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Folding and bioassembly of secretory proteins in health and disease
Forefronts in Nephrology
397
Folding and bioassembly of secretory proteins in health and disease SANJAY K. NIGAM and KEVIN T. BUSH Departments of Pediatrics and Medicine, University of California-San Diego, La Jolla, California, USA
Our work has focused on the identification of endoplasmic reticulum (ER) molecular chaperones and understanding their role in cell injury and cytoprotection. The ER is the site of the initial folding and bioassembly of membrane and secreted proteins [reviewed in 1]. This is considered to be a rate-limiting step in secretion. The folding and bioassembly process is mediated by an ERspecific set of molecular chaperones that include glucose regulated proteins (BiP, grp94, grp170), calcium binding proteins (calnexin, calreticulin), disulfide isomerases (PDI, ERp72) and possibly immunophilins (FKBP13) [1, 2]. Together, these proteins catalyze folding reactions, disulfide isomerization, peptidylprolyl isomerization and the assembly of multisubunit complexes in the ER. When secretory proteins are misfolded, they accumulate in the ER and are often bound to one or more of these molecular chaperones. We have studied this process using the folding of thyroglobulin in a thyroid epithelial cell line as a model system. Thyroglobulin is a large dimeric glycoprotein with multiple disulfide bonds. Its folding appears to require the participation of a number of ER chaperones, including BiP, grp 94, Erp72, grp170 and possibly others [3-5]. Misfolding of membrane or secreted proteins can lead to an ER stress/overload response characterized by dramatic increases in the mRNAs encoding ER chaperones. Misfolding can occur because of single amino acid changes in a membrane or secreted protein, leading to genetic disease [reviewed in 1]. Well-studied examples include cystic fibrosis and alpha-1-antitrypsin deficiency. Our recent focus, however, has been on the misfolding in the ER due to cell “stress.” An example relevant to the kidney is acute epithelial tissue ischemia [4, 7, 8]. Since many of the molecular and cellular disturbances characteristic of the ischemic epithelial phenotype are
attributable to changes in membrane proteins (including their degradation), and since ischemia also perturbs ER folding and bioassembly, full recovery of the ER is likely to be essential for restoration of the epithelial cell to its normal state [7, 8]. It is worth noting in this context that preinduction of ER chaperones with agents such as tunicamycin and proteasome inhibitors correlates with an enhanced capacity of the cell to survive a subsequent insult, although it is likely that other mechanisms are also involved in the cell protection conferred by these agents [6, 7]. Thus, both in the case of certain inheritable diseases as well as more common clinical syndromes (acute renal failure), it may be useful to enhance the folding capacity of the ER to facilitate cell recovery. REFERENCES 1. Kuznetsov G, Nigam SK: Mechanisms of Disease: Folding of Secretory and Membrane Proteins. N Engl J Med 339:1688–1695, 1998 2. Nigam SK, Goldberg AL, Ho S, et al: A set of ER proteins with properties of molecular chaperones includes calcium binding proteins and members of the thioredoxin superfamily. J Biol Chem 269:1744–1749, 1994 3. Kuznetsov G, Chen LB, Nigam SK: Several endoplasmic reticulum stress proteins, including ERp72, interact with thyroglobulin during its maturation. J Biol Chem 269:22990–22995, 1994 4. Kuznetsov GK, Bush KT, Zhang PL, Nigam SK: Perturbations in maturation of secretory proteins and their association with endoplasmic reticulum molecular chaperones in a cell culture model for epithelial ischemia. Proc Natl Acad Sci 93:8584–8589, 1996 5. Kuznetsov G, Chen LB, Nigam SK: Multiple molecular chaperones complex with misfolded large oligomeric glycoproteins in the endoplasmic reticulum. J Biol Chem 272:3057–3063, 1997 6. Bush KT, Goldberg AL, Nigam SK: Proteasome inhibition leads to a heat-shock response, induction of endoplasmic reticulum chaperones, and thermotolerance. J Biol Chem 272:9086–9092, 1997 7. Bush KT, George SK, Zhang PL, Nigam SK: Pretreatment with inducers of ER molecular chaperones protects epithelial cells subjected to ATP depletion. Am J Physiol 277:F211–F218, 1999 8. Bush KT, Keller SH, Nigam SK: Genesis and reversal of the ischemic phenotype in epithelial cells. J Clin Invest 106:621–626, 2000
397
Folding and bioassembly of secretory proteins in health and disease SANJAY K. NIGAM and KEVIN T. BUSH Departments of Pediatrics and Medicine, University of California-San Diego, La Jolla, California, USA
Our work has focused on the identification of endoplasmic reticulum (ER) molecular chaperones and understanding their role in cell injury and cytoprotection. The ER is the site of the initial folding and bioassembly of membrane and secreted proteins [reviewed in 1]. This is considered to be a rate-limiting step in secretion. The folding and bioassembly process is mediated by an ERspecific set of molecular chaperones that include glucose regulated proteins (BiP, grp94, grp170), calcium binding proteins (calnexin, calreticulin), disulfide isomerases (PDI, ERp72) and possibly immunophilins (FKBP13) [1, 2]. Together, these proteins catalyze folding reactions, disulfide isomerization, peptidylprolyl isomerization and the assembly of multisubunit complexes in the ER. When secretory proteins are misfolded, they accumulate in the ER and are often bound to one or more of these molecular chaperones. We have studied this process using the folding of thyroglobulin in a thyroid epithelial cell line as a model system. Thyroglobulin is a large dimeric glycoprotein with multiple disulfide bonds. Its folding appears to require the participation of a number of ER chaperones, including BiP, grp 94, Erp72, grp170 and possibly others [3-5]. Misfolding of membrane or secreted proteins can lead to an ER stress/overload response characterized by dramatic increases in the mRNAs encoding ER chaperones. Misfolding can occur because of single amino acid changes in a membrane or secreted protein, leading to genetic disease [reviewed in 1]. Well-studied examples include cystic fibrosis and alpha-1-antitrypsin deficiency. Our recent focus, however, has been on the misfolding in the ER due to cell “stress.” An example relevant to the kidney is acute epithelial tissue ischemia [4, 7, 8]. Since many of the molecular and cellular disturbances characteristic of the ischemic epithelial phenotype are
attributable to changes in membrane proteins (including their degradation), and since ischemia also perturbs ER folding and bioassembly, full recovery of the ER is likely to be essential for restoration of the epithelial cell to its normal state [7, 8]. It is worth noting in this context that preinduction of ER chaperones with agents such as tunicamycin and proteasome inhibitors correlates with an enhanced capacity of the cell to survive a subsequent insult, although it is likely that other mechanisms are also involved in the cell protection conferred by these agents [6, 7]. Thus, both in the case of certain inheritable diseases as well as more common clinical syndromes (acute renal failure), it may be useful to enhance the folding capacity of the ER to facilitate cell recovery. REFERENCES 1. Kuznetsov G, Nigam SK: Mechanisms of Disease: Folding of Secretory and Membrane Proteins. N Engl J Med 339:1688–1695, 1998 2. Nigam SK, Goldberg AL, Ho S, et al: A set of ER proteins with properties of molecular chaperones includes calcium binding proteins and members of the thioredoxin superfamily. J Biol Chem 269:1744–1749, 1994 3. Kuznetsov G, Chen LB, Nigam SK: Several endoplasmic reticulum stress proteins, including ERp72, interact with thyroglobulin during its maturation. J Biol Chem 269:22990–22995, 1994 4. Kuznetsov GK, Bush KT, Zhang PL, Nigam SK: Perturbations in maturation of secretory proteins and their association with endoplasmic reticulum molecular chaperones in a cell culture model for epithelial ischemia. Proc Natl Acad Sci 93:8584–8589, 1996 5. Kuznetsov G, Chen LB, Nigam SK: Multiple molecular chaperones complex with misfolded large oligomeric glycoproteins in the endoplasmic reticulum. J Biol Chem 272:3057–3063, 1997 6. Bush KT, Goldberg AL, Nigam SK: Proteasome inhibition leads to a heat-shock response, induction of endoplasmic reticulum chaperones, and thermotolerance. J Biol Chem 272:9086–9092, 1997 7. Bush KT, George SK, Zhang PL, Nigam SK: Pretreatment with inducers of ER molecular chaperones protects epithelial cells subjected to ATP depletion. Am J Physiol 277:F211–F218, 1999 8. Bush KT, Keller SH, Nigam SK: Genesis and reversal of the ischemic phenotype in epithelial cells. J Clin Invest 106:621–626, 2000