Curcuminoids Metabolite Tetrahydrocurcumin Suppresses Melanoma Cell Growth and Alters G2/M-Phase Cell Cycle Regulation Without Apoptosis Induction

Background: Melanoma is an aggressive malignancy characterized by rapid proliferation and the potential to develop resistance to conventional therapies. Tetrahydrocurcumin (THC), a metabolite of curcumin, has demonstrated various biological activities; however, its effects on melanoma cell proliferation and cell cycle regulation remain insufficiently characterized. Methods: This study investigated the antiproliferative effects of THC and the associated molecular changes in human melanoma A375 and C32 cells. Cell viability was assessed using the CCK-8 assay, while Ki-67 immunofluorescence was used to evaluate cellular proliferation. Apoptosis was examined by Annexin V/propidium iodide staining, and cell cycle distribution was analyzed by flow cytometry. The expression of cell cycle-related proteins was evaluated by Western blotting. Results: THC treatment inhibited the viability of A375 and C32 cells in a dose- and time-dependent manner, with 24-h IC₅₀ values of 158.1 μM and 154.8 μM, respectively. Consistently, Ki-67 expression decreased following THC treatment, supporting its antiproliferative effect. However, Annexin V/propidium iodide analysis did not reveal a significant increase in apoptosis under the tested conditions, suggesting that growth inhibition was not primarily associated with the induction of apoptosis. Flow cytometric analysis demonstrated an increased proportion of cells in the G2/M phase, accompanied by a reduction in the G1-phase population. At the molecular level, THC treatment was associated with increased Cyclin B1 and CDK1 expression, together with alterations in other cell cycle regulatory proteins. These findings suggest that THC suppresses melanoma cell growth and promotes G2/M-phase accumulation, potentially through dysregulation of cell cycle progression. Further investigation of Cyclin B1–CDK1 complex activity and the phosphorylation status of CDK1 is required to clarify the underlying molecular mechanism. Conclusion: This study identifies THC as a potential inhibitor of melanoma cell proliferation and provides a basis for further investigation of its effects on cell cycle regulation.