INHIBITION OF AN ERYTHROID-DIFFERENTIATION SWITCH BY THE HELIX-LOOP-HELIX PROTEIN ID1

The Id proteins function as negative regulators of basic-helix-loop he lix transcription factors, which play important roles in determination of cell lineage and in tissue-specific differentiation. Down-regulati on of Id1 mRNA is associated with dimethyl sulfoxide-induced terminal differentiation of mouse erythroleukemia cells. To examine the signifi cance of Id1 down-regulation in erythroid differentiation, we generate d stable mouse erythroleukemia cell lines that constitutively express a ''marked'' form of the murine Id1 gene. Terminal erythroid different iation was inhibited in these lines, as indicated by a block in activa tion of the erythroid-spe cific genes alpha-globin, beta-globin, and b and 3 and continued proliferation in the presence of dimethyl sulfoxid e. Interestingly, this block, occurred even in the presence of normal levels of the lineage-specific transcription factors GATA-1, NF-E2, an d EKLF. Constitutive expression of Id1 did not interfere with DNase I hypersensitivity at site HS2 of the locus control region, expression o f the erythropoietin receptor gene, or down-regulation of the endogeno us Id1 or c-myc genes. The differentiation block is reversible in thes e lines and can be rescued by fusion with human erythroleukemia cells. These findings suggest that in vivo, Id1 functions as an antagonist o f terminal erythroid differentiation.