| Mitochondria, previously only recognized as power houses in eukaryotic cells, attracted great attention for their important functions in apoptosis, aging, cancer and genetic diseases in the past ten years. Recently, it has been found that mitochondria could be a novel target in chemotherapy as they play a crucial regulatory role in apoptosis.;Drug resistance is the single most important stumbling block in the fight against cancer by chemotherapy. A number of mechanisms such as the over-expression of P-glycoprotein (Pgp) and alteration of apoptosis pathway have been suggested to associate with the development of multidrug resistance (MDR). However, how mitochondria respond to anticancer drugs during chemotherapy and whether mitochondria contribute to the multifactorial MDR remain unclear.;In my studies, a human hepatocellular carcinoma cell line HepG2 and its derivative, R-HepG2, were chosen to be an in vitro model to study the activity of mitochondria during chemotherapy with emphasis on their roles in MDR. Effects of conventional anticancer drugs doxorubicin (DOX), cisplatin (CDDP) and taxol, as well as mitochondria target drugs oligomycin and lonidamine (LND) on the changes of mitochondrial activity in HepG2 and R-HepG2 were studied.;Results in my studies indicated that R-HepG2 showed a multi-drug resistance to a number of conventional anticancer drugs such as cisplatin, taxol in addition to DOX. Western blot illustrated that a higher level of Pgp was detected in R-HepG2 while no detectable Pgp was found in HepG2. Studies of DOX uptake by flow cytometry further confirmed a much lower cellular DOX in R-HepG2 cells. Meanwhile, a higher ATP content, mitochondrial mass and a higher expression of mitochondrial proteins such as ATPase alpha and cytochrome c oxidase II were found in R-HepG2 than that in HepG2. Mitochondria target drugs, oligomycin, an inhibitor of FoF1-ATPase, and LND, an inhibitor of glucose phosphorylation, reduced the ATP production in R-HepG2. Moreover, oligomycin increased the DOX uptake in R-HepG2 while no such results were observed when R-HepG2 cells were treated with LND.;Furthermore, mitochondrial activities during apoptosis were studied. When more DOX was added to R-HepG2 to overcome the Pgp-mediated drug efflux, cytochrome c (Cyt c) release was found in R-HepG2 but with a less extent when compared to that of HepG2 with the same intracellular DOX level. Meanwhile, when exogenous Cyt c was introduced to HepG2 and R-HepG2 to trigger the death pathway, almost the same cytotoxicity was found. These indicate that an intact post-mitochondrial apoptotic pathway existed in both cell lines while the pre-mitochondrial apoptotic pathway in R-HepG2 has been altered. Similarly, CDDP and taxol induced a mitochondrial-mediated apoptosis in HepG2 whereas a higher dose of these drugs were needed for R-HepG2 to execute apoptosis.;Mitochondria target drugs oligomycin and LND were effective in killing both HepG2 and R-HepG2. These drugs damaged mitochondria by causing mitochondrial membrane potential depolarization, Cyt c release, an increase in ROS production and a reduced cardiolipin content in HepG2 and R-HepG2.;Taken together, mitochondria participate in MDR, at least in part, by modulating both the energy supply and apoptotic pathway in our model. As mitochondrial target drugs are found to be promising anticancer candidates, detailed mechanisms involved await further investigation. |
