Biosynthesis, assembly and secretion of fibrinogen in cultured rat hepatocytes.

The biosynthesis, assembly and secretion of fibrinogen were investigated in cultured rat hepatocytes which were incubated with [35S]methionine. When initial rates of the synthesis of three fibrinogen subunits were compared, the A alpha-subunit was found to be synthesized significantly slower than the B beta- and gamma-subunits. Pulse-chase experiments revealed that the secreted fibrinogen contained different proportions of the newly synthesized subunits, depending upon the chase times. Radioactivity in the A alpha subunit, which initially had the highest level of the three, was rapidly decreased in parallel with the chase time. The gamma-subunit had an increasing amount of the radioactivity in the secreted molecule during the chase periods, whereas that in the B beta-subunit was gradually decreased at the later stages of chase. Analysis of intracellular components of fibrinogen confirmed that the nascent A alpha-subunit was most rapidly exhausted, and the gamma-subunit occupied the largest proportion among the non-assembled subunits at later stages of chase. Taken together, these results suggest that the synthesis of A alpha-subunit, which has the lowest rate, could be the rate-limiting step in the production and secretion of fibrinogen in cultured rat hepatocytes, in contrast with what has been proposed for human and rabbit fibrinogen, namely that the synthesis of B beta-subunit is the rate-limiting step. The results also indicate that there is a large intracellular pool of gamma-subunit.     

Studies on the biosynthesis, assembly and secretion of vitellogenin, an oestrogen-induced multicomponent protein.

1. The process by which the egg-yolk protein precursor vitellogenin is biosynthesized, assembled and secreted by Xenopus laevis (South African clawed toad) liver was studied. It was previously shown in other laboratories that vitellogenin contains the two egg-yolk proteins lipovitellin (mol.wt. 140 000) and phosvitin (mol.wt. 35 000). 2. Evidence is presented which shows that Xenopus liver microsomal fractions synthesize precursors of vitellogenin. These precursors were solubilized from the membranes with detergent and analysed by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. This analysis indicated that there is only one precursor polypeptide, and this has mol.wt. approx. 200 000 +/- 20 000. This demonstrates that the egg-yolk proteins are translated as part of this larger polypeptide. 3. Experiments also demonstrate the existence of a microsomal proteinase which is able to cleave the precursor into smaller fragments. The nature of these fragments provided some indirect evidence that phosvitin and lipovitellin light chains are situated together within the precursor molecule. 4. These precursor data fit in well with structural studies on serum vitellogenin, since it has been shown that the latter protein consists of two identical subunits each with a mobility on sodium dodecyl sulphate/polyacrylamide gels identical with that shown by the microsomal precursor. This indicates that both the intracellular precursor and subunit of vitellogenin have similar (but not necessarily identical) molecular weights. 5. It was also shown that trypsin or chymotrypsin can cleave the serum vitellogenin into leucine- and serine-rich fragments which resemble lipovitellin and phosvitin respectively. Attention is, however, drawn to the fact that the serine-rich fragment is not identical with phosvitin, since it contains eight times more leucine than that expected for the authentic phosvitin molecule [Penning (1976) Ph.D. Thesis, University of Southampton].     

Intracellular assembly of human fibrinogen

Hep-G2 cells, pulse-labeled with L-[35S]methionine, incorporate radioactivity within 2 min into precursor forms of fibrinogen and into fibrinogen. Pulse-labeled intracellular fibrinogen is first composed of radioactive B beta chains, followed by nascent A alpha chains. Radioactive gamma chains accumulate in the cells and later contribute, via intermediate forms, to the assembly of fibrinogen. Following a pulse-chase incubation with L-[35S]methionine, the radioactive composition of newly secreted fibrinogen also reflects the fact that there is a large intracellular pool of gamma chains.     

Studies on the assembly and secretion of a multicomponent protein. The biosynthesis of vitellogenin, an oestrogen-induced glycolipophosphoprotein

1. Incorporation of [(32)P]P(i) and [(3)H]leucine into vitellogenin secreted in vitro by liver slices from oestrogen-treated Xenopus laevis is accompanied by a 2h lag; no lag is apparent for the incorporation into total tissue protein. 2. The addition of cycloheximide was found immediately to inhibit further incorporation of radioactive leucine into total tissue protein. The incorporation into secreted vitellogenin, however, continued for 2h after the addition of cycloheximide. 3. Pulse-labelling of liver slices with [(3)H]leucine for 30min, followed by a chase with a large excess of unlabelled leucine, resulted in the appearance of radioactivity in secreted vitellogenin from 90min after the end of the pulse period. 4. Evidence is presented which suggests that of the radioactivity from [(3)H]leucine incorporated into proteins by the liver of oestrogen-treated Xenopus some 70% is present in the single protein vitellogenin. 5. The incorporation of [(32)P]P(i) into vitellogenin followed a pattern identical with that found for [(3)H]leucine in the pulse-labelling experiments and this indicates that synthesis of the polypeptide chain and incorporation of P(i) are closely linked processes. 6. The cumulative evidence suggests that the 2h lag phase represents the time required for the assembly and secretion of this multicomponent protein.     

Assembly and secretion of fibrinogen. Degradation of individual chains.

Hep G2 cells produce surplus A alpha and gamma fibrinogen chains. These excess chains, which are not secreted, exist primarily as free gamma chains and as an A alpha-gamma complex. We have determined the intracellular location and the degradative fate of these polypeptides by treatment with endoglycosidase-H and by inhibiting lysosomal enzyme activity, using NH4Cl, chloroquine, and leupeptin. Free gamma chain and the gamma component of A alpha-gamma are both cleaved by endoglycosidase-H, indicating that the gamma chains accumulate in a pre-Golgi compartment. Lysosomal enzyme inhibitors did not affect the disappearance of free gamma chains but inhibited A alpha-gamma by 50%, suggesting that A alpha-gamma is degraded in lysosomes. The degradative fate of individual chains was determined in transfected COS cells which express but do not secrete single chains. Leupeptin did not affect B beta chain degradation, had very little affect on gamma chain, but markedly inhibited A alpha chain degradation. Antibody to immunoglobulin heavy chain-binding protein (GRP 78) co-immunoprecipitated B beta but not A alpha or gamma chains. Preferential binding of heavy chain-binding protein to B beta was also noted in Hep G2 cells and in chicken hepatocytes. Taken together these studies indicate that B beta and gamma chains are degraded in the endoplasmic reticulum, but only B beta is bound to BiP. By contrast A alpha chains and the A alpha-gamma complex undergo lysosomal degradation.     

Characterization of the inhibition of fibrin assembly by fibrinogen fragment D.

Fragment D (Mr 100 000) prepared from a terminal plasmin digest of fibrinogen was isolated and used to study its effect on fibrin formation. Increasing amounts of fragment D added to a solution of fibrinogen and thrombin decrease the rigidity of the resultant gel (10% of control at 2 mol of fragment D/mol of fibrinogen). Half-maximal inhibition is achieved at 1 mol of fragment D/mol of fibrinogen for non-cross-linked clots and at 1/2 mol of fragment D/mol of fibrinogen for cross-linked clots. "Clottability' decreases concomitantly with the rigidity. Only small amounts of fragment D (less than 10% for non-cross-linked gels) are incorporated into the gel. Light-scattering shows an increase in the final fibre thickness at fragment D concentrations up to 2 mol of fragment D/mol of fibrinogen, from 60 molecules/cross-section for the control to 120 molecules/cross-section. Higher fragment D concentrations lead to a decrease in the final fibre thickness. The limit fibre thickness is 8 nm, with a length of 80 nm, which is equivalent to a fibrin trimer. On the basis of results of synthetic-substrate and fibrinopeptide-release assays, it is clear that thrombin inactivation is not responsible for this effect. These data suggest that fragment D may inhibit fibrin formation by blocking the bimolecular polymerization of activated fibrin monomer molecules to form protofibrils, although additional effects on subsequent assembly steps may also be involved.     

Assembly and secretion of recombinant human fibrinogen

Expression vectors containing full-length cDNAs for each of the human fibrinogen chains were constructed. COS-1 cells were transfected with single vectors, mixtures of two, or with all three vectors and stable cell lines selected. Cells transfected with single vectors, or with mixtures of any two vectors, expressed the appropriate fibrinogen chains but did not secrete them. COS cells transfected with three vectors expressed all of the chains and secreted fibrinogen. COS cells transfected with three vectors contained, intracellularly, a mixture of fibrinogen-related proteins. The four main intracellular products were nascent fibrinogen, an A alpha.gamma complex, free A alpha chains, and free gamma chains. This is a similar pattern to that noted in Hep G2 cells. The intracellular forms of fibrinogen were sensitive to endoglycosidase H, indicating that they reside in a pre-Golgi compartment. Secreted fibrinogen was endoglycosidase H-insensitive, suggesting that the secreted glycoprotein moieties were processed in the normal manner. When mixed with plasma fibrinogen, radiolabeled recombinant fibrinogen was incorporated into a thrombin-induced clot. These studies demonstrate that COS cells transfected with all three fibrinogen chain cDNAs are capable of assembling and secreting a functional fibrinogen molecule.     

The assembly and secretion of apolipoprotein B-containing lipoproteins.

The assembly of lipoproteins containing apolipoprotein B is a complex process that occurs in the lumen of the secretory pathway. The process consists of two relatively well-identified steps. In the first step, two VLDL precursors are formed simultaneously and independently: an apolipoprotein B-containing VLDL precursor (a partially lipidated apolipoprotein B) and a VLDL-sized lipid droplet that lacks apolipoprotein B. In the second step, these two precursors fuse to form a mature VLDL particle. The apolipoprotein B-containing VLDL precursor is formed during the translation and concomitant translocation of the protein to the lumen of the endoplasmic reticulum. The VLDL precursor is completed shortly after the protein is fully synthesized. The process is dependent on the microsomal triglyceride transfer protein (MTP). Although the mechanism by which the lipid droplets are formed is unknown, recent observations indicate that the process is dependent on MTP. The fusion of the two precursors is not dependent on MTP, but the mechanism remains to be elucidated. The conversion of the apolipoprotein B-containing precursor to VLDL seems to be dependent on the ADP ribosylation factor 1 (ARF 1) and its activation of phospholipase D. During their assembly, nascent apolipoprotein B chains undergo quality control and are sorted to degradation. Such sorting, which occurs cotranslationally during the formation of the apolipoprotein B-containing precursor, involves cytosolic chaperons and ubiquitination that targets apolipoprotein B to proteasomal degradation. Other levels of sorting occur in the secretory pathway. Thus, lysosomal enzymes are involved as well as the LDL receptor.     

Procollagen assembly and secretion in embryonic chick bone.

Chick embryo skull bones incorporated radioactive proline and cystein into procollagen in short term organ culture. Pulse-chase experiments showed that individual precursor chains (pro-alpha1 and pro-alpha2) were formed first and that these were subsequently linked by disulfide bounds into trimers. Radioautography showed that labeled material was secreted 30 min after adding label to the cells, and electrophoretic analyses showed that after this time completed labeled collagen molecules appeared. Conversion from disulfide-linked procollagen to collagen proceeded in more than one step. An intermediate form consisting of shorter chains, which were still trimerically linked, was found.     

Very-low-density lipoprotein assembly and secretion

The assembly of apolipoprotein B (apoB) into VLDL is broadly divided into two steps. The first involves transfer of lipid by the microsomal triglyceride transfer protein (MTP) to apoB during translation. The second involves fusion of apoB-containing precursor particles with triglyceride droplets to form mature VLDL. ApoB and MTP are homologs of the egg yolk storage protein, lipovitellin. Homodimerization surfaces in lipovitellin are reutilized in apoB and MTP to achieve apoB-MTP interactions necessary for first step assembly. Structural modeling predicts a small lipovitellin-like lipid binding cavity in MTP and a transient lipovitellin-like cavity in apoB important for nucleation of lipid sequestration. The formation of triglyceride droplets in the endoplasmic reticulum requires MTP however, their fusion with apoB may be MTP-independent. Second step assembly is modulated by phospholipase D and A2. Phospholipases may prime membrane transport steps required for second step fusion and/or channel phospholipids into a pathway for VLDL triglyceride production. The enzymology of VLDL triglyceride synthesis is still poorly understood; however, it appears that ACAT2 is the sole source of cholesterol esters for VLDL and chylomicron assembly. VLDL production is controlled primarily at the level of presecretory degradation. Recently, it was discovered that the LDL receptor modulates VLDL production through its interactions with nascent VLDL in the secretory pathway.     

Studies on the dynamics and mechanism of glibenclamide-induced insulin secretion.

Sustained, 60-minute perfusion of glibenclamide (0.5, 1.5 and 10 mug/ml) elicits a one-phase insulin release profile, formed by a rapid secretion peak followed by a second peak with lower insulin levels than the former. Basal insulin secretion values are observed during the period comprised between 13 and 60 minutes of perfusion. Concurrent stimulation with glucose (100, 150, 200 and 300 mg%) plus glibenclamide (1 mug/ml) causes a marked rise in both phases of insulin secretion. The addition of glibenclamide does not modify the biphasic secretion pattern caused by maximal glucose concentration (400 mg%). The maximal values of both phases of secretion in the dose-response curve elicited by different glucose concentrations shift to the left when glibenclamide is added to the perfusate. The increase in insulin secretion caused by glibenclamide is not inhibited by puromycin. Both theophylline and phentolamine modify and increase the glibenclamide-induced insulin release pattern. Propranolol and imidazole inhibit glibenclamide-induced insulin release. Our results suggest that: 1. Glibenclamide increases beta cell sensitivity to glucose stimulation. 2. Glibenclamide and glucose induce secretion of insulin originating in the same compartment. 3. Modification of alpha and beta adrenergic receptors may modify glibodulate the beta cell response to glibenclamide.     

Studies on the mechanism of assembly of tobacco mosaic virus.

Sedimentation and proton binding studies on the endothermic self-association of tobacco mosaic virus (TMV) protein indicate that the so-called "20S" sedimenting protein is an interaction system involving at least the 34-subunit two-turn yield cylindrical disk aggregate and the 49-subunit three-turn helical rod. The pH dependence of this overall equilibrium suggests that disk formation is proton-linked through the binding of protons to the two-turn helix which is not present as significant concentrations near pH 7. There is a temperature-induced intramolecular conformation change in the protein leading to a difference spectrum which is complete in 5 x 10(-6) s at pH 7 and 20 degrees C and is dominated at 300 nm by tryptophan residues. Kinetics measurements of protein polymerization, from 10(-6) to 10(3) s, reveal three relaxation processes at pH 7.0, 20 degrees C, 0.10 M ionic strength K (H) PO4. The fastest relaxation time is a few milliseconds and represents reactions within the 4S protein distribution. The second fastest relaxation is 50-100 x 10(-3) s and represents elementary polymerization steps involved in the formation of the approximately 20 S protein. Analysis of the slowest relaxation, approximately 5 x 10(4) s, suggests that this very slow formation of approximately 20 S protein may be dominated by some first order process in the overall dissociation of approximately 20S protein. Sedimentation measurements of the rate of TMV reconstitution, under the same conditions, show by direct measurements of 4S and approximately 20S incorporation at various 4S to approximately 20S weight ratios that the relative rate of approximately 20S incorporation decreases almost linearly, from 0 to 50% 4S. There appears to be one or more regions of TMV-RNA, approximately 1-1.5 kilobases long, which incorporates approximately 20S protein exclusively. Solutions of approximately 95-100% approximately 20S protein have been prepared for the first time and used for reconstitution with RNA. Such protein solutions yield full size TMV, but at a slower rate than if 4S protein is added. Thus the elongation reaction in TMV assembly, following nucleation with approximately 20S protein, is not exclusively dependent upon the presence of either 4S or approximately 20S protein aggregates. The initial, maximum, rate of reconstitution increases about threefold when the protein composition is changed from 5% to 30% 4S protein, at constant total protein concentration at pH 7.0, 20 degrees C in 0.10 M ionic strength K (H)PO4. The probable binding frame at the internal assembly nucleation site of TMV-RNA has been determined by measuring the association constants for the binding of various trinucleoside diphosphates to helical TMV protein rods. The -CAG-AAG-AAG-sequence at the nucleation site is capable of providing at least 10-14 kcal/mol of sites of binding free energy for the nucleation event in TMV self-assembly.     

Identification of B beta chain domains involved in human fibrinogen assembly.

Fibrinogen chains are assembled in a stepwise manner in the rough endoplasmic reticulum prior to secretion of the final six-chain dimeric molecule. Previous studies indicated that the synthesis of B beta may be a rate-limiting factor in the assembly of human fibrinogen. To determine the domains of B beta which interact with the other two component chains of fibrinogen, deletion mutants of B beta were transiently co-expressed, together with A alpha and gamma chains, in COS cells, and fibrinogen assembly and secretion were measured. Deletion of the COOH-terminal half of the B beta chain (amino acids 208-461) did not affect assembly and secretion. Assembly of A alpha, gamma, and B beta also occurred when the first NH2-terminal 72 amino acids of B beta were deleted, but not when 93 amino acids were deleted. This indicates that the B beta domain between amino acids 73 and 93 is necessary for the assembly of the three fibrinogen chains. This domain marks the start of the alpha-helical "coiled-coil" region of fibrinogen.     

Postnatal development of hepatocellular apolipoprotein B assembly and secretion in the rat.

In this study, we explored the paradox that in suckling rats the serum concentration of LDL is high although the liver secretes only minimal quantities of VLDL, the presumed precursor of LDL. Freshly isolated hepatocytes and hepatocytes in primary culture obtained from adult (90 days old) and suckling (17 days old) rats were used to investigate the synthesis and secretion of apolipoprotein B (apoB) and lipids as well as the density profile of secreted apoB-containing lipoproteins. Furthermore, the effects of dexamethasone and oleate on apoB biogenesis were investigated in primary cultures of hepatocytes from adult and suckling rats. Hepatocytes from suckling rats were unable to assemble mature VLDL but secreted apoB as primordial lipoprotein particles in the LDL-HDL density range. Intracellular degradation of apoB was also reduced in hepatocytes from suckling rats compared with that in hepatocytes from adults. The immaturity in VLDL assembly and apoB degradation of hepatocytes from suckling rats could be overcome by treating the cultures with dexamethasone plus oleate or dexamethasone alone. The lower microsomal triacylglycerol transfer protein (MTP) mRNA concentrations in hepatocytes from suckling rats in comparison with hepatocytes from adult rats were not reflected in lower MTP activity levels. Furthermore, dexamethasone plus oleate treatment had no effect on MTP activity although VLDL assembly and secretion were clearly stimulated. We conclude that, during the suckling period of the rat, serum LDL is directly produced by the liver. This is a result of impaired hepatic VLDL assembly, which is a consequence of low triglyceride synthesis and an inefficient mobilization of bulk lipids in the second step of VLDL assembly.