Combining metabolic starvation and PI3K/AKT/mTOR pathway inhibition to chemosensitize prostate cancer cells

Prostate cancer is characterized by resistance to chemotherapy and is often associated with loss of the tumor suppressor PTEN, a negative regulator of the PI3K/AKT/mTOR pathway. Currently, taxanes are the first line chemotherapy for advanced prostate cancer, however effective therapeutic strategies are still needed that extend survival or alter the outcome of the disease. The inhibition of PI3K/AKT/mTOR pathway has been reported to enhance the efficacy of chemotherapeutic agents in different experimental models. Moreover, it has been reported that the over-expression of oncogenes under nutrient stress can activate a death program. Therefore metabolic stress can be a useful strategy to sensitize AKT up-regulated prostate cancer cells to anti-tumor drugs. We have previously demonstrated that: • mTOR is involved not only in PI3K/AKT survival pathway, but also in apoptotic pathway, induced by anti-microtubule agents, such as taxanes; • AKT down-regulation enhances the antitumor effects of taxanes; • metabolic starvation increases the sensitivity to death, induced by taxanes; • down-regulating Bcl-2 levels raises the sensitization to cell death, induced by association of starvation plus taxanes. Here different cancer’s capabilities, such as sustained proliferative signaling, deregulated cellular energetics, cell death resistance, have been targeted by various mechanism-directed treatments in cellular model systems of human prostate cancer. The specific aim of our research project is to overcome therapeutic resistance and restore the sensitivity to anti-cancer drugs in prostate cancer cells, through selective inhibition of different components of the PI3K/AKT/mTOR cascade, in association with treatment with taxanes and starvation. The PI3K/AKT/mTOR pathway has been targeted using either Rapamycin or small interference RNAs (siRNAs). LNCaP, PC3 (both PTEN-null) and DU145 (wild type PTEN) cell lines have been treated with Rapamycin as a single agent or in combination with metabolic starvation and/or taxanes. Cell viability of LNCaP and PC3 prostate cells, with high levels of phosphorylated AKT, was not significantly affected by Rapamycin treatment alone, but the percentage of growth inhibition significantly increased in combined treatments with starvation and taxanes. DU145 cells, with lower levels of activated AKT, showed high inhibition of cell growth following Rapamycin treatment alone (45%), that increased to 75% when Rapamycin was associated with starvation and/or taxanes. PTEN-null and PTEN-wild type prostate cancer cells were also treated with siRNAs directed towards mTOR (simTOR), as a single agent or in association with starvation and taxanes. The combination of treatments caused a dramatic reduction of viable cell number in DU145 (up to 90%) and, to a lesser extent, in LNCaP cells (about 50%). These results have been confirmed by Annexin V assays analyzing cell death in the same samples. Therefore we have established that acting at the same time on different cancer’s capabilities is a promising strategy to induce the apoptotic program, initiated by anti-microtubule drugs, in prostate cancer cells. We demonstrated that it is possible to induce prostate cancer cell death through selective inhibition of specific components of PI3K/AKT/mTOR signaling cascade, in association with treatment with taxanes and metabolic starvation. The identification of the most effective and safe combination of targets to achieve a specific and maximal sensitization to anti-proliferative drugs might provide new therapeutic models in clinical applications.