Isolation and characterization of a CHO amino acid transport mutant resistant to melphalan (L-phenylalanine mustard)

Decreased melphalan accumulation in a human breast cancer cell line selected for resistance to melphalan

An in vitro model of acquired melphalan resistance was developed by serial incubation of an MCF-7 human breast cancer cell line in increasing concentrations of melphalan. The resulting derivative cell line, Me1R MCF-7, was 30-fold resistant to melphalan. Uptake studies demonstrated decreased initial melphalan accumulation in Me1R MCF-7 cells. Inverse-reciprocal plots of initial melphalan uptake revealed a 4-fold decrease in the apparent Vmax of Me1R MCF-7 compared with WT MCF-7 (516 amol cell-1 min-1 vs 2110 amol cell-1 min-1 respectively) as well as a decrease in the apparent Kt (36 microM vs 70 microM respectively). Two amino acid transporters have previously been identified as melphalan transporters: system L, which is sodium-independent and inhibited by 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid (BCH), and system ASC which is sodium dependent and unaffected by BCH. At low concentrations of melphalan (3-30 microM), 1mM BCH competition eliminated the differences between the two cell lines, thus implicating an alteration of the system L transporter in the transport defect in the resistant cells. Me1R MCF-7 cells were also evaluated for glutathione-mediated detoxification mechanisms associated with melphalan resistance. There was no difference between Me1R MCF-7 and WT MCF-7 in glutathione content, glutathione-S-transferase activity and expression of pi class glutathione S-transferase RNA. In addition, buthionine sulfoximine did not reverse melphalan resistance in Me1R MCF-7 cells. Therefore, Me1R MCF-7 cells provide an in vitro model of transport-mediated melphalan resistance in human breast cancer cells.

Selection of a lysine-resistant CHO-K1 mutant with reduced amino acid transport through multiple systems

High levels of L-lysine were used to select for resistant variants of Chinese hamster ovary (CHO-K1) cells. Surviving colonies were screened for altered lysine transport and two with reduced uptake were picked. Clone CH-Kr, derived from the more severely affected colony, was analyzed in detail. In starved cells the Vmax of lysine uptake in CH-Kr was half that of CHO while Km was unaltered. The intracellular pool of lysine, a substrate of cationic amino acid transport system y+, was significantly lower in CH-Kr. However, transport and pools of other amino acids, which are not substrates of y+, were also reduced in CH-Kr, as was the internal sodium concentration, while hexose import was increased. It appears that the mutation in CH-Kr is pleiotropic, affecting some general aspects of amino acid transport.

Comparative study of two human melanoma cell lines with different sensitivities to mustine and cisplatin

The cytotoxic effects of the bifunctional DNA-reactive drugs cisplatin, mustine and melphalan were studied in two human melanoma cell lines, RPMI 8322 and A 375. A 375 cells were 3.0 times more sensitive to cisplatin and 1.5 times more sensitive to mustine than RPMI 8322 cells. In contrast, A 375 cells were less sensitive to melphalan than RPMI 8322 cells. The increased sensitivity of A 375 cells was parallelled by an increased induction of DNA interstrand crosslinks following exposure to cisplatin and mustine. After cisplatin exposure A 375 cells also showed higher levels of platinum-DNA intrastrand adducts than RPMI 8322 cells. The increased effect of cisplatin in A 375 cells was not due to an increased drug accumulation in these cells. The higher sensitivity of A 375 cells to cisplatin may be related to lower intranuclear levels of glutathione, compared to RPMI 8322 cell nuclei, while the sensitivity to mustine may depend on lower overall levels of glutathione than in RPMI 8322 cells.

Membrane potential and amino acid transport in a mutant Chinese hamster ovary cell line

The bioenergetics of amino acid transport system A was studied in two Chinese hamster ovary (CHO) cell lines, the parent line CHO-PEOT/1 and CHY-1, a mutant of the former exhibiting a low activity of the same transport system. The steady-state transmembrane distribution ratio of the cationic amino acid L-arginine (RARG) was employed as an indicator of membrane potential (delta psi). Evidence for the reliability of RARG to measure delta psi can be summarized as follows: (1) L-arginine transmembrane distribution increased under conditions of cell hyperpolarization and decreased under conditions of cell depolarization; (2) L-arginine distribution conformed closely to that expected for a probe of delta psi in conditions in which delta psi depends largely on the transmembrane potassium gradient; and (3) the value of delta psi obtained through a valinomycin null point experiment (-72.7 mV) was very similar to the value calculated from L-arginine distribution using the Nernst equation (-73.4 mV). The transmembrane gradient of sodium electrochemical potential (delta mu Na), the driving force for the operation of system A, was slightly higher in the mutant cell line CHY-1. In the same line, the intracellular level of the specific system A substrate MeAIB at steady state was also higher. Studies of the rheogenicity of system A in the two lines indicated that the depolarization associated with the entry of substrates of system A was proportional to the amount of amino acid taken up by the cells. Kinetic analysis showed that the low activity of system A in the mutant cell line was referrable to a decrease in transport Vmax. It is concluded that neither a decrease in energy available for the operation of system A nor a decreased efficiency of coupling of the system to delta psi is responsible for the defect observed in the mutant line.

The transport of L-arginine in Chinese hamster ovary cells

The transport of L-arginine has been characterized in Chinese hamster ovary cells (CHO). In the absence of Na+ the influx of the amino acid decreased. Both in the presence and in the absence of Na+ L-arginine influx was trans-stimulated and cis-inhibited by cationic amino acids. The amino acid entered CHO cells through an apparently non saturable mechanism and a single saturable agency whose Km increased in the absence of Na+. These results indicate that the agency devoted to transport cationic amino acids in CHO cells resembles system y+, the Na+-independent route that transports cationic amino acids in a number of mammalian models, although its activity is lowered by the replacement of extracellular sodium.

Differentiation in vitro of primary cultures and transfected cell lines of epithelial cells derived from the thick ascending limb of Henle's loop

The use of primary cell cultures derived from defined locations of the kidney has enabled the study of certain kidney cell type-specific characteristics under defined environmental conditions. The use of primary cell cultures, however, has a number of inherent disadvantages, many of which may be overcome by the use of differentiated cell lines of defined origin. In this paper I describe in detail an approach to: (a) the isolation and culture of primary cultures derived from the thick ascending limb of Henle's loop (TALH), and (b) the production of differentiated cell lines by the transfection of these primary cell cultures with early region SV40 virus genes. The characteristics of these cultures and other TALH-derived cell lines are described.

Isolation and characterization of a glycine transport mutant in an established mammalian cell line, CHO(PEOT/1)

Glycine uptake in CHO(PEOT/1) cells is mediated by at least three systems, of which two have been identified and partially characterized in this study: (1) a low affinity "A" system that transports a number of small neutral amino acids including glycine and methylaminoisobutyric acid (MeAIB), and (2) a high-affinity system, specific for glycine and sarcosine. By a combination of tritium suicide and replica plating, we have isolated a mutant (CHY-3) with a 47% decrease in glycine transport at the standard test concentration of 2.5 microM. Uptake studies with radioactive glycine, MeAIB, and sarcosine revealed that the mutant lacks the glycine-sarcosine system, but has undergone a compensatory 30-50% increase in the A system. Thus, there appears to be a regulatory interaction between these two systems for glycine uptake by CHO cells.