Increased de novo purine synthesis by insulin through selective enzyme induction in primary cultured rat hepatocytes.
The proliferative effect of insulin on de novo purine synthesis and on the expression of various enzymes of purine metabolism were studied in primary cultured rat hepatocytes. Insulin greater than 1.5 x 10(-8) M increased DNA and de novo purine synthesis to 260-390 and 270-420%, respectively, 24 and 8 h after the administration. Insulin at 1.5 x 10(-7) M increased the specific activity of amidophosphoribosyltransferase (ATase) to 154-180%, hypoxanthine-guanine phosphoribosyltransferase to 129%, and adenine phosphoribosyltransferase (APRT) to 205%, in contrast to unchanged xanthine dehydrogenase at 80%. Enzyme induction was supported by the results of kinetic analysis and the inhibition of the insulin-induced increase in enzyme activities by protein synthesis inhibitors. Insulin increased ATP to 127% and decreased AMP, ADP, 5'-guanylic acid (GMP), and guanosine 5'-diphosphate (GDP), respectively, to 73, 69, 73, and 69%. Insulin increased adenylate energy charge from 0.83 to 0.90 without changing total feedback inhibitory potential on ATase. No obvious increase of 5-phosphoribosyl-1-pyrophosphate supply was suggested, although its apparent availability for purine ribonucleotide synthesis was increased to 208-245%, reflecting mainly induced APRT activity to 205%. It is concluded that hepatocyte proliferation by insulin, as evidenced by purine metabolism, is mediated by the selective gene activation of anabolic enzymes and increased ATP as the basis to activate multiple metabolic pathways without remarkable changes of substrate availability or feedback inhibition.
Division of Endocrinology and Metabolism, University of Tsukuba, Ibaraki, Japan.
M
M
Tsuchiya
H
H
Yoshikawa
M
M
Itakura
K
K
Yamashita
D000228:Adenine Phosphoribosyltransferase / Q000096:biosynthesis; D000582:Amidophosphoribosyltransferase / Q000096:biosynthesis; D000818:Animals; D002478:Cells, Cultured; D004261:DNA Replication / Q000187:drug effects*; D004790:Enzyme Induction; D007041:Hypoxanthine Phosphoribosyltransferase / Q000096:biosynthesis; D007328:Insulin / Q000494:pharmacology*; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000201:enzymology*; D008297:Male; D011687:Purines / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012265:Ribonucleotides / Q000378:metabolism; D013936:Thymidine / Q000378:metabolism; D014968:Xanthine Dehydrogenase / Q000096:biosynthesis
D007328:Insulin; D011687:Purines; D012265:Ribonucleotides; D014968:Xanthine Dehydrogenase; D000582:Amidophosphoribosyltransferase; D000228:Adenine Phosphoribosyltransferase; D007041:Hypoxanthine Phosphoribosyltransferase; D013936:Thymidine