Addition of mycophenolic acid to cultures of HL-60 cells results in a decreased cellular level of guanine nucleotides and the induction of cell differentiation. During the early stages of this induction, steady- state levels of cellular IMP dehydrogenase (IMPDH), messenger RNA (mRNA), and protein are increased, perhaps because of cellular compensation for the inhibition of IMPDH activity. The subsequent decrease in IMPH mRNA and protein levels after several days of treatment suggests a change in the control of IMPDH expression. In contrast to the pattern of increased IMPDH expression observed in the mycophenolic acid-treated cells, treatment of HL-60 cells with two other inducers of differentiation, namely retinoic acid and phorbol 12- myristate 13-acetate, resulted in stable or decreased levels of cellular IMPDH mRNA and protein. However, the kinetics of this expression were different. These results suggest that a number of factors influence the regulation of IMPDH expression during the induction of HL-60 cell differentiation, including the nature of the inducer. A decrease in the cellular IMPDH activity was observed for all of the inducers, suggesting that this decreased activity may be a determining factor in the acquisition of a mature phenotype in the HL- 60 cells.

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