THE MYELOID ZINC-FINGER GENE (MZF-1) DELAYS RETINOIC ACID-INDUCED APOPTOSIS AND DIFFERENTIATION IN MYELOID-LEUKEMIA CELLS

The myeloid zinc finger gene, MZF-1, is a hematopoietic transcription factor expressed in developing myeloid cells. To characterize further the role of MZF-1 in myelopoiesis, we used retroviral gene transductio n to overexpress MZF-1 in HL-60 cells to produce HL-60-MZF-1 cells. HL -60 cells respond to retinoic acid (RA) with growth inhibition, granul ocytic differentiation and apoptosis. However, HL-60-MZF-1 cells expos ed to RA continue to proliferate in response to RA as evidenced by a h igher percentage of cells in S phase, higher peak cell counts, and lat er peak cell counts. Morphologic differentiation of the HA-induced HL- 60-MZF-1 cells is delayed with half as many of the HL-60-MZF-1 cells c ompared to the wild-type HL-60 cells that are differentiated after 3 d ays of RA, although both cells types responded with 80-95% mature gran ulocytes after 6 days of RA. Apoptosis was delayed in the MZF-1 transd uced cells as measured by internucleosomal DNA fragmentation patterns, the terminal transferase end labeling reaction (TUNEL), and quantitat ion of fragmented DNA by the diphenylamine reaction. Several markers o f differentiation were identical in both HL-60 and HL-60 MZF-1 cells i ncluding CD11b, CD33, CD34, CD13, CD16 and CD14. However, following 6 days of RA, only half as many HL-60-MZF-1 cells expressed CD18 compare d to the wild-type HL-60 cells. Expression of the bcl-2 proto-oncogene transcript and protein was higher in the HL-60-MZF-1 cells compared t o wild-type HL-60s and expression persisted for 5 days following RA in the HL-60-MZF-1 cells compared to only 3 days in the parental HL-60 c ells suggesting that bcl-2 may contribute to the inhibition of apoptos is. Overexpression of MZF-1 had no effect on PMA-induced monocyte/macr ophage differentiation of HL-60 cells. Together these findings indicat e that MZF-1 can stimulate cell proliferation and delay RA-induced dif ferentiation and apoptosis in HL-60 cells. MZF-1 may function in a sim ilar role in myelopoiesis allowing myeloid precursors to expand their numbers before going on to terminally differentiate.