ENDOGENOUS SEROTONIN-2A AND SEROTONIN-2C RECEPTORS IN BALB C-3T3 CELLS REVEALED IN SEROTONIN-FREE MEDIUM - DESENSITIZATION AND DOWN-REGULATION BY SEROTONIN/
C. Saucier et al., ENDOGENOUS SEROTONIN-2A AND SEROTONIN-2C RECEPTORS IN BALB C-3T3 CELLS REVEALED IN SEROTONIN-FREE MEDIUM - DESENSITIZATION AND DOWN-REGULATION BY SEROTONIN/, Biochemical pharmacology, 56(10), 1998, pp. 1347-1357
We studied the endogenous expression of the serotonin-2A (5-hydroxytry
ptamine(2A), 5-HT2A) 5-HT2C, and a splice-variant of the 5-HT2C recept
or in murine Balb/c-3T3 fibroblast cells that is revealed when these c
ells are maintained in medium containing 5-HT-free serum. RNA editing
of the 5-HT2C receptor was exclusively at a single brain-specific site
. Addition of 5-HT (EC50 = 23 +/- 29 nM) induced an immediate release
of calcium from an ionomycin-sensitive intracellular store by coupling
to a pertussis toxin-insensitive pathway. The 5-HT-induced calcium mo
bilization displayed a 5-HT-(2)-like pharmacology, and ligand binding
analyses indicated the presence of specific binding sites (27.5 +/- 2
fmol/mg protein) with a 5-HT2A-like pharmacology. Although the 5-HT2A
receptor site was predominant, the smaller component of 5-HT2C recepto
rs alone was sufficient to mediate a maximal calcium response. The 5-H
T-induced increase in [Ca2+](i) was reversibly inhibited by >75% follo
wing a 12-hr pretreatment (T-1/2 = 2 hr) with 5-HT (EC50 = 400 nM). Ex
tended treatment (24-96 hr) with 5-HT induced a complete functional de
sensitization that was associated with a partial (60%) reduction in 5-
HT2 receptor number, implicating both receptor down-regulation and pos
t-receptor mechanisms in 5-HT-induced desensitization. Long-term (hour
s to days) treatment with 5-HT did not modulate DNA synthesis, cell pr
oliferation, or transformation in Balb/c-3T3 cells. These results demo
nstrate that Balb/c-3T3 cells express endogenous 5-HT2 receptors that
are desensitized by the 5-HT present in normal serum, illustrating the
importance of growth conditions in the identification of receptor res
ponsiveness. The lack of proliferative response to 5-HT in Balb/c-3T3
suggests a putative role of desensitization as a ''safety valve'' to p
revent abnormal cell growth during sustained 5-HT2 receptor activation
.; BIOCHEM PHARMACOL 56;10:1347-1357, 1998. (C) 1998 Elsevier Science
Inc.