mTOR inhibition reverses acquired endocrine therapy resistance of breast cancer cells at the cell proliferation and gene-expression levels

Apoptosis Deregulation and the Development of Cancer Multi-Drug Resistance

Simple Summary

Despite recent therapeutic advances against cancer, many patients do not respond well or respond poorly, to treatment and develop resistance to more than one anti-cancer drug, a term called multi-drug resistance (MDR). One of the main factors that contribute to MDR is the deregulation of apoptosis or programmed cell death. Herein, we describe the major apoptotic pathways and discuss how pro-apoptotic and anti-apoptotic proteins are modified in cancer cells to convey drug resistance. We also focus on our current understanding related to the interactions between survival and cell death pathways, as well as on mechanisms underlying the balance shift towards cancer cell growth and drug resistance. Moreover, we highlight the role of the tumor microenvironment components in blocking apoptosis in MDR tumors, and we discuss the significance and potential exploitation of epigenetic modifications for cancer treatment. Finally, we summarize the current and future therapeutic approaches for overcoming MDR.

Abstract

The ability of tumor cells to evade apoptosis is established as one of the hallmarks of cancer. The deregulation of apoptotic pathways conveys a survival advantage enabling cancer cells to develop multi-drug resistance (MDR), a complex tumor phenotype referring to concurrent resistance toward agents with different function and/or structure. Proteins implicated in the intrinsic pathway of apoptosis, including the Bcl-2 superfamily and Inhibitors of Apoptosis (IAP) family members, as well as their regulator, tumor suppressor p53, have been implicated in the development of MDR in many cancer types. The PI₃K/AKT pathway is pivotal in promoting survival and proliferation and is often overactive in MDR tumors. In addition, the tumor microenvironment, particularly factors secreted by cancer-associated fibroblasts, can inhibit apoptosis in cancer cells and reduce the effectiveness of different anti-cancer drugs. In this review, we describe the main alterations that occur in apoptosis-and related pathways to promote MDR. We also summarize the main therapeutic approaches against resistant tumors, including agents targeting Bcl-2 family members, small molecule inhibitors against IAPs or AKT and agents of natural origin that may be used as monotherapy or in combination with conventional therapeutics. Finally, we highlight the potential of therapeutic exploitation of epigenetic modifications to reverse the MDR phenotype.

Single-cell immunoblotting resolves estrogen receptor-α isoforms in breast cancer

An array of isoforms of the nuclear estrogen receptor alpha (ER-α) protein contribute to heterogeneous response in breast cancer (BCa); yet, a single-cell analysis tool that distinguishes the full-length ER-α66 protein from the activation function-1 deficient ER-α46 isoform has not been reported. Specific detection of protein isoforms is a gap in single-cell analysis tools, as the de facto standard immunoassay requires isoform-specific antibody probes. Consequently, to scrutinize hormone response heterogeneity among BCa tumor cells, we develop a precision tool to specifically measure ER-α66, ER- α46, and eight ER-signaling proteins with single-cell resolution in the highly hetero-clonal MCF-7 BCa cell line. With a literature-validated pan-ER immunoprobe, we distinguish ER-α66 from ER-α46 in each individual cell. We identify ER-α46 in 5.5% of hormone-sensitive (MCF-7) and 4.2% of hormone-insensitive (MDA-MB-231) BCa cell lines. To examine whether the single-cell immunoblotting can capture cellular responses to hormones, we treat cells with tamoxifen and identify different sub-populations of ER-α46: (i) ER-α46 induces phospho-AKT at Ser473, (ii) S6-ribosomal protein, an upstream ER target, activates both ER-α66 and ER-α46 in MCF-7 cells, and (iii) ER-α46 partitions MDA-MB-231 subpopulations, which are responsive to tamoxifen. Unlike other single-cell immunoassays, multiplexed single-cell immunoblotting reports–in the same cell–tamoxifen effects on ER signaling proteins and on distinct isoforms of the ER-α protein.

Estrogen Receptor-Alpha and p53 Status as Regulators of AMPK and mTOR in Luminal Breast Cancer

Luminal breast cancer (LBC) driven by dysregulated estrogen receptor-alpha (ERα) signaling accounts for 70% of the breast cancer cases diagnosed. Although endocrine therapy (ET) is effective against LBC, about one-third of these patients fail to respond to therapy owing to acquired or inherent resistance mechanisms. Aberrant signaling via ERα, oncogenes, growth factor receptors, and mutations in tumor suppressors such as p53 impinge on downstream regulators such as AMPK and mTOR. While both AMPK and mTOR have been reported to play important roles in determining sensitivity of LBC to ET, how the ERα-p53 crosstalk impinges on regulation of AMPK and mTOR, thereby influencing therapeutic efficacy remains unknown. Here, we have addressed this important issue using isogenic breast cancer cell lines, siRNA-mediated RNA knockdown, and different modes of drug treatments. Interaction of p53 with ERα and AMPK was determined by in situ proximity ligation assay (PLA), and endogenous gene transcripts were analyzed by quantitative real-time polymerase chain reaction (qRT-PCR). Further, the effect of concurrent and sequential administration of Fulvestrant-Everolimus combination on colony formation was determined. The studies showed that in cells expressing wild type p53, as well as in cells devoid of p53, ERα represses AMPK, whereas in cells harboring mutant p53, repression of AMPK is sustained even in the absence of ERα. AMPK is a major negative regulator of mTOR, and to our knowledge, this is the first study on the contribution of AMPK-dependent regulation of mTOR by ERα. Furthermore, the studies revealed that independent of the p53 mutation status, combination of Fulvestrant and Everolimus may be a viable first line therapeutic strategy for potentially delaying resistance of ERα+/HER2- LBC to ET.

Association of Sperm-Associated Antigen 5 and Treatment Response in Patients With Estrogen Receptor-Positive Breast Cancer

Importance

There is no proven test that can guide the optimal treatment, either endocrine therapy or chemotherapy, for estrogen receptor-positive breast cancer.

Objective

To investigate the associations of sperm-associated antigen 5 (SPAG5) transcript and SPAG5 protein expressions with treatment response in systemic therapy for estrogen receptor-positive breast cancer.

Design, Settings, and Participants

This retrospective cohort study included patients with estrogen receptor-positive breast cancer who received 5 years of adjuvant endocrine therapy with or without neoadjuvant anthracycline-based combination chemotherapy (NACT) derived from 11 cohorts from December 1, 1986, to November 28, 2019. The associations of SPAG5 transcript and SPAG5 protein expression with pathological complete response to NACT were evaluated, as was the association of SPAG5 mRNA expression with response to neoadjuvant endocrine therapy. The associations of distal relapse-free survival with SPAG5 transcript or SPAG5 protein expressions were analyzed. Data were analyzed from September 9, 2015, to November 28, 2019.

Main Outcomes and Measures

The primary outcomes were breast cancer-specific survival, distal relapse-free survival, pathological complete response, and clinical response. Outcomes were examined using Kaplan-Meier, multivariable logistic, and Cox regression models.

Results

This study included 12 720 women aged 24 to 78 years (mean [SD] age, 58.46 [12.45] years) with estrogen receptor-positive breast cancer, including 1073 women with SPAG5 transcript expression and 361 women with SPAG5 protein expression of locally advanced disease stage IIA through IIIC. Women with SPAG5 transcript and SPAG5 protein expressions achieved higher pathological complete response compared with those without SPAG5 transcript or SPAG5 protein expressions (transcript: odds ratio, 2.45 [95% CI, 1.71-3.51]; P < .001; protein: odds ratio, 7.32 [95% CI, 3.33-16.22]; P < .001). Adding adjuvant anthracycline chemotherapy to adjuvant endocrine therapy for SPAG5 mRNA expression in estrogen receptor-positive breast cancer was associated with prolonged 5-year distal relapse-free survival in patients without lymph node involvement (hazard ratio, 0.34 [95% CI, 0.14-0.87]; P = .03) and patients with lymph node involvement (hazard ratio, 0.35 [95% CI, 0.18-0.68]; P = .002) compared with receiving 5-year endocrine therapy alone. Mean (SD) SPAG5 transcript was found to be downregulated after 2 weeks of neoadjuvant endocrine therapy compared with pretreatment levels in 68 of 92 patients (74%) (0.23 [0.18] vs 0.34 [0.24]; P < .001).

Conclusions and Relevance

These findings suggest that SPAG5 transcript and SPAG5 protein expressions could be used to guide the optimal therapies for estrogen receptor-positive breast cancer. Retrospective and prospective clinical trials are warranted.

Everolimus versus alpelisib in advanced hormone receptor-positive HER2-negative breast cancer: targeting different nodes of the PI3K/AKT/mTORC1 pathway with different clinical implications

Background

The PI3K/AKT/mTORC1 axis is implicated in hormone receptor-positive HER2-negative metastatic breast cancer (HR+ HER2− mBC) resistance to anti-estrogen treatments. Based on results of the BOLERO-2 trial, the mTORC1 inhibitor everolimus in combination with the steroidal aromatase inhibitor (AI) exemestane has become a standard treatment for patients with HR+ HER2− mBC resistant to prior non-steroidal AI therapy. In the recent SOLAR-1 trial, the inhibitor of the PI3K alpha subunit (p110α) alpelisib in combination with fulvestrant prolonged progression-free survival (PFS) when compared to fulvestrant alone in patients with PIK3CA-mutated HR+ HER2− mBC that progressed after/on previous AI treatment. Therefore, two different molecules targeting the PI3K/AKT/mTORC1 axis, namely everolimus and alpelisib, are available for patients progressing on/after previous AI treatment, but it is unclear how to optimize their use in the clinical practice.

Main body of the abstract

Here, we reviewed the available clinical evidence deriving from the BOLERO-2 and SOLAR-1 trials to compare efficacy and safety profiles of everolimus and alpelisib in advanced HR+ HER2− BC treatment. Adding either compound to standard endocrine therapy provided similar absolute and relative PFS advantage. In the SOLAR-1 trial, a 76% incidence of grade (G) 3 or 4 (G3/G4) adverse events was reported, while G3/G4 toxicities occurred in 42% of patients in the BOLERO-2 trial. While alpelisib was only effective in patients with PIK3CA-mutated neoplasms, retrospective analyses indicate that everolimus improves exemestane efficacy independently of PIK3CA mutational status.

Conclusions

Based on the available efficacy and safety data, the “new” alpelisib may be burdened by higher incidence of severe adverse events, higher costs, and anticancer efficacy that is limited to PIK3CA-mutated tumors when compared to the “old” everolimus. Therefore, the everolimus-exemestane combination remains an effective and reasonably well-tolerated therapeutic option for HR+ HER2− mBC patients progressing after/on previous AI treatment, independently of PIK3CA mutational status.

The combination of NVP-BEZ235 and rapamycin regulates nasopharyngeal carcinoma cell viability and apoptosis via the PI3K/AKT/mTOR pathway

Nasopharyngeal carcinoma (NPC) is a rare malignancy with a remarkable geographical distribution. Regarding NPC treatment, improving the survival rate of advanced patients seems promising. Phosphoinositide 3-kinase (PI3K)/protein kinase B (AKT)/mammalian target of rapamycin (mTOR) pathway deregulation is closely associated with tumorigenesis. In the present study, the NPC cell line SUNE1 was divided into four groups: Control, NVP-BEZ235, rapamycin, and NVP-BEZ235+rapamycin. SUNE1 cells in the NVP-BEZ235 group were incubated with NVP-BEZ235; cells in the rapamycin group were incubated with rapamycin, whereas the NVP-BEZ235+rapamycin group refers to SUNE1 cells incubated with a mixture of NVP-BEZ235 and rapamycin. The control group was treated with the same amount of vehicle. Morphological, MTT, terminal deoxynucleotidyl-transferase-mediated dUTP nick end labeling and flow cytometry assays demonstrated that NVP-BEZ235 and rapamycin caused morphological changes, inhibited cell viability and induced cellular apoptosis. In addition, reverse transcription-quantitative polymerase chain reaction and western blot revealed that the combination of NVP-BEZ235 and rapamycin affected the activation of the PI3K/AKT/mTOR pathway. The combination of NVP-BEZ235 and rapamycin significantly improved the effect of the drug therapy. The potential underlying mechanism may comprise the joint effects of inhibiting cell viability, promoting cellular apoptosis and reducing relative signal protein expression levels in SUNE1 cells. These findings provided novel evidence that NVP-BEZ235 suppresses NPC development, and indicated a promising potential application of combination drug therapy (NVP-BEZ235+rapamycin) for the clinical treatment of NPC.

Resistance to the mTOR inhibitor everolimus is reversed by the downregulation of survivin in breast cancer cells

Everolimus (RAD001) is an inhibitor of mammalian target of rapamycin used in combination with exemestane to treat hormone receptor-positive advanced breast cancer. However, not all patients are equally sensitive to RAD001 and certain patients develop resistance. Therefore, the present study analyzed the mechanisms involved in the resistance of breast cancer cells to RAD001 in order to identify a potential tool to overcome it. The effects of RAD001 on the inhibition of cell viability, on the induction of apoptosis and autophagy and on the regulation of survivin, an anti-apoptotic protein, were evaluated in two breast cancer cell lines: BT474 (luminal B) and MCF7 (luminal A). RAD001 was demonstrated to induce autophagy in the two cell lines at following a short period of treatment (4 h) and to induce apoptosis exclusively in BT474 cells following longer periods of treatment (48 h). RAD001 induced the downregulation of survivin in BT474 cells and its upregulation in MCF7 cells. Consequently, inhibiting survivin with YM155 resulted in the acquired resistance of MCF7 cells to RAD001 being reverted, restoring RAD001-induced apoptosis. These data demonstrated that RAD001 exerted anti-proliferative and pro-apoptotic effects on breast cancer cells, but that these effects were repressed by the simultaneous up-regulation of survivin. Finally, the results demonstrated that inhibiting the expression of survivin resulted in the restoration of the anti-neoplastic activity of RAD001.

Efficacy and safety of everolimus in Chinese metastatic HR positive, HER2 negative breast cancer patients: a real-world retrospective study

Background

Everolimus combined with endocrine therapy has been proved to be effective among postmenopausal women with hormone receptor-positive human epidermal growth factor receptor-2 negative (HR+/HER2-) metastatic breast cancer (MBC). We aimed to evaluate the efficacy and safety of everolimus plus endocrine therapy in Chinese real-world practice for the first time, and investigate factors associated with efficacy.

Methods

Seventy-five HR+/HER2- MBC patients were included in this retrospective study who received everolimus plus endocrine therapy after progression on prior endocrine therapy in Fudan University Shanghai Cancer Center (FUSCC) between June 2013 and February 2016. Main outcome measures are progression free survival (PFS), overall survival (OS), objective response rate (ORR), clinical benefit rate (CBR) and safety profile.

Results

After a median follow up of 10.3 (range: 2.1-32.2) months, median PFS was 5.9 months (95%CI 4.6-7.2), and median OS was not reached. The CBR was 38.8% (95%CI, 26.8-50.8) and ORR was 9.0% (95%CI, 2.0-16.0). Most common all-grade adverse events were stomatitis (57.1%), fatigue (25.7%), infection (24.3%) and hyperglycemia (21.4%). The most common ≥3 grade adverse events were stomatitis (9.3 %) and thrombocytopenia (5.7%). No treatment-related death was documented during and one month after the drug administration.

Conclusions

The combination of everolimus and endocrine therapy proved to be effective in Chinese population. The safety profiles were similar to previous studies but incidences were lower. In conclusion, everolimus combined with endocrine therapy provides a reasonable option for Chinese HR+/HER2- metastatic breast cancer patients.

Inhibitors of PI3K/ERK1/2/p38 MAPK Show Preferential Activity Against Endocrine-Resistant Breast Cancer Cells

Current mainstream pharmacological options for the treatment of endocrine-resistant breast cancer have limitations in terms of their side effect profile and lack of discrimination between normal and cancer cells. In the current study, we assessed the responses of normal breast epithelial cells MCF10A, estrogen receptor-positive (ER⁺) MCF-7, and ER-silenced pII breast cancer cells to inhibitors (either individually or in combination) of downstream signaling molecules. The expression/activity of ERK1/2, p38 MAPK, and Akt was determined by Western blotting. Cell proliferation, motility, and invasion were determined using MTT, wound healing, and Matrigel assays, respectively. Morphological changes in response to variation in external pH were assessed by light microscopy. Our results demonstrated that the inhibitors of ERK1/2 (PD0325901), p38 MAPK (SB203580), and PI3K (LY294002) preferentially reduce breast cancer cell proliferation. In pII cells, they also reduced motility, invasion, and bleb formation induced by alkaline conditions. Combination treatment with lower concentrations of inhibitors was significantly more effective than single agents and was more effective against the cancer cell lines than the normal MCF10A. In contrast, the commonly used cytotoxic agent paclitaxel did not sufficiently discriminate between the MCF10A and the cancer cells. We concluded that combination therapy using ERK1/2 inhibitor and either p38 MAPK or PI3K inhibitor may provide a greater therapeutic benefit in treating breast cancer by specifically targeting cancer cells with lower doses of each drug than needed individually, potentially reducing unwanted side effects.

Acyl-CoA synthetase-4, a new regulator of mTOR and a potential therapeutic target for enhanced estrogen receptor function in receptor-positive and -negative breast cancer

Although the role of acyl-CoA synthetase 4 (ACSL4) in mediating an aggressive phenotype is well accepted, there is little evidence as to the early steps through which ACSL4 increases tumor growth and progression. In this study, and by means of the stable transfection of MCF-7 cells with ACSL4 using the tetracycline Tet-Off system (MCF-7 Tet-Off/ACSL4), we identify the mTOR pathway as one of the main specific signatures of ACSL4 expression and demonstrate the partial involvement of the lipoxygenase pathway in the activation of mTOR. The specificity of ACSL4 action on mTOR signaling is also determined by doxycycline inhibition of ACSL4 expression in MCF-7 Tet-Off/ACSL4 cells, by the expression of ACSL4 in the non-aggressive T47D breast cancer cell line and by knocking down this enzyme expression in the MDA-MB-231 breast cancer cells, which constitutively express ACSL4. ACSL4 regulates components of the two complexes of the mTOR pathway (mTORC1/2), along with upstream regulators and substrates.

We show that mTOR inhibitor rapamycin and ACSL4 inhibitor rosiglitazone can act in combination to inhibit cell growth. In addition, we demonstrate a synergistic effect on cell growth inhibition by the combination of rosiglitazone and tamoxifen, an estrogen receptor α (ERα) inhibitor. Remarkably, this synergistic effect is also evident in the triple negative MDA-MB-231 cells in vitro and in vivo.

These results suggest that ACSL4 could be a target to restore tumor hormone dependence in tumors with poor prognosis for disease-free and overall survival, in which no effective specifically targeted therapy is readily available.

miR-3188 regulates nasopharyngeal carcinoma proliferation and chemosensitivity through a FOXO1-modulated positive feedback loop with mTOR-p-PI3K/AKT-c-JUN

The biological role of miR-3188 has not yet been reported in the context of cancer. In this study, we observe that miR-3188 not only reduces cell-cycle transition and proliferation, but also significantly prolongs the survival time of tumour-bearing mice as well as sensitizes cells to 5-FU. Mechanistic analyses indicate that miR-3188 directly targets mTOR to inactivate p-PI3K/p-AKT/c-JUN and induces its own expression. This feedback loop further suppresses cell-cycle signalling through the p-PI3K/p-AKT/p-mTOR pathway. Interestingly, we also observe that miR-3188 direct targeting of mTOR is mediated by FOXO1 suppression of p-PI3K/p-AKT/c-JUN signalling. In clinical samples, reduced miR-3188 is an unfavourable factor and negatively correlates with mTOR and c-JUN levels but positively correlates with FOXO1 expression. Our studies demonstrate that as a tumour suppressor, miR-3188 directly targets mTOR to stimulate its own expression and participates in FOXO1-mediated repression of cell growth, tumorigenesis and NPC chemotherapy resistance.

Although miR-related mechanisms have been implicated in nasopharyngeal carcinoma (NPC), a precise role for miR-3188 has not been reported in this context. Here, Zhao et al. show that FOXO1-induced miR-3188 acts as a tumour suppressor in NPC by regulating the axis mTOR/PI3K/Akt/c-Jun.

Occurrence and characterization of everolimus adverse events during first and subsequent cycles in the treatment of metastatic breast cancer

PURPOSE

Endocrine therapy remains the standard therapy for patients with metastatic hormone receptor (HR)-positive breast cancer. The novel combination of everolimus and exemestane has been shown to prolong progression-free survival but with increased adverse events compared to exemestane alone. In this study, we aimed to describe the frequency and timing of everolimus dose reductions and/or interruptions due to adverse events.

METHODS

This is a single-center retrospective case series including all patients who received everolimus in combination with exemestane from May 1, 2012, through July 31, 2013. The primary objective was to determine the incidence of first-cycle interruptions or dose reductions with everolimus.

RESULTS

Forty-six patients were included in the analysis. First-cycle dose reductions or interruptions were observed in 21 (45.6 %) patients. The most common adverse events leading to dose reduction or interruption was stomatitis (57.1 %), fatigue (14.3 %), and diarrhea (14.3 %). The median time to dose reduction was 14 days, and the median duration of the interruption was 14 days. The median progression-free survival was 6.2 months, and the median time to treatment failure was 4.4 months.

CONCLUSIONS

In this case series, almost half of the patients treated with everolimus and exemestane required a dose reduction or interruption of everolimus during the first cycle of treatment. This early onset of adverse events requires thorough patient education and close clinical monitoring during the first 28 days of therapy.

Inhibition of Aerobic Glycolysis Represses Akt/mTOR/HIF-1α Axis and Restores Tamoxifen Sensitivity in Antiestrogen-Resistant Breast Cancer Cells

Tamoxifen resistance is often observed in the majority of estrogen receptor–positive breast cancers and it remains as a serious clinical problem in breast cancer management. Increased aerobic glycolysis has been proposed as one of the mechanisms for acquired resistance to chemotherapeutic agents in breast cancer cells such as adriamycin. Herein, we report that the glycolysis rates in LCC2 and LCC9—tamoxifen-resistant human breast cancer cell lines derived from MCF7— are higher than those in MCF7S, which is the parent MCF7 subline. Inhibition of key glycolytic enzyme such as hexokinase-2 resulted in cell growth retardation at higher degree in LCC2 and LCC9 than that in MCF7S. This implies that increased aerobic glycolysis even under O₂-rich conditions, a phenomenon known as the Warburg effect, is closely associated with tamoxifen resistance. We found that HIF-1α is activated via an Akt/mTOR signaling pathway in LCC2 and LCC9 cells without hypoxic condition. Importantly, specific inhibition of hexokinase-2 suppressed the activity of Akt/mTOR/HIF-1α axis in LCC2 and LCC9 cells. In addition, the phosphorylated AMPK which is a negative regulator of mTOR was decreased in LCC2 and LCC9 cells compared to MCF7S. Interestingly, either the inhibition of mTOR activity or increase in AMPK activity induced a reduction in lactate accumulation and cell survival in the LCC2 and LCC9 cells. Taken together, our data provide evidence that development of tamoxifen resistance may be driven by HIF-1α hyperactivation via modulation of Akt/mTOR and/or AMPK signaling pathways. Therefore, we suggest that the HIF-1α hyperactivation is a critical marker of increased aerobic glycolysis in accordance with tamoxifen resistance and thus restoration of aerobic glycolysis may be novel therapeutic target for treatment of tamoxifen-resistant breast cancer.

MicroRNA-125b upregulation confers aromatase inhibitor resistance and is a novel marker of poor prognosis in breast cancer

Introduction

Increasing evidence indicates that microRNAs (miRNAs) are important players in oncogenesis. Considering the widespread use of aromatase inhibitors (AIs) in endocrine therapy as a first-line treatment for postmenopausal estrogen receptor α–positive breast cancer patients, identifying deregulated expression levels of miRNAs in association with AI resistance is of utmost importance.

Methods

To gain further insight into the molecular mechanisms underlying the AI resistance, we performed miRNA microarray experiments using a new model of acquired resistance to letrozole (Res-Let cells), obtained by long-term exposure of aromatase-overexpressing MCF-7 cells (MCF-7aro cells) to letrozole, and a model of acquired anastrozole resistance (Res-Ana cells). Three miRNAs (miR-125b, miR-205 and miR-424) similarly deregulated in both AI-resistant cell lines were then investigated in terms of their functional role in AI resistance development and breast cancer cell aggressiveness and their clinical relevance using a cohort of 65 primary breast tumor samples.

Results

We identified the deregulated expression of 33 miRNAs in Res-Let cells and of 18 miRNAs in Res-Ana cells compared with the sensitive MCF-7aro cell line. The top-ranked Kyoto Encyclopedia of Genes and Genomes pathways delineated by both miRNA signatures converged on the AKT/mTOR pathway, which was found to be constitutively activated in both AI-resistant cell lines. We report for the first time, to our knowledge, that ectopic overexpression of either miR-125b or miR-205, or the silencing of miR-424 expression, in the sensitive MCF-7aro cell line was sufficient to confer resistance to letrozole and anastrozole, to target and activate the AKT/mTOR pathway and to increase the formation capacity of stem-like and tumor-initiating cells possessing self-renewing properties. Increasing miR-125b expression levels was also sufficient to confer estrogen-independent growth properties to the sensitive MCF-7aro cell line. We also found that elevated miR-125b expression levels were a novel marker for poor prognosis in breast cancer and that targeting miR-125b in Res-Let cells overcame letrozole resistance.

Conclusion

This study highlights that acquisition of specific deregulated miRNAs is a newly discovered alternative mechanism developed by AI-resistant breast cancer cells to achieve constitutive activation of the AKT/mTOR pathway and to develop AI resistance. It also highlights that miR-125b is a new biomarker of poor prognosis and a candidate therapeutic target in AI-resistant breast cancers.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-015-0515-1) contains supplementary material, which is available to authorized users.

Efficacy of everolimus with exemestane versus exemestane alone in Asian patients with HER2-negative, hormone-receptor-positive breast cancer in BOLERO-2

BACKGROUND

The addition of mTOR inhibitor everolimus (EVE) to exemestane (EXE) was evaluated in an international, phase 3 study (BOLERO-2) in patients with hormone-receptor-positive (HR(+)) breast cancer refractory to letrozole or anastrozole. The safety and efficacy of anticancer treatments may be influenced by ethnicity (Sekine et al. in Br J Cancer 99:1757-62, 2008). Safety and efficacy results from Asian versus non-Asian patients in BOLERO-2 are reported.

METHODS

Patients were randomized (2:1) to 10 mg/day EVE + EXE or placebo (PBO) + EXE. Primary endpoint was progression-free survival (PFS). Secondary endpoints included overall survival, response rate, clinical benefit rate, and safety.

RESULTS

Of 143 Asian patients, 98 received EVE + EXE and 45 received PBO + EXE. Treatment with EVE + EXE significantly improved median PFS versus PBO + EXE among Asian patients by 38 % (HR = 0.62; 95 % CI, 0.41-0.94). Median PFS was also improved among non-Asian patients by 59 % (HR = 0.41; 95 % CI, 0.33-0.50). Median PFS duration among EVE-treated Asian patients was 8.48 versus 4.14 months for PBO + EXE, and 7.33 versus 2.83 months, respectively, in non-Asian patients. The most common grade 3/4 adverse events (stomatitis, anemia, elevated liver enzymes, hyperglycemia, and dyspnea) occurred at similar frequencies in Asian and non-Asian patients. Grade 1/2 interstitial lung disease occurred more frequently in Asian patients. Quality of life was similar between treatment arms in Asian patients.

CONCLUSION

Adding EVE to EXE provided substantial clinical benefit in both Asian and non-Asian patients with similar safety profiles. This combination represents an improvement in the management of postmenopausal women with HR(+)/HER2(-) advanced breast cancer progressing on nonsteroidal aromatase inhibitors, regardless of ethnicity.

Mechanisms and therapeutic advances in the management of endocrine-resistant breast cancer

The estrogen receptor (ER) pathway plays a critical role in breast cancer development and progression. Endocrine therapy targeting estrogen action is the most important systemic therapy for ER positive breast cancer. However its efficacy is limited by intrinsic and acquired resistance. Mechanisms responsible for endocrine resistance include deregulation of the ER pathway itself, including loss of ER expression, post-translational modification of ER, deregulation of ER co-activators; increased receptor tyrosine kinase signaling leading to activation of various intracellular pathways involved in signal transduction, proliferation and cell survival, including growth factor receptor tyrosine kinases human epidermal growth factor receptor-2, epidermal growth factor receptor, PI3K/AKT/mammalian target of rapamycin (mTOR), Mitogen activated kinase (MAPK)/ERK, fibroblast growth factor receptor, insulin-like growth factor-1 receptor; alterations in cell cycle and apoptotic machinery; Epigenetic modification including dysregulation of DNA methylation, histone modification, and nucleosome remodeling; and altered expression of specific microRNAs. Functional genomics has helped us identify a catalog of genetic and epigenetic alterations that may be exploited as potential therapeutic targets and biomarkers of response. New treatment combinations targeting ER and such oncogenic signaling pathways which block the crosstalk between these pathways have been proven effective in preclinical models. Results of recent clinical studies suggest that subsets of patients benefit from the combination of inhibitor targeting certain oncogenic signaling pathway with endocrine therapy. Especially, inhibition of the mTOR signaling pathway, a key component implicated in mediating multiple signaling cascades, offers a promising approach to restore sensitivity to endocrine therapy in breast cancer. We systematically reviewed important publications cited in PubMed, recent abstracts from ASCO annual meetings and San Antonio Breast Cancer Symposium, and relevant trials registered at ClinicalTrials.gov. We present the molecular mechanisms contributing to endocrine resistance, in particular focusing on the biological rationale for the clinical development of novel targeted agents in endocrine resistant breast cancer. We summarize clinical trials utilizing novel strategies to overcome therapeutic resistance, highlighting the need to better identify the appropriate patients whose diseases are most likely to benefit from these specific strategies.

Clinical potential of novel therapeutic targets in breast cancer: CDK4/6, Src, JAK/STAT, PARP, HDAC, and PI3K/AKT/mTOR pathways

Breast cancers expressing estrogen receptor α, progesterone receptor, or the human epidermal growth factor receptor 2 (HER2) proto-oncogene account for approximately 90% of cases, and treatment with antiestrogens and HER2-targeted agents has resulted in drastically improved survival in many of these patients. However, de novo or acquired resistance to antiestrogen and HER2-targeted therapies is common, and many tumors will recur or progress despite these treatments. Additionally, the remaining 10% of breast tumors are negative for estrogen receptor α, progesterone receptor, and HER2 (“triple-negative”), and a clinically proven tumor-specific drug target for this group has not yet been identified. Therefore, the identification of new therapeutic targets in breast cancer is of vital clinical importance. Preclinical studies elucidating the mechanisms driving resistance to standard therapies have identified promising targets including cyclin-dependent kinase 4/6, phosphoinositide 3-kinase, poly adenosine diphosphate–ribose polymerase, Src, and histone deacetylase. Herein, we discuss the clinical potential and status of new therapeutic targets in breast cancer.

Endocrine resistance in breast cancer: focus on the phosphatidylinositol 3-kinase/akt/mammalian target of rapamycin signaling pathway

Breast cancer is the most common cancer among women. Up to 75% of breast cancers express the estrogen receptor (ER)α and/or the progesterone receptor (PR). Patients with hormone receptor-positive metastatic breast cancer are typically treated with endocrine therapy. Yet, not all patients with metastatic breast cancer respond to endocrine treatments and are considered to have primary (de novo) resistance. Furthermore, all patients who initially respond to endocrine treatment will eventually develop acquired resistance. Several mechanisms have been linked to the development of endocrine resistance, including reduced expression of ERα, altered regulation of the ER pathway, and activation of various growth factor signaling pathways, among them the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) signaling pathway. This pathway is involved in critical processes including cell survival, proliferation, and angiogenesis, and plays a central role in breast cancer development. Recent laboratory and clinical data implicate this pathway as mediating endocrine resistance, and agents directed against critical components of this pathway are either already approved for clinical use in breast cancer patients or are currently being tested in clinical trials. In this review, we describe the interaction between the PI3K/Akt/mTOR pathway and the ER cascade, its role in mediating endocrine resistance, and the clinical implications of this interaction.

Impact of dual mTORC1/2 mTOR kinase inhibitor AZD8055 on acquired endocrine resistance in breast cancer in vitro

Introduction

Upregulation of PI3K/Akt/mTOR signalling in endocrine-resistant breast cancer (BC) has identified mTOR as an attractive target alongside anti-hormones to control resistance. RAD001 (everolimus/Afinitor®), an allosteric mTOR inhibitor, is proving valuable in this setting; however, some patients are inherently refractory or relapse during treatment requiring alternative strategies. Here we evaluate the potential for novel dual mTORC1/2 mTOR kinase inhibitors, exemplified by AZD8055, by comparison with RAD001 in ER + endocrine resistant BC cells.

Methods

In vitro models of tamoxifen (TamR) or oestrogen deprivation resistance (MCF7-X) were treated with RAD001 or AZD8055 alone or combined with anti-hormone fulvestrant. Endpoints included growth, cell proliferation (Ki67), viability and migration, with PI3K/AKT/mTOR signalling impact monitored by Western blotting. Potential ER cross-talk was investigated by immunocytochemistry and RT-PCR.

Results

RAD001 was a poor growth inhibitor of MCF7-derived TamR and MCF7-X cells (IC₅₀ ≥1 μM), rapidly inhibiting mTORC1 but not mTORC2/AKT signalling. In contrast AZD8055, which rapidly inhibited both mTORC1 and mTORC2/AKT activity, was a highly effective (P <0.001) growth inhibitor of TamR (IC₅₀ 18 nM) and MCF7-X (IC₅₀ 24 nM), and of a further T47D-derived tamoxifen resistant model T47D-tamR (IC₅₀ 19 nM). AZD8055 significantly (P <0.05) inhibited resistant cell proliferation, increased cell death and reduced migration. Furthermore, dual treatment of TamR or MCF7-X cells with AZD8055 plus fulvestrant provided superior control of resistant growth versus either agent alone (P <0.05). Co-treating with AZD8055 alongside tamoxifen (P <0.01) or oestrogen deprivation (P <0.05) also effectively inhibited endocrine responsive MCF-7 cells. Although AZD8055 inhibited oestrogen receptor (ER) ser167 phosphorylation in TamR and MCF7-X, it had no effect on ER ser118 activity or expression of several ER-regulated genes, suggesting the mTOR kinase inhibitor impact was largely ER-independent. The capacity of AZD8055 for ER-independent activity was further evidenced by growth inhibition (IC₅₀18 and 20 nM) of two acquired fulvestrant resistant models lacking ER.

Conclusions

This is the first report demonstrating dual mTORC1/2 mTOR kinase inhibitors have potential to control acquired endocrine resistant BC, even under conditions where everolimus fails. Such inhibitors may prove of particular benefit when used alongside anti-hormonal treatment as second-line therapy in endocrine resistant disease, and also potentially alongside anti-hormones during the earlier endocrine responsive phase to hinder development of resistance.

The phosphoinositide-3-kinase-Akt-mTOR pathway as a therapeutic target in breast cancer

The phosphoinositide-3-kinase (PI3-kinase)-Akt-mTOR pathway is a central signal transduction pathway that regulates many critical aspects of normal and cancer physiology, including cell proliferation, apoptosis, cell morphology and migration, protein synthesis, and integration of metabolism. In breast cancer, somatic mutations that activate the pathway occur in more than 50% of tumors, underscoring the potentially broad impact of targeting the pathway for therapy. A vast body of preclinical data demonstrates the efficacy of pathway inhibition on tumor growth, and evidence also shows that activation of the pathway occurs in models of acquired resistance to hormonal therapy. This preclinical work led to the investigation of allosteric mTOR inhibitors, everolimus and temsirolimus, in metastatic hormone receptor-positive breast cancer. The recent BOLERO-2 trial comparing everolimus plus exemestane versus placebo plus exemestane in women with resistance to nonsteroidal aromatase inhibitors demonstrated a 6-month improvement in progression-free survival and led to FDA approval of everolimus for this indication in the United States. This landmark trial is the first demonstration of significant clinical benefit using drugs targeting this pathway in breast cancer. Many questions remain about the role of everolimus and other pathway-targeting drugs in clinical development in breast cancer treatment. This article reviews the role of the PI3-kinase-Akt-mTOR pathway in breast cancer biology and the clinical trial evidence available to date.

Combining mTOR Inhibitors with Chemotherapy and Other Targeted Therapies in Advanced Breast Cancer: Rationale, Clinical Experience, and Future Directions

Improvements in survival of patients with breast cancer have been attributed to the development of agents that target key components of dysregulated pathways involved in oncogenesis and progression of breast cancer. Aberrant mammalian target of rapamycin (mTOR) activation has been implicated in oncogenesis, angiogenesis, and the development of estrogen independence and resistance to chemotherapy in breast tumors. Several mTOR inhibitors (sirolimus, everolimus, temsirolimus, and ridaforolimus) have demonstrated antitumor activity in breast cancer cells. Combining mTOR inhibitors with endocrine therapies has demonstrated clinical antitumor activity in patients with metastatic breast cancer. In addition, mTOR inhibitor combinations with various targeted biologic agents or cytotoxic chemotherapeutic agents are being examined in more than 40 clinical trials with some early promising results. Combination therapies targeting multiple components of these central signaling pathways may be an optimal treatment strategy for patients with advanced breast cancer.

New and emerging treatments for estrogen receptor-positive breast cancer: focus on everolimus

Management of patients with metastatic hormone receptor-positive breast cancer poses a challenge due to the inevitable development of endocrine resistance. Hormone resistance is associated with a complex interaction of the estrogen receptor with growth factors, transmembrane receptors, and intracellular growth cascades. The PI3K/Akt/mTOR pathway plays a major role in hormone resistance and proliferation of breast cancer. Preclinical and clinical data indicate that inhibitors of human epidermal growth factor receptor-2, epidermal growth factor receptor, insulin-like growth factor-1 receptor, and the mammalian target of rapamycin pathway may act synergistically with hormone therapy to circumvent endocrine resistance. Everolimus is currently approved for combination with exemestane in postmenopausal women with advanced hormone receptor-positive breast cancer. However, we still need to unfold the full potential of targeted agents in the hormone-refractory setting and to identify the subsets of patients who will benefit from combination hormonal therapy using targeted agents.

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.

Activation of Akt, mTOR, and the estrogen receptor as a signature to predict tamoxifen treatment benefit

The frequent alterations of the PI3K/Akt/mTOR-growth signaling pathway are proposed mechanisms for resistance to endocrine therapy in breast cancer, partly through regulation of estrogen receptor α (ER) activity. Reliable biomarkers for treatment prediction are required for improved individualized treatment. We performed a retrospective immunohistochemical analysis of primary tumors from 912 postmenopausal patients with node-negative breast cancer, randomized to either tamoxifen or no adjuvant treatment. Phosphorylated (p) Akt-serine (s) 473, p-mTOR-s2448, and ER phosphorylations-s167 and -s305 were evaluated as potential biomarkers of prognosis and tamoxifen treatment efficacy. High expression of p-mTOR indicated a reduced response to tamoxifen, most pronounced in the ER+/progesterone receptor (PgR) + subgroup (tamoxifen vs. no tamoxifen: hazard ratio (HR), 0.86; 95 % confidence interval (CI), 0.31–2.38; P = 0.78), whereas low p-mTOR expression predicted tamoxifen benefit (HR, 0.29; 95 % CI, 0.18–0.49; P = 0.000002). In addition, nuclear p-Akt-s473 as well as p-ER at -s167 and/or -s305 showed interaction with tamoxifen efficacy with borderline statistical significance. A combination score of positive pathway markers including p-Akt, p-mTOR, and p-ER showed significant association with tamoxifen benefit (test for interaction; P = 0.029). Cross-talk between growth signaling pathways and ER-signaling has been proposed to affect tamoxifen response in hormone receptor-positive breast cancer. The results support this hypothesis, as an overactive pathway was significantly associated with reduced response to tamoxifen. A clinical pre-treatment test for cross-talk markers would be a step toward individualized adjuvant endocrine treatment with or without the addition of PI3K/Akt/mTOR pathway inhibitors.

Effectiveness and molecular interactions of the clinically active mTORC1 inhibitor everolimus in combination with tamoxifen or letrozole in vitro and in vivo

INTRODUCTION

Strategies to improve the efficacy of endocrine agents in breast cancer (BC) therapy and to delay the onset of resistance include concomitant targeting of the estrogen receptor alpha (ER) and the mammalian target of rapamycin complex 1 (mTORC1), which regulate cell-cycle progression and are supported by recent clinical results.

METHODS

BC cell lines expressing aromatase (AROM) and modeling endocrine-sensitive (MCF7-AROM1) and human epidermal growth factor receptor 2 (HER2)-dependent de novo resistant disease (BT474-AROM3) and long-term estrogen-deprived (LTED) MCF7 cells that had acquired resistance associated with HER2 overexpression were treated in vitro and as subcutaneous xenografts with everolimus (RAD001-mTORC1 inhibitor), in combination with tamoxifen or letrozole. End points included proliferation, cell-cycle arrest, cell signaling, and effects on ER-mediated transactivation.

RESULTS

Everolimus caused a concentration-dependent decrease in proliferation in all cell lines, which was associated with reductions in S6 phosphorylation. Everolimus plus letrozole or tamoxifen enhanced the antiproliferative effect and G1-accumulation compared with monotherapy, as well as increased phosphorylation (Ser10) and nuclear accumulation of p27 and pronounced dephosphorylation of Rb. Sensitivity was greatest to everolimus in the LTED cells but was reduced by added estrogen. Increased pAKT occurred in all circumstances with everolimus and, in the BT474 and LTED cells, was associated with increased pHER3. Decreased ER transactivation suggested that the effectiveness of everolimus might be partly related to interrupting cross-talk between growth-factor signaling and ER. In MCF7-AROM1 xenografts, letrozole plus everolimus showed a trend toward enhanced tumor regression, versus the single agents. In BT474-AROM3 xenografts, everolimus alone was equally effective at reducing tumor volume as were the combination therapies.

CONCLUSIONS

The results provide mechanistic support for recent positive clinical data on the combination of everolimus and endocrine therapy, as well as data on potential routes of escape via enhanced HER2/3 signaling. This merits investigation for further improvements in treatment efficacy.

The role of S6K1 in ER-positive breast cancer

The 40S ribosomal S6 kinase 1 (S6K1) is a conserved serine/threonine protein kinase that belongs to the AGC family of protein kinases, which also includes Akt and many others. S6K1 is the principal kinase effector downstream of the mammalian target of rapamycin complex 1 (mTORC1). S6K1 is sensitive to a wide range of signaling inputs, including growth factors, amino acids, energy levels and hypoxia. S6K1 relays these signals to regulate a growing list of substrates and interacting proteins in control of oncogenic processes, such as cell growth and proliferation, cell survival and apoptosis and cell migration and invasion. Several lines of evidence suggest an important role for S6K1 in estrogen receptor (ER)-positive breast cancer. S6K1 directly phosphorylates and activates ERα. Furthermore, S6K1 expression is estrogenically regulated. Therefore, hyperactivation of mTORC1/S6K1 signaling may be closely related to ER-positive status in breast cancer and may be utilized as a marker for prognosis and a therapeutic target.

Mammalian target of rapamycin expression in poorly differentiated endocrine carcinoma: clinical and therapeutic future challenges

While the mammalian target of rapamycin (mTOR) signaling pathway is a promising target for well-differentiated endocrine carcinoma therapy with the mTOR inhibitor everolimus (RAD001), poorly differentiated endocrine carcinomas (PDECs) are usually excluded from clinical trials due to their aggressiveness. So far, mTOR activity in PDECs has only been tested in cell lines. This study reviewed 36 mono-institutional PDECs to determine mTOR expression. Slides of normal kidney as positive control were used to optimize mTOR staining. To ensure antibody specificity, consecutive sections were incubated in the absence of primary antibody. Immunoreactivity was evaluated on a semi-quantitative scale scoring the extent and intensity of staining. The product of these two scores was used to obtain a total immunostaining score. The main primary site of disease was the pancreas, and 83% of patients had stage IV disease. In 80% of samples, mTOR expression was maintained at similar levels, with no relationship to tumor origin or proliferation rate determined by MIB-1. This study seems to demonstrate that mTOR is expressed in human PDECs regardless of tumor site. Its role in relation to the activity of everolimus in this subset of patients needs to be confirmed.

A gene expression signature of acquired chemoresistance to cisplatin and fluorouracil combination chemotherapy in gastric cancer patients

Background

We initiated a prospective trial to identify transcriptional alterations associated with acquired chemotherapy resistance from pre- and post-biopsy samples from the same patient and uncover potential molecular pathways involved in treatment failure to help guide therapeutic alternatives.

Methodology/Principal Findings

A prospective, high-throughput transcriptional profiling study was performed using endoscopic biopsy samples from 123 metastatic gastric cancer patients prior to cisplatin and fluorouracil (CF) combination chemotherapy. 22 patients who initially responded to CF were re-biopsied after they developed resistance to CF. An acquired chemotherapy resistance signature was identified by analyzing the gene expression profiles from the matched pre- and post-CF treated samples.

The acquired resistance signature was able to segregate a separate cohort of 101 newly-diagnosed gastric cancer patients according to the time to progression after CF. Hierarchical clustering using a 633-gene acquired resistance signature (feature selection at P<0.01) separated the 101 pretreatment patient samples into two groups with significantly different times to progression (2.5 vs. 4.7 months). This 633-gene signature included the upregulation of AKT1, EIF4B, and RPS6 (mTOR pathway), DNA repair and drug metabolism genes, and was enriched for genes overexpressed in embryonic stem cell signatures. A 72-gene acquired resistance signature (a subset of the 633 gene signature also identified in ES cell-related gene sets) was an independent predictor for time to progression (adjusted P = 0.011) and survival (adjusted P = 0.034) of these 101 patients.

Conclusion/Significance

This signature may offer new insights into identifying new targets and therapies required to overcome the acquired resistance of gastric cancer to CF.

The growth-inhibition effect of tamoxifen in the combination chemotherapeutics on the human cholangiocarcinoma cell line QBC939

In the individual application of adriamycin, mitomycin, vindesine and their combined application with tamoxifen for the pre-treatment of the human cholangiocarcinoma cell line QBC939, QBC939 was determined by MTT assay to investigate the inhibitive effect and its initial mechanism of TAM on cell growth. Growth cycle and apoptosis of each group were determined by flow cytometry. Concentration of ADM in QBC939 was detected by flow cytometry. The levels of their P-glycoprotein were detected by immunohistochemistry. The mRNA and protein levels of apoptotic-associated genes Bcl-2 and Bax were determined by western blot and real-time PCR. The inhibitive rates of adriamycin, mitomycin, vindesine to QBC939 and the apoptosis rates of QBC939 were enhanced after the pre-treatment of tamoxifen. Influence of tamoxifen in their growth cycle was not so obvious except vindesine group because of the increasing cell numbers of G (2)/M phase in which cells may be blocked. The contents of adriamycin in cells rose after the pre-treatment of tamoxifen. Expression level of the multi-drug resistant protein on cell surface was shown as (+). Furthermore, real-time PCR and Western blot analysis revealed an upregulation of Bcl-2 and a downregulation of Bax in QBC939 after the pre-treatment of tamoxifen. Therefore, tamoxifen may have the ability to enhance the relative sensitivity of QBC939 to chemotherapeutics.

Multiple roles and therapeutic implications of Akt signaling in cancer

The prominence of the PI3K-Akt signaling pathway in several tumors indicates a relationship with tumor grade and proliferation. Critical cellular processes are driven through this pathway. More detailed knowledge of the pathogenesis of tumors would enable us to design targeted drugs to block both membrane tyrosine kinase receptors and the intracellular kinases involved in the transmission of the signal. The newly approved molecular inhibitors sunitinib (an inhibitor of vascular endothelial growth factor receptor, platelet-derived growth factor receptor, and other tyrosine kinase receptors), sorafenib (a serine-threonine kinase inhibitor that acts against B-Raf) and temsirolimus (an mTOR inhibitor) shown clinical activity in advanced kidney cancer. Chronic myeloid leukemia has changed its natural history thanks to imatinib and dasatinib, both of which inhibit the intracellular bcr/abl protein derived from the alteration in the Philadelphia chromosome. Intracellular pathways are still important in cancer development and their blockade directly affects outcome. Cross-talk has been observed but is not well understood. Vertical and horizontal pathway blockade are promising anticancer strategies. Indeed, preclinical and early clinical data suggest that combining superficial and intracellular blocking agents can synergize and leverage single-agent activity. The implication of the Akt signaling pathway in cancer is well established and has led to the development of new anticancer agents that block its activation.