Drug enhanced autophagy to fight mutant protein overload (2010)

Safe drugs to fight mutant protein overload and alpha-1-antitrypsin deficiencyJournal of HepatologyVol. 55Issue 4PreviewWe read with interest the commentary, “Drug enhanced autophagy to fight mutant protein overload”, by Mehrpour and Codogno [1]. The authors discuss recent experimental findings by Hidvegi et al., in a in vitro and in an animal model of alpha-1-antitrypsin (AT) deficiency, which raise the interesting possibility that the iminostilbene derivative carbamazepine (CBZ) could be used to treat this severe liver disease in humans [1,2]. The authors correctly conclude that these findings have provided the rationale for studies on CBZ in patients with AT deficiency [1]. Full-Text PDF Open Access COMMENTARY ON: An autophagy-enhancing drug promotes degradation of mutant alpha1-antitrypsin Z and reduces hepatic fibrosis. Hidvegi T, Ewing M, Hale P, Dippold C, Beckett C, Kemp C, Maurice N, Mukherjee A, Goldbach C, Watkins S, Michalopoulos G, Perlmutter DH. Science 2010;329(5988):229–32. Abstract reprinted with permission from the AAAS. http://www.ncbi.nlm.nih.gov/pubmed/20522742 Abstract: In the classical form of alpha1-antitrypsin (AT) deficiency, a point mutation in AT alters the folding of a liver-derived secretory glycoprotein and renders it aggregation-prone. In addition to decreased serum concentrations of AT, the disorder is characterized by accumulation of the mutant alpha1-antitrypsin Z (ATZ) variant inside cells, causing hepatic fibrosis and/or carcinogenesis by a gain-of-toxic function mechanism. The proteasomal and autophagic pathways are known to mediate degradation of ATZ. Here we show that the autophagy-enhancing drug CBZ (CBZ) decreased the hepatic load of ATZ and hepatic fibrosis in a mouse model of AT deficiency-associated liver disease. These results provide a basis for testing CBZ, which has an extensive clinical safety profile, in patients with AT deficiency and also provides a proof of principle for therapeutic use of autophagy enhancers. Toxic accumulation of misfolded protein aggregates is a common feature of various disorders (proteinopathies) including (i) hepatic fibrosis and/or carcinogenesis caused by alpha1-antitrypsin (AT) deficiency [[1]Perlmutter D.H. Brodsky J.L. Balistreri W.F. Trapnell B.C. Molecular pathogenesis of alpha-1-antitrypsin deficiency-associated liver disease: a meeting review.Hepatology (Baltimore Md). 2007; 45: 1313-1323Crossref PubMed Scopus (89) Google Scholar], (ii) cystic fibrosis caused by cystic fibrosis transmembrane conductance regulator (CFTR) [[2]Ratjen F. Doring G. Cystic fibrosis.Lancet. 2003; 361: 681-689Abstract Full Text Full Text PDF PubMed Scopus (915) Google Scholar], and (iii) neurodegenerative diseases [[3]Renna M. Jimenez-Sanchez M. Sarkar S. Rubinsztein D.C. Chemical inducers of autophagy that enhance the clearance of mutant proteins in neurodegenerative diseases.J Biol Chem. 2010; 285: 11061-11067Crossref PubMed Scopus (171) Google Scholar]. In Mendelian forms of these diseases, the mutations generally confer novel toxic functions to the proteins involved. Thus, one potential strategy for dealing with these mutant proteins would be to boost their degradation. This could be achieved by up-regulating macroautophagy (hereafter called autophagy), by which intracellular components are sequestered and delivered to lysosomes for degradation (Fig. 1) (reviewed in [[4]Mehrpour M. Esclatine A. Beau I. Codogno P. Overview of macroautophagy regulation in mammalian cells.Cell Res. 2010; 20: 748-762Crossref PubMed Scopus (395) Google Scholar]). Current studies suggest that basal autophagy, the routine method by which the cytosol is cleared of protein aggregates and damaged organelles, is in fact crucial for maintaining healthy cells [5Hara T. Nakamura K. Matsui M. Yamamoto A. Nakahara Y. Suzuki-Migishima R. et al.Suppression of basal autophagy in neural cells causes neurodegenerative disease in mice.Nature. 2006; 441: 885-889Crossref PubMed Scopus (3229) Google Scholar, 6Komatsu M. Waguri S. Chiba T. Murata S. Iwata J. Tanida I. et al.Loss of autophagy in the central nervous system causes neurodegeneration in mice.Nature. 2006; 441: 880-884Crossref PubMed Scopus (2931) Google Scholar, 7Luciani A. Villella V.R. Esposito S. Brunetti-Pierri N. Medina D. Settembre C. et al.Defective CFTR induces aggresome formation and lung inflammation in cystic fibrosis through ROS-mediated autophagy inhibition.Nat Cell Biol. 2010; 12: 863-875Crossref PubMed Scopus (384) Google Scholar]. AT is a plasma serine protease inhibitor belonging to the serpin superfamily. It protects tissues against enzymes produced by inflammatory cells, especially neutrophil elastase, a protease that can degrade all constituents of the pulmonary connective tissue matrix. AT is primarily synthesized in hepatocytes. It is inserted into the endoplasmic reticulum (ER), where only the correctly-folded, soluble monomeric form of AT is secreted through the ER membrane and Golgi complex to be released from the cell (Fig. 1). Misfolded monomeric forms of AT are extracted into the cytoplasm and degraded via the proteasome pathways. Consequently, the circulating concentration of AT is low, leaving elastase free to breakdown elastin, resulting in respiratory complications such as pulmonary emphysema, or chronic obstructive pulmonary disease in adults, by a loss-of-function mechanism. A point mutation (lysine for glutamate 342) known as ATZ can lead to non-functional proteins that can polymerize and accumulate in the ER. This accumulation in the ER causes inflammation of the liver (infantile hepatic cirrhosis), and carcinogenesis by a gain-of-toxic function mechanism. Thus, AT deficiency can be viewed as a prototype, a naturally-occurring form of the “ER stress” state. In terms of cellular response pathways, accumulation of ATZ in the ER specifically activates the autophagy response, but not the unfolded-protein response. Because the autophagy response is involved in both the degradation of ATZ and the cellular response to the accumulation of ATZ in the ER, it is interesting to consider the possibility that drugs enhancing autophagy could help to attenuate the hepatotoxicity associated with hepatic fibrosis and/or carcinogenesis caused by ATZ. In a study published in January in Science [[8]Hidvegi T. Ewing M. Hale P. Dippold C. Beckett C. Kemp C. et al.An autophagy-enhancing drug promotes degradation of mutant alpha1-antitrypsin Z and reduces hepatic fibrosis.Science (New York, NY). 2010; 329: 229-232Crossref PubMed Scopus (504) Google Scholar], Perlmutter’s team addressed this issue using an elegant strategy. They selected carbamazepine (CBZ) from a list of drugs recently shown to enhance the mTOR-independent autophagic degradation of aggregation-prone proteins [[9]Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. et al.Lithium induces autophagy by inhibiting inositol monophosphatase.J Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (836) Google Scholar]. They showed how CBZ, a drug currently used as an anticonvulsant and a mood stabilizer, could have beneficial effects in genetic liver disease. Results from cell-based studies suggest that CBZ produces its effects by changing the rate of intracellular degradation of insoluble ATZ, but that does not alter the fate of wild type AT. This change occurs by increasing the stimulation of autophagy. CBZ was earlier reported to enhance in neuron the clearance of aggregation-prone protein by mTOR-independent and inositol-phosphate dependent autophagy (Fig. 1) [[3]Renna M. Jimenez-Sanchez M. Sarkar S. Rubinsztein D.C. Chemical inducers of autophagy that enhance the clearance of mutant proteins in neurodegenerative diseases.J Biol Chem. 2010; 285: 11061-11067Crossref PubMed Scopus (171) Google Scholar]. However, the role of an mTOR-dependent pathway sensitive to rapamycin cannot be totally excluded in the inhibition of ATZ accumulation via polymerized conformation, as reported by Teckman’s team [[10]Kaushal S. Annamali M. Blomenkamp K. Rudnick D. Halloran D. Brunt E.M. et al.Rapamycin reduces intrahepatic alpha-1-antitrypsin mutant Z protein polymers and liver injury in a mouse model.Exp Biol Med (Maywood, NJ). 2010; 235: 700-709Crossref PubMed Scopus (104) Google Scholar]. The authors went onto determine whether the effects of CBZ are specific to ATZ. They have investigated its effect on the disposal of a variant form of AT, known as AT Saar, which accumulates in ER without aggregating, and is predominantly degraded via a proteasomal mechanism. CBZ degraded ATZ and AT Saar in an identical manner, suggesting that CBZ also has an effect on the proteasome (Fig. 1). Next, the authors examined the effect of CBZ on the hepatic load of ATZ, in a mouse model of liver disease associated with AT deficiency. Administration of CBZ decreased the levels of total, insoluble and soluble ATZ in the liver, and also of the intrahepatocytic ATZ-containing globules, whereas it increased hepatic autophagosomes. In models of hepatic fibrosis, CBZ reduced both fibrosis and liver damage. Although there is strong evidence implicating the insoluble polymers, the available data fall short of establishing the identity of the toxic species. It has recently been shown that in cystic fibrosis, defective CFTR induces aggresome formation by inhibiting ROS-mediated autophagy [[7]Luciani A. Villella V.R. Esposito S. Brunetti-Pierri N. Medina D. Settembre C. et al.Defective CFTR induces aggresome formation and lung inflammation in cystic fibrosis through ROS-mediated autophagy inhibition.Nat Cell Biol. 2010; 12: 863-875Crossref PubMed Scopus (384) Google Scholar]. Findings reported by Hidvegi et al. have provided the rationale for studies on CBZ in patients with AT deficiency, and confirmed that autophagy enhancers could be used to fight protein overload in genetic diseases. It would be interesting to find out whether the combination of CBZ with mTOR-dependent autophagy inducers results in a greater up-regulation of autophagy and clearance of ATZ [[3]Renna M. Jimenez-Sanchez M. Sarkar S. Rubinsztein D.C. Chemical inducers of autophagy that enhance the clearance of mutant proteins in neurodegenerative diseases.J Biol Chem. 2010; 285: 11061-11067Crossref PubMed Scopus (171) Google Scholar]. The question is now whether this approach will be useful for treating genetics diseases associated with aggregation-prone protein in human. The authors who have taken part in this study declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript.