Immunocytochemical characterization of human NOR-90 (upstream binding factor) and associated antigens reactive with autoimmune sera

Abstract

The 90-kDa nucleolus organizer region autoantigen (NOR-90) was previously shown to be identical to the human upstream binding factor (hUBF) and composed of twoMr forms. In this study, thirteen human anti-NOR-90/hUBF autoimmune sera were used to further characterize NOR-90/hUBF and its associated autoantigens. Nucleolar and nucleoplasmic staining of interphase cells and NOR staining in mitosis were observed with all sera by immunofluorescence. All sera showed equal reactivity with both high and lowMr forms in Western blotting and immunoprecipitation, suggesting that the cellular content and distribution for bothMr forms were approximately equal. Using extracts of [³⁵S]methionine- and [³²P]orthophosphate-labeled cells, phosphorylated and nonphosphorylated NOR-90/hUBF were identified for bothMr forms and these two populations were recognized by human autoantibodies. In immunoprecipitation analyses, the nonphosphorylated population was readily extracted while the phosphorylated population was tightly bound. Clinical data were available for 8 patients in whom anti-NOR-90/hUBF autoantibodies were present. They had diverse diagnoses including SLE, rheumatoid arthritis and malignancies. Although only one patient was diagnosed as scleroderma, Raynaud's phenomenon was observed in 4 of the 8 patients. Interestingly, one NOR-90/hUBF serum was shown to contain additional antibodies to RNA polymerases I and II.Abbreviations: ANA=Antinuclear antibody; HCC=hepatocellular carcinoma; hUBF=human upstream binding factor; NOR=nucleolus organizer region; RNA pol I=RNA polymerase I; RNA pol II=RNA polymerase II; rRNA=ribosomal RNA; SLE=systemic lupus erythematosus.