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

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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 α‐(methylamino) isobutyric acid (mAIB), a trans‐inhibitor, and glutamate, an effective repressor. The apparent initial T1/2 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.

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