The multidrug-resistance phenotype expressed in mammalian cell lines is complex. Cells selected with a single agent can acquire cross-resistance to a remarkably wide range of compounds which have no obvious structural or functional similarities. The basis for cross-resistance seems to be a decreased net cellular accumulation of the drug involved, and has been attributed to alterations in the plasma membrane. An over-expressed plasma membrane glycoprotein of relative molecular mass (Mr) 170,000 (P-glycoprotein) is consistently found in different multidrug-resistant human and animal cell lines, and in transplantable tumours. Consequently, it has been postulated that P-glycoprotein directly or indirectly mediates multidrug resistance. Here we report the cloning of a complementary DNA encoding P-glycoprotein. Southern blot analysis of hamster, mouse and human DNA using this cDNA as a probe showed that P-glycoprotein is conserved and is probably encoded by a gene family, and that members of this putative family are amplified in multidrug-resistant cells.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMedDate not supplied

Detection of P-glycoprotein in multidrug-resistant cell lines by monoclonal antibodies.

One reason for the failure of chemotherapy in the treatment of advanced cancers may be the outgrowth of multidrug-resistant tumour cells. Multidrug resistance has been modelled in numerous mammalian cell lines in which the phenotype is characterized by a pleiotropic cross-resistance to unrelated drugs. In the study reported here, we have produced monoclonal antibodies whose binding to plasma membranes of different multidrug-resistant mammalian cells correlates with the degree of drug resistance. All these antibodies are specific for P-glycoprotein, a cell surface component of relative molecular mass (Mr) 170,000 (170K) that has been described previously, and are directed against three spatially distinct epitopes which define a conserved cytoplasmic domain in the C-terminal region of the P-glycoprotein polypeptide. The conserved nature of P-glycoprotein and its low-level expression is drug-sensitive cells suggest that it has an important function at the cell surface. The monoclonal antibodies against P-glycoprotein described here might serve as diagnostic reagents for clinically unresponsive tumours.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMedDate not supplied

Homology between P-glycoprotein and a bacterial haemolysin transport protein suggests a model for multidrug resistance.

Increased expression of P-glycoprotein, a plasma membrane glycoprotein of relative molecular mass (Mr) 170,000 (170K), occurs in a wide variety of cell lines that exhibit pleiotropic resistance to unrelated drugs. The presence of P-glycoprotein in human cancers refractory to chemotherapy suggests that tumour cells with multidrug resistance can arise during malignant progression. We have discovered striking homology between P-glycoprotein and the HlyB protein, a 66K Escherichia coli membrane protein required for the export of haemolysin (protein of Mr 107K). P-glycoprotein can be viewed as a tandem duplication of the HlyB protein. The hydropathy profiles of the two proteins are similar and reveal an extensive transmembrane region resembling those found in pore-forming plasma membrane proteins. The C-terminal region of P-glycoprotein and the HlyB protein contain sequences homologous to the nucleotide-binding domains of a group of closely related bacterial ATP-binding proteins. We propose a model for multidrug resistance in which P-glycoprotein functions as an energy-dependent export pump to reduce intracellular levels of anticancer drugs.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMedDate not supplied

Toremifene resistance in a rat mammary tumour model.

The effect of Toremifene on cell growth in vitro was tested on the R3230AC rat mammary adenocarcinoma as model. Toremifene differed from Tamoxifen in that it did not induce resistance; some cross-resistance was observed in Tamoxifen tolerant tumour cell lines. Toremifene does not influence P-glycoprotein expression in this model and at the levels studied.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMedDate not supplied

R-verapamil decreases anti-estrogen resistance in a breast cancer model.

Drug resistance eventually limits the effectiveness of antiestrogens in breast cancer treatment. Pharmacological reversal of this refractoriness has been attempted with R-Verapamil, a well tolerated calcium channel blocker. This drug significantly decreased the incidence of lung foci after intravenous seeding of the R3230AC rat adenocarcinoma; this effect was correlated with reduction in the expression of P-glycoprotein. The simultaneous administration of antiestrogens with a non-toxic enantiomer of Verapamil was beneficial in the tumour model investigated.

ATP Binding Cassette Transporter, Subfamily B, Mem↗

PubMedDate not supplied

Decreased P-glycoprotein in a tamoxifen-tolerant breast carcinoma model.

The initial benefits of antiestrogen treatment in mammary cancer tend to decrease with time as antiestrogen resistant subpopulations of cancer cells predominate. The role of P-glycoprotein in this phenomenon is not known. In the tumour model investigated, estradiol increased this efflux pump, while prolonged Tamoxifen exposure resulted in cell populations tolerant to the drug and with decreased P-glycoprotein expression. While Tamoxifen resistance is clinically undesirable, selective pressure by this drug may result in cancer cell lines with increased vulnerability to other chemotherapeutic modalities.

ATP Binding Cassette Transporter, Subfamily B, Mem↗