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. 2008 May;7(5):445-55.
doi: 10.1016/j.cmet.2008.03.005.

IRE1beta inhibits chylomicron production by selectively degrading MTP mRNA

Jahangir Iqbal  1 , Kezhi Dai, Tracie Seimon, Rivka Jungreis, Miho Oyadomari, George Kuriakose, David Ron, Ira Tabas, M Mahmood Hussain
Affiliations

IRE1beta inhibits chylomicron production by selectively degrading MTP mRNA

Jahangir Iqbal et al. Cell Metab. 2008 May.

Abstract

Microsomal triglyceride transfer protein (MTP) is needed to assemble chylomicrons in the endoplasmic reticulum (ER) of enterocytes. We explored the role of an ER stress protein, inositol-requiring enzyme 1beta (IRE1beta), in regulating this process. High-cholesterol and high-fat diets decreased intestinal IRE1beta mRNA in wild-type mice. Ire1b(-/-) mice fed high-cholesterol and high-fat diets developed more pronounced hyperlipidemia because these mice secreted more chylomicrons and expressed more intestinal MTP, though not hepatic MTP, than wild-type mice did. Chylomicron secretion and MTP expression also were increased in primary enterocytes isolated from cholesterol-fed Ire1b(-/-) mice. There was no correlation between ER stress and MTP expression. Instead, cell culture studies revealed that IRE1beta, but not its ubiquitous homolog IRE1alpha, decreased MTP mRNA through increased posttranscriptional degradation. Conversely, knockdown of IRE1beta enhanced MTP expression. These studies show that IRE1beta plays a role in regulating MTP and in chylomicron production.

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Figures

Figure 1
Figure 1. Plasma Non-HDL Lipids Are Elevated and Intestinal Lipids Are Decreased in Ire1b−/− Mice Fed High-Cholesterol and High-Fat Diets
Wild-type Ire1b+/+ (+/+) and Ire1b−/− (−/−) male mice were fed normal chow, chow supplemented with 2% w/w cholesterol (average of five experiments with n = 3–5 animals per group), or a high-fat diet (n = 3 per group) for 2 weeks. (A–F) Plasma samples were used to measure total, non-HDL, and HDL cholesterol as well as triglyceride. (G–J) Plasma samples from high-cholesterol (G and H) and high-fat (I and J) fed animals were separated by gel filtration to measure lipids in different lipoproteins. (K–N) Liver and intestinal samples (n = 3) were used to measure cholesterol and triglyceride mass. Values are mean ± SD. *p < 0.05, **p < 0.01, ***p < 0.001 versus Ire1b+/+.
Figure 2
Figure 2. Intestines from Ire1b−/− Mice Secrete More Lipids with Chylomicrons
Ire1b+/+ (+/+; white bars) and Ire1b−/− (−/−; black bars) mice were fed a high-cholesterol diet for 2 weeks and fed 1 μCi of [3H]cholesterol or [3H]triolein as well as 0.1 mg of cholesterol in 15 μl of olive oil. (A and C) After 2 hr, plasma was used to measure radioactivity. (B and D) Liver slices were digested with OptiSolv and used for radioactivity determinations. (E–J) In another experiment, Ire1b+/+ and Ire1b−/− mice fasted overnight were injected intraperitoneally with poloxamer 407 (30 μg/mouse). After 1 hr, mice were fed 1 μCi [3H]cholesterol and 0.1 mg unlabeled cholesterol. Plasma was collected after 2 hr and used to measure radioactivity (E) as well as cholesterol (G) and triglyceride (I) mass in total plasma, HDL, and non-HDL lipoproteins. Plasma was separated by gel filtration to determine lipids in different lipoproteins (F, H, and J). (K) Enterocytes were isolated from cholesterol-fed male Ire1b+/+ and Ire1b−/− mice and incubated with 1 μCi/ml of [3H]cholesterol for different lengths of time to study uptake. At each time point, cellular lipids were extracted with isopropanol and counted. (L) To study secretion, cells were labeled with [3H]cholesterol for 1 hr, washed, and chased for various lengths of time in the presence of 1.2 mM oleic acid. Radioactivity was measured in conditioned media. (M and N) To identify lipoproteins carrying cholesterol, enterocytes from cholesterol-fed male Ire1b+/+ and Ire1b−/− mice were labeled for 1 hr with 1 μCi/ml of [3H]cholesterol, washed, and chased for 2 hr in the presence of 1.2 mM oleic acid-containing micelles. Conditioned media were used to measure radioactivity (M) or for density gradient ultracentrifugation. Fractions were collected from the top and assayed for radioactivity (N). Each measurement was performed in triplicate with n = 3 mice per group. Values are mean ± SD. *p < 0.05, **p < 0.01, ***p < 0.001 versus Ire1b+/+.
Figure 3
Figure 3. MTP mRNA, Protein, and Activity Are Increased in the Intestines of Ire1b−/− Mice
Intestinal and liver samples from Ire1b+/+ (+/+; white bars) and Ire1b−/− (−/−; black bars) mice (n = 3 per group) fed a cholesterol-enriched or fat-enriched diet were used for MTP activity assays (A, E, I, and M). RNA was used to measure MTP (B, F, J, and N), apoB (C, G, K, and O), GAPDH (D, H, L, and P), and ARPp0 mRNA in triplicate. The ratio between the gene of interest and ARPp0 in Ire1b+/+ mice was used to normalize mRNA levels in all samples. Values are mean ± SD. **p < 0.01, ***p < 0.001 compared to Ire1b+/+. Data are representative of two experiments.
Figure 4
Figure 4. XBP-1 Splicing Does Not Regulate MTP
RNA was isolated from the intestinal tissues of Ire1b+/+ and Ire1b−/− mice (n = 3) fed with or without high-cholesterol or high-fat diet for 2 weeks. (A) Spliced and unspliced XBP-1 mRNA levels. (B–D, G, and H) Intestinal RNA was used to determine in triplicate the levels of IRE1α (B), BiP (C), CHOP (D), MTP (G), and IRE1β (H) mRNAs. Values are mean ± SD. *p < 0.05, **p < 0.01, ***p < 0.001 versus chow-fed animals. (E and F) RNA also was isolated from the primary enterocytes of Ire1b+/+ and Ire1b−/− mice fed a high-cholesterol diet and used to quantify mRNA levels of ER stress and lipoprotein genes (E) and XBP-1 splicing (F). RNA from Huh7 cells treated with tunicamycin (5 μg/ml) was used as a control (F). Values are mean ± SD. ***p < 0.001 versus Ire1b+/+. (I and K) Differentiated Caco-2 cells were treated with tunicamycin (5 μg/ml) or thapsigargin (1 μM) for 17 hr and used to quantify spliced as well as unspliced XBP-1 mRNA (I) and to measure MTP, IRE1α, BiP, CHOP, and ARPp0 mRNA (K). (J and L) Enterocytes isolated from Ire1b+/+ mice were treated with tunicamycin (10 μg/ml) or thapsigargin (5 μM) for 3 hr and used to measure XBP-1 splicing (J) and MTP, IRE1α, BiP, CHOP, and ARPp0 mRNA (L).
Figure 5
Figure 5. Overexpression of IRE1β Decreases MTP mRNA Levels
(A) Huh7 cells transfected with different c-Myc-tagged IRE1β expression plasmids were used to determine protein expression using c-Myc antibodies. (B) Total RNA from IRE1β-ΔC- and IRE1β-WT-transfected Huh7 cells was loaded on 1% agarose gels to visualize changes in 18S and 28S rRNA. (C–E) Cells from three wells were used to measure MTP (C) and PDI (E) mRNA in triplicate. Media were used to measure apoB secretion (D).*p < 0.05, **p < 0.01 compared to pcDNA3.1-treated cells. (F) RNA interference was used to knock down IRE1β in undifferentiated Caco-2 cells. siGL2 served as a control. Cells were treated with siRNA targeting IRE1β and used to measure IRE1β, MTP, IRE1α, and PDI mRNAs. ***p < 0.001 compared to siGL2-treated cells. (G and H) To study the effect of cholesterol on MTP expression, Huh7 cells were transfected with either IRE1β-WT or IRE1β-ΔC expression plasmids. After 12 hr, cells were supplemented with or without cholesterol (40 μg/ml), and MTP and ARPp0 mRNA were quantitated (G). Cells treated in parallel were used for MTP activity assays (H). **p < 0.01, ***p < 0.001 compared to IRE1β-ΔC-overexpressing cells without cholesterol treatment. (I and J) Huh7 cells were transfected with IRE1β expression plasmids or treated with 5 μg/ml tunicamycin. Total RNA was isolated and used to measure MTP, IRE1α, BiP, and CHOP mRNA (I). ***p < 0.001 compared to IRE1β-ΔC-overexpressing cells. In addition, RNA was used to determine XBP-1 splicing (J). Data are representative of two experiments performed in triplicate. Values are mean ± SD.
Figure 6
Figure 6. IRE1β Enhances Posttranscriptional Degradation of MTP mRNA
(A) Caco-2, Huh7, and HEK293 cells in 12-well plates were cotransfected with 40 ng of control plasmid (pCMV-RL) and 1 μg of pGL2 plasmid DNAs with luciferase under the control of various lengths (−204 to −1483 bp) of 5′ sequences upstream of the human MTTP gene. Cells were collected 48 hr later and analyzed with a Dual-Luciferase Reporter kit (Promega). (B) Promoter activity assays were performed using the dual-luciferase assay system. pCMV-Renilla luciferase served as a control for transfection efficiency. Huh7 cells were transfected with 20 ng pCMV-Renilla luciferase and 1 μg pMTP-1483-luciferase (expressing luciferase activity under the control of a 1.5 kb MTTP promoter) with pcDNA3.1, IRE1β-ΔC (100 ng), or IRE1β-WT (100 ng). After 48 hr, luciferase activity was measured. (C and D) Mouse fibroblasts L cells stably expressing mouse MTP under the control of the cytomegalovirus promoter were transfected with different plasmids. After 48 hr, MTP mRNA (C) and activity (D) were measured in triplicate. Inset in (D) shows western blot of a representative sample. *p < 0.05, **p < 0.01 versus pcDNA3.1-treated cells. (E) Huh7 cells were transfected in triplicate with either IRE1β-WT or IRE1β-ΔC expression plasmid DNA. After overnight incubation, cells were cultured in the presence of actinomycin D (1 μg/ml) for different lengths of time in triplicate. RNA was used to quantify MTP, PDI, and ARPp0 mRNA. The MTP/ARPp0 or PDI/ARPp0 ratio at time 0 was normalized to 100%, and loss of MTP was plotted versus time. (F) Schematic representation of possible internal cleavage of MTP mRNA by IRE1β and its subsequent degradation by different exonucleases. (G) Huh7 cells were transfected with IRE1β-WT. After 24 hr, cells were transfected with siRNAs against Ski2, XRN1, or XRN2. RNA was isolated after 48 hr and used to quantify Ski2, XRN1, or XRN2 mRNA in control (siGL2) and RNAi-treated cells to determine the efficacy and specificity of RNA interference. (H) Huh7 cells were transfected in triplicate with IRE1β-ΔC or IRE1β-WT expression plasmids. After 24 hr, cells were transfected with indicated siRNAs. Total RNA from these cells was used to amplify different regions (exons 1–2 and exons 7–8) of MTP mRNA using primers listed in Table S1. Data are representative of two experiments performed in triplicate. Values are mean ± SD.
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