Enhancement of serotonin transporter function by tumor necrosis factor alpha but not by interleukin-6
Introduction
Interactions between the nervous system and the immune system are of great interest. A key player in this regard may be serotonin (5-HT), which acts both as a neurotransmitter and is released at high concentrations at sites of inflammation (Sternberg et al., 1987).Neurotransmission mediated by 5-HT influences many physiologic functions including motor activity, food intake, reproductive activity, sleep, and neuroendocrine rhythms, as well as cognition and emotional states, including mood and anxiety (Lesch et al., 1996). The serotonin transporter (5-HTT) is central to the fine-tuning of serotonergic neurotransmission and regulates the magnitude and duration of serotonergic responses (Lesch et al., 1996). Modulation of 5-HTT function is therefore an important mechanism to influence serotonergic neurotransmission (Heils et al., 1998).
Tumor necrosis factor alpha (TNF-α) is an inflammatory mediator that is now recognized as a critical component of the immune system. It initiates many different aspects of host inflammatory defenses including cellular proliferation, differentiation, induction of cell surface molecules, antiviral activity, and the induction of cell death (Beutler, 1990; Wong et al., 1996). It is rapidly released by macrophages and lymphocytes upon activation (Beutler, 1992). Interleukin-6 is another inflammatory mediator that is released early during immune responses and plays an important part in host defense (Akira et al., 1993). We therefore investigated whether TNF-α and interleukin-6 as key mediators of the immune system influence the function of the 5-HTT, using as cellular model system the JAR choriocarcinoma cell line, which constitutively expresses the 5-HTT.
Section snippets
Cell culture
The human choriocarcinoma cell line JAR was obtained from American Type Culture Collection (Rockville, Md.) and grown in RPMI 1640 medium supplemented with 10% fetal calf serum (FCS), glutamine, and gentamicin at 37°C in a humidified atmosphere at 5% CO2. For treatment with recombinant human TNF-α or interleukin-6 (R+D Systems, Wiesbaden, Germany), JAR cells were washed once before plating at a density of 2×105 cells⧹well into 24-well plates (Falcon, Becton-Dickinson, Heidelberg, Germany), 1 ml
Effect of TNF-α on the 5-HTT
JAR cells incubated with TNF-α (50 ng⧹ml) for two days showed markedly enhanced 5-HT uptake, compared to control cells (Fig. 1). This effect is characterized by an increase in the maximal velocity (Vmax) of 5-HT uptake, with Vmax approx. 22 pmol for TNF-treated and Vmax approx. 11 pmol for untreated cells. The affinity of the transporter (Km) showed no significant change, with a Km of between 0.5 and 0.6 μM. To assess the time course of the effect of TNF-α , JAR cells were incubated with the
Discussion
We have shown that exposure of the JAR cell line to the cytokine TNF-α enhances 5-HT transporter-mediated uptake of 5-HT. This effect is characterized by an increase in the maximal velocity of 5-HT uptake, with only a slight change in the affinity of the 5-HTT. The effect is dose- and time-dependent, with a maximum observed after exposure to TNF-α for 48 h. In contrast to TNF-α, interleukin-6 had no effect on 5-HTT function.
We have excluded the possibility that the increased 5-HT uptake after
Acknowledgements
This work was supported by the Bundesministerium für Bildung und Forschung (BMBF, Nr. 512-4001-01KS9603).
References (22)
- Akira, S., Taga, T., Kishimoto, T., 1993. Interleukin-6 in biology and medicine. Advances in Immunology 54,...
- Beutler, B., 1990. The complex regulation and biology of TNF (cachectin). Critical Reviews of Oncogenes 2,...
- Beutler, B., 1992. Tumor necrosis factors: the molecules and their emerging role in medicine. Raven Press, New...
- Carlson, S. L., Felten, D. L., Livnat, S., Felten, S. Y., 1987. Alterations of monoamines in specific central autonomic...
- Feinberg, B. B., Anderson, D. J., Steller, M. A., Fulop, V., Berkowitz, R. S., Hill, J. A., 1994. Cytokine regulation...
- Gardier, A. M., Kachaner, S., Shaghaghi, E. K., Blot, C., Bohuon, C., Jacquot, C., Pallardy, M. J., 1994. Effects of a...
- Heils, A., Wichems, C., Mössner, R., Petri, S., Glatz, K., Bengel, D., Murphy, D. L., Lesch, K.-P., 1998. Functional...
- Hunt, J. S., 1989. Cytokine networks in the uteroplacental unit: macrophages as pivotal regulatory cells. Journal of...
- Jarousseau, A.-C., Thibault, G., Reverdiau, P., Rodriguez, A.-M., Lacord, M., de Russe, J., Watier, H., Degenne, D.,...
- Jayanthi, L. D., Ramamoorthy, S., Mahesh, V. B., Leibach, F. H., Ganapathy, V., 1994. Calmodulin-dependent regulation...