The Biological Effects of C/EBPa in K562 Cells Depend on the Potency of the N-terminal Regulatory Region, Not on Specificity of the DNA Binding Domain

The transcription factor C/EBPa is more potent than C/EBPb in inducing granulocitic differentiation and inhibiting BCR/ABL-expressing cells. We took a "domain swapping" approach to assess biological effects, modulation of gene expression, and binding to C/EBPa-regulated promoters by wild-type and chimeric C/EBPa/C/EBPb proteins. Wild-type and N-C/EBPa+ C/EBPb-DBD induced transcription of the granulocyte-colony stimulating factor receptor (G-CSFR) gene, promoted differentiation, and suppressed proliferation of K562 cells vigorously; instead, wild-type C/EBPb and N-C/EBPb+C/EBPa-DBD had modest effects, although they bound the G-CSFR promoter like wild-type C/EBPa and N-C/EBPa+C/EBPb-DBD. Chimeric proteins consisting of the TAD of VP16 and the DBD of C/EBPa or C/EBPb inhibited proliferation and induced differentiation of K562 cells as effectively as wild-type C/EBPa. Gene expression profiles induced by C/EBPa resembled those modulated by N-C/EBPa+C/EBPb-DBD, whereas C/EBPb induced a pattern similar to that of N-C/EBPb+C/EBPa-DBD. C/EBPa activation induced changes in the expression of more cell cycle- and apoptosis-related genes than the other proteins and enhanced Imatinib-induced apoptosis of K562 cells. Expression of FOXO3a, a novel C/EBPa-regulated gene, was required for apoptosis but not for differentiation induction or proliferation inhibition of K562 cells.