Site-specific cleavage of CD59 mRNA by endoplasmic reticulum-localized ribonuclease, IRE1

IRE1, an ER-localized transmembrane-RNase, plays a central role in ER stress response. Upon ER stress, IRE1 induces various adaptive genes through the processing of mRNA encoding the transcription factor XBP1. Moreover, it was recently reported that in fly IRE1 attenuates the expression of several genes by cleaving mRNAs, but it has been unclear whether such a mechanism also exists in mammal. In this study, we searched for IRE1alpha-cleaved mRNAs in mammalian cells and identified human CD59 (complement defense 59) mRNA as a novel cleavage target. In addition, the expression of CD59 was significantly attenuated by overexpression of IRE1alpha or ER stress. These results suggest that IRE1alpha-mediated mRNA cleavage functions even in mammals as a common system to regulate gene expression.

Analysis of the XBP1 splicing mechanism using endoplasmic reticulum stress-indicators

Under endoplasmic reticulum (ER) stress conditions, XBP1 mRNA is processed by unconventional splicing and translated into a functional transcription factor. ER stress-specific XBP1 splicing is also known to be activated by IRE1. However, many aspects of the molecular mechanism of XBP1 splicing remain to be elucidated. We previously developed an indicator system that enabled detection of XBP1 splicing using fluorescent proteins as the reporter signals. Here, we use a modification of this method that employs modified ER stress-indicators and mutant IRE1 in vivo and in vitro to analyze XBP1 splicing mechanisms. Our analyses suggest that the 506-579 nt region of the XBP1 mRNA is necessary and sufficient for XBP1 splicing, that XBP1 splicing can occur in the cytoplasm, and that cleavage and ligation of XBP1 mRNA during splicing may occur as a coupled reaction.

Regulation of human STARD4 gene expression under endoplasmic reticulum stress

STARD4 belongs to the STARD family of proteins that have steroidogenic acute regulatory protein-related lipid transfer domains and function in the transport and metabolism of lipids. We isolated STARD4 as a novel endoplasmic reticulum (ER) stress-responsive gene using a subtracted, ER stress-specific human cDNA library, and analyzed its transcriptional regulation under ER stress. Northern blot analysis revealed that the induction of STARD4 by ER stress was limited to the early phase. Luciferase reporter assay showed that the induction of STARD4 depended on both transcription factor ATF6 and an ERSE-like element in its promoter. To date, no other genes that are induced only during the early phase of ER stress have been identified, although the mammalian ER stress response is known to be regulated multiphasically and to induce expression of other genes. This study is a first step in elucidating the relationship between lipid metabolism and the ER stress response.

A transgenic mouse model for monitoring endoplasmic reticulum stress

Endoplasmic reticulum (ER) stress is caused by the accumulation of unfolded proteins in the ER lumen, and is associated with vascular and neurodegenerative diseases. Although the connection between ER stress and some disease-related proteins has been studied using animal models of these diseases, no in vivo data concerning ER stress are available. Here we report a new method for monitoring ER stress in vivo, based on XBP-1 mRNA splicing by inositol requiring-1 (IRE-1) during ER stress. The stress indicator was constructed by fusing XBP-1 and venus, a variant of green fluorescent protein. During stress, the spliced indicator mRNA is translated into an XBP-1-venus fusion protein, which can be detected by its fluorescence. We used transgenic animals expressing the ER stress indicator to show that it can be used to monitor physiological and pathological ER stress in vivo.

Liver regeneration in heparin-binding EGF-like growth factor transgenic mice after partial hepatectomy

BACKGROUND & AIMS

Heparin-binding epidermal growth factor-like growth factor (HB-EGF), a member of the EGF family, is synthesized in the form of a membrane-anchored precursor (proHB-EGF), which subsequently is processed proteolytically to mature HB-EGF. This study describes the effects of HB-EGF on liver regeneration after 70% partial hepatectomy in proHB-EGF transgenic mice with liver-specific expression.

METHODS & RESULTS

No significant differences in liver/body weight ratios and in bromodeoxyuridine (BrdU)-labeling index (the ratios of BrdU-positive hepatocyte nuclei) were found between adult transgenic and wild-type mice. However, in regenerating liver after partial hepatectomy, transgenic mice had higher liver/body weight ratios than wild-type mice and at 120 hours reached a level equal to that before partial hepatectomy. The BrdU-labeling index was about 5 times higher in the livers of transgenic mice compared with the wild type (51.5% vs. 10.2%, respectively; P < 0.01) at 48 hours after partial hepatectomy. Activation of microtubule-associated protein kinase after partial hepatectomy was higher and earlier in the transgenic mice as compared with the wild-type mice. Soluble HB-EGF was increased in the liver (at 8 min) after partial hepatectomy, indicating that the shedding of proHB-EGF occurred after partial hepatectomy.

CONCLUSIONS

The transgenic expression of HB-EGF accelerates the proliferation of hepatocytes after partial hepatectomy, suggesting that HB-EGF functions as a hepatotrophic factor in vivo.

Diphtheria toxin receptor-mediated conditional and targeted cell ablation in transgenic mice

Specific cell ablation is a useful method for analyzing the in vivo function of cells. We have developed a simple and sensitive method for conditional cell ablation in transgenic mice, called "toxin receptor-mediated cell knockout." We expressed the diphtheria toxin (DT) receptor in transgenic mice using a hepatocyte-specific promoter and found that injection of DT caused fulminant hepatitis. Three independently established transgenic lines demonstrated a good correlation between the sensitivity of hepatocytes to DT and the expression level of the DT receptors. Moreover, the degree of hepatocyte damage was easily controlled over a wide range of doses of injected DT without any obvious abnormalities in other cells or tissues. This system is useful for generating mouse models of disease and for studying the recovery or regeneration of tissues from cell damage or loss. As DT is a potent inhibitor of protein synthesis in both growing and non-growing cells, the method is applicable to a wide range of cells and tissues in mice or in other DT-insensitive animals.

Translational control by the ER transmembrane kinase/ribonuclease IRE1 under ER stress

Under conditions of endoplasmic reticulum (ER) stress, mammalian cells induce both translational repression and the unfolded protein response that transcriptionally activates genes encoding ER-resident molecular chaperones. To date, the only known pathway for translational repression in response to ER stress has been the phosphorylation of eIF-2alpha by the double-stranded RNA-activated protein kinase (PKR) or the transmembrane PKR-like ER kinase (PERK). Here we report another pathway in which the ER transmembrane kinase/ribonuclease IRE1beta induces translational repression through 28S ribosomal RNA cleavage in response to ER stress. The evidence suggests that both pathways are important for efficient translational repression during the ER stress response.

Identification of a potential nurr1 response element that activates the tyrosine hydroxylase gene promoter in cultured cells

Expression of the gene encoding tyrosine hydroxylase (TH), the initial and rate-limiting enzyme of catecholamine biosynthesis, is regulated at the transcriptional level during neuronal development and in response to a variety of environmental stimuli. Nur-related factor 1 (Nurr1), a member of the orphan nuclear receptor superfamily, is required for development of dopamine-producing neurons in the ventral midbrain and for expression of TH in these neurons. In the present study, we found a direct activation of the rat TH gene promoter by Nurr1 in cultured cell lines. This activation appeared to be dependent on multiple regulatory elements conferring Nurr1 responsiveness to the promoter. We identified a Nurr1 response element (TH-NBRE1) in the proximal region of the TH promoter that mediates a moderate activation of the promoter. The sequence of TH-NBRE1 was highly homologous to that of the typical NGFI-B response element. Our findings suggest that Nurr1 may be implicated in the transcriptional control of TH gene expression during development or in response to altered physiological states.

Phyllodes tumor in a patient with a preexistent fibrocystic disease

A phyllodes tumor appeared at the site of a resected fibrocystic disease focus. Administration of danazol resulted in temporary regression, but the tumor resumed growth. Relative hyperestrogenism also continued to increase in spite of continued long-term administration of danazol. After 12 months a simple mastectomy was performed. Most of the resected mass consisted of the phyllodes tumor, but it also contained fibrocystic disease.