Mallory body--a disease-associated type of sequestosome

Mallory bodies (MBs) consist of abnormal keratins, ubiquitin, heat shock proteins, and the protein p62. p62 is encoded by an immediate-early response gene that rapidly responds to a variety of extracellular signals involved in cell proliferation, differentiation, and particularly oxidative stress. It acts as an adapter in signal transduction and binds noncovalently to ubiquitin, possibly being involved in the regulation of the fate of ubiquitinated proteins by segregation (i.e., sequestosome or aggresome formation). The presence of p62 together with ubiquitinated abnormal keratins in the MB characterizes MBs as a disease-associated type of sequestosome. A detailed study on the expression of p62 and its relationship to MB formation in the 3,5-diethoxycarbonyl-1,4-dihydrocollidine (DDC)-treated mouse liver is reported based on immunohistochemical, immunoblot, and Northern blot analyses. The results indicate that p62 is rapidly induced in hepatocytes of intoxicated animals preceding MB formation. As suggested by experiments with short-term DDC-treated naive mice and mice refed DDC after recovery from long-term DDC treatment (primed mice), p62 does not exert an initiating effect on MB formation but the appearance of MBs requires the presence of abnormal keratins, which associate with p62 after ubiquitination. The rapid induction of p62 and its association with MBs further support the role of oxidative stress in MB formation. In conclusion, the constant presence of p62 in MBs suggests that binding of p62 to abnormal keratins may allow hepatocytes to dispose potentially harmful proteins in a biologically inert manner.