The autophagy inducer trehalose stimulates macropinocytosis in NF1-deficient glioblastoma cells

Background

Glioblastoma is a highly aggressive brain tumor. A big effort is required to find novel molecules which can cross the blood–brain barrier and efficiently kill these tumor cells. In this perspective, trehalose (α-glucopyranosyl‐[1→1]‐α‐d‐glucopyranoside), found in various dietary sources and used as a safe nutrient supplement, attracted our attention for its pleiotropic effects against tumor cells.

Methods

Human glioblastoma cell lines U373-MG and T98G were exposed to trehalose and analyzed at different time points. Cell proliferation was evaluated at medium term, and clonogenic capacity and cell morphology were evaluated at long term. Western blot was used to evaluate biochemical markers of autophagy (also measured in cells co-treated with EIPA or chloroquine), and mTOR, AMPK and ERK 1/2 signalling. Macropinocytosis was evaluated morphologically by bright-field microscopy; in cells loaded with the fluorescein-conjugated fluid-phase tracer dextran, macropinocytic vacuoles were also visualized by fluorescence microscopy, and the extent of macropinocytosis was quantified by flow cytometry.

Results

The long-term effect of trehalose on U373-MG and T98G cell lines was impressive, as indicated by a dramatic reduction in clonogenic efficiency. Mechanistically, trehalose proved to be an efficient autophagy inducer in macropinocytosis-deficient T98G cells and an efficient inducer of macropinocytosis and eventual cell death by methuosis in U373-MG glioblastoma cells, proved to be poorly responsive to induction of autophagy. These two processes appeared to act in a mutually exclusive manner; indeed, co-treatment of U373-MG cells with the macropinocytosis inhibitor, EIPA, significantly increased the autophagic response. mTOR activation and AMPK inhibition occurred in a similar way in the two trehalose-treated cell lines. Interestingly, ERK 1/2 was activated only in macropinocytosis-proficient U373-MG cells harbouring loss-of-function mutations in the negative RAS regulator, NF1, suggesting a key role of RAS signalling.

Conclusions

Our results indicate that trehalose is worthy of further study as a candidate molecule for glioblastoma therapy, due to its capacity to induce a sustained autophagic response, ultimately leading to loss of clonogenic potential, and more interestingly, to force macropinocytosis, eventually leading to cell death by methuosis, particularly in tumor cells with RAS hyperactivity. As a further anticancer strategy, stimulation of macropinocytosis may be exploited to increase intracellular delivery of anticancer drugs.

The SGLT2-inhibitor dapagliflozin improves neutropenia and neutrophil dysfunction in a mouse model of the inherited metabolic disorder GSDIb

Glycogen Storage Disease type 1b (GSDIb) is a genetic disorder with long term severe complications. Accumulation of the glucose analog 1,5-anhydroglucitol-6-phosphate (1,5AG6P) in neutrophils inhibits the phosphorylation of glucose in these cells, causing neutropenia and neutrophil dysfunctions. This condition leads to serious infections and inflammatory bowel disease (IBD) in GSDIb patients. We show here that dapagliflozin, an inhibitor of the renal sodium-glucose co-transporter-2 (SGLT2), improves neutrophil function in an inducible mouse model of GSDIb by reducing 1,5AG6P accumulation in myeloid cells.

Role of medical treatment of endometriosis

Endometriosis is a chronic benign disease that affects women of reproductive age. Medical therapy is often the first line of management for women with endometriosis in order to ameliorate symptoms or to prevent post-surgical disease recurrence. Currently, there are several medical options for the management of patients with endometriosis and long-term treatments should balance clinical efficacy (controlling pain symptoms and preventing recurrence of disease after surgery) with an acceptable safety-profile. Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used in the treatment of chronic inflammatory conditions, being efficacious in relieving primary dysmenorrhea. Combined oral contraceptives and progestins, available for multiple routes of administration, are commonly administered as first-line hormonal therapies. Several studies demonstrated that they succeed in improving pain symptoms in the majority of patients; moreover, they are well tolerated and not expensive. Gonadotropin-releasing hormone-agonists are prescribed when first line therapies are ineffective, not tolerated or contraindicated. Even if these drugs are efficacious in treating women not responding to COCs or progestins, they are not orally available and have a less favorable tolerability profile (needing an appropriate add-back therapy). Because few data are available on long-term efficacy and safety of aromatase inhibitors they should be reserved only for women with symptoms who are refractory to other treatments only in a research environment. Almost all of the currently available treatment options for endometriosis suppress ovarian function and are not curative. For this reason, research into new drugs is unsurprisingly demanding. Amongst the drugs currently under investigation, gonadotropin-releasing hormone antagonists have shown most promise, currently in late-stage clinical development. There is a number of potential future therapies currently tested only in vitro, in animal models of endometriosis or in early clinical studies with a small sample size. Further studies are necessary to conclude whether these treatments would be of value for the treatment of endometriosis.

Inter- and Transgenerational Effects of Paternal Exposure to Inorganic Arsenic

The rise of metabolic disorders in modern times is mainly attributed to the environment. However, heritable effects of environmental chemicals on mammalian offsprings' metabolic health are unclear. Inorganic arsenic (iAs) is the top chemical on the Agency for Toxic Substances and Disease Registry priority list of hazardous substances. Here, we assess cross-generational effects of iAs in an exclusive male-lineage transmission paradigm. The exposure of male mice to 250 ppb iAs causes glucose intolerance and hepatic insulin resistance in F1 females, but not males, without affecting body weight. Hepatic expression of glucose metabolic genes, glucose output, and insulin signaling are disrupted in F1 females. Inhibition of the glucose 6-phosphatase complex masks the intergenerational effect of iAs, demonstrating a causative role of hepatic glucose production. F2 offspring from grandpaternal iAs exposure show temporary growth retardation at an early age, which diminishes in adults. However, reduced adiposity persists into middle age and is associated with altered gut microbiome and increased brown adipose thermogenesis. In contrast, F3 offspring of the male-lineage iAs exposure show increased adiposity, especially on a high-calorie diet. These findings have unveiled sex- and generation-specific heritable effects of iAs on metabolic physiology, which has broad implications in understanding gene-environment interactions.

Effects of Aqueous Extract of Lycopersicum esculentum L. var. “Camone” Tomato on Blood Pressure, Behavior and Brain Susceptibility to Oxidative Stress in Spontaneously Hypertensive Rats

Behavioral disorders affect millions of people worldwide. Hypertension contributes to both the development and progression of brain damage and cognitive dysfunction and could represent the most powerful modifiable risk factor for cerebral vessel dysfunction and consequent behavioral impairment. Tomato contains antioxidants and bioactive molecules that might play an important role in the prevention of cardiovascular and brain diseases. The effects of the combined gel and serum from Lycopersicum esculentum L. var. “Camone” tomatoes and those of purified tomato glycoalkaloids (tomatine) and an antihypertensive drug (captopril) were investigated in male spontaneously hypertensive rats (SHRs) and compared with normotensive Wistar Kyoto (WKY) rats. Body weight, systolic blood pressure, behavioral parameters, as well as brain susceptibility to oxidative stress and brain cytokine contents, were assessed. Treating hypertensive rats with tomato gel/serum or captopril for four weeks caused a significant reduction in blood pressure, decreased locomotor activity and increased grooming behavior; the last two parameters were also significantly affected by tomatine treatment. Brain slices obtained from hypertensive rats treated with tomato gel/serum were more resistant to oxidative stress and contained lower levels of inflammatory cytokines than vehicle-treated ones. In contrast, tomatine treatment had no effect. In conclusion, the tomato-derived gel/serum can be considered a dietary supplement able to drive in vivo blood pressure towards healthier values and also control some central effects such as behavior and brain oxidative stress.

Pinocembrin and its linolenoyl ester derivative induce wound healing activity in HaCaT cell line potentially involving a GPR120/FFA4 mediated pathway

Wound healing represents an urgent need from the clinical point of view. Several diseases result in wound conditions which are difficult to treat, such as in the case of diabetic foot ulcer. Starting from there, the medicinal research has focused on various targets over the years, including GPCRs as new wound healing drug targets. In line with this, GPR120, known to be an attractive target in type 2 diabetes drug discovery, was studied to finalize the development of new wound healing agents. Pinocembrin (HW0) was evaluated as a suitable compound for interacting with GPR120, and was hybridized with fatty acids, which are known endogenous GPR120 ligands, to enhance the wound healing potential and GPR120 interactions. HW0 and its 7-linolenoyl derivative (HW3) were found to be innovative wound healing agents. Immunofluorescence and functional assays suggested that their activity was mediated by GPR120, and docking simulations showed that the compounds could share the same pocket occupied by the known GPR120 agonist, TUG-891.

Chemical Characterisation and Antihypertensive Effects of Locular Gel and Serum of Lycopersicum esculentum L. var. "Camone" Tomato in Spontaneously Hypertensive Rats

Blood pressure control in hypertensive subjects calls for changes in lifestyle, especially diet. Tomato is widely consumed and rich in healthy components (i.e., carotenoids, vitamins and polyphenols). The aim of this study was to evaluate the chemical composition and antihypertensive effects of locular gel reconstituted in serum of green tomatoes of "Camone" variety. Tomato serum and locular gel were chemically characterised. The antihypertensive effects of the locular gel in serum, pure tomatine, and captopril, administered by oral gavage, were investigated for 4 weeks in male spontaneously hypertensive and normotensive rats. Systolic blood pressure and heart rate were monitored using the tail cuff method. Body and heart weight, serum glucose, triglycerides and inflammatory cytokines, aorta thickness and liver metabolising activity were also assessed. Locular gel and serum showed good tomatine and polyphenols content. Significant reductions in blood pressure and heart rate, as well as in inflammatory blood cytokines and aorta thickness, were observed in spontaneously hypertensive rats treated both with locular gel in serum and captopril. No significant effects were observed in normotensive rats. Green tomatoes locular gel and serum, usually discarded during tomato industrial processing, are rich in bioactive compounds (i.e., chlorogenic acid, caffeic acid and rutin, as well as the glycoalkaloids, α-tomatine and dehydrotomatine) that can lower in vivo blood pressure towards healthier values, as observed in spontaneously hypertensive rats.

Autophagy up-regulation by ulipristal acetate as a novel target mechanism in the treatment of uterine leiomyoma: an in vitro study

OBJECTIVE

To assess the effect of ulipristal acetate (UPA) on the autophagic process of uterine leiomyoma cells.

DESIGN

In vitro study in primary cultures of leiomyoma and myometrial cells isolated from biopsy specimen, and gene expression evaluation in biopsy material.

SETTING

Cellular pathology laboratory.

PATIENT(S)

Premenopausal women (without hormonal treatment) undergoing myomectomy or hysterectomy for symptomatic leiomyomas.

INTERVENTION(S)

Surgical specimens collected from uterine leiomyomas and matched normal myometria.

MAIN OUTCOME MEASURE(S)

After treatment of myometrial and leiomyoma cells with UPA, autophagy was evaluated by Western blot analysis of the typical biochemical markers, LC3-II, LC3-II:LC3-I ratio, and p62/SQSTM1. The expression level of Atg7 and Atg4D proteins was also assessed by Western blot.

RESULT(S)

The increase of LC3-II protein, LC3-II:LC3-I ratio, and p62/SQSTM1 protein indicates that UPA treatment up-regulates the autophagic response in leiomyoma cells, whereas these markers were almost unchanged in myometrial cells. Consistently, an increased level of Atg7 and Atg4D proteins was observed only in UPA-treated leiomyoma cells. The autophagic machinery is put into motion selectively in these cells, despite that the basal messenger RNA levels of LC3, SQSTM1, and ATG7 in leiomyoma biopsy specimen were not significantly different from those found in normal myometrial biopsy material.

CONCLUSION(S)

In vitro UPA treatment stimulates the autophagic response selectively in leiomyoma cells, which adds a novel indication for the clinical use of this selective P receptor (PR) modulator. Autophagy up-regulation may potentially contribute to the leiomyoma shrinkage occurring in UPA-treated patients and warrants further study.

Glucose Transport and Transporters in the Endomembranes

Glucose is a basic nutrient in most of the creatures; its transport through biological membranes is an absolute requirement of life. This role is fulfilled by glucose transporters, mediating the transport of glucose by facilitated diffusion or by secondary active transport. GLUT (glucose transporter) or SLC2A (Solute carrier 2A) families represent the main glucose transporters in mammalian cells, originally described as plasma membrane transporters. Glucose transport through intracellular membranes has not been elucidated yet; however, glucose is formed in the lumen of various organelles. The glucose-6-phosphatase system catalyzing the last common step of gluconeogenesis and glycogenolysis generates glucose within the lumen of the endoplasmic reticulum. Posttranslational processing of the oligosaccharide moiety of glycoproteins also results in intraluminal glucose formation in the endoplasmic reticulum (ER) and Golgi. Autophagic degradation of polysaccharides, glycoproteins, and glycolipids leads to glucose accumulation in lysosomes. Despite the obvious necessity, the mechanism of glucose transport and the molecular nature of mediating proteins in the endomembranes have been hardly elucidated for the last few years. However, recent studies revealed the intracellular localization and functional features of some glucose transporters; the aim of the present paper was to summarize the collected knowledge.

Development and characterization of an inducible mouse model for glycogen storage disease type Ib

BACKGROUND AND AIMS

Glycogen storage disease type Ib (GSD1b) is a rare metabolic and immune disorder caused by a deficiency in the glucose-6-phosphate transporter (G6PT) and characterized by impaired glucose homeostasis, myeloid dysfunction, and long-term risk of hepatocellular adenomas. Despite maximal therapy, based on a strict diet and on granulocyte colony-stimulating factor treatment, long-term severe complications still develop. Understanding the pathophysiology of GSD1b is a prerequisite to develop new therapeutic strategies and depends on the availability of animal models. The G6PT-KO mouse mimics the human disease but is very fragile and rarely survives weaning. We generated a conditional G6PT-deficient mouse as an alternative model for studying the long-term pathophysiology of the disease. We utilized this conditional mouse to develop an inducible G6PT-KO model to allow temporally regulated G6PT deletion by the administration of tamoxifen (TM).

METHODS

We generated a conditional G6PT-deficient mouse utilizing the CRElox strategy. Histology, histochemistry, and phenotype analyses were performed at different times after TM-induced G6PT inactivation. Neutrophils and monocytes were isolated and analyzed for functional activity with standard techniques.

RESULTS

The G6PT-inducible KO mice display the expected disturbances of G6P metabolism and myeloid dysfunctions of the human disorder, even though with a milder intensity.

CONCLUSIONS

TM-induced inactivation of G6PT in these mice leads to a phenotype which mimics that of human GSD1b patients. The conditional mice we have generated represent an excellent tool to study the tissue-specific role of the G6PT gene and the mechanism of long-term complications in GSD1b.

11β-hydroxysteroid dehydrogenase type 1 and pregnancy: Role in the timing of labour onset and in myometrial contraction

Glucocorticoids play a primary role in the maturation of fetal organs and may contribute to the onset of labour. Glucocorticoid activity depends on the 11β-hydroxysteroid dehydrogenase family (11β-HSDs), catalysing the interconversion between "active" cortisol into inactive cortisone. No definitive study exists on 11β-HSD expression profile in human decidua and myometrium during pregnancy. We investigated the implications of 11β-HSD1 in the regulation of uterine activity in pregnancy, examining its role on contraction of a myocyte cell line and murine 11β-hsd1 levels in utero. Murine 11β-hsd1 mRNA and protein levels in utero progressively increased until the last day of gestation and significantly decreased at the onset of labour (P < 0.0001) (n = 3 to 5 in the various gestational days analysed). Experiments on human myometrial samples confirm the significant fall in 11β-hsd1 mRNA levels at labour, compared to end pregnancy samples (n = 5 to 8). In vitro experiments showed that human myometrial contraction is inhibited by using a non-selective inhibitor of 11β-HSD1. The present study shows the temporal localisation of 11β-HSD1 in uterus, highlighting its importance in the timing of gestation and suggesting its contribution in the myometrium contraction.

Fructose, Glucocorticoids and Adipose Tissue: Implications for the Metabolic Syndrome

The modern Western society lifestyle is characterized by a hyperenergetic, high sugar containing food intake. Sugar intake increased dramatically during the last few decades, due to the excessive consumption of high-sugar drinks and high-fructose corn syrup. Current evidence suggests that high fructose intake when combined with overeating and adiposity promotes adverse metabolic health effects including dyslipidemia, insulin resistance, type II diabetes, and inflammation. Similarly, elevated glucocorticoid levels, especially the enhanced generation of active glucocorticoids in the adipose tissue due to increased 11β-hydroxysteroid dehydrogenase 1 (11β-HSD1) activity, have been associated with metabolic diseases. Moreover, recent evidence suggests that fructose stimulates the 11β-HSD1-mediated glucocorticoid activation by enhancing the availability of its cofactor NADPH. In adipocytes, fructose was found to stimulate 11β-HSD1 expression and activity, thereby promoting the adipogenic effects of glucocorticoids. This article aims to highlight the interconnections between overwhelmed fructose metabolism, intracellular glucocorticoid activation in adipose tissue, and their metabolic effects on the progression of the metabolic syndrome.

Expression and regulation of 11β-hydroxysteroid dehydrogenase type 1 in first trimester human decidua cells: Implication in preeclampsia

Glucocorticoids are implicated in successful blastocyst implantation, whereas alterations in glucocorticoid levels are associated with various pregnancy disorders including preeclampsia. Tissue concentration of active glucocorticoids depends on the expression of 11β-hydroxysteroid dehydrogenase (11β-HSD). This study investigated the contribution of first trimester decidua to glucocorticoid availability at the fetal-maternal interface by assessing the expression and regulation of 11β-HSD in human first trimester decidual tissues and cells and by evaluating 11β-HSD levels in preeclamptic vs. gestational age-matched decidua. 11β-HSD1 was the predominant isoform in first trimester decidua. In vitro, decidual cell 11β-HSD1 levels and enzymatic activity were up-regulated by ovarian steroids and inflammatory cytokines. Higher levels of 11β-HSD1 were found in preeclamptic decidua compared to controls. The present study indicates the predominance of 11β-HSD oxoreductase isoform in early decidua. Observations that ovarian hormones and inflammatory cytokines up-regulate 11β-HSD1, together with increased 11β-HSD1 expression in preeclampsia, highlight a role for decidual cells in controlling biologically active glucocorticoids in early pregnancy.

Glucose transporter type 10-lacking in arterial tortuosity syndrome-facilitates dehydroascorbic acid transport

Loss-of-function mutations in the gene encoding GLUT10 are responsible for arterial tortuosity syndrome (ATS), a rare connective tissue disorder. In this study GLUT10-mediated dehydroascorbic acid (DAA) transport was investigated, supposing its involvement in the pathomechanism. GLUT10 protein produced by in vitro translation and incorporated into liposomes efficiently transported DAA. Silencing of GLUT10 decreased DAA transport in immortalized human fibroblasts whose plasma membrane was selectively permeabilized. Similarly, the transport of DAA through endomembranes was markedly reduced in fibroblasts from ATS patients. Re-expression of GLUT10 in patients' fibroblasts restored DAA transport activity. The present results demonstrate that GLUT10 is a DAA transporter and DAA transport is diminished in the endomembranes of fibroblasts from ATS patients.

Transient knockdown of presenilin-1 provokes endoplasmic reticulum stress related formation of autophagosomes in HepG2 cells

The involvement of presenilins in the endoplasmic reticulum (ER) related autophagy was investigated by their transient knockdown in HepG2 cells. The silencing of PSEN1 but not of PSEN2 led to cell growth impairment and decreased viability. PSEN1 silencing resulted in ER stress response as evidenced by the elevated levels of glucose regulated protein 78 (Grp78), protein disulfide isomerase (PDI), and CCAAT/enhancer-binding protein homologous protein (CHOP) and by the activation of activating transcription factor 6 (ATF6). The activation of autophagy was indicated by the increased procession of microtubule-associated light chain 3 protein isoform B (LC3B) and by decreased phosphorylation of mammalian target of rapamycin (mTOR) and 70kDa ribosomal protein S6 kinase (p70S6K). Formation of ER-related cytoplasmic vacuolization colocalizing with the autophagic marker LC3B was also observed. The morphological effects and LC3B activation in presenilin-1 knockdown cells could be prevented by using the phosphoinositide 3-kinase (PI3K) inhibitor wortmannin or by calcium chelation. The results show that presenilin-1 hampers the ER stress dependent initiation of macroautophagy.

The glucose-6-phosphate transport is not mediated by a glucose-6-phosphate/phosphate exchange in liver microsomes

A phosphate-linked antiporter activity of the glucose-6-phosphate transporter (G6PT) has been recently described in liposomes including the reconstituded transporter protein. We directly investigated the mechanism of glucose-6-phosphate (G6P) transport in rat liver microsomal vesicles. Pre-loading with inorganic phosphate (Pi) did not stimulate G6P or Pi microsomal inward transport. Pi efflux from pre-loaded microsomes could not be enhanced by G6P or Pi addition. Rapid G6P or Pi influx was registered by light-scattering in microsomes not containing G6P or Pi. The G6PT inhibitor, S3483, blocked G6P transport irrespectively of experimental conditions. We conclude that hepatic G6PT functions as an uniporter.

Expression of hexose-6-phosphate dehydrogenase in rat tissues

Hexose-6-phosphate dehydrogenase (H6PD) is the main NADPH generating enzyme in the lumen of the endoplasmic reticulum. H6PD is regarded as an ancillary enzyme in prereceptorial glucocorticoid activation and probably acts as a nutrient sensor and as a prosurvival factor. H6PD expression was determined in a variety of rat and human tissues by detecting mRNA and protein levels, and by measuring its dehydrogenase and lactonase activities. It was found that H6PD was present in all investigated tissues; both expression and activity remained within an order of magnitude. Correlation was found between the dehydrogenase activity and protein or mRNA levels. The results confirmed the supposed housekeeping feature of the enzyme.

Contribution of fructose-6-phosphate to glucocorticoid activation in the endoplasmic reticulum: possible implication in the metabolic syndrome

Both fructose consumption and increased intracellular glucocorticoid activation have been implicated in the pathogenesis of the metabolic syndrome. Glucocorticoid activation by 11β-hydroxysteroid dehydrogenase type 1 (11β-HSD1) depends on hexose-6-phosphate dehydrogenase (H6PD), which physically interacts with 11β-HSD1 at the luminal surface of the endoplasmic reticulum (ER) membrane and generates reduced nicotinamide adenine dinucleotide phosphate for the reduction of glucocorticoids. The reducing equivalents for the reaction are provided by glucose-6-phosphate (G6P) that is transported by G6P translocase into the ER. Here, we show that fructose-6-phosphate (F6P) can substitute for G6P and is sufficient to maintain reductase activity of 11β-HSD1 in isolated microsomes. Our findings indicate that the mechanisms of F6P and G6P transport across the ER membrane are distinct and provide evidence that F6P is converted to G6P in the ER lumen, thus yielding substrate for H6PD-dependent reduced nicotinamide adenine dinucleotide phosphate generation. Using the purified enzyme, we show that F6P cannot be directly dehydrogenated by H6PD, and we also excluded H6PD as a phosphohexose isomerase. Therefore, we postulate the existence of an ER luminal hexose-phosphate isomerase different from the cytosolic enzyme. The results suggest that cytosolic F6P promotes prereceptor glucocorticoid activation in white adipose tissue, which might have a role in the pathophysiology of the metabolic syndrome.

Hexose-6-phosphate dehydrogenase in the endoplasmic reticulum

Hexose-6-phosphate dehydrogenase (H6PD) is a luminal enzyme of the endoplasmic reticulum that is distinguished from cytosolic glucose-6-phosphate dehydrogenase by several features. H6PD converts glucose-6-phosphate and NADP(+) to 6-phosphogluconate and NADPH, thereby catalyzing the first two reactions of the pentose-phosphate pathway. Because the endoplasmic reticulum has a separate pyridine nucleotide pool, H6PD provides NADPH for luminal reductases. One of these enzymes, 11beta-hydroxysteroid dehydrogenase type 1 responsible for prereceptorial activation of glucocorticoids, has been the focus of much attention as a probable factor in the pathomechanism of several human diseases including insulin resistance and the metabolic syndrome. This review summarizes recent advances related to the functions of H6PD.

Vitamin E supplementation improves neutropenia and reduces the frequency of infections in patients with glycogen storage disease type 1b

BACKGROUND

Neutropenia and/or neutrophil dysfunction are part of glycogen storage disease type 1b (GSD1b) phenotype. Recent studies indicated that activation of apoptosis and increased reactive oxygen species are implicated in the pathogenesis of neutropenia in GSD1b.

METHODS

We studied seven GSD1b patients over a 2-year-period to evaluate the efficacy of vitamin E, a known antioxidant, in preventing or improving the clinical manifestations associated with neutropenia and neutrophil dysfunction. Frequency and severity of infections, neutrophil counts and function, ileocolonoscopy and intestinal histology, were monitored. During the first year, patients did not assume vitamin E; during the second year of the study, vitamin E supplementation was added to their therapeutic regimens.

RESULTS

During vitamin E supplementation, the mean values of neutrophil counts were significantly higher (p < 0.05) and neutrophil counts lower than 500/mm(3) were found less frequently (p < 0.05); the frequency and severity of infections, mouth ulcers and perianal lesions, was reduced (p < 0.05); ileocolonoscopy and histology showed a mild improvement. Vitamin E supplementation did not result in changes in neutrophil function.

CONCLUSIONS

These results suggest that vitamin E supplementation might be beneficial in GSD1b patients and may alleviate disease manifestations associated with neutropenia.

Constant expression of hexose-6-phosphate dehydrogenase during differentiation of human adipose-derived mesenchymal stem cells

The reductase activity of 11beta-hydroxysteroid dehydrogenase type 1 (HSD11B1) plays an important role in the growth and differentiation of adipose tissue via the prereceptorial activation of glucocorticoids. This enzyme colocalizes with hexose-6-phosphate dehydrogenase (H6PD) at the luminal surface of the endoplasmic reticulum membrane, and the latter enzyme provides NADPH to the former, which can thus act as an 11beta-reductase. It was suggested that, during adipogenesis, the increased expression of H6PD causes a dehydrogenase-to-reductase switch in the activity of HSD11B1. However, only the expression of the HSD11B1 has been extensively studied, and little is known about the expression of H6PD. Here, we investigated the expression and the activity of H6PD in the course of the differentiation of human adipose-derived mesenchymal stem cells (ADMSCs) and murine 3T3-L1 cells. It was found that H6PD is already present in adipose-derived stem cells and in 3T3-L1 fibroblasts even before the induction of adipogenesis. Moreover, mRNA and protein levels, as well as the microsomal H6PD activities remained unchanged during the differentiation. At the same time a great induction of HSD11B1 was observed in both cell types. The observed constant expression of H6PD suggests that HSD11B1 acts as a reductase throughout the adipogenesis process in human ADMSCs and murine 3T3-L1 cells.

Metyrapone prevents cortisone-induced preadipocyte differentiation by depleting luminal NADPH of the endoplasmic reticulum

Preadipocyte differentiation is greatly affected by prereceptorial glucocorticoid activation catalyzed by 11beta-hydroxysteroid dehydrogenase type 1 in the lumen of the endoplasmic reticulum. The role of the local NADPH pool in this process was investigated using metyrapone as an NADPH-depleting agent. Metyrapone administered at low micromolar concentrations caused the prompt oxidation of the endogenous NADPH, inhibited the reduction of cortisone and enhanced the oxidation of cortisol in native rat liver microsomal vesicles. However, in permeabilized microsomes, it only slightly decreased both NADPH-dependent cortisone reduction and NADP(+)-dependent cortisol oxidation. Accordingly, metyrapone administration caused a switch in 11beta-hydroxysteroid dehydrogenase activity from reductase to dehydrogenase in both 3T3-L1-derived and human stem cell-derived differentiated adipocytes. Metyrapone greatly attenuated the induction of 11beta-hydroxysteroid dehydrogenase type 1 and the accumulation of lipid droplets during preadipocyte differentiation when 3T3-L1 cells were stimulated with cortisone, while it was much less effective in case of cortisol or dexamethasone. In conclusion, the positive feedback of glucocorticoid activation during preadipocyte differentiation is interrupted by metyrapone, which depletes NADPH in the endoplasmic reticulum. The results also indicate that the reduced state of luminal pyridine nucleotides in the endoplasmic reticulum is important in the process of adipogenesis.

Maintenance of luminal NADPH in the endoplasmic reticulum promotes the survival of human neutrophil granulocytes

The present study demonstrates the expression of hexose-6-phosphate dehydrogenase and 11 beta-hydroxysteroid dehydrogenase type 1 in human neutrophils, and the presence and activity of these enzymes in the microsomal fraction of the cells. Their concerted action together with the previously described glucose-6-phosphate transporter is responsible for cortisone-cortisol interconversion detected in human neutrophils. Furthermore, the results suggest that luminal NADPH generation by the cortisol dehydrogenase activity of 11 beta-hydroxysteroid dehydrogenase type 1 prevents neutrophil apoptosis provoked by the inhibition of the glucose-6-phosphate transporter. In conclusion, the maintenance of the luminal NADPH pool is an important antiapoptotic factor in neutrophil granulocytes.

The glucose-6-phosphate transporter-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system of the adipose tissue

11beta-hydroxysteroid dehydrogenase type 1, expressed mainly in the endoplasmic reticulum of adipocytes and hepatocytes, plays an important role in the prereceptorial activation of glucocorticoids. In liver endoplasmic reticulum-derived microsomal vesicles, nicotinamide adenine dinucleotide phosphate reduced supply to the enzyme is guaranteed by a tight functional connection with hexose-6-phosphate dehydrogenase and the glucose-6-phosphate transporter (G6PT). In adipose tissue, the proteins and their activities supporting the action of 11beta-hydroxysteroid dehydrogenase type 1 have not been explored yet. Here we report the occurrence of the hexose-6-phosphate dehydrogenase in rat epididymal fat, as detected at the level of mRNA, protein, and activity. In the isolated microsomes, the activity was evident only on the permeabilization of the membrane because of the poor permeability to the cofactor nicotinamide adenine dineucleotide phosphate (NADP(+)), which is consistent with the intralumenal compartmentation of both the enzyme and a pool of pyridine nucleotides. In fat cells, the access of the substrate, glucose-6-phosphate to the intralumenal hexose-6-phosphate dehydrogenase appeared to be mediated by the liver-type G6PT. In fact, the G6PT expression was revealed at the level of mRNA and protein. Accordingly, the transport of glucose-6-phosphate was demonstrated in microsomal vesicles, and it was inhibited by S3483, a prototypic inhibitor of G6PT. Furthermore, isolated adipocytes produced cortisol on addition of cortisone, and the production was markedly inhibited by S3483. The results show that adipocytes are equipped with a functional G6PT-hexose-6-phosphate dehydrogenase-11beta-hydroxysteroid dehydrogenase type 1 system and indicate that all three components are potential pharmacological targets for modulating local glucocorticoid activation.

Transport and transporters in the endoplasmic reticulum

Enzyme activities localized in the luminal compartment of the endoplasmic reticulum are integrated into the cellular metabolism by transmembrane fluxes of their substrates, products and/or cofactors. Most compounds involved are bulky, polar or even charged; hence, they cannot be expected to diffuse through lipid bilayers. Accordingly, transport processes investigated so far have been found protein-mediated. The selective and often rate-limiting transport processes greatly influence the activity, kinetic features and substrate specificity of the corresponding luminal enzymes. Therefore, the phenomenological characterization of endoplasmic reticulum transport contributes largely to the understanding of the metabolic functions of this organelle. Attempts to identify the transporter proteins have only been successful in a few cases, but recent development in molecular biology promises a better progress in this field.

Immunodetection of the expression of microsomal proteins encoded by the glucose 6-phosphate transporter gene

Glucose 6-phosphate transport has been well characterized in liver microsomes. The transport is required for the functioning of the glucose-6-phosphatase enzyme that is situated in the lumen of the hepatic endoplasmic reticulum. The genetic deficiency of the glucose 6-phosphate transport activity causes a severe metabolic disease termed type 1b glycogen storage disease. The cDNA encoding a liver transporter for glucose 6-phosphate was cloned and was found to be mutated in patients suffering from glycogen storage disease 1b. While related mRNAs have been described in liver and other tissues, the encoded protein(s) has not been immunologically characterized yet. In the present study, we report (using antibodies against three different peptides of the predicted amino acid sequence) that a major protein encoded by the glucose 6-phosphate transporter gene is expressed in the endoplasmic reticulum membranes of rat and human liver. The protein has an apparent molecular mass of approx. 33 kDa using SDS/PAGE, but several lines of evidence indicate that its real molecular mass is 46 kDa, as expected. The glucose 6-phosphate transporter protein was also immunodetected in kidney microsomes, but not in microsomes derived from human fibrocytes, rat spleen and lung, and a variety of cell lines. Moreover, little or no expression of the glucose 6-phosphate transporter protein was found in liver microsomes obtained from three glycogen storage disease 1b patients, even bearing mutations that do not directly interfere with protein translation, which can be explained by a (proteasome-mediated) degradation of the mutated transporter.

Genotype/phenotype correlation in glycogen storage disease type 1b: a multicentre study and review of the literature

We studied the genotype/phenotype correlation in a cohort of glycogen storage disease type (GSD) 1b patients. A total of 25 GSD1b patients, 13 females and 12 males, age range: 4.3-28.4 years, mean:14.6+/-6.8 years; median: 15 years, representing the entire case load of Italian GSD1b patients, were enrolled in the study. Molecular analysis of the glucose 6-phosphate translocase (G6PT1) gene was performed in all patients. We analysed the presence of a correlation among both the clinical features associated with GSD1b (neutropenia, frequency of admission to the hospital for severe infections) and the presence of systemic complications (liver adenomas, nephropathy, bone mineral density defect, polycystic ovaries, short stature, inflammatory bowel disease) and the mutations detected in each patient. Nine patients were homozygous or compound heterozygous for mutations causing stop codons. In particular, three patients were homozygous for the same mutation (400X); of these patients, one showed chronic neutropenia with severe and frequent infections and severe inflammatory bowel disease, another patient cyclic neutropenia associated with rare bacterial infections and mild bowel involvement and the last one normal neutrophil count. Two patients were homozygous for the mutation 128X; one of these patients did not show neutropenia, whereas the other one had severe neutropenia needing frequent hospital admission and was under granulocyte-colony stimulating factor treatment. In three patients no mutations were detected.

CONCLUSION

No correlation was found between individual mutations and the presence of neutropenia, bacterial infections and systemic complications. These results suggest that different genes and proteins modulate neutrophil differentiation, maturation and apoptosis and thus the severity and frequency of infections. The absence of detectable mutations in three patients could suggest that a second protein plays a role in microsomal phosphate transport.

Inhibition of microsomal glucose-6-phosphate transport in human neutrophils results in apoptosis: a potential explanation for neutrophil dysfunction in glycogen storage disease type 1b

Mutations in the gene of the hepatic glucose-6-phosphate transporter cause glycogen storage disease type 1b. In this disease, the altered glucose homeostasis and liver functions are accompanied by an impairment of neutrophils/monocytes. However, neither the existence of a microsomal glucose-6-phosphate transport, nor the connection between its defect and cell dysfunction has been demonstrated in neutrophils/monocytes. In this study we have characterized the microsomal glucose-6-phosphate transport of human neutrophils and differentiated HL-60 cells. The transport of glucose-6-phosphate was sensitive to the chlorogenic acid derivative S3483, N-ethylmaleimide, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, known inhibitors of the hepatic microsomal glucose-6-phosphate transporter. A glucose-6-phosphate uptake was also present in microsomes from undifferentiated HL-60 and Jurkat cells, but it was insensitive to S3483. The treatment with S3484 of intact human neutrophils and differentiated HL-60 cells mimicked some leukocyte defects of glycogen storage disease type 1b patients (ie, the drug inhibited phorbol myristate acetate-induced superoxide anion production and reduced the size of endoplasmic reticulum Ca(2+) stores). Importantly, the treatment with S3484 also resulted in apoptosis of human neutrophils and differentiated HL-60 cells, while undifferentiated HL-60 and Jurkat cells were unaffected by the drug. The proapoptotic effect of S3483 was prevented by the inhibition of nicotinamide adenine dinucleotide phosphate oxidase or by antioxidant treatment. These results suggest that microsomal glucose-6-phosphate transport has a role in the antioxidant protection of neutrophils, and that the genetic defect of the transporter leads to the impairment of cellular functions and apoptosis.

Histone 2A stimulates glucose-6-phosphatase activity by permeabilization of liver microsomes

Histone 2A increases glucose-6-phosphatase activity in liver microsomes. The effect has been attributed either to the conformational change of the enzyme, or to the permeabilization of microsomal membrane that allows the free access of substrate to the intraluminal glucose-6-phosphatase catalytic site. The aim of the present study was the critical reinvestigation of the mechanism of action of histone 2A. It has been found that the dose-effect curve of histone 2A is different from that of detergents and resembles that of the pore-forming alamethicin. Inhibitory effects of EGTA on glucose-6-phosphatase activity previously reported in histone 2A-treated microsomes have been also found in alamethicin-permeabilized vesicles. The effect of EGTA cannot therefore simply be an antagonization of the effect of histone 2A. Histone 2A stimulates the activity of another latent microsomal enzyme, UDP-glucuronosyltransferase, which has an intraluminal catalytic site. Finally, histone 2A renders microsomal vesicles permeable to non-permeant compounds. Taken together, the results demonstrate that histone 2A stimulates glucose-6-phosphatase activity by permeabilizing the microsomal membrane.

Glucose 6-phosphate transport in fibroblast microsomes from glycogen storage disease type 1b patients: evidence for multiple glucose 6-phosphate transport systems

In liver endoplasmic reticulum the intralumenal glucose-6-phosphatase activity requires the operation of a glucose 6-phosphate transporter (G6PT1). Mutations in the gene encoding G6PT1 cause glycogen storage disease type 1b, which is characterized by a loss of glucose-6-phosphatase activity and impaired glucose homoeostasis. We describe a novel glucose 6-phosphate (G6P) transport activity in microsomes from human fibroblasts and HeLa cells. This transport activity is unrelated to G6PT1 since: (i) it was similar in microsomes of skin fibroblasts from glycogen storage disease type 1b patients homozygous for mutations of the G6PT1 gene, and in microsomes from human control subjects; (ii) it was insensitive to the G6PT1 inhibitor chlorogenic acid; and (iii) it was equally active towards G6P and glucose 1-phosphate, whereas G6PT1 is highly selective for G6P. Taken together, our results provide evidence for the presence of multiple transporters for G6P (and other hexose phosphoesters) in the endoplasmic reticulum.

Glucose-6-phosphatase in the insulin secreting cell line INS-1

The glucose-6-phosphatase system of the glucose sensitive insulin secreting rat insulinoma cells (INS-1) was investigated. INS-1 cells contain easily detectable levels of glucose-6-phosphatase enzyme protein (assessed by Western blotting) and have a very significant enzymatic activity. The features of the enzyme (Km and Vmax values, sensitivity to acidic pH, partial latency, and double immunoreactive band) are similar to those of the hepatic form. On the other hand, hardly detectable levels of glucose-6-phosphatase activity and protein were present in the parent glucose insensitive RINm5F cell line. The mRNA of the glucose-6-phosphate transporter was also more abundant in the INS-1 cells. The results support the view that the glucose-6-phosphatase system of the beta-cell is associated with the regulation of insulin secretion.

Mutations in the glucose-6-phosphate transporter (G6PT) gene in patients with glycogen storage diseases type 1b and 1c

Glycogen storage diseases type 1 (GSD 1) are a group of autosomal recessive disorders characterized by impairment of terminal steps of glycogenolysis and gluconeogenesis. Mutations of the glucose-6-phosphatase gene are responsible for the most frequent form of GSD 1, the subtype 1a, while mutations of the glucose-6-phosphate transporter gene (G6PT) have recently been shown to cause the non 1a forms of GSD, namely the 1b and 1c subtypes. Here, we report on the analysis by single-stranded conformation polymorphism (SSCP) and/or DNA sequencing of the exons of the G6PT in 14 patients diagnosed either as affected by the GSD 1b or 1c subtypes. Mutations in the G6PT gene were found in all patients. Four of the detected mutations were novel mutations, while the others were previously described. Our results confirm that the GSD 1b and 1c forms are due to mutations in the same gene, i.e. the G6PT gene. We also show that the same kind of mutation can be associated or not with evident clinical complications such as neutrophil impairment. Since no correlation between the type and position of the mutation and the severity of the disease was found, other unknown factors may cause the expression of symptoms, such as neutropenia, which dramatically influence the severity of the disease.

Caffeine releases a glucose-primed endoplasmic reticulum Ca2+ pool in the insulin secreting cell line INS-1

Caffeine mobilized an intracellular Ca2+ pool in intact fura-2-loaded INS-1 cells in suspension exposed to high (16 mM) [glucose], while a minor effect was observed with low (2 mM) [glucose]. Cells were kept in a medium containing diaxozide or no Ca2+ to prevent the influx of extracellular Ca2+. The caffeine-sensitive intracellular Ca2+ pool was within the endoplasmic reticulum since it was depleted by the inhibitor of the reticular Ca2+ pumps thapsigargin and the InsP3-dependent agonist carbachol. No effect of caffeine was observed in the parent glucose-insensitive RINmF5 cells. In microsomes from INS-1 but not RINmF5 cells, the type 2 ryanodine receptor was present as revealed by Western blotting. It was concluded that the endoplasmic reticulum of INS-1 cells possesses caffeine-sensitive type 2 ryanodine receptors Ca2+ channels.

Conformational change of the catalytic subunit of glucose-6-phosphatase in rat liver during the fetal-to-neonatal transition

The glucose-6-phosphatase system was investigated in fetal rat liver microsomal vesicles. Several observations indicate that the orientation of the catalytic subunit is different in the fetal liver in comparison with the adult form: (i) the phosphohydrolase activity was not latent using glucose-6-phosphate as substrate, and in the case of other phosphoesters it was less latent; (ii) the intravesicular accumulation of glucose upon glucose-6-phosphate hydrolysis was lower; (iii) the size of the intravesicular glucose-6-phosphate pool was independent of the glucose-6-phosphatase activities; (iv) antibody against the loop containing the proposed catalytic site of the enzyme inhibited the phosphohydrolase activity in fetal but not in adult rat liver microsomes. Glucose-6-phosphate, phosphate, and glucose uptake could be detected by both light scattering and/or rapid filtration method in fetal liver microsomes; however, the intravesicular glucose-6-phosphate and glucose accessible spaces were proportionally smaller than in adult rat liver microsomes. These data demonstrate that the components of the glucose-6-phosphatase system are already present, although to a lower extent, in fetal liver, but they are functionally uncoupled by the extravesicular orientation of the catalytic subunit.

Heterogeneity of glucose transport in rat liver microsomal vesicles

Glucose transport across the membrane of rat liver microsomal vesicles was studied by a rapid filtration method in three different experimental systems: (i) inward transport in the presence of extravesicular glucose, (ii) efflux from passively preloaded vesicles, and (iii) efflux of glucose generated intravesicularly by glucose-6-phosphatase upon addition of glucose 6-phosphate were investigated. The apparent intravesicular glucose space estimated with the rapid filtration method was lower than the total microsomal glucose accessible space both the in the steady-state phase of uptake and at the starting point of efflux: 0.5 versus 2.3 microl/mg protein. The initial rate of influx/efflux was dependent on the extravesicular/intravesicular glucose concentration and was much lower than the rate of influx estimated previously by the light-scattering technique. Both influx and efflux could be inhibited by N-ethylmaleimide and possibly became saturable at high (>100 mM) glucose concentration. Known inhibitors of GLUT transporters (genistein, cytochalasin B, phloretin, and hexoses) did not affect glucose influx. The time course of glucose efflux from vesicles preincubated in the presence of glucose 6-phosphate was similar to that from glucose-loaded vesicles. These data together with that obtained previously (by a light-scattering technique; Marcolongo, P., Fulceri, R., Giunti, R., Burchell, A., and Benedetti, A. (1996) Biochem. Biophys. Res. Commun. 219, 916-922) indicate that microsomal vesicles are heterogeneous regarding their glucose-transporting properties and that glucose transport is bidirectional and its feature meets the requirements of a facilitative transport.

Structure and mutation analysis of the glycogen storage disease type 1b gene

Glycogen storage disease (GSD) 1b is the deficiency of endoplasmic reticulum glucose-6-phosphate (G6P) transport. We here report the structure of the gene encoding a protein likely to be responsible for G6P transport, and its mapping to human chromosome 11q23.3. The gene is composed of nine exons spanning a genomic region of approximately 4 kb. Primers based on the genomic sequence were used in single strand conformation polymorphism (SSCP) analysis and mutations were found in six out of seven GSD 1b patients analysed.

Gulonolactone oxidase activity-dependent intravesicular glutathione oxidation in rat liver microsomes

The orientation of gulonolactone oxidase activity was investigated in rat liver microsomes. Ascorbate formation upon gulonolactone addition resulted in higher intravesicular than extravesicular ascorbate concentrations in native microsomal vesicles. The intraluminal ascorbate accumulation could be prevented or the accumulated ascorbate could be released by permeabilising the vesicles with the pore-forming alamethicin. The formation of the other product of the enzyme, hydrogen peroxide caused the preferential oxidation of intraluminal glutathione in glutathione-loaded microsomes. In conclusion, these results suggest that the orientation of the active site of gulonolactone oxidase is intraluminal and/or the enzyme releases its products towards the lumen of the endoplasmic reticulum.

Histones and basic polypeptides activate Ca2+/cation influx in various cell types

Histone H2A (1-10 microg/ml) added to Ehrlich ascite cell suspensions promoted: (i) Ca2+ influx, but no apparent intracellular Ca2+ mobilization; (ii) plasma-membrane depolarization and Na+ influx in Ca2+-free medium, which were recovered by Ca2+ readmission; (iii) influx of other cations such as Ba2+, Mn2+, choline+ and N-methyl-d-glucamine+, but not of propidium+, ethidium bromide and Trypan Blue. H2A-induced Ca2+ influx and cell depolarization were: (i) blocked by La3+ and Gd3+, but not by various inhibitors of receptor-activated Ca2+-influx pathways/channels; (ii) mimicked by various basic polypeptides, with Mr>4000; (iii) prevented or reversed by polyanions such as polyglutamate or heparin; (iv) present in other cell types, such as Jurkat, PC12 and Friend erythroleukaemia cells, but virtually absent from rat hepatocytes and thymocytes. We conclude that cationic proteins/polypeptides, by interacting in a cell-specific manner with the cell surface, can activate in those cells putative non-selective Ca2+ channels and membrane depolarization.

Dehydroascorbate and ascorbate transport in rat liver microsomal vesicles

Ascorbate and dehydroascorbate transport was investigated in rat liver microsomal vesicles using radiolabeled compounds and a rapid filtration method. The uptake of both compounds was time- and temperature-dependent, and saturable. Ascorbate uptake did not reach complete equilibrium, it had low affinity and high capacity. Ascorbate influx could not be inhibited by glucose, dehydroascorbate, or glucose transport inhibitors (phloretin, cytochalasin B) but it was reduced by the anion transport inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and by the alkylating agent N-ethylmaleimide. Ascorbate uptake could be stimulated by ferric iron and could be diminished by reducing agents (dithiothreitol, reduced glutathione). In contrast, dehydroascorbate uptake exceeded the level of passive equilibrium, it had high affinity and low capacity. Glucose cis inhibited and trans stimulated the uptake. Glucose transport inhibitors were also effective. The presence of intravesicular reducing compounds increased, while extravesicular reducing environment decreased dehydroascorbate influx. Our results suggest that dehydroascorbate transport is preferred in hepatic endoplasmic reticulum and it is mediated by a GLUT-type transporter. The intravesicular reduction of dehydroascorbate leads to the accumulation of ascorbate and contributes to the low intraluminal reduced/oxidized glutathione ratio.

Liver microsomal transport of glucose-6-phosphate, glucose, and phosphate in type 1 glycogen storage disease

The transport of glucose-6-phosphate (G6P), glucose, and orthophosphate into liver microsomes, isolated from six patients with various subtypes of type 1 glycogen storage disease (GSD), was measured using a light-scattering method. We found that G6P, glucose, and phosphate could all cross the microsomal membrane, in four cases of type 1a GSD. In contrast, liver microsomal transport of G6P and phosphate was deficient in the GSD 1b and 1c patients, respectively. These results support the involvement of multiple proteins (and genes) in GSD type 1. The results obtained with the light-scattering method are in accordance with conventional kinetic analysis of the microsomal glucose-6-phosphatase system. Therefore, this technique could be used to directly diagnose type 1b and 1c GSD.

Demonstration of a metabolically active glucose-6-phosphate pool in the lumen of liver microsomal vesicles

Glucose-6-phosphate transport was investigated in rat or human liver microsomal vesicles using rapid filtration and light-scattering methods. Upon addition of glucose-6-phosphate, rat liver microsomes accumulated the radioactive tracer, reaching a steady-state level of uptake. In this phase, the majority of the accumulated tracer was glucose, but a significant intraluminal glucose-6-phosphate pool could also be observed. The extent of the intravesicular glucose pool was proportional with glucose-6-phosphatase activity. The relative size of the intravesicular glucose-6-phosphate pool (irrespective of the concentration of the extravesicular concentration of added glucose-6-phosphate) expressed as the apparent intravesicular space of the hexose phosphate was inversely dependent on glucose-6-phosphatase activity. The increase of hydrolysis by elevating the extravesicular glucose-6-phosphate concentration or temperature resulted in lower apparent intravesicular glucose-6-phosphate spaces and, thus, in a higher transmembrane gradient of glucose-6-phosphate concentrations. In contrast, inhibition of glucose-6-phosphate hydrolysis by vanadate, inactivation of glucose-6-phosphatase by acidic pH, or genetically determined low or absent glucose-6-phosphatase activity in human hepatic microsomes of patients suffering from glycogen storage disease type 1a led to relatively high intravesicular glucose-6-phosphate levels. Glucose-6-phosphate transport investigated by light-scattering technique resulted in similar traces in control and vanadate-treated rat microsomes as well as in microsomes from human patients with glycogen storage disease type 1a. It is concluded that liver microsomes take up glucose-6-phosphate, constituting a pool directly accessible to intraluminal glucose-6-phosphatase activity. In addition, normal glucose-6-phosphate uptake can take place in the absence of the glucose-6-phosphatase enzyme protein, confirming the existence of separate transport proteins.

Evidence for an UDP-glucuronic acid/phenol glucuronide antiport in rat liver microsomal vesicles

The transport of glucuronides synthesized in the luminal compartment of the endoplasmic reticulum by UDP-glucuronosyltransferase isoenzymes was studied in rat liver microsomal vesicles. Microsomal vesicles were loaded with p-nitrophenol glucuronide (5 mM), phenolphthalein glucuronide or UDP-glucuronic acid, by a freeze-thawing method. In was shown that: (i) the loading procedure resulted in millimolar intravesicular concentrations of the different loading compounds; (ii) addition of UDP-glucuronic acid (5 mM) to the vesicles released both intravesicular glucuronides within 1 min; (iii) glucuronides stimulated the release of UDP-glucuronic acid from UDP acid-loaded microsomal vesicles; (iv) trans-stimulation of UDP-glucuronic acid entry by loading of microsomal vesicles with p-nitrophenol glucuronide, phenolphthalein glucuronide, UDP-glucuronic acid and UDP-N-acetyl-glucosamine almost completely abolished the latency of UDP-glucuronosyltransferase, although mannose 6-phosphatase latency remained unaltered; (v) the loading compounds by themselves did not stimulate UDP-glucuronosyltransferase activity. This study indicates that glucuronides synthesized in the lumen of endoplasmic reticulum can leave by an antiport, which concurrently transports USP-glucuronic acid into the lumen of the endoplasmic reticulum.