Insulin Concentration during Preconditioning Mediates the Regulation of Urea Synthesis during Exposure to Amino Acid-Supplemented Plasma

Abstract

Understanding the effects of preconditioning on cellular metabolism is important in providing insight into how cells and tissues may behave when confronted with alterations in their environment. Previously, elevated accumulation of lipids and a correspondingly reduced rate of urea synthesis were found in primary heptocytes preconditioned in culture medium containing high levels of insulin and then exposed to plasma. Subsequent studies found that preconditioning primary hepatocytes in medium containing low levels of insulin before exposure to plasma supplemented with amino acids was able to confer resistance to the lipid-accumulating effect of plasma exposure and restored urea synthesis. In the current study, we investigated the effects of insulin preconditioning and amino acid supplementation on the gene expression profile of the urea cycle and fatty acid metabolism enzymes. We found that insulin preconditioning mediates the effects of amino acids in the plasma exposure period on urea synthesis. Urea synthesis is regulated by insulin and amino acids through both short-term and long-term control. Possible mechanisms of long-term control through transcriptional regulation of urea synthesis by insulin, amino acids, and enzymes and transcription factors associated with lipid metabolism are discussed.

Genetic Networks of Liver Metabolism Revealed by Integration of Metabolic and Transcriptional Profiling

“…Our network identifies a specific metabolite, glx that regulates gene expression. This is consistent with previous studies where glutamine alone increases hepatic expression of argininosuccinate synthetase and phosphoenolpyruvate carboxykinase, but when combined with other essential amino acids, alters additional transcripts of urea cycle and gluconeogenic pathways [36]–[38],[52]. Our work extends these prior observations by showing that glutamine also changes expression of Agxt , Arg1 , Ivd , and Slc1a2 , but does not alter Slc38a3 , despite the positive correlation with this transcript.…”

A rational design approach for amino acid supplementation in hepatocyte culture

“…(1) Although it is only one of many hepatic functions, urea production is an often used marker of hepatocyte function that typically correlates with a healthy, differentiated phenotype (Chan et al, 2002;Higuchi et al, 2006;Kane et al, 2006;Li et al, 2004;Washizu et al, 2000a,b) and therefore one that hepatocytes may seek to satisfy directly or indirectly; therefore, urea maximization is a reasonable case study for the approach.…”

“…However, hepatocytes are anchoragedependent cells, and they rapidly lose liver-specific functions once they are removed from their host and cultured in an artificial environment (Allen et al, 2001). Some culture techniques have been introduced to help stabilize liverspecific in vitro functions, which include culturing hepatocytes in sandwich collagen gels (Dunn et al, 1991(Dunn et al, , 1992, co-culturing hepatocytes with other types of cells such as 3T3 fibroblasts (Bhatia et al, 1997), culturing hepatocytes on extracellular matrix (Page et al, 2007), and addition of insulin to the culture medium (Li et al, 2004). Maintenance of liver-specific functions in hepatocytes during plasma exposure is very important to BAL devices.…”

Diabetes-induced Coronary Vascular Dysfunction Involves Increased Arginase Activity

“…17,18 However, transcriptional upregulation of enzyme expression can also occur. 19 It is well known that insulin represses expression of genes for urea synthesis pathways and that insulin signaling is impaired in both type 1 and type 2 diabetes. Thus, diabetes-induced increases in arginase activity could explain Original received April 27, 2007; revision received September 5, 20007; accepted October 15, 2007 20 Acute administration of supplemental L-arginine is reported to prevent or reverse endothelial dysfunction and restore endothelial-dependent vasodilation in diabetes, hypertension, and heart failure.…”

“…17,18 However, transcriptional upregulation of enzyme expression can also occur. 19 It is well known that insulin represses expression of genes for urea synthesis pathways and that insulin signaling is impaired in both type 1 and type 2 diabetes. Thus, diabetes-induced increases in arginase activity could explain the decreased L-arginine levels reported in plasma from diabetic animals and patients, 20,21 and in vascular tissue of streptozotocin (STZ) diabetic rats.…”