Polyphyllin D activates mitochondrial and lysosomal apoptotic pathway in drug resistant RHepG2 cells.

摘要

Cancer is one of the leading causes of death in the world. During cancer treatment, development of multidrug resistance (MDR) is always the major cause of failures of chemotherapy in human cancers. In our project, hepatocarcinoma HepG2 and its drug-resistant derivatives RHepG2 with MDR towards doxorubicin (Dox), fenretinide and Taxol were used to examine the differences in their response towards various anti-cancer agents.;Polyphyllin D (PD) is a saponin found in a tradition Chinese herb, Paris polyphylla, which has been used to treat liver cancers in China for many years. Interestingly, from the MTT assays, we found out that RHepG2 (IC50: 2.0 muM) was more sensitive towards PD when compared to that of its parental cells (IC50: 3.9 muM). To keep the MDR properties, RHepG2 cells were routinely cultured with 1.2 muM of Dox. When we cultured RHepG2 in the absence of Dox but with 1.2 muM of PD for 28 days, the Pgp expression could not be maintained. However, such high expression level of Pgp was maintained when RHepG2 cells were treated with vincristine (1.2 muM) in the absence of Dox. This indicates that vincristine was a substrate of Pgp to keep the Pgp expression in RHepG2 cells while PD was not.;Next, we investigated the underlying killing mechanism and found out that PD switched on both the mitochondrial and lysosomal apoptotic pathway in both cell lines. Our results indicate that PD was able to depolarize mitochondrial membrane potential and release apoptosis inducing factor (AIF) and cytochrome c (cyt c) from the mitochondria to cytosol. Also, PD was able to act on isolated mitochondria directly, causing a stronger mitochondrial membrane permeabilization and more AIF release from the RHepG2 than that of the parental cells.;By using the acridine orange (AO) staining method to examine the release of contents from lysosomes, it was found that PD released AO into the cytosol in both cell lines. However, the releasing pattern of HepG2 and RHepG2 was quite different. Upon PD treatment, the release of AO in HepG2 cells was graduate and slow while that in RHepG2 was sudden and sharp.;When incubated with different concentrations of Dox, RHepG2 accumulated less Dox than that of its parental HepG2 cells. When probed by the antibody against P-glycoprotein (Pgp), RHepG2 showed a strong Pgp expression. With the addition of Pgp modulator, verapamil, RHepG2 accumulated more Dox. All these findings indicate that Pgp is a mediator giving rise the MDR in RHepG2 cells. However, RHepG2 had a higher resistance to Dox than its parental line even co-cultured with verapamil. RHepG2 remained viable at the intracellular Dox concentration that was toxic to HepG2 cells. These observations suggest that the MDR properties of RHepG2 involved multiple mechanisms in addition to the effect of Pgp.;From the AO staining, most of the lysosomes were found in the cytosol near the nucleus. However, some lysosomes were found inside the nucleus occasionally. When we double stained the HepG2 cells with DiOC6(3), it was found that the lysosomes were actually located inside the nuclear tubules. However, no such lysosome migration was observed after treating the HepG2 cells with PD. Thus, lysosomes inside the nuclear tubules might not be involved in the PD-induced lysosomal pathway. The mechanism that leads to the migration of lysosomes into the nuclear tubules is still unclear.;From the Western blot analysis, cathepsin D (Cat D) and cathepsin L (Cat L) were both released from the lysosomes after treating the two cell lines with PD. Also, it seemed likely that Cat L was released earlier than that of cyt c. This implies that lysosomal permeabilization is an early event in apoptosis. With the use of siRNA technology, it was found that RHepG2 with the knockdown of Cat D and Cat L were more tolerant and vulnerable towards PD, respectively. These suggest that Cat D and Cat L might act oppositely in the apoptotic pathway. Furthermore, the addition of Cat D inhibitor, pepstatin A, blocked the PD-mediated cell death in RHepG2 cells further confirms that Cat D is a pro-apoptotic protein that is involved in the apoptotic pathway.;In conclusion, PD was a potent anti-cancer agent that could reverse the MDR properties of RHepG2 and kill more RHepG2 cells through lysosomal and mitochondrial apoptotic pathway.