura-2 fluorescence, however, indicated that both
TSH and elevated [Ca2+]e increased cytosolic calcium ([Ca2+]i), as did
elevation of [K+]e. It is concluded that exocytosis can be triggered in MTC
cells by multiple signal transduction mechanisms. Either cAMP or elevated
[Ca2+]i can stimulate secretion; however, a secretogogue that increases
cAMP has yet to be identified.
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