| The unusual balanced chromosomal translocation t(11;17) (q23;q21) is associated with acute promyelocytic leukemia which unlike the more typical t(15;17) APL, fails to respond to treatment with all-trans retinoic acid. In t(11;17) the PLZF gene on chromosome 11 is fused to the retinoic acid receptor {dollar}alpha{dollar} gene on chromosome 17 and yields two reciprocal fusion proteins. Using anti-PLZF antibodies, I characterized the nature of the wildtype PLZF protein and its fusion proteins with the retinoic acid receptor. PLZF gene yields a 90 kd nuclear protein. The PLZF-RAR{dollar}alpha{dollar} fusion protein is also localized exclusively to the nucleus, while the reciprocal RAR{dollar}alpha{dollar}-PLZF protein is found in both the nucleus and cytoplasm. I also demonstrated that PLZF is phosphorylated; upon further examination it was determined that PLZF is phosphorylated on serine and threonine residues. I investigated the effect of PLZF on differentiation of the nontumorigenic mouse myeloid cell line 32DCL3G/GM. Overexpression of PLZF had a dramatic growth suppressive effect on these cells. Severe growth suppression was accompanied by accumulation of cells in the G0/G1 compartment of the cell cycle, an increased incidence of apoptosis and downregulation of IL-3 receptor expression. There is also evidence of cell-autonomous mechanism of growth suppression through a putative autocrine factor secreted by pools of PLZF expressing cells, which inhibits growth of cells not expressing PLZF. PLZF overexpression inhibits the granulocytic and monocytic differentiation of 32DCL3G/GM cells in response to G-CSF and GM-CSF respectively. Furthermore, cells that express PLZF become more immature as demonstrated by their morphology--increased expression of Sca-1 and decreased expression of Gr-1. These findings suggest that PLZF plays an important role in control of the cell cycle, cell death and cell differentiation. Disruption of PLZF in t(11;17) may be a critical event leading to hematopoietic malignancy. |
