Effects of short-term and prolonged aerogenic hypoxia on gamma-glutamyl transpeptidase activity in the brain, liver, and biological fluids of young rats

Early elevated serum gamma glutamyl transpeptidase after liver transplantation is associated with better survival

BACKGROUND

Gamma glutamyl transpeptidase (GGT) is a membrane bound enzyme that plays a key role in the synthesis of the antioxidant glutathione. Epidemiological studies have linked high GGT with an increased risk of morbidity and cardiovascular mortality. In contrast, GGT is usually elevated in liver transplant recipients that experience good outcomes.

AIMS

To study if and how GGT is correlated with mortality following liver transplantation.

METHODS

We analyzed the prognostic relevance of serum GGT levels during the early and late postoperative period after liver transplantation in 522 consecutive adults. We also studied alanine aminotransferase, aspartate aminotransferase, and total bilirubin levels.

RESULTS

Early after transplantation, the peak median (interquartile range) GGT levels were significantly higher in patients who survived more than 90 days compared to non-survivors: 293 (178-464) vs. 172 (84-239) U/l, p<0.0001. In contrast, late after transplantation, GGT levels were significantly lower in patients who survived more than 5 years than those who did not ( p<0.01). The pattern of GGT levels also differed from those of alanine aminotransferase, aspartate aminotransferase, and total bilirubin early after transplantation, while these patterns were congruent late after transplantation. Kaplan-Meier survival analysis showed that early after transplantation the higher the GGT levels, the better the 90-day survival ( p<0.001). In contrast, late after transplantation, higher GGT levels were associated with a lower 5-year survival ( p<0.001).

CONCLUSIONS

  These paradoxical findings may be explained by the time-dependent role of GGT in glutathione metabolism. Immediate postoperative elevation of GGT may indicate a physiological systemic response while chronic elevation reflects a pathological response.

The PDGF B-chain is involved in the ontogenic susceptibility of the developing rat brain to NMDA toxicity

Hypoxic-ischemic (H-I) injury to neonatal brains can cause a life-long neuronal deficit because of increased susceptibility in the neonatal period. Excitotoxicity due to overstimulation of the N-methyl-d-aspartate receptor (NMDAR) is assumed to be the basis of the injury. However, the ontogenic profile of the susceptibility does not directly correlate with the levels of NMDAR expression. Platelet-derived growth factor B-chain (PDGF-B) has been reported to protect neurons by suppressing the NMDA-evoked current and translocating the glutamate transporter to the cell membrane. Thus, we assessed the relationship between the susceptibility to H-I injury and the expression of PDGF-B in neonatal rat brain. PDGF-B infusion before and after an intrastriatal NMDA injection significantly reduced the size of the lesions in 7-day-old rats, when they are most susceptible and the neuronal expression of PDGF-B is low. Fourteen-day-old neonatal rats were found to be resistant to NMDA injury, even though NMDARs are expressed at high levels in the brain at this age. Inhibition of PDGF-B protein synthesis by antisense oligodeoxynucleotides increased the size of the NMDA-induced lesions up to 6-fold at postnatal day 14, when PDGF-B is expressed at high levels in neurons. These data suggest that PDGF-B is an important physiological modulator of NMDAR excitability in the developing brain, and that the balance between the expression of NMDAR and PDGF-B partly determines the ontogenic susceptibility to brain injury. Enhancement of the PDGF-B/receptor signal pathway might rescue neonatal brains at risk of H-I injury.

Cell transplantation causes loss of gap junctions and activates GGT expression permanently in host liver

Cell transplantation into hepatic sinusoids, which is necessary for liver repopulation, could cause hepatic ischemia. To examine the effects of cell transplantation on host hepatocytes, we transplanted Fisher 344 rat hepatocytes into syngeneic dipeptidyl peptidase IV-deficient rats. Within 24 h of cell transplantation, areas of ischemic necrosis, along with transient disruption of gap junctions, appeared in the liver. Moreover, host hepatocytes expressed gamma-glutamyl transpeptidase (GGT) extensively, which was observed even 2 years after cell transplantation. GGT expression was not associated with alpha-fetoprotein activation, which is present in progenitor cells. Increased GGT expression was apparent after transplantation of nonparenchymal cells and latex beads but not after injection of saline, fragmented hepatocytes, hepatocyte growth factor, or turpentine. Some host hepatocytes exhibited apoptosis, as well as DNA synthesis, between 24 and 48 h after cell transplantation. Changes in gap junctions, GGT expression, DNA synthesis, and apoptosis after cell transplantation were prevented by vasodilators. The findings indicated the onset of ischemic liver injury after cell transplantation. These hepatic perturbations must be considered when transplanted cells are utilized as reporters for biological studies.

Biochemical characteristics of gamma-glutamyl transpeptidase in capillaries from entorhinohippocampal complex of quinolinate-lesioned rat brain

Quinolinic acid (QUIN) is an endogenous excitotoxic agonist of the N-methyl-D-aspartate (NMDA) type of glutamate receptor, which causes slowly progressing degeneration of vulnerable neurons in some brain regions. Using changes in the activity of membrane-bound gamma-glutamyl transpeptidase (GGT) as a marker of cell damage, we found a significant decrease of this enzyme activity, which was preferentially located in the ipsilateral hippocampal formation and entorhinal cortex, 4 d after the unilateral intracerebroventricular (icv) injection of 0.5 mumol QUIN. The dose of QUIN divided into two half-doses injected bilaterally led to a symmetrical decline of GGT activity in hippocampal areas. The lesion was characterized by a suppression of GGT activity in hippocampal and entorhinal capillaries, corresponding to 60 and 81% of their initial value, respectively, but no significant changes were ascertained in synaptosomal membranes. The changes in the activity of capillary GGT were associated with the decrease of apparent maximal velocity Vmaxapp, whereas apparent Michaelis constant K(m)app (0.69-0.79 mM) remained unaffected. In the nonlesioned brain, concanavalin A (Con A) affinity chromatography revealed five glycoforms of synaptosomal GGT in contrast to only one found in hippocampal and entorhinal capillaries. The results document that neither the saccharide moiety of GGT nor the value of enzyme K(m)app is significantly affected by the QUIN-induced lesion of the rat brain. However, the suppression of GGT activity, which is accompanied by a decrease in the value of Vmaxapp in brain microvessels, may suggest dysfunction of the blood-brain barrier (BBB) in the QUIN-injured rat brain.

Developmental changes in the activity of membrane-bound gamma-glutamyl transpeptidase and in the sialylation of synaptosomal membranes from the chick embryonic brain

gamma-Glutamyl transpeptidase (GGT) is a membrane-bound sialoglycoprotein. The developmental changes in GGT activity and in sialic acid content were determined in a crude synaptosomal membrane fraction from the cerebral hemispheres of the chick embryo between days 11 and 19 of incubation. The GGT activity increased almost eightfold during the examined developmental period, while sialic acid content rose significantly only between days 11 and 15. Cortical administered on day 13 significantly increased GGT activity. On the other hand, the content of membrane bound sialic acid was not substantially affected. The value of the GGT apparent Michaelis constant (Kmapp) for gamma-glutamyl-p-nitroanilide in the presence of 20 mmol.l-1 glycylglycine was 1.5 mmol.l-1 and cortisol did not influence it. However, Vmax was increased by this hormone. The affinity of GGT to concanavalin A (ConA) did not change during development. Neither the administration of cortisol nor neuroaminidase treatment had any effect on the interaction of GGT with ConA. Desialylation of crude synaptosomal fraction did not change GGT activity. The results presented here suggest no developmental nor functional relationship between the activity of GGT and the level of sialylation in synaptosomal membranes from the cerebral hemispheres of the chick embryo.

The effect of acute and repeated hyperammonemia on gamma-glutamyl transpeptidase in homogenates and capillaries of various rat brain regions

The effect of hyperammonemia of varying degree and duration on the gamma-glutamyl-transpeptidase (GGT) activity was studied in the homogenates and capillaries of different brain regions of the rat. "Acute" hyperammonemia (750 and 600 mg of ammonium acetate per kg b.w. were injected i.p. at 30 min interval, and the animals were decapitated immediately), in which blood ammonia was increased 14-fold, and brain ammonia six-fold above the control level, produced a 20% increase of the enzyme activity in cerebellum, and a 17% decrease in gyrus dentatus, but had no effect in the frontal cortex and the CA1 and CA3 regions of hippocampus. "Subchronic" hyperammonemia (two injections of 600 mg ammonium acetate/kg were given at 24 h intervals, and tissue samples were removed 24 h later), that was accompanied by only a 60% increase of blood or brain ammonia, increased the activity in cerebellum to 38% above control, but produced no effect in the other brain regions. "Chronic" hyperammonemia (three injections of 600 mg ammonium acetate/kg at 24 h intervals and excision of tissue samples 30 min after the last injection), in which blood and brain ammonia were, respectively, 60 and 100% higher than in control animals, elevated the GGT activity in the cerebellum by 57%, in CA1 by 15%, and in CA3 by 21%, but produced no effect in the frontal cortex or gyrus dentatus. By contrast, "chronic" hyperammonemia produced a 30% increase of GGT activity in cerebral cortical capillaries, but only a 10% increase in hippocampal capillaries, and no change in cerebellar capillaries. The results suggest that, hyperammonemia of relatively long duration may contribute to the enhancement of brain GGT activity observed in chronic forms of hepatic encephalopathy. However, ammonia does not appear to activate the enzyme directly.

Changes in the activity of gamma-glutamyl transpeptidase induced by kainic acid and surgical lesions of the hippocampal formation in young rats

To study possible functional involvement of gamma-glutamyl transpeptidase (GGT) in glutamate transmitter metabolism we lesioned putative glutamatergic structures of the rat hippocampal formation by intracerebroventricular (i.c.v.) injection of kainic acid (KA) or by surgical CA3 axotomy. Unilateral injection of KA into the left lateral cerebral ventricle of 30-day-old rats resulted in decreased GGT activity in hippocampal areas CA3, Ca1 ipsilaterally, and in the contralateral area CA1, four hours after the induction of the chemical lesion. Four days after the injection, the enzyme activity was decreased in all hippocampal areas with the exception of the contralateral dentate gyrus. Four days after bilateral i.c.v. injection of KA, lower GGT levels were found than was seen after bilateral surgical lesion of the CA3 pyramidal cell axons (Schaffer's collaterals). The surgical lesion was followed by a decrease of GGT only in the stratum pyramidale and stratum radiatum of area CA1. In contrast to the effects in 30-day-old rats, unilateral i.c.v. injection of KA on postnatal day 12 did not alter the GGT activity in any studied hippocampal area presumably because of incomplete maturation of structures required for KA vulnerability.