Adaptive regulation of the cationic amino acid transporter-1 (Cat-1) in Fao cells

Post-transcriptional regulation of the arginine transporter Cat-1 by amino acid availability

The regulation of the high affinity cationic amino acid transporter (Cat-1) by amino acid availability has been studied. In C6 glioma and NRK kidney cells, cat-1 mRNA levels increased 3.8-18-fold following 2 h of amino acid starvation. The transcription rate of the cat-1 gene remained unchanged during amino acid starvation, suggesting a post-transcriptional mechanism of regulation. This mechanism was investigated by expressing a cat-1 mRNA from a tetracycline-regulated promoter. The cat-1 mRNA contained 1.9 kilobase pairs (kb) of coding sequence, 4.5 kb of 3'-untranslated region, and 80 base pairs of 5'-untranslated region. The full-length (7.9 kb) mRNA increased 5-fold in amino acid-depleted cells. However, a 3.4-kb species that results from the usage of an alternative polyadenylation site was not induced, suggesting that the cat-1 mRNA was stabilized by cis-acting RNA sequences within the 3'-UTR. Transcription and protein synthesis were required for the increase in full-length cat-1 mRNA level. Because omission of amino acids from the cell culture medium leads to a substantial decrease in protein synthesis, the translation of the increased cat-1 mRNA was assessed in amino acid-depleted cells. Western blot analysis demonstrated that cat-1 mRNA and protein levels changed in parallel. The increase in protein level was significantly lower than the increase in mRNA level, supporting the conclusion that cat-1 mRNA is inefficiently translated when the supply of amino acids is limited, relative to amino acid-fed cells. Finally, y(+)-mediated transport of arginine in amino acid-fed and -starved cells paralleled Cat-1 protein levels. We conclude that the cat-1 gene is subject to adaptive regulation by amino acid availability. Amino acid depletion initiates molecular events that lead to increased cat-1 mRNA stability. This causes an increase in Cat-1 protein, and y(+) transport once amino acids become available.

Control of expression of the gene for the arginine transporter Cat-1 in rat liver cells by glucocorticoids and insulin

Hepatic arginine and lysine uptake is partly regulated by changes in the transport activity of a group of cell surface proteins exhibiting properties of the transport system y+. The Cat-1 gene encodes a sodium-independent high-affinity cationic amino acid transporter of the y+ system which is nearly undetectable in the quiescent liver. In this paper we investigate the regulation of expression of Cat-1 in the quiescent rat liver by glucocorticoids and insulin, two hormones which play a critical role in amino acid dependent pathways of hepatic metabolism. Injection of insulin and glucocorticoids resulted in a rapid (15-30 min, 4-5 fold) increase in transcription which returned to basal levels within 4 hours. In contrast to the rapid single peak of transcriptional induction of the Cat-1 gene, the accumulation of the Cat-1 mRNAs occurred transiently with two peaks, the first at 30 minutes and the second at 2-4 hours following hormone treatment. These data indicate that expression of the Cat-1 gene in the quiescent liver can be transiently induced by both transcriptional and post-transcriptional mechanisms. In FTO2B rat hepatoma cells, expression of the gene is constitutive and accumulation of Cat-1 mRNAs in response to dexamethasone and insulin was dependent on transcription and protein synthesis. Furthermore, the accumulation of the basal level of the Cat-1 mRNAs was reduced by 70%, upon treatment of cells with inhibitors of protein synthesis for 6 h, when the transcription rate of the gene did not decrease significantly. We conclude the following: (i) under normal physiologic conditions, expression of the Cat-1 gene in the quiescent liver is negligible, probably to prevent unnecessary transport and metabolism of arginine by the hepatic arginase in the hepatocytes. (ii) in the cases when hepatic cationic amino acid transport is needed, such as following feeding, cellular growth and illness, glucocorticoids and insulin induce expression of the Cat-1 gene in liver cells through induction of transcription and stabilization of the mRNA. (iii) constitutive Cat-1 mRNA accumulation in rat hepatoma cells depends on protein synthesis through a labile regulated factor. Overall, constitutive expression of Cat-1 is associated with hepatic cellular growth and transformation.