Regulation of expression of the multidrug resistance protein MRP1 by p53 in human prostate cancer cells.

The expression of several drug-resistance genes, including MRP and p53, increases with advancing stage of human prostate cancer. Altered transcription could account for the genotypic alterations associated with prostate cancer progression, and it was recently reported that the promoter of MRP1 is activated in the presence of mutant p53. To determine whether there is a relationship between p53 status and the expression of MRP1, a human, temperature-sensitive p53 mutant (tsp Val(138)) was transfected into LNCaP human prostate cancer cells. In the transfected cell line (LVCaP), the wild-type p53 produced growth arrest at the G1/S interface of the cell cycle, inhibited colony formation, and induced p21(waf1/cip1). Temperature shifting to 38 degrees C (p53 mutant) produced a time-dependent increase in expression of MRP1. This change in MRP1 expression was also seen in isogenic cell lines in which p53 was inactivated by human papilloma virus (HPV)16E6 protein or by a dominant-negative mutant. Functional assays revealed a decrease in drug accumulation and drug sensitivity associated with mutant p53 and increased MRP1 expression. These results provide the first mechanistic link between expression of MRP1 and mutation of p53 in human prostate cancer and support recent clinical associations. Furthermore, these data suggest a mechanism tying accumulation of p53 mutations to the multidrug resistance phenotype seen in this disease.

The expression of drug resistance gene products during the progression of human prostate cancer.

Prostate cancer progresses from a localized disease to a widely disseminated malignancy. Each step along this progression pathway involves multiple genetic alterations that impart a survival advantage to the tumor cell over its normal counterparts and may confer resistance to therapy. Because metastatic prostate cancer is one of the most therapy-resistant human neoplasms, we studied the expression of certain molecular determinants of drug resistance in the context of tumor progression. Paraffin-embedded formalin-fixed resected prostates were chosen based on Gleason grade and surgical stage. Immunohistochemistry was used to detect the expression of multidrug resistance protein (MRP), topoisomerase II alpha, p53, glutathione S-transferase pi, Bcl-2, and P-glycoprotein in these specimens. We found that all of the proteins were expressed in resected prostate except for P-glycoprotein. The expression of MRP, topoisomerase II alpha, p53, and Bcl-2 increased with the Gleason grade. In addition, the expression of MRP, topoisomerase II alpha, and p53 increased with the surgical stage. In contrast, the glutathione S-transferase pi and Bcl-2 expression decreased with the increasing surgical stage. Stage was the strongest indicator of protein expression. These results suggest that drug resistance gene products are expressed in prostate cancer at the time of surgical resection. Thus, although the emergence of the "pan-resistance" phenotype in prostate cancer may partly be a function of the selection pressure exerted by therapeutic interventions, certain determinants of chemoresistance may be caused by genetic changes accompanying tumorigenesis.

Flumazenil in the management of acute drug overdosage with benzodiazepines and other agents.

A double-blind, randomized, placebo-controlled trial was employed to evaluate the place of flumazenil, a benzodiazepine antagonist, in the early management of 60 patients who went to an accident and emergency center with overdosage of sedatives. The level of consciousness was monitored by a modified Glasgow Coma Scale, and the response to the intravenous administration of up to 1 mg flumazenil or placebo was followed for periods between 1 and 24 hours. The increases in the glasgow coma scale at 5 minutes in the flumazenil-treated group were significant in the group as a whole (+4.9; P less than 0.005), in those who had taken benzodiazepines only (+5.3; P = 0.005), and in those with mixed overdosages (+5.6; P less than 0.005). There were no significant changes in the placebo-treated group. Some patients with overdosage with ethanol also responded to flumazenil, but there was no effect in patients with overdosage of barbiturates alone or tricyclic antidepressants. Flumazenil was well tolerated although three patients had mild withdrawal reactions. The need for intensive physiologic support was avoided in several cases, and the differential diagnosis of the unconscious patient was facilitated.

Regulation of the function of P-glycoprotein by epidermal growth factor through phospholipase C.

Many multidrug-resistant (MDR) cell lines overexpress the epidermal growth factor receptor (EGFR) as well as P-glycoprotein (P-gp). However, the role of the increased EGFR in P-gp-mediated drug resistance remains unclear. Since recent studies suggest that activation of phospholipase C (PLC) could increase the phosphorylation of P-gp, and activation of the EGFR would also activate PLC, we investigated whether the effect of epidermal growth factor (EGF) on the phosphorylation of P-gp was mediated through PLC. Treatment of the human MDR breast cancer cell line, MCF-7/AdrR, with EGF increased the phosphorylation of P-gp by 20-50%. The increased phosphorylation of P-gp was accompanied by stimulation of PLC activity, as measured by the production of inositol, 1,4,5-trisphosphate and diacylglycerol, products of phosphatidylinositol-4,5-bisphosphate hydrolysis. Treatment of MDR cells with EGF also had detectable effects on P-gp function. For example, following incubation of MCF-7/AdrR cells with ECF, we observed a consistent decrease in total vinblastine (VBL) accumulation. Kinetic analysis revealed this change to be due to an increase in membrane efflux. The latter was measured by the initial uptake velocity, which was inhibited by EGF. VBL uptake measured at 0-320 sec was inhibited by 20-40%, which was associated with a similar increase in VBL efflux. EGF had no effect on drug accumulation, uptake, or efflux in sensitive MCF-7 cells. These data indicate that EGF can modulate the phosphorylation and function of P-gp, and suggest that this effect may be initiated by the activation of PLC.

Inhibitory effect of alkylating modulators on the function of P-glycoprotein.

Modulators of P-glycoprotein (P-gp) are often themselves transported out of cells, thereby limiting their effectiveness. It may be possible to develop more effective modulators of multidrug resistance by designing drugs that irreversibly block the function of P-gp. Therefore, we studied the effect of the mustard derivatives of fluphenazine (FPN) and trans-flupenthixol (FPT) on P-gp function. Both fluphenazine-mustard (FPN-M) and trans-flupenthixol-mustard (FPT-M) possessed alkylating activity, as assayed using 4-(p-nitrobenzyl) pyridine. Multidrug-resistant MCF-7/AdrR cells were incubated with FPN or FPN-M, or FPT or FPT-M for 1 h, washed for varying number of times in phosphate-buffered saline (PBS), then resuspended in medium containing [3H]vinblastine (VBL), and assayed for steady-state accumulation of the drug. Washing had far less of an effect on the ability of FPN-M and FPT-M to increase VBL accumulation compared to their parent compounds. After eight washes in excess PBS, the cells initially exposed to FPN or FPT accumulated only 30% and 50% of the initially accumulated drug, whereas the FPN-M- or FPT-M-treated cells accumulated approximately 75% and 90% of the control, respectively. FPN-M and FPT-M also increased the uptake and decreased the efflux of VBL from MDR cells despite repeated washing. We also examined the effects of these modulators on sensitivity of MDR cells to cytotoxic agents. FPN-M and FPT-M sensitized MCF-7/AdrR cells to VBL and doxorubicin to a greater extent than their parent compounds. These studies point out the potential of "irreversible" P-gp modulators to produce prolonged chemosensitization.