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The effects of introduction of mutant PTEN constructs which lack lipid phosphatase [PTEN(G129E)] or lipid and protein phosphatase [PTEN(C124S)] activity on in vitro sensitivity of PTEN-positive MCF-7 human breast cancer cells to the chemotherapeutic drug doxorubicin and ionizing radiation were investigated. Ectopic expression of a mutant form of PTEN lacking lipid phosphatase activity [PTEN(G129E)] decreased sensitivity of MCF-7 cells to doxorubicin and ionizing radiation indicating that this mutant PTEN gene could act in a dominant negative fashion to suppress wild type PTEN activity. Furthermore, introduction of a mutant PTEN gene lacking both lipid and protein phosphatase activity [PTEN(C124S)] further increased the resistance of the cells to doxorubicin and ionizing radiation indicating that the protein phosphatase activity of PTEN was also important in the tumor suppressor effects of PTEN. Interference with the lipid phosphatase domain of PTEN was sufficient to activate Akt/p70S6K as MCF-7 cells transfected with the mutant PTEN gene lacking the lipid phosphatase activity [PTEN(G129E)] displayed elevated levels of activated Akt and p70S6K compared to empty vector transfected cells. These studies indicate that disruption of the normal activity of the PTEN phosphatase can have dramatic effects on activity of Akt/p70S6K and alter the therapeutic sensitivity of breast cancer cells. Our studies demonstrate for the first time that the protein phosphatase activity of PTEN, as well as the lipid phosphatase activity, are important in the ability of PTEN to inhibit growth after chemo- and radio-therapy and that mutations in the key residues which control lipid and protein phosphatase activity can act as dominant negative mutants to suppress endogenous PTEN. These results are relevant to the development of and therapy of breast cancer as PTEN mutations are often present in a hemizygous state.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA