James K. Petell

Expression of dipeptidyl peptidase IV in rat tissues is mainly regulated at the mRNA levels.

Dipeptidyl peptidase IV (DPPIV) is a serine peptidase that cleaves N-terminal dipeptides from polypeptides when the second residue is a proline or an alanine. We have recently cloned cDNAs for rat gp110, a membrane glycoprotein with Mr of 110,000 isolated initially from rat liver. Studies reported here establish that the gp110 for which we have cloned cDNAs is DPPIV. Using the antibodies against and cDNA for DPPIV, we have assessed the tissue distribution of DPPIV by molecular approaches. Immunoblot analysis demonstrated that DPPIV is present in the kidney, lung, and small intestine at high levels, in the liver and spleen at moderate levels, and in the heart at low levels. The highest levels of mRNA for DPPIV were detected in the kidney and small intestine as compared to moderate levels found in the lung, liver, and spleen. The lowest levels of DPPIV mRNA were found in the stomach, testis, and heart. No detectable DPPIV protein and mRNA were found in brain or muscle. LDPPIV protein and mRNA are present at much lower levels in fetal livers as compared to the adult liver. Indirect immunofluorescence microscopy demonstrated that DPPIV is localized in the bile canaliculus of hematocytes and in the apical membrane domains of kidney tubule and small intestine. Further studies by Southern blot analysis indicate that DPPIV is encoded by a single gene.

Developmental regulation of the hepatocyte receptor for galactose-terminated glycoproteins☆

The receptor which recognizes glycoproteins that have had their terminal sialic acids removed, thus exposing penultimate galactose residues (asialoglycoproteins), was examined for expression in rat liver during development. The level of asialoglycoprotein receptor binding activity in fetal rat livers was present in very low amounts but rose dramatically at the time of birth and reached adult levels by the second day after birth. Using immunoquantitation methods, it was found that the increased binding capacity of rat liver for asialoglycoproteins during development reflected accumulation of receptor molecules rather than activation of previously existing ones. The relative rates of synthesis of the predominant polypeptide of Mr 42,000 and the lesser abundant polypeptides of Mr 50,000 and 58,000 which comprise asialoglycoprotein receptor were found to increase in livers of fetuses near term and attain adult synthesis rates around birth. Thus, the accumulation of receptor protein molecules during development reflected increased synthesis of receptor polypeptides. These results suggest that the different gene products which code for the three forms of the receptor are coordinately expressed during development. Copurifying with asialoglycoprotein receptor during ligand affinity chromatography were polypeptides of Mr 25,000 and 27,000. These polypeptides display several characteristics similar to hepatic mannose binding lectin described by others. Onset of synthesis of the mannose binding lectin during development was analogous to asialoglycoprotein receptor but, in contrast, did not reach adult synthesis rates immediately after birth.

Alteration in the regulation of plasma membrane glycoproteins of the hepatocyte during ontogeny

The expression of four integral membrane glycoproteins was examined in detail utilizing monospecific antibodies during liver development. These included asialoglycoprotein receptor, a hepatocyte glycoprotein residing in the sinusoidal domain, and three bile canalicular glycoproteins, leucine aminopeptidase, dipeptidyl peptidase IV, and a Mr 110,000 glycoprotein denoted GP 110. It was observed that asialoglycoprotein receptor, GP 110, and dipeptidyl peptidase IV were present in low amounts in fetal liver and reached adult levels between 1 to 3 weeks. In contrast, leucine aminopeptidase was present in nearly adult amounts in 18-day-old fetal livers. These observations were qualitatively confirmed by indirect immunofluorescent staining of frozen thin liver sections obtained from fetal and adult rats. Further, in fetal livers it was found that leucine aminopeptidase was not localized to typical bile canalicular areas. Immunoprecipitation studies performed in the presence of proteolytic inhibitors using detergent-solubilized extracts of metabolically labeled liver minces revealed that GP 110 was present in low amounts as Mr 110,000 and Mr 105,000 polypeptides in 17-day fetal livers but by 21 days of gestation the larger polypeptide was the major synthesis product. Conversely, the apparent molecular weights of leucine aminopeptidase and dipeptidyl peptidase IV were not altered during development. Experiments determining relative rates of synthesis using excess amounts of antibodies showed that the concentrations of the three bile canalicular glycoproteins in liver during ontogeny reflect their rates of synthesis. These results underscore that plasma membrane constituents of the hepatocyte undergo dramatic changes in expression and localization as the liver changes its physiological role at birth.

Levels of Cu-Zn and Mn superoxide dismutases in rat liver during development

Superoxide dismutase is the main scavenger of superoxide radicals in the mammalian body. The liver has high levels of two types of superoxide dismutase enzymes, cytosolic Cu-Zn and mitochondrial Mn enzymes. The aim of the present study was to investigate the content of two distinct superoxide dismutases in liver during the perinatal transition from a hypoxic to a hyperoxic environment. Both isozymes were purified to homogeneity and used as immunogens in rabbits. Antisera raised were found to recognize only polypeptides of molecular weight 16,900 or 23,400, which correspond to Cu-Zn and Mn superoxide dismutases, respectively. It was found that the level of Cu-Zn superoxide dismutase enzymatic activity and protein as assessed by immunoquantitation increased 10-fold during the postnatal period, reaching adult levels by 3 weeks. In contrast, the amount of Mn superoxide dismutase content increased only twofold to adult levels during the first week of life. Neither of the superoxide dismutases showed an alteration in specific activity or apparent molecular weight in rat livers during ontogeny. These results show that the levels of two intracellular superoxide dismutases are differentially elevated during the perinatal period. It is suggested that each dismutase plays a different physiological role for superoxide scavenging in liver as a function of the hypoxic/hyperoxic environment at birth.

Isolation of domains of the plasma membrane of hepatocytes

Several recent studies have demonstrated the ability of techniques based on immunoadsorption to selectively isolate specialized subregions of membranes, termed domains, which are derived from a larger more complex parent membrane like the plasma membrane. The immunoadsorbent is directed against a specific antigen that resides exclusively or predominantly in the membrane domain to be isolated. Thus, a monospecific antibody to the domain-specific antigen is required. In the present study we developed a method employing a modified immunoblotting strategy which could utilize polyspecific antibodies to isolate membrane vesicles derived from a specific membrane domain of the hepatocyte plasma membrane. We also used specific cell surface labeling of the hepatocyte plasma membrane by lactoperoxidase-catalyzed iodination at 4 degrees C and preparation of different sized vesicles by sonication to facilitate isolation of the specific domain. For this study, polyspecific antisera were raised in goats against a membrane fraction, denoted N2u, which is enriched in bile canalicular proteins. This antiserum recognizes, among other antigens, a 110,000 Mr polypeptide previously shown to be localized in the bile canaliculus (J. Cook et al. (1983) J. Cell. Biol. 97, 1823-1833). A monospecific antiserum was raised in rabbits against the rat hepatocyte asialoglycoprotein receptor, a sinusoidal domain-specific set of glycoproteins whose major form has a Mr of 43,000. These antisera were each coupled indirectly to different pieces of nitrocellulose by the immunoblotting protocol and were used to isolate membrane vesicles from a crude extract of liver plasma membrane prepared by sonication. The ratio of iodinated asialoglycoprotein receptor to the 110,000 Mr polypeptide in vesicles isolated by the affinity nitrocellulose immunoadsorbent method indicate a 10- to 15-fold enrichment of sinusoidal-derived vesicles relative to bile canalicular-derived membrane vesicles. These results show that the affinity nitrocellulose immunoadsorbent method can be used to isolate domain-specific vesicles. Further, the affinity immunoadsorbent method described here for the isolation of domains of the plasma membrane is an integrative one allowing isolation of vesicles present in relatively small concentration in crude cell extracts and it requires minimal ultracentrifugation time.

Reduced Levels of Galactose-Terminated Glycoproteins in Rat Serum during Perinatal Development

Asialoglycoprotein receptor is an abundant protein localized in the sinusoidal domain of the hepatocyte plasma membrane. Its principle function is the clearance of serum glycoproteins which include several acute phase reactant proteins that have lost their terminal sialic acid residue. It has been reported that asialoglycoprotein receptor is nearly absent in mammalian fetal liver but rises to adult levels during the early postpartum period. The hypothesis to be tested was to determine whether defective glycoproteins, those lacking terminal sialic acid residues, are accumulated in fetal serum as a result of reduced receptor content. It was found that low levels of asialoglycoprotein receptor in 18-day-old fetal liver correlated to a threefold higher level of total asialoglycoproteins in fetal serum than adult serum as determined by a modified Western blot protocol employing radioiodinated ricinus communis agglutinin, a galactose-binding lectin. As hepatic asialoglycoprotein receptor accumulated at the time of birth, the elevated level of serum asialoglycoproteins decreased to adult values in an opposite manner. This study indicates that the hepatic asialoglycoprotein receptor plays an important role in reducing the amount of circulating defective glycoproteins at the time of birth.

93. DIFFERENTIAL REGULATION OF SOD1 AND SOD2 IN RAT LIVER DURING ONTOGENY

Superoxidase dismutase (SOD) is the main scavenger of superoxidase radicals in the mammalian body. The liver contains high levels of two SOD enzymes; Cu-Zn (SOD1) localized in cytosol and Mn SOD (SOD2) localized in mitochondria. The aim of the present study was to investigate the levels of SOD1 and SOD2 during development using enzymatic and immunological methodologies. Monospecific antibodies were prepared in rabbit against purified rat SOD1 and SOD2 polypeptides. Homogenates of fresh rat livers obtained from newborn, young, and adult rats were assessed for levels of SOD1 and SOD2 activity. It was found that the levels of SOD1 activity increased greater than ten-fold after birth, reaching adult levels after weaning. In contrast, the SOD2 level increased only four-fold after birth and reached essentially adult levels at weaning. SOD1 and SOD2 in newborn and young rat livers were not different in their molecular weight or charge from adult liver. Immunoblot quantitation indicated that changes in activity during ontogeny levels reflected the number of SOD1 and SOD2 molecules. These results show that SOD1 and SOD2 are differentially and independently regulated during ontogeny. It may be suggested that cytosolic and mitochrondrial SOD may play different roles in superoxide scavaging during liver development.

HORMONAL REGULATION OF LIVER PLASMA MEMBRANE PROTEIN EXPRESSION DURING DEVELOPMENT

In the past, dexamethasone and glucagon has been shown to have a variable effect on the expression of soluble liver enzymes during development. The aim of the present study was to investigate the effect of dexamethasone and glucagon on the expression of plasma membrane proteins during ontogeny. Our previous studies have demonstrated a significant increase of the asialoglycoprotein receptor (ASGR) and 110,000 Mr glycoprotein at birth, decrease in alkaline phosphatase (AP) and gammaglutamyltranspeptidase (GT) and very little change in leucine aminopeptidase (LAP) after birth. Buffalo rat fetuses at 18 days of gestation and 1-day-old newborns were injected (single and daily, respectively) with dexamethasone (2 and 4 μg, respectively) or glucagon (25 and 50 μg, respectively). Fetuses were sacrificed at 22 days of gestation and newborns at 3, 5, and 7 days of age. Total post-nuclei membrane vesicles were prepared from rat livers using a sucrose gradient method. Quantitation of levels of membrane proteins was performed using immunological and/or enzymatic methods. In fetal rats, dexamethasone did not affect the expression of membrane proteins. In contrast, glucagon increased significantly the levels of AP, LAP, and GT. In newborns, glucagon did not alter membrane protein levels. However, dexamethasone dramatically increased six-fold the level of GT while the amounts of ASGR, AP, and LAP were reduced by two-to four-fold. These data indicate that there is a differential effect of dexamethasone and glucagon on the expression of membrane proteins during late intrauterine and early extrauterine stages of development.