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T he pathogenesis of the eosinophilia myalgia syndrome (EMS) remains unclear. Several abnormal constituents have been found in the L-tryptophan lots responsible for the illness, particularly, 1,1-ethylidenebis[L-tryptophan], also called peak E or EBT, and 3-phenylamino-alanine or peak 5. However, the role of these contaminants in the pathogenesis of EMS and in the development of fibrosis is unknown. We now report that peak E, a dimer of L-tryptophan, is a potent stimulus for human dermal fibroblast DNA and collagen synthesis. Peak E (0.1-1.0 microM) increased DNA synthesis up to four-fold (P = 0.0001) in a dose-dependent manner (r = 0.987). When added to monolayer cultures for 2 to 24 h, peak E (0.5 to 100 microM) caused a progressive, more than threefold increase in alpha 1(I) procollagen mRNA levels and collagenous protein. No increase in procollagen mRNA levels was found after the addition of another major L-tryptophan contaminant, peak 5, or with L-tryptophan itself. Transient transfection with a 2.5-kb alpha 1(I) procollagen promoter-luciferase construct showed that peak E causes a twofold upregulation of promoter activity (P = 0.022). Contraction of collagen gels, consisting of human dermal fibroblasts incorporated into a type I collagen lattice, was enhanced two-fold by exposure to peak E (P = 0.001). We conclude that a major constituent of contaminated batches of L-tryptophan, peak E, is a potent stimulus for fibroblast activation and collagen synthesis. This stimulatory action of peak E may provide a direct mechanism for the development of fibrosis in EMS.