P-glycoprotein (P-gp) is a transmembrane glycoprotein that confers multidrug resistance (MDR). It has been demonstrated that the Gly185 residue within the cytoplasmic loop between predicted transmembrane portions 2 and 3 plays an important role in substrate specificity of human P-gp. Derivatives of cyclosporin interact with and reverse the ability of P-gp to act as a drug efflux pump. To determine if the Gly185 residue of human P-gp is also important for the interaction of P-gp with closely related cyclosporin derivatives, we examined the effect of PSC-833 and CsA on P-gp in KB3-1 cells transfected with human wild-type P-gp (GSV-2) or with the mutant P-gp (VSV-1) that habored the Gly185-->Val substitution. While the ability of CsA to sensitize VSV-1 cells to anticancer agents was enhanced, no changes in the potency of PSC-833 against cells transfected with either the wild-type or mutant P-gp were observed. In addition, VSV-1 transfected cells were more sensitive to CsA inhibition of verapamil-stimulated ATPase activity than cells transfected with wild-type P-gp. Furthermore, the intracellular accumulation of CsA was low in GSV-2 P-gp-expressing cells, compared with its accumulation in VSV-1 cells and it was found to be as high as in non-P-gp expressing KB3-1 cells. These results indicated an enhanced sensitivity of Val185-P-gp expressing cells to CsA that correlated with increased intracellular accumulation in these cells. In contrast, no significant difference in the accumulation of PSC-833 was observed among the parental, wild-type or resistant cells. Since PSC-833 was found to be more potent than CsA, these studies provided insight into the effects of the structure of MDR modulators in mediating sensitivity to anticancer drugs.
The effect of 7 specific endonucleases EcoRI, BamHI, BglII, SalI HindIII, XhoI, SmaI, on the genome of the phage Sd was studied. The molecular weight of the genome was estimated as 45.10(6). BamHI, EcoRI, HindIII, BglII, SmaI have altogether 15 sites of restriction SalI and XhoI do not hydrolyse the phage DNA. Fragmentation of the phage DNA in conditions of partial hydrolysis and simultaneous action of several enzymes have allowed to draw a physical man to the phage Sd DNA is more resistent to the action of restriction enzymes than DNAs from other phages. The mean size of Sd DNA fragments exceeds the statistical value almost by one order of magnitude.
Three-dimensional cell cultures (spheroids) provide an in vitro approximation of solid tumors in vivo. Some mechanisms of drug resistance were reported to differ in their effects on monolayer and spheroid cells. We have compared the effect on the expression of P-glycoprotein (Pgp), the MDR1 gene product, on vinblastine resistance in two glioblastoma lines, U87 and U251, grown as either monolayers or spheroids. U87 cells form spheroids spontaneously and the spheroids continue to grow, while U251 cells form spheroids only when plated over agarose and do not proliferate in spheroids. U87 cells are equally resistant to vinblastine in monolayer and in spheroids, while U251 cells are 4.5 times more resistant in spheroids than in monolayers. The distribution of a fluorescent vinblastine derivative in cells of spheroids was more heterogeneous than in monolayers. MDR1-expressing U87 cells increased their resistance 9-fold in monolayer and 50-fold in spheroids, while MDR1 transduction of U251 cells made them 20-fold more resistant in monolayer and 160-fold more resistant in spheroids. MDR1 expression in both cell lines decreased the accumulation of 3H-vinblastine and fluorescent vinblastine derivative. These results indicate that MDR1 is more efficient in conferring drug resistance in spheroids than in monolayer cells.
The ability of tumor cells to develop simultaneous resistance to multiple lipophilic cytotoxic compounds represents a major problem in cancer chemotherapy. This review describes recent molecular biological studies which resulted in the identification and cloning of the gene responsible for multidrug resistance in human tumor cells. This gene, designated mdr1, is overexpressed in all and amplified in many of the multidrug-resistant cell lines analyzed. Gene transfer and expression assays have indicated that the mdr1 gene is both necessary and sufficient for multidrug resistance. The product of the mdr1 gene is P-glycoprotein, a transmembrane protein which shares homology with several bacterial proteins involved in active membrane transport. P-glycoprotein appears to function as an energy-dependent efflux pump responsible for the removal of drugs from multidrug-resistant cells. The functions of the mdr system in normal cells and its potential clinical implications are discussed.
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