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XBP1: a link between the unfolded protein response, lipid biosynthesis, and biogenesis of the endoplasmic reticulum.

Sriburi R, Jackowski S, Mori K, Brewer JW - J. Cell Biol. (2004)

Bottom Line: When the protein folding capacity of the endoplasmic reticulum (ER) is challenged, the unfolded protein response (UPR) maintains ER homeostasis by regulating protein synthesis and enhancing expression of resident ER proteins that facilitate protein maturation and degradation.Cells overexpressing XBP1(S) exhibit elevated levels of membrane phospholipids, increased surface area and volume of rough ER, and enhanced activity of the cytidine diphosphocholine pathway of phosphatidylcholine biosynthesis.These data suggest that XBP1(S) links the mammalian UPR to phospholipid biosynthesis and ER biogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology, Stritch School of Medicine, Loyola University Chicago, Maywood, IL 60153, USA.

ABSTRACT
When the protein folding capacity of the endoplasmic reticulum (ER) is challenged, the unfolded protein response (UPR) maintains ER homeostasis by regulating protein synthesis and enhancing expression of resident ER proteins that facilitate protein maturation and degradation. Here, we report that enforced expression of XBP1(S), the active form of the XBP1 transcription factor generated by UPR-mediated splicing of XBP1 mRNA, is sufficient to induce synthesis of phosphatidylcholine, the primary phospholipid of the ER membrane. Cells overexpressing XBP1(S) exhibit elevated levels of membrane phospholipids, increased surface area and volume of rough ER, and enhanced activity of the cytidine diphosphocholine pathway of phosphatidylcholine biosynthesis. These data suggest that XBP1(S) links the mammalian UPR to phospholipid biosynthesis and ER biogenesis.

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Enzymatic activities in the CDP-choline pathway of PtdCho synthesis. The relative activities of CK, CCT, and CPT, which constitute the CDP-choline pathway of PtdCho biosynthesis (right side of figure), were determined using lysates or microsomes prepared from NIH-3T3 cells (▪), or NIH-3T3 cells transduced with XBP1(S) (•) or empty vector (○). The rates of production of phosphocholine (P-Choline), CDP-Choline, and PtdCho were compared as a function of total protein in each assay. (A) Data for CK are averaged from triplicate determinations and are representative of two independent experiments. (B) Data for CCT are averaged from five determinations obtained in two independent experiments. (C) Data for CPT are averaged from duplicate determinations and are representative of three independent experiments.
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fig3: Enzymatic activities in the CDP-choline pathway of PtdCho synthesis. The relative activities of CK, CCT, and CPT, which constitute the CDP-choline pathway of PtdCho biosynthesis (right side of figure), were determined using lysates or microsomes prepared from NIH-3T3 cells (▪), or NIH-3T3 cells transduced with XBP1(S) (•) or empty vector (○). The rates of production of phosphocholine (P-Choline), CDP-Choline, and PtdCho were compared as a function of total protein in each assay. (A) Data for CK are averaged from triplicate determinations and are representative of two independent experiments. (B) Data for CCT are averaged from five determinations obtained in two independent experiments. (C) Data for CPT are averaged from duplicate determinations and are representative of three independent experiments.

Mentions: The predominant pathway for PtdCho biosynthesis in mammalian cells is the cytidine diphosphocholine (CDP-choline) or Kennedy pathway (Lykidis and Jackowski, 2001). First, choline kinase (CK) phosphorylates choline in the presence of ATP to yield phosphocholine. Second, choline cytidylyltransferase (CCT) converts phosphocholine to CDP-choline in the presence of CTP, and this is considered to be the rate-limiting step in the CDP-choline pathway (Lykidis and Jackowski, 2001). Third, cholinephosphotransferase 1 (CPT1) transfers the phosphocholine moiety of CDP-choline to DAG, producing PtdCho. This final step can also be catalyzed by choline/ethanolaminephosphotransferase (CEPT1), a bifunctional enzyme that can synthesize both choline- and ethanolamine-containing phospholipids. Here, we refer to the activities of CPT1 (Henneberry et al., 2000) and CEPT1 (Henneberry and McMaster, 1999) collectively as CPT activity. In XBP1(S)-transduced cells, CK activity was unchanged (Fig. 3 A), CCT activity increased ∼30% (Fig. 3 B), and CPT activity elevated approximately fivefold (Fig. 3 C). Remarkably similar to these data, increased PtdCho biosynthesis in lipopolysaccharide-stimulated splenic B cells was previously shown to correlate with unchanged CK activity, a small increase in CCT activity, and an approximately sixfold increase in CPT activity (Rush et al., 1991). Therefore, it follows that XBP1(S) might mediate the increased synthesis of phospholipids necessary for ER biogenesis as B cells transition into high-rate antibody secretion.


XBP1: a link between the unfolded protein response, lipid biosynthesis, and biogenesis of the endoplasmic reticulum.

Sriburi R, Jackowski S, Mori K, Brewer JW - J. Cell Biol. (2004)

Enzymatic activities in the CDP-choline pathway of PtdCho synthesis. The relative activities of CK, CCT, and CPT, which constitute the CDP-choline pathway of PtdCho biosynthesis (right side of figure), were determined using lysates or microsomes prepared from NIH-3T3 cells (▪), or NIH-3T3 cells transduced with XBP1(S) (•) or empty vector (○). The rates of production of phosphocholine (P-Choline), CDP-Choline, and PtdCho were compared as a function of total protein in each assay. (A) Data for CK are averaged from triplicate determinations and are representative of two independent experiments. (B) Data for CCT are averaged from five determinations obtained in two independent experiments. (C) Data for CPT are averaged from duplicate determinations and are representative of three independent experiments.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2172532&req=5

fig3: Enzymatic activities in the CDP-choline pathway of PtdCho synthesis. The relative activities of CK, CCT, and CPT, which constitute the CDP-choline pathway of PtdCho biosynthesis (right side of figure), were determined using lysates or microsomes prepared from NIH-3T3 cells (▪), or NIH-3T3 cells transduced with XBP1(S) (•) or empty vector (○). The rates of production of phosphocholine (P-Choline), CDP-Choline, and PtdCho were compared as a function of total protein in each assay. (A) Data for CK are averaged from triplicate determinations and are representative of two independent experiments. (B) Data for CCT are averaged from five determinations obtained in two independent experiments. (C) Data for CPT are averaged from duplicate determinations and are representative of three independent experiments.
Mentions: The predominant pathway for PtdCho biosynthesis in mammalian cells is the cytidine diphosphocholine (CDP-choline) or Kennedy pathway (Lykidis and Jackowski, 2001). First, choline kinase (CK) phosphorylates choline in the presence of ATP to yield phosphocholine. Second, choline cytidylyltransferase (CCT) converts phosphocholine to CDP-choline in the presence of CTP, and this is considered to be the rate-limiting step in the CDP-choline pathway (Lykidis and Jackowski, 2001). Third, cholinephosphotransferase 1 (CPT1) transfers the phosphocholine moiety of CDP-choline to DAG, producing PtdCho. This final step can also be catalyzed by choline/ethanolaminephosphotransferase (CEPT1), a bifunctional enzyme that can synthesize both choline- and ethanolamine-containing phospholipids. Here, we refer to the activities of CPT1 (Henneberry et al., 2000) and CEPT1 (Henneberry and McMaster, 1999) collectively as CPT activity. In XBP1(S)-transduced cells, CK activity was unchanged (Fig. 3 A), CCT activity increased ∼30% (Fig. 3 B), and CPT activity elevated approximately fivefold (Fig. 3 C). Remarkably similar to these data, increased PtdCho biosynthesis in lipopolysaccharide-stimulated splenic B cells was previously shown to correlate with unchanged CK activity, a small increase in CCT activity, and an approximately sixfold increase in CPT activity (Rush et al., 1991). Therefore, it follows that XBP1(S) might mediate the increased synthesis of phospholipids necessary for ER biogenesis as B cells transition into high-rate antibody secretion.

Bottom Line: When the protein folding capacity of the endoplasmic reticulum (ER) is challenged, the unfolded protein response (UPR) maintains ER homeostasis by regulating protein synthesis and enhancing expression of resident ER proteins that facilitate protein maturation and degradation.Cells overexpressing XBP1(S) exhibit elevated levels of membrane phospholipids, increased surface area and volume of rough ER, and enhanced activity of the cytidine diphosphocholine pathway of phosphatidylcholine biosynthesis.These data suggest that XBP1(S) links the mammalian UPR to phospholipid biosynthesis and ER biogenesis.

View Article: PubMed Central - PubMed

Affiliation: Department of Microbiology and Immunology, Stritch School of Medicine, Loyola University Chicago, Maywood, IL 60153, USA.

ABSTRACT
When the protein folding capacity of the endoplasmic reticulum (ER) is challenged, the unfolded protein response (UPR) maintains ER homeostasis by regulating protein synthesis and enhancing expression of resident ER proteins that facilitate protein maturation and degradation. Here, we report that enforced expression of XBP1(S), the active form of the XBP1 transcription factor generated by UPR-mediated splicing of XBP1 mRNA, is sufficient to induce synthesis of phosphatidylcholine, the primary phospholipid of the ER membrane. Cells overexpressing XBP1(S) exhibit elevated levels of membrane phospholipids, increased surface area and volume of rough ER, and enhanced activity of the cytidine diphosphocholine pathway of phosphatidylcholine biosynthesis. These data suggest that XBP1(S) links the mammalian UPR to phospholipid biosynthesis and ER biogenesis.

Show MeSH