Histochem Cell Biol (2008) 129:735–749 active in the liver. For example, conditional knock-out of the autophagy-related gene 7 (Atg7) leads to development of ubiquitin-positive inclusions in hepatocytes (Komatsu et al. 2005). Similarly, proteasomes seem to interact with MDBs and inhibition of proteasomal degradation leads to formation of MDB-like structures both in vitro and in DDC-primed mice (Harada et al. 2003; Bardag-Gorce et al. 2002; Riley et al. 2002; French et al. 2001). Furthermore, autophagic degradation is upregulated in DDC-fed mice and its further stimulation with rapamycin attenuates spon- taneous MDB formation in K8 overexpressing mice (Harada et al. 2008). In contrast, MDB formation might be facilitated by pro- teasomal impairment, since MDBs are preferentially seen in proteasome-depleted hepatocytes (Bardag-Gorce et al. 2001). In addition, an aberrant form of ubiquitin, termed [UBB + 1] arising as a consequence of molecular misread- ing, is observed in MDBs and may inhibit proteasomal function (McPhaul et al. 2002; Lindsten et al. 2002). Simi- larly, alcohol and aging, both potential inducers of MDBs can be associated with decreased proteasome function (Hayashi and Goto 1998; Fataccioli et al. 1999). However, aggregate formation must not always be caused by a general inhibition in protein degradation. The degradative capacity may simply be overwhelmed by increased amounts of misfolded proteins. Alternatively, certain proteins may not be easily degradable, such as pro- teins with long polyglutamine stretches (Venkatraman et al. 2004). The latter scenario may apply to DDC-fed mice, which exhibit an accumulation of ubiquitinated protein only in the highly insoluble protein fraction (Strnad et al. 2007). This fraction is characterized by highly cross-linked protein species generated through extensive transamidation, which may make them resistant to proteolytic degradation (Strnad et al. 2007). Pathologic signiWcance of MDBs In principle, cytoplasmic protein aggregates can be beneW- cial, detrimental or inert depending on the context. In many pathologic situations aggregation seems to be a protective response if the Wrst lines of defense, that is, refolding and degradation, fail by sequestration of potentially harmful proteins (Arrasate et al. 2004; Bodner et al. 2006). On the other hand, large protein aggregates can be detrimental either by mechanical interference with cellular transport processes (e.g., shown with microtubule-dependent trans- port in neurons), deprivation of the cell of vital components by aggregation or coaggregation and by overwhelming/ inhibiting the capacity of the chaperone system and/or the protein degradation machinery by indigestible material (Alonso et al. 1997; Stenoien et al. 1999; Suhr et al. 2001; 745 Bence et al. 2001; Lee et al. 2004; Grune et al. 2004). It can be expected, therefore, that in diVerent cell systems and pathologic conditions inclusion bodies diVer regarding their cellular eVects and consequences. MDBs per se do not seem to compromise the viability of transfected tissue cul- ture cells (Stumptner et al. 2007) or hepatocytes in vivo, who even exhibit an activated phenotype (Denk et al. 2000). Further studies are needed to deWnitely characterize the molecular consequences of MDB formation. References Alonso AD, Grundke-Iqbal I, Barra HS, Iqbal K (1997) Abnormal phosphorylation of tau and the mechanism of Alzheimer neuroW- brillary degeneration: sequestration of microtubule-associated proteins 1 and 2 and the disassembly of microtubules by the abnormal tau. Proc Natl Acad Sci USA 94:298–303 Arrasate M, Mitra S, Schweitzer ES, Segal MR, Finkbeiner S (2004) Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronal death. Nature 431:805–810 Bantel H, Lugering A, Heidemann J, Volkmann X, Poremba C, Strass- burg CP, Manns MP, Schulze-OsthoV K (2004) Detection of apoptotic caspase activation in sera from patients with chronic HCV infection is associated with Wbrotic liver injury. Hepatology 40:1078–1087 Barak V, Goike H, Panaretakis KW, Einarsson R (2004) Clinical utility of cytokeratins as tumor markers. Clin Biochem 37:529–540 Bardag-Gorce F, French BA, Lue YH, Nguyen V, Wan YJ, French SW (2001) Mallory bodies formed in proteasome-depleted hepato- cytes: an immunohistochemical study. Exp Mol Pathol 70:7–18 Bardag-Gorce F, van Leeuwen FW, Nguyen V, French BA, Li J, Riley N, McPhaul LW, Lue YH, French SW (2002) The role of the ubiquitin-proteasome pathway in the formation of Mallory bod- ies. Exp Mol Pathol 73:75–83 Bence NF, Sampat RM, Kopito RR (2001) Impairment of the ubiqui- tin-proteasome system by protein aggregation. Science 292:1552–1555 Bershadsky AD, Vasiliev JM (1988) Cytoskeleton. In: Cellular Organ- elles, Plenum Press, New York, pp 133–154 Bjorkoy G, Lamark T, Brech A, Outzen H, Perander M, Overvatn A, Stenmark H, Johansen T (2005) P62/SQSTM1 forms protein aggregates degraded by autophagy and has a protective eVect on huntingtin-induced cell death. J Cell Biol 171:603–614 Bodner RA, Outeiro TF, Altmann S, Maxwell MM, Cho SH, Hyman BT, McLean PJ, Young AB, Housman DE, Kazantsev AG (2006) Pharmacological promotion of inclusion formation: a therapeutic approach for Huntington’s and Parkinson’s diseases. Proc Natl Acad Sci U S A 103:4246–4251 Brunt EM (2004) Nonalcoholic steatohepatitis. Semin Liver Dis 24:3–20 Bukau B, Weissman J, Horwich A (2006) Molecular chaperones and protein quality control. Cell 125:443–451 Cadrin M, Kawahara H, Ohta M, Katsuma Y, Marceau N, French SW (1990) Mallory bodies in hepatomas and hyperplastic nodules: in vitro and in vivo studies. Prog Clin Biol Res 331:231–248 Cadrin M, French SW, Wong PT (1991) Alteration in molecular struc- ture of cytoskeleton proteins in griseofulvin-treated mouse liver: a pressure tuning infrared spectroscopy study. Exp Mol Pathol 55:170–179 Cadrin M, Hovington H, Marceau N, McFarlane-Anderson N (1995) Early perturbations in keratin and actin gene expression and Wbril- lar organisation in griseofulvin-fed mouse liver. J Hepatol 33:199–207 123