Adaptive regulatory control of system a transport activity in a kidney epithelial cell line (MDCK) and in a transformed variant (MDCK‐T₁)

Abstract

Adaptive regulatory control of System A activity was investigated using MDCK cells and a chemically induced, oncogenic transformant of MDCK cells, MDCK-T1. Within 7 hours after transfer to an amino-acid-deficient medium, A activity of subconfluent MDCK cells had maximally derepressed, but this activity in confluent MDCK cells and in subconfluent transformed cells showed little capacity for derepression. Amino-acid-starved, subconfluent MDCK cells were used to study trans-inhibition and repression of A activity by individual amino acids. Trans-inhibition and repression were defined as the cycloheximide-insensitive and cycloheximide-sensitive components, respectively, of the total inhibition. Trans-inhibition correlated well with substrate affinity, but repression did not. Trans-inhibition and repression were further characterized using alpha-(methylamino) isobutyric acid (mAIB), a trans-inhibitor, and glutamate, an effective repressor. The apparent initial T 1/2 for inhibition by mAIB in the presence of cycloheximide was 0.5 hours, while that for repression by glutamate was 4.7 hours. Half-maximal inhibition by mAIB and repression by glutamate occurred at approximately 0.02 mM and 0.07 mM, respectively. Reversal of trans-inhibition by methionine occurred in the presence of cycloheximide within 1-4 hours after removal of methionine. The A system of the transformed MDCK-T1 cells showed elevated activity, little capacity for derepression, resistance to repression by amino acids, but retention of sensitivity to trans-inhibition. Kinetic analysis of mAIB uptake indicated that the A system of MDCK-T1 cells has become kinetically more complex in a manner which resembled amino-acid-starved rather than amino-acid-fed MDCK cells. These results suggest that the A system of MDCK-T1 cells has become resistant to adaptive regulatory control.

Tumor necrosis factor stimulates amino acid transport in plasma membrane vesicles from rat liver.

“…In addition, we observed the appearance of a second low affinity system A transport component not observed in control animals. This dichotomy in system A-mediated transport has been observed by others (28,29), particularly under conditions derepressing system A, such as amino acid starvation and cellular transformation (28). Such a TNF-induced heterogeneity in system A transport may translate into an additional augmentation of hepatic amino acid uptake in vivo during sepsis.…”

“…6 B). The observed increase in uptake was due to a fourfold increase in the maximal velocity (28,29). Similarly, Na'-dependent glutamine transport was increased in vesicles from TNFtreated rats across all assay glutamine concentrations (Fig.…”

Effects of 5‐azacytidine, sodium butyrate, and phorbol esters on amino acid transport system A in a kidney epithelial cell line, MDCK: Evidence for multiple mechanisms of regulation

“…Both phases are referred to as adaptive regulatory control and are protein synthesis dependent, a feature which distinguishes adaptive control from transinhibition. System A activity of chemically transformed MDCK-T1 118 BOERNER AND SAIER cells was severalfold higher than observed with MDCK cells (Boerner and Saier, 1982a1, showed no capacity for derepression, was resistant to repression (i.e., irreversible inactivation) by amino acids, and retained sensitivity to transinhibition (Boerner and Saier, 1985a). Kinetic analyses indicated that MeAIB uptake by MDCK-TI cells, measured as a function of MeAIB concentration, exhibited biphasic kinetics, a feature similar to that observed with amino-acid-starved but not amino-acidfed MDCK cells (Boerner and Saier, 1985a).…”

Kidney‐specific enzyme expression by human kidney cell lines generated through oncogene transfection

“…Differentiated characteristics are often expressed as a result of cell-cell communication (Lever, 1985;Pfaller et al, 1990). The MDCK and LLC-PK1 cells are heterogeneous populations (Boerner and Saier, 1985;Lever, 1985;Wohlwend et al, 1986;Gstraunthaler and Handler, 1987), which could increase their ability to express differentiated characteristics. The ST-li and STt-4i cells were derived from clonal populations and have been surrounded by the same kidney cell type throughout their in vitro culture history.…”