Background: WT1 was originally identified in Wilms tumor, a childhood kidney cancer. This gene was expressed in wide variety of solid cancers. Alternative splicing of WT1 transcript generates four major protein isoforms and thirty-six minor protein isoforms, each having different functional properties. WT1 gene has been considered as a tumor suppressor gene and anti-apoptotic protein. However, the mechanism of WT1 in breast cancer remains unclear. Objective: Evaluate the role of truncated WT1

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... oforms (T-KTS+ and T-KTS-) and two major WT1 isoforms (+/+ and +/-) in apoptosis in breast cancer cell line, MCF-7. Materials and methods: RNA interference (RNAi) was employed in an attempt to define the role of WT1 in a breast cancer cell line (MCF-7). Furthermore, MCF-7 overe-xpressing cells that stably expressed two truncated WT1 isoforms (T-KTS+ and T-KTS-) or two major WT1 isoforms (+/+ and +/-) were generated and exposed to Doxorubicin. The mortality of cells was determined as a percentage of trypan blue-stained cells in total cells. The apoptotic molecules in apoptosis pathway were detected using RT-PCR, caspase-7 activity assay and Western blot analysis techniques. Results: Transfection of siRNAWT1 into MCF-7 cells resulted in decreasing of WT1 protein and related to the increasing in number of cell death and caspase-7 activity. Over-expression of T-KTS+, T-KTS-, WT1+/+ and WT1+/- isoforms protected cells from cell death induced by apoptosis-inducing agent, doxorubicin. Moreover, the expression of apoptotic p53, Bak and caspase-7 were decreased by the expression of all four WT1 isoforms, especially T-KTS- and T-KTS+ isoforms. Conclusion: T-KTS+ and T-KTS- isoforms as well as WT1+/+ and WT1+/- isoforms could function as an antiapoptotic protein in breast cancer cell line, MCF-7.