Abstract
Betulin has a wide range of biological and pharmacological properties with its anticancer activity attracting most of the attention as it offers a possible alternative treatment to chemotherapy. However, betulin's in vivo biological effectiveness is limited by its poor solubility. As such, we synthesized polar glycosylated derivatives to increase its hydrosolubility and enhance its pharmacological properties. Among these synthesized compounds, 28-O-α-l-rhamnopyranosylbetulin 3β-O-α-l-rhamnopyranoside (Bi-L-RhamBet) was assessed for its cytotoxic effects against a suite of lung cancer cell lines. We also investigated its mechanism of action using an A549 lung cancer cell line. Our results showed that Bi-L-RhamBet exhibited potent cytotoxic activity toward lung cancer cell lines including A549, NCI-H2087, NCI-H522, NCI-H1993 NCI-H1755, and LLC1 having IC50 values ranging from 2.9 to 5.9 μM. Moreover, Bi-L-RhamBet (50 mg/kg) significantly inhibited tumor growth with a treatment-to-control ratio (T/C) of 0.54 and a tumor growth inhibition rate of 46% at day 18 (p < 0.05). Microscopic observations of A549 cells, double stained with acridine orange and ethidium bromide, showed apoptotic features. Bi-L-RhamBet induced activation of pro-apoptotic caspases 8, 9, and 3/7 as well as causing DNA fragmentation. Moreover, a marked increase in mitochondrial ROS (mROS) was coupled with a reduction of mitochondrial potential. Interestingly, the presence of mitochondrial electron transport chain (ETC) inhibitors, including rotenone, malonate, and antimycin A, reduced mROS production, and the activation of caspases suggesting that Bi-L-RhamBet disturbs the ETC. Finally, dichloroacetate, a pyruvate dehydrogenase kinase inhibitor potentiated the cytotoxicity of Bi-L-RhamBet against A549 cells. Taken together, these data suggest that Bi-L-RhamBet can induce apoptotic cell death via disturbance of mitochondrial electron transfer chain, reduced ROS production, and decreased membrane potential.