Isoforms of Wilms' tumor suppressor gene (WT1) have distinct effects on mammary epithelial cells.

The role of WT1 (Wilm's tumor suppressor gene) in breast cancer is controversial, with evidence for both tumor-promoting and tumor-suppressing activities. In order to address this question, we expressed different WT1 isoforms in the mammary epithelial cell line H16N-2, which does not express endogenous WT1. Cells were stably transfected with either WT1 (-Ex5/-KTS) or WT1 (+Ex5/+KTS) under the control of the inducible metallothionein promoter. Induction of WT1 (-Ex5/-KTS) upregulated p21, causing a slowing of proliferation and a G2-phase cell cycle arrest. In artificial basement membrane, the WT1 (-Ex5/-KTS) isoform promoted the appearance of highly organized acinar cellular aggregates. In contrast, WT1 (+Ex5/+KTS) had no effect on p21 or proliferation, but rather caused an epithelial-mesenchymal transition and a redistribution of E-cadherin from the cell membrane to the cytoplasm. This isoform also causes the cellular aggregates growing in artificial basement membrane to appear significantly less organized than control cells. Thus, different WT1 isoforms have distinct effects in this cell line, suggesting that depending on the ratio of WT1 isoform expression in mammary epithelial cells, WT1 could function to either promote or suppress a transformed phenotype.

An isoform of the Wilms' tumor suppressor gene potentiates granulocytic differentiation.

WT1 is expressed in hematopoietic progenitor cells and in acute leukemia, but its role in normal and malignant hematopoiesis has not been clearly defined. Alternative splicing of the WT1 mRNA yields several protein isoforms with distinct DNA binding and transcriptional regulatory activities. In this study, we investigated the effect of the WT1 isoform lacking two alternatively spliced sequences (WT1 (-/-)) in 32D cl3 cells, a murine myeloid progenitor cell line. The expression of WT1 (-/-) accelerated the granulocyte-colony stimulating factor (G-CSF)-mediated differentiation of these cells, as judged by morphology and by the expression of differentiation-associated genes and cell surface antigens. WT1 (-/-) inhibited G1/S progression in G-CSF but not in interleukin-3, potentially accounting for its ability to accelerate differentiation. It is likely that dominant-negative mutants previously reported in leukemia patients participate in leukemogenesis by inhibiting this function of the wild-type protein.