Antagonism of the serotonin pathway – a novel antifibrotic approach?

The neurotransmitter serotonin (5‐Hydroxytryptamin, 5‐HT) has been implicated in the pathogenesis of scleroderma and scleroderma‐like skin changes for years. In particular, a role of serotonin has been suggested as an important vasoactive mediator of the Raynaud's phenomenon in patients with systemic sclerosis. Accordingly, anti‐serotoninergic therapies have been utilized clinically to treat Raynaud's phenomenon. There is also recent evidence that multiple cell types of the skin have the capacity to synthesize serotonin. The biological effects of serotonin are mediated by at least 15 different 5‐HT‐receptor subtypes. One of them is the 5‐HT3‐receptor, which binds the 5‐HT‐antagonist tropisetron. Here we investigated the effect of tropisetron on transforming growth factor‐beta1 (TGF‐beta1)‐mediated collagen synthesis in human dermal fibroblasts (HDF) in vitro. In addition, we elucidated the impact of tropisetron on activation of SMAD, a signaling transduction pathway implicated in TGF‐beta1‐mediated collagen synthesis. Tropisetron at non‐cytotoxic doses significantly suppressed TGF‐beta1‐mediated collagen type I (alpha1 and 2) and type III (alpha1) expression at RNA level in HDF as shown by real‐time PCR analysis. This effect of tropisetron was paralleled by reduced amounts of intracellular collagen type I protein as evidenced by Western immunoblotting. In addition, tropisetron significantly decreased secreted procollagen‐I‐C‐peptide as demostrated by ELISA. In contrast, tropisetron alone did not have any detectable effect on collagen expression. Analysing the SMAD pathway we could further show that tropisetron in contrasts to TGF‐beta1 neither affects SMAD3 phosphorylation, nuclear translocation of SMAD2/3 nor SMAD3/4‐dependent promoter activity in HDF. Our results for the first time demonstrate a potent suppressive effect of tropisetron on TGF‐beta1‐induced collagen synthesis in vitro. Regardless of the molecular mechanism of TGF‐beta1 antagonism by tropisetron, these findings should be of special relevance towards the exploitation of tropisetron and related agents as a novel treatment for patients with fibrotic diseases.