The development of phenylalanine hydroxylase in rat liver; in vivo, and in vitro studies utilizing fetal hepatocyte cultures

Interactive effects of neonatal exposure to monosodium glutamate and aspartame on glucose homeostasis

BACKGROUND

Recent evidence suggests that the effects of certain food additives may be synergistic or additive. Aspartame (ASP) and Monosodium Glutamate (MSG) are ubiquitous food additives with a common moiety: both contain acidic amino acids which can act as neurotransmitters, interacting with NMDA receptors concentrated in areas of the Central Nervous System regulating energy expenditure and conservation. MSG has been shown to promote a neuroendocrine dysfunction when large quantities are administered to mammals during the neonatal period. ASP is a low-calorie dipeptide sweetener found in a wide variety of diet beverages and foods. However, recent reports suggest that ASP may promote weight gain and hyperglycemia in a zebrafish nutritional model.

METHODS

We investigated the effects of ASP, MSG or a combination of both on glucose and insulin homeostasis, weight change and adiposity, in C57BL/6 J mice chronically exposed to these food additives commencing in-utero, compared to an additive-free diet. Pearson correlation analysis was used to investigate the associations between body characteristics and variables in glucose and insulin homeostasis.

RESULTS

ASP alone (50 mg/Kgbw/day) caused an increase in fasting blood glucose of 1.6-fold, together with reduced insulin sensitivity during an Insulin Tolerance Test (ITT) P < 0.05. Conversely MSG alone decreased blood triglyceride and total cholesterol (T-CHOL) levels. The combination of MSG (120 mg/Kgbw/day) and ASP elevated body weight, and caused a further increase in fasting blood glucose of 2.3-fold compared to Controls (prediabetic levels); together with evidence of insulin resistance during the ITT (P < 0.05). T-CHOL levels were reduced in both ASP-containing diets in both genders. Further analysis showed a strong correlation between body weight at 6 weeks, and body weight and fasting blood glucose levels at 17 weeks, suggesting that early body weight may be a predictor of glucose homeostasis in later life.

CONCLUSIONS

Aspartame exposure may promote hyperglycemia and insulin intolerance. MSG may interact with aspartame to further impair glucose homeostasis. This is the first study to ascertain the hyperglycemic effects of chronic exposure to a combination of these commonly consumed food additives; however these observations are limited to a C57BL/6 J mouse model. Caution should be applied in extrapolating these findings to other species.

Partial biopterin deficiency disturbs postnatal development of the dopaminergic system in the brain

Postnatal development of dopaminergic system is closely related to the development of psychomotor function. Tyrosine hydroxylase (TH) is the rate-limiting enzyme in the biosynthesis of dopamine and requires tetrahydrobiopterin (BH4) as a cofactor. To clarify the effect of partial BH4 deficiency on postnatal development of the dopaminergic system, we examined two lines of mutant mice lacking a BH4-biosynthesizing enzyme, including sepiapterin reductase knock-out (Spr(-/-)) mice and genetically rescued 6-pyruvoyltetrahydropterin synthase knock-out (DPS-Pts(-/-)) mice. We found that biopterin contents in the brains of these knock-out mice were moderately decreased from postnatal day 0 (P0) and remained constant up to P21. In contrast, the effects of BH4 deficiency on dopamine and TH protein levels were more manifested during the postnatal development. Both of dopamine and TH protein levels were greatly increased from P0 to P21 in wild-type mice but not in those mutant mice. Serotonin levels in those mutant mice were also severely suppressed after P7. Moreover, striatal TH immunoreactivity in Spr(-/-) mice showed a drop in the late developmental stage, when those mice exhibited hind-limb clasping behavior, a type of motor dysfunction. Our results demonstrate a critical role of biopterin in the augmentation of TH protein in the postnatal period. The developmental manifestation of psychomotor symptoms in BH4 deficiency might be attributable at least partially to high dependence of dopaminergic development on BH4 availability.

Phenylalanine hydroxylase expression in primary rat hepatocytes is modulated by oxygen concentration

In this work we have investigated the regulation of rat phenylalanine hydroxylase (rPAH) expression by oxygen in primary cultures of rat hepatocytes. We show that rPAH is negatively modulated at the mRNA, protein and activity levels by pO(2) of 16% (periportal hepatic levels) compared to 8% (perivenous hepatic levels). Our results suggest that PAH might be metabolically zonated in vivo, and preferentially found in perivenous hepatocytes with high glucose consumption and largely influenced by insulin levels.

Sequence analysis of the rat phenylalanine hydroxylase gene promoter

We have characterized the 5'-end (3218 bp) of the rat phenylalanine hydroxylase (PAH) gene. Within this PAH promoter sequence, we have identified a number of putative regulatory sites analogous to those present in the human and murine PAH promoters. In particular, potential HNF 1 binding sites and a CRE have been identified. These sequences respectively bind HNF1 and CREB transcription factors present in rat nuclear extracts and may be significant in the tissue-specific and hormonal control of PAH expression.

Conserved as well as divergent regulatory elements account for expression of the human and rodent phenylalanine hydroxylase genes

We have uncovered a fundamental difference in the regulation of the rodent and the human phenylalanine hydroxylase (PAH) genes: expression of human PAH is independent of glucocorticoids and/or cAMP in contrast to the mouse gene which is not only highly inducible but dependent upon hormones for expression. Nevertheless, the two genes do exhibit similarities: DNaseI hypersensitive sites are identically located in the regulatory regions, and the sequences around these sites are partially conserved and associated with regulatory elements sharing similar function. In transient transfections, the human proximal promoter is tissue-specific and presents significant activity compared to the extremely low and ubiquitous activity of the mouse promoter. DNA fragments corresponding to the two upstream hypersensitive sites of both genes have enhancer activity that depends upon the liver-enriched transcription factor binding sites for hepatocyte nuclear factor (HNF) 1 and/or CCAAT/enhancer binding protein (C/EBP). While expression of the rodent gene relies upon two modules in the HSIII enhancer, one activated by HNF1 and C/EBP and the other required for the hormone response, the human equivalent has conserved only the liver-specific transcription factor binding module. Even though the more proximal enhancer is not necessary for full reporter gene activity in transient transfection assays in Pah-expressing hepatoma cells, this enhancer could be required in both species for activation during development.

The relationship between differentiation and proliferation in late gestation fetal rat hepatocytes

Hepatocyte proliferation and differentiation occur simultaneously during late mammalian gestation. We hypothesized that regulation of hepatocyte growth and differentiation would be coordinated in late gestation fetal hepatocyte cultures such that proliferation would be most active in a population of less well-differentiated cells. Cultured fetal hepatocytes (embryonic d 19 and 21; E19 and E21) were studied using double staining immunofluorescent microscopy. Differentiation was assessed as staining for alpha-fetoprotein (AFP), three markers of enzymic differentiation (glucokinase [GK], phosphoenolpyruvate carboxykinase [PEPCK], and carbamoyl phosphate synthase [CPS]), and a hepatocyte cell-cell adhesion molecule (C-CAM). Proliferation was assessed using immunocytochemical detection of proliferating cell nuclear antigen (PCNA) or 5-bromo-2'-deoxy-uridine (BrdU) incorporation into DNA. Fetal hepatocyte cultures consisted of a heterogeneous population of cells, slightly more than half of which were proliferative under defined, growth factor-free conditions. These cultures were heterogeneous for AFP expression. There was no correlation between the expression of AFP and PCNA or AFP and S-phase entry (BrdU staining) during the first 48 h in culture. Similar results were obtained in staining for the enzymic differentiation markers and C-CAM. In addition, the differentiation status of cultured fetal hepatocytes was unrelated to a presumed indicator of mature growth regulation, mitogenic responsiveness to transforming growth factor alpha (TGFalpha), and hepatocyte growth factor (HGF). Finally, absence of any correlation between proliferation and differentiated phenotype was supported by in vivo studies using staining for PCNA, AFP, CPS, and PEPCK in liver sections. These results indicate that the developmental program governing differentiation of late gestation fetal rat hepatocytes is independent from mechanisms controlling proliferation.

Developmental pattern of phenylalanine hydroxylase activity in the chicken

Experiments were conducted to determine the conditions for assay of hepatic phenylalanine hydroxylase (PAH) activity in the chicken and to determine the developmental pattern of PAH activity in liver 25,000 x g supernatant. PAH activity was detected in liver supernatant and (postnuclear) 25,000 x g particulate fraction. Optimum assay conditions differed for the two cell fractions, the most notable difference being a broad pH optimum of 7.7 to 9.2 for the supernatant and 4.7 and 5.6 for the particulate fraction. The PAH activity in the supernatant increased to a maximum as L-phenylalanine concentration in the assay medium increased from 0.02 to 0.5 mM and 1.0 mM. Activity increased in the particulate fraction as the Phe concentration increased to 0.5 mM. Substrate inhibition of PAH activity occurred at Phe concentrations of 3 to 5 mM in the supernatant but not in the particulate fraction. Concentrations of the cofactor, 6(R)-5,6,7,8-tetrahydrobiopterin, ranging from 0.09 to 0.75 mM, resulted in maximal PAH activity. The developmental pattern of PAH in supernatant was determined using a modified assay in which substrate and cofactor concentrations and pH were optimum. The PAH activity in liver supernatant was present at a low level in 11 d chick embryos and increased several fold between Days 15 and 17 to a maximum at Days 17 to 21. Activity declined at hatching to levels that were present in 11 to 15 d embryos and remained at this level in male chicks through 4 wk of age. Mature males had higher PAH activity than mature laying females.

Identification of differentially expressed genes during hepatocytes development and characterization of their prenatal hormonal induction

Upon birth, the liver acquires new functions as a result of the initiation of expression of key enzymes. One example is the initiation of gluconeogenesis which depends on the induced appearance of phosphoenolpyruvate carboxykinase (P-pyruvate-CK) at birth. To characterize other genes that undergo such regulation, a differential screening was performed on a cDNA library from well-differentiated hepatoma cells. The pattern of tissue-specific and developmental-specific expression was determined for seven genes. Three clones, out of which two encode for the known genes alcohol dehydrogenase class I (ADH) and phenylalanine 4-monooxygenase (PAH) and a new gene (clone 116-3), exhibited a pattern of expression similar to that of the P-pyruvate-CK gene, i.e. their expression was liver and kidney specific and induced in the liver upon birth. Determination of the sequence of clone 116-3 revealed that it belonged to the UDP-glucuronosyltransferases type 2 (UGT2) family and thus was named UGT2B-rH4. To examine whether expression of the various genes could be prematurely induced by hormones in the fetal liver, either high levels of cAMP or low levels of insulin were induced in utero. The results demonstrated that cAMP induced a marked expression only of the genes for P-pyruvate-CK and ADH but not of those for PAH or UGT2B-rH4, while insulin deficiency induced premature expression of all four genes. We suggest that a set of genes whose expression is specifically induced in the liver upon birth can be prematurely induced by the hormones in utero.

Differential effects of streptozotocin-induced diabetes on phenylalanine hydroxylase protein and mRNA abundance in isolated rat liver cells

Phenylalanine hydroxylase catalyzes the major regulatory step of the phenylalanine degradation pathway. In view of the glucogenic nature of phenylalanine breakdown, and hence its potential contribution to glucose homeostasis, we have investigated the impact of streptozotocin-induced diabetes upon the expression of rat phenylalanine hydroxylase. Northern blot analysis revealed that induction of diabetes was associated with an increase in the in vivo abundance of hepatic phenylalanine hydroxylase-specific mRNA. This increase in mRNA abundance was maintained for at least 8 hr in liver cells isolated from diabetic animals. In contrast, phenylalanine hydroxylase immunoreactivity and enzymic activity decreased, over the 8 hr incubation period, to levels similar to those observed in liver cells from normal animals. These changes were retarded, but not prevented, by the presence of dexamethasone in incubation media. In liver cells from normal animals the abundance of phenylalanine hydroxylase-specific mRNA, immunoreactivity and enzymic activity, were largely insensitive to treatment with dexamethasone and/or glucagon over an 8 hr incubation period. It is concluded that, whereas diabetes-related alterations in phenylalanine hydroxylase-specific mRNA abundance persist after isolation of liver cells, changes in phenylalanine hydroxylase protein abundance do not. Additionally, in contrast to certain other enzymes (e.g. phosphoenolpyruvate carboxykinase) it is not possible to mimic diabetes-related alterations in the expression of phenylalanine hydroxylase, in liver cells from normal animals, by simple hormonal manipulation of incubation media. This implies that other additional factors must also contribute to diabetes-related alterations in hepatic enzyme expression.

Calcium phosphate transfection and cell-specific expression of heterologous genes in primary fetal rat hepatocytes

In order to study transcriptional regulation of hepatic genes during development, a method for transfer of fusion genes to primary cultures of fetal hepatocytes was required. The aim of this study was to assess currently available transfection methods and optimize the best method for use with cultured fetal hepatocytes. The Rous sarcoma virus 5' long terminal repeat controlling transcription of the beta-galactosidase reporter gene (pRSV lac Z II) was used to assess electroporation, lipofection, DEAE-dextran and calcium phosphate transfection in cultured primary fetal hepatocytes. The success of transfection was determined by histochemical detection and quantitation of beta-galactosidase activity. Results showed that calcium phosphate transfection was optimal for fetal hepatocytes with respect to beta-galactosidase activity and cell survival. For maximum transfection of cells, 10 micrograms/ml DNA, HEPES buffered saline transfection buffer at pH 7.05 and a 24 hr expression period for the reporter gene were employed. Glycerol shock did not increase transfection efficiency significantly. The method was simplified by adding calcium chloride solution to DNA diluted in transfection buffer and the resulting co-precipitate added directly to the medium covering the cells. Transfection 24 hr after initial culture and a precipitate incubation time of 20 hr were optimal. The suitability of this method was confirmed with a liver-specific promoter controlling beta-galactosidase and chloramphenicol acetyltransferase expression. In conclusion this study shows that a modified calcium phosphate transfection method is most effective for transferring DNA to primary cultured fetal hepatocytes. It is concluded that this method is appropriate for use with fetal hepatocytes and will facilitate studies of gene regulation during liver development.

The activity of the highly inducible mouse phenylalanine hydroxylase gene promoter is dependent upon a tissue-specific, hormone-inducible enhancer

Expression of the phenylalanine hydroxylase gene in livers and kidneys of rodents is activated at birth and is induced by glucocorticoids and cyclic AMP in the liver. Regulatory elements in a 10-kb fragment upstream of the mouse gene have been characterized. The promoter lacks TAATA and CCAAT consensus sequences and shows only extremely weak activity in transitory expression assays with phenylalanine hydroxylase-producing hepatoma cells. No key elements for regulation of promoter activity are localized within 2 kb of upstream sequences. However, a liver-specific DNase I-hypersensitive site at kb -3.5 comprises a tissue-specific and hormone-inducible enhancer. This enhancer contains multiple protein binding sites, including sites for ubiquitous factors (NF1 and AP1), the glucocorticoid receptor, and the hepatocyte-enriched transcription factors hepatocyte nuclear factor 1 (HNF1) and C/EBP. Mutation revealed that the last two sites are critical not only for basal activity but also for obtaining a maximal hormone response. Efficient transcription from the highly inducible promoter shows absolute dependence upon the enhancer at kb - 3.5, which in turn requires HNF1 and C/EBP as well as hormones. The regulatory region of the mouse phenylalanine hydroxylase gene differs totally from that of humans, even though the genes of both species are expressed essentially in the liver. Furthermore, the phenylalanine hydroxylase gene of mice shows an expression pattern very similar to those of the rodent tyrosine aminotransferase and phosphoenolpyruvate carboxykinase genes, yet each shows a different organization of its regulatory region.

The immediate 5'-flanking region of the rat phenylalanine hydroxylase-encoding gene

We have characterized the immediate (465 bp) 5'-flanking region of the rat phenylalanine hydroxylase (PAH)-encoding gene. This sequence shows considerable similarity to the 5'-flanking region of the human PAH gene [Konecki et al., Biochemistry 31 (1992) 8363-8368]. Both sequences lack obvious TATA elements; however, putative regulatory sites, including a potential cyclic AMP-response element and glucocorticoid response elements, are present.

The effect of streptozotocin-induced diabetes on phenylalanine hydroxylase expression in rat liver

The impact of experimentally induced diabetes on the expression of rat liver phenylalanine hydroxylase has been investigated. A significant elevation in maximal enzymic activity was observed in diabetes. This was associated with significant increases in the amount of enzyme, the phenylalanine hydroxylase-specific translational activity of hepatic RNA and the abundance of phenylalanine hydroxylase-specific mRNA. These changes in phenylalanine hydroxylase expression were not observed when diabetes was controlled by daily injections of insulin. These results are discussed in relation to the hormonal control of phenylalanine hydroxylase gene expression.