Abstract
In this article, we review the biological and clinical implication of the Recruitment-Secretory Block (“R-SB”) phenomenon. The phenomenon refers to the reaction of the liver with regard to protein secretion in conditions of clinical stimulation. Our basic knowledge of the process is due to the experimental work in animal models. Under basal conditions, the protein synthesis is mainly carried out by periportal (zone 1) hepatocytes that are considered the “professional” synthesizing protein cells. Under stimulation, midlobular and centrolobular (zones 2 and 3) hepatocytes, are progressively recruited according to lobular gradients and contribute to the increase of synthesis and secretion. The block of secretion, operated by exogenous agents, causes intracellular retention of all secretory proteins. The Pi MZ phenotype of Alpha-1-antitrypsin deficiency (AATD) has turned out to be the key for in vivo studies of the reaction of the liver, as synthesis and block of secretion are concomitant. Indeed, the M fraction of AAT is stimulated for synthesis and regularly exported while the Z fraction is mostly retained within the cell. For that reason, the phenomenon has been designated “Recruitment-Secretory Block” (“R-SB”). The “R-SB” phenomenon explains why: (a) the MZ individuals can correct the serum deficiency; (b) the resulting immonohistochemical and electron microscopic (EM) patterns are very peculiar and specific for the diagnosis of the Z mutation in tissue sections in the absence of genotyping; (c) the term carrier is no longer applicable for the heterozygous condition as all Pi MZ individuals undergo storage and the storage predisposes to liver damage. The storage represents the true elementary lesion and consequently reflects the phenotype-genotype correlation; (d) the site and function of the extrahepatic AAT and the relationship between intra and extracellular AAT; (e) last but not least, the concept of Endoplasmic Reticulum Storage Disease (ERSD) and of a new disease, hereditary hypofibrinogenemia with hepatic storage (HHHS). In the light of the emerging phenomenon, described in vitro, namely that M and Z AAT can form heteropolymers within hepatocytes as well as in circulation, we have reviewed the whole clinical and experimental material collected during forty years, in order to evaluate to what extent the polymerization phenomenon occurs in vivo. The paper summarizes similarities and differences between AAT and Fibrinogen as well as between the related diseases, AATD and HHHS. Indeed, fibrinogen gamma chain mutations undergo an aggregation process within the RER of hepatocytes similar to AATD. In addition, this work has clarified the intriguing phenomenon underlying a new syndrome, hereditary hypofibrinogenemia and hypo-APO-B-lipoproteinemia with hepatic storage of fibrinogen and APO-B lipoproteins. It is hoped that these studies could contribute to future research and select strategies aimed to simultaneously correct the hepatocytic storage, thus preventing the liver damage and the plasma deficiency of the two proteins.
Highlights
The liver synthesizes the vast majority of secretory plasma proteins and represents the only source of circulating alpha-1-antitrypsin (AAT) and fibrinogen [1]
This information has contributed to understand the mechanisms for acquired protein retention in humans as well but, the rationale is inadequate to explore the retention mechanism of mutant proteins occurring in Hepatic Endoplasmic Reticulum Storage Diseases (ERSD), i.e., Alpha-1-antitrypsin Deficiency (AATD) and Hereditary Hypofibrinogenemia with Hepatic
We have shown how experimental animal models have contributed to the understanding of the morphodynamics of the synthesis and block of secretory proteins in liver tissue
Summary
The liver synthesizes the vast majority of secretory plasma proteins and represents the only source of circulating alpha-1-antitrypsin (AAT) and fibrinogen [1]. Experimental animal models have shown that: (i) hepatocytes exert the protein synthesis to different degrees, according to the lobular gradients and intensity of stimuli [7]; (ii) they synthesize more than one acute phase protein simultaneously [8] (iii) the block of secretion by exogenous agents results in a collective retention of all secretory proteins This information has contributed to understand the mechanisms for acquired protein retention in humans as well but, the rationale is inadequate to explore the retention mechanism of mutant proteins occurring in Hepatic Endoplasmic Reticulum Storage Diseases (ERSD), i.e., Alpha-1-antitrypsin Deficiency (AATD) and Hereditary Hypofibrinogenemia with Hepatic. MZ individuals, the secretory block is intrinsic in the Z mutation and does not require external intervention
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