Background and Aims: Tryptophan is an essential amino acid and an important source for protein synthesis, and its various metabolites also participate in diverse physiological functions. We have previously shown that one of the tryptophan metabolites, 5-methoxytryptophan (5-MTP), reduces inflammatory cytokine-induced p38 MAPK activation in vascular smooth muscle cells (VSMCs) and protects against vascular remodeling in animal models. Our goal is to investigate whether hydroxyindole O-methyltransferase (HIOMT) is the synthetic enzyme for 5-MTP and functions as 5-MTP in protecting against intimal hyperplasia and whether it alters tryptophan metabolism in VSMCs. Methods: HIOMT transgenic mice were generated to evaluate the function of HIOMT in vivo using a neointima formation model. Immunohistochemistry was performed to evaluate intimal hyperplasia and expression levels of select tryptophan metabolites. Primary wild-type and transgenic VSMCs were used in vitro for mechanistic studies. Results: Compared with wild-type, HIOMT transgenic mice had increased 5-MTP levels in the medial layer of the arteries. After arterial injury, transgenic mice had smaller neointima and reduced MMP2 levels in the arterial wall. Furthermore, arterial levels of the tryptophan metabolite serotonin and its synthetic enzyme aromatic amino acid decarboxylase (AADC) were lower than those of wild-types. Interestingly, serotonin promotes VSMC proliferation and reduces the expression of VSMC markers. Transient transfection of HIOMT expression plasmid in VSMCs or primary HIOMT transgenic VSMCs exhibited higher 5-MTP levels, reduced IL1-β-induced p38 phosphorylation, and prevented IL1-β-reduced VSMC marker downregulation. Moreover, HIOMT overexpression decreased basal and IL1-β-induced serotonin and AADC levels in VSMCs. Conclusions: Our results indicate that HIOMT is the synthetic enzyme of 5-MTP in VSMCs and protects against injury-induced occlusive vascular disease. Tryptophan metabolic pathways are altered in VSMCs when 5-MTP is present, or HIOMT is overexpressed. Taken together, 5-MTP and its synthetic enzyme HIOMT are important targets for treating occlusive vascular disease.
