Thyroid hormones and the precocious induction of hepatic glucokinase in the neonatal rat

Effect of primary congenital hypothyroidism upon expression of genes mediating murine brain glucose uptake

Using hyt/hyt mice that exhibit naturally occurring primary hypothyroidism (n = 72) and Balb/c controls (n = 66), we examined the mRNA, protein, and activity of brain glucose transporters (Glut 1 and Glut 3) and hexokinase I enzyme at various postnatal ages (d 1, 7, 14, 21, 35, and 60). The hyt/hyt mice showed an age-dependent decline in body weight (p < 0.04) and an increase in serum TSH levels (p < 0.001) at all ages. An age-dependent translational/posttranslational 40% decline in Glut 1 (p = 0.02) with no change in Glut 3 levels was observed. These changes were predominant during the immediate neonatal period (d 1). A posttranslational 70% increase in hexokinase enzyme activity was noted at d 1 alone (p < 0.05) with no concomitant change in brain 2-deoxy-glucose uptake. This was despite a decline in the hyt/hyt glucose production rate. We conclude that primary hypothyroidism causes a decline in brain Glut 1 associated with no change in Glut 3 levels and a compensatory increase in hexokinase enzyme activity. These changes are pronounced only during the immediate neonatal period and disappear in the postweaned stages of development. These hypothyroid-induced compensatory changes in gene products mediating glucose transport and phosphorylation ensure an adequate supply of glucose to the developing brain during transition from fetal to neonatal life.

Effects of triiodothyronine and retinoic acid on glucokinase gene expression in neonatal rat hepatocytes

Glucokinase (EC 2.7.1.2) first appears in rat liver two weeks after birth and increases rapidly after weaning on to a high-carbohydrate diet. We investigated the role of triiodothyronine and retinoic acid in the absence of insulin on the first expression of the glucokinase gene in primary cultures of hepatocytes from 10 day-old rats. These two hormones were able to induce a rapid accumulation of liver glucokinase mRNA, secondarily to a stimulation of gene transcription during the first 24 h of culture. Moreover, the effects of individual hormones were not additive. Finally, glucokinase mRNA stability was not modified by these hormones. This suggests that triiodothyronine and retinoic acid act on glucokinase gene at the transcriptional.

Hormonal regulation of liver phosphoenolpyruvate carboxykinase and glucokinase gene expression at weaning in the rat

During the suckling period, the rats are fed continuously with milk, which is a high-fat low-carbohydrate diet (HF). At weaning, milk is progressively replaced by the rat's laboratory chow which is a high-carbohydrate low-fat diet (HCHO), and this is accompanied by large hormonal modifications: an increase in plasma insulin and a decrease in plasma glucagon concentrations, and by marked changes in metabolic pathways in liver: decrease in hepatic gluconeogenesis and increase in glycolysis and lipogenesis. Most of the data concerning these changes are related to maximal activity of enzymes. The recent availability of specific cDNA probes for phosphoenolpyruvate carboxykinase (PEPCK), and glucokinase (GK) has allowed the study of the role of pancreatic hormones and nutrition in the changes of the expression of these genes at weaning in the rat. Regarding phosphoenolpyruvate carboxykinase gene transcription, the concentration of mRNA as well as the activity of PEPCK are elevated in the liver of suckling rat until the onset of weaning, 21 d after delivery. After weaning to a HCHO diet, both mRNA and activity of PEPCK rapidly decrease to a very low level. In contrast, weaning on an HF diet, which maintains high plasma glucagon and low plasma insulin levels, does not decrease in plasma glucagon concentration and a 90% decrease in PEPCK gene transcription and PEPCK mRNA concentration in 1 h. Regarding glucokinase gene transcription, the concentration of mRNA as well as the activity of GK are not detectable before 15 d after birth in the liver of the rat. They markedly increase when the newborn are weaned on an HCHO diet but not when they are weaned on an HF diet.(ABSTRACT TRUNCATED AT 250 WORDS)

Hormonal control of specific gene expression in the rat liver during the suckling-weaning transition

In the rat, the suckling-weaning transition is accompanied by marked changes in nutrition. During the suckling period, the pups are fed with milk which is a high-fat low-carbohydrate diet. At weaning, milk is progressively replaced by the rat chow which is a high-carbohydrate low-fat diet. This is accompanied by considerable hormonal modifications: an increase in plasma insulin and a decrease in plasma glucagon concentrations, as well as by marked changes in metabolic pathways in liver: decrease in hepatic gluconeogenesis, increase in lipogenesis, and appearance of liver glucokinase. Most of the data concerning these changes are related to maximal activity of enzymes. The recent availability of specific cDNA probes for phosphoenolpyruvate carboxykinase, acetyl-CoA carboxylase, fatty acid synthase and glucokinase has allowed study of the role of pancreatic hormones and of nutrition in the changes of the expression of these genes at weaning in the rat.

Regulation of glucokinase activity in the domestic fowl

The factors which regulate soluble and particulate glucokinase and hexokinase activity in the liver of domestic chickens has been investigated. Pretreatment with oral administration (via tube feeding) of glucose plus injection of insulin resulted in a significant increase in the activity of soluble (p less than 0.01) and particulate (p less than 0.01) glucokinase activity whereas fasting for 48 hours reduced glucokinase and hexokinase activity (p less than 0.01) in the particulate fraction only. Treatment of fed chickens for 2 weeks with thyroxine (50 micrograms: i.m. daily) plus triiodothyronine (50 micrograms) resulted in a marginal decrease (NS) in soluble glucokinase activity but significantly increased soluble hexokinase (p less than 0.05) activity. Thyroidectomized animals showed a decline in both soluble glucokinase (p less than 0.01) and hexokinase (p less than 0.025) activity. There was no effect of thyroid hormone manipulation on particulate glucokinase activity although there was a significant reduction in particulate hexokinase activity (p less than 0.05) in thyroidectomized birds. These data establish a physiological role for the glucokinase enzyme activity in avian carbohydrate metabolism and suggest that in contrast with the mammal, the particulate fraction is the more physiologically important enzyme.

Role of the pituitary and neonatal androgenic imprinting in the hormonal regulation of liver alcohol dehydrogenase activity

Liver alcohol dehydrogenase activity is increased by thyroidectomy, orchidectomy, or hypophysectomy. We investigated the mechanisms of these hormonal effects by examining the effects of testosterone, dexamethasone and thyroid hormone on liver alcohol dehydrogenase activity in hypophysectomized rats and in cultured hepatocytes, and the effect of administration of androgens to neonatal female rats. Testosterone did not lower alcohol dehydrogenase activity in hypophysectomized rats, whereas dexamethasone and thyroxine produced moderate decreases in activity. Triiodothyronine reduced alcohol dehydrogenase activity of cultured hepatocytes from male and hypothyroid female rats in a dose-dependent fashion, confirming that thyroid hormone had pituitary-independent effects on the enzyme activity. Dexamethasone was required for the expression of alcohol dehydrogenase activity in cultured cells, and it increased the enzyme activity when present at supraphysiologic concentrations. Treatment of neonatal female rats with testosterone reduced the activity of the enzyme in adulthood. The difference in alcohol dehydrogenase activity in adult male and female rats appears to be determined in part by neonatal imprinting by androgens and in part by an effect of testosterone that is either mediated by or dependent upon the pituitary. Thyroid hormone reduces alcohol dehydrogenase activity by a direct effect on the liver.

Precocious induction of hepatic glucokinase and malic enzyme in artificially reared rat pups fed a high-carbohydrate diet

Glucokinase and NADP:malate dehydrogenase (malic enzyme) first appear in liver when rat pups are weaned from milk which is high in fat to lab chow which is high in carbohydrate. To examine the influence of diet during the early neonatal period, before developmental changes in the circulating concentrations of thyroid and adrenocortical hormones occur, high-carbohydrate formula (56% of calories from carbohydrate), isocaloric and isonitrogenous with rat milk, was intermittently infused via gastrostomy starting on the second day of life. Pups had no further access to their dams. Body weights attained by these pups were at least 90% of those attained by mother-fed pups, which served as controls. In artificially reared rats fed the high-carbohydrate formula, on Day 4, glucokinase and malic enzyme were 30 and 18% of adult activity, respectively; on Day 10, glucokinase and malic enzyme were 71 and 96% of adult activity, respectively. On Days 4 and 10 glucose-6-phosphate dehydrogenase was elevated four- to fivefold in pups fed the high-carbohydrate formula compared to mother-fed pups. A second isocaloric formula, with 22% of calories from carbohydrate but low in protein, resulted in intermediate levels of all three enzymes on Day 10. Pups fed the high-carbohydrate formula has plasma insulin concentrations four- to fivefold greater than mother-fed pups on both Days 4 and 10. Triiodothyronine administration (1 microgram/g body wt) on Day 1 enhanced the induction of malic enzyme but not glucokinase on Day 4 in pups fed the high-carbohydrate formula. The results demonstrate that neonatal rat liver is competent to respond to high carbohydrate intake by induction of glucokinase and malic enzyme.

Developmental studies on the uptake of tyrosine by synaptosomes and plasma membrane vesicles derived from rat brain. Effect of thyroid hormones

The uptake of L-tyrosine at various stages of development was examined in synaptosomes and in plasma membrane vesicles derived from rat brain. The total uptake has two components, Na+-dependent and Na+-independent, respectively. The Na+-dependent component of the transport system appears around the 5th postnatal day and increases with age. The affinity of the transport system for tyrosine does not vary substantially during development. The Vmax increases more than six-fold between day 15 and adulthood. Plasma membrane vesicles derived from T3-treated rats accumulate more tyrosine than those obtained from the control animals. The results support the view that thyroid hormones during development promote the establishment of the systems implicated in neurotransmission in the developing nervous system.

Ontogenetic studies on tryptophan transport into plasma membrane vesicles derived from rat brain synaptosomes: effect of thyroid hormones

The uptake of tryptophan at various stages of development was examined in plasma membrane vesicles derived from rat brain. The total uptake has two components Na+-dependent and Na+-independent respectively. The Na+-dependent component of the transport system appears around the 5th postnatal day and increases with the age. The Km value of the system does not vary during development. The Vmax increases five-fold between 14 and 35 day of postnatal life. Plasma membrane vesicles derived from T3-treated rats are able to accumulate nearly three-fold more tryptophan than nontreated rats. The results support the idea that thyroid hormones at the earlier stages of life, promote the establishment of neurotransmission in the developing nervous system.

Stability of hexokinases A, B and C and N-acetylglucosamine kinase in liver cells isolated from rats submitted to diabetes and several dietary conditions

The effect of dietary and hormonal variations on the specific activities of hexokinase isoenzymes, N-acetylglucosamine kinase and pyruvate kinase isoenzymes in parenchymal and non-parenchymal liver cells was studied. Hexokinase D was markedly decreased in hepatocytes from animals fasted or fed on the carbohydrate-free diet as well as from diabetic rats, attaining a constant low level of about 17% of normal values. Pyruvate kinase L was also diminished in hepatocytes under the same experimental conditions. In contrast, the three high-affinity hexokinase isoenzymes A, B and C remained without variation in total amount or in their relative proportions in hepatocytes and non-parenchymal liver cells isolated from animals under the various conditions studied. N-Acetylglucosamine kinase activities also did not change either in parenchymal or in non-parenchymal liver cells under all conditions. The results are discussed in relation to the significance of N-acetylglucosamine kinase and the various hexokinase isoenzymes for the phosphorylation of glucose after dietary and hormonal manipulations.

Hypothyroidism in the fetal and neonatal rat does not impair the insulin secretory response to glucose

Fetal and neonatal hypothyroidism was induced by treating pregnant rats with propylthiouracil (PTU) in the drinking water from day 12 of gestation to day 7 postnatally. The serum levels of thyroxine and triiodothyronine were lowered in both mother and offspring and the neonates showed a 20% reduction in weight gain. An i.p. glucose tolerance test on the 7-day old neonates revealed no major disturbances in glucose tolerance or insulin response. The data suggest that thyroid hormones are not essential for the neonatal development of the B-cell secretory response to glucose.

Biochemistry of liver development in the perinatal period

Just before birth, changes occur in the metabolic capacities of rat liver so that the animal can adapt to changes in the substrate supply. In utero, glucose is the main energy-generating fuel and the liver metabolism is directed towards glucose degradation. The activities of the rate-limiting enzymes of glycolysis, hexokinase and phosphofructokinase, are high. In preparation for post-natal life, when the continuous glucose supply from the mother is interrupted, very large amounts of glycogen are stored in the late fetal liver. With the intake of the fat-rich and carbohydrate-poor milk diet, the animal develops the ability to synthesize glucose de novo from non-carbohydrate precursors. During suckling, metabolic energy is derived mainly from the beta-oxidation of fatty acids, which in turn is an essential prerequisite for the high rate of gluconeogenesis, by yielding acetyl-CoA for the activation of pyruvate carboxylase and by generating a high NADH/NAD ratio for the shift of the glyceraldehyde 3-phosphate dehydrogenase reaction in the direction of glucose formation.--The developmental adaptation of metabolism and the process of enzymatic differentiation are closely connected with the maturation of the endocrine system and the changes in the concentration of circulating hormones. The neonatal regulation of phosphoenolpyruvate carboxykinase and of tyrosine aminotransferase by variations in the hormonal milieu around birth, and also the interaction of hormonal and nutritional factors in the induction of serine dehydratase and glucokinase at the end of the suckling period, will be discussed in detail.

Lipogenesis at the suckling-weaning transition in liver and brown adipose tissue of the rat

The responses of rat hepatic and brown adipose tissue in vivo lipogenesis to premature (15 days) and normal (21 days) weaning have been correlated to changes in the activities of acetyl-CoA carboxylase and two NADPH-producing enzymes, malic enzyme and glucose-6-phosphate dehydrogenase. Both tissues show an induction of lipogenesis in response to weaning. In the liver, lipogenic flux is closely linked to the activity of acetyl-CoA carboxylase, but not necessarily that of malic enzyme or glucose-6-phosphate dehydrogenase, whereas no such dissociation between enzyme activity and flux rate occurs in brown adipose tissue. Thyroid hormones, implicated in many physiological changes around weaning, do not seem to play a primary role in the adaptation of lipogenesis to the dietary change at this time, although a permissive role in both tissues is possible.

The separate roles of glucose and insulin in the induction of glucokinase in hepatocytes isolated from neonatal rats

1. The specificity of the effect of glucose on the induction of glucokinase activity that occurs when hepatocytes freshly isolated from 13-day-old rats are incubated in Medium 199 together with insulin [Wakelam & Walker (1980) FEBS Lett. 111, 115-119] was examined. A pattern that is different from other known effects of glucose is found, and metabolism of this compound is not necessarily to account for this particular effect. 2. The effects of a raised glucose concentration and of insulin on the induction can be separated. The hexose initiates the process in the absence of insulin in a manner that is sensitive to actinomycin D but not to cycloheximide. The subsequent effect of insulin is dependent on the prior effect of glucose or other positive analogue, does not require the presence of glucose and is inhibited by cycloheximide but not by actinomycin D. 3. Induction of glucokinase in vitro in hepatocytes from neonatal animals is inhibited by adrenaline, glucagon and dibutyryl cyclic AMP, but not by vasopressin or angiotensin II. The inhibition by cyclic AMP is on the stage requiring insulin and is comparatively specific, because total protein synthesis is not apparently diminished. 4. The implications of these results are discussed with reference to possible mechanisms of induction and to the situation in vivo.

Apparent 'glucokinase' activity in non-hepatic tissues due to N-acetyl-D-glucosamine kinase

1. Electrophoretic examination of tissue extracts from rat intestinal mucosa, kidney, lung, spleen, mammary gland, adipose tissue, heart muscle and placenta in agarose gels did not reveal the presence of any glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) activity corresponding to that present in rat liver. 2. All these tissues do contain an enzyme that possesses very high-Km glucose-phosphorylating activity but which has a slightly lower electrophoretic mobility than glucokinase and can be separated from it by various means. 3. This phosphotransferase activity is due to N-acetyl-D-glucosamine kinase (ATP:2-acetamido-2-deoxy-D-glucose 6-phosphotransferase, EC 2.7.1.59), which has been partialyy purified from intestinal mucosa tissue and shown to have similar kinetic properties to the same enzyme previously purified more extensively from liver and kidney. 4. It is suggested that many of the effects reported in the literature of 'glucokinase' activity in non-hepatic tissues are probably due to N-acetyl-D-glucosamine kinase.

Factors that prevent the premature appearance of glucokinase in neonatal rat liver

1. The physiological factors that prevent the precocious appearance of glucokinase activity in the 13-day-old rat that can be induced by oral glucose administration were explored. 2. Evidence is presented that the galactose component of milk sugar is inhibitory. In the absence of this inhibitory galactose, the amount of glucose necessary to effect appreciable induction is greater than that present in milk. 3. The induction is prevented both by administration of mannoheptulose, which inhibits insulin release, and by excess insulin; the amount of insulin available therefore seems to be critical. 4. The inhibition of induction by galactose does not appear to be via competition with glucose but by enhancing insulin release and thereby making this excessive. The relative amounts of glucose and insulin appear to be important in regulating glucokinase induction. 5. The precocious induction of glucokinase by glucose is inhibited by simultaneous treatment with approriate amounts of adrenaline, glucagon, dibutyryl cyclic AMP or isoprenaline but not by vasopressin or angiotensin II. 6. No single cause of glucokinase induction in neonatal rat liver can be recognized. The process is subject to regulation by many factors at a time subsequent to when competence to synthesize the enzyme has been established.