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 su bunits and the enzymatic activity of proteasomes purified from tumor derive d and normal B lymphocytes representing different stages of B-cell activati on/ differentiation. The catalytic beta subunits (Lmp2 and Lmp7) and the re gulatory subunits (PA28 alpha and PA28 beta) 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 no rmal CD23(-) B lymphocytes. Lmp2 and Lmp7 were selectively down-regulated i n germinal center cell derived Burkitt's lymphoma (BL) and Hodgkin's lympho ma (HD) cell lines. There was a direct correlation between the expression o f Lmp2/7 and the chymotrypsin and trypsin-like activities in proteasomes pu rified from LCLs, BLs and CLL cells, whereas 5 HD cell lines expressing B o r T-cell markers exhibited a variable pattern of subunit expression and enz ymatic activity, Poor hydrolysis of the fluorogenic substrates by proteasom es from BL cells correlated with a distinct pattern of cleavage of a refere nce 50mer peptide, production of different sets of degradation products and significantly reduced recovery of a known cytotoxic T-lymphocyte (CTL) tar get epitope, The enzymatic activity of proteasomes from normal CD23(-) "res ting" B lymphocytes resembled that of BL cells in spite of high Lmp2/7 expr ession, This pattern was not reversed by treatment with the B-cell mitogen, lipopolysaccharide (LPS), The results suggest that different stages of B-c ell activation/differentiation are associated with distinct profiles of IFN -gamma -regulated subunit composition and enzymatic activity of the proteas ome. This may have important implications for the analysis and manipulation of tumor-specific immune responses. Int. J. Cancer 88:881-888, 2000. (C) 2 000 Wiley-Liss, Inc.