Mice with a Deletion in the Gene for CCAAT/Enhancer-binding Protein β Have an Attenuated Response to cAMP and Impaired Carbohydrate Metabolism

Abstract

Fifty percent of the mice homozygous for a deletion in the gene for CCAAT/enhancer-binding protein ␤ (C/ EBP␤؊/؊ mice; B phenotype) die within 1 to 2 h after birth of hypoglycemia. They do not mobilize their hepatic glycogen or induce the cytosolic form of phosphoenolpyruvate carboxykinase (PEPCK). Administration of cAMP resulted in mobilization of glycogen, induction of PEPCK mRNA, and a normal blood glucose; these mice survived beyond 2 h postpartum. Adult C/EBP␤؊/؊ mice (A phenotype) also had difficulty in maintaining blood glucose levels during starvation. Fasting these mice for 16 or 30 h resulted in lower levels of hepatic PEPCK mRNA, blood glucose, ␤-hydroxybutyrate, blood urea nitrogen, and gluconeogenesis when compared with control mice. The concentration of hepatic cAMP in these mice was 50% of controls, but injection of theophylline, together with glucagon, resulted in a normal cAMP levels. Agonists (glucagon, epinephrine, and isoproterenol) and other effectors of activation of adenylyl cyclase were the same in liver membranes isolated from C/EBP␤؊/؊ mice and littermates. The hepatic activity of cAMP-dependent protein kinase was 80% of wild type mice. There was a 79% increase in the concentration of RI␣ and 27% increase in RII␣ in the particulate fraction of the livers of C/EBP␤؊/؊ mice relative to wild type mice, with no change in the catalytic subunit (C␣). Thus, a 45% increase in hepatic cAMP (relative to the wild type) would be required in C/EBP␤؊/؊ mice to activate protein kinase A by 50%. In addition, the total activity of phosphodiesterase in the livers of C/EBP␤؊/؊ mice, as well as the concentration of mRNA for phosphodiesterase 3A (PDE3A) and PDE3B was approximately 25% higher than in control animals, suggesting accelerated degradation of cAMP. C/EBP␤ influences the regulation of carbohydrate metabolism by altering the level of hepatic cAMP and the activity of protein kinase A.

Loss of c/EBP-β activity promotes the adaptive to apoptotic switch in hypoxic cortical neurons

“…These reports suggest several potential mechanisms responsible for the observed induction of c/EBP-β in cortical neurons after hypoxic stress. Data from knockout studies also indicate that c/EBP-β plays an essential role in regulating carbohydrate metabolism (Croniger et al, 2001;Liu et al, 1999). It is interesting to speculate that in addition to activating mitochondrial heat shock responses though CHOP-10, and responding to amino acid limitation in cooperation with ATF4, c/EBP-β may function as an effector loop linking ER responses to hypoglycemia with the mobilization of energy stores (Scheuner et al, 2001).…”

Muscle glycogenosis with low phosphorylase kinase activity: mutations in PHKA1, PHKG1 or six other candidate genes explain only a minority of cases

“…Mutations in transcription factors acting on Phk subunit genes (cf. the impact of C/EBP isoforms on glycogen metabolism), 53,54 in scaffolding proteins that assemble glycogen particles with their metabolizing and regulatory enzymes including Phk (Lerin et al 55 ), or in one of the many proteins involved in the transmission of regulatory signals to Phk such as protein kinase A, might also affect the expression, the stability or the activity regulation of Phk.…”

Differences in regulation of type I collagen synthesis in primary and passaged hepatic stellate cell cultures: the role of α5β1-integrin

“…The small decrease in CAT activity after day 7 may be explained by the fact that cells at day 7 are confluent and following time points contain significant amount of cellular debris, suggesting some cell death in very confluent cultures. B: real-time PCR analysis of type I collagen COL1A1 mRNA in the primary cultures of HSC at days 4,6, and 8 compared with the expression at day 2 (quiescent HSC). Values are expressed as a fold change compared with levels measured in quiescent HSC.…”

“…C/EBP␤ Ϫ/Ϫ mice and heterozygous (wild-type phenotype) littermates were generated by Dr. Valeria Poli (6) and provided by Dr. Colleen M. Croniger and Dr. Richard W. Hanson from Case Western Reserve University, Cleveland, OH. The generation of C/EBP␤ Ϫ/Ϫ mice and their genetic background has been described previously (6).…”

“…Primary HSC cultures on days 2,3,4,5,6,7, and 9, as well as passaged HSC cultures, were lysed with ice cold lysis buffer containing 25 mM Tris pH 7.4, 50 mM NaCl, 25 mM NaF, 10% glycerol, 1% Triton X-100, PMSF (1 mM), protease inhibitors cocktails 10 l/ml (Sigma no. P8340, Sigma); scraped; and left on ice for 30 min.…”