Abstract
The cancer stem cell hypothesis predicts that standard prostate cancer monotherapy eliminates bulk tumor cells but not a tumor-initiating cell population, eventually leading to relapse. Many studies have sought to determine the underlying differences between bulk tumor and cancer stem cells. Our previous data suggest that the PTEN/PI3K/AKT pathway is critical for the in vitro maintenance of CD133(+)/CD44(+) prostate cancer progenitors and, consequently, that targeting PI3K signaling may be beneficial in treatment of prostate cancer. Here, we show that inhibition of PI3K activity by the dual PI3K/mTOR inhibitor NVP-BEZ235 leads to a decrease in the population of CD133(+)/CD44(+) prostate cancer progenitor cells in vivo. Moreover, the combination of the PI3K/mTOR modulator NVP-BEZ235, which eliminates prostate cancer progenitor populations, and the chemotherapeutic drug Taxotere, which targets the bulk tumor, is significantly more effective in eradicating tumors in a prostate cancer xenograft model than monotherapy. This combination treatment ultimately leads to the expansion of cancer progenitors with a PTEN E91D mutation, suggesting that the analysis of PTEN mutations could predict therapeutic response to the dual therapy.
Highlights
We showed previously that PI3K/AKT/FOXO3a signaling plays a critical role in the maintenance and viability of CD133þ/CD44þ prostate cancer progenitors
Depletion of FOXO3a and PTEN increased the tumor-initiating cell population, and inhibition of PI3K activity by small molecules led to growth inhibition of prostate cancer progenitors while sparing differentiated cells [8]. This finding is consistent with previous studies showing an important role for the PTEN/PI3K/AKT pathway in the regulation of malignant progenitor cell populations in acute myeloid leukemia and breast and brain tumors [9,10,11]. These results suggest that inhibition of the PI3K/AKT and mTOR pathways in prostate cancer progenitors together with conventional chemotherapy may provide improved therapeutic outcomes [12, 13]
Recent evidence showed that the PTEN/PI3K/AKT pathway is critical for stem cell maintenance but that aberrant activation of PI3K signaling contributes to transformation of quiescent normal stem cells to cancer stem cells in the hematopoietic system, brain, intestine, mammary gland, and prostate [9,10,11,12, 17,18,19,20]
Summary
Reagents, and animals DU145, PC3, 22RV1, and LNCaP (prostate cancer cells) were obtained from ATCC and were cultured in Dulbecco’s Minimal Essential Medium (DU145 cells), Ham’s/F12 Medium (PC3 cells), or RPMI-1640 Medium (22RV1 and LNCaP cells), supplemented with 10% fetal calf serum, 300 mg/mL of glutamine, 100 IU/mL of penicillin, and 100 mg/mL of streptomycin. PI3K inhibitor NVP-BEZ235 was obtained from Novartis, Taxotere (docetaxel) was purchased from LC Laboratories, and 5-FU and Oxaliplatin were purchased from Sigma. Cells were dissociated with Accutase (Innovative Cell Tech Inc.) and washed twice in staining solution containing Ca2þ- and Mg2þ-free PBS with 1 mmol/L of EDTA, 25 mmol/L of HEPES (pH 7.0; Invitrogen), 0.5% FBS. Cells were stained with conjugated antiCD44 (mouse anti-human CD44-APC-H7; BD Pharmingen) and anti-CD133 (mouse anti-human CD133/2 (293C3)-PE; Miltenyi Biotec) antibody (50 minutes at 4C). Samples were analyzed on a BD LSR II flow cytometer (Beckton Dickinson Immunocytometry Systems). 2 Â 107 cells were processed for CD44 and CD133 staining along with appropriate negative controls, single color-positive controls, and isotype negative control (mouse IgG2b; Miltenyi Biotec). The CD133þ/CD44þ and CD133À/CD44À populations were sorted on a BD FACS Diva cell sorter
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