Down-regulation of ubiquitin gene expression during differentiation of human leukemia cells

Ubiquitin, which is ligated covalently to target proteins for their acquisition of a variety of functions, is encoded by multiple unique genes in human cells: two distinct poly-ubiquitin genes with tandemly repeated sequences of 3 or 9 moieties and two mono-ubiquitin genes fused with small and large ribosomal proteins. We found that all classes of ubiquitin genes as well as the two genes encoding the ribosomal proteins S17 and L31 were expressed at abnormally high levels in various hematopoietic malignant tumor cells. In contrast, in vitro terminal differentiation of various immature leukemic cell lines, such as HL-60 promyelocytic leukemia cells and K562 erythroleukemia cells into monocytic, granulocytic and erythroid cells, induced by various agents was found to cause rapid and marked down-regulation of ubiquitin expression, irrespective of the cell type, direction of differentiation or type of signal. These findings suggest that the expressions of the multiple ubiquitin genes, coordinated with those of the ribosomal protein genes, are in a dynamic state during growth and differentiation of leukemia cells.