Search PubMed⌕ Search

Biomedical subjects

G Xu

Publications and source records attributed to G Xu.

At least 307 records · Page 17Linked to original sources
PubMed1998

Nutritional and hormonal regulation of the gene for malic enzyme.

In vivo, refeeding starved chickens stimulates transcription of the avian gene for malic enzyme in liver; in hepatocytes in culture, triiodothyronine (T3) and insulin stimulate transcription of this gene. In vivo, starvation, and in hepatocytes in culture, glucagon, medium-chain fatty acids (MCFA) and long-chain fatty acids (LCFA) inhibit transcription of the malic enzyme gene. We have defined a T3-response unit in the 5'-flanking DNA of the malic enzyme gene; it contains one major T3 response element and several minor ones; maximum responsiveness is dependent on the presence of all of these elements. LCFA probably act by inhibiting binding of T3 to its nuclear receptor. MCFA appear to act by a different mechanism. Inhibitory MCFA have chain lengths of six, seven or eight carbons; a common feature of other inhibitory compounds is that they can be metabolized to MCFA. Eight-carbon fatty acids with a hydroxyl on the 2- or 3-carbon are more potent inhibitors than octanoate, whereas 2-bromo-fatty acids and 2-hydroxy hexanoate are not inhibitory. In transfection experiments with a large variety of constructs derived from the malic enzyme 5'-flanking DNA, the ability of fatty acids to inhibit promoter function localizes to regions of DNA that contain T3REs. Promoter function of artificial T3REs also is inhibited by MCFA. Inhibition of promoter function using malic enzyme DNA is relatively constant in magnitude irrespective of the size of the T3 response. We postulate that MCFA directly regulates one of the functions of the T3 receptor.

Animal Nutritional Physiological Phenomena↗
PubMed1998

Fetal Smith-Lemli-Opitz syndrome can be detected accurately and reliably by measuring amniotic fluid dehydrocholesterols.

The Smith-Lemli-Opitz syndrome, characterized by limb, face and organ abnormalities, and mental retardation, is caused by an inherited block in the step of cholesterol biosynthesis in which the delta 7 double bond of 7-dehydrocholesterol is reduced. It is diagnosed by the presence of markedly elevated levels of 7-dehydrocholesterol and 8-dehydrocholesterol in plasma and tissue. We measured amniotic fluid sterols in 15 pregnancies in 13 women who had previously carried an affected fetus. Cholesterol, 7-dehydrocholesterol and 8-dehydrocholesterol concentrations averaged 18 +/- 3, 9.8 +/- 2.9 and 5.0 +/- 1.7 micrograms/ml, respectively, in seven pregnancies with an affected fetus or child. In contrast, these levels were 19 +/- 3, 0.05 +/- 0.01 and < 0.005 micrograms/ml, respectively, in eight increased-risk pregnancies with normal outcomes and 16 +/- 2, 0.07 +/- 0.01 and < 0.005 micrograms/ml in normal controls. 7-dehydrocholesterol concentrations, 2.2-26 and 0.05-0.10 micrograms/ml in pregnancies with an affected and unaffected fetus, respectively, did not overlap. Thus, abnormally elevated amniotic fluid dehydrocholesterol concentrations are an accurate predictor of fetal Smith-Lemli-Opitz syndrome. A false-positive or a false-negative result is highly unlikely.

Adult↗
PubMed1998

Regulation of classic and alternative bile acid synthesis in hypercholesterolemic rabbits: effects of cholesterol feeding and bile acid depletion.

The effect of cholesterol feeding (3 g/day) on bile acid synthesis was examined in 10 New Zealand white rabbits (NZW), 8 Watanabe heterozygous and 10 homozygous rabbits with partial and complete deficiencies of LDL receptors. After 10 days of cholesterol feeding, bile fistulas were constructed and bile acid pool sizes were measured. Cholesterol feeding increased plasma and hepatic cholesterol levels in all rabbit groups. Baseline bile acid pool sizes were smaller (P < 0.01) in heterozygotes (139 +/- 3 mg) and homozygotes (124 +/- 30 mg) than NZW rabbits (254 +/- 44 mg). After feeding cholesterol, bile acid pool sizes doubled with increased cholic acid synthesis in NZW and, to a lesser extent, in Watanabe heterozygous rabbits but not in homozygotes. Baseline cholesterol 7alpha-hydroxylase activity in NZW and heterozygotes declined 69% and 53% (P < 0.001), respectively, after cholesterol feeding. Sterol 27-hydroxylase activity reflecting alternative bile acid synthesis increased 66% (P < 0.01) in NZW and 37% in Watanabe heterozygotes but not in homozygotes after feeding cholesterol. Bile fistula drainage stimulated cholesterol 7alpha-hydroxylase activity but not sterol 27-hydroxylase activity in all three rabbit groups. These results demonstrated that dietary cholesterol increased hepatic sterol 27-hydroxylase activity and alternative bile acid synthesis to expand the bile acid pool and inhibited cholesterol 7alpha-hydroxylase in NZW and in Watanabe heterozygous rabbits but not in homozygotes with absent hepatic LDL receptor function. Thus, in rabbits, sterol 27-hydroxylase is up-regulated by the increased hepatic cholesterol that enters the liver via LDL receptors whereas cholesterol 7alpha-hydroxylase is controlled by the circulating hepatic bile acid flux.

Animals↗
PubMed1998

Synthesis and intestinal metabolism of ursodeoxycholic acid conjugate with an antiinflammatory agent, 5-aminosalicylic acid.

5-Aminosalicylic acid conjugate of ursodeoxycholic acid was synthesized in above 90% yield by adding a basic solution of 5-aminosalicylic acid into the mixed anhydride formed with ursodeoxycholic acid and ethyl chloroformate. The 5-aminosalicylic acid conjugate of ursodeoxycholic acid was poorly secreted into the bile and was deconjugated with cholylglycine hydrolase and Clostridium perfringens, that deconjugate naturally occurring glycine and taurine conjugates of bile acids. However, ursodeoxycholic acid 5-aminosalicylic acid conjugate was not absorbed from the duodenum but was concentrated in the colon where it was partially hydrolyzed by the intestinal bacteria to ursodeoxycholic acid and 5-aminosalicylic acid. We believe that this unique conjugation of ursodeoxycholic acid with 5-aminosalicylic acid may facilitate the transport of both 5-aminosalicylic acid and ursodeoxycholic acid to the colon and may be useful for the treatment of colonic inflammatory bowel diseases, ulcerative colitis and Crohn's disease.

Amidohydrolases↗
PubMed1997

Separate bisphosphatase domain of chicken liver 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase: the role of the C-terminal tail in modulating enzyme activity.

The separate bisphosphatase domain (amino acid residues 243-468) of the chicken liver bifunctional enzyme 6-phosphofructo-2-kinase-fructose-2,6-bisphosphatase was expressed in Escherichia coli and purified to homogeneity. The fructose-2, 6-bisphosphatase activity of the separate domain was 7-fold higher than that of the native bifunctional enzyme, and exhibited substrate inhibition characteristic of the native enzyme. The inhibition of the enzymes by fructose 2,6-bisphosphate could be overcome by Pi, glycerol 3-phosphate and GTP. Deletion of 30 amino acid residues from the C-terminus of the separate domain resulted in around a 5-fold increase in the Vmax of the bisphosphatase. Also, the truncated form was more accessible to chemical modification by diethyl pyrocarbonate and N-ethylmaleimide, suggesting a more open structure than the wild-type form. In addition, the mutation of cysteine-389 to alanine increased bisphosphatase activity by 20% and the Km value for fructose 2,6-bisphosphate by 3-fold, and both the point mutation at cysteine-389 and the deletional mutation led to the predominantly insoluble expression of the enzyme. The results indicated that the C-terminal tail plays a role in modulating the enzyme activity and suggested that the difference in the C-terminal tail sequence is responsible for the difference in activity of the chicken and rat liver fructose-2,6-bisphosphatases. It is postulated that an interaction between the C-terminal tail and the active site might be present.

Animals↗
PubMed1997

A study on biliary ductal system and bile fistula in the American alligator, Alligator mississippiensis.

The anomalous arrangement of bile ducts in the Crocodylia has not been fully appreciated. A clear understanding of biliary anatomy is necessary in order to create complete bile drainage in these reptiles. The object of this study was to clarify the anatomy of the bile ductal system and to establish total bile fistulas in the American alligator, Alligator mississippiensis. Bile duct anatomy was studied in 104 juvenile alligators, and bile fistulas were constructed in seven alligators. In 93 out of 104 (89%) of the juveniles dissected there was an interconnection between the right and left hepatic duct before the right hepatic duct emptied into the gallbladder. The left hepatic duct then entered the duodenum independently of the cystic duct which drained the gallbladder directly into the duodenum. In 8% of the animals, the left hepatic duct did not enter the duodenum but joined with the right duct, forming a common hepatic duct that emptied into the gallbladder. In 3% of the cases, the right hepatic duct emptied into the gallbladder, while the left duct had no communication with the right hepatic duct and drained separately into the duodenum. This arrangement of bile ducts is similar to that seen in birds and reflects the common ancestry of crocodiles and birds. In other reptiles, the biliary system shows much more variability and is different from the alligator. In five of seven alligators in which total biliary diversion was attempted, the biliary catheter remained in place and stayed patent from 2-7 weeks. Bile flow was extremely low (1.5-2.5 ml/24 h) when compared to that of mammals (80-100 ml/24 h). This study demonstrates the variable nature of the biliary ductal system in Alligator mississippiensis and suggest a method for constructing an effective total bile fistula in these animals.

Alligators and Crocodiles↗
PubMed1997

Human DNA-(cytosine-5) methyltransferase-PCNA complex as a target for p21WAF1.

DNA-(cytosine-5) methyltransferase (MCMT) methylates newly replicated mammalian DNA, but the factors regulating this activity are unknown. Here, MCMT is shown to bind proliferating cell nuclear antigen (PCNA), an auxiliary factor for DNA replication and repair. Binding of PCNA requires amino acids 163 to 174 of MCMT, occurs in intact cells at foci of newly replicated DNA, and does not alter MCMT activity. A peptide derived from the cell cycle regulator p21(WAF1) can disrupt the MCMT-PCNA interaction, which suggests that p21(WAF1) may regulate methylation by blocking access of MCMT to PCNA. MCMT and p21(WAF1) may be linked in a regulatory pathway, because the extents of their expression are inversely related in both SV40-transformed and nontransformed cells.

Amino Acid Sequence↗
PubMed1997

Influences of dietary omega-3 polyunsaturated fatty acids on the recovery of cardiac and renal functions after preservation in hyperlipidemic rats.

BACKGROUND: The effects of a soybean oil diet and a high-cholesterol oil (HC) diet, and an HC diet with eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA) supplementation, on basal and postpreservative cardiac function of the hearts and on postpreservative renal function of the kidneys from older rats were examined. METHODS: Groups 1 through 4 of 100-week-old rats were fed either soybean oil, HC, HC with EPA, or HC with DHA, respectively, for 12 weeks. Blood was collected for analysis of plasma fatty acids, and the heart and left kidney were removed from the rat. In experiment 1, the heart was perfused on a Langendorff apparatus. After evaluation of the cardiac function of each rat, the heart was stored in histidine-tryptophan-ketoglutarate solution for 8 hr at 4 degrees C. The heart was reperfused and the recovery of cardiac function was evaluated. The coronary perfusate during reperfusion was collected to measure 6-keto prostaglandin F1alpha and thromboxane B2. Coronary flow (CF) perfused with Krebs-Henseleit bicarbonate (KHB) solution containing 5-hydroxytryptamine (5-HT) and nitroglycerin were evaluated in the Langendorff mode with atrial pacing (330 beats/min). In experiment 2, the excised left kidney was immediately flushed and preserved with University of Wisconsin solution for 8 hr at 4 degrees C. The kidney was then reperfused with KHB solution and renal function was evaluated. RESULTS: The plasma and cardiac EPA levels in group 3 were significantly higher than the levels found in the other groups. The plasma and cardiac ratios of EPA to arachidonic acid were significantly higher in groups 3 and 4 than in groups 1 and 2. There were no significant differences in basal cardiac function among any of the diet-fed rats. The percentage values of the recovery of aortic flow, cardiac output (CO), and left ventricular max dp/dt in group 3 and CO in group 4 were significantly higher than in group 2. In addition, the recovery of CF in group 3 tended to be higher than in group 2 (P=0.07). The percentage values of the recovery of aortic flow, CF, CO, and left ventricular max dp/dt in group 1 were significantly lower than in the other dietary groups. CF reperfused with KHB solution containing 5-HT was significantly higher in group 3 than in groups 1 and 2. CF reperfused with KHB solution containing 5-HT was significantly higher in group 4 than in group 1. CF reperfused with KHB solution containing nitroglycerin in group 3 tended to be higher than in groups 1 and 2 (P=0.07). The thromboxame B2 concentrations in the coronary perfusate during reperfusion in groups 3 and 4 were significantly lower than in groups 1 and 2. Fractional sodium reabsorption in group 3 was significantly higher than in group 2. Inulin clearance in groups 3 and 4 was significantly higher than in group 1. The postpreservative urinary flow in group 3 was significantly higher than in groups 1 and 2. The urinary flow was significantly higher in group 4 than in group 1. CONCLUSIONS: These results suggest that EPA administration may attenuate preservation and reperfusion injury and improve the recovery of cardiac and renal functions in hyperlipidemic and older rats. DHA administration may also show beneficial effects on kidney preservation in hyperlipidemic rats.

6-Ketoprostaglandin F1 alpha↗
PubMed1997

Activation of spinach chloroplast fructose-1,6-bisphosphatase by monovalent cations.

Spinach chloroplast fructose 1,6-bisphosphatase, like its counterparts from animal sources, can be activated by monovalent cations such as potassium and ammonium ions. The extents of activation are closely related to the pH values and the concentrations of magnesium ions. The activation effect is most prominent when the concentrations of magnesium ions are high enough to exert inhibitory side effect on the enzyme. Similar to the cases of enzymes from mammalian tissues, activation of the chloroplast enzyme by monovalent cations is, at least partially, due to overcoming of the inhibitory effect by the excess of magnesium ions. The enzyme can also be activated by low concentrations of guanidine hydrochloride, which probably involves a similar mechanism compared with that by monovalent cations.

Ammonium Chloride↗
PubMed1997

Gene expression and synthesis of fibronectin isoforms in rat hepatic stellate cells. Comparison with liver parenchymal cells and skin fibroblasts.

Fibronectins are multifunctional glycoproteins that are important components of the extracellular matrix in normal and fibrotic liver. Multiple fibronectin isoforms are generated from a single gene by alternative splicing of the primary transcript at the domains EIIIA, EIIIB, and V. The aim of this study was to investigate the fibronectin isoforms expressed by activated hepatic stellate cells, the most important connective tissue-producing cells in injured liver. Hepatocytes and skin fibroblasts were also studied for comparison. Activation of hepatic stellate cells in vivo was induced by injecting CCl4 twice weekly for 3 weeks. Activation in vitro was achieved by culturing cells on plastic. The level of activation was evaluated by alpha-smooth muscle actin immunocytochemistry. Steady-state levels of fibronectin isoform messenger RNA were examined by Northern hybridization analysis using specific cDNA probes for the EIIIA, EIIIB, and V domains. The de novo synthesis of fibronectin isoforms was examined by metabolic labelling and immunoprecipitation using domain-specific monoclonal antibodies. Fibronectin transcripts were not detectable in freshly isolated hepatic stellate cells from normal liver. Cultured hepatic stellate cells, as well as skin fibroblasts, expressed EIIIA+, EIIIB+, and V95+ transcripts. They were detectable as early as day 3 and increased with time in culture. At 3 days in culture, more than 90 per cent of stellate cells were alpha-smooth muscle actin-positive. In vivo activated hepatic stellate cells expressed EIIIA+ and V95+ transcripts; EIIIB+ fibronectin mRNA was absent. Less than 20 per cent of in vivo activated stellate cells expressed alpha-smooth muscle actin. Freshly isolated parenchymal cells from normal liver as well as from CCl4-treated liver expressed V95+ transcripts, but were negative for EIIIA or EIIIB fibronectin mRNA. Immunoprecipitation results were in accordance with Northern hybridization analysis. Hepatic stellate cells in culture synthesized and secreted fibronectin molecules that contained EIIIA, EIIIB, and V fragments. Our results indicate that hepatic stellate cells synthesize and secrete fibronectin isoforms that are distinct from those of parenchymal cells.

Animals↗
PubMed1997

Localization and cellular sources of activins in normal and fibrotic rat liver.

UNLABELLED: Activins are dimeric proteins, members of the transforming growth factor beta (TGF-beta) gene superfamily, consisting of the beta-subunits of inhibin (betaA and betaB). Recently, it was shown that activin A (betaA:betaA) inhibits DNA replication and induces apoptosis in rat parenchymal cells in vitro and in vivo. Cryostat sections of normal livers and livers of rats treated with intraperitoneal injections of CCl4 were stained for the different inhibin subunits and desmin, a marker for stellate cells (SC). Staining for inhibin-alpha was invariably negative, both in normal and fibrotic rat liver. In normal liver, inhibin-betaA subunit immunoreactivity was localized in parenchymal cells (PC). Staining for inhibin-betaB was weaker but similarly distributed. In fibrotic livers, connective tissue septa were strongly immunoreactive for inhibin-betaA. Desmin-positive stellate cells (SC) accumulated in areas strongly immunoreactive for inhibin-betaA and several cells were positive for both desmin and inhibin-betaA. Staining for inhibin-betaB was weaker but similarly distributed. As above data pointed to a possible role for PC and SC, we examined by Northern blot analysis, the expression of inhibin-alpha, -betaA, and -betaB messenger RNA (mRNA) in total RNA extracted from freshly isolated SC and PC of normal and CCl4-treated liver and in cultured SC. Inhibin-betaA mRNA was predominantly expressed in PC of normal liver. Expression was lost in PC of CCl4-treated liver. Inhibin-betaB mRNA expression was induced in SC of CCl4-treated liver. Inhibin-betaA mRNA, and to a lesser extent, inhibin-betaB mRNA expression was rapidly induced in cultured SC. The presence of activin A in conditioned media of cultured SC was shown by Western blotting. Apoptotic cells, identified by terminal deoxy-transferase mediated X-dUTP nick end labeling (TUNEL)-staining, were found predominantly in and near the fibrotic septa. IN CONCLUSION: 1) while activin A was constitutively expressed in PC of normal liver, its expression was lost in PC of fibrotic liver; 2) expression of activins was induced in activated SC; and 3) apoptotic cells were found predominantly near the septa, in support of the hypothesis that activin A, derived from activated SC in the septa, contributes to the induction of cell death in neighboring PC.

Activins↗
PubMed1997

Pharmacokinetics and metabolism of dichloroacetate in the F344 rat after prior administration in drinking water.

The effect of prior administration of dichloroacetate (DCA) in drinking water on the pharmacokinetics of DCA in male F344 rats was studied. Rats were provided with DCA in their drinking water at 0.2 and 2.0 g/liter for 14 days and then challenged with iv bolus iv or gavage doses of [14C1,2]DCA, 16 hr after pretreatment withdrawal. The blood concentration-time profiles of DCA and the disposition of 14C was characterized and compared with controls. The effect of pretreatment on the in vitro metabolism of DCA in hepatic cytosol was also evaluated. Pretreatment caused a significant increase in the blood concentration and AUC0-->infinity of DCA (433.3 versus 2406 microg ml-1 hr). Pharmacokinetic analysis indicated that pretreatment significantly decreased total body clearance (267.4 versus 42.7 ml hr-1 kg-1), which was largely due to decreased metabolism since only modest differences in the urinary clearance of DCA were observed. Pretreatment significantly decreased the formation of 14CO2 after both iv and oral doses of [14C]DCA. The decrease in CO2 formation was also observed after pretreatment with DCA at 0.2 g/liter. Pretreatment also increased the urinary elimination of DCA and several metabolites, particularly glycolate. The in vitro experiments demonstrated that DCA pretreatment inhibited the conversion of DCA to glyoxylate, oxalate, and glycolate in hepatic cytosol. These results indicate that DCA has an auto-inhibitory effect on its metabolism and that pharmacokinetic studies using single doses in naïve rats will underestimate the concentration of DCA at the target tissue during chronic or repeated exposures.

Administration, Oral↗
PubMed1997

Experimental investigation of the role of the splenic vein in gastric venous obstruction after pylorus-preserving pancreatoduodenectomy in the dog.

CONCLUSION: This study showed that the splenic vein (SV) plays a major role in maintaining gastric blood flow after ligature of peripyloric vessels in the dog. Therefore, SV preservation or alternative venous drainage may be recommended to maintain sufficient gastric blood flow in pylorus-preserving pancreatoduodenectomy (PPPD). BACKGROUND: In PPPD, we often encounter the problem of gastric venous drainage obstruction, if the splenic-portal vein confluence needs to be resected. This study investigated significance of venous drainage via the left gastric and splenic veins in gastric tissue blood flow. METHODS: In seven mongrel dogs, gastric tissue blood flow was measured by both laser Doppler flowmetry (LDF) in the antrum and hydrogen gas clearance (HGC) in the antrum and body. Blood flow was measured after the following treatments: 1. No treatment; 2. Duodenal transection; 3. Ligature of the right gastric and gastroepiploic arteries and veins; 4. Transient obstruction of the left gastric vein (LGV); 5. Transient obstruction of the SV with the LGV open; and 6. Obstruction of both the LGV and the SV. RESULTS: As judged by LDF, treatments 2, 3, and 4 did not decrease blood flow, but treatments 5 and 6 significantly decreased it compared with both treatments 1 and 3. By HGC, changes in the antrum and the body were parallel. Duodenal transection did not, but treatments 3 and 4 did decrease blood flow compared with no treatment. Obstruction of the left gastric vein did not, but treatments 5 and 6 did significantly decrease it compared with treatment 3.

Animals↗
PubMed1997

Biosynthetic retrovectoring systems for gene therapy.

Chemical and physical methods of introducing genes into cells (transfection) are being combined with viral transduction as one possible approach toward overcoming the shortcomings of current gene transfer methods. Although several different strategies are being developed worldwide, this article focuses on modification of retroviral vectoring systems. One goal of this work is to combine the safety and ease of transfection methods with the permanency that is presently achieved only by integrating viruses. Work with retrotransposon pseudotypes, synthetic retrovectors, and liposomal delivery of retrovirus vectors is discussed.

Animals↗
PubMed1997

Increased bile acid pool inhibits cholesterol 7 alpha-hydroxylase in cholesterol-fed rabbits.

BACKGROUND & AIMS: Cholesterol feeding unexpectedly inhibits cholesterol 7 alpha-hydroxylase in rabbits. The aim of this study was to explore the mechanism. METHODS: Twenty male New Zealand white rabbits were fed regular chow with and without 2% cholesterol for 10 days followed by 7 days of bile drainage. The activities of hepatic cholesterol 7 alpha-hydroxylase and sterol 27-hydroxylase that control bile acid synthesis in classic and alternative pathways were related to the size and composition of bile acid pool. RESULTS: After feeding cholesterol, plasma and hepatic cholesterol concentrations increased, the bile acid pool doubled (from 254 +/- 44 to 533 +/- 51 mg; P < 0.001), cholesterol 7 alpha-hydroxylase activity decreased 68% (P < 0.01), but sterol 27-hydroxylase activity increased 66% (P < 0.05) with increased cholic acid synthesis (P < 0.01). Bile drainage in the cholesterol-fed rabbits depleted the bile acid pool and stimulated down-regulated cholesterol 7 alpha-hydroxylase activity 11.4-fold (P < 0.001), although hepatic cholesterol remained elevated. Hepatic sterol 27-hydroxylase activity was unaffected. CONCLUSIONS: Feeding cholesterol increased hepatic cholesterol and stimulated sterol 27-hydroxylase and alternative bile acid synthesis, which expanded the bile acid pool and inhibited cholesterol 7 alpha-hydroxylase in rabbits. In distinction, hepatic sterol 27-hydroxylase was insensitive to changes in the bile acid pool.

Animals↗