Suppression of PTEN function increases breast cancer chemotherapeutic drug resistance while conferring sensitivity to mTOR inhibitors

Abstract

Ectopic expression of mutant forms of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) lacking lipid (G129E) or lipid and protein (C124S) phosphatase activity decreased sensitivity of MCF-7 breast cancer cells, which have wild-type PTEN, to doxorubicin and increased sensitivity to the mammalian target of rapamycin (mTOR) inhibitor rapamycin. Cells transfected with a mutant PTEN gene lacking both lipid and protein phosphatase activities were more resistant to doxorubicin than cells transfected with the PTEN mutant lacking lipid phosphatase activity indicating that the protein phosphatase activity of PTEN was also important in controlling the sensitivity to doxorubicin, while no difference was observed between the lipid (G129E) and lipid and protein (C124S) phosphatase PTEN mutants in terms of sensitivity to rapamycin. A synergistic inhibitory interaction was observed when doxorubicin was combined with rapamycin in the phosphatase-deficient PTENtransfected cells. Interference with the lipid phosphatase activity of PTEN was sufficient to activate Akt/mTOR/ p70S6K signaling. These studies indicate that disruption of the normal activity of the PTEN phosphatase can have dramatic effects on the therapeutic sensitivity of breast cancer cells. Mutations in the key residues which control PTEN lipid and protein phosphatase may act as dominant-negative mutants to suppress endogenous PTEN and alter the sensitivity of breast cancer patients to chemo-and targeted therapies.

Targeting the translational apparatus to improve leukemia therapy: roles of the PI3K/PTEN/Akt/mTOR pathway

“…Deregulation of Akt/PTEN expression in various cancers has been shown to make the cells or tumors resistant to chemotherapy, yet hyper-responsive to the mTOR inhibitor rapamycin. 30,153,158,159 Many 5 0 -UTR on mRNAs involved in cell survival are G þ C rich and highly structured and can affect leukemic transformation. To translate these survival mRNAs efficiently requires the assembly to a translation complex, which binds to the 5 0 -UTR.…”

“…29 Some cells become therapy resistant by inactivation of PTEN. 30 Thus, reactivating PTEN could potentially be very useful in those tumors driven by PTEN inactivation.…”

Involvement of p53 and Raf/MEK/ERK pathways in hematopoietic drug resistance

“…Inhibitory concentration 50% (IC 50 ) is defined in this context as the drug concentration that causes the cells to proliferate at a rate that is half as rapid as cells incubated in the absence of drugs. 18,39,40 Western blot analysis Cells were cultured and then protein lysates prepared as described. 36,39,40 Western blots were performed with antibodies specific for ERK; p53; caspases 3, 8 and 10; and PARP (poly(ADP-ribose)polymerase), as previously described.…”

“…18,36,39 Proliferation assays were performed to measure cellular growth under various conditions over a period of 5 days. Cells were resuspended in phenol-red-free RPMI containing 10% fetal bovine serum and IL-3.…”

“…36,39,40 Western blots were performed with antibodies specific for ERK; p53; caspases 3, 8 and 10; and PARP (poly(ADP-ribose)polymerase), as previously described. [39][40][41] Antibodies were purchased from Cell Signaling (Beverly, MA, USA).…”