Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR Cascade Inhibitors: How Mutations Can Result in Therapy Resistance and How to Overcome Resistance

The Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades are often activated by genetic alterations in upstream signaling molecules such as receptor tyrosine kinases (RTK). Targeting these pathways is often complex and can result in pathway activation depending on the presence of upstream mutations (e.g., Raf inhibitors induce Raf activation in cells with wild type (WT) RAF in the presence of mutant, activated RAS) and rapamycin can induce Akt activation. Targeting with inhibitors directed at two constituents of the same pathway or two different signaling pathways may be a more effective approach. This review will first evaluate potential uses of Raf, MEK, PI3K, Akt and mTOR inhibitors that have been investigated in pre-clinical and clinical investigations and then discuss how cancers can become insensitive to various inhibitors and potential strategies to overcome this resistance.     

Mutations and Deregulation of Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR Cascades Which Alter Therapy Response

The Ras/Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR cascades are often activated by genetic alterations in upstream signaling molecules such as receptor tyrosine kinases (RTK). Certain components of these pathways, RAS, NF1, BRAF, MEK1, DUSP5, PP2A, PIK3CA, PIK3R1, PIK3R4, PIK3R5, IRS4, AKT, NFKB1, MTOR, PTEN, TSC1, and TSC2 may also be activated/inactivated by mutations or epigenetic silencing. Upstream mutations in one signaling pathway or even in downstream components of the same pathway can alter the sensitivity of the cells to certain small molecule inhibitors. These pathways have profound effects on proliferative, apoptotic and differentiation pathways. Dysregulation of components of these cascades can contribute to: resistance to other pathway inhibitors, chemotherapeutic drug resistance, premature aging as well as other diseases. This review will first describe these pathways and discuss how genetic mutations and epigenetic alterations can result in resistance to various inhibitors.     

Cooperative effects of Akt-1 and Raf-1 on the induction of cellular senescence in doxorubicin or tamoxifen treated breast cancer cells

Escape from cellular senescence induction is a potent mechanism for chemoresistance. Cellular senescence can be induced in breast cancer cell lines by the removal of estrogen signaling with tamoxifen or by the accumulation of DNA damage induced by the chemotherapeutic drug doxorubicin. Long term culturing of the hormone-sensitive breast cancer cell line MCF-7 in doxorubicin (MCF-7/DoxR) reduced the ability of doxorubicin, but not tamoxifen, to induce senescence. Two pathways that are often upregulated in chemo- and hormonal-resistance are the PI3K/PTEN/Akt/mTOR and Ras/Raf/MEK/ERK pathways. To determine if active Akt-1 and Raf-1 can influence drug-induced senescence, we stably introduced activated ΔAkt-1(CA) and ΔRaf-1(CA) into drug-sensitive and doxorubicin-resistant cells. Expression of a constitutively-active Raf-1 construct resulted in higher baseline senescence, indicating these cells possessed the ability to undergo oncogene-induced-senescence. Constitutive activation of the Akt pathway significantly decreased drug-induced senescence in response to doxorubicin but not tamoxifen in MCF-7 cells. However, constitutive Akt-1 activation in drug-resistant cells containing high levels of active ERK completely escaped cellular senescence induced by doxorubicin and tamoxifen. These results indicate that up regulation of the Ras/PI3K/PTEN/Akt/mTOR pathway in the presence of elevated Ras/Raf/MEK/ERK signaling together can contribute to drug-resistance by diminishing cell senescence in response to chemotherapy. Understanding how breast cancers containing certain oncogenic mutations escape cell senescence in response to chemotherapy and hormonal based therapies may provide insights into the design of more effective drug combinations for the treatment of breast cancer.     

Synthetic lethal interaction between PI3K/Akt/mTOR and Ras/MEK/ERK pathway inhibition in rhabdomyosarcoma

Rhabdomyosarcoma (RMS) frequently exhibits concomitant activation of the PI3K/Akt/mTOR and the Ras/MEK/ERK pathways. Therefore, we investigated whether pharmacological cotargeting of these two key survival pathways suppresses RMS growth. Here, we identify a synthetic lethal interaction between PI3K/Akt/mTOR and Ras/MEK/ERK pathway inhibition in RMS. The dual PI3K/mTOR inhibitor PI103 and the MEK inhibitor UO126 synergize to trigger apoptosis in several RMS cell lines in a highly synergistic manner (combination index <0.1), whereas either agent alone induces minimal cell death. Similarly, genetic knockdown of p110α and MEK1/2 cooperates to induce apoptosis. Molecular studies reveal that cotreatment with PI103/UO126 cooperates to suppress PI3K/Akt/mTOR and Ras/MEK/ERK signaling, whereas either compound alone is not only less effective to inhibit signaling, but even cross-activates the other pathway. Accordingly, PI103 alone increases ERK phosphorylation, while UO126 enhances Akt phosphorylation, consistent with negative crosstalks between these two signaling pathways. Furthermore, PI103/UO126 cotreatment causes downregulation of several antiapoptotic proteins such as XIAP, Bcl-x_L and Mcl-1 as well as increased expression and decreased phosphorylation of the proapoptotic protein Bim_EL, thus shifting the balance towards apoptosis. Consistently, PI103/UO126 cotreatment cooperates to trigger Bax activation, loss of mitochondrial membrane potential, caspase activation and caspase-dependent apoptosis. This identification of a synthetic lethal interaction between PI3K/mTOR and MEK inhibitors has important implications for the development of novel treatment strategies in RMS.     

Interactions between the PI3K and Raf signaling pathways can result in the transformation of hematopoietic cells.

The PI3K/Akt and Raf/MEK/ERK signal transduction cascades are pivotal in transmitting signals from membrane receptors to downstream targets that regulate apoptosis, gene expression, and cell growth. The abilities of activated PI3K, Akt, Raf, and MEK proteins to abrogate the cytokine dependence of three different hematopoietic cell lines were determined. Activated PI3K or Akt expression by themselves did not efficiently annul cytokine dependence. Raf and MEK could abrogate the cytokine dependence of murine FDC-PI and human TF-1 cells; however, the frequency of transformation was dependent on the particular oncogene examined, as more factor-independent cells were isolated after infection with activated retroviruses encoding A-Raf or Raf-1 than were with MEK1 or B-Raf. Cytokine-independent deltaRaf-1-infected cells formed tumors on injection into immunocompromised mice, whereas cytokine-dependent cell lines did not, demonstrating the oncogenic effects of activation of the Raf/MEK/ERK pathway. Overexpression of the antiapoptotic Bcl-2 protein synergized with activation of the Raf/MEK/ERK cascade and increased the efficiency of transformation of FDC-PI and TF-1 cells. In contrast to the results observed with FDC-P1 and TF-I cells, the activated Raf genes did not relieve the cytokine dependence of murine FL5.12 cells. The abilities of the Raf and PI3K pathways to interact and annul the cytokine dependence of FL5.12 cells were determined. The combination of Raf and either PI3K or Akt expression relieved cytokine dependence of some FL5.12 cells, and the efficiency of transformation could be enhanced further by Bcl-2 or Bcl-XL overexpression. Thus, the antiapoptotic PI3K/Akt and Bcl-2/Bcl-XL proteins can interact with the growth-promoting Raf/MEK/ERK pathway and annul the cytokine dependence of certain hematopoietic cells.     

Differential effects of kinase cascade inhibitors on neoplastic and cytokine-mediated cell proliferation.

The Raf/MEK/ERK and PI3K/Akt pathways regulate proliferation and prevent apoptosis, and their altered expression is commonly observed in human cancer due to the high mutation frequency of upstream regulators. In this study, the effects of Raf, MEK, and PI3K inhibitors on conditionally transformed hematopoietic cells were examined to determine if they would display cytotoxic differences between cytokine- and oncogene-mediated proliferation, and whether inhibition of both pathways was a more effective means to induce apoptosis. In the hematopoietic model system employed, proliferation was conditional and occurred when either interleukin-3 (IL-3) or the estrogen receptor antagonist 4-hydroxytamoxifen (4HT), which activates the conditional oncoprotein (DeltaRaf:ER), were provided. Thus, upon the addition of the signal transduction inhibitors and either IL-3 or 4HT, the effects of these drugs were examined in the same cell under 'cytokine-' and 'oncoprotein' -mediated growth conditions avoiding genetic and differentiation stage heterogeneity. At drug concentrations around the reported IC(50) for the Raf inhibitor L-779,450, it suppressed DNA synthesis and induced apoptosis in hematopoietic FDC-P1 cells transformed to grow in response to either Raf-1 or A-Raf (FD/DeltaRaf-1:ER and FD/DeltaA-Raf:ER), but it displayed less effects on DNA synthesis and apoptosis when the cells were cultured in IL-3. This Raf inhibitor was less effective on B-Raf- or MEK1-responsive cells, demonstrating the specificity of this drug. MEK inhibitors also suppressed DNA synthesis and induced apoptosis in Raf-responsive cells and the effects were more significant on Raf-responsive compared to cytokine-mediated growth. The PI3K inhibitor LY294002 suppressed Raf-mediated growth, indicating that part of the long-term proliferative effects mediated by Raf are PI3K dependent. Simultaneous inhibition of both Raf/MEK/ERK and PI3K/Akt pathways proved a more efficient means to suppress DNA synthesis and induce apoptosis at lower drug concentrations.     

Cisplatin activates survival signals in UM-SCC-23 squamous cell carcinoma and these signal pathways are amplified in cisplatin-resistant squamous cell carcinoma

cis-Diaminodichloroplatinum (II) (cisplatin) is one of the most effective anticancer drugs and is widely used for the treatment of squamous cell carcinoma (SCC). However, its efficacy is often limited due to the development of resistance. Although several factors implicated in cisplatin resistance have been identified, the resistance mechanisms in detail are not fully understood yet. In the present study, we have examined the implication of survival signaling pathways in cisplatin-resistance. Cisplatin induced activation of Ras and its downstream effector kinases, Raf/MEK/ERK in UM-SCC-23 human squamous cell carcinoma, suggesting that this anticancer drug activates survival signal pathway in addition to apoptosis signals. In cisplatin-resistant UM-SCC-23 in culture, which we have established, the protein levels of Ras, Raf-1 and MEK were drastically elevated compared to parent UM-SCC-23, and ERK and Akt signals were constitutively activated. U0126, an inhibitor for MEK and LY294002, an inhibitor for phosphatidylinositol 3-kinase (PI3K), sensitized resistant UM-SCC-23 to cisplatin-induced cell death. These results indicate that Raf/MEK/ERK and PI3K/Akt signal cascades may play a considerable role in cisplatin resistance in SCC.     

Role of dual PI3/Akt and mTOR inhibition in Waldenstrom's Macroglobulinemia

Tumorigenesis occurs due to synergistic interactions from a complex of signal transduction processes, including multiple onco-proteins and tumor suppressors such as Ras, Myc, PI3K/Akt/mTOR, Her-2/Neu, p53 and PTEN. Specifically, the PI3K/Akt and mTOR pathways have been shown to play a pivotal role on the initiation and progression of malignancies, enhancing cell survival by stimulating cell proliferation, and inhibiting apoptosis. Therefore, it is critical to examine therapeutic agents that explicitly target both the PI3K/Akt and mTOR signaling cascades in diseases, such as Waldenstrom Macroglobulinemia (WM), that harbor activation of the PI3K/Akt pathway. We demonstrated that dual targeting of the PI3K and mTOR pathways by the novel inhibitor NVP-BEZ235, exhibited toxicity on WM cells by directly targeting the tumor clone and indirectly through an effect on the bone marrow milieu. These findings suggest that dual targeting of the PI3K and mTOR pathways is a better modality of targeted therapy for tumors that harbor activation of the PI3K/mTOR pathways, such as in WM.     

Elevated levels of ornithine decarboxylase cooperate with Raf/ERK activation to convert normal keratinocytes into invasive malignant cells

Ornithine decarboxylase (ODC) overexpression coupled with activated Ras is fully sufficient to oncogenically transform primary keratinocytes. To determine the Ras effector pathways that represent the minimal essential contribution to full oncogenic transformation in this context, we evaluated the cooperativity of different Ras effector mutants with overexpressed ODC in an in vivo tracheal xenotransplantation assay for epithelial cell invasiveness. Primary keratinocytes, isolated from either K6/ODC transgenic mouse skin (expressing increased ODC) or from normal littermate skin were infected with retrovirus producing an activated RasV12 or partial loss-of-function effector mutants of RasV12 that selectively induce only the Raf/ERK, RalGDS, or the PI3-kinase signaling pathway. Whereas keratinocytes expressing a fully activated RasV12 are not invasive in tracheal xenotransplants, ODC-overexpressing keratinocytes acquire an invasive phenotype with additional expression of either RasV12 or activation of the Raf/ERK pathway. Independent of a mutated ras, elevated levels of ODC activate the Akt/mTOR signaling pathway as well as the Rho/Rac pathway in primary keratinocytes. Thus, Raf/ERK signaling is sufficient to cooperate with increased ODC activity in the conversion of normal keratinocytes to invasive cells. In order to promote invasiveness in keratinocytes, elevated levels of ODC may cooperate with Raf/ERK via activation of the Akt and Rho/Rac signaling pathway.     

Oridonin in combination with imatinib exerts synergetic anti-leukemia effect in Ph+ acute lymphoblastic leukemia cells in vitro by inhibiting activation of LYN/mTOR signaling pathway

Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph+ ALL) is triggered by constitutively activated BCR-ABL and SRC family tyrosine kinases.They account for the activations of multiple growth-signaling pathways, including Raf/MEK/ERK, Akt/mTOR and STAT5 pathways. The BCR-ABL tyrosine kinase inhibitor imatinib is the standard treatment for Ph+ leukemia and plays efficacious role in CML. However, imatinib has few inhibitory effects on SRC tyrosine kinase with response rate of Ph+ ALL lower and relapse more frequent and quicker compared with CML. Previous studies showed that oridonin inhibits proliferation and induces apoptosis in many tumor cells. However, the anticancer activity and mechanism of oridonin in Ph+ ALL is unknown. To investigate the anticancer activity of oridonin, we examined its role in constitutively activated Akt/mTOR, Raf/MEK/ERK, STAT5 and SRC pathway, mRNA level of bcr/abl gene, cell viability and apoptosis in Ph+ ALL SUP-B15 cells. Furthermore, we detected synergetic effect of oridonin plus imatinib. Our results showed that oridonin inhibiting activations of LYN (one of SRC family kinases) and ABL and their downstream Akt/mTOR, Raf/MEK/ERK and STAT5 pathways, downregulated Bcl-2 but upregulated Bax protein and then induced apoptosis in Ph+ ALL cells. Oridonin plus imatinib exerted synergetic effects by overcoming imatinib defect of upregulating Akt/mTOR and LYN signaling. Additionally, we examined the effect of oridonin on the signaling pathways in the primary specimens from Ph+ ALL patients. Our data showed that oridonin remarkably suppressed activations of Akt/mTOR, Raf/MEK and STAT5 pathway in these primary specimens and oridonin with imatinib exerted synergetic suppressive effects on mTOR, STAT5 and LYN signaling in one imatinib resistant patient specimen. Additional evaluation of oridonin as a potential therapeutic agent for Ph+ ALL seems warranted.     

Nitric oxide in physiologic concentrations targets the translational machinery to increase the proliferation of human breast cancer cells: involvement of mammalian target of rapamycin/eIF4E pathway

Nitric oxide (NO) in nanomolar (nmol/L) concentrations is consistently detected in tumor microenvironment and has been found to promote tumorigenesis. The mechanism by which NO enhances tumor progression is largely unknown. In this study, we investigated the possible mechanisms and identified cellular targets by which NO increases proliferation of human breast cancer cell lines MDA-MB-231 and MCF-7. DETA-NONOate, a long acting NO donor, with a half-life of 20 h, was used. We found that NO (nmol/L) dramatically increased total protein synthesis in MDA-MB-231 and MCF-7 and also increased cell proliferation. NO specifically increased the translation of cyclin D1 and ornithine decarboxylase (ODC) without altering their mRNA levels or half-lives. Critical components in the translational machinery, such as phosphorylated mammalian target of rapamycin (mTOR) and its downstream targets, phosphorylated eukaryotic translation initiation factor and p70 S6 kinase, were up-regulated following NO treatment, and inhibition of mTOR with rapamycin attenuated NO induced increase of cyclin D1 and ODC. Activation of translational machinery was mediated by NO-induced up-regulation of the Raf/mitogen-activated protein/extracellular signal-regulated kinase (ERK) kinase/ERK (Raf/MEK/ERK) and phosphatidylinositol 3-kinase (PI-3 kinase)/Akt signaling pathways. Up-regulation of the Raf/MEK/ERK and PI-3 kinase/Akt pathways by NO was found to be mediated by activation of Ras, which was cyclic guanosine 3',5'-monophosphate independent. Furthermore, inactivation of Ras by farnesyl transferase inhibitor or K-Ras small interfering RNA attenuated NO-induced increase in proliferation signaling and cyclin D1 and ODC translation, further confirming the involvement of Ras activation during NO-induced cell proliferation.     

Molecular targeting for malignant gliomas (Review)

With tendency to invade rapidly in the brain, malignant gliomas are very resistant to conventional therapies including radiation and chemotherapy. Recent advances in genetic and molecular techniques have made it possible to define characteristic molecular profiles of malignant gliomas. Based on the list of the molecules closely related to glioblastoma tissues, we reviewed strategies targeting them. Target molecules extensively studied include EGFR, PTEN, telomerase and signal pathway modulators for Ras/Raf/MAPK and PI3K/Akt/mTOR pathways. Therapies targeting specific molecules may result in killing tumor cells effectively while keeping normal cells intact.     

Requirement for the PI3K/Akt pathway in MEK1-mediated growth and prevention of apoptosis: identification of an Achilles heel in leukemia.

The Raf/MEK/ERK kinase cascade plays a critical role in transducing growth signals from activated cell surface receptors. Using DeltaMEK1:ER, a conditionally active form of MEK1 which responds to either beta-estradiol or the estrogen receptor antagonist 4 hydroxy-tamoxifen (4HT), we previously documented the ability of this dual specificity protein kinase to abrogate the cytokine-dependency of human (TF-1) and murine (FDC-P1 and FL5.12) hematopoietic cells lines. Here we demonstrate the ability of DeltaMEK1:ER to activate the phosphatidylinositol 3-kinase (PI3K)/Akt/p70 ribosomal S6 kinase (p70(S6K)) pathway and the importance of this pathway in MEK1-mediated prevention of apoptosis. MEK1-responsive cells can be maintained long term in the presence of beta-estradiol, 4HT or IL-3. Removal of hormone led to the rapid cessation of cell proliferation and the induction of apoptosis in a manner similar to cytokine deprivation of the parental cells. Stimulation of DeltaMEK1:ER by 4HT resulted in ERK, PI3K, Akt and p70(S6K) activation. Treatment with PI3K, Akt and p70(S6K) inhibitors prevented MEK-responsive growth. Furthermore, the apoptotic effects of PI3K/Akt/p70(S6K) inhibitors could be enhanced by cotreatment with MEK inhibitors. Use of a PI3K inhibitor and a constitutively active form of Akt, [DeltaAkt(Myr(+))], indicated that activation of PI3K was necessary for MEK1-responsive growth and survival as activation of Akt alone was unable to compensate for the loss of PI3K activity. Cells transduced by MEK or MEK+Akt displayed different sensitivities to signal transduction inhibitors, which targeted these pathways. These results indicate a requirement for the activation of the PI3K pathway during MEK-mediated transformation of certain hematopoietic cells. These experiments provide important clues as to why the identification of mutant signaling pathways may be the Achilles heel of leukemic cell growth. Leukemia treatment targeting multiple signal transduction pathways may be more efficacious than therapy aimed at inhibiting a single pathway.     

Deregulation of the EGFR/PI3K/PTEN/Akt/mTORC1 pathway in breast cancer: possibilities for therapeutic intervention

The EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway plays prominent roles in malignant transformation, prevention of apoptosis, drug resistance and metastasis. The expression of this pathway is frequently altered in breast cancer due to mutations at or aberrant expression of: HER2, ERalpha, BRCA1, BRCA2, EGFR1, PIK3CA, PTEN, TP53, RB as well as other oncogenes and tumor suppressor genes. In some breast cancer cases, mutations at certain components of this pathway (e.g., PIK3CA) are associated with a better prognosis than breast cancers lacking these mutations. The expression of this pathway and upstream HER2 has been associated with breast cancer initiating cells (CICs) and in some cases resistance to treatment. The anti-diabetes drug metformin can suppress the growth of breast CICs and herceptin-resistant HER2+ cells. This review will discuss the importance of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway primarily in breast cancer but will also include relevant examples from other cancer types. The targeting of this pathway will be discussed as well as clinical trials with novel small molecule inhibitors. The targeting of the hormone receptor, HER2 and EGFR1 in breast cancer will be reviewed in association with suppression of the EGFR/PI3K/PTEN/Akt/mTORC1/GSK-3 pathway.     

Enhanced cytotoxicity induced by gefitinib and specific inhibitors of the Ras or phosphatidyl inositol-3 kinase pathways in non-small cell lung cancer cells

In this study, we have characterized a panel of NSCLC cell lines with differential sensitivity to gefitinib for activating mutations in egfr, pik3ca, and k-ras, and basal protein expression levels of PTEN. The egfr mutant NSCLC cell line H1650 as well as the egfr wild type cell lines H292 and A431 were highly sensitive to gefitinib treatment, indicating that other factors determine gefitinib-sensitivity in egfr wild type cells. Activating k-ras mutations were specifically detected in gefitinib-resistant cells, suggesting that the occurrence of k-ras mutations is correlated with resistance to EGFR antagonists. No pik3ca mutations were detected within the panel of cell lines, and PTEN protein expression levels did not correlate with gefitinib sensitivity. Gefitinib effectively blocked Akt and Erk phosphorylation in two gefitinib-sensitive NSCLC cell lines, further supporting our previous findings that persistent activity of the PI3K/Akt and/or Ras/Erk pathways is associated with gefitinib-resistance of NSCLC cell lines. Gefitinib-resistant NSCLC cell lines, showing EGFR-independent activity of the PI3K/Akt or Ras/Erk pathways, were treated with gefitinib in combination with specific inhibitors of mTOR, P13K, Ras, and MEK. Additive cytotoxicity was observed in A549 cells co-treated with gefitinib and the MEK inhibitor U0126 or the farnesyl transferase inhibitor SCH66336 and in H460 cells treated with gefitinib and the PI3K inhibitor LY294002, but not in H460 cells treated with gefitinib and rapamycin. These data suggest that combination treatment of NSCLC cells with gefitinib and specific inhibitors of the PI3K/Akt and Ras/Erk pathways may provide a successful strategy.     

PI3 kinase integrates Akt and MAP kinase signaling pathways in the regulation of prostate cancer

PI3 kinase (PI3K), Akt and MAP kinase (MAPK) pathways are central to many classical signaling cascades and are often de-regulated in many cancers. Due to this, inhibitors for a number of key signaling molecules in these pathways such as PI3K, Akt, mTOR, Raf and ERK are currently in clinical trials. In the current study, we investigated the effects of specific inhibition of these signaling molecules, alone or in combinations, on prostate cancer cells. Our study showed that integration of Akt-mTOR and MAPK signaling by PI3K was essential for the EGF-stimulated TRAMP cell migration, proliferation, survival and invasion as well as PC3 and LNCaP C4-2 (C4-2) colony/foci formation. Adenovirus-mediated expression of constitutively active Akt (Ad-myrAkt) in PC3 cells resulted in significant increase in number of foci. Even though PI3K inhibition significantly reduced foci formed by C4-2 cells, none of the Akt, ERK or mTOR inhibitors showed any significant inhibition. This indicated that functional redundancies and/or feed back loops between Akt-mTOR and MAPK signaling exist in prostate cancer. Further studies on cotargeting these signaling molecules revealed that combined inhibition of Akt (or mTOR) and ERK, but not Akt and mTOR, resulted in significant reduction in number of foci formed by the C4-2 cells. Overall, our study demonstrated that the effects of PI3K-mediated prostate cancer growth necessitates a synergism between the Akt and MAPK pathways and suggests cotargeting Akt (or mTOR) and MAPK as an effective method for prostate cancer therapeutic interventions.     

Targeting Mnks for cancer therapy

Deregulation of protein synthesis is a common event in human cancer and a key player in translational control is eIF4E. Elevated expression levels of eIF4E promote cancer development and progression. Recent findings suggest that eIF4E activity is a key determinant of the PI3K/Akt/mTOR and Ras/Raf/MEK/ERK mediated tumorigenic activity and targeting eIF4E should have a major impact on these pathways in human cancer. The function of eIF4E is modulated through phosphorylation of a conserved serine (Ser209) by Mnk1 and Mnk2 downstream of ERK. While the phosphorylation event is necessary for oncogenic transformation, it seems to be dispensable for normal development. Hence, pharmacologic Mnk inhibitors may provide non-toxic and effective anti-cancer strategy. Strong circumstantial evidence indicates that Mnk inhibition presents attractive therapeutic potential, but the lack of selective Mnk inhibitors has so far confounded pharmacological target validation and clinical development.     

GSK-3 as potential target for therapeutic intervention in cancer

The serine/threonine kinase glycogen synthase kinase-3 (GSK-3) was initially identified and studied in the regulation of glycogen synthesis. GSK-3 functions in a wide range of cellular processes. Aberrant activity of GSK-3 has been implicated in many human pathologies including: bipolar depression, Alzheimer''s disease, Parkinson''s disease, cancer, non-insulin-dependent diabetes mellitus (NIDDM) and others. In some cases, suppression of GSK-3 activity by phosphorylation by Akt and other kinases has been associated with cancer progression. In these cases, GSK-3 has tumor suppressor functions. In other cases, GSK-3 has been associated with tumor progression by stabilizing components of the beta-catenin complex. In these situations, GSK-3 has oncogenic properties. While many inhibitors to GSK-3 have been developed, their use remains controversial because of the ambiguous role of GSK-3 in cancer development. In this review, we will focus on the diverse roles that GSK-3 plays in various human cancers, in particular in solid tumors. Recently, GSK-3 has also been implicated in the generation of cancer stem cells in various cell types. We will also discuss how this pivotal kinase interacts with multiple signaling pathways such as: PI3K/PTEN/Akt/mTORC1, Ras/Raf/MEK/ERK, Wnt/beta-catenin, Hedgehog, Notch and others.