738 tions with release into blood upon liver injury (Omary et al. 2002; Ku et al. 2007). However, one important caveat is the fact, that K8/K18/K19 are expressed in most simple epithe- lial cells and are therefore not liver-speciWc (Moll et al. 1982; Ku et al. 1999). The K8/K18/K19 epitopes used in serologic diagnosis can be divided into two classes, that is, non-speciWc and apoptosis-generated epitopes. The former class constitutes established tumor markers such as tissue polypeptide anti- gen (TPA, represents total K8/K18/K19), tissue polypep- tide speciWc antigen (TPS, derived from K18), and CYtokeratin FRAgment 21-1 (CYFRA 21-1, derived from K19). Their original clinical use was to monitor treatment response and to detect recurring tumors (Barak et al. 2004). However, later studies showed that these epitope serum lev- els are also elevated in non-malignant diseases and might be a general marker of tissue injury (Gonzalez-Quintela et al. 2006a, b; Tarantino et al. 2007). The apoptosis-speciWc keratin antibodies are based on the Wnding, that type I keratins are cleaved at a conserved VEMD/VEVD residue during apoptosis. In addition to that, K18 posseses a second, K18 speciWc caspase-cleav- age site at Asp396, which is an early event during apopto- sis preceding the cleavage at the VEMD/VEVD motif (Oshima 2002; Marceau et al. 2007). The cleavage of human K18 at Asp396 can be monitored using the M30 antibody (Leers et al. 1999) and M30-Ab ELISA has become a useful serologic test for determining liver dis- ease severity. For example, elevated serum M30 titers can distinguish simple steatosis from non-alcoholic steatohep- atitis (Wieckowska et al. 2006) and predict several impor- tant prognostic parameters in patients with chronic hepatitis C infection (Bantel et al. 2004; Volkmann et al. 2006). As another tool for detecting apoptotic keratin frag- ments, an antibody speciWc to the conserved K18/K19 cleavage site at Asp237 was recently generated (Tao et al. 2008). It detects both mouse and human K18/K19 frag- ments and appears to be more sensitive than the established Asp396-Ab (Tao et al. 2008). Measuring the serum levels of apoptosis-speciWc keratin fragments should also improve our understanding of chronic liver disease, where apoptotic cell death is an important pathogenic feature (Malhi et al. 2006). In addition to that, monitoring the phosphorylation status of the circulating keratin fragments might be useful. Kera- tins undergo dynamic phosphorylation during mitosis, apoptosis and a variety of stress situations (Omary et al. 1998; Omary et al. 2006) and their in situ phosphorylation status is a marker of human liver disease progression (Toi- vola et al. 2004). However, it is currently unknown whether the phosphorylation status of the circulating keratin frag- ments correlates with the situation in situ. 123 Histochem Cell Biol (2008) 129:735–749 Hepatic phenotype in keratin-deWcient transgenic animals The liver consists of diVerent cell types with characteristic IF composition (Table 3; Omary et al. 2002). Adult hepato- cytes are unique among simple epithelial cells in that they express exclusively K8 and K18, whereas other glandular epithelia exhibit a more complex keratin expression pattern (Omary et al. 2002; Ku et al. 2007). The hepatocytic keratin IF network is dense, particularly around bile canaliculi and at the cell periphery, and acts as cytoskeletal backbone to the functionally more dynamic and contractile actin micro- Wlament system (Zatloukal et al. 2004). Biliary epithelial cells diVer from hepatocytes by additional expression of keratin 7 and 19 (Omary et al. 2002; Zatloukal et al. 2004). Keratins in cholangiocytes, but not hepatocytes, exhibit polarized and compartment-speciWc expression pattern (Omary et al. 2002; Zatloukal et al. 2004). The biological signiWcance of such an expression is enigmatic, but it may be related to polarity and secretory processes. Among non- epithelial cells, stellate cells express variable amounts of GFAP, desmin, vimentin, and nestin dependent on their activation status, localization, and other parameters (Table 3; Geerts 2001). Studies in keratin knock-out mice revealed that the regu- lar liver development does not require the presence of K8, K18, or K19 (Ku et al. 2007). However, K8/K18 knockout mice exhibited mild chronic hepatitis, hepatocyte fragility and were markedly more sensitive to a variety of stress con- ditions (Omary et al. 2002; Zatloukal et al. 2004; Ku et al. 2007). Furthermore, K8/K18 transgenic mice were developed, which over-express diVerent single-amino-acid variants (Ku et al. 2007). Among them, the K18 R89C variant resulted in disruption of hepatocyte IF network and exhibited a phenotype reminiscent of the situation in K8/K18- knockout mice (Ku et al. 1995, 1996). K18 R89C mice also Table 3 IFs of liver cell populations Cell type Hepatocyte Oval cells Cholangiocyte KupVer cell Stellate cell Endothelial cell IF composition K8/K18a K7/K8/K18/K19 K7/K8/K18/K19 Vimentin GFAP, Vimentin, Desmin, Nestinb Vimentin ModiWed from Omary et al. (2002) a During embryogenesis, hepatocytes also express variable levels of K19 (Vassy et al. 1997) b Stellate cells represent a highly heterogeneous population with var- iable IF expression dependent on species, activation status of the cell, location within the hepatic lobe and many other parameters (Geerts 2001)