Tropisetron inhibits serotonin‐induced PGE2 release from macrophage‐like synovial cells in serum‐free tissue culture

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Tropisetron has shown potent analgesic and antiphlogistic effects in patients with rheumatic diseases such as osteoarthritis (1) or tendinopathies (2). These observations suggest that serotonin (5-HT) plays an important role in pain and inflammation. Objectives: The objective of our study was to determine if 5-HT stimulates the release of further inflammatory mediators such as prostaglandin E2 (PGE2) and if this effect can be blocked by tropisetron. To resolve these questions, we used a novel serum-free tissue culture technique for dissociated synovial tissue (3). This method permits in vitro experiments without perturbation by serum factors. Our cultures are characterized by 80% synoviocytes expressing the macrophage-specific marker CD68. Methods: Dissociated synovial tissue from osteoarthritis patients (n~5, mean age 67.2i6.2 years) was cultured for 11 days in Iscove’s Modified Dulbecco’s Eagle Medium supplemented with insulin. Cultures were then stimulated with 10, 10, and 10 M 5-HT and antagonized with 50 mg/mL tropisetron. Positive controls consisted of stimulation with 100 U/ml interleukin 1b (IL-1b). Negative controls included unstimulated cells or co-culture with tropisetron alone. After 48 h, PGE2 levels were determined using a conventional enzymeimmunoassay (EIA). PGE2 concentrations/mg protein were expressed as percent of controls and compared using the Mann-Whitney rank sum test. In addition, RT-PCR analyses for 5-HT1A, 5-HT1B, 5-HT2A, 5-HT3, and 5-HT4 receptor subtypes (4) were performed using synovial tissue (n~5). Results: The results (Figure 1) showed an increase of PGE2 in response to 10 25 M 5-HT by 421.8i84.9% (meanistandard error, p~v0.0001), whereas 10 and 10 M did not show significant effects (126.9i28.7%; p~0.37 and 96.8i28.7%; p~0.87, respectively). Stimulation by IL-1b caused an increase of 1347.5i481.4% (p~0.0094). Preincubation with tropisetron completely suppressed the serotonin-induced PGE2 release. RT-PCR showed an expression of the 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT3 but not 5-HT4 receptor subtype. Conclusions: The increase of PGE2 shows that serotonin is a potent proinflammatory mediator. The findings indicate the presence of 5-HT3 receptors and a tropisetron-induced suppression of this response. Additional analyses are necessary to further elucidate the underlying inflammatory mechanisms and inhibitory effect of tropisetron.