The immunoproteasome in steatohepatitis: its role in Mallory-Denk body formation

Recently it has been shown that the expression of the immunoproteasome increased in proportion to the degree of chronic inflammation in both the liver cell cytoplasm and nuclei in liver biopsies from patients who had chronic active hepatitis or cirrhosis. In the present study, biopsies from patients with steatohepatitis, with or without Mallory-Denk body (MDB) formation, were studied by immunofluorescent staining. Normal liver showed colocalization of FAT10, LMP2, LMP7, and MECL-1 at the mitochondria. Only LMP2 and LMP7 were found in the cell nuclei. Liver biopsies from patients with steatohepatitis and MDB formation, and a case of hepatocellular carcinoma forming MDBs in the tumor cells, showed colocalization of FAT10 and ubiquitin with LMP2, LMP7 and MECL-1 within the MDB. This indicates involvement of the immunoproteasome in MDB formation in steatohepatitis cases and in a case of HCC forming MDBs. Prior studies have shown that the immunoproteasome was involved in drug-induced MDB formation using the same immunofluorescent colocalization approach as was used on these human liver biopsies. The increase in the immunoproteasome subunit proteins was made at the expense of the 26S proteasome. This indicates that the shift from the 26S to the immunoproteasome had occurred in the MDB positive hepatocytes.