Dermcidin expression in hepatic cells improves survival without N-glycosylation, but requires asparagine residues.

Proteolysis-inducing factor, a cachexia-inducing tumour product, is an N-glycosylated peptide with homology to the unglycosylated neuronal survival peptide Y-P30 and a predicted product of the dermcidin gene, a pro-survival oncogene in breast cancer. We aimed to investigate whether dermcidin is pro-survival in liver cells, in which proteolysis-inducing factor induces catabolism, and to determine the role of potentially glycosylated asparagine residues in this function. Reverse cloning of proteolysis-inducing factor demonstrated approximately 100% homology with the dermcidin cDNA. This cDNA was cloned into pcDNA3.1+ and both asparagine residues removed using site-directed mutagenesis. In vitro translation demonstrated signal peptide production, but no difference in molecular weight between the products of native and mutant vectors. Immunocytochemistry of HuH7 cells transiently transfected with V5-His-tagged dermcidin confirmed targeting to the secretory pathway. Stable transfection conferred protection against oxidative stress. This was abrogated by mutation of both asparagines in combination, but not by mutation of either asparagine alone. These findings suggest that dermcidin may function as an oncogene in hepatic as well as breast cells. Glycosylation does not appear to be required, but the importance of asparagine residues suggests a role for the proteolysis-inducing factor core peptide domain.

Skeletal muscle myocytes undergo protein loss and reactive oxygen-mediated NF-kappaB activation in response to tumor necrosis factor alpha.

Skeletal muscle atrophy and weakness are thought to be stimulated by tumor necrosis factor alpha (TNF-alpha) in a variety of chronic diseases. However, little is known about the direct effects of TNF-alpha on differentiated skeletal muscle cells or the signaling mechanisms involved. We have tested the effects of TNF-alpha on the mouse-derived C2C12 muscle cell line and on primary cultures from rat skeletal muscle. TNF-alpha treatment of differentiated myotubes stimulated time- and concentration-dependent reductions in total protein content and loss of adult myosin heavy chain (MHCf) content; these changes were evident at low TNF-alpha concentrations (1-3 ng/ml) that did not alter muscle DNA content and were not associated with a decrease in MHCf synthesis. TNF-alpha activated binding of nuclear factor kappaB (NF-kappaB) to its targeted DNA sequence and stimulated degradation of I-kappaBalpha, an NF-kappaB inhibitory protein. TNF-alpha stimulated total ubiquitin conjugation whereas a 26S proteasome inhibitor (MG132 10-40 microM) blocked TNF-alpha activation of NF-kappaB. Catalase 1 kU/ml inhibited NF-kappaB activation by TNF-alpha; exogenous hydrogen peroxide 200 microM activated NF-kappaB and stimulated I-kappaBalpha degradation. These data demonstrate that TNF-alpha directly induces skeletal muscle protein loss, that NF-kappaB is rapidly activated by TNF-alpha in differentiated skeletal muscle cells, and that TNF-alpha/NF-kappaB signaling in skeletal muscle is regulated by endogenous reactive oxygen species.

Low levels of glucose-6-phosphate hydrolysis in the sarcoplasmic reticulum of skeletal muscle: involvement of glucose-6-phosphatase.

Glucose-6-phosphate hydrolysis was measured in a fraction obtained from rabbit fast-twitch skeletal muscle and corresponding to total sarcoplasmic reticulum, as well as in three subfractions containing longitudinal tubules, terminal cisternae or both structures. In all cases the levels of hydrolysis measured both in native and disrupted membranes were approximately 60-100 times lower than the microsomal glucose-6-phosphatase activity of the corresponding livers. In contrast to liver microsomes, most (up to 80%) of the glucose-6-phosphate hydrolysing activity in muscle sarcoplasmic reticulum membranes was not inactivated by pH 5.0 pre-incubation indicating that it was not catalysed by the specific glucose-6-phosphatase enzyme. Osmotically induced changes in light-scattering intensity of sarcoplasmic reticulum vesicles revealed that, in contrast to liver microsomes, sarcoplasmic reticulum vesicles were not selectively permeable to glucose-6-phosphate as mannose-6-phosphate was also permeable and in addition they were poorly permeable to glucose. Immunoblot experiments using antibodies raised against the glucose-6-phosphatase enzyme, and liver endoplasmic reticulum glucose and Pi translocases, failed to detect the presence of these protein components in sarcoplasmic reticulum membranes. Southern blot analysis of reverse transcriptase-polymerase chain reaction products from rat muscle revealed that glucose-6-phosphatase mRNA is present in muscle. Quantification of Northern blot analysis of liver and muscle mRNA indicated that muscle contains less than 2% of the amount of glucose-6-phosphate mRNA found in corresponding livers. We conclude that very low levels of specific glucose-6-phosphatase (e.g. as in liver; E.C. 3.1.3.9) are present in muscle sarcoplasmic reticulum and that the muscle and liver glucose-6-phosphatase systems have several different properties.

An altered T2 beta translocase of the glucose-6-phosphatase system in the membrane of the endoplasmic reticulum from livers of Ehrlich-ascites-tumour-bearing mice.

The inhibitory interactions of orthophosphate (P1) with the glucose-6-phosphatase system of intact microsomes derived from the livers of normal and Ehrlich-ascites-tumour-bearing mice reveal the appearance of a novel form of the T2 beta translocase component of the glucose-6-phosphatase system in tumour-stressed mice. Kinetic studies, with and without 20 mM P1, show a strictly classical competitive inhibition, with a K1,P1 of 4.2 mM, with disrupted microsomes from both control and tumour-bearing mouse liver. Inhibition was also observed with intact microsomes from livers of control mice, and contributions by both competitive and non-competitive components of inhibition were quantified by calculation of Kis,P1 and Kii,P1 values respectively. However, little inhibition was noted with intact microsomes from the livers of tumour-bearing mice. It is concluded that this novel form of T2 beta is less able to transport Pi, from the cytosol to the endoplasmic reticulum lumen, perhaps because of the tumour-related increased Km for Pi transport in this direction.

Evidence for glucuronide (small molecule) sorting by human hepatic endoplasmic reticulum.

The entry of substrates into, and the export of glururonides from, the lumen of hepatic endoplasmic reticulum (ER) in vitro (sealed microsomes) has been measured using radioactivity-labelled materials and a rapid filtration assay. Analysis of liver microsomes from a jaundiced patient showed the accumulation of bilirubin glucuronides within the lumen of the ER. Further analysis of these hepatic microsomes revealed that newly synthesized 1-naphthol glucuronide could exit from the microsomes whereas bilirubin glucuronide was accumulated within the microsomes. These results suggest the existence of mechanisms for the sorting of small molecules, destined for export through bile canalicular or basolateral plasma membranes, by ER. Furthermore, these sorting processes may be regulated by specific transporters within the ER.

The hepatic glucose-6-phosphatase system in Ehrlich-ascites-tumour-bearing mice.

To examine the effects of the presence of Ehrlich ascites tumours on both the catalytic unit and the substrate/product translocase components of the glucose-6-phosphatase system in vivo, we isolated microsomes from the livers of control and tumour-bearing mice. Samples were analysed immunochemically for the quantity of catalytic unit, stabilizing protein and translocases T2 and T3 proteins. In comparison experiments, a variety of kinetic studies were performed. The most striking findings in tumour-bearing mice were: a 2.5-fold increase in the quantity of translocase T2 protein; increases in the Km and Vmax. for glucose 6-phosphate phosphohydrolase; and a decrease in the Km value for carbamoyl phosphate (carbamoyl-P) of carbamoyl-P:glucose phosphotransferase, all with intact microsomes. The percentage latency at Vmax. decreased for PPi phosphohydrolase and for glucose 6-phosphate phosphohydrolase, but was unaffected for carbamoyl-P:glucose phosphotransferase. These observations support a tumour-related increase in translocase T2 capacity in vivo, as it transports Pi from the microsomal lumen to the medium and carbamoyl-P or PPi from the medium to the microsomal lumen.

Identification, purification and genetic deficiencies of the glucose-6-phosphatase system transport proteins.

Hepatic microsomal glucose-6-phosphatase (Glc-6-P'ase) is a complex multicomponent system containing at least three transport proteins, in addition to the catalytic subunit and a Ca2+ binding regulatory protein. The transport proteins have been designated T1 the glucose-6-phosphate transport protein, T2 a phosphate/pyrophosphate transport protein and T3 a glucose transport protein. Diagnosis of the genetic deficiencies of these transport proteins at present requires a complex kinetic analysis of the Glc-6-P'ase system as a whole. Here we describe the progress to date in our attempts to identify, purify and clone each transport protein with the ultimate aim of isolating specific cDNA probes for each transport protein which can be used for the diagnosis of types 1b, 1c and the putative 1d glycogen storage diseases.

The molecular basis of the genetic deficiencies of five of the components of the glucose-6-phosphatase system: improved diagnosis.

The understanding of type 1 glycogen storage diseases (GSDs) has been greatly hindered by a lack of knowledge of the molecular basis of glucose-6-phosphatase (Glc-6-P'ase). The problem has been the complete failure of many laboratories, including our own, to purify to homogeneity a single polypeptide with high levels of Glc-6-P'ase activity. The best preparations to date all contain five or six different polypeptide bands and have specific activities in the range 17-50 mumoles/min per milligram. The two major reasons for failure have been that Glc-6-P'ase is extremely difficult to solubilise from the microsomal membrane (large amounts of detergents are needed) and that it is not a single polypeptide as originally thought, but a multicomponent system. Recent studies of patients with type 1 GSD have proved that Glc-6-P'ase comprises at least five different polypeptides. Four of the proteins have now been purified and three have been cloned. We have assayed the Glc-6-P'ase system in over 600 human biopsy samples and developed microassays to diagnose deficiencies of each of the proteins. Ways of avoiding possible problems which have the potential to lead to the wrong diagnosis will be discussed.

Glucose-6-phosphatase in normal adult human intestinal mucosa.

1. The existence of specific glucose-6-phosphatase activity in human intestinal mucosa has been somewhat controversial. 2. We have demonstrated the presence of low levels of specific glucose-6-phosphatase activity in normal human adult intestinal mucosa. Activity was found in oesophagus, stomach, duodenum and colon. 3. Immunoblot analysis using antibodies monospecific for the 36.5 kDa liver glucose-6-phosphatase catalytic subunit demonstrated that intestinal mucosa contains low levels of the glucose-6-phosphatase enzyme protein. 4. The low levels of activity together with problems of proteolysis make human intestinal biopsies unsuitable for use in the diagnosis of type 1 glycogen-storage disease.

Cloning and expression of a hepatic microsomal glucose transport protein. Comparison with liver plasma-membrane glucose-transport protein GLUT 2.

Antibodies raised against a 52 kDa rat liver microsomal glucose-transport protein were used to screen a rat liver cDNA library. Six positive clones were isolated. Two clones were found to be identical with the liver plasma-membrane glucose-transport protein termed GLUT 2. The sequence of the four remaining clones indicates that they encode a unique microsomal facilitative glucose-transport protein which we have termed GLUT 7. Sequence analysis revealed that the largest GLUT 7 clone was 2161 bp in length and encodes a protein of 528 amino acids. The deduced amino acid sequence of GLUT 7 shows 68% identity with the deduced amino acid sequence of rat liver GLUT 2. The GLUT 7 sequence is six amino acids longer than rat liver GLUT 2, and the extra six amino acids at the C-terminal end contain a consensus motif for retention of membrane-spanning proteins in the endoplasmic reticulum. When the largest GLUT 7 clone was transfected into COS 7 cells the expressed protein was found in the endoplasmic reticulum and nuclear membrane, but not in the plasma membrane. Microsomes isolated from the transfected COS 7 cells demonstrated an increase in their microsomal glucose-transport capacity, demonstrating that the GLUT 7 clone encodes a functional endoplasmic-reticulum glucose-transport protein.

Glycogen storage disease diagnosed in adults.

Glycogen storage diseases are usually identified in childhood. We present the clinical, biochemical and histological features of 10 patients first diagnosed in adult life. Five had glycogen storage disease type 1a, one type 1c, two type IX, and in two patients there were previously unreported abnormalities of hepatic glucose-6-phosphatase system activity. Of the latter, one patient had an inhibitor of liver glucose-6-phosphatase (pseudo-1b glycogen storage disease) the other having abnormal glucose-6-phosphatase activity and microsomal pyrophosphate transport. A glucagon test is suggested as a useful screening procedure. Glycogen storage disease should be considered in adults with symptoms suggesting hypoglycaemia.

Analysis of human hepatic microsomal glucose-6-phosphatase in clinical conditions where the T2 pyrophosphate/phosphate transport protein is absent.

The availability of a rare set of human hepatic microsomes in which T2, a pyrophosphate/phosphate transport protein of the glucose-6-phosphatase system, has been shown immunologically to be completely absent, has permitted further characterization of multicomponent glucose-6-phosphatase (EC 3.1.3.9). Pyrophosphatase activity in intact microsomes was found to be totally absent, but was normal in disrupted microsomes. However, Pi did not accumulate within the lumen of the microsomes when glucose 6-phosphate was the substrate. This was not as predicted if there is only one transport protein in the endoplasmic reticulum capable of transporting Pi, produced by glucose-6-phosphatase, out of the lumen. The results suggest that the pyrophosphate/phosphate transport system of human hepatic endoplasmic reticulum must be more complex than previously thought, as it must comprise at least two protein components.

Transverse topology of glucose-6-phosphatase in rat hepatic endoplasmic reticulum.

Antibodies raised against purified components of glucose-6-phosphatase were used to study the transmembrane orientation of the complex. Measurements of glucose-6-phosphatase activities and immunoblot analysis of sealed microsomes and detergent-solubilized microsomes after treatment with proteases suggested that most of the catalytic subunit resides within the lumen of the endoplasmic reticulum. In contrast, other components of glucose-6-phosphatase are accessible to the cytoplasm. Treatment of the partially purified glucose-6-phosphatase enzyme with glycopeptide N-glycosidase indicated that the catalytic subunit of the enzyme was a glycoprotein.