Genetic variants in genes involved in mechanisms of chemoresistance to anticancer drugs.

Refractoriness to the pharmacological treatment of cancer is dependent on the expression levels of genes involved in mechanisms of chemoresistance and on the existence of genetic variants that may affect their function. Thus, changes in genes encoding solute carriers may account for considerable inter-individual variability in drug uptake and the lack of sensitivity to the substrates of these transporters. Moreover, changes in proteins involved in drug export can affect their subcellular localization and transport ability and hence may also modify the bioavailability of antitumor agents. Regarding pro-drug activation or drug inactivation, genetic variants are responsible for changes in the activity of drug-metabolizing enzymes, which affect drug clearance and may determine the lack of response to anticancer chemotherapy. The presence of genetic variants may also decrease the sensitivity to pharmacological agents acting through molecular targets or signaling pathways. Recent investigations suggest that changes in genes involved in DNA repair may affect the response to platinum-based drugs. Since most anticancer agents activate cell death pathways, the evasion of apoptosis plays an important role in chemoresistance. Several genetic variants affecting death-receptor pathways, the mitochondrial pathway, downstream caspases and their natural modulators, and the p53 pathway, whose elements are mutated in more than half of tumors, and survival pathways, have been reported. The present review summarizes the available data regarding the role of genetic variants in the different mechanisms of chemoresistance and discusses their potential impact in clinical practice and in the development of tools to predict and overcome chemoresistance.

Strategies for overcoming chemotherapy resistance in enterohepatic tumours.

When considered together, enterohepatic tumours, i.e., those affecting the liver, the biliary tree and gallbladder and the intestine, constitute the first cause of death due to cancer. Although in many cases surgery and radiotherapy are efficacious, these therapeutic strategies cannot always be implemented. Moreover, even when the removal of tumours is possible, pre- and post-operative pharmacological adjuvant regimens are often needed. However, one important limitation to the use of cytostatic drugs to treat enterohepatic tumours is that they generally exhibit marked refractivity to currently available pharmacological approaches. In addition, most of them increase their chemoresistance during treatment. In view of the high refractivity of these tumours to anti-cancer drugs and the existence of undesirable side effects, both of which are drawbacks in the available chemotherapy, several novel therapeutic approaches have been devised. The purpose of the present review is to offer some insight into the different types of strategies that have already been evaluated and incorporated into clinical practice, such as therapies based on the use of molecular targets, as well as into the approaches that are still under experimental development, such as the chemosensitization of cancer cells, genetic manipulation of tumour or host cells, and cell-specific enhancement of intracellular concentrations of the active agent by efficient targeting of pro-drugs or by using inhibitors of efflux pumps.

Expression in human trophoblast and choriocarcinoma cell lines, BeWo, Jeg-3 and JAr of genes involved in the hepatobiliary-like excretory function of the placenta.

Using cytokeratin-7-positive trophoblast cells (hTr) isolated from human term placentas and the choriocarcinoma cell lines (hCC) BeWo, Jeg-3 and JAr, the expression of genes involved in the hepatobiliary excretion of cholephilic compounds was investigated by RT-PCR/sequencing followed by measurement of the absolute abundance of mRNA by real-time RT-PCR. Although mRNA of BSEP was detectable and its expression confirmed by Western blotting, its very low expression (higher in hTr than in whole placenta and hCC) did not permit its detection by immunohistochemistry. In hTr, the expression was high for OATP-B/2B1, OATP-8/1B3, MRP1, MRP3, BCRP, FIC1, RARalpha, FXR and SHP, low for OSTalpha, MRP2, MRP4, MRP8, MDR1, CAR and SXR, very low for OATP-A/1A2 and MDR3, and not detectable for OATP-C/1B1, HNF1alpha and HNF4. Expression patterns in hCC mimicked those in hTr, although some important cell line-specific differences were found. The functionality of transporters expressed in hCC was confirmed by their ability to take up and export estradiol 17beta-d-glucuronide in a self-inhibitable and temperature-sensitive manner. In conclusion, several transporters, export pumps, and nuclear receptors involved in the liver excretory function may play a similar role in the placenta, whose specific aspects can be studied by selectively using BeWo, Jeg-3 or JAr cells.