Induction in primary culture of 'gluconeogenic' and 'glycolytic' hepatocytes resembling periportal and perivenous cells

Towards improved hepatocyte cultures: Progress and limitations

Hepatotoxicity is among the most frequent reasons for drug withdrawal from the market. Therefore, there is an urgent need for reliable predictive in vitro tests, which unfailingly identify hepatotoxic drug candidates, reduce drug development time, expenses and the number of test animals. Currently, human hepatocytes represent the gold standard. However, the use of hepatocytes is challenging since the cells are not constantly available and lose their metabolic activity in culture. To solve these problems many different approaches have been developed in the past decades. The aim of this review is to present these approaches and to discuss the possibilities and limitations as well as future opportunities and directions.

Dynamic Metabolic Zonation of the Hepatic Glucose Metabolism Is Accomplished by Sinusoidal Plasma Gradients of Nutrients and Hormones

Being the central metabolic organ of vertebrates, the liver possesses the largest repertoire of metabolic enzymes among all tissues and organs. Almost all metabolic pathways are resident in the parenchymal cell, hepatocyte, but the pathway capacities may largely differ depending on the localization of hepatocytes within the liver acinus-a phenomenon that is commonly referred to as metabolic zonation. Metabolic zonation is rather dynamic since gene expression patterns of metabolic enzymes may change in response to nutrition, drugs, hormones and pathological states of the liver (e.g., fibrosis and inflammation). This fact has to be ultimately taken into account in mathematical models aiming at the prediction of metabolic liver functions in different physiological and pathological settings. Here we present a spatially resolved kinetic tissue model of hepatic glucose metabolism which includes zone-specific temporal changes of enzyme abundances which are driven by concentration gradients of nutrients, hormones and oxygen along the hepatic sinusoids. As key modulators of enzyme expression we included oxygen, glucose and the hormones insulin and glucagon which also control enzyme activities by cAMP-dependent reversible phosphorylation. Starting with an initially non-zonated model using plasma profiles under fed, fasted and diabetic conditions, zonal patterns of glycolytic and gluconeogenetic enzymes as well as glucose uptake and release rates are created as an emergent property. We show that mechanisms controlling the adaptation of enzyme abundances to varying external conditions necessarily lead to the zonation of hepatic carbohydrate metabolism. To the best of our knowledge, this is the first kinetic tissue model which takes into account in a semi-mechanistic way all relevant levels of enzyme regulation.

Mechanical Bridge to Transplantation with the Vienna Heart in TAH and LVAD Configuration

The Vienna heart uses a vacuum formed, pellethane pulsatile ventricle and is available in left ventricular assist (LVAD) and total artificial heart (TAH) configurations. This device was used as mechanical support of the failing heart in nine patients intended for heart transplantation. In two patients with cardiomyopathy an orthotopic TAH was implanted; one survived despite severe preoperative ischemic liver damage, and the other died of sepsis.

In seven patients an atrio-aortic LVAD was implanted; six had suffered an acute myocardial infarction with cardiogenic shock, and one could not be weaned off bypass. Three patients survived. These included one 65-year-old with incipient ARDS at operation, and a 40-year-old with preoperative liver and kidney insufficiency who was transplanted in septicemia. In this patient the septic focus, natural and artificial heart, were removed at transplantation.

Four patients died. In one we were unable to establish satisfactory circulation, one died after failure of the transplanted heart, one suffered a lethal cerebral embolism and one developed multi-organ failure after repeated attacks of ventricular fibrillation.

With the Vienna heart sufficient circulatory support could be established with cardiac outputs between 6 and 8 l/min for the TAH and 3.5 to 4.5 I/min for the LVAD. With this type of support an overall survival rate of 44% could be achieved. Mechanical hemolysis was not a clinical problem and no device failure occurred.

A multiscale modelling approach to assess the impact of metabolic zonation and microperfusion on the hepatic carbohydrate metabolism

The capacity of the liver to convert the metabolic input received from the incoming portal and arterial blood into the metabolic output of the outgoing venous blood has three major determinants: The intra-hepatic blood flow, the transport of metabolites between blood vessels (sinusoids) and hepatocytes and the metabolic capacity of hepatocytes. These determinants are not constant across the organ: Even in the normal organ, but much more pronounced in the fibrotic and cirrhotic liver, regional variability of the capillary blood pressure, tissue architecture and the expression level of metabolic enzymes (zonation) have been reported. Understanding how this variability may affect the regional metabolic capacity of the liver is important for the interpretation of functional liver tests and planning of pharmacological and surgical interventions. Here we present a mathematical model of the sinusoidal tissue unit (STU) that is composed of a single sinusoid surrounded by the space of Disse and a monolayer of hepatocytes. The total metabolic output of the liver (arterio-venous glucose difference) is obtained by integration across the metabolic output of a representative number of STUs. Application of the model to the hepatic glucose metabolism provided the following insights: (i) At portal glucose concentrations between 6–8 mM, an intra-sinusoidal glucose cycle may occur which is constituted by glucose producing periportal hepatocytes and glucose consuming pericentral hepatocytes, (ii) Regional variability of hepatic blood flow is higher than the corresponding regional variability of the metabolic output, (iii) a spatially resolved metabolic functiogram of the liver is constructed. Variations of tissue parameters are equally important as variations of enzyme activities for the control of the arterio-venous glucose difference.

Glucose homeostasis is one of the central liver functions. The liver extracts glucose from the blood when plasma glucose levels are high and produces glucose when plasma glucose levels are low. To fulfill this function the liver is organized in smallest functional units, the sinusoidal tissue units (STUs). These STUs consist of a single sinusoid surrounded by linear arranged hepatocytes. Liver zonation describes the spatial separation of metabolic pathways along the STUs. As blood flows through the sinusoid the plasma nutrient and hormone composition changes and in conjunction with the heterogeneous endowment of metabolic enzymes this leads to big differences in the metabolic performance of hepatocytes depending on their position within the sinusoid. This makes liver zonation and blood flow two central determinants for the functional output of the liver. In this work we present a tissue model of hepatic carbohydrate metabolism that combines liver zonation and microperfusion within the STU. We show that structural properties, enzymatic properties and regional bloodflow are equally important for the understanding of liver functionality. With our work we provide a true multi-scale model bridging the scale from the cellular to the tissue level.

Hedgehog signaling is a potent regulator of liver lipid metabolism and reveals a GLI-code associated with steatosis

Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease in industrialized countries and is increasing in prevalence. The pathomechanisms, however, are poorly understood. This study assessed the unexpected role of the Hedgehog pathway in adult liver lipid metabolism. Using transgenic mice with conditional hepatocyte-specific deletion of Smoothened in adult mice, we showed that hepatocellular inhibition of Hedgehog signaling leads to steatosis by altering the abundance of the transcription factors GLI1 and GLI3. This steatotic 'Gli-code' caused the modulation of a complex network of lipogenic transcription factors and enzymes, including SREBP1 and PNPLA3, as demonstrated by microarray analysis and siRNA experiments and could be confirmed in other steatotic mouse models as well as in steatotic human livers. Conversely, activation of the Hedgehog pathway reversed the "Gli-code" and mitigated hepatic steatosis. Collectively, our results reveal that dysfunctions in the Hedgehog pathway play an important role in hepatic steatosis and beyond.

DOI:http://dx.doi.org/10.7554/eLife.13308.001

The liver is one of the main organs responsible for processing everything that mammals eat and drink. Nutrients absorbed by the gut like sugars and lipids (fats) are processed by the liver and are stored or distributed to provide energy to other organs. Sometimes these metabolic processes become unbalanced. This can lead to lipids accumulating in the liver – a process known as steatosis, which is a feature of human non-alcoholic fatty liver disease.

In organs like the liver, cells are instructed how to behave via signaling pathways. A protein outside the cell signals to specific proteins inside, which switch on a set of target genes. One such pathway is the Hedgehog pathway, which primarily regulates tissue regeneration and the development of embryos. A component of this pathway is the Smoothened gene, which indirectly switches on proteins called GLI factors that regulate metabolic genes, including those involved in lipid metabolism. The Hedgehog pathway has been found to control the metabolism of lipids in fat tissue but it is not known whether it is important for lipid metabolism in the liver.

Matz-Soja et al. investigated this possible role of the Hedgehog pathway in the liver using mice with a Smoothened gene that could be deleted specifically in that organ. This deletion disrupted Hedgehog signaling and led to lipids accumulating in the liver and eventually to steatosis. These changes were associated with an increase in the amounts and activityof several enzymes (and the proteins that regulate these enzymes) that help to synthesize lipids. Steatosis was also associated with low amounts of two of the three GLI factors; indeed, this seems to be key for triggering problems with lipid metabolism. Human livers with steatosis showed the same changes in levels of the GLI factors.

Increasing the amount of GLI factors in liver cells taken from mice with steatosis reduced the accumulation of lipids and brought lipid metabolism back to its normal balance. A focus of future studies will be to understand how the Hedgehog signaling pathway interacts with other signaling pathways known to regulate liver lipid metabolism, such as insulin signaling. This knowledge will help clinicians to design new treatments for lipid-associated diseases like non-alcoholic fatty liver disease.

DOI:http://dx.doi.org/10.7554/eLife.13308.002

Insulin resistance in the liver: deficiency or excess of insulin?

In insulin-resistant states (obesity, pre-diabetes, and type 2 diabetes), hepatic production of glucose and lipid synthesis are heightened in concert, implying that insulin deficiency and insulin excess coexists in this setting. The fact that insulin may be inadequate or excessive at any one point in differing organs and tissues has many biologic ramifications. In this context the concept of metabolic compartmentalization in the liver is offered herein as one perspective of this paradox. In particular, we focus on the hypothesis that insulin resistance accentuates differences in periportal and perivenous hepatocytes, namely periportal glucose production and perivenous lipid synthesis. Subsequently, excessive production of glucose and accumulation of lipids could be expected in the livers of patients with obesity and insulin resistance. Overall, in this review, we provide our integrative perspective regarding how excessive production of glucose in periportal hepatocytes and accumulation of lipids in perivenous hepatocytes interact in insulin resistant states.

Differentiation and selection of hepatocyte precursors in suspension spheroid culture of transgenic murine embryonic stem cells

Embryonic stem cell-derived hepatocyte precursor cells represent a promising model for clinical transplantations to diseased livers, as well as for establishment of in vitro systems for drug metabolism and toxicology investigations. This study aimed to establish an in vitro culture system for scalable generation of hepatic progenitor cells. We used stable transgenic clones of murine embryonic stem cells possessing a reporter/selection vector, in which the enhanced green fluorescent protein- and puromycin N-acetyltransferase-coding genes are driven by a common alpha-fetoprotein gene promoter. This allowed for "live" monitoring and puromycin selection of the desired differentiating cell type possessing the activated alpha-fetoprotein gene. A rotary culture system was established, sequentially yielding initially partially selected hepatocyte lineage-committed cells, and finally, a highly purified cell population maintained as a dynamic suspension spheroid culture, which progressively developed the hepatic gene expression phenotype. The latter was confirmed by quantitative RT-PCR analysis, which showed a progressive up-regulation of hepatic genes during spheroid culture, indicating development of a mixed hepatocyte precursor-/fetal hepatocyte-like cell population. Adherent spheroids gave rise to advanced differentiated hepatocyte-like cells expressing hepatic proteins such as albumin, alpha-1-antitrypsin, cytokeratin 18, E-cadherin, and liver-specific organic anion transporter 1, as demonstrated by fluorescent immunostaining. A fraction of adherent cells was capable of glycogen storage and of reversible up-take of indocyanine green, demonstrating their hepatocyte-like functionality. Moreover, after transplantation of spheroids into the mouse liver, the spheroid-derived cells integrated into recipient. These results demonstrate that large-scale hepatocyte precursor-/hepatocyte-like cultures can be established for use in clinical trials, as well as in in vitro screening assays.

Regulation of hepatic GLUT8 expression in normal and diabetic models

GLUT8 is a novel glucose transporter protein that is widely distributed in tissues including liver, a central organ of regulation of glucose homeostasis. The purpose of the current study was to investigate expression and regulation of hepatic GLUT8 mRNA and protein. Therefore, Northern and immunoblot analysis, semiquantitative RT-PCR, and immunofluorescence microscopy were performed using mouse livers at different stages of embryonic and postnatal development and in type 1 (streprozotocin treated) and type 2 (GLUT4 heterozygous) diabetes. GLUT8 mRNA and protein expression in embryonic liver was differentially regulated depending on the prenatal and postnatal developmental stage of the mice. Immunofluorescence microscopy of liver from wild-type mice demonstrated the highest levels of GLUT8 protein in perivenous hepatocytes pointing to its role in regulation of glycolytic flux. In diabetic scenarios, GLUT8 mRNA levels were correlated with circulating insulin; specifically, GLUT8 mRNA decreased in a type 1 diabetes model and increased in a type 2 diabetes model, suggesting a regulatory role for insulin in GLUT8 mRNA expression. While up-regulation of GLUT8 protein occurred in both models of diabetes, only in streptozotocin diabetic livers was GLUT8 zonation altered. These data demonstrate that GLUT8 mRNA and protein are differentially regulated in liver in response to physiologic and pathologic (diabetes) milieu and suggests that GLUT8 is intimately linked to glucose homeostasis.

Differential modulation of insulin actions by dexamethasone: studies in primary cultures of adult rat hepatocytes

BACKGROUND/AIMS

Steroid diabetes is associated with hepatic insulin resistance; in hepatic cell models, however, mainly insulin-permissive effects have been described. Here we investigate modulation by dexamethasone of a larger number of insulin actions.

METHODS

Adult rat hepatocytes were cultured+/-dexamethasone for 48 h; insulin actions were studied subsequently.

RESULTS

Stimulation of glycolysis by insulin but not by glucose required culture with dexamethasone. Activation of glycogen synthesis by insulin or glucose was strongly enhanced by dexamethasone, the insulin effects on glycogenolysis and amino acid uptake were not modulated. When dexamethasone was omitted from the culture, insulin was incapable to activate glycogen synthase, inactivate glycogen phosphorylase or elevate the level of fructose 2,6-bisphosphate. Dexamethasone did not alter insulin binding, insulin receptor number or kinase activity, insulin receptor substrate-1 and Akt protein expression/phosphorylation. Insulin-stimulated association of phosphatidylinositol 3-kinase with insulin receptor substrates-1 and -2 was increased with dexamethasone, the increased association with IRS-2 may, at least partially, be explained by higher IRS-2 protein expression.

CONCLUSIONS

The steroid does not cause hepatic resistance in vitro. The differential attenuation under steroid deprivation points to defects in branches of the insulin signal chain and/or loss of hormonal regulation at the level of target enzymes.

Retinoid regulation of the phosphoenolpyruvate carboxykinase gene in liver

The cytosolic PEPCK gene is a model gene for assessing retinoid regulation of liver-specific genes encoding enzymes of carbohydrate metabolism. In vivo, we have demonstrated that the PEPCK gene is inhibited by vitamin A deficiency. Specifically, under conditions of food deprivation, induction of the PEPCK gene is inhibited in the vitamin A deficient mouse. Inhibition of the PEPCK gene by vitamin A deficiency is reversed by all-trans or 9-cis retinoic acid (RA) treatment. In a transgenic mouse model, a -460 and -355 bp PEPCK promoter fragment confers susceptibility to inhibition by vitamin A deficiency and responsiveness to all-trans RA treatment. However, there is a differential effect of 9-cis RA on the PEPCK promoter; the -460 fragment confers responsiveness to 9-cis RA, but the -355 fragment does not. Taken together, these results indicate that the PEPCK retinoic acid response element (RARE)1 is required for 9-cis RA induction-but not all-trans RA induction-of the PEPCK gene. In order to determine if vitamin A deficiency alters specific localized expression of the PEPCK gene in the periportal cells of the liver, the effect of vitamin A status on PEPCK localization in the liver was also measured. The PEPCK transgenes were expressed specifically in the periportal region of the liver acinus and although vitamin A deficiency caused a decrease in PEPCK transgene mRNA levels in periportal cells, it did not alter the periportal cell-specific pattern of expression. Retinoid treatment induced PEPCK transgene mRNA levels in the same population of cells, however, the -355 bp PEPCK promoter fragment did not respond to 9-cis RA treatment. In order to determine the nuclear transcription factor(s) responsible for retinoid regulation of the PEPCK gene in the liver, we investigated retinoic acid receptor (RAR)alpha and beta and the retinoid X receptor (RXR)alpha-the major retinoid receptors in liver-in terms of expression and the ability of the receptors to bind the PEPCK RAREs. Vitamin A deficiency significantly decreased hepatic RAR beta, but not RAR alpha or RXR alpha mRNA levels. In situ hybridization showed that RAR alpha, RAR beta and RXR alpha mRNAs were localized in the periportal region, however, immunohistochemistry showed that RAR alpha and RXR alpha were distributed evenly across the liver acinus, whereas only RAR beta levels were higher in periportal cells. The binding of nuclear receptors to PEPCK RARE1, RARE2 and RARE3 indicates a complex pattern of retinoid receptor and orphan nuclear receptor binding.

Activation of glucokinase gene expression by hepatic nuclear factor 4alpha in primary hepatocytes

Glucokinase (GK) is a key enzyme for glucose utilization in liver and shows a higher expression in the perivenous zone. In primary rat hepatocytes, the GK gene expression was activated by HNF (hepatic nuclear factor)-4alpha via the sequence -52/-39 of the GK promoter. Venous pO2 enhanced HNF-4 levels and HNF-4 binding to the GK-HNF-4 element. Thus, HNF-4alpha could play the role of a regulator for zonated GK expression.

Effects of glucagon on glycogenolysis and gluconeogenesis are region-specific in periportal and perivenous hepatocytes

It has been established, mainly by histochemical and immunohistochemical studies, that liver cells are functionally heterogeneous, with periportal hepatocytes (PPHs) being predominantly gluconeogenic and perivenous hepatocytes (PVHs) being glycolytic. We therefore investigated the region-specific functional effects of glucagon on glycogenolysis and gluconeogenesis in isolated PPHs and PVHs prepared by the digitonin-collagenase method. BB rats, a model of insulin-dependent diabetes, were used to study the region-specific heterogeneity of gluconeogenesis in the diabetic state. Although glycogen content was not different between PVHs and PPHs in rats fed the normal diet, basal glucose release was 1.37 times greater in PVHs than in PPHs (P <.05). The increase in glucose release stimulated by 0.01 to 0.1 nmol/L glucagon was 1.52 times greater in PVHs than in PPHs (P < .05), whereas no differences were seen in response to 1 to 100 nmol/L glucagon. Glucose release from gluconeogenic substrates was 1.57 times greater in the PPHs than in the PVHs of fasted normal rats (P < .05), whereas the increase in gluconeogenesis produced by glucagon was not different between PPHs and PVHs. The glucagon-binding capacity, the cAMP release, and the increase in intracellular Ca2+ stimulated by glucagon were not different between PPHs and PVHs in the fed or fasted states. Gluconeogenesis from gluconeogenic substrates was 1.52 times greater in the PPHs than in the PVHs of fasted nondiabetic BB rats (P < .05). After the development of diabetes, the gluconeogenic capacity in PVHs increased to the level observed in PPHs, but that in PPHs did not change. Thus there was no difference in gluconeogenesis between the PPHs and PVHs of diabetic BB rats. In both the PPHs and PVHs of diabetic BB rats, the 0.01 to 100 nmol/L glucagon-induced increase in gluconeogenesis was greater than that in PPHs from nondiabetic BB rats (2.30 and 3.07 times, P < .01, respectively). We conclude that PPHs and PVHs of normal rat liver express region-specific differences in their glycogenolytic and gluconeogenic responses to glucagon. In diabetic BB rats, the difference in the gluconeogenic capacity between PPHs and PVHs disappeared, whereas glucagon-induced gluconeogenesis was enhanced.

Role of oxygen in the zonation of carbohydrate metabolism and gene expression in liver

Hepatocytes around the afferent (periportal) vessels differ from those around the efferent (perivenous) vessels in their contents of key enzymes, and therefore have different metabolic capacities. Thus, the model of "metabolic zonation" proposes that the periportal cells produce glucose via glycogenolysis and gluconeogenesis and that the perivenous cells utilize glucose via glycogen synthesis and glycolysis. The periportal and perivenous cells receive different signal patterns, because substrates including oxygen and hormones are degraded and products and mediators are formed during passage of blood through the liver. The different signal patterns should be important for both short-term regulation of metabolic rates and for long-term induction and maintenance of the enzyme equipments by control of gene expression. From the periportal to the perivenous zone, the concentration of the signal oxygen falls corresponding to a drop from about 13 (arterial) to 9 (mixed periportal) and then to 4 (hepatovenous) volume% gas atmosphere. For short-term regulation of metabolism, in perivenous-like cells net glucose production measured over a period of two hours was observed below 2%, net glycogen synthesis above 4%, and net lactate utilization above 6% oxygen. In periportal-like cells net glucose formation and net lactate utilization increased sharply from anoxia to 6% oxygen and then only moderately. For long-term regulation of gene expression, the glucagon (cAMP)-dependent activation of the PCK gene was modulated by oxygen. The transcriptional rate, the abundance of mRNA and the enzyme activity were increased to higher levels under arterial rather than under venous oxygen. Conversely, the insulin-dependent activation of the glucokinase gene was negatively modulated by oxygen. A heme protein appeared to be involved in oxygen sensing, since CO mimicked the effects of oxygen on the PCK gene. Hydrogen peroxide was produced by hepatocytes as a function of oxygen tension; exogenously added, it mimicked the effects of oxygen on PCK gene induction. Therefore, the heme protein containing an oxygen sensor could be a peroxide producing oxidase. It is not known at present whether the same oxygen sensor is also involved in the short-term regulation by oxygen of hepatic carbohydrate metabolism. Transfection of PCK promoter-CAT gene constructs into primary hepatocytes showed that oxygen modulated PCK gene activation in the region of -277/+73. This modulation was not mediated by isolated cAMP responsive elements.

The contribution of glucose cycling to the maintenance of steady-state levels of lactate by hepatocytes during glycolysis and gluconeogenesis

When hepatocytes from fasted rats were incubated with 10 mM glucose, there was a linear accumulation of lactate and pyruvate for about 80 min after which steady-state concentrations of these metabolites became established. The rate of glycolysis, determined with [6-3H]glucose, was constant over the entire incubation period and was 50% greater than that calculated from carbon balance studies. This suggests that one-third of the glycolytic products formed were recycled to glucose. To enable study of the factors associated with the generation and maintenance of the lactate steady state and to measure accurately the carbon balance, incubations were performed using supraphysiological concentrations of glucose (20-80 mM). Under these conditions the initial rate of lactate accumulation and its concentration at steady state were shown to be dependent on the concentration of extracellular glucose. Rates of glycolysis were also measured using 40 mM [6-3H]glucose and [U-14C]glucose added alone, or in combination with a steady-state lactate concentration (3 mM). There was no effect on the rate of glycolysis determine with [6-3H]glucose, even when lactate was present in the medium. The difference in rates between measurements with the two isotopes reflect the apparent degree of glucose recycling which in the absence and presence of added lactate increased from 0.26 to 0.54 mumol C3 equivalents min-1.g-1 respectively. Identical studies employing [U-14C]lactate showed that glucose and CO2 were the major products of lactate metabolism under steady-state conditions and that the formation of lactate from [U-14C]glucose exactly balanced the rate of lactate removal as a result of oxidation and gluconeogenesis. These studies provide evidence for the concomitant operation of glycolysis and gluconeogenesis, even in the presence of high glucose concentrations. They also demonstrate that lactate steady states are achieved not by the cessation of glycolysis but rather by the removal of lactate and pyruvate at a rate equal to that of their production.

Cultured rat hepatocytes adapt their cellular glycolytic activity and adenylate energy status to tissue oxygen tension: influences of extracellular matrix components, insulin and glucagon

The influence of extracellular matrix components, insulin, and glucagon on the cellular response to periportal- or pericentral-equivalent tissue oxygen tension was investigated in freshly isolated rat hepatocytes cultured at 13% O2 or 4% O2 in Teflon membrane dishes. With extended culture time, significant increases in lactate release and cellular lactate content were observed in cultures at 4% O2 compared with 13% O2. This shift toward glycolysis was detectable when hepatocytes were cultured on dishes coated with rat liver crude membrane fraction (CMF/COL) but not in collagen type I-coated dishes. This indicates that extracellular matrix components are involved in the process of adaptation. ATP and total adenylate content in cells cultured at 4% O2 were up to 40% lower than in cells cultured at 13% O2. However, the adenylate energy charge was not affected, suggesting that an adequate energy supply was maintained also in hepatocytes cultured at pericentral-equivalent oxygen tension. This adaptation was reversible. When hepatocytes were transferred either from 4% to 13% O2 or from 13% to 4% O2, they adapted the corresponding metabolic profile to the new oxygen tension within 2 days. This demonstrates that hepatocytes are not fully unidirectionally programmed. The modulation of the glycolytic activity by insulin and glucagon was effective in cultures at pericentral-equivalent oxygen tension (4% O2) only. Insulin (0.1-100 nM) shifted cellular metabolism toward the glycolytic pathway and glucagon (1-100 nM) counteracted the effect of insulin in a dose-dependent manner. Clearly, oxygen tension is the principal regulator in the hepatic glycolytic activity, whereas the hormones (insulin and glucagon) act as secondary modulators.

Physiological oxygen tension modulates the chemically induced mitogenic response of cultured rat hepatocytes

Freshly isolated rat hepatocytes were cultured at periportal- (13% O2) or perivenous-like (4% O2) oxygen tension and exposed to subtoxic exposure levels of cyproterone acetate (CPA: 10-330 microM), phenobarbital (PB: 0.75-6 mM), and dimethylsulfoxide (DMSO: 0.1-3.3%) from 24-72 h after seeding. Induced alterations in ploidy, in the number of S-phase cells, the degree of binuclearity, and cellular protein content were determined by twin parameter protein/DNA flow cytometry analysis of intact cells and isolated nuclei. CPA and PB increased whereas DMSO decreased dose dependently the total number of S-phase cells. The changes differed within individual ploidy classes and were modulated by the oxygen tension. CPA increased and DMSO decreased the number of S-phase cells preferentially among the diploid hepatocytes at periportal-like oxygen tension. In contrast, PB increased binuclearity and S-phase cells mainly among the tetraploid hepatocytes at perivenous-like oxygen tension. Cellular protein content increased dose dependently after exposure to the hepatomitogens (CPA, PB) and decreased after exposure to DMSO at both oxygen tensions. Comparison with in vitro data proves that chemicals which interact with cells from the progenitor liver compartment (CPA, DMSO) exert their mitogenic activity best in cultures at periportal-like oxygen tension preferentially in diploid hepatocytes, whereas chemicals which affect cells from the functional compartment show a higher activity at perivenous-like oxygen tension. Physiological oxygen tension seems to be an effective modulator of the proliferative response of cultured rat hepatocytes similar to that expected for periportally or perivenously derived hepatocytes.

Turnover and transformation of mitochondrial acetyl-CoA acetyltransferase into CoA-modified forms

Rat liver mitochondrial acetyl-CoA acetyltransferase (acetoacetyl-CoA thiolase, EC 2.3.1.9) exists additionally in the CoA-modified forms A1 and A2. After a pulse of radioactivity using [35S]methionine in hepatocytes, the highest radioactivity was obtained in the unmodified enzyme. Over the chase time, the radioactivity in the unmodified enzyme decreased, but simultaneously increased in both CoA-modified forms, thus proving that the fully active unmodified enzyme exists before the partially active modified forms A1 and A2. Also, the specific radioactivity (ratio % radioactivity/% immunoreactive area) of A1 > A2 demonstrates a sequential CoA modification of form A1 to form A2. Acetyl-CoA acetyltransferase was degraded with an apparent half-life of 38.0 h: the modified forms A1 and A2 have half-lives of 24.5 and 7.2 h. The physiological meaning of the CoA modification of acetyl-CoA acetyltransferase is not yet understood.

Insulin-mimetic actions of phorbol ester in cultured adult rat hepatocytes. Lack of phorbol-ester-elicited inhibition of the insulin signal

The actions of the phorbol ester phorbol 12-myristate 13-acetate (PMA) on glucose metabolism, amino acid transport and enzyme inductions were studied in primary cultures of adult-rat hepatocytes and compared with the effects of insulin. PMA and insulin stimulated glycolysis 5- and 7-fold respectively. The half-maximal effective dose of PMA was 60 nM. Stimulation of glycolysis was accompanied by an insulin- or PMA-dependent and okadaic acid-sensitive activation of 6-phosphofructo-2-kinase and pyruvate kinase, as well as by an increase in fructose 2,6-bisphosphate. Glucose production from glycogen was decreased to 50% by PMA and to 15% by insulin, whereas glycogen synthesis was stimulated 2- and 7-fold respectively. PMA also increased aminoisobutyrate uptake, induced ornithine decarboxylase and counteracted the glucagon-dependent induction of phosphoenolpyruvate carboxykinase. PMA strongly antagonized the hormonal activation of glycogen synthesis, but all other insulin actions assayed were not decreased by the phorbol ester. Whereas additive effects of PMA and insulin were not detected, PMA and a simultaneous increase in the glucose concentration had additive effects on glycolysis and glycogen metabolism. Cell exposure to insulin resulted in receptor autophosphorylation and a more than 10-fold activation of the receptor tyrosine kinase. PMA did not alter these effects, and also had no effect on the receptor phosphorylation status in the absence of insulin. Long-term (15 h) pretreatment of the cells with PMA abolished all PMA effects, but not the insulin effects. It is concluded that PMA does not generally antagonize the action of insulin in differentiated adult hepatocytes, and that insulin and PMA may use related signal-transduction pathways.

Evidence for dissociation of gluconeogenesis stimulated by non-esterified fatty acids and changes in fructose 2,6-bisphosphate in cultured rat hepatocytes

In order to examine the role of fructose 2,6-bisphosphate (Fru-2,6-P2) in non-esterified-fatty-acid-stimulated gluconeogenesis, Fru-2,6-P2 levels were measured in cultured rat hepatocytes under conditions mimicking the fasted state. After addition of either 1.5 mM-palmitate or 10 nM-glucagon, [U-14C]lactate incorporation into glucose increased 2-fold, but only glucagon suppressed Fru-2,6-P2. Prevention of palmitate oxidation with a carnitine palmitoyltransferase-I inhibitor (2-bromopalmitate) diminished glucose production and Fru-2,6-P2 levels. Addition of exogenous glucose to the media increased Fru-2,6-P2 in a dose-related manner, which was further augmented by addition of palmitate. When Fru-2,6-P2 levels were examined in cells cultured under conditions mimicking the fed state (significantly higher basal Fru-2,6-P2 levels and lower glucose production), palmitate oxidation was associated with a significant fall in Fru-2,6-P2. In conclusion, the present studies have demonstrated a dissociation between fatty-acid-stimulated gluconeogenesis and changes in Fru-2,6-P2 in cultured rat hepatocytes. Further experiments suggest that the accumulation of intracellular hexose 6-phosphate as a result of fatty-acid-stimulated gluconeogenesis masks a putative inhibitory effect of fatty acids on Fru-2,6-P2 concentrations.

A perifusion system for cultured hepatocytes

A perifusion system for primary cultures of hepatocytes is described. The system accommodates 20 rotated petri dishes (60 mm) and allows individual medium composition and sampling for each dish. Cell number and insulin (15 pM to 7.7 nM) were stable in the system for at least 24 h. The dose-response relationship for induction by insulin of glucokinase and pyruvate kinase was shifted to the left by a factor of 9 and 5, respectively, as compared to conventional, stationary cultures. The system is useful for studies at low and/or constant concentrations of substrates, hormones, growth factors, etc., with monolayers of cells having a high metabolic capacity.

Metabolic actions of insulin-like growth factor II in cultured adult rat hepatocytes are not mediated through the insulin-like growth factor II receptor

Short- and long-term regulation of hepatic carbohydrate metabolism by insulin-like growth factor II was studied in primary cultures of adult rat hepatocytes and compared to the metabolic potency of insulin. Insulin-like growth factor II stimulated glycogen synthesis from [14C]glucose, uptake of [3H]aminoisobutyric acid and [14C]lactate formation from [14C]glucose up to three-fold. Basal glycogenolysis was inhibited to about 10%, and glucagon-activated glycogenolysis was blocked completely. The enzymatic activity of glucokinase and pyruvate kinase was induced two-fold, the glucagon-dependent induction of phosphoenolpyruvate carboxykinase was antagonized. Compared to insulin, half-maximal responses required up to 50 times higher insulin-like growth factor II concentrations ranging from 10-20 nmol/l. A similar difference was observed for binding affinity of insulin-like growth factor II to the insulin receptor. The interaction with the insulin-like growth factor II/mannose 6-phosphate (IGF-II/Man-6-P) receptor was examined by studying 125I-insulin-like growth factor II binding and uptake of lysosomal enzymes. The affinity of insulin-like growth factor II to the IGF-II/Man-6-P receptor was considerably higher than for the insulin receptor. Antibodies against the IGF-II/Man-6-P receptor did not affect metabolic responses to insulin-like growth factor II, while binding to its receptor and the receptor-mediated endocytosis of arylsulphatase A were strongly inhibited. Thus, in adult rat liver insulin-like growth factor II appeared to exert metabolic actions not via interaction with its own receptor but through low affinity binding to hepatic insulin receptors.

Metabolic zonation of the liver: regulation and implications for liver function

Liver parenchyma shows a remarkable heterogeneity of the hepatocytes along the porto-central axis with respect to ultrastructure and enzyme activities resulting in different cellular functions within different zones of the liver lobuli. According to the concept of metabolic zonation, the spatial organization of the various metabolic pathways and functions forms the basis for the efficient adaptation of liver metabolism to the different nutritional requirements of the whole organism in different metabolic states. The present review summarizes current knowledge about this heterogeneity, its development and determination, as well as about its significance for the understanding of all aspects of liver function and pathology, especially of intermediary metabolism, biotransformation of drugs and zonal toxicity of hepatotoxins.

Modulation by oxygen of the glucagon-dependent activation of the phosphoenolpyruvate carboxykinase gene in rat hepatocyte cultures

In liver phosphoenolpyruvate carboxykinase (PCK) activity, protein and mRNA are localized predominantly in the periportal zone. The activation of the PCK gene by glucagon was studied in primary rat hepatocyte cultures under physiological arterial and venous oxygen tensions [16% and 8% (by vol.)]. PCK gene expression was monitored on the level of transcription, mRNA abundance and enzyme activity as well as enzyme synthesis and degradation. 1. Transcription of the PCK gene was increased by 10 nM glucagon maximally after 0.5 h; it reached nearly basal levels again after 2 h. The increase in transcription was 45% lower under 8% oxygen than under 16% oxygen. 2. PCK mRNA was maximally increased after 2 h under 16% oxygen and after 4 h under 8% oxygen; it subsequently declined to twice the basal values after 8 h. The maximal increase after 2 h was 50% lower under 8% oxygen than under 16% oxygen. 3. PCK enzyme activity was maximally increased after 4-6 h. The maximal enhancement after 4 h was 50% lower under 8% oxygen than under 16% oxygen. 4. The increase in PCK enzyme activity was due to an enhanced synthesis rate of PCK protein. The rate increased after 3 h was 35% lower under 8% oxygen than under 16% oxygen. 5. The degradation of PCK protein was equal under both oxygen tensions. The results show that in cultured rat hepatocytes the induction of PCK gene expression is modulated by physiological concentrations of oxygen. The modulation occurred at the level of gene transcription, mRNA abundance, enzyme protein synthesis and enzyme activity. The periportal to perivenous oxygen gradient could be the major factor responsible for the predominant expression of the PCK gene in the periportal zone.

Metabolic actions of insulin-like growth factor-I in cultured hepatocytes from adult rats

Short-term and long-term regulation of hepatic carbohydrate metabolism by insulinlike growth factor-I was studied in primary cultures of adult rat hepatocytes and compared with the metabolic potency of insulin. Insulinlike growth factor-I stimulated the formation of [14C]lactate from [14C]glucose up to three-fold with a half-maximally effective concentration of approximately 50 nmol/L. Basal glycogenolysis was inhibited by about 20%, and glucagon-activated glycogenolysis was blocked completely by insulinlike growth factor-I with half-maximally effective concentrations of about 1.5 to 2 nmol/L. The activity of the key glycolytic enzymes glucokinase and pyruvate kinase were induced twofold. The glucagon-dependent induction of phosphoenolpyruvate carboxykinase--the key gluconeogenic enzyme--was antagonized with a half-maximally effective concentration of about 5 nmol/L. This inhibition of the glucagon-dependent induction of the enzyme was accompanied by a similar reduction of the increase in phosphoenolpyruvate carboxykinase-mRNA level as assessed by Northern blot analysis. The potency of insulinlike growth factor-I at half-maximally effective concentrations was approximately 2% to 4% that of insulin. Because binding studies demonstrated a comparably low affinity of insulinlike growth factor-I to the insulin receptor, it is suggested that in adult liver--in contrast to fetal and regenerating liver--insulinlike growth factor-I could exert short-term and long-term metabolic effects on parenchymal cells only through interaction with the insulin receptor.

Stimulation of glucose production from glycogen by glucagon, noradrenaline and non-degradable adenosine analogues is counteracted by adenosine and ATP in cultured rat hepatocytes

The glycogenolytic potency of adenosine and ATP was studied in adult rat hepatocytes and compared with the action of glucagon and noradrenaline. In cells cultured for 48 h, adenosine and ATP as well as their analogues 2-chloroadenosine, phenylisopropyladenosine, N-ethylcarboxamidoadenosine and beta-gamma-methylene-substituted ATP (p[CH2]ppA) increased glycogen phosphorylase alpha to levels indistinguishable from those obtained by the addition of glucagon or noradrenaline. The P1 receptor antagonist 8-phenyltheophylline abolished the activation of phosphorylase by adenosine and by p[CH2]ppA, but not that by ATP. Protein kinase A was activated by p[CH2]ppA and ATP via their breakdown to adenosine. [14C]Glucose production from glycogen was stimulated only 3-fold by ATP and adenosine, compared with a 7-fold increase produced by the hormones. Stimulation of glucose production by glucagon or noradrenaline was almost completely abolished by ATP or adenosine, with half-maximal effects at around 10 microM. The non-degradable adenosine analogues were equipotent with glucagon with respect to stimulation of glucose production, and their action was also inhibited by adenosine. ATP and p[CH2]ppA, which were both degraded to adenosine, showed comparable metabolic effects, whereas the alpha, beta-methylene analogue was without biological action and also was not degraded to adenosine. In the presence of the adenosine transport inhibitor nitrobenzyl thioinosine (NBTI), adenosine exerted an increased glycogenolytic potency, reaching 80% of the maximal stimulation obtained by glucagon. The glucagon-antagonistic effect of adenosine could be completely abolished by NBTI, but was not affected by phenyltheophylline. It is concluded that, in the hepatocyte culture system, adenosine and ATP decrease the catalytic efficiency of phosphorylase alpha through signals arising from their uptake into the cell.

In vitro studies on the action of CS-045, a new antidiabetic agent

The mechanism of action of CS-045, a new orally active antidiabetic agent, was studied in vitro using cultured hepatoma cells (Hep G2) and muscle cells (BC3H-1). Treatment of both types of cultured cells with varying doses of CS-045 did not significantly alter insulin receptor binding. Basal and insulin-stimulated glucose transport in BC3H-1 cells was also unaltered by the drug. In contrast, CS-045 increased glycogen synthase I activity in both cell types. This effect was maximal after 24 hours and in Hep G2 cells was associated with a threefold increase in the apparent affinity of the enzyme for glucose-6-phosphate. Gluconeogenesis from lactate in Hep G2 cells was greatly reduced by CS-045 treatment. We conclude that CS-045 may act directly on muscle and liver cells to increase glucose utilization. It is also effective in reducing glucose production. These multiple effects may account in part for the ability of CS-045 to reduce blood sugar levels in vivo.

Restricted expression of the erythroid/brain glucose transporter isoform to perivenous hepatocytes in rats. Modulation by glucose

The "erythroid/brain" glucose transporter (GT) isoform is expressed only in a subset of hepatocytes, those forming the first row around the terminal hepatic venules, while the "liver" GT is expressed in all hepatocytes. After 3 d of starvation, a three- to fourfold elevation of expression of the erythroid/brain GT mRNA and protein is detected in the liver as a whole; this correlates with the expression of this GT in more hepatocytes, those forming the first three to four rows around the hepatic venules. Starvation-dependent expression of the erythroid/brain GT on the plasma membrane of these additional hepatocytes is lost within 3 h of glucose refeeding; however, by immunoblotting we show that the protein is still present. Its loss from the surface is possibly explained by internalization.

Depressed gluconeogenesis and ureogenesis in isolated hepatocytes after intermittent hypoxia in rats

1. Rats were exposed to hypobaric hypoxia (equivalent altitude 4500 m), 2 x 2 hr per day, for 5 days. Isolated hepatocytes were prepared on day 6 after 18 hr of fast and also from control normoxic animals. The hepatocytes were incubated (120 min) with various substrates. 2. ATP contents were lower in hepatocytes from exposed as compared to control animals whether at the beginning (14%) or at the end (-6 to -33%) of incubation depending on the substrate. 3. Gluconeogenesis from all precursors (lactate, alanine, pyruvate, glutamine) was significantly reduced (40-50%) in exposed as compared to control animals. 4. Ureogenesis from alanine and from pyruvate + NH4Cl was also markedly depressed in exposed animals but no differences were noticed with glutamine or lactate + NH4Cl and alanine + NH4Cl. 5. Results are discussed in relation to known effects of acute and chronic hypoxia, interrelationship between gluconeogenesis and ureogenesis, taking into account the inhomogeneity of liver and the metabolic properties of periportal and perivenous hepatocytes.

Central nervous system control of glycogenolysis and gluconeogenesis in fed and fasted rat liver

The influence of brain cholinergic activation on hepatic glycogenolysis and gluconeogenesis was studied in fed and 48-hour fasted rats. Neostigmine was injected into the third cerebral ventricle and hepatic venous plasma glucose, glucagon, insulin, and epinephrine were measured. The activity of hepatic phosphorylase-a and phosphoenolpyruvate-carboxykinase (PEP-CK) was also measured. Experimental groups: 1, intact rats; 2, rats infused with somatostatin through the femoral vein; 3, bilateral adrenodemedullated (ADMX) rats; 4, somatostatin infused ADMX rats; 5, 5-methoxyindole-2-carboxylic acid (MICA) was injected intraperitoneally 30 minutes before injection of neostigmine into the third cerebral ventricle of intact rats. MICA treatment completely suppressed the increase in hepatic glucose in fasted rats, but had no effect in fed rats. Phosphorylase-a activity was not changed in fasted rats, but increased in fed rats, intact rats, somatostatin-infused rats, somatostatin-infused ADMX rats, and ADMX rats in that order. PEP-CK was not changed in fed rats, but increased at 60 and 120 minutes after neostigmine injection into the third cerebral ventricle in fasted rats. We conclude that, in fed states, brain cholinergic activation causes glycogenolysis by epinephrine, glucagon, and direct neural innervation. In fasted states, on the other hand, gluconeogenesis is dependent on epinephrine alone to increase hepatic glucose output.

Biological activity of des-(B26-B30)-insulinamide and related analogues in rat hepatocyte cultures

Short-term and long-term biological activities were studied in adult rat hepatocytes cultured in the presence of the insulin analogues des-(B26-B30)-insulinamide, [TyrB25]des-(B26-B30)-insulinamide and [HisB25]des-(B26-B30)-insulinamide. When compared to insulin, full potency of des-(B26-B30)-insulinamide has been reported in rat adipocytes and an enhanced potency has been reported for the other analogues. Steady state binding characteristics of the analogues to hepatocytes were indistinguishable from those of native insulin with half-maximal binding occurring at concentrations of about 0.8 nmol/l. Half-maximal effects for the stimulation of glycolysis and inhibition of basal and glucagon-activated glycogenolysis required identical concentrations for insulin and all 3 analogues. Induction of the key glycolytic enzymes glucokinase and pyruvate kinase as well as the inhibition of glucagon-dependent induction of phosphenolpyruvate carboxy-kinase also required identical concentrations of insulin and the 3 analogues. These data confirm that in cultured hepatocytes the C-terminal amidation of des-(B26-B30)-insulin results in a molecule with full in vitro potency. In contrast to data obtained in adipocytes, the des-(B26-B30)-insulin-amidated analogues with tyrosine or histidine substitutions at position B25 are equally as potent as native insulin in eliciting biological responses in rat hepatocyte culture.

Zonation of hepatic lipogenic enzymes identified by dual-digitonin-pulse perfusion

The zonal distribution within rat liver of acetyl-CoA carboxylase, ATP citrate-lyase and fatty acid synthase, the principal enzymes of fatty acid synthesis, was investigated by using dual-digitonin-pulse perfusion. Analysis of enzyme mass by immunoblotting revealed that, in normally feeding male rats, the periportal/perivenous ratio of acetyl-CoA carboxylase mass was 1.9. The periportal/perivenous ratio of ATP citrate-lyase mass was 1.4, and fatty acid synthase exhibited the largest periportal/perivenous mass gradient, having a ratio of 3.1. This pattern of enzyme distribution was observed in male rats only; in females, the periportal/perivenous ratio of enzyme mass was nearly equal. The periportal/perivenous gradients for acetyl-CoA carboxylase, ATP citrate-lyase and fatty acid synthase observed in fed (and fasted) males were abolished when animals were fasted (48 h) and refed (30 h) with a high-carbohydrate/low-fat diet. As determined by enzyme assay of eluates obtained from the livers of normally feeding male rats, there is also periportal zonation of acetyl-CoA carboxylase activity, expressed either as units per mg of eluted protein or units per mg of acetyl-CoA carboxylase protein, suggesting the existence of gradients in both enzyme mass and specific activity. From these results, we conclude that the enzymes of fatty acid synthesis are zonated periportally in the liver of the normally feeding male rat.

Adult rat hepatocyte microcarrier culture. Comparison to the conventional dish culture system

A technique has been devised to attach adult rat hepatocytes to collagen-coated dextran microcarriers. Cells were cultured serum-free for 2 d and their viability, enzyme activities, glucose metabolism, and hormone responsiveness were compared to data obtained from conventional dish cell culture. The two different culture methods showed no difference in cell viability and morphology. Microcarrier-cultured cells exhibited hormone responsiveness comparable to dish cultures; glycolysis could be activated three-fold by the sole addition of insulin, and gluconeogenesis was increased by 40 to 50% by glucagon. During the 48-h culture glucokinase and phosphoenolpyruvate carboxykinase activities declined at a similar rate in both culture systems. Long-term culture with 0.1 microM insulin prevented the decrease of glucokinase activity. Insulin responsiveness (activation of glycolysis) was still pronounced after 48 h in culture. The microcarrier technique establishes a new in vitro liver system in which acute and long-term hormonal actions can be investigated using the technical advantages of a suspension culture.

Activation of phosphofructokinase 2 by insulin in cultured hepatocytes without accompanying changes of effector levels or cAMP-stimulated protein kinase activity ratios

Activation of glycolysis by insulin in cultured adult rat hepatocytes is accompanied by an activation of phosphofructokinase 2 (PFK 2). PFK 2 activation might be caused by insulin-dependent changes of (a) metabolite levels, (b) basal and (c) Br8cAMP-stimulated cAMP-dependent protein kinase activity; this problem was investigated. 1. Cells cultured with 0.1 nM insulin for 48 h exhibited a low glycolytic rate and low fructose 2,6-bisphosphate [Fru(2,6)P2] levels. Addition of insulin increased Fru(2,6)P2 and Fru(1,6)P2 levels sequentially which points to PFK 2 as first target enzyme of insulin action. 2. Concentrations of Glc6P, Fru6P, phosphoenolpyruvate, glycerol 3-phosphate and citrate, which modulate PFK 2/fructose 2,6-bisphosphatase 2 activity, were not altered by insulin. 3. Activation of PFK 2 by insulin occurred without changes in the levels of total and protein-bound cAMP. Bound cAMP amounted to about 14% of total cAMP. 4. Insulin neither decreased the basal dissociation state of the cAMP-dependent protein kinase nor lowered the sensitivity of the kinase towards cAMP in cell extracts. 5. Addition of the phosphodiesterase-resistant Br8cAMP to the cultures increased cAMP levels 3-4-fold, elevated the protein kinase activity ratio from 0.14 to 0.6 and decreased the Fru(2,6)P2 level and the rate of glycolysis. When Br8cAMP and insulin were given together, insulin was capable of counteracting Br8cAMP in that it activated glycolysis and PFK 2 and elevated the Fru(2,6)P2 level; however, it did not decrease the elevated protein kinase activity ratio. It is concluded that insulin presumably does not activate PFK 2 through changes in cAMP and effector levels or through inhibition of cAMP-dependent protein kinase dissociation. The data support the hypothesis that insulin may act via activation of PFK 2 phosphatase.

Active glycolysis and glycogenolysis in early stages of primary cultured hepatocytes. Role of AMP and fructose 2,6-bisphosphate

This study examines the factors involved in the rapid glycolysis and glycogenolysis that occur during the first stages of hepatocyte culture: a) Shortly after seeding glycolysis, estimated as lactate released to culture medium, increased 10 times in comparison to that reported in vivo. By 8 to 9 h of culture, hepatocytes were nearly glycogen-depleted even in the presence of insulin. b) 6-Phosphofructo-2-kinase remained 100% active during this period. The proportion of the initial active phosphorylase (87%) decreased to 57% by 7 h of culture. c) Fructose 2,6-bisphosphate content was initially similar to that found in liver of fed animals, decreased after seeding and increased thereafter up to four times the initial concentration. In spite of changes in the concentration of this activator, the glycolytic rate remained high and constant. d) ADP and AMP increased sharply after cell plating, reaching values 1.7 and 3.5 times higher. The rise in AMP levels may be involved in the activation of glycolysis and glycogenolysis, because this metabolite is known to act as an allosteric activator of phosphofructokinase and glycogen phosphorylase. This metabolic situation resembles that of cells under hypoxia.

Compartmentation of glucose 6-phosphate in hepatocytes

Rat hepatocytes were incubated with 14C-labelled hexoses, and the specific radioactivities of glucose 6-phosphate, glucose 1-phosphate and fructose 6-phosphate were determined. (1) When suspensions of freshly isolated hepatocytes were incubated with [14C]glucose, the specific radioactivities of glucose 1-phosphate and fructose 6-phosphate were severalfold higher than that of glucose 6-phosphate. The ratios of the specific radioactivities decreased with time of incubation. These relationships were also found when incubations were carried out with primary cultures of rat hepatocytes or with crude homogenates of hepatocytes, but not with isolated nuclei. (2) When cells were incubated with [14C]fructose, the ratios of the specific radioactivities were higher than with [14C]glucose, and also decreased with time. (3) Paired incubations were carried out with a mixture of galactose and fructose, with one or other sugar being labelled with 14C. The specific radioactivity of glucose released into the medium was greater than that of glucose 6-phosphate when fructose was labelled, but not when galactose was labelled. Furthermore, glucose 6-phosphate and glucose in the medium differed with regard to the distribution of 14C between C-1 and C-6. These results are interpreted as evidence that glucose 6-phosphate in hepatocytes does not exist as a homogeneous pool, but that subcompartments exist which are associated with glucose phosphorylation, gluconeogenesis and glycogenolysis.

Sub-compartmentation of the 'cytosolic' glucose 6-phosphate pool in cultured rat hepatocytes

[14C]Glucose release either from endogenous 14C-prelabelled glycogen or from added 14C-labelled glucose 6-phosphate was measured in filipin-treated, permeabilized hepatocytes in 48 h culture. [14C]Glucose output from prelabelled glycogen was not altered by the addition of 5 mM glucose 6-phosphate to the incubation medium. Conversely, [14C]glucose release from 5 mM labelled glucose 6-phosphate was not influenced by different glycogen concentrations in the cells. Moreover, in the permeabilized cells the anion transport inhibitor DIDS (4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid) inhibited only the liberation of [14C]glucose from labelled glucose 6-phosphate but not from glycogen. It is therefore concluded that there exist at least 2 separate, mutually non-accessible glucose 6-phosphate pools in cultured rat hepatocytes, one linked to glycogenolysis and the other to gluconeogenesis.

Antagonistic regulation of the glucose/glucose 6-phosphate cycle by insulin and glucagon in cultured hepatocytes

Flux through the glucose/glucose 6-phosphate cycle in cultured hepatocytes was measured with radiochemical techniques. Utilization of [2-3H]glucose was taken as a measure of glucokinase flux. Liberation of [14C]glucose from [U-14C]glycogen and from [U-14C]lactate, as well as the difference between the utilization of [2-3H]glucose and of [U-14C]glucose, were taken as measures of glucose-6-phosphatase flux. At constant 5 mM-glucose and 2 mM-lactate concentrations insulin increased glucokinase flux by 35%; it decreased glucose-6-phosphatase flux from glycogen by 50%, from lactate by 15% and reverse flux from external glucose by 65%, i.e. overall by 40%. Glucagon had essentially no effect on glucokinase flux; it enhanced glucose-6-phosphatase flux from glycogen by 700%, from lactate by 45% and reverse flux from external glucose by 20%, i.e. overall by 110%. At constant glucose concentrations cellular glucose 6-phosphate concentrations were essentially not altered by insulin, but were increased by glucagon by 230%. In conclusion, under basic conditions without added hormones the glucose/glucose 6-phosphate cycle showed only a minor net glucose uptake, of 0.03 mumol/min per g of hepatocytes; this flux was increased by insulin to a net glucose uptake of 0.21 mumol/min per g and reversed by glucagon to a net glucose release of 0.22 mumol/min per g. Since the glucose 6-phosphate concentrations after hormone treatment did not correlate with the glucose-6-phosphatase flux, it is suggested that the hormones influenced the enzyme activity directly.

Gluconeogenic-glycolytic capacities and metabolic zonation in liver of rats with streptozotocin, non-ketotic as compared to alloxan, ketotic diabetes

Activities (mumol X min-1 X g liver) and zonal distributions of key enzymes of carbohydrate metabolism were studied in livers of streptozotocin-diabetic rats and compared to the values in alloxan-diabetes. Streptozotocin led to a non-ketotic diabetes with blood glucose being increased by more than fivefold but ketone bodies being in the normal range, while alloxan produced a ketotic diabetes with blood glucose, acetoacetate and beta-hydroxybutyrate being elevated by more than fivefold. Portal insulin was decreased to about 20% in streptozotocin- and more drastically to about 7% in alloxan-diabetes. Conversely, portal glucagon was increased in the two states to about 250% and 180%, respectively. The glucogenic key enzyme phosphoenolpyruvate carboxykinase (PEPCK) was enhanced in streptozotocin- and alloxan-diabetes to over 300%, while the glycolytic pyruvate kinase L (PKL) was lowered to 65% and 80%, respectively. The normal periportal to perivenous gradient of PEPCK of about 3:1, as measured in microdissected tissue samples, was maintained with elevated activities in the two zones. The normal periportal to perivenous gradient of PKL of 1:1.7 was diminished with lowered activities in the two zones. The glucogenic glucose-6-phosphatase (G6Pase) was increased in streptozotocin- and alloxan-diabetes to 130% and 140%, respectively, while the glucose utilizing glucokinase (GK) was decreased to 60% and 50%, respectively. The normal periportal to perivenous gradient of G6Pase, demonstrated histochemically, remained unaffected. Carnitine palmitoyltransferase (CPT) was increased to over 190% and acetyl-CoA carboxylase (ACC) was decreased to 60% in streptozotocin, non-ketotic diabetes, while the two enzymes were altered more drastically to 400% and 50%, respectively, in alloxan, ketotic diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of glycolysis by insulin with a sequential increase of the 6-phosphofructo-2-kinase activity, fructose-2,6-bisphosphate level and pyruvate kinase activity in cultured rat hepatocytes

The involvement of 6-phosphofructo-2-kinase, fructose 2,6-bisphosphate [Fru(2,6)P2] and pyruvate kinase in the insulin-dependent short-term activation of glycolysis was studied in primary cultures of rat hepatocytes. The short-term influence of insulin on these parameters was dependent on the insulin concentration used for the long-term culture. Cells were cultured either with 10 nM or 0.1 nM insulin for 48 h, and are referred to as 'insulin cells' and 'control cells', respectively. Insulin cells exhibited a high level of Fru(2,6)P2. Addition of insulin to insulin cells led to an immediate stimulation of glycolysis (two-fold) and activation of pyruvate kinase. The concentration of Fru(2,6)P2 and activity of 6-phosphofructo-2-kinase remained constant. Control cells exhibited a very low level of Fru(2,6)P2 and low activity of 6-phosphofructo-2-kinase directly after the medium change. However, both parameters increased during a 1-2-h incubation in the absence of insulin. Although the level of Fru(2,6)P2 thus changed up to tenfold the glycolytic rate remained at a constant value. Addition of insulin to control cells led to a 5-8-fold stimulation of glycolysis but only after a 30-90-min lag phase. During this lag period insulin strongly increased sequentially the 6-phosphofructo-2-kinase, the level of Fru(2,6)P2 and the pyruvate kinase activity. The activation of the latter enzyme slightly preceded the onset of the insulin-stimulated glycolysis. Addition of insulin to control cells, which were preincubated for 3 h in the absence of insulin and in which the Fru(2,6)P2 level had risen insulin-independently, led to an immediate increase in glycolysis without a lag phase. It is concluded that in this insulin-sensitive cell system: the changes of glycolytic flux did not correlate with changes in the level of total Fru(2,6)P2 either in insulin or in control cells; an increase in the Fru(2,6)P2 concentration was not obligatory for the insulin-dependent stimulation of glycolysis in insulin cells; activation of pyruvate kinase and thus glycolysis by insulin did not proceed unless the Fru(2,6)P2 level had been elevated above a threshold level. The lack of correlation between total Fru(2,6)P2 levels and the glycolytic flux and the apparent existence of a threshold concentration for Fru(2,6)P2 suggest a permissive action for this effector in enzyme interconversion.

Long-term effects of physiological oxygen concentrations on glycolysis and gluconeogenesis in hepatocyte cultures

Primary cultures of adult rat hepatocytes were kept for 46 h with either insulin ('insulin cells') or glucagon ('glucagon cells') as the dominant hormone under different oxygen concentrations with 13% (v/v) O2 mimicking arterial and 4% hepatovenous levels. Thereafter metabolic rates were measured for a 2 h period under the same ('overall long-term O2 effects') or a different ('short-term O2 effects') oxygen concentration. From the differences of the two effects the 'intrinsic long-term O2 effects' were derived. Glycolysis, as measured in 'insulin-cells', was stimulated by low O2 levels. It was about threefold faster in cells cultured and tested under 4% O2 as compared to cells cultured and tested under 13% O2, indicating the overall long-term effect. Glycolysis was about twofold faster in cells cultured and tested under 4% O2 as compared to cells cultured under 4% O2 but tested under 13% O2, demonstrating the short-term effect. Glycolysis was about 1.5-fold faster in cells cultured and tested under 4% O2 as compared to cells cultured under 13% O2 but tested under 4% O2, showing the intrinsic long-term effect. This difference was roughly parallel to the difference in levels of glucokinase and pyruvate kinase. Gluconeogenesis, as measured in 'glucagon cells', was stimulated by high O2 levels. Similar to glycolysis overall long-term, short-term and intrinsic long-term effects could be distinguished. The intrinsic long-term effects determined under 13% O2 corresponded to a 1.5-fold stimulation and paralleled the difference in phosphoenolpyruvate carboxykinase levels. The present results show that physiological oxygen concentrations also modulate hepatic carbohydrate metabolism by long-term effects and that the O2 gradient over the liver parenchyma thus contributes to the metabolic differences between periportal and perivenous hepatocytes in vivo.

Gluconeogenesis in periportal and perivenous hepatocytes of rat liver, isolated by a new high-yield digitonin/collagenase perfusion technique

A technique is described which allows preparations of hepatocytes, enriched in either periportal or perivenous hepatocytes ('PP-cells' and 'PV-cells' respectively), in a yield of about 30-50% compared with control cell preparations. The liver is first perfused for 40-60s with digitonin (4 mg/ml) to destroy selectively either the periportal or the perivenous part of the microcirculatory unit, and then the remaining hepatocytes are isolated by the ordinary collagenase perfusion technique. In periportal cells the activities of alanine aminotransferase and pyruvate kinase were 29.4 and 18.7 mumol/min per mg of DNA respectively. The rate of gluconeogenesis was 0.402 mumol/min per mg of DNA. In perivenous cells the corresponding values were 9.55, 22.1 and 0.244 mumol/min per mg of DNA respectively. These data support the concept of a zonation of glucose metabolism within the microcirculatory unit of the liver, with the afferent part (periportal zone) having a 2-fold, more active gluconeogenesis than the efferent part (perivenous zone).