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

Adipose tissue and liver metabolic responses to different levels of dietary carbohydrates in gilthead sea bream (Sparus aurata)

This study analyzes the effects of replacing dietary lipids by carbohydrates and carbohydrates by fiber on gilthead sea bream growth, as well as lipid and glucose metabolism in adipose tissue and liver over the course of a 15-week feeding trial. Six different diets were formulated and fish were classified into two experimental groups sharing one diet. In the first group (LS), fish were fed four diets where lipids were reduced (23%-17%) by increasing carbohydrates (12%-28%) and, the second group (SF) consisted on three diets where the amount of carbohydrates (28%-11%) was exchanged at expenses of fiber (1%-18%). Differences in growth were not observed; nevertheless, the hepatosomatic index was positively related to dietary starch levels, apparently not due to enhanced hepatic lipogenesis, partly supported by unchanged G6PDH expression. In the LS group, lipogenic activity of adipose tissue was stimulated with low-lipid/high-carbohydrate diets by up-regulating G6PDH expression and a tendency to increase FAS, and promoted carbohydrate utilization versus fatty acid oxidation by modulating the transcription factors LXRα, PPARα and PPARβ expression. In the SF group, PPARs and LXRα increased parallel to fiber levels in adipose tissue. Furthermore, an adaptation of hepatic GK to dietary starch inclusion was observed in both groups; however, the lack of effects on G6Pase expression indicated that gluconeogenesis was not nutritionally regulated under the conditions examined. Overall, metabolic adaptations directed to an efficient use of dietary carbohydrates are present in gilthead sea bream, supporting the possibility of increasing carbohydrate or fiber content in diets for aquaculture sustainability.

Dietary carbohydrate dictates development of Type 2 diabetes in the Nile rat

Amount and type of dietary carbohydrate (CHO), as well as the CHO:fat ratio, are thought to be critical for both the rate of development and severity of Type 2 diabetes mellitus. Thus, these nutritional considerations were examined in the previously described "spontaneous" model of diabetes and metabolic syndrome, the Nile rat. Weanling male Nile rats (n=92) were fed semipurified diets, modifying glycemic index and load by changing the amount of fiber or altering the CHO:fat ratio. Random and fasting blood glucose and body weight were assessed, and diabetes was characterized in terms of blood glucose, relevant plasma and liver parameters, food and water intake and terminal organ weights. Nile rats fed with hiCHO became more hyperglycemic than rats fed with modCHO (P<.05), while loCHO and hiCHO+hiFiber rats remained essentially normoglycemic. Liver lipid and glycogen accumulation was associated with severe hyperlipemia in diabetic rats, analogous to metabolic syndrome in humans. Advanced diabetes was linked to liver and kidney damage and elevated blood urea nitrogen with weight loss. Dispersing dietary CHO by fiber or replacing it by moderate fat (reducing the glycemic index and load) delayed the onset of diabetes but did not prevent signs of insulin resistance. A very low content of dietary CHO (high fat) seemed to prevent even these early indicators of insulin resistance. Thus, the Nile rat represents a novel CHO-sensitive model for study of Type 2 diabetes that reliably follows the course of disease in humans.

Comparative effects of short-term and long-term insulin-induced hypoglycemia on glucose production in the perfused livers of weaned rats

The liver glucose production (LGP) levels of 15-h overnight fasted weaned rats submitted to short-term insulin-induced hypoglycemia (ST-IIH) and long-term IIH (LT-IIH) were compared. Experiments to characterize ST-IIH or LT-IIH that followed an intraperitoneal (ip) injection (1.0 U/kg) of regular (ST-IIH) or insulin detemir (LT-IIH) were performed and glycemia were measured 0 (normoglycemic control), 0.5 h (ST-IIH), 4 h and 6 h (LT-IIH) later. The values of glycemia (mg/dl) were 77.8 ±l 7.2 (normoglycemic control), 26.2 ±l 6.1 (ST IIH 0.5 h), 21.2 ±l 7.6 (LT-IIH 4 h) and 35.3 ±l 14.5 (LT-IIH 6.0). The LGP levels were measured in the rats submitted to ST-IIH (0.5 h) and LT-IIH (4 h or 6 h). The rats that received ip saline were used as the normoglycemic control group (COG). The livers from the COG and IIH groups (ST-IIH or LT-IIH) were perfused in situ with infusion of L-alanine (5 mM), L-glutamine (10 mM), glutamine dipeptide (5 mM), L-lactate (2 mM) or glycerol (2 mM). The ST-IIH rats showed a higher LGP level than COG group following the L-glutamine infusion (p < 0.05), but the LGP levels that were measured following the L-lactate, L-alanine, glutamine dipeptide (5 mM), L-lactate (2 mM) or glycerol infusion remained unchanged. Moreover, if the period of IIH was expanded to 4 h following insulin injection, the LGP levels induced by L-alanine, glutamine dipeptide or glycerol infusion also increased (p < 0.05, LT-IIH vs. COG). However, the LGP from the L-lactate infusion remained unchanged until 6 h after insulin injection. In conclusion, these results suggest that the intensification of liver gluconeogenesis during ST-IIH and LT-IIH in weaned rats is not a synchronous "all or nothing" process; instead, this process integrated in a temporal manner and is specific for each gluconeogenic substrate.

The novel anti-hyperglycemic effect of Paeoniae radix via the transcriptional suppression of phosphoenopyruvate carboxykinase (PEPCK)

The antidiabetic actions of Paeoniae Radix involve stimulating glucose uptake and reducing glucose absorption. However, the importance of this herb in the transcriptional regulation of hepatic gluconeogenesis has not previously been investigated, although hepatic gluconeogenesis contributes the most to fasting hyperglycemia. Using rats with streptozotocin-induced diabetes and db/db mice, the dose- and time-dependent suppressive effects of the ethanol extract of Paeoniae Radix (PR-Et) on diabetic hyperglycemia and phosphoenopyruvate carboxykinase (PEPCK) transcription are first demonstrated. Second, by employing H4IIE cells, the inhibitory action of PR-Et on both dexamethasone- and 8-bromo-cAMP-induced-PEPCK expression was also confirmed without causing any cytotoxicity. In addition, this inhibitory effect could be sustained for over 24 h with repeated treatment. Most importantly, PR-Et's action was unaffected by either insulin desensitization or palmitate stimulation. Finally, paeonol and paeoniflorin, two well-known constituents in Paeoniae Radix, did not suppress PEPCK expression at testing concentration. In conclusion, it was clearly demonstrated that transcriptional inhibition of gluconeogenesis is one of the important antidiabetic actions of Paeoniae Radix. Future development of this herb as a dietary supplement or drug should bring substantial benefits for the diabetic population.

Differential regulation of sterol regulatory element-binding protein 1c transcriptional activity by insulin and liver X receptor during liver development

Sterol regulatory element-binding proteins (SREBPs) are transcription factors involved in the synthesis of cholesterol and fatty acids. In adults, the isoform SREBP-1c is the predominant transcript in the liver of fed animals, and it activates triglyceride production from glucose when diet is enriched in carbohydrates. Studies have shown that SREBP-1c expression is dependent on insulin but also on the availability of oxysterols, ligands of the nuclear liver X receptor (LXR). The aim of this study was to investigate the regulation of the hepatic SREBP-1c expression in vivo in situations where drastic nutritional and hormonal changes occur, from the gestation to the weaning period. In this paper, we report the discovery of LXR-independent SREBP-1c transcriptional activity during late gestation. In utero insulin injection prior to the natural rise in insulin in late gestation triggers SREBP-1c mRNA elevation, nuclear SREBP-1c binding activity, and expression of its target genes independently of LXR transactivation. On the other hand, during suckling, we observed strong SREBP-1c mRNA expression despite very low plasma insulin, an expression that may be due to LXR transactivation. In contrast to insulin, LXR is not sufficient to trigger nuclear SREBP-1c binding activity and target gene induction. This could be due to the concomitant induction of INSIG-2a by LXR and subsequent retention of SREBP-1c in the endoplasmic reticulum.

Free fatty acids increase basal hepatic glucose production and induce hepatic insulin resistance at different sites

To investigate the sites of the free fatty acid (FFA) effects to increase basal hepatic glucose production and to impair hepatic insulin action, we performed 2-h and 7-h Intralipid + heparin (IH) and saline infusions in the basal fasting state and during hyperinsulinemic clamps in overnight-fasted rats. We measured endogenous glucose production (EGP), total glucose output (TGO, the flux through glucose-6-phosphatase), glucose cycling (GC, index of flux through glucokinase = TGO - EGP), hepatic glucose 6-phosphate (G-6-P) content, and hepatic glucose-6-phosphatase and glucokinase activities. Plasma FFA levels were elevated about threefold by IH. In the basal state, IH increased TGO, in vivo glucose-6-phosphatase activity (TGO/G-6-P), and EGP (P < 0.001). During the clamp compared with the basal experiments, 2-h insulin infusion increased GC and in vivo glucokinase activity (GC/TGO; P < 0.05) and suppressed EGP (P < 0.05) but failed to significantly affect TGO and in vivo glucose-6-phosphatase activity. IH decreased the ability of insulin to increase GC and in vivo glucokinase activity (P < 0.01), and at 7 h, it also decreased the ability of insulin to suppress EGP (P < 0.001). G-6-P content was comparable in all groups. In vivo glucose-6-phosphatase and glucokinase activities did not correspond to their in vitro activities as determined in liver tissue, suggesting that stable changes in enzyme activity were not responsible for the FFA effects. The data suggest that, in overnight-fasted rats, FFA increased basal EGP and induced hepatic insulin resistance at different sites. 1) FFA increased basal EGP through an increase in TGO and in vivo glucose-6-phosphatase activity, presumably due to a stimulatory allosteric effect of fatty acyl-CoA on glucose-6-phosphatase. 2) FFA induced hepatic insulin resistance (decreased the ability of insulin to suppress EGP) through an impairment of insulin's ability to increase GC and in vivo glucokinase activity, presumably due to an inhibitory allosteric effect of fatty acyl-CoA on glucokinase and/or an impairment in glucokinase translocation.

Folate cofactors regulate serine metabolism in fetal ovine hepatocytes

The fetal liver is the primary site of fetal serine production. The regulation of this unique fetal hepatic serine production is unknown. We hypothesized that serine production would be responsive to folate cofactor supply or hormonal regulation. To test this hypothesis, we determined the effect of key folate cofactors and insulin and glucagon on serine and glycine metabolism in primary culture of fetal ovine hepatocytes. Hepatocytes were cultured in serum-free, low-folate media [5 nM 5-methyl-tetrahydrofolate (THF)] with or without 50 nM 5,10-methylene-THF (MTHF) or 5-formyl-THF (FTHF). Serine and glycine production (P) and utilization (U) were determined by stable isotope dilution with [1-(13)C]serine and [1-(13)C]glycine for 24 h. The effect of insulin (1 microM) or glucagon (1 micro M) was determined in a similar manner. Under basal conditions, serine P (43.2 +/- 5.1 micromol/mg DNA per 24 h) is greater than serine U (24.1 +/- 3.1 micromol/mg DNA per 24 h), whereas glycine U (27.3 +/- 3.0 micromol/mg DNA per 24 h) exceeds glycine P (16.7 +/- 1.9 micromol/mg DNA per 24 h). MTHF results in a significant decrease in serine U (16.0 +/- 2.7 micromol/mg DNA per 24 h; p = 0.02 versus low folate), with no change in serine P. FTHF reduces serine P (36.2 +/- 4.9 micromol/mg DNA per 24 h; p = 0.01), but does not alter serine U. There were no effects on glycine metabolism with 50 nM MTHF or FTHF. Serine P and U were inversely correlated whereas glycine P and U were directly correlated with the media concentration of MTHF or FTHF. Glucagon treatment increased serine U by 260 +/- 65% versus low folate (p = 0.0004) but did not change serine P. Insulin treatment led to parallel increases in both serine P and U. Both folate cofactor availability and hormone concentrations regulate serine metabolism in the fetal liver. We speculate that serine metabolism may be a marker of fetal hepatic folate cofactor supply.

Postnatal expression and induction by pregnenolone-16alpha-carbonitrile of the organic anion-transporting polypeptide 2 in rat liver

Newborn rats are more sensitive to the toxic effects of cardiac glycosides than are adult rats. This is associated with a decreased ability to remove cardiac glycosides from blood into the liver. Pregnenolone-16alpha-carbonitrile (PCN), a prototypical rodent CYP3A inducer and pregnane-X-receptor (PXR) ligand, stimulates the hepatic clearance of cardiac glycosides in newborn rats, which results in decreased toxicity of the cardiac glycosides. The mechanism responsible for this phenomenon is not clear; however, if elucidated, it would help in understanding and preventing potential drug-drug interactions. The recently cloned rat organic anion-transporting polypeptide 2 (oatp2) (Slc21a5) is a sinusoidal hepatic uptake transporter, with very high affinities for cardiac glycosides, and thus it was hypothesized that rat oatp2 increases during postnatal development and is inducible by PCN. In the present study, livers were removed from Sprague-Dawley rats from postnatal days (pnd) 0 to 45, in 5-day increments; as well as from pnd 10 to 90, in 10-day increments, after PCN (75 mg/kg i.p., for 4 days) or corn oil (vehicle for PCN) treatment. The protein and mRNA levels of rat oatp2 were determined by Western blot analysis and branched DNA signal amplification technique, respectively. Expression of rat oatp2 protein and mRNA increased gradually during postnatal development. PCN treatment increased liver to body weight ratio in both genders, and dramatically accelerated the maturation of hepatic oatp2 protein and mRNA levels. In summary, rat oatp2 undergoes age-dependent and chemical regulation during postnatal development, and is a potential target for drug-drug and age-drug interactions.

Region-specific ontogeny of alpha-2,6-sialyltransferase during normal and cortisone-induced maturation in mouse intestine

Regional differences in the ontogeny of mouse intestinal alpha-2,6-sialyltransferase activities (alpha-2,6-ST) and the influence of cortisone acetate (CA) on this expression were determined. High ST activity and alpha-2,6-ST mRNA levels were detected in immature small and large intestine, with activity increasing distally from the duodenum. As the mice matured, ST activity (predominantly alpha-2,6-ST) in the small intestine decreased rapidly to adult levels by the fourth postnatal week. CA precociously accelerated this region-specific ontogenic decline. A similar decline of ST mRNA levels reflected ST activity in the small, but not the large, intestine. Small intestinal sialyl alpha-2,6-linked glycoconjugates displayed similar developmental and CA induced-precocious declines when probed using Sambucus nigra agglutinin (SNA) lectin. SNA labeling demonstrated age-dependent diminished sialyl alpha2,6 glycoconjugate expression in goblet cells in the small (but not large) intestine, but no such regional specificity was apparent in microvillus membrane. This suggests differential regulation of sialyl alpha-2,6 glycoconjugates in absorptive vs. globlet cells. These age-dependent and region-specific differences in sialyl alpha-2,6 glycoconjugates may be mediated in part by altered alpha-2,6-ST gene expression regulated by trophic factors such as glucocorticoids.

Maternal dietary glucose modifies phosphoenolpyruvate carboxykinase (PEPCK) gene expression in the kidney of newborn rats

The consequence of low maternal dietary glucose on perinatal phosphoenolpyruvate carboxykinase (PEPCK EC 4.1.32) gene expression was investigated. Pregnant rats were fed isoenergetic diets containing graded levels of glucose (0, 12, 24, and 60%) from gestation day 2 to lactation day 15. The postnatal developmental profile of PEPCK mRNA in the neonatal kidney was analysed by Northern blot and presented as PEPCK/GAPDH mRNA ratios. In comparison with the 24 and 60% dietary groups, maternal dietary glucose restriction (0 or 12%) during pregnancy resulted in a significant delay in postnatal renal PEPCK gene expression. In these glucose restricted pups, renal PEPCK mRNA was barely detected at birth and was fully visualized only at 4-6 hr; it peaked 24 hr after birth, which was 12 hr later than pups born to dams fed 24 or 60% glucose diets. These results demonstrate for the first time that maternal dietary glucose can modify postnatal renal PEPCK gene expression during perinatal development when glucose homeostasis via gluconeogenesis is critical for neonatal survival.

Effect of delayed weaning on opioid receptor control of swim stress-induced antinociception in the developing rat

1. The opioid type of swim-stress induced antinociception (SIA) is mediated via mu-sites in preweanling rats and predominantly by delta-sites in postweanling animals. We have studied the effect of delay of weaning on the receptor transition of this behaviour in the developing rat. 2. Litters were weaned normally at day 21 or allowed to remain with their mothers until assessment of swim SIA. Animals were stressed by warm water (20 degrees C) swimming for 3 min periods and antinociception assessed by the tail immersion test (50 degrees C). 3. Naloxone (10 mg kg-1) partially reversed swim SIA in both 25 day old weaned and non-weaned rats. 4. Naltrindole (1 mg kg-1) partially reversed swim SIA in 25 day old weaned rats but had no effect in non-weaned animals. Naltrindole (5 mg kg-1) completely abolished swim SIA in weaned rats but was without effect in non-weaned groups. Antinociceptive responses to the mu-agonist, alfentanil (60 micrograms kg-1) were unaffected by naltrindole at 1 mg kg-1 but were partially reversed at 5 mg kg-1. 5. In 30 day old non-weaned rats, naltrindole (5 mg kg-1) abolished the swim SIA. 6. In conclusion, transition from mu to delta-receptor control of swim SIA in rat pups can be delayed by between 5 and 10 days by delay of weaning. The environmental stimulus of weaning can activate opioid receptor subtype operation of biological responses in the developing animal.