Intracellular and Transcellular Transport of Secretory Component and Albumin in Rat Hepatocytes
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View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by PubMed Central Intracellular and Transcellular Transport of Secretory Component and Albumin in Rat Hepatocytes ELIZABETH S. SZTUL, KATHRYN E. HOWELL,* and GEORGE E. PALADE Section of Cell Biology, Yale University School of Medicine, New Haven, Connecticut 06510; and *European Molecular Biology Laboratory, Heidelberg, Federal Republic of Germany ABSTRACT The intra- and transcellular transports of hepatic secretory and membrane proteins were studied in rats in vivo using [3H]fucose and [35S]cysteine as metabolic precursors. Incorporated radioactivity in plasma, bile, and liver subcellular fractions was measured and the labeled proteins of the Golgi complex, bile, and plasma were separated by SDS PAGE and identified by fluorography. 3H-radioactivity in Golgi fractions peaked at 10 rain postinjection (p.i.) and then declined concomitantly with the appearance of labeled glycoproteins in plasma. Maximal secretion of secretory fucoproteins from Golgi occurred between 10 and 20 min p.i. In contrast, the clearance of labeled proteins from Golgi membrane subfractions occurred past 30 min p.i., indicating that membrane proteins leave the Golgi complex at least 30 min later than the bulk of content proteins. A major 80,000-dalton form of secretory component (SC) was identified in the bile by co-precipitation with (IgA)2 by an anti-lgA antibody. An antibody (raised in rabbit) against the biliary 80,000-dalton peptide recognized two larger forms (116,000 and 94,000 dalton), presumably precursors, in Golgi membranes. A compara- tive study of kinetics of transport of 35S-SC and 35S-albumin showed that albumin peaked in bile at ,-,45 min p.i., whereas the SC peak occurred at 80 min p.i., suggesting that the transit time differs for plasma and membrane proteins that are delivered to the bile canaliculus. Hepatocytes secrete proteins into two distinct extracellular or not with IgA [23]), presumably after being proteolytically compartments: the space of Disse (associated with sinusoidal cleaved from membranes. Available evidence suggests that capillaries), and the bile capillaries. The output of proteins the SC-(IgA)2 complex is transported across the cell from the secreted into the blood plasma in the spaces of Disse is basolateral to the luminal domain of the plasmalemma, i.e., considerably greater than the biliary counterpart. Hepatic in the case of the hepatocyte, from the sinusoidal to the biliary proteins secreted into the bile fall into two distinct categories aspects of the cell membrane (8, 16, 38). (21). The first group comprises the same components that are SC is an interesting research topic: it is originally a mem- in the blood plasma, presumably transported from blood into brane protein; it is apparently involved in the transcellular bile via a transcellular pathway. Albumin is the predominant transport of a physiologically significant ligand, (IgA)2; and it species in this category (17). The second group consists of appears to be converted into a soluble, secretory protein before specific components, i.e., proteins not found in the plasma. or soon after reaching the luminal plasmalemma. SC appears, The secretory component (SC)' of polymeric IgA is the major therefore, as a representative of a new category of proteins protein in this class. that are physiologically converted, within their lifetime, from SC is a protein of special interest. It is synthesized by integral membrane proteins to soluble secretory entities (33). epithelial cells as a transmembrane protein (20); it is found In addition, SC can be conveniently studied since it is synthe- on the basolateral domain of their plasmalemma (13) where sized at a higher rate than most other membrane proteins and it acts as receptor for IgA dimers (3, 26), but it is also found does not seem to be involved in extensive recycling. in exocrine secretions, e.g., sputum (35), milk (15), saliva (6, We have decided to investigate (a) the kinetics of intracel- 19), and bile (22) as a soluble secretory protein (complexed lular transport of secretory and membrane proteins, from their sites of synthesis to the sinusoidal aspect of hepatocytes, Abbreviations used in this paper: p.i., postinjection; PTA, phos- for either secretion or incorporation into the plasmalemma; photungstic acid; and SC, secretory component. (b) the kinetics of appearance of newly synthesized proteins THE JOURNAL OF CELL BIOLOGY • VOLUME 97 November 1983 1582-1591 1 582 © The Rockefeller University Press • 0021-9525/83/11/1582/10 $1.00 in the bile; and (c) the nature and origin of these biliary prepared for electrophoresis as in (12), except that bile samples (200 ~1) were proteins. treated with 2 ml of ice-cold acetone to precipitate proteins; the latter were resuspended in electrophoresis buffer prior to boiling. Gels were processed for With the exception of albumin, practically all plasma pro- fluorography with EN3HANCE (New England Nuclear). teins are glycoproteins and many of them are fucosylated (2). SC is also fucosylated and so are other biliary secretory RADIOIODINATION glycoproteins. Hence, [3H]fucose can be used, in addition to radioactive amino acids, for the comparative studies men- Peptides to be radioiodinated (10-50 ag) were mixed with carrier-free Natal (2 mCi), and 0.5 M sodium phosphate buffer, pH 7.2, in a final volume of 100 tioned above. ul. The mixtures were transferred into tubes plated with Iodo-gen (Pierce In this paper we report data obtained using [3H]fucose and Chemical Co., Rockford, IL) and the reaction was allowed to proceed for l0 [35S]cysteine in a study of the pathways and kinetics involved min on ice. Free Na~251 was separated from labeled peptides by desalting on in the transport of cohorts of secretory and membrane pro- Sephadex G-25 medium (Pharmacia Fine Chemicals Div., Pharmacia Inc., teins to the sinusoidal and biliary aspects of hepatocytes. We Piscataway, N J) columns. Rat albumin was labeled to a specific radioactivity of 0.30 mM ~251/mM albumin (1.2 x l0 ~° cpm/mg), while rat bile proteins intend to use this information as background against which contained 7.06 x l0 ~ mM ~2sI/200 #g protein (1.47 x l0 s cpm/mg). to study the kinetics and transport pathways of the SC from its site of synthesis to its final discharge into bile, IMMUNE PRECIPITATION MATERIALS AND METHODS Samples were made 2% in SDS, boiled 2 min, and made up to a final concentration of 2% Triton X-100, 150 mM NaCI, 2 mM EDTA, 30 mM Tris- Materials HC1, pH 7.6 (immunoprecipitation buffer). To each sample 50 #1 of sheep anti- rat IgA serum (alpha chain specific) (Cappel Laboratories, Inc., Cochranville, Specific biochemical compounds were purchased from Sigma Chemical Co. PA) was added, and antigen-antibody complexes were allowed to form over- (St. Louis, MO); L-[6-3H]fucose, 27 Ci/mmol and [a~S]cysteine, 1,080 Ci/mmol night at 4"C. Immune complexes were adsorbed on protein A-Sepharose were purchased from Amersham Corp. (Arlington Heights, IL). Na~251 was (Pharmacia Fine Chemicals) as follows: to each sample, 60 #1 of a solution obtained from New England Nuclear (Boston, MA). consisting of 50% beads/50% immunoprecipitation buffer was added, and the mixture was incubated for 1 h at room temperature. The beads were sedi- mented, washed three times with immunoprecipitation buffer, two times with Methods the same buffer without Triton X-100, and processed for SDS PAGE as described above. IN VIVO LABELING EXPERIMENTS 120-180-g male Sprague-Dawley rats (Charles River Breeding Laboratories, ANALYTICAL PROCEDURES Inc., Wilmington, MA) were anesthetized with intraperitoneally injected Nem- butal (Abbott Laboratories, North Chicago, IL) at 0.1 ml/100 g animal wt, and RADIOACTIVITY DETERMINATION: An equal volume (1 ml) of ice- received the label by injection into the saphenous vein. At various times after cold 1% phosphotungstic acid (PTA) in 0.5 M HCI was added to each sample label administration, the livers were removed for further processing. of interest? The precipitate was washed three times by resuspension in the same PTA solution followed by low speed centrifugation and the final pellet was resuspended in 0.5 ml of 1% SDS. After addition of Liquiscint (National SAMPLING OF BLOOD AND BILE Diagnostics, Inc., Somerville, NJ), the samples were counted in a Beckman LS- Blood was collected (under anesthesia) from the tail vein and allowed to 250 liquid scintillation system (Beckman Instruments, Palo Alto, CA). clot. The serum was separated from the clot by low speed centrifugation. Bile LIPID EXTRACTION: Lipids were extracted from ceU fractions accord- was collected (under anesthesia) via a canula (PE-50 tubing, Clay Adams, ing to Salto and Hakamori (34), first with 20 vol of chloroform/methanol (2:1), Parsippany, N J) inserted into the common bile duct. and then with 20 vol of chloroform/methanol (1:2). The organic phases were pooled, dried under N2, and the residue was redissolved in chloroform. Chlo- roform solution samples were counted upon addition of Liquiscint. LIVER HOMOGENIZATION AND CELL PROTEIN ASSAY: Protein was estimated using the Bio-Rad Protein FRACTIONATION Assay (Bio-Rad Laboratories, Richmond, CA), with BSA as the standard. Livers were removed from anesthetized rats, placed in ice-cold 0.25 M sucrose, and minced. The mince was washed free of blood and then homoge- RESULTS nized with a motor-driven Teflon pestle in a Brendler homogenizer to give Clearance of [3H]Fucose Radioactivity from the ~30% wt/vol homogenate. Golgi fractions were isolated by a modification (12) of the procedure of Blood Plasma Ehrenreich et al. (7) omitting in all cases ethanol administration to the animals. Three Golgi fractions were obtained: GF~+2, GF~t, and GFa.