Intracellular Processing of α1-Antitrypsin

α1-Antitrypsin (AAT) secreted from hepatocytes is an inhibitor of neutrophil elastase. Its normal circulating concentration functions to maintain the elasticity of the lung by preventing the hydrolytic destruction of elastin fibers. Severely diminished circulating concentrations of AAT, resulting from the impaired secretion of genetic variants that exhibit distinct polypeptide folding defects, can function as an etiologic agent for the development of chronic obstructive pulmonary disease. In addition, the inappropriate accumulation of structurally aberrant AAT within the hepatocyte endoplasmic reticulum can contribute to the etiology of liver disease. This article focuses on the discovery and characterization of a biosynthetic quality control system that contributes to the secretion of AAT by first facilitating its proper structural maturation, and then by orchestrating the selective elimination of those molecules that fail to attain structural maturation. Mechanistic elucidation of these interconnected quality control events recently led to the identification of an underlying genetic modifier capable of accelerating the onset of end-stage liver disease by impairing the efficiency of an initial step in the protein disposal process

Nuclear receptor LRH-1/NR5A2 is required and targetable for liver endoplasmic reticulum stress resolution

Chronic endoplasmic reticulum (ER) stress results in toxicity that contributes to multiple human disorders. We report a stress resolution pathway initiated by the nuclear receptor LRH-1 that is independent of known unfolded protein response (UPR) pathways. Like mice lacking primary UPR components, hepatic Lrh-1-null mice cannot resolve ER stress, despite a functional UPR. In response to ER stress, LRH-1 induces expression of the kinase Plk3, which phosphorylates and activates the transcription factor ATF2. Plk3-null mice also cannot resolve ER stress, and restoring Plk3 expression in Lrh-1-null cells rescues ER stress resolution. Reduced or heightened ATF2 activity also sensitizes or desensitizes cells to ER stress, respectively. LRH-1 agonist treatment increases ER stress resistance and decreases cell death. We conclude that LRH-1 initiates a novel pathway of ER stress resolution that is independent of the UPR, yet equivalently required. Targeting LRH-1 may be beneficial in human disorders associated with chronic ER stress

Nuclear receptor LRH-1/NR5A2 is required and targetable for liver endoplasmic reticulum stress resolution.

Chronic endoplasmic reticulum (ER) stress results in toxicity that contributes to multiple human disorders. We report a stress resolution pathway initiated by the nuclear receptor LRH-1 that is independent of known unfolded protein response (UPR) pathways. Like mice lacking primary UPR components, hepatic Lrh-1-null mice cannot resolve ER stress, despite a functional UPR. In response to ER stress, LRH-1 induces expression of the kinase Plk3, which phosphorylates and activates the transcription factor ATF2. Plk3-null mice also cannot resolve ER stress, and restoring Plk3 expression in Lrh-1-null cells rescues ER stress resolution. Reduced or heightened ATF2 activity also sensitizes or desensitizes cells to ER stress, respectively. LRH-1 agonist treatment increases ER stress resistance and decreases cell death. We conclude that LRH-1 initiates a novel pathway of ER stress resolution that is independent of the UPR, yet equivalently required. Targeting LRH-1 may be beneficial in human disorders associated with chronic ER stress. DOI: http://dx.doi.org/10.7554/eLife.01694.001

ERManI Is a Target of miR-125b and Promotes Transformation Phenotypes in Hepatocellular Carcinoma (HCC)

<div><p>The MAN1B1 gene product, designated ER alpha-1, 2-mannosidase (ERManI), is an enzyme localized in the Golgi complex of mammalian cells. By functioning as a “gate keeper” to prevent the inappropriate secretion of misfolded glycoproteins, it plays a critical role in maintaining protein homeostasis in the mammalian secretory pathway. In the present study, we identified that a conserved motif within the 3’UTR of ERManI is a target of miR-125b, a microRNA frequently down-regulated in numerous types of cancers, including hepatocellular carcinoma (HCC). As predicted, the expression of ERManI is significantly elevated in HCC, as measured by immunohistochemistry in a liver spectrum tissue microarray. Additional analyses using several hepatoma cell lines demonstrated that the elevated ERManI inversely correlates with a diminished intracellular concentration of miR-125b. Moreover, functional studies indicated that RNAi-mediated knock-down of endogenous ERManI was sufficient to inhibit proliferation, migration, and invasion of hepatoma cells. These phenotypical changes occurred in the absence of alterations in global glycoprotein secretion or ER-stress status. Together, these results revealed a novel post-transcriptional regulatory mechanism for ERManI and implied that this molecule contributes to the regulation of carcinogenesis in HCC independent of its function in glycoprotein quality control. </p> </div

ERManI is upregulated in HCC.

<p><b>A</b>. Representative images of different liver tissues stained with anti-ERManI antibodies. <b>B</b>. Statistical analyses of the HCC and cirrhosis/hepatitis/tumor adjacent tissues using Mann-Whitney Rank Sum Test. The analyses were based on the sample number (N), median, and intensity score at 25% and 75% percentile of each indicated tissue group. <b>C</b>. The percentage of miR-125a and miR-125b is expressed in PLC/PRF5, Huh7, and HepG2 cells lines as compared to that expressed in normal liver tissues. The data are based on the results obtained from real-time RT-PCR. <b>D</b>. Western blotting of ERManI and actin in normal liver and indicated cell lines. Numbers on the bottom represent the relative expression of ERManI normalized against actin.</p