A naturally occurring nonpolymerogenic mutant of alpha 1-antitrypsin characterized by prolonged retention in the endoplasmic reticulum

The Emerging Roles of Intracellular PCSK9 and Their Implications in Endoplasmic Reticulum Stress and Metabolic Diseases

The importance of the proprotein convertase subtilisin/kexin type-9 (PCSK9) gene was quickly recognized by the scientific community as the third locus for familial hypercholesterolemia. By promoting the degradation of the low-density lipoprotein receptor (LDLR), secreted PCSK9 protein plays a vital role in the regulation of circulating cholesterol levels and cardiovascular disease risk. For this reason, the majority of published works have focused on the secreted form of PCSK9 since its initial characterization in 2003. In recent years, however, PCSK9 has been shown to play roles in a variety of cellular pathways and disease contexts in LDLR-dependent and -independent manners. This article examines the current body of literature that uncovers the intracellular and LDLR-independent roles of PCSK9 and also explores the many downstream implications in metabolic diseases.

The loss-of-function PCSK9Q152H variant increases ER chaperones GRP78 and GRP94 and protects against liver injury

Individuals harboring the loss-of-function (LOF) proprotein convertase subtilisin/kexin type 9 Gln152His variation (PCSK9Q152H) have low circulating low-density lipoprotein cholesterol levels and are therefore protected against cardiovascular disease (CVD). This uncleavable form of proPCSK9, however, is retained in the endoplasmic reticulum (ER) of liver hepatocytes, where it would be expected to contribute to ER storage disease (ERSD), a heritable condition known to cause systemic ER stress and liver injury. Here, we examined liver function in members of several French-Canadian families known to carry the PCSK9Q152H variation. We report that PCSK9Q152H carriers exhibited marked hypocholesterolemia and normal liver function despite their lifelong state of ER PCSK9 retention. Mechanistically, hepatic overexpression of PCSK9Q152H using adeno-associated viruses in male mice greatly increased the stability of key ER stress-response chaperones in liver hepatocytes and unexpectedly protected against ER stress and liver injury rather than inducing them. Our findings show that ER retention of PCSK9 not only reduced CVD risk in patients but may also protect against ERSD and other ER stress-driven conditions of the liver. In summary, we have uncovered a cochaperone function for PCSK9Q152H that explains its hepatoprotective effects and generated a translational mouse model for further mechanistic insights into this clinically relevant LOF PCSK9 variant.

Calcium as a reliable marker for the quantitative assessment of endoplasmic reticulum stress in live cells

Calcium (Ca²⁺) is an essential mineral of endoplasmic reticulum (ER) luminal biochemistry because of the Ca²⁺ dependence of ER-resident chaperones charged with folding de novo proteins that transit this cellular compartment. ER Ca²⁺ depletion reduces the ability of chaperones to properly fold the proteins entering the ER, thus leading to an accumulation of misfolded proteins and the onset of a state known as ER stress. However, not all conditions that cause ER stress do so in a manner dependent on ER Ca²⁺ depletion. Agents such as tunicamycin inhibit the glycosylation of de novo polypeptides, a key step in the maturation process of newly synthesized proteins. Despite this established effect of tunicamycin, our understanding of how such conditions modulate ER Ca²⁺ levels is still limited. In the present study, we report that a variety of ER stress–inducing agents that have not been known to directly alter ER Ca²⁺ homeostasis can also cause a marked reduction in ER Ca²⁺ levels. Consistent with these observations, protecting against ER stress using small chemical chaperones, such as 4-phenylbutyrate and tauroursodeoxycholic acid, also attenuated ER Ca²⁺ depletion caused by these agents. We also describe a novel high-throughput and low-cost assay for the rapid quantification of ER stress using ER Ca²⁺ levels as a surrogate marker. This report builds on our understanding of ER Ca²⁺ levels in the context of ER stress and also provides the scientific community with a new, reliable tool to study this important cellular process in vitro.

The endoplasmic reticulum co-chaperone ERdj3/DNAJB11 promotes hepatocellular carcinoma progression through suppressing AATZ degradation

AIM

The co-chaperone ERdj3/DNAJB11 is involved in the endoplasmic reticulum stress response observed in cancer cells. We hypothesized that ERdj3 functions as a hepatocellular carcinoma (HCC) oncogene by inhibiting AATZ degradation.

MATERIALS & METHODS

ERdj3 and AATZ expressions were analyzed in 84 HCC patients. Cell proliferation, epithelial-mesenchymal transition marker expression, migration and invasiveness were assessed in HepG2 and Huh-7 cells. A murine xenograft tumor model was constructed.

RESULTS

ERdj3 is upregulated in HCC tumors and cell lines. Tumor ERdj3 levels are positively associated with cirrhosis, enhanced HCC status, inferior survival outcomes and AATZ levels. ERdj3 suppresses AATZ degradation. ERdj3 overexpression enhances proliferation, epithelial-mesenchymal transition marker expression, migration, invasiveness and xenograft tumor growth in an AATZ-dependent manner.

CONCLUSION

ERdj3 enhances HCC progression through suppressing AATZ degradation.

Semiquantitation of Monomer and Polymer Alpha-1 Antitrypsin by Centrifugal Separation and Assay by Western Blot of Soluble and Insoluble Components

Alpha-1 antitrypsin (a1AT) deficiency, in its classical form, is an autosomal recessive disease associated with an increased risk of liver disease in adults and children, and with lung disease in adults. The vast majority of liver disease is associated with homozygosity for the Z mutant allele, also called PiZZ. This homozygous allele synthesizes large quantities of a1AT mutant Z protein in the liver, but the mutant protein also folds improperly during biogenesis. As a result, approximately 85% of the molecules are retained within the hepatocytes instead of being appropriately secreted. The resulting low, or "deficient," serum level leaves the lungs vulnerable to inflammatory injury from uninhibited neutrophil proteases. Most of the mutant Z protein retained within hepatocytes is directed into intracellular proteolysis pathways, but some molecules remain in the endoplasmic reticulum for long periods of time and others adopt an unusual aggregated or "polymerized" conformation. It is thought that these intracellular polymers trigger a cascade of intracellular injury which can lead to end organ liver injury including chronic hepatitis, cirrhosis, and hepatocellular carcinoma. It is widely accepted that the disease causing factor in mutant Z-alpha-1 antitrypsin deficiency (AATD-Z) is the toxic build-up of the mutant Z protein. Since misfolding of some but not all of the Z protein during its maturation leads to homopolymerization, an assay to assess the amount of normally folded ATZ and accumulated polymeric ATZ would be very useful. Here we describe a method to semiquantitatively assess these two fractions in a tissue or cell culture source.

Loss-of-function PCSK9 mutants evade the unfolded protein response sensor GRP78 and fail to induce endoplasmic reticulum stress when retained

The proprotein convertase subtilisin/kexin type-9 (PCSK9) plays a central role in cardiovascular disease (CVD) by degrading hepatic low-density lipoprotein receptor (LDLR). As such, loss-of-function (LOF) PCSK9 variants that fail to exit the endoplasmic reticulum (ER) increase hepatic LDLR levels and lower the risk of developing CVD. The retention of misfolded protein in the ER can cause ER stress and activate the unfolded protein response (UPR). In this study, we investigated whether a variety of LOF PCSK9 variants that are retained in the ER can cause ER stress and hepatic cytotoxicity. Although overexpression of these PCSK9 variants caused an accumulation in the ER of hepatocytes, UPR activation or apoptosis was not observed. Furthermore, ER retention of endogenous PCSK9 via splice switching also failed to induce the UPR. Consistent with these in vitro studies, overexpression of PCSK9 in the livers of mice had no impact on UPR activation. To elucidate the cellular mechanism to explain these surprising findings, we observed that the 94-kDa glucose-regulated protein (GRP94) sequesters PCSK9 away from the 78-kDa glucose-regulated protein (GRP78), the major activator of the UPR. As a result, GRP94 knockdown increased the stability of GRP78-PCSK9 complex and resulted in UPR activation following overexpression of ER-retained PCSK9 variants relative to WT secreted controls. Given that overexpression of these LOF PCSK9 variants does not cause UPR activation under normal homeostatic conditions, therapeutic strategies aimed at blocking the autocatalytic cleavage of PCSK9 in the ER represent a viable strategy for reducing circulating PCSK9.

ERAD defects and the HFE-H63D variant are associated with increased risk of liver damages in Alpha 1-Antitrypsin Deficiency

BACKGROUND

The most common and severe disease causing allele of Alpha 1-Antitrypsin Deficiency (1ATD) is Z-1AT. This protein aggregates in the endoplasmic reticulum, which is the main cause of liver disease in childhood. Based on recent evidences and on the frequency of liver disease occurrence in Z-1AT patients, it seems that liver disease progression is linked to still unknown genetic factors.

METHODS

We used an innovative approach combining yeast genetic screens with next generation exome sequencing to identify and functionally characterize the genes involved in 1ATD associated liver disease.

RESULTS

Using yeast genetic screens, we identified HRD1, an Endoplasmic Reticulum Associated Degradation (ERAD) associated protein, as an inducer of Z-mediated toxicity. Whole exome sequencing of 1ATD patients resulted in the identification of two variants associated with liver damages in Z-1AT homozygous cases: HFE H63D and HERPUD1 R50H. Functional characterization in Z-1AT model cell lines demonstrated that impairment of the ERAD machinery combined with the HFE H63D variant expression decreased both cell proliferation and cell viability, while Unfolded Protein Response (UPR)-mediated cell death was hyperstimulated.

CONCLUSION

This powerful experimental pipeline allowed us to identify and functionally validate two genes involved in Z-1AT-mediated severe liver toxicity. This pilot study moves forward our understanding on genetic modifiers involved in 1ATD and highlights the UPR pathway as a target for the treatment of liver diseases associated with 1ATD. Finally, these findings support a larger scale screening for HERPUD1 R50H and HFE H63D variants in the sub-group of 1ATD patients developing significant chronic hepatic injuries (hepatomegaly, chronic cholestasis, elevated liver enzymes) and at risk developing liver cirrhosis.

α1-antitrypsin Deficiency: A Misfolded Secretory Protein Variant with Unique Effects on the Endoplasmic Reticulum

In the classical form of α1-antitrypsin deficiency (ATD) a point mutation leads to accumulation of a misfolded secretory glycoprotein in the endoplasmic reticulum (ER) of liver cells and so ATD has come to be considered a prototypical ER storage disease. It is associated with two major types of clinical disorders, chronic obstructive pulmonary disease (COPD) by loss-of-function mechanisms and hepatic cirrhosis and carcinogenesis by gain-of-function mechanisms. The lung disease predominantly results from proteolytic damage to the pulmonary connective tissue matrix because of reduced levels of protease inhibitor activity of α1-anitrypsin (AT) in the circulating blood and body fluids. Cigarette smoking is a powerful disease-promoting modifier but other modifiers are known to exist because variation in the lung disease phenotype is still found in smoking and non-smoking homozygotes. The liver disease is highly likely to be caused by the proteotoxic effects of intracellular misfolded protein accumulation and a high degree of variation in the hepatic phenotype among affected homozygotes has been hypothetically attributed to genetic and environmental modifiers that alter proteostasis responses. Liver biopsies of homozygotes show intrahepatocytic inclusions with dilation and expansion of the ER and recent studies of iPS-derived hepatocyte-like cells from individuals with ATD indicate that the changes in the ER directly vary with the hepatic phenotype i.e there is much lesser alteration in the ER in cells derived from homozygotes that do not have clinically significant liver disease. From a signaling perspective, studies in mammalian cell line and animal models expressing the classical α1-antitrypsin Z variant (ATZ) have found that ER signaling is perturbed in a relatively unique way with powerful activation of autophagy and the NFκB pathway but relatively limited, if any, UPR signaling. It is still not known how much these unique structural and functional changes and the variation among affected homozygotes relate to the tendency of this variant to polymerize and aggregate and/or to the repertoire of proteostasis mechanisms that are activated.

Deficient and Null Variants of SERPINA1 Are Proteotoxic in a Caenorhabditis elegans Model of α1-Antitrypsin Deficiency

α1-antitrypsin deficiency (ATD) predisposes patients to both loss-of-function (emphysema) and gain-of-function (liver cirrhosis) phenotypes depending on the type of mutation. Although the Z mutation (ATZ) is the most prevalent cause of ATD, >120 mutant alleles have been identified. In general, these mutations are classified as deficient (<20% normal plasma levels) or null (<1% normal levels) alleles. The deficient alleles, like ATZ, misfold in the ER where they accumulate as toxic monomers, oligomers and aggregates. Thus, deficient alleles may predispose to both gain- and loss-of-function phenotypes. Null variants, if translated, typically yield truncated proteins that are efficiently degraded after being transiently retained in the ER. Clinically, null alleles are only associated with the loss-of-function phenotype. We recently developed a C. elegans model of ATD in order to further elucidate the mechanisms of proteotoxicity (gain-of-function phenotype) induced by the aggregation-prone deficient allele, ATZ. The goal of this study was to use this C. elegans model to determine whether different types of deficient and null alleles, which differentially affect polymerization and secretion rates, correlated to any extent with proteotoxicity. Animals expressing the deficient alleles, Mmalton, Siiyama and S (ATS), showed overall toxicity comparable to that observed in patients. Interestingly, Siiyama expressing animals had smaller intracellular inclusions than ATZ yet appeared to have a greater negative effect on animal fitness. Surprisingly, the null mutants, although efficiently degraded, showed a relatively mild gain-of-function proteotoxic phenotype. However, since null variant proteins are degraded differently and do not appear to accumulate, their mechanism of proteotoxicity is likely to be different to that of polymerizing, deficient mutants. Taken together, these studies showed that C. elegans is an inexpensive tool to assess the proteotoxicity of different AT variants using a transgenic approach.

Role of Drosophila EDEMs in the degradation of the alpha-1-antitrypsin Z variant

The synthesis of proteins in the endoplasmic reticulum (ER) that exceeds the protein folding capacity of this organelle is a frequent cause of cellular dysfunction and disease. An example of such a disease is alpha-1-antitrypsin (A1AT) deficiency, caused by destabilizing mutations in this glycoprotein. It is considered that the mutant proteins are recognized in the ER by lectins and are subsequently degraded through the proteasome, leading to a deficiency in this enzyme in the afflicted patients. We previously established a Drosophila model of this disease by overexpressing the null Hong Kong (NHK) allele of this gene and found that the Drosophila lectin, ER degradation-enhancing α-mannosidase-like protein 2 (EDEM2), can accelerate the degradation of A1AT when overexpressed. NHK is a rare allele, and in this study, we investigated in depth the mechanisms through which Drosophila EDEMs affect the degradation of the Z variant, which is the predominant disease allele. Specifically, we report that the Z allele does not activate ER stress signaling as prominently as the NHK allele, but similarly requires both Drosophila EDEM1 and EDEM2 for the degradation of the protein. We demonstrate that EDEMs are required for their ubiquitination, and without EDEMs, glycosylated A1AT mutants accumulate in cells. These results support the role of the EDEM-mediated ubiquitination of the alpha-1-antitrypsin Z (ATZ) allele, and establish a Drosophila model for the study of this protein and disease.

Large protein complexes retained in the ER are dislocated by non-COPII vesicles and degraded by selective autophagy

Multisubunit protein complexes are assembled in the endoplasmic reticulum (ER). Existing pools of single subunits and assembly intermediates ensure the efficient and rapid formation of complete complexes. While being kinetically beneficial, surplus components must be eliminated to prevent potentially harmful accumulation in the ER. Surplus single chains are cleared by the ubiquitin-proteasome system. However, the fate of not secreted assembly intermediates of multisubunit proteins remains elusive. Here we show by high-resolution double-label confocal immunofluorescence and immunogold electron microscopy that naturally occurring surplus fibrinogen Aα-γ assembly intermediates in HepG2 cells are dislocated together with EDEM1 from the ER to the cytoplasm in ER-derived vesicles not corresponding to COPII-coated vesicles originating from the transitional ER. This route corresponds to the novel ER exit path we have previously identified for EDEM1 (Zuber et al. Proc Natl Acad Sci USA 104:4407-4412, 2007). In the cytoplasm, detergent-insoluble aggregates of fibrinogen Aα-γ dimers develop that are targeted by the selective autophagy cargo receptors p62/SQSTM1 and NBR1. These aggregates are degraded by selective autophagy as directly demonstrated by high-resolution microscopy as well as biochemical analysis and inhibition of autophagy by siRNA and kinase inhibitors. Our findings demonstrate that different pathways exist in parallel for ER-to-cytoplasm dislocation and subsequent proteolytic degradation of large luminal protein complexes and of surplus luminal single-chain proteins. This implies that ER-associated protein degradation (ERAD) has a broader function in ER proteostasis and is not limited to the elimination of misfolded glycoproteins.

The endosomal protein-sorting receptor sortilin has a role in trafficking α-1 antitrypsin

Up to 1 in 3000 individuals in the United States have α-1 antitrypsin deficiency, and the most common cause of this disease is homozygosity for the antitrypsin-Z variant (ATZ). ATZ is inefficiently secreted, resulting in protein deficiency in the lungs and toxic polymer accumulation in the liver. However, only a subset of patients suffer from liver disease, suggesting that genetic factors predispose individuals to liver disease. To identify candidate factors, we developed a yeast ATZ expression system that recapitulates key features of the disease-causing protein. We then adapted this system to screen the yeast deletion mutant collection to identify conserved genes that affect ATZ secretion and thus may modify the risk for developing liver disease. The results of the screen and associated assays indicate that ATZ is degraded in the vacuole after being routed from the Golgi. In fact, one of the strongest hits from our screen was Vps10, which can serve as a receptor for the delivery of aberrant proteins to the vacuole. Because genome-wide association studies implicate the human Vps10 homolog, sortilin, in cardiovascular disease, and because hepatic cell lines that stably express wild-type or mutant sortilin were recently established, we examined whether ATZ levels and secretion are affected by sortilin. As hypothesized, sortilin function impacts the levels of secreted ATZ in mammalian cells. This study represents the first genome-wide screen for factors that modulate ATZ secretion and has led to the identification of a gene that may modify disease severity or presentation in individuals with ATZ-associated liver disease.

The ubiquitin ligase Hrd1 promotes degradation of the Z variant alpha 1-antitrypsin and increases its solubility

Alpha 1-antitrypsin (AAT) deficiency is an autosomal recessive disorder that is characterized by the retention of misfolded AAT in the endoplasmic reticulum (ER) of hepatocytes and a significant decrease in the serum levels of AAT. Previous studies have demonstrated that the ubiquitin-proteasome pathway is involved in the degradation of the Z variant of AAT (ATZ). However, the detailed mechanisms of ATZ degradation are not fully understood. We investigated whether the ER membrane-embedded ubiquitin ligase (E3) Hrd1 promotes the removal of ATZ through ER-associated degradation (ERAD). Our results indicate that Hrd1 decreases intracellular levels of ATZ, especially the detergent-insoluble fraction, in cells transfected with a plasmid-encoding ATZ. The degradation of ATZ was also found to be dependent on the functional E3 activity of Hrd1. In addition, we demonstrated that Hrd1 increases the solubility of ATZ. Cycloheximide (CHX) chase and proteasome inhibition experiments showed that the ubiquitin-proteasome pathway is involved in Hrd1-mediated ATZ degradation. Furthermore, we found that Hrd1 helped to maintain normal morphology of ATZ expressing cells. These data indicate that Hrd1 enhances the removal of ATZ through ERAD and attenuates intracellular ATZ accumulation and toxicity, which implies a potential value for Hrd1 in the treatment of AAT deficiency diseases.

Blood Outgrowth Endothelial Cells as a Vehicle for Transgene Expression of Hepatocyte-Secreted Proteins viaSleeping Beauty

The therapeutic use of autologous cells with the capacity for extensive in vitro expansion and manipulation prior to host administration has been an area of significant investigation over the last decade. Blood outgrowth endothelial cells (BOECs) are derived from the circulation and exhibit proliferative growth, in vivo engraftment, and survival characteristics for long-term expression of endogenously secreted proteins, such as factor VIII (FVIII). The authors describe a modified method for the isolation, culture, and expansion of these cells that is readily accomplished using standard laboratory methods. Using a commercially available transfection reagent, approximately 30% of these primary cells can be routinely transfected with the nonviral Sleeping Beauty transposon for long-term, stable transgene expression. Moreover, the results indicate that these cells have the ability to secrete functionally active proteins that are synthesized endogenously by hepatocytes and require post-translational modification including alpha1-antitrypsin and clotting factors VII and IX. This, coupled with their notably long half-life of years, suggests that these cells may provide an appropriate vehicle for secretion of a variety of proteins produced by different cell types in vivo. Thus, BOECs have the potential to provide clinically relevant secreted proteins for diseases other than those of endothelial origin.

Regulator of G Signaling 16 is a marker for the distinct endoplasmic reticulum stress state associated with aggregated mutant alpha1-antitrypsin Z in the classical form of alpha1-antitrypsin deficiency

In the classical form of alpha(1)-antitrypsin deficiency, a mutant protein accumulates in a polymerized form in the endoplasmic reticulum (ER) of liver cells causing liver damage and carcinogenesis by a gain-of-toxic function mechanism. Recent studies have indicated that the accumulation of mutant alpha(1)-antitrypsin Z in the ER specifically activates the autophagic response but not the unfolded protein response and that autophagy plays a critical role in disposal of insoluble alpha(1)-antitrypsin Z. In this study, we used genomic analysis of the liver in a novel transgenic mouse model with inducible expression to screen for changes in gene expression that would potentially define how the liver responds to accumulation of this mutant protein. There was no unfolded protein response. Of several distinct gene expression profiles, marked up-regulation of regulator of G signaling (RGS16) was particularly notable. RGS16 did not increase when model systems were exposed to classical inducers of ER stress, including tunicamycin and calcium ionophore, or when a nonpolymerogenic alpha(1)-antitrypsin mutant accumulated in the ER. RGS16 was up-regulated in livers from patients with alpha(1)-antitrypsin deficiency, and the degree of up-regulation correlated with the hepatic levels of insoluble alpha(1)-antitrypsin Z protein. Taken together, these results indicate that expression of RGS16 is an excellent marker for the distinct form of "ER stress" that occurs in alpha(1)-antitrypsin deficiency, presumably determined by the aggregation-prone properties of the mutant protein that characterizes the deficiency.

Molecular pathogenesis of alpha-1-antitrypsin deficiency-associated liver disease: a meeting review

In recent years, we have witnessed several important paradigm shifts in understanding the molecular basis of liver disease in alpha-1-antitrypsin (AT) deficiency. These shifts have become possible as a result of a number of advances in research on the cell biology of aggregation-prone mutant proteins and in research on the pathobiological mechanisms of liver disease in general. Late-breaking research in these areas was the subject of an AASLD/Alpha-1 Foundation Single Topic Conference in Atlanta, Georgia, on January 26 to 28, 2006. The conference was titled "Alpha-1-Antitrypsin Deficiency and Other Liver Diseases Caused by Aggregated Proteins." Investigators from all over the world, representing a broad array of scientific disciplines and perspectives, discussed the pathobiology of AT deficiency, mechanisms of cell injury in diseases associated with aggregation-prone proteins, pathways by which cells respond to protein aggregation and mislocalization, and mechanisms of liver injury in general and in diseases related to AT deficiency. A session of the meeting was devoted to novel therapeutic strategies being developed for AT deficiency as well as to strategies either in development or already being applied to the class of diseases associated with mutant proteins.

Update on research, diagnosis and management of alphal-antitrypsin deficiency

Formed in response to a World Health Organization recommendation, the Alpha One International Registry (AIR) is a multinational research program focused on alphal-antitrypsin (AAT) deficiency. Each of the nearly 20 participating countries maintains a National Registry of patients with AAT deficiency and contributes to an international database in Malmö, Sweden, that is designed to increase understanding of AAT deficiency as well as safeguard patient confidentiality. AIR members are engaged in active and wide-ranging investigations to improve the diagnosis, monitoring and therapy of the disease. The AIR membership meets biennially to exchange views and research findings. The third biennial meeting was held in Barcelona, Spain, June 11 -13, 2003. A wide range of AAT deficiency-related topics were addressed, encompassing molecular and cellular pathophysiologic mechanisms, clinical epidemiology, diagnostic advances, current and investigational therapeutic approaches, and progress in registry development. Valuable cross-fertilization of concepts and scientific observations was apparent between AAT deficiency research and other fields of biomedicine. The proceedings of the meeting are summarized in this report.

Low incidence of alpha 1-antitrypsin deficiency among Filipinos with neonatal cholestatis

AIM

Alpha 1-antitrypsin (AAT) deficiency is the most common genetic cause of liver disease in children. The Pi*S carrier rate among Filipinos is <1%. Its significance in Filipino infants with neonatal cholestasis has not been investigated. The aim of the study was to determine the incidence of AAT deficiency among Filipino infants presenting with neonatal cholestasis.

METHODS

Genotype determination that detects Pi*S and Pi*Z alleles was performed using Elucigene AAT reagents (Cellmark Diagnostics, UK). AAT inclusions were identified by light microscopy using periodic acid-Schiff (PAS) stain.

RESULTS

Ninety-six infants (mean age: 89 days, 48 males) with a history of jaundice since 2 weeks old and a direct bilirubin level>20% of the total were recruited. Only one patient (1 month old, male) was positive for Pi*S allele and 95 were negative for Pi*S and Pi*Z alleles, with an annual incidence of 0.7%. Of the 96, 49 infants underwent diagnostic percutaneous liver biopsy. All liver biopsy specimen were subjected to PAS stain and two infants, 2 and 4 months old, both with idiopathic neonatal hepatitis, had suspicious findings of AAT globules that was confirmed on immunostain. Both infants were negative for Pi*S alleles. The only patient positive for Pi*S allele was negative for PAS globule on liver biopsy.

CONCLUSION

Our results showed a low incidence of AAT deficiency caused by the Pi*S and Pi*Z alleles among Filipino infants presenting with neonatal cholestasis, similar to the low carrier rate in the population.

Pathogenesis of chronic liver injury and hepatocellular carcinoma in alpha-1-antitrypsin deficiency

Alpha-1-antitrypsin (AT) deficiency is the most common genetic cause of liver disease in children. In addition to chronic liver inflammation and injury, it has a predilection to cause hepatocellular carcinoma later in life. The deficiency is caused by a mutant protein, ATZ, which is retained in the endoplasmic reticulum (ER) in a polymerized form rather than secreted into the blood in its monomeric form. The histologic hallmark of the disease is ATZ-containing globules in some, but not all, hepatocytes. Liver injury results from a gain-of-toxic function mechanism in which mutant ATZ retained in the ER initiates a series of pathologic events, but little is known about the mechanism by which this leads to carcinogenesis. Several recent observations from my laboratory have led to a novel hypothetical paradigm for carcinogenesis in AT deficiency in which globule-containing hepatocytes are "sick," relatively growth suppressed, but also elaborating trans-acting regenerative signals. These signals are received and transduced by globule-devoid hepatocytes, which, because they are younger and have a lesser load of accumulated ATZ, have a selective proliferative advantage. Chronic regeneration in the presence of tissue injury leads to adenomas and ultimately carcinomas. Aspects of this hypothetical paradigm may also explain the proclivity for hepatocarcinogenesis in other chronic liver diseases, including other genetic diseases, viral hepatitis, and nonalcoholic steatohepatitis.

Alpha-1-antitrypsin deficiency: diagnosis, pathophysiology, and management

Alpha-1-antitrypsin deficiency is a relatively common but under-recognized genetic disease in which individuals homozygous for the mutant Z disease-associated allele are at risk for the development of liver disease and emphysema. The protein product of the mutant Z gene is synthesized in hepatocytes but accumulates intracellularly rather than being appropriately secreted. The downstream effects of the intracellular accumulation of the mutant Z protein include the formation of unique protein polymers, activation of autophagy, mitochondrial injury, endoplasmic reticulum stress, and caspase activation, which subsequently progress in a cascade, causing chronic hepatocellular injury. The variable clinical presentations among affected individuals suggest an important contribution of genetic and environmental disease modifiers, which are only now being identified. The heterozygous carrier state for the mutant Z gene, found in 1.5% to 3% of the population, is not itself a common cause of liver injury but may be a modifier gene for other liver diseases.

The Selective Advantage of α1-Antitrypsin Deficiency

The S- and Z-deficiency alleles of alpha1-antitrypsin are found in more than 20% of some white populations. This high gene frequency suggests that these mutations confer a selective advantage, but the biologic mechanism of this has remained obscure. It is now well recognized that the S and Z alleles result in a conformational transition within the alpha1-antitrypsin molecule and the formation of polymers that are retained within the endoplasmic reticulum of hepatocytes. Polymers of mutant alpha1-antitrypsin can also form within the alveoli and small airways of the lung where they may drive the inflammation that underlies emphysema in individuals with alpha1-antitrypsin deficiency. This local production of polymers by mutant S and Z alpha1-antitrypsin may have also provided protection against infectious disease in the preantibiotic era by focusing and amplifying the inflammatory response to limit invasive respiratory and gastrointestinal infection. It is only since the discovery of antibiotics, the widespread adoption of smoking, and increased longevity that these protective, proinflammatory properties of alpha1-antitrypsin mutants have become detrimental to cause the emphysema and systemic inflammatory diseases associated with alpha1-antitrypsin deficiency.

Characterization of an ERAD gene as VPS30/ATG6 reveals two alternative and functionally distinct protein quality control pathways: one for soluble Z variant of human alpha-1 proteinase inhibitor (A1PiZ) and another for aggregates of A1PiZ

The Z variant of human alpha-1 proteinase inhibitor (A1PiZ) is a substrate for endoplasmic reticulum-associated protein degradation (ERAD). To identify genes required for the degradation of this protein, A1PiZ degradation-deficient (add) yeast mutants were isolated. The defect in one of these mutants, add3, was complemented by VPS30/ATG6, a gene that encodes a component of two phosphatidylinositol 3-kinase (PtdIns 3-kinase) complexes: complex I is required for autophagy, whereas complex II is required for the carboxypeptidase Y (CPY)-to-vacuole pathway. We found that upon overexpression of A1PiZ, both PtdIns 3-kinase complexes were required for delivery of the excess A1PiZ to the vacuole. When the CPY-to-vacuole pathway was compromised, A1PiZ was secreted; however, disruption of autophagy led to an increase in aggregated A1PiZ rather than secretion. These results suggest that excess soluble A1PiZ transits the secretion pathway to the trans-Golgi network and is selectively targeted to the vacuole via the CPY-to-vacuole sorting pathway, but excess A1PiZ that forms aggregates in the endoplasmic reticulum is targeted to the vacuole via autophagy. These findings illustrate the complex nature of protein quality control in the secretion pathway and reveal multiple sites that recognize and sort both soluble and aggregated forms of aberrant or misfolded proteins.

Intracellular inclusions containing mutant alpha1-antitrypsin Z are propagated in the absence of autophagic activity

Mutant alpha(1)-antitrypsin Z (alpha(1)-ATZ) protein, which has a tendency to form aggregated polymers as it accumulates within the endoplasmic reticulum of the liver cells, is associated with the development of chronic liver injury and hepatocellular carcinoma in hereditary alpha(1)-antitrypsin (alpha(1)-AT) deficiency. Previous studies have suggested that efficient intracellular degradation of alpha(1)-ATZ is correlated with protection from liver disease in alpha(1)-AT deficiency and that the ubiquitin-proteasome system accounts for a major route, but not the sole route, of alpha(1)-ATZ disposal. Yet another intracellular degradation system, autophagy, has also been implicated in the pathophysiology of alpha(1)-AT deficiency. To provide genetic evidence for autophagy-mediated disposal of alpha(1)-ATZ, here we used cell lines deleted for the Atg5 gene that is necessary for initiation of autophagy. In the absence of autophagy, the degradation of alpha(1)-ATZ was retarded, and the characteristic cellular inclusions of alpha(1)-ATZ accumulated. In wild-type cells, colocalization of the autophagosomal membrane marker GFP-LC3 and alpha(1)-ATZ was observed, and this colocalization was enhanced when clearance of autophagosomes was prevented by inhibiting fusion between autophagosome and lysosome. By using a transgenic mouse with liver-specific inducible expression of alpha(1)-ATZ mated to the GFP-LC3 mouse, we also found that expression of alpha(1)-ATZ in the liver in vivo is sufficient to induce autophagy. These data provide definitive evidence that autophagy can participate in the quality control/degradative pathway for alpha(1)-ATZ and suggest that autophagic degradation plays a fundamental role in preventing toxic accumulation of alpha(1)-ATZ.

Accumulation of mutant alpha1-antitrypsin Z in the endoplasmic reticulum activates caspases-4 and -12, NFkappaB, and BAP31 but not the unfolded protein response

In alpha(1)-antitrypsin (alpha1AT) deficiency, a polymerogenic mutant form of the secretory glycoprotein alpha1AT, alpha1ATZ, is retained in the endoplasmic reticulum (ER) of liver cells. It is not yet known how this results in liver injury in a subgroup of deficient individuals and how the remainder of deficient individuals escapes liver disease. One possible explanation is that the "susceptible" subgroup is unable to mount the appropriate protective cellular responses. Here we examined the effect of mutant alpha1ATZ on several potential protective signaling pathways by using cell lines with inducible expression of mutant alpha1AT as well as liver from transgenic mice with liver-specific inducible expression of mutant alpha1AT. The results show that ER retention of polymerogenic mutant alpha1ATZ does not result in an unfolded protein response (UPR). The UPR can be induced in the presence of alpha1ATZ by tunicamycin excluding the possibility that the pathway has been disabled. In striking contrast, ER retention of nonpolymerogenic alpha1AT mutants does induce the UPR. These results indicate that the machinery responsible for activation of the UPR can distinguish the physical characteristics of proteins that accumulate in the ER in such a way that it can respond to misfolded but not relatively ordered polymeric structures. Accumulation of mutant alpha1ATZ does activate specific signaling pathways, including caspase-12 in mouse, caspase-4 in human, NFkappaB, and BAP31, a profile that was distinct from that activated by nonpolymerogenic alpha1AT mutants.

Grp78, Grp94, and Grp170 interact with alpha1-antitrypsin mutants that are retained in the endoplasmic reticulum

In alpha1-antitrypsin (alpha1-AT) deficiency, a mutant form of alpha1-AT polymerizes in the endoplasmic reticulum (ER) of liver cells resulting in chronic hepatitis and hepatocellular carcinoma by a gain of toxic function mechanism. Although some aspects of the cellular response to mutant alpha1-AT Z have been partially characterized, including the involvement of several proteasomal and nonproteasomal mechanisms for disposal, other parts of the cellular response pathways, particularly the chaperones with which it interacts and the signal transduction pathways that are activated, are still not completely elucidated. The alpha1-AT Z molecule is known to interact with calnexin, but, according to one study, it does not interact with Grp78. To carry out a systematic search for the chaperones with which alpha1-AT Z interacts in the ER, we used chemical cross-linking of several different genetically engineered cell systems. Mutant alpha1-AT Z was cross-linked with Grp78, Grp94, calnexin, Grp170, UDP-glucose glycoprotein:glucosyltransferase, and two unknown proteins of approximately 110-130 kDa. Sequential immunoprecipitation/immunoblot analysis and coimmunoprecipitation techniques demonstrated each of these interactions without chemical cross-linking. The same chaperones were found to interact with two nonpolymerogenic alpha1-AT mutants that are retained in the ER, indicating that these interactions are not specific for the alpha1-AT Z mutant. Moreover, sucrose density gradient centrifugation studies suggest that approximately 85% of alpha1-AT Z exists in heterogeneous soluble complexes with multiple chaperones and approximately 15% in extremely large polymers/aggregates devoid of chaperones. Agents that perturb the synthesis and/or activity of ER chaperones such as tunicamycin and calcium ionophore A23187, have different effects on the solubility and degradation of alpha1-AT Z as well as on its residual secretion.

Alpha-1-antitrypsin deficiency: a new paradigm for hepatocellular carcinoma in genetic liver disease

Liver disease in alpha-1-antitrypsin (alpha1AT) deficiency is caused by a gain-of-toxic function mechanism engendered by the accumulation of a mutant glycoprotein in the endoplasmic reticulum (ER). The extraordinary degree of variation in phenotypical expression of this liver disease is believed to be determined by genetic modifiers and/or environmental factors that influence the intracellular disposal of the mutant glycoprotein or the signal transduction pathways that are activated. Recent investigations suggest that a specific repertoire of signaling pathways are involved, including the autophagic response, mitochondrial- and ER-caspase activation, and nuclear factor kappaB (NFkappaB) activation. Whether activation of these signaling pathways, presumably to protect the cell, inadvertently contributes to liver injury or perhaps protects the cell from one injury and, in so doing, predisposes it to another type of injury, such as hepatocarcinogenesis, is not yet known. Recent studies also suggest that hepatocytes with marked accumulation of alpha1ATZ, globule-containing hepatocytes, engender a cancer-prone state by surviving with intrinsic damage and by chronically stimulating in 'trans' adjacent relatively undamaged hepatocytes that have a selective proliferative advantage. Further, this paradigm may apply to other genetic and infectious liver diseases that are predisposed to hepatocellular carcinoma.

Quantitative isolation of alphalAT mutant Z protein polymers from human and mouse livers and the effect of heat

Alpha-1-antitrypsin (alpha1AT) deficiency in its most common form is caused by homozygosity for the alpha1AT mutant Z gene. This gene encodes a mutant Z secretory protein, primarily synthesized in the liver, that assumes an abnormal conformation and accumulates within hepatocytes causing liver cell injury. Studies have shown that mutant alpha1ATZ protein molecules form unique protein polymers. These Z protein polymers have been hypothesized to play a critical role in the pathophysiology of liver injury in this disease, although a lack of quantitative methods to isolate the polymers from whole liver has hampered further analysis. In this study, we demonstrate a quantitative alpha1ATZ polymer isolation technique from whole liver and show that the hepatocellular periodic acid-Schiff-positive globular inclusions that are the histopathological hallmark of this disease are composed almost entirely of the polymerized alpha1ATZ protein. Furthermore, we examine the previously proposed but untested hypothesis that induction of alpha1ATZ polymerization by the heat of physiological fever is part of the mechanism of hepatic alpha1ATZ protein accumulation. The results, however, show that fever-range temperature elevations have no detectable effect on steady-state levels of intrahepatic Z protein polymer in a model in vivo system. In conclusion, methods to separate insoluble protein aggregates from liver can be used for quantitative isolation of alpha1ATZ protein polymers, and the effect of heat from physiological fever may be different in vivo compared with in vitro systems.

Alpha-1-antitrypsin deficiency: diagnosis and treatment

Alpha-1-antitrypsin (AT) deficiency was first described in the late 1960s in patients with severe pulmonary emphysema. The recognition of AT deficiency as a cause of emphysema then led to what is still the prevailing theory for the pathogenesis of emphysema, the protease-antiprotease theory. Soon it was found that AT deficiency accounted for a significant number of cases of neonatal liver disease that were previously categorized as idiopathic. We now know that AT deficiency is the most common genetic cause of neonatal liver disease and the most frequent diagnosis necessitating liver transplantation. It has also been shown to cause chronic liver disease, cryptogenic cirrhosis, and hepatocellular carcinoma in adults never previously known to have liver disease in infancy or childhood. Observations indicate that genetic traits unlinked to the AT gene or environmental factors predispose to or protect AT-deficient individuals from liver disease.

Mutant forms of tumour necrosis factor receptor I that occur in TNF-receptor-associated periodic syndrome retain signalling functions but show abnormal behaviour

Tumour necrosis factor (TNF)-receptor-associated periodic syndrome (TRAPS) is a hereditary autoinflammatory disorder involving autosomal-dominant missense mutations in TNF receptor superfamily 1A (TNFRSF1A) ectodomains. To elucidate the molecular effects of TRAPS-related mutations, we transfected HEK-293 cells to produce lines stably expressing high levels of either wild-type (WT) or single mutant recombinant forms of TNFRSF1A. Mutants with single amino acid substitutions in the first cysteine-rich domain (CRD1) were produced both as full-length receptor proteins and as truncated forms lacking the cytoplasmic signalling domain (deltasig). High-level expression of either WT or mutant full-length TNFRSF1A spontaneously induced apoptosis and interleukin-8 production, indicating that the mutations in CRD1 did not abrogate signalling. Consistent with this, WT and mutant full-length TNFRSF1A formed cytoplasmic aggregates that co-localized with ubiquitin and chaperones, and with the signal transducer TRADD, but not with the inhibitor, silencer of death domain (SODD). Furthermore, as expected, WT and mutant deltasig forms of TNFRSF1A did not induce apoptosis or interleukin-8 production. However, whereas the WT full-length TNFRSF1A was expressed both in the cytoplasm and on the cell surface, the mutant receptors showed strong cytoplasmic expression but reduced cell-surface expression. The WT and mutant deltasig forms of TNFRSF1A were all expressed at the cell surface, but a proportion of the mutant receptors were also retained in the cytoplasm and co-localized with BiP. Furthermore, the mutant forms of surface-expressed deltasig TNFRSF1A were defective in binding TNF-alpha. We conclude that TRAPS-related CRD1 mutants of TNFRSF1A possess signalling properties associated with the cytoplasmic death domain, but other behavioural features of the mutant receptors are abnormal, including intracellular trafficking and TNF binding.

Intracellular accumulation of antithrombin Morioka (C95R), a novel mutation causing type I antithrombin deficiency

Antithrombin (AT) is a major plasma protease inhibitor with three intramolecular disulfide bonds, and its deficiency is associated with increased venous thrombosis. Recently, we found a novel missense mutation named AT Morioka (C95R), which causes the loss of one of the three disulfide bonds. In this study, we prepared Chinese hamster ovary cells stably overexpressing wild type or mutant AT and examined the intracellular fate of the ATs. In pulse-chase experiments, newly synthesized wild type AT was secreted into the medium with a half-life of approximately 1.5 h. In contrast, most of the mutant type AT was not secreted during the chase period of 9 h and, surprisingly, was not degraded in the cells. The kinetics of the secretion suggests that the mutant was secreted about 50 times more slowly into the medium. Most of the mutant AT in the cells had high mannose type oligosaccharides, suggesting that it was retained in the endoplasmic reticulum (ER). In addition, half of the mutant AT existed in a dimeric form with an intermolecular disulfide bond. On immunoelectron microscopy, the mutant AT was found to have accumulated in variously sized structures surrounded by a single membrane in the cytoplasm. Immunogold particles exhibiting calnexin immunoreactivity were detected on the membranes. Ribosomes were attached to some of the small structures that had accumulated the mutant AT. Further, we prepared Chinese hamster ovary cells stably overexpressing another mutant AT in which two cysteine residues at 21 and 95, responsible for disulfide bond formation, were substituted for arginines. In pulse-chase experiments, the mutant AT (C21C,C95R) was secreted faster than that of AT Morioka (C95R) into the medium. These results suggest that AT Morioka remained for a long time in ER without being degraded and accumulated in newly formed membrane structures derived from the ER. The dimerization of AT Morioka (C95R) through Cys-21 seems to be critical for its intracellular accumulation.

The proteasome participates in degradation of mutant alpha 1-antitrypsin Z in the endoplasmic reticulum of hepatoma-derived hepatocytes

Because retention of mutant alpha(1)-antitrypsin (alpha(1)-AT) Z in the endoplasmic reticulum (ER) is associated with liver disease in alpha(1)-AT-deficient individuals, the mechanism by which this aggregated glycoprotein is degraded has received considerable attention. In previous studies using stable transfected human fibroblast cell lines and a cell-free microsomal translocation system, we found evidence for involvement of the proteasome in degradation of alpha(1)-ATZ (Qu, D., Teckman, J. H., Omura, S., and Perlmutter, D. H. (1996) J. Biol. Chem. 271, 22791-22795). In more recent studies, Cabral et al. (Cabral, C. M., Choudhury, P., Liu, Y., and Sifers, R. N. (2000) J. Biol. Chem. 275, 25015-25022) found that degradation of alpha(1)-ATZ in a stable transfected murine hepatoma cell line was inhibited by tyrosine phosphatase inhibitors, but not by the proteasomal inhibitor lactacystin and concluded that the proteasome was only involved in ER degradation of alpha(1)-ATZ in nonhepatocytic cell types or in cell types with levels of alpha(1)-AT expression that are substantial lower than that which occurs in hepatocytes. To examine this important issue in further detail, in this study we established rat and murine hepatoma cell lines with constitutive and inducible expression of alpha(1)-ATZ. In each of these cell lines degradation of alpha(1)-ATZ was inhibited by lactacystin, MG132, epoxomicin, and clasto-lactacystin beta-lactone. Using the inducible expression system to regulate the relative level of alpha(1)-ATZ expression, we found that lactacystin had a similar inhibitory effect on degradation of alpha(1)-ATZ at high and low levels of alpha(1)-AT expression. Although there is substantial evidence that other mechanisms contribute to ER degradation of alpha(1)-ATZ, the data reported here indicate that the proteasome plays an important role in many cell types including hepatocytes.