III. 5HTlc and 5HT2 Receptors Function as Proto- 
oncogenes in Fibroblasts. 
Although fibroblasts express intracellular signal- 
ing pathways activated by the 5HTlc and 5HT2 re- 
ceptors, this second messenger pathway is likely to 
couple to effector proteins that differ from those in 
neurons and that may mediate distinct cellular 
functions. Dr. Axel's laboratory therefore examined 
the functional consequences of activating these re- 
ceptors in NIH 3T3 fibroblasts. These studies dem- 
onstrated that 3T3 cells expressing high levels of ei- 
ther the 5HTlc or 5HT2 receptor form foci in cell 
culture. Several observations indicate that the for- 
mation of foci results from the expression and acti- 
vation of 5HT receptors on the cell surface. First, 
the introduction of functional 5HTlc or 5HT2 re- 
ceptor cDNA into NIH 3T3 cells results in the gen- 
eration of transformed foci at high frequency. Foci 
are not observed after transfection with expression 
vectors lacking the 5HT receptor cDNA. Second, 
cells within transformed foci exhibit a high density 
of functional 5HT receptors on the cell surface. 
Third, the generation of foci after DNA transfer is 
completely blocked by serotonin antagonists. 
Fourth, transformed foci expressing the 5HT recep- 
tors fail to reestablish foci when plated in serotonin 
antagonists, such as mesulergine. Finally, the injec- 
tion of cells derived from transformed foci obtained 
after expression of the 5HTlc receptor into nude 
mice results in the generation of tumors. Thus acti- 
vation of the same receptor in fibroblasts and neu- 
rons elicits distinct phenotypes. In neurons the 
5HT receptors are involved in neurotransmission 
via regulation of ion channel function, whereas in 
fibroblasts the same receptor alters the growth 
properties of cells and results in malignant transfor- 
mation. The distinct phenotypic consequences of 
receptor activation in fibroblasts and neurons may 
reflect the different ways in which different cell 
types are programmed to respond to the same set 
of signaling events. 
The observation that expression of 5HTlc and 
5HT2 receptors elicits focus formation only in the 
PUBLICATIONS 
presence of ligand permits the selection of mutants 
affecting components of the signal transduction 
machinery. This receptor-mediated transformation 
system should facilitate a somatic cell genetic analy- 
sis of neurotransmitter-mediated signaling. 
IV Activation of the ^-Adrenergic Receptor Pro- 
motes Growth and Differentiation in Epithelial 
Cells. 
Studies with serotonin receptors indicate that a 
single ligand may interact with the same receptor 
subtype in different cells to elicit distinct cellular 
responses. The (B^-adrenergic receptor was there- 
fore introduced into the unnatural environment of 
the thyroid cell to demonstrate that the activation 
of this receptor also initiates diverse cellular pro- 
grams in different cell types. The cell line FRTL5 is a 
continuous line of thyroid cells that depends on 
thyroid-stimulating hormone (TSH) for growth and 
differentiation. Activation of the TSH receptor stim- 
ulates adenylate cyclase, resulting in an increase in 
cAMP. The p^-adrenergic receptor also stimulates 
adenylate cyclase, but this receptor subtype is not 
expressed in thyroid cells. Experiments were per- 
formed to ask whether thyroid cells transfected 
with cDNA encoding the P^-^drenergic receptor will 
undergo growT:h and differentiation in response to 
the adrenergic ligand isoproterenol. In thyroid cells 
transfected with the p^-adrenergic receptor, 
isoproterenol elicits the same program of thyroid- 
specific functions observed with TSH. The p^-^dren- 
ergic receptor that contributes to autonomic neu- 
rotransmission in the sympathetic nervous system 
regulates growth and differentiation in the unnatu- 
ral environment of a thyroid cell. Thus the func- 
tional distinction between a neurotransmitter re- 
ceptor and a growth factor (or even an oncogene 
product) may depend critically on the cellular envi- 
ronment. 
Dr. Axel is also Higgins Professor of Biochemistry 
and Pathology at the Columbia University College 
of Physicians and Surgeons. 
Articles 
Arthos, J., Deen, K.C., Chaikin, M.A., Fornwald, J.A., Sathe, G., Sattentau, Q.J., Clapham, PR., Weiss, R.A., Mc- 
Dougal, J.S., Pietropaolo, C, Axel, R. , Truneh, A., Maddon, RJ., and Sweet, R.W 1989. Identification of the 
residues in human CD4 critical for the binding of HIV Cell 57:469-481. 
Hen, R. , Axel, R. , and Obici, S. 1989. Activation of the P2-adrenergic receptor promotes growth and differenti- 
ation in thyroid cells. Proc Natl Acad Set USA 86:4785-4788. 
Continued 
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