Rapamycin enhances docetaxel-induced cytotoxicity in a androgen-independent prostate cancer xenograft model by survivin downregulation

Phase I Study of Docetaxel and Temsirolimus in Refractory Solid Tumors

INTRODUCTION

The mammalian target of rapamycin (mTOR) is a downstream mediator in the phosphatidylinositol 3-kinase/Akt signaling pathway, and plays a central role in cell proliferation, growth, differentiation, migration, and survival. Temsirolimus (CCI-779), a selective inhibitor of the mTOR, is an ester analog of rapamycin (sirolimus) with improved aqueous solubility and pharmacokinetic (PK) properties. Preclinical studies have confirmed additive and synergistic antitumor activity in cancer cell lines (breast, prostate cancer) with combinations of taxanes and mTOR inhibitors. We conducted a phase I open-label, dose-escalation study to determine the maximal tolerated dose (MTD) of docetaxel in combination with temsirolimus in patients with refractory solid tumors.

PATIENTS AND METHODS

Eligible patients had a diagnosis of a refractory solid malignancy, measurable disease, and adequate organ function. Patients were sequentially enrolled in 4 dose level intravenous combinations of docetaxel and temsirolimus. Temsirolimus was administered weekly with docetaxel administered every 3 weeks. Laboratory data for tumor markers and radiologic imaging were conducted prestudy and then after every 2 cycles of the treatment. Radiologic response was assessed by Response Evaluation Criteria in Solid Tumors (RECIST) criteria. Blood samples for PK and pharmacodynamic analysis were planned to be drawn at MTD. Apart from the traditional 3+3 design, we also implemented Bayesian Optimal Interval design which uses isotonic regression method to select MTD. We proceeded with isotonic regression analysis by using 20% dose-limiting toxicity (DLT) rate as target.

RESULTS

Twenty-six patients were treated in this study in 4 cohorts and dose levels. Fourteen males and 12 females were enrolled with a median age of 50 years (range of 27 to 72 y) and median Eastern Cooperative Oncology Group performance score of 1. Tumor histologies included pancreas (6), colon (5), rectum (3), gallbladder (2), non-small cell lung (2), endometrium (1), neuroendocrine (1), esophagus (1), stomach (1), pharynx (1), small intestine (1), and duodenum (1). Stable disease was observed in 2/4 (50%), 3/7 (43%), 4/10 (40%), and 3/5 (60%) patients in cohorts 1, 2, 3, and 4, respectively. Dose escalation in cohorts 2, 3, and 4 was complicated by DLTs such as grade 4 neutropenia and grade 3 diarrhea and an inability for patients to tolerate treatments during and beyond cycle 1 without dose reductions. Therefore, we could not determine an MTD or recommended phase II dose using the traditional 3+3 study analysis. Blood samples for PK and pharmacodynamic analysis were not collected since MTD was not determined. By using 20% DLT rate closest to the target, isotonic regression analysis showed identical estimated DLT rates in dose -1 (docetaxel 50 mg/m2 and temsirolimus 15 mg/m2) and dose level 1 (docetaxel 60mg/m2 and temsirolimus 15 mg/m2).

CONCLUSIONS

Dose escalation of docetaxel and temsirolimus was limited by severe myelosuppressive toxicity in this phase I study. Most of the DLTs occurred after cycle 1 of therapy hence, we were unable to determine MTD or collect blood samples for PK and pharmacodynamic analysis. Our trial did not meet its objectives due to significant DLTs with this chemotherapy combination. Although our novel use of Bayesian Optimal Interval design using isotonic regression method to select MTD showed identical estimated DLT rates in dose levels 1 and -1, clinically our patients were not able to complete 2 cycles of this regimen without dose reductions due to myelosuppressive toxicity in either of these dose levels, and hence, escaped clinical validity. This combination regimen should not be studied further at the dose levels and schedules tested in our study.

Depletion of Survivin suppresses docetaxel-induced apoptosis in HeLa cells by facilitating mitotic slippage

The anticancer effects of taxanes are attributed to the induction of mitotic arrest through activation of the spindle assembly checkpoint. Cell death following extended mitotic arrest is mediated by the intrinsic apoptosis pathway. Accordingly, factors that influence the robustness of mitotic arrest or disrupt the apoptotic machinery confer drug resistance. Survivin is an inhibitor of apoptosis protein. Its overexpression is associated with chemoresistance, and its targeting leads to drug sensitization. However, Survivin also acts specifically in the spindle assembly checkpoint response to taxanes. Hence, the failure of Survivin-depleted cells to arrest in mitosis may lead to taxane resistance. Here we show that Survivin depletion protects HeLa cells against docetaxel-induced apoptosis by facilitating mitotic slippage. However, Survivin depletion does not promote clonogenic survival of tumor cells but increases the level of cellular senescence induced by docetaxel. Moreover, lentiviral overexpression of Survivin does not provide protection against docetaxel or cisplatin treatment, in contrast to the anti-apoptotic Bcl-xL or Bcl-2. Our findings suggest that targeting Survivin may influence the cell response to docetaxel by driving the cells through aberrant mitotic progression, rather than directly sensitizing cells to apoptosis.

Cellular growth factors as prospective therapeutic targets for combination therapy in androgen independent prostate cancer (AIPC)

Cancer is the second leading cause of death worldwide, with prostate cancer, the second most commonly diagnosed cancer among men. Prostate cancer develops in the peripheral zone of the prostate gland, and the initial progression largely depends on androgens, the male reproductive hormone that regulates the growth and development of the prostate gland and testis. The currently available treatments for androgen dependent prostate cancer are, however, effective for a limited period, where the patients show disease relapse, and develop androgen-independent prostate cancer (AIPC). Studies have shown various intricate cellular processes such as, deregulation in multiple biochemical and signaling pathways, intra-tumoral androgen synthesis; AR over-expression and mutations and AR activation via alternative growth pathways are involved in progression of AIPC. The currently approved treatment strategies target a single cellular protein or pathway, where the cells slowly develop resistance and adapt to proliferate via other cellular pathways over a period of time. Therefore, an increased research aims to understand the efficacy of combination therapy, which targets multiple interlinked pathways responsible for acquisition of resistance and survival. The combination therapy is also shown to enhance efficacy as well as reduce toxicity of the drugs. Thus, the present review focuses on the signaling pathways involved in the progression of AIPC, comprising a heterogeneous population of cells and the advantages of combination therapy. Several clinical and pre-clinical studies on a variety of combination treatments have shown beneficial outcomes, yet further research is needed to understand the potential of combination therapy and its diverse strategies.

Rapamycin inhibits AR signaling pathway in prostate cancer by interacting with the FK1 domain of FKBP51

Reactivation of the androgen receptor signaling pathway in the emasculated environment is the main reason for the occurrence of castration-resistant prostate cancer (CRPC). The immunophilin FKBP51, as a co-chaperone protein, together with Hsp90 help the correct folding of AR. Rapamycin is a known small-molecule inhibitor of FKBP51, but its effect on the FKBP51/AR signaling pathway is not clear. In this study, the interaction mechanism between FKBP51 and rapamycin was investigated using steady-state fluorescence quenching, X-ray crystallization, MTT assay, and qRT-PCR. Steady-state fluorescence quenching assay showed that rapamycin could interact with FKBP51. The crystal of the rapamycin-FKBP51 complex indicated that rapamycin occupies the hydrophobic binding pocket of FK1 domain which is vital for AR activity. The residues involving rapamycin binding are mainly hydrophobic and may overlap with the AR interaction site. Further assays showed that rapamycin could inhibit the androgen-dependent growth of human prostate cancer cells by down-regulating the expression levels of AR activated downstream genes. Taken together, our study demonstrates that rapamycin suppresses AR signaling pathway by interfering with the interaction between AR and FKBP51. The results of this study not only can provide useful information about the interaction mechanism between rapamycin and FKBP51, but also can provide new clues for the treatment of prostate cancer and castration-resistant prostate cancer.

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Rapamycin occupies the hydrophobic binding pocket of FK1 domain of FKBP51.

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Rapamycin suppresses the AR signaling pathway by interacting with FKBP51.

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Rapamycin inhibits the growth of prostate cancer cells via the AR signaling pathway.

Disulfiram potentiates docetaxel cytotoxicity in breast cancer cells through enhanced ROS and autophagy

BACKGROUND

Recent studies have demonstrated that autophagy plays a critical role in reducing the drug sensitivity of docetaxel (DTX) therapy. Disulfiram (DSF) has exhibited potent autophagy inducing activity in multiple studies. We hypothesized that DSF co-treatment could sensitize breast cancer cells to DTX therapy via autophagy modulation.

METHODS

Breast cancer cells, MCF7, and 4T1, were treated with DTX and DSF, alone and in combination. The effects were analyzed by evaluating cytotoxicity, induction of apoptosis, induction of autophagy, and reactive oxygen species (ROS) generation. In addition, the consequence of autophagy and ROS inhibition on the DTX + DSF mediated cytotoxicity was also evaluated.

RESULTS

Significant synergism in cytotoxicity was observed with DTX + DSF combination in breast cancer cells, MCF7, and 4T1. Hyper induction of ROS and autophagy was also found with the combination treatment. ROS inhibition by N-Acetyl Cysteine (NAC), as well as autophagy inhibition by ATG5 silencing significantly reduced the autophagy level as well as cytotoxicity of the DTX + DSF combination, indicating that the induction of autophagy mediated by high ROS generation played a critical role behind the synergistic cytotoxicity.

CONCLUSIONS

This study indicates that DTX + DSF combination therapy can effectively sensitize cancer cells by hyper inducing autophagy through ROS generation and can be developed as a therapeutic strategy for cancer treatment in the future.

Dual role of autophagy on docetaxel-sensitivity in prostate cancer cells

Prostate cancer (PC) is one of the leading causes of death in males. Available treatments often lead to the appearance of chemoresistant foci and metastases, with mechanisms still partially unknown. Within tumour mass, autophagy may promote cell survival by enhancing cancer cells tolerability to different cell stresses, like hypoxia, starvation or those triggered by chemotherapic agents. Because of its connection with the apoptotic pathways, autophagy has been differentially implicated, either as prodeath or prosurvival factor, in the appearance of more aggressive tumours. Here, in three PC cells (LNCaP, PC3, and DU145), we tested how different autophagy inducers modulate docetaxel-induced apoptosis. We selected the mTOR-independent disaccharide trehalose and the mTOR-dependent macrolide lactone rapamycin autophagy inducers. In castration-resistant PC (CRPC) PC3 cells, trehalose specifically prevented intrinsic apoptosis in docetaxel-treated cells. Trehalose reduced the release of cytochrome c triggered by docetaxel and the formation of aberrant mitochondria, possibly by enhancing the turnover of damaged mitochondria via autophagy (mitophagy). In fact, trehalose increased LC3 and p62 expression, LC3-II and p62 (p62 bodies) accumulation and the induction of LC3 puncta. In docetaxel-treated cells, trehalose, but not rapamycin, determined a perinuclear mitochondrial aggregation (mito-aggresomes), and mitochondria specifically colocalized with LC3 and p62-positive autophagosomes. In PC3 cells, rapamycin retained its ability to activate autophagy without evidences of mitophagy even in presence of docetaxel. Interestingly, these results were replicated in LNCaP cells, whereas trehalose and rapamycin did not modify the response to docetaxel in the ATG5-deficient (autophagy resistant) DU145 cells. Therefore, autophagy is involved to alter the response to chemotherapy in combination therapies and the response may be influenced by the different autophagic pathways utilized and by the type of cancer cells.

Everolimus (RAD001) sensitizes prostate cancer cells to docetaxel by down-regulation of HIF-1α and sphingosine kinase 1

Resistance to docetaxel is a key problem in current prostate cancer management. Sphingosine kinase 1 (SK1) and phosphoinositide 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathways have been implicated in prostate cancer chemoresistance. Here we investigated whether their combined targeting may re-sensitize prostate cancer cells to docetaxel.In hormone-insensitive PC-3 and DU145 prostate cancer cells the mTOR inhibitor everolimus (RAD001) alone did not lead to significant cell death, however, it strongly sensitized cells to low levels (5 nM) of docetaxel. We show that mTOR inhibition has led to a decrease in hypoxia-inducible factor-1α (HIF-1α) protein levels and SK1 mRNA. HIF-1α accumulation induced by CoCl2 has led to a partial chemoresistance to RAD001/docetaxel combination. SK1 overexpression has completely protected prostate cancer cells from RAD001/docetaxel effects. Using gene knockdown and CoCl2 treatment we showed that SK1 mRNA expression is downstream of HIF-1α. In a human xenograft model in nude mice single RAD001 and docetaxel therapies induced 23% and 15% reduction in prostate tumor volume, respectively, while their combination led to a 58% reduction. RAD001 alone or in combination with docetaxel has suppressed intratumoral mTOR and SK1 signaling, however as evidenced by tumor size, it required docetaxel for clinical efficacy. Combination therapy was well tolerated and had similar levels of toxicity to docetaxel alone.Overall, our data demonstrate a new mechanism of docetaxel sensitization in prostate cancer. This provides a mechanistic basis for further clinical application of RAD001/docetaxel combination in prostate cancer therapy.

In vitro and in vivo effects of a recombinant anti-PSMA immunotoxin in combination with docetaxel against prostate cancer

Docetaxel (DOC) is used for the first-line treatment of castration resistant prostate cancer (CPRC). However, the therapeutic effects are limited, only about one half of patients respond to the therapy and severe side effects possibly lead to discontinuation of treatment. Therefore, actual research is focused on the development of new DOC-based combination treatments.

In this study we investigated the antitumor effects of a recombinant immunotoxin targeting the prostate specific membrane antigen (PSMA) in combination with DOC in vitro and in vivo. The immunotoxin consists of an anti-PSMA single chain antibody fragment (scFv) as binding and a truncated form of Pseudomonas aeruginosa Exotoxin A (PE40) as toxin domain. The immunotoxin induced apoptosis and specifically reduced the viability of androgen-dependent LNCaP and androgen-independent C4-2 prostate cancer cells. A synergistic cytotoxic activity was observed in combination with DOC with IC₅₀ values in the low picomolar or even femtomolar range. Moreover, combination treatment resulted in an enhanced antitumor activity in a C4-2 SCID mouse xenograft model. This highlights the immunotoxin as a promising therapeutic agent for a future DOC-based combination therapy of CPRC.

Radiofrequency heat-enhanced direct intratumoral chemotherapy for prostate cancer

A novel, minimally invasive interventional technique, radiofrequency heat (RFH), has been suggested to improve the efficacy of chemotherapy for solid organ tumors. However, the treatment for prostate cancer has not been completely characterized. The aim of the present study was to investigate the in vitro and in vivo efficiency of chemotherapy in combination with RFH for the treatment of prostate cancer. The following four treatment groups were included: i) No treatment (control); ii) RFH-only; iii) chemotherapy (docetaxel)-only; and iv) combination therapy of docetaxel and RFH in human prostate cancer (HPC) cell lines and mice with HPC xenografts. In the in vitro experiments, a heating guidewire was attached under the bottom of the last chamber of the four-chamber cell culture slide, and was then connected to a radiofrequency (RF) generator. In the in vivo experiments, a tumor model was generated by subcutaneously injecting human prostate cancer cells into 24 male nu/nu mice. RFH was conducted by inserting the 0.022-inch heating-guidewire into the tumor. The follow-up magnetic resonance imaging demonstrated a significant reduction in the average tumor size in animals treated with combination therapy compared with those receiving RFH-only and chemotherapy-only. The number of apoptotic cells and the average apoptotic index of the combination therapy group were significantly higher compared with those of the other three treatment groups. In conclusion, the results of the present study suggested that RFH is able to increase the therapeutic efficiency of docetaxel in prostate cancer, and this study serves as a foundation for the future development of an interventional molecular image-guided local treatment strategy for prostate cancer that integrates RF technology, interventional oncology and direct intratumoral chemotherapy, as a replacement for systemic chemotherapy.

Kavalactone yangonin induces autophagy and sensitizes bladder cancer cells to flavokawain A and docetaxel via inhibition of the mTOR pathway

Consumption of kava (Piper methysticum Forst) has been linked to reduced cancer risk in the South Pacific Islands. Kavalactones are major bioactive components in kava root extracts, which have recently demonstrated anti-cancer activities. However, molecular mechanisms of kavalactones' anti-cancer action remain largely unknown. We have identified two kavalactones, yangonin and 5′ 6'-dehydrokawain, as potent inducers of autophagic cell death in bladder cancer cells. The effect of yangonin inducing autophagy is associated with increased expression of beclin and ATG5. In addition, yangonin increases the expression of LKB1 and decreases the phosphorylation of Akt, PRAS40, rpS6, p70S6K and 4E-BP1, leading to increased binding of 4E-BP1 to m7 GTP. The growth inhibitory effects of yangonin were attenuated inTSC1 or LKB1 knockout mouse embryonic fibroblasts, suggesting that TSC1 and LKB1 expression may contribute to optimal growth inhibition by yangonin. Furthermore, yangonin reduces the viability of bladder cancer cell lines derived from different stages of human bladder cancer, and acts synergistically with apoptosis-inducing agents such as docetaxel and flavokawain A. Our results support a novel anti-bladder cancer mechanism by yangonin and further studies are needed to assess the potential use of yangonin for bladder cancer prevention and treatment

Resveratrol directly targets DDX5 resulting in suppression of the mTORC1 pathway in prostate cancer

Resveratrol has various attractive bioactivities, such as prevention of cancer, neurodegenerative disorders, and obesity-related diseases. Therefore, identifying its direct binding proteins is expected to discover druggable targets. Sirtuin 1 and phosphodiesterases have so far been found as the direct molecular targets of resveratrol. We herein identified 11 novel resveratrol-binding proteins, including the DEAD (Asp-Glu-Ala-Asp) box helicase 5 (DDX5, also known as p68), using resveratrol-immobilized beads. Treatment with resveratrol induced degradation of DDX5 in prostate cancer cells. Depletion of DDX5 caused apoptosis by inhibiting mammalian target of rapamycin complex 1 (mTORC1) signaling. Moreover, knockdown of DDX5 attenuated the inhibitory activities of resveratrol against mTORC1 signaling and cancer cell growth. These data show that resveratrol directly targets DDX5 and induces cancer cell death by inhibiting the mTORC1 pathway.

Effect of Metformin, Rapamycin, and Their Combination on Growth and Progression of Prostate Tumors in HiMyc Mice

In this study, we compared the effect of oral administration of metformin (MET) and rapamycin (RAPA) alone or in combination on prostate cancer development and progression in HiMyc mice. MET (250 mg/kg body weight in the drinking water), RAPA (2.24 mg/kg body weight microencapsulated in the diet), and the combination inhibited progression of prostatic intraepithelial neoplasia lesions to adenocarcinomas in the ventral prostate (VP). RAPA and the combination were more effective than MET at the doses used. Inhibition of prostate cancer progression in HiMyc mice by RAPA was associated with a significant reduction in mTORC1 signaling that was further potentiated by the combination of MET and RAPA. In contrast, treatment with MET alone enhanced AMPK activation, but had little or no effect on mTORC1 signaling pathways in the VP of HiMyc mice. Further analyses revealed a significant effect of all treatments on prostate tissue inflammation as assessed by analysis of the expression of cytokines, the presence of inflammatory cells and NFκB signaling. MET at the dose used appeared to reduce prostate cancer progression primarily by reducing tissue inflammation whereas RAPA and the combination appeared to inhibit prostate cancer progression in this mouse model via the combined effects on both mTORC1 signaling as well as on tissue inflammation. Overall, these data support the hypothesis that blocking mTORC1 signaling and/or tissue inflammation can effectively inhibit prostate cancer progression in a relevant mouse model of human prostate cancer. Furthermore, combinatorial approaches that target both pathways may be highly effective for prevention of prostate cancer progression in men.

Rapamycin-Induced apoptosis in HGF-stimulated lens epithelial cells by AKT/mTOR, ERK and JAK2/STAT3 pathways

Hepatocyte growth factor (HGF) induced the proliferation of lens epithelial cells (LECs) and may be a major cause of posterior capsule opacification (PCO), which is the most frequent postoperative complication of cataract surgery. To date, several agents that can block LECs proliferation have been studied, but none have been used in clinic. Recently, accumulating evidence has suggested rapamycin, the inhibitor of mTOR (mammalian target of Rapamycin), was associated with the induction of apoptosis in LECs. The purpose of our study was to investigate the potential effects of rapamycin on HGF-induced LECs and the underlying mechanisms by which rapamycin exerted its actions. Using cell proliferation, cell viability and flow cytometric apoptosis assays, we found that rapamycin potently not only suppressed proliferation but also induced the apoptosis of LECs in a dose-dependent manner under HGF administration. Further investigation of the underlying mechanism using siRNA transfection revealed that rapamycin could promote apoptosis of LECs via inhibiting HGF-induced phosphorylation of AKT/mTOR, ERK and JAK2/STAT3 signaling molecules. Moreover, the forced expression of AKT, ERK and STAT3 could induce a significant suppression of apoptosis in these cells after treatment of rapamycin. Together, these findings suggested that rapamycin-induced apoptosis in HGF-stimulated LECs is accompanied by inhibition of AKT/mTOR, ERK and JAK2/STAT3 pathways, which supports its use to inhibit PCO in preclinical studies and provides theoretical foundation for future possible practice.

Inhibition of Hsp90 augments docetaxel therapy in castrate resistant prostate cancer

First line treatment of patients with castrate resistant prostate cancer (CRPC) primarily involves administration of docetaxel chemotherapy. Unfortunately, resistance to docetaxel therapy is an ultimate occurrence. Alterations in androgen receptor (AR) expression and signaling are associated mechanisms underlying resistance to docetaxel treatment in CRPC. Heat shock protein 90 (Hsp90) is a molecular chaperone, which regulates the activation, maturation and stability of critical signaling proteins involved in prostate cancer, including the AR. This knowledge and recent advances in compound design and development have highlighted Hsp90 as an attractive therapeutic target for the treatment of CRPC. We recently reported the development of a MYC-CaP castrate resistant (MYC-CaP/CR) transplant tumor model, which expresses amplified wild type AR. Within, we report that a second generation Hsp90 inhibitor, NVP-AUY922, inhibits cell growth and significantly induces cell death in MYC-CaP/CR and Pten-CaP/cE2 cell lines. NVP-AUY922 induced proteasome degradation of AR, though interestingly does not require loss of AR protein to inhibit AR transcriptional activity. Further, NVP-AUY922 increased docetaxel toxicity in MYC-CaP/CR and Pten-CaP/cE2 cell lines in vitro. Finally, NVP-AUY922/docetaxel combination therapy in mice bearing MYC-CaP/CR tumors resulted in greater anti-tumor activity compared to single treatment. This study demonstrates that NVP-AUY922 elicits potent activity towards AR signaling and augments chemotherapy response in a mouse model of CRPC, providing rationale for the continued clinical development of Hsp90 inhibitors in clinical trials for treatment of CRPC patients.

YM155 reverses rapamycin resistance in renal cancer by decreasing survivin

PURPOSE

Mammalian target of rapamycin inhibitor has exhibited promising anticancer activity for the treatment of renal cell carcinoma (RCC). However, many patients acquire resistance to therapeutic agents leading to treatment failure. The objective of this study was to determine whether treatment with YM155, a novel small molecule inhibitor of survivin, could reverse rapamycin resistance in a rapamycin-resistant RCC.

METHODS

We induced a rapamycin-resistant clear cell carcinoma cell line (Caki-1-RapR). We showed that survivin gene expression was significantly up-regulated in Caki-1-RapR compared with that in its parent cells (Caki-1). Therefore, we hypothesized that targeting of survivin in Caki-1-RapR could reverse the resistant phenotype in tumor cells, thereby enhancing the therapeutic efficacy of rapamycin. We used both in vitro and in vivo models to test the efficacy of YM155 either as a single agent or in combination with rapamycin.

RESULTS

In Caki-1-RapR cells, YM155 significantly decreased survivin gene and protein expression levels and cell proliferation in a dose-dependent manner in vitro. In addition, YM155 treatment significantly reversed rapamycin resistance in cancer cells. In a nude mouse tumor xenograft model, YM155 significantly inhibited the growth of Caki-1-RapR tumor. In addition, YM155 significantly enhanced the antitumor effects of rapamycin in Caki-1-RapR tumor.

CONCLUSIONS

Our results suggest a potentially novel strategy to use YM155 to overcome the resistance in tumor cells, thereby enhancing the effectiveness of molecular target therapy in RCC.

Targeting the PI3K/Akt/mTOR pathway in castration-resistant prostate cancer

The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway is a key signaling pathway that has been linked to both tumorigenesis and resistance to therapy in prostate cancer and other solid tumors. Given the significance of the PI3K/Akt/mTOR pathway in integrating cell survival signals and the high prevalence of activating PI3K/Akt/mTOR pathway alterations in prostate cancer, inhibitors of this pathway have great potential for clinical benefit. Here, we review the role of the PI3K/Akt/mTOR pathway in prostate cancer and discuss the potential use of pathway inhibitors as single agents or in combination in the evolving treatment landscape of castration-resistant prostate cancer.

Targeted inhibition of mammalian target of rapamycin (mTOR) enhances radiosensitivity in pancreatic carcinoma cells

The mammalian target of rapamycin (mTOR) is a protein kinase that regulates protein translation, cell growth, and apoptosis. Rapamycin (RPM), a specific inhibitor of mTOR, exhibits potent and broad in vitro and in vivo antitumor activity against leukemia, breast cancer, and melanoma. Recent studies showing that RPM sensitizes cancers to chemotherapy and radiation therapy have attracted considerable attention. This study aimed to examine the radiosensitizing effect of RPM in vitro, as well as its mechanism of action. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and colony formation assay showed that 10 nmol/L to 15 nmol/L of RPM had a radiosensitizing effects on pancreatic carcinoma cells in vitro. Furthermore, a low dose of RPM induced autophagy and reduced the number of S-phase cells. When radiation treatment was combined with RPM, the PC-2 cell cycle arrested in the G2/M phase of the cell cycle. Complementary DNA (cDNA) microarray and reverse transcription polymerase chain reaction (RT-PCR) revealed that the expression of DDB1, RAD51, and XRCC5 were downregulated, whereas the expression of PCNA and ABCC4 were upregulated in PC-2 cells. The results demonstrated that RPM effectively enhanced the radiosensitivity of pancreatic carcinoma cells.

Discovery of survivin inhibitors and beyond: FL118 as a proof of concept

Survivin, a novel antiapoptotic protein molecule, plays a central role in cancer cell survival/proliferation networks and has therefore become a therapeutic target for cancer drug discovery efforts. There are two strategies for discovering survivin inhibitors. One is based on survivin interactions within the cell and the other strategy is based on blocking survivin expression. Survivin inhibitors developed by the first strategy would disrupt a particular survivin function. These survivin inhibitors could also be useful tools for delineating the mechanism of action of survivin. The second strategy may use a reporter system of the survivin gene to screen drug libraries. To date, two molecules, YM155 and FL118, have been identified using this strategy. These two examples provide a proof of concept that screens for inhibitors of survivin expression using survivin gene reporter assays as surrogate markers will uncover versatile small molecules that not only inhibit survivin but also inhibit other essential cancer survival/proliferation-associated targets and/or signaling pathways. This review provides an overview of current information in the area relevant to survivin inhibitors that may facilitate future studies.

Antitumor effects of rapamycin in pancreatic cancer cells by inducing apoptosis and autophagy

Rapamycin (Rapa), an inhibitor of mammalian target of Rapamycin (mTOR), is an immunosuppressive agent that has anti-proliferative effects on some tumors. This study aims to investigate the effects of Rapa suppressing proliferation of pancreatic carcinoma PC-2 cells in vitro and its molecular mechanism involved in antitumor activities. MTT assays showed that the inhibition of proliferation of PC-2 cells in vitro was in a time- and dose-dependent manner. By using transmission electron microscopy, apoptosis bodies and formation of abundant autophagic vacuoles were observed in PC-2 cells after Rapa treatment. Flow cytometry assays also showed Rapa had a positive effect on apoptosis. MDC staining showed that the fluorescent density was higher and the number of MDC-labeled particles in PC-2 cells was greater in the Rapa treatment group than in the control group. RT-PCR revealed that the expression levels of p53, Bax and Beclin 1 were up-regulated in a dose-dependent manner, indicating that Beclin 1 was involved in Rapa induced autophagy and Rapa induced apoptosis as well as p53 up-regulation in PC-2 cells. The results demonstrated that Rapa could effectively inhibit proliferation and induce apoptosis and autophagy in PC-2 cells.