Variations in proteasome subunit composition and enzymatic activity in B-lymphoma lines and normal B cells.

We investigated the expression of interferon gamma (IFN-gamma)-regulated subunits and the enzymatic activity of proteasomes purified from tumor-derived and normal B lymphocytes representing different stages of B-cell activation/differentiation. The catalytic beta subunits (Lmp2 and Lmp7) and the regulatory subunits (PA28alpha and PA28beta) were expressed at equally high levels in Epstein-Barr virus (EBV)-transformed lymphoblastoid cell lines (LCLs), freshly isolated B-chronic lymphocytic leukemia (B-CLL) cells and normal CD23(-) B lymphocytes. Lmp2 and Lmp7 were selectively down-regulated in germinal center cell-derived Burkitt's lymphoma (BL) and Hodgkin's lymphoma (HD) cell lines. There was a direct correlation between the expression of Lmp2/7 and the chymotrypsin and trypsin-like activities in proteasomes purified from LCLs, BLs and CLL cells, whereas 5 HD cell lines expressing B or T-cell markers exhibited a variable pattern of subunit expression and enzymatic activity. Poor hydrolysis of the fluorogenic substrates by proteasomes from BL cells correlated with a distinct pattern of cleavage of a reference 50mer peptide, production of different sets of degradation products and significantly reduced recovery of a known cytotoxic T-lymphocyte (CTL) target epitope. The enzymatic activity of proteasomes from normal CD23(-) "resting" B lymphocytes resembled that of BL cells in spite of high Lmp2/7 expression. This pattern was not reversed by treatment with the B-cell mitogen, lipopolysaccharide (LPS). The results suggest that different stages of B-cell activation/differentiation are associated with distinct profiles of IFN-gamma-regulated subunit composition and enzymatic activity of the proteasome. This may have important implications for the analysis and manipulation of tumor-specific immune responses.