Letter to the editor

Suppression of Wilms' tumor gene (WT1) expression induces $\text{G}{}_2\text{M}$ arrest in leukemic cells

Earlier studies have revealed one function of the inhibitory mechanism of curcumin. Activating PKCα induces WT1 gene expression via signalling through downstream JNK and c-JUN. In the present study, the effect of c-JUN/AP-1 binding and transcriptional regulation of the WT1 gene promoter was investigated in K562 leukaemic cells. The non-cytotoxic dose (IC₂₀ values) of curcumin (WT1 and AP-1 inhibitors) was employed to examine its effect on WT1 gene-mediated WT1 and AP-1 protein expression. Non-cytotoxic doses of both tanshinone IIA (AP-1 DNA-binding inhibitor) and SP600125 (JNK inhibitor) were used to test the role of c-JUN/AP-1 in WT1 gene expression. Curcumin, tanshinone IIA, and SP600125 inhibited WT1 protein expression in a dose-dependent manner (5–15 μM) at 24 h as shown by immunoblotting. A ChIP assay showed that curcumin and tanshinone IIA inhibited AP-1 and WT1 binding to the proximal WT1 promoter (−301 bp), and a luciferase reporter assay showed that the WT1 luciferase gene reporter activity was decreased after curcumin, tanshinone IIA, and SP600126 treatments. Furthermore, depletion of c-JUN abrogated WT1 gene expression. In summary, AP-1 contributes to the WT1 autoregulation of WT1 gene expression in leukaemic K562 cells.

Wilms’ tumor gene 1 (WT1) is overexpressed in various hematological malignancies and has been proposed as a target for minimal residual disease (MRD) detection and for immunotherapy. Although WT1 is known as a key molecule for tumor cell proliferation, the expression pattern of WT1 in leukemic cells in dependency of proliferation has not yet been investigated. Furthermore, WT1 expression was mostly studied by reverse transcriptase PCR and the expression of WT1 protein has not been extensively studied.

Here, we analyzed WT1 protein expression in the human myeloid leukemia cell lines K562 and HL-60 by indirect immunofluorescence and flow cytometry. Both cell lines exhibited varying nuclear WT1 immunoreactivity pointing to a cell cycle-dependent and/or proliferation-dependent WT1 expression. In rapidly proliferating cells high levels of WT1 protein were detected by flow cytometry. A reduced proliferation rate was associated with a low WT1 protein expression and an accumulation of cells in G₀/G₁ phase. During G₀/G₁ phase cells expressed WT1 at a lower level than in S or G₂/M phase. Moreover, WT1 expression was diminished in all cell cycle phases in slowly proliferating cells.

We conclude that WT1 protein expression is dependent on the cell cycle phase as well as on the proliferation rate. This finding might be relevant for MRD studies and immunotherapeutic strategies targeting WT1.

WT1 has been implicated in human leukemia and hematopoiesis, but its role in stem cell differentiation is not yet fully defined. We show that Wt1-null murine fetal liver cells are capable of reconstituting functional hematopoiesis following transplantation into irradiated recipients. There was also no significant difference between the in vitro colony-forming ability of wild-type and Wt1-null cells. Using a reporter gene assay in a transgenic mouse system, expression from the WT1 promoter was detectable in adult bone marrow, but undetectable in subsets of different hematopoietic cells. We conclude that Wt1 is not essential for murine hematopoiesis and that there may be significant differences in its role between mouse and man.

TheWT1tumor-suppressor gene is expressed by many forms of acute myeloid leukemia. Inhibition of this expression can lead to the differentiation and reduced growth of leukemia cells and cell lines, suggesting that WT1 participates in regulating the proliferation of leukemic cells. However, the role of WT1 in normal hematopoiesis is not well understood. To investigate this question, we have used murine cells in which theWT1gene has been inactivated by homologous recombination. We have found that cells lacking WT1 show deficits in hematopoietic stem cell function. Embryonic stem cells lackingWT1, although contributing efficiently to other organ systems, make only a minimal contribution to the hematopoietic system in chimeras, indicating that hematopoietic stem cells lackingWT1compete poorly with healthy stem cells. In addition, fetal liver cells lackingWT1have an approximately 75% reduction in erythroid blast-forming unit (BFU-E), erythroid colony-forming unit (CFU-E), and colony-forming unit–granulocyte macrophage–erythroid–megakaryocyte (CFU-GEMM). However, transplantation of fetal liver hematopoietic cells lackingWT1will repopulate the hematopoietic system of an irradiated adult recipient in the absence of competition. We conclude that the absence of WT1 in hematopoietic cells leads to functional defects in growth potential that may be of consequence to leukemic cells that have alterations in the expression of WT1.