Several pathways could be involved in these mechanisms including activation of anti-apoptotic factors, inactivation of pro-apoptotic effectors, and/or reinforcement of survival signals [122]. Based on an understanding of their characteristics, the refractory response of CSCs to drugs and radiation treatments may be attributed to: drug effluxion glutathione (GSH) system apoptosis; enrichment of CSCs during disease progression tumor dormancy and CSC quiescence Drug effluxion It can be caused by an altered uptake or efflux of drug in the target

cell. Platinum compounds enter the cell, primarily by passive diffusion, however several different ways have been described such as copper transporter proteins (CTR), organic cation transporters (OCTs) from de SLC22 family, ATP-binding cassette (ABC) multidrug transporters, copper-transporting ATPases, and multidrug and toxin extrusion from the SLC47 subfamily members that might Napabucasin concentration facilitate the active efflux of anticancer platinum agents. Some of most frequently studied drug transporters associated with acquisition of resistance in normal SCs as well as in CSCs are multifunctional efflux transporters from the ABC gene family [123]. These contribute to tumor resistance by actively transporting drugs across cell membranes

through ATP hydrolysis [83, 124–127]. Efflux transporters in the ABC family such as ABCG2 are cell surface drug-resistance markers involved in the transport of Selleck TSA HDAC substances and cellular products [128–133]. The resistance gene BCRP/MXR/ABCP has been studied for its involvement in development of chemoresistance. ABCG2/BCRP plays a key role in cellular homeostasis and GW-572016 in vivo tissue integrity. It has been observed that ovarian CSCs exposed to chemotherapy overexpress this ABC family of transporters. Consequently, ABCG2/BCRP acts as a xenobiotic drug transported

by promoting expulsion through an ejection system. Glutathione (GSH) system Also inflammatory processes can contribute to multiple CSC capabilities by supplying bioactive molecules to the tumor microenvironment and, additionally, inflammatory cells can release reactive oxygen species that are actively mutagenic for nearby cancer cells and accelerate their genetic evolution toward states of heightened malignancy [134]. GSH system protects cells against the effect of external 2-hydroxyphytanoyl-CoA lyase cytotoxic agents, including platinum [135, 136]. The GSH system can suppress oxidative stress and maintain cellular redox homeostasis [137]. The contribution of GSH and GSH-related enzymes to chemoresistance has been demonstrated in different types of tumor, including ovarian cancer and brain tumor [138]. GSH is also involved in the detoxification of various xenobiotics [139]. Upon metabolism of chemotherapeutic agents, the enzymes of glutathione-S-transferase (GST) family could prompt the formation of GSH-drug conjugates. Many chemotherapeutic agents have been shown to conjugate with GSH, including chloroethylnitrosoureas, platinum compounds, and other alkylating agents.