A novel synthetic melanin as a potential anticancer agent that induces apoptosis and cyclin D downregulation through distinct pathways

Despite extensive development of anticancer agents, there remains a critical need for therapeutics with novel mechanisms of action. This study reports the potent anticancer activity and mechanistic analysis of highly water-soluble dopa-melanin (DM) prepared through a unique synthetic process. DM exhibits distinctive structural and chemical properties that contribute to its exceptional aqueous solubility (≥50 mg/ml) and may underlie its enhanced biological activity. DM reduced cell viability in multiple cancer cell lines. Using HeLa cells as a representative model, DM suppressed cell migration and three-dimensional growth. Fractionation revealed that activity was associated with polymers larger than 30 kDa, suggesting a critical role for high-molecular-weight species. Mechanistic studies showed that DM induced S/G₂/M arrest followed by cell death with minimal apoptotic body formation. DM rapidly decreased cyclin D1 and D3 protein and mRNA levels. A pan-caspase inhibitor significantly suppressed DM's inhibitory effect on cell viability but did not prevent cyclin D degradation. Cyclin D degradation was mediated through calcium-dependent calpain activation triggered by endoplasmic reticulum Ca2+ release via IP₃ receptors, and inhibition of this pathway attenuated DM's activity. In a mouse syngeneic tumor model, oral administration of DM significantly inhibited tumor growth without apparent toxicity. These results demonstrate that both caspase-dependent apoptosis and a caspase-independent cyclin D degradation pathway contribute to DM-induced cell death. Its high solubility, oral efficacy, and unique mechanism of action make DM a promising candidate for therapeutic development.