ATP independent proteasomal degradation of NQO1 in BL cell lines

Human NAD(P)H: quinone oxidoreductase 1 (NQO1) catalyzes the obligatory two-electron reduction of quinones. For this peculiar catalytic mechanism, the enzyme is considered an important cytoprotector. The NQO1 gene is expressed in all human tissues, unless a polymorphism due to C609T point mutation is present. This polymorphism produces a null phenotype in the homozygous condition and reduced enzyme activity in the heterozygous one. We previously demonstrated that two cell lines of haematopoietic origin, HL60 and Raji cells, possess the same heterozygous genotype, but different phenotypes; as expected for a heterozygous condition the HL60 cell line showed a low level of enzyme activity, while the Raji cell line appeared as null phenotype. The level of NQO1 mRNA was similar in the two cell lines and the different phenotype was not due to additional mutations or to expression of alternative splicing products. Here we show that in Raji BL cell line with heterozygous genotype the null NQO1 phenotype is due to 20S proteasome degradation of wild type and mutant protein isoforms and is not directly linked to C609T polymorphism. This finding may have important implications in B-cell differentiation, in leukaemia risk evaluation and in chemotherapy based on proteasome inhibitors.