Proapoptotic and TRAIL-sensitizing constituents isolated from Salvia militiorrhiza (Danshen)

Natural compounds isolated from medicinal plants are invaluable resources for drug discovery. Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent unique by its cancer cell-specific proapoptotic action, but its potential is heavily curbed by acquired resistance. We herein reported for the first time the identification of cytotoxic and TRAIL-sensitizing components of Salvia miltiorrhiza (Danshen), a traditional medicinal plant effective for treating cardiovascular disorders. Specifically, we found that the ethanol extract and its group 5 fraction of S. miltiorrhiza showed evident cytotoxicity against the human lung adenocarcinoma cell line A549 and ovarian adenocarcinoma cell line TOV-21G in a concentration-dependent manner. Likewise, a dose-dependent cytotoxicity was exerted by the standard solutions of cryptotanshinone, tanshinone I and tanshinone IIA, the major components of the group 5 fraction, where tanshinone IIA were most potent and displayed an IC₅₀ of 2.00 ± 0.36 μM and 2.75 ± 0.23 μM for A549 and TOV-21G, respectively. Induction of apoptosis represents an essential mechanism underlying tanshinone IIA-mediated cytotoxic action, as evidenced by the proteolytic processing of PARP upon tanshinone IIA stimulation and, importantly, a marked rescue of the viability of tanshinone IIA-treated cells when co-treatment with the pan-caspase inhibitor z-VAD-fmk. Noteworthy, stimulation with cryptotanshinone, tanshinone I or tanshinone IIA all effectively potentiated TRAIL to reduce viability and inhibit the colony formation capacity of TRAIL-resistant TOV-21G and SKOV3. Collectively, we revealed the proapoptotic and TRAIL-sensitizing components of S. miltiorrhiza and further implicated the potential of developing these active compounds as monotherapeutic agent or TRAIL-based therapy for cancer chemoprevention or chemotherapy.