| Abstract: | When mamalian cells are starved for amino acids, the activity of the A amino acid transport system increases, a phenomenon called adaptive regulation. We have examined the effects of those factors which support Madin-Darby canine kidney (MDCK) cell growth in a defined medium on the derepression of System A activity. Of the five factors which supported MDCK cell growth, insulin was found to be an absolute requirement for derepression. In contrast, PGE1 was a negative controlling factor for the transport system. Growth of MDCK cells in the absence of PGE1 resulted in elevated System A activity which derepressed poorly upon amino acid starvation. Kinetic analysis of α-(methylamino) isobutyric acid (mAIB) uptake as a function of substrate concentration showed that the elevated A activity observed when cells were grown in the absence of PGE1 was kinetically similar to the activity induced by starvation for amino acids. Transport of mAIB by amino-acid-fed cells grown in the presence of PGE1 was characterized by a linear Eadie-Hofstee graph and by a relatively low Vmax. Transport by cells starved for amino acids or by cells grown in the absence of PGE1 was characterized by biphasic kinetics for mAIB transport and by elevated Vmax values. An influence of growth factors on the inactivation of derepressed A activity was also observed. In the presence of cycloheximide the rate of loss of A activity in amino-acid-starved cells was 1/4–1/2 that of amino-acid-fed cells. Insulin slowed inactivation in the absence of most amino acids in a protein-synthesis-independent manner, but insulin did not influence the more rapid inactivation observed in amino-acid-fed cells. These results indicate that the level of System A activity observed in response to regulation by amino acids represents a balance between carrier synthesis and inactivation, which can be positively or negatively influenced by growth factors. |
