Identification of the translation products of alpha -1-antitrypsin mRNA from baboon liver polysomes

Post-translational processing of the inter-alpha-trypsin inhibitor in the human hepatoma HepG2 cell line

In human hepatoma HepG2 cells, the serum inter-alpha-trypsin inhibitor (ITI)-like protein is synthesized from two protein precursors, the heavy chain (H) H2 and the light chain (L). Both of them carry sulphate groups involved in the chondroitin sulphate glycosaminoglycan (GAG) linkage, as demonstrated by [35S]sulphate labelling, chondroitinase digestion and inhibition with beta-D-xyloside, an artificial GAG acceptor. While inhibition of N-glycosylation prevented neither the maturation nor the secretion of the ITI-related entities, brefeldin A induced the accumulation of H and L precursors in the cells, therefore blocking subsequent association and maturation of the precursors before their secretion. The enzyme system involved in the ester linkage between H and L chains is localized in the trans-Golgi network since no ITI-like protein could be obtained in the presence of monensin; instead free heavy-chain protein forms and bikunin were secreted in culture supernatants. The ITI-like protein synthesized by HepG2 cells is therefore composed of two heavy chains HC2 linked to two bikunin chains by chondroitin sulphate bridges, although the GAG linkage between HC2 chains is presumably different. Further, a different maturation route leading to restricted heavy-chain forms, Hm and Hd, could be shown.

Human inter-alpha-trypsin inhibitor. Synthesis and maturation in hepatoma HepG2 cells

In hepatoma HepG2 cells, human inter-alpha-trypsin inhibitor (ITI) was synthesized as three heavy chains, H-1 (100 kDa), H-2 (110 kDa) and H-3 (113 kDa), and light hybrid chain (49.5 kDa) composed of alpha 1-microglobulin and HI-30 (ITI derivative, human inhibitor of 30 kDa). The association of at least two heavy chains, H-1 and H-3, with the HI-30 part of the light chain gave rise to a molecule similar to serum ITI. A composite protein (approximately 250 kDa) including heavy and light chains was also secreted, while alpha 1-microglobulin and ITI H-2 protein were released as separate entities. Light chain synthesis could be the limiting factor for ITI maturation.

Expression of the alpha-1-antitrypsin gene in mononuclear phagocytes of normal and alpha-1-antitrypsin-deficient individuals

To evaluate the contribution of mononuclear phagocytes, and particularly alveolar macrophages, to alpha-1-antitrypsin (alpha 1AT) production in normal and alpha 1AT-deficient individuals, Northern analysis with a human alpha 1AT complementary DNA was used to demonstrate that alpha 1AT messenger RNA (mRNA) can be detected in liver, blood monocytes, and alveolar macrophages. Quantification of alpha 1AT mRNA expression demonstrated that: (a) type PiMM monocytes and alveolar macrophages expressed, respectively, 200-fold and 70-fold less alpha 1AT mRNA per cell than the liver; (b) the level of expression of the alpha 1AT gene was increased during the in vitro maturation of blood monocytes; and (c) blood monocyte and alveolar macrophage levels of expression of the alpha 1AT gene were the same in PiMM and PiZZ individuals. However, the amount of newly synthesized alpha 1AT secreted by ZZ alveolar macrophages was 10 times lower than that secreted by MM alveolar macrophages. Thus, mononuclear phagocytes of PiZZ individuals express a secretory defect in alpha 1AT in a fashion similar to hepatocytes. Not only do mononuclear phagocytes provide a readily accessible cell to evaluate the regulation of alpha 1AT gene expression, but these cells may contribute to the levels of alpha 1AT present in the lower respiratory tract in the normal and ZZ states.

The effects of 17 alpha-ethynyloestradiol and of acute inflammation on the plasma concentration of rat alpha 1-acid glycoprotein and on the induction of its hepatic mRNA

We measured the serum concentration of alpha 1-acid glycoprotein (alpha 1-AGP) and we evaluated the content of its hepatic mRNA in rats after 17 alpha-ethynyloestradiol treatment or after turpentine-induced acute inflammation, or after both treatments performed simultaneously. We have also studied the affinity of serum alpha 1-AGP for concanavalin A under these conditions. Both types of stimuli induce a marked retention of the glycoprotein on free concanavalin A. The serum concentration of alpha 1-AGP is increased about 14-fold compared with that in control rats when a single pharmacological dose (50 micrograms) or multiple injections of 17 alpha-ethynyloestradiol are administered. This increase is greater in turpentine-oil-injected rats (about 21-fold) and reaches a maximum (about 32-fold) in rats injected with 17 alpha-ethynyloestradiol plus turpentine oil; this increase in alpha 1-AGP corresponds to the addition of the effects of the two inducing agents. Similar changes are also observed either in the alpha 1-AGP mRNA content as estimated by using an alpha 1-AGP-specific cDNA probe, or in the amount of translatable alpha 1-AGP mRNA. The results indicate that: after a high dose of 17 alpha-ethynyloestradiol and after acute inflammation, the increase of the alpha 1-AGP serum concentration is due to an accumulation of the alpha 1-AGP mRNA; different mechanisms and/or pathways are probably involved in regulating the synthesis of alpha 1-AGP under various stimuli; 17 alpha-ethynyloestradiol as well as acute inflammation seem to control the glycosylation process of alpha 1-AGP in an identical manner.

Induction of rat alpha 2-macroglobulin in vivo and in hepatocyte primary cultures: synergistic action of glucocorticoids and a Kupffer cell-derived factor

Turpentine injection into rats elicits enhanced secretion of acute phase proteins including alpha 2-macroglobulin (alpha 2M). Hypophysectomized rats, however, do not respond in this way unless dexamethasone is given together with turpentine. On the other hand, dexamethasone injection alone did not result in an induction of alpha 2M synthesis. When a medium of Kupffer cell cultures was added to hepatocytes, a dose-dependent stimulation of alpha 2M synthesis of up to 4-fold after 10-12 h was observed. However, the presence of low concentrations (10(-9)M) of dexamethasone was essential for the stimulatory effect. We conclude that the acute phase induction of alpha 2M in hepatocytes requires the synergistic action of glucocorticoids and a non-dialysable factor secreted by Kupffer cells.

Cell-free synthesis of rat alpha 2-macroglobulin and induction of its mRNA during experimental inflammation

Poly(A)-rich RNA was isolated from the livers of acutely inflamed rats by extraction with guanidinium HCl and oligo(dT)-cellulose chromatography. After translation in a recticulocyte lysate and immunoprecipitation with a specific antiserum to alpha 2-macroglobulin a polypeptide with an apparent molecular weight of 162000 could be detected. The cell-free synthesis of alpha 2-macroglobulin was stimulated 8-fold by the addition of RNase inhibitor. Full-length alpha 2-macroglobulin polypeptide chains appeared after 35 min in the presence of 1.85 mM Mg2+ and 100 mM K+. A nucleotide number of about 5100 was estimated for alpha 2-macroglobulin by means of sucrose gradient centrifugation of poly(A)-rich RNA followed by translation in vitro and immunoprecipitation of alpha 2-macroglobulin. In normal liver alpha 2-macroglobulin mRNA represented about 0.0007% of total translatable RNA. Acute inflammation generated by intramuscular injection of turpentine led to a 66-fold increase in translatable alpha 2-macroglobulin mRNA after 18 h, followed by a rapid decrease. In accordance to the induction of alpha 2-macroglobulin mRNA serum concentrations of alpha 2-macroglobulin increased to about 2 mg/ml. Unlike alpha 2-macroglobulin mRNA serum alpha 2-macroglobulin levels remained unchanged up to 60 h.

Inter-alpha-trypsin-inhibitor (ITI): two distinct mRNAs in baboon liver argue for a discrete synthesis of ITI and ITI derivatives

Human serum inter-alpha-trypsin-inhibitor (ITI) has so far been assumed to be comprised of a single polypeptide chain which can undergo fragmentation, whereby inhibitory ITI derivatives are released into the blood stream. In contrast, the analysis of the baboon liver mRNA translation products showed that ITI is made up of heavy and light chain(s). The latter may be excreted independently and very likely corresponds to the so-called ITI derivatives.