Tryptophan and Its Selected Metabolites Modulate Melanoma-Cell Biology: A Preliminary In Vitro Study

Tryptophan (TRP) and its metabolites are involved in numerous physiological and pathological processes within the skin. They may influence melanoma biology, but their role remains insufficiently understood. This study investigated the effects of TRP, anthranilic acid (AA), and cinnabarinic acid (CBA) in melanoma cells in vitro and their potential interaction with the B-Raf proto-oncogene, serine/threonine kinase (BRAF) inhibitor vemurafenib. A375, G361, and SK-MEL-3 melanoma cell lines and a THP-1-derived macrophage model were used. Cell proliferation and metabolic activity were assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, migration using a scratch assay, and cell-cycle distribution by flow cytometry. Western blotting was used to analyze selected signaling proteins. Reactive oxygen species (ROS) and nitric oxide (NO) production were evaluated. Bioinformatic analyses investigated associations between BRAF and genes related to tryptophan metabolism and the kynurenine pathway. All compounds showed dose-dependent antiproliferative effects, with prolonged exposure increasing efficacy. TRP and AA increased G0/G1 cell-cycle arrest, whereas CBA increased the G2/M population. TRP also reduced A375 cell migration. All three compounds enhanced the antiproliferative effect of vemurafenib, with CBA showing the strongest effect (approximately 20%). Western blot analysis indicated modulation of p21 expression and ERK1/2 phosphorylation, suggesting implications for cell cycle and stress signaling pathways. Moreover, conditioned medium from A375 cells exposed to tested compounds affected cell viability, ROS, and NO production in THP-1-derived macrophages. TRP and its metabolites may be considered potential supportive agents for melanoma BRAF-targeted therapy. However, further in vivo studies are needed to clarify their therapeutic value and to elucidate the underlying molecular mechanisms.