P21(Cip1) induced by Raf is associated with increased Cdk4 activity in hematopoietic cells

Unraveling the interplay between RAS/RAF/MEK/ERK signaling pathway and autophagy in cancer: From molecular mechanisms to targeted therapy

RAS/RAF/MEK/ERK signaling pathway is one of the most important pathways of Mitogen-activated protein kinases (MAPK), which widely participate in regulating cell proliferation, differentiation, apoptosis and signaling transduction. Autophagy is an essential mechanism that maintains cellular homeostasis by degrading aged and damaged organelles. Recently, some studies revealed RAS/RAF/MEK/ERK signaling pathway is closely related to autophagy regulation and has a dual effect in tumor cells. However, the specific mechanism by which RAS/RAF/MEK/ERK signaling pathway participates in autophagy regulation is not fully understood. This article provides a comprehensive review of the research progress with regard to the RAS/RAF/MEK/ERK signaling pathway and autophagy, as well as their interplay in cancer therapy. The impact of small molecule inhibitors that target the RAS/RAF/MEK/ERK signaling pathway on autophagy is discussed in this study. The advantages and limitations of the clinical combination of these small molecule inhibitors with autophagy inhibitors are also explored. The findings from this study may provide additional perspectives for future cancer treatment strategies.

ZNF424 Induces Apoptosis and Inhibits Proliferation in Lung Carcinoma Cells

Background:

Previously, we showed that the Zinc finger-containing transcription factor ZNF424 inhibits p21 transcription, which has been widely associated with various cancers. However, because the roles of ZNF424 in tumorigenesis have not been characterized, we correlated ZNF424 expression with tumorigenesis in lung cancer.

Results:

The present immunohistochemical analyses show significantly lower ZNF424 expression levels in 43 of 60 lung cancer tissues compared with adjacent tissues. Moreover, flow cytometry assays indicated that overexpression of ZNF424 induces apoptosis in A549 human lung carcinoma cells, and overexpression of ZNF424 significantly increases numbers of G1 phase cells and decreases numbers of S phase cells, suggesting that ZNF424 inhibits proliferation. Western Blot analyses show that overexpression of ZNF424 decreases protein expression levels of the mitogen-activated protein kinase (MAPK) signaling proteins P-P38 and P-ERK in A549 cells.

Conclusion:

These are the first data to associate ZNF424 with tumorigenesis and demonstrate an inhibitory role in lung cancer, indicating the potential of ZNF424 expression as a diagnostic marker of lung tumorigenesis.

Sorafenib inhibited cell growth through the MEK/ERK signaling pathway in acute promyelocytic leukemia cells

The present study assessed the mechanism underlying the effect of sorafenib on the proliferation and apoptosis of the acute promyelocytic leukemia (APL) cell line NB4. NB4 cells were treated with different concentrations of sorafenib (0, 1.5, 3, 6, and 12 µM) for 24, 48 and 72 h. Cell proliferation, cell cycle, and apoptosis were analyzed using an MTT assay and flow cytometry analysis, respectively. Reverse transcription-semi-quantitative polymerase chain reaction and western blot analysis were performed to assess the expression of caspase-3, caspase-8, myeloid cell leukemia (MCL)1, cyclin D1, mitogen-activated protein kinase (MEK), phosphorylated (P)-MEK, extracellular signal-regulated kinase (ERK) and P-ERK. The results of the MTT assay demonstrated that, compared with untreated cells, the proliferation of sorafenib-treated NB4 cells was inhibited dose- and time-dependently. Furthermore, cell cycle arrest was induced in the G0/G1 phase and cell apoptosis was promoted in a dose-dependent manner in sorafenib-treated NB4 cells compared with untreated cells. In addition, the expression of the proapoptotic molecules caspase-3 and caspase-8 was significantly upregulated, and the expression of the antiapoptotic molecule MCL1 and the cell cycle-associated cyclin D1 was downregulated in sorafenib-treated NB4 cells compared with untreated cells. Furthermore, the phosphorylation of MEK and ERK was inhibited in sorafenib-treated NB4 cells compared with untreated cells. Sorafenib may inhibit proliferation and induce cell cycle arrest and apoptosis in APL cells. The underlying mechanisms of such effects may be associated with alterations to the expression of apoptosis-associated and cell cycle-associated molecules via MEK/ERK signaling pathway inhibition.

MAP kinases and the inflammatory signaling cascade as targets for the treatment of endometriosis?

INTRODUCTION

The pathogenesis of endometriosis, a common benign disease, remains ill-defined, although it is clear that chronic inflammation plays a crucial role through mitogen-activated protein kinase (MAPK) signaling pathways. All current medical therapies for endometriosis are antigonadotropic, and therefore have a contraceptive effect. A concerted research effort is hence warranted with the aim of delivering novel therapeutics that reduces disease symptoms without blocking ovulation.

AREAS COVERED

The authors review the complex pathogenic mechanisms of chronic inflammation in endometriosis and their relationships with MAPK pathways. The authors conducted a literature search of descriptive and functional targeted validation of MAPK in the pathogenesis of endometriosis. The effects of MAPK inhibitors, which constitute potential agents for future treatments, are also described.

EXPERT OPINION

Preliminary studies have highlighted a crucial role for MAPK in driving endometriosis-related inflammation. MAPK inhibitors exhibit potent activity in terms of controlling growth of endometriosis lesions both in vitro and in animal models. As MAPK inhibitors are known to have a multitude of undesirable side effects, their use in humans has to be approached with great care. Indeed, use of these drugs would probably be limited to short exposures prior to surgery in cases involving the most severe disease phenotypes.

Latexin sensitizes leukemogenic cells to gamma-irradiation-induced cell-cycle arrest and cell death through Rps3 pathway

Leukemia is a leading cause of cancer death. Recently, the latexin (Lxn) gene was identified as a potential tumor suppressor in several types of solid tumors and lymphoma, and Lxn expression was found to be absent or downregulated in leukemic cells. Whether Lxn functions as a tumor suppressor in leukemia and what molecular and cellular mechanisms are involved are unknown. In this study, the myeloid leukemogenic FDC-P1 cell line was used as a model system and Lxn was ectopically expressed in these cells. Using the protein pull-down assay and mass spectrometry, ribosomal protein subunit 3 (Rps3) was identified as a novel Lxn binding protein. Ectopic expression of Lxn inhibited FDC-P1 growth in vitro. More surprisingly, Lxn enhanced gamma irradiation-induced DNA damages and induced cell-cycle arrest and massive necrosis, leading to depletion of FDC-P1 cells. Mechanistically, Lxn inhibited the nuclear translocation of Rps3 upon radiation, resulting in abnormal mitotic spindle formation and chromosome instability. Rps3 knockdown increased the radiation sensitivity of FDC-P1, confirming that the mechanism of action of Lxn is mediated by Rps3 pathway. Moreover, Lxn enhanced the cytotoxicity of chemotherapeutic agent, VP-16, on FDC-P1 cells. Our study suggests that Lxn itself not only suppresses leukemic cell growth but also potentiates the cytotoxic effect of radio- and chemotherapy on cancer cells. Lxn could be a novel molecular target that improves the efficacy of anti-cancer therapy.

Identification of the PKR nuclear interactome reveals roles in ribosome biogenesis, mRNA processing and cell division

The double-strand RNA-dependent protein kinase, PKR, plays a central role in inflammatory/chronic stress-mediated pathologies such as cancer, diabetes, and neuro/muscular degenerative diseases. Although a significant amount of research has been conducted to elucidate the role of PKR signaling in the cytosol, only recently has attention been paid to the role of PKR in the nuclear compartment. Previously our group reported that phosphorylated forms of PKR are present in the nucleus of acute leukemic cell lines, representing a reservoir of active kinase that responds to stress. Using the CCRF-CEM acute T-cell leukemia cell line, a PKR-specific inhibitor, co-immunoprecipitation and a proteomics approach, which included affinity purified mass spectrometry analysis (AP/MS), we identified the proteins present in active and inactive PKR nuclear complexes. Of the proteins identified in the PKR complexes, sixty-nine (69) were specific to the active complex, while thirty-eight (38) were specific to the inactive complex. An additional thirteen (13) proteins associated specifically with both complexes. The majority of the proteins identified are involved in, ribosome biogenesis, RNA splicing, mRNA stability, gene expression, cell cycle, or chromatin organization, including several with known significance to normal hematopoiesis and/or hematological disease. In agreement with the AP/MS data, basal- or over-expression of PKR under normal growth conditions favored cell proliferation in the tested cell lines, whereas pharmacological inhibition of PKR or shRNA-mediated knock-down did not. PKR was also found to influence the isoform and the level of expression of the proto-oncogene MYC.

The TRAIL of oncogenes to apoptosis

Despite the significant advances in clinical research, surgical resection, radiotherapy and chemotherapy are still used as the primary method for cancer treatment. As compared to conventional therapies that often induce systemic toxicity and eventually contribute to tumor resistance, the TNF-related apoptosis-inducing ligand (TRAIL) is a promising anticancer agent that selectively triggers apoptosis in various cancer cells by interacting with its proapoptotic receptors DR4 and KILLER/DR5, while sparing the normal surrounding tissue. The intensive studies of TRAIL signaling pathways over the past decade have provided clues for understanding the molecular mechanisms of TRAIL-induced apoptosis in carcinogenesis and identified an array of therapeutic responses elicited by TRAIL and its receptor agonists. Analysis of its activity at the molecular level has shown that TRAIL improves survival either as monotherapies or combinatorial therapies with other mediators of apoptosis or anticancer chemotherapy. Combinatorial treatments amplify the activities of anticancer agents and widen the therapeutic window by overcoming tumor resistance to apoptosis and driving cancer cells to self-destruction. Although TRAIL sensitivity varies widely depending on the cell type, nontransformed cells are largely resistant to death mediated by TRAIL Death Receptors (DRs). Genetic alterations in cancer can contribute in tumor progression and often play an important role in evasion of apoptosis by tumor cells. Remarkably, RAS, MYC and HER2 oncogenes have been shown to sensitise tumor cells to TRAIL induced cell death. Here, we summarise the cross-talk of oncogenic and apoptotic pathways and how they can be exploited toward efficient combinatorial therapeutic protocols.

Enhanced beta cell proliferation in mice overexpressing a constitutively active form of Akt and one allele of p21Cip

AIMS/HYPOTHESIS

The ability of pancreatic beta cells to proliferate is critical both for normal tissue maintenance and in conditions where there is an increased demand for insulin. Protein kinase B(Akt) plays a major role in promoting proliferation in many cell types, including the insulin-producing beta cells. We have previously reported that mice overexpressing a constitutively active form of Akt(caAkt (Tg)) show enhanced beta cell proliferation that is associated with increased protein levels of cyclin D1, cyclin D2 and cyclin-dependent kinase inhibitor 1A (p21(Cip)). In the present study, we sought to assess the mechanisms responsible for augmented p21(Cip) levels in caAkt(Tg) mice and test the role of p21(Cip) in the proliferative responses induced by activation of Akt signalling.

METHODS

To gain a greater understanding of the relationship between Akt and p21(Cip), we evaluated the mechanisms involved in the modulation of p2(Cip) by Akt and the in vivo role of reduced p21(Cip) in proliferative responses induced by Akt.

RESULTS

Our experiments showed that Akt signalling regulates p21(Cip) transcription and protein stability. caAkt(Tg) /p21(Cip+/-) mice exhibited fasting and fed hypoglycaemia as well as hyperinsulinaemia when compared with caAkt(Tg) mice. Glucose tolerance tests revealed improved glucose tolerance in caAkt(Tg)/p21(Cip+/-) mice compared with caAkt (Tg). These changes resulted from increased proliferation, survival and beta cell mass in caAkt(Tg)/p21(Cip+/-) compared with caAkt(Tg) mice.

CONCLUSIONS/INTERPRETATION

Our data indicate that increased p21(Cip) levels in caAkt(Tg) mice act as a compensatory brake, protecting beta cells from unrestrained proliferation. These studies imply that p21(Cip) could play important roles in the adaptive responses of beta cells to proliferate in conditions such as in insulin resistance.

Pro-oncogenic and anti-oncogenic pathways: opportunities and challenges of cancer therapy

Carcinogenesis is the uncontrolled growth of cells gaining the potential to invade and disrupt vital tissue functions. This malignant process includes the occurrence of 'unwanted' gene mutations that induce the transformation of normal cells, for example, by overactivation of pro-oncogenic pathways and inactivation of tumor-suppressive or anti-oncogenic pathways. It is now recognized that the number of major signaling pathways that control oncogenesis is not unlimited; therefore, suppressing these pathways can conceivably lead to a cancer cure. However, the clinical application of cancer intervention has not matched up to scientific expectations. Increasing numbers of studies have revealed that many oncogenic-signaling elements show double faces, in which they can promote or suppress cancer pathogenesis depending on tissue type, cancer stage, gene dosage and their interaction with other players in carcinogenesis. This complexity of oncogenic signaling poses challenges to traditional cancer therapy and calls for considerable caution when designing an anticancer drug strategy. We propose future oncology interventions with the concept of integrative cancer therapy.

Enhancing therapeutic efficacy by targeting non-oncogene addicted cells with combinations of signal transduction inhibitors and chemotherapy

The effects of inhibition of the Raf/MEK/ERK and PI3K/Akt/mTOR signaling pathways and chemotherapeutic drugs on cell cycle progression and drug sensitivity were examined in cytokine-dependent FL5.12 hematopoietic cells. We examined their effects, as these cells resemble normal hematopoietic precursor cells as they do not exhibit "oncogene-addicted" growth, while they do display "cytokine-addicted" proliferation as cytokine removal resulted in apoptosis in greater than 80% of the cells within 48 hrs. When cytokine-dependent FL5.12 cells were cultured in the presence of IL-3, which stimulated multiple proliferation and anti-apoptotic cascades, MEK, PI3K and mTOR inhibitors transiently suppressed but did not totally inhibit cell cycle progression or induce apoptosis while chemotherapeutic drugs such as doxorubicin and paclitaxel were more effective in inducing cell cycle arrest and apoptosis. Doxorubicin induced a G(1) block, while paclitaxel triggered a G(2)/M block. Doxorubicin was more effective in inducing cell death than paclitaxel. Furthermore the effects of doxorubicin could be enhanced by addition of MEK, PI3K or mTOR inhibitors. Cytokine-dependent cells which proliferate in vitro and are not "oncogene-addicted" may represent a pre-malignant stage, more refractory to treatment with targeted therapy. However, these cells are sensitive to chemotherapeutic drugs. It is important to develop methods to inhibit the growth of such cytokine-dependent cells as they may resemble the leukemia stem cell and other cancer initiating cells. These results demonstrate the enhanced effectiveness of targeting early hematopoietic progenitor cells with combinations of chemotherapeutic drugs and signal transduction inhibitors.

The Raf/MEK/ERK pathway can govern drug resistance, apoptosis and sensitivity to targeted therapy

The effects of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways on proliferation, drug resistance, prevention of apoptosis and sensitivity to signal transduction inhibitors were examined in FL/DeltaAkt-1:ER*(Myr(+)) + DeltaRaf-1:AR cells which are conditionally-transformed to grow in response to Raf and Akt activation. Drug resistant cells were isolated from FL/DeltaAkt-1:ER*(Myr(+)) + DeltaRaf-1:AR cells in the presence of doxorubicin. Activation of Raf-1, in the drug resistant FL/DeltaAkt-1:ER*(Myr(+)) + DeltaRaf-1:AR cells, increased the IC(50) for doxorubicin 80-fold, whereas activation of Akt-1, by itself, had no effect on the doxorubicin IC50. However, Akt-1 activation enhanced cell proliferation and clonogenicity in the presence of chemotherapeutic drugs. Thus the Raf/MEK/ERK pathway had profound effects on the sensitivity to chemotherapeutic drugs, and Akt-1 activation was required for the long term growth of these cells as well as resistance to chemotherapeutic drugs. The effects of doxorubicin on the induction of apoptosis in the drug resistant cells were enhanced by addition of either mTOR and MEK inhibitors. These results indicate that targeting the Raf/MEK/ERK and PI3K/Akt/mTOR pathways may be an effective approach for therapeutic intervention in drug resistant cancers that have mutations activating these cascades.

Dominant roles of the Raf/MEK/ERK pathway in cell cycle progression, prevention of apoptosis and sensitivity to chemotherapeutic drugs

The effects of the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR signaling pathways on cell cycle progression, gene expression, prevention of apoptosis and sensitivity to chemotherapeutic drugs were examined in FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells which are conditionally-transformed to grow in response to Raf-1 and Akt-1 activation by treatment with testosterone or tamoxifen respectively. In these cells we can compare the effects of normal cytokine vs. oncogene mediated signaling in the same cells by changing the culture conditions. Raf-1 was more effective than Akt-1 in inducing cell cycle progression and preventing apoptosis in the presence and absence of chemotherapeutic drugs. The normal cytokine for these cells, interleukin-3 induced/activated most downstream genes transiently, with the exception of p70S6K that was induced for prolonged periods of time. In contrast, most of the downstream genes induced by either the activate Raf-1 or Akt-1 oncogenes were induced for prolonged periods of time, documenting the differences between cytokine and oncogene mediated gene induction which has important therapeutic consequences. The FL/ΔAkt-1:ER*(Myr(+)) + ΔRaf-1:AR cells were sensitive to MEK and PI3K/mTOR inhibitors. Combining MEK and PI3K/mTOR inhibitors increased the induction of apoptosis. The effects of doxorubicin on the induction of apoptosis could be enhanced with MEK, PI3K and mTOR inhibitors. Targeting the Raf/MEK/ERK and PI3K/PTEN/Akt/mTOR pathways may be an effective approach for therapeutic intervention in those cancers which have upstream mutations which result in activation of these pathways.

Glutamine deprivation induces abortive s-phase rescued by deoxyribonucleotides in k-ras transformed fibroblasts

Background

Oncogene activation plays a role in metabolic reprogramming of cancer cells. We have previously shown that K-ras transformed fibroblasts have a stronger dependence on glycolysis and a reduced oxidative phosphorylation ability as compared to their normal counterparts. Another metabolic adaptation of cancer cells, that has long been established, is their propensity to exhibit increased glutamine consumption, although the effects induced by glutamine deprivation on cancer cells are still controversial.

Methodology and Principal Findings

Here, by using nutritional perturbations and molecular physiology, we show that reduction or complete depletion of glutamine availability in K-ras transformed fibroblasts causes a strong decrease of proliferation ability and a slower re-entry of synchronized cells into the cell cycle. The reduced proliferation is accompanied by sustained expression of cyclin D and E, abortive S phase entrance and is dependent on Ras signalling deregulation, since it is rescued by expression of a dominant negative guanine nucleotide exchange factor. The growth potential of transformed cells as well as the ability to execute the G₁ to S transition is restored by adding the four deoxyribonucleotides, indicating that the arrest of proliferation of K-ras transformed cells induced by glutamine depletion is largely due to a reduced supply of DNA in the presence of signalling pathways promoting G₁ to S transition.

Conclusions and Significance

Our results suggest that the differential effects of glutamine and glucose on cell viability are not a property of the transformed phenotype per se, but rather depend on the specific pathway being activated in transformation. For instance, myc-overexpressing cells have been reported to die under glutamine depletion and not under glucose shortage, while the opposite holds for ras-transformed fibroblasts as shown in this paper. These different responses of transformed cells to nutritional stress should be taken into account when designing anti-cancer therapies that aim to exploit metabolic differences between normal and transformed cells.

Crystal structure of human CDK4 in complex with a D-type cyclin

The cyclin D1-cyclin-dependent kinase 4 (CDK4) complex is a key regulator of the transition through the G(1) phase of the cell cycle. Among the cyclin/CDKs, CDK4 and cyclin D1 are the most frequently activated by somatic genetic alterations in multiple tumor types. Thus, aberrant regulation of the CDK4/cyclin D1 pathway plays an essential role in oncogenesis; hence, CDK4 is a genetically validated therapeutic target. Although X-ray crystallographic structures have been determined for various CDK/cyclin complexes, CDK4/cyclin D1 has remained highly refractory to structure determination. Here, we report the crystal structure of CDK4 in complex with cyclin D1 at a resolution of 2.3 A. Although CDK4 is bound to cyclin D1 and has a phosphorylated T-loop, CDK4 is in an inactive conformation and the conformation of the heterodimer diverges from the previously known CDK/cyclin binary complexes, which suggests a unique mechanism for the process of CDK4 regulation and activation.

Roles of the Raf/MEK/ERK pathway in cell growth, malignant transformation and drug resistance

Growth factors and mitogens use the Ras/Raf/MEK/ERK signaling cascade to transmit signals from their receptors to regulate gene expression and prevent apoptosis. Some components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf). Mutations also occur at genes encoding upstream receptors (e.g., EGFR and Flt-3) and chimeric chromosomal translocations (e.g., BCR-ABL) which transmit their signals through these cascades. Even in the absence of obvious genetic mutations, this pathway has been reported to be activated in over 50% of acute myelogenous leukemia and acute lymphocytic leukemia and is also frequently activated in other cancer types (e.g., breast and prostate cancers). Importantly, this increased expression is associated with a poor prognosis. The Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt pathways interact with each other to regulate growth and in some cases tumorigenesis. For example, in some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade due to the ability of activated Akt to phosphorylate and inactivate different Rafs. Although both of these pathways are commonly thought to have anti-apoptotic and drug resistance effects on cells, they display different cell lineage specific effects. For example, Raf/MEK/ERK is usually associated with proliferation and drug resistance of hematopoietic cells, while activation of the Raf/MEK/ERK cascade is suppressed in some prostate cancer cell lines which have mutations at PTEN and express high levels of activated Akt. Furthermore the Ras/Raf/MEK/ERK and Ras/PI3K/PTEN/Akt pathways also interact with the p53 pathway. Some of these interactions can result in controlling the activity and subcellular localization of Bim, Bak, Bax, Puma and Noxa. Raf/MEK/ERK may promote cell cycle arrest in prostate cells and this may be regulated by p53 as restoration of wild-type p53 in p53 deficient prostate cancer cells results in their enhanced sensitivity to chemotherapeutic drugs and increased expression of Raf/MEK/ERK pathway. Thus in advanced prostate cancer, it may be advantageous to induce Raf/MEK/ERK expression to promote cell cycle arrest, while in hematopoietic cancers it may be beneficial to inhibit Raf/MEK/ERK induced proliferation and drug resistance. Thus the Raf/MEK/ERK pathway has different effects on growth, prevention of apoptosis, cell cycle arrest and induction of drug resistance in cells of various lineages which may be due to the presence of functional p53 and PTEN and the expression of lineage specific factors.

Regulation of CDK4

Cyclin-dependent kinase (CDK)4 is a master integrator that couples mitogenic and antimitogenic extracellular signals with the cell cycle. It is also crucial for many oncogenic transformation processes. In this overview, we address various molecular features of CDK4 activation that are critical but remain poorly known or debated, including the regulation of its association with D-type cyclins, its subcellular location, its activating Thr172-phosphorylation and the roles of Cip/Kip CDK "inhibitors" in these processes. We have recently identified the T-loop phosphorylation of CDK4, but not of CDK6, as a determining target for cell cycle control by extracellular factors, indicating that CDK4-activating kinase(s) might have to be reconsidered.

Roles of the RAF/MEK/ERK and PI3K/PTEN/AKT pathways in malignant transformation and drug resistance

The Ras/Raf/MEK/ERK and PI3K/PTEN/AKT signaling cascades play critical roles in the transmission of signals from growth factor receptors to regulate gene expression and prevent apoptosis. Components of these pathways are mutated or aberrantly expressed in human cancer (e.g., Ras, B-Raf, PI3K, PTEN, Akt). Also, mutations occur at genes encoding upstream receptors (e.g., EGFR and Flt-3) and chimeric chromosomal translocations (e.g., BCR-ABL) which transmit their signals through these cascades. These pathways interact with each other to regulate growth and in some cases tumorigenesis. For example, in some cells, PTEN mutation may contribute to suppression of the Raf/MEK/ERK cascade due to the ability of elevated activated Akt levels to phosphorylate and inactivate Raf-1. We have investigated the genetic structures and functional roles of these two signaling pathways in the malignant transformation and drug resistance of hematopoietic, breast and prostate cancer cells. Although both of these pathways are commonly thought to have anti-apoptotic and drug resistance effects on cells, they display different cell-lineage-specific effects. Induced Raf expression can abrogate the cytokine dependence of certain hematopoietic cell lines (FDC-P1 and TF-1), a trait associated with tumorigenesis. In contrast, expression of activated PI3K or Akt does not abrogate the cytokine dependence of these hematopoietic cell lines, but does have positive effects on cell survival. However, activated PI3K and Akt can synergize with activated Raf to abrogate the cytokine dependence of another hematopoietic cell line (FL5.12) which is not transformed by activated Raf expression by itself. Activated Raf and Akt also confer a drug-resistant phenotype to these cells. Raf is more associated with proliferation and the prevention of apoptosis while Akt is more associated with the long-term clonogenicity. In breast cancer cells, activated Raf conferred resistance to the chemotherapeutic drugs doxorubicin and paclitaxel. Raf induced the expression of the drug pump Mdr-1 (a.k.a., Pgp) and the Bcl-2 anti-apoptotic protein. Raf did not appear to induce drug resistance by altering p53/p21Cip-1 expression, whose expression is often linked to regulation of cell cycle progression and drug resistance. Deregulation of the PI3K/PTEN/Akt pathway was associated with resistance to doxorubicin and 4-hydroxyl tamoxifen, a chemotherapeutic drug and estrogen receptor antagonist used in breast cancer therapy. In contrast to the drug-resistant breast cancer cells obtained after overexpression of activated Raf, cells expressing activated Akt displayed altered (decreased) levels of p53/p21Cip-1. Deregulated expression of the central phosphatase in the PI3K/PTEN/Akt pathway led to breast cancer drug resistance. Introduction of mutated forms of PTEN, which lacked lipid phosphatase activity, increased the resistance of the MCF-7 cells to doxorubicin, suggesting that these lipid phosphatase deficient PTEN mutants acted as dominant negative mutants to suppress wild-type PTEN activity. Finally, the PI3K/PTEN/Akt pathway appears to be more prominently involved in prostate cancer drug resistance than the Raf/MEK/ERK pathway. Some advanced prostate cancer cells express elevated levels of activated Akt which may suppress Raf activation. Introduction of activated forms of Akt increased the drug resistance of advanced prostate cancer cells. In contrast, introduction of activated forms of Raf did not increase the drug resistance of the prostate cancer cells. In contrast to the results observed in hematopoietic cells, Raf may normally promote differentiation in prostate cells which is suppressed in advanced prostate cancer due to increased expression of activated Akt arising from PTEN mutation. Thus in advanced prostate cancer it may be advantageous to induce Raf expression to promote differentiation, while in hematopoietic cancers it may be beneficial to inhibit Raf/MEK/ERK-induced proliferation. These signaling and anti-apoptotic pathways can have different effects on growth, prevention of apoptosis and induction of drug resistance in cells of various lineages which may be due to the expression of lineage-specific factors.

Ras-dependent carbon metabolism and transformation in mouse fibroblasts

Mutational activation of ras genes is required for the onset and maintenance of different malignancies. Here we show, using a combination of molecular physiology, nutritional perturbations and transcriptional profiling, that full penetrance of phenotypes related to oncogenic Ras activation, including the shift of carbon metabolism towards fermentation and upregulation of key cell cycle regulators, is dependent upon glucose availability. These responses are induced by Ras activation, being specifically reverted by downregulation of the Ras pathway obtained through the expression of a dominant-negative Ras-specific guanine nucleotide exchange protein. Our data allow to link directly to ras activation the alteration in energy metabolism of cancer cells, their fragility towards glucose shortage and ensuing apoptotic death.

Oncogenes as novel targets for cancer therapy (part II): Intermediate signaling molecules

This is the second part of a four-part review on potential therapeutic targeting of oncogenes. The previous part introduced the new technologies responsible for the advancement of oncogene identification, target validation, and drug design. Because of such advances, new specific and more efficient therapeutic agents can be developed for cancer. This part of the review continues the exploration of various oncogenes, which we have grouped within seven categories: growth factors, tyrosine kinases, intermediate signaling molecules, transcription factors, cell cycle regulators, DNA damage repair genes, and genes involved in apoptosis. Part I included a discussion of growth factors and tyrosine kinases. This portion of the review covers intermediate signaling molecules and the various strategies used to inhibit their expression or decrease their activities.

Cyclin A is a c-Jun target gene and is necessary for c-Jun-induced anchorage-independent growth in RAT1a cells

Overexpression of c-Jun enables Rat1a cells to grow in an anchorage-independent manner. We used an inducible c-Jun system under the regulation of doxycycline in Rat1a cells to identify potential c-Jun target genes necessary for c-Jun-induced anchorage-independent growth. Induction of c-Jun results in sustained expression of cyclin A in the nonadherent state with only minimal expression in the absence of c-Jun. The promoter activity of cyclin A2 was 4-fold higher in Rat1a cells in which c-Jun expression was induced compared with the control cells. Chromatin immunoprecipitation demonstrated that c-Jun bound directly to the cyclin A2 promoter. Mutation analysis of the cyclin A2 promoter mapped the c-Jun regulatory site to an ATF site at position -80. c-Jun was able to bind to this site both in vitro and in vivo, and mutation of this site completely abolished promoter activity. Cyclin A1 was also elevated in c-Jun-overexpressing Rat1a cells; however, c-Jun did not regulate this gene directly, since it did not bind directly to the cyclin A1 promoter. Suppression of cyclin A expression via the introduction of a cyclin A antisense sequences significantly reduced the ability of c-Jun-overexpressing Rat1a cells to grow in an anchorage-independent fashion. Taken together, these results suggest that cyclin A is a target of c-Jun and is necessary but not sufficient for c-Jun-induced anchorage-independent growth. In addition, we demonstrated that the cytoplasmic oncogenes Ras and Src transcriptionally activated the cyclin A2 promoter via the ATF site at position -80. Using a dominant negative c-Jun mutant, TAM67, we showed that this transcriptional activation of cyclin A2 requires c-Jun. Thus, our results suggest that c-Jun is a mediator of the aberrant cyclin A2 expression associated with Ras/Src-induced transformation.

Raf promotes human herpesvirus-8 (HHV-8/KSHV) infection

Human herpesvirus-8 (HHV-8/KSHV) is etiologically associated with Kaposi's sarcoma (KS) and other tumors. Constitutive activation of the mitogen-activated protein kinase (MAPK) signaling pathway has been associated with a variety of tumors, including AIDS-related KS. The oncoprotein Raf is situated at a pivotal position in regulating the MAPK pathway. Hence, we analysed the effect of oncoprotein Raf on HHV-8 infectious entry into target cells. Here we report Raf expression to significantly enhance HHV-8 infection of target cells. These findings implicate a role for Raf not only in the infectious entry of HHV-8 but also in modulating KS pathogenesis.

The role of cyclin D2 and p21/waf1 in human T-cell leukemia virus type 1 infected cells

Background

The human T-cell leukemia virus type 1 (HTLV-1) Tax protein indirectly influences transcriptional activation, signal transduction, cell cycle control, and apoptosis. The function of Tax primarily relies on protein-protein interactions. We have previously shown that Tax upregulates the cell cycle checkpoint proteins p21/waf1 and cyclin D2. Here we describe the consequences of upregulating these G₁/S checkpoint regulators in HTLV-1 infected cells.

Results

To further decipher any physical and functional interactions between cyclin D2 and p21/waf1, we used a series of biochemical assays from HTLV-1 infected and uninfected cells. Immunoprecipitations from HTLV-1 infected cells showed p21/waf1 in a stable complex with cyclin D2/cdk4. This complex is active as it phosphorylates the Rb protein in kinase assays. Confocal fluorescent microscopy indicated that p21/waf1 and cyclin D2 colocalize in HTLV-1 infected, but not in uninfected cells. Furthermore, in vitro kinase assays using purified proteins demonstrated that the addition of p21/waf1 to cyclin D2/cdk4 increased the kinase activity of cdk4.

Conclusion

These data suggest that the p21/cyclin D2/cdk4 complex is not an inhibitory complex and that p21/waf1 could potentially function as an assembly factor for the cyclin D2/cdk4 complex in HTLV-1 infected cells. A by-product of this assembly with cyclin D2/cdk4 is the sequestration of p21/waf1 away from the cyclin E/cdk2 complex, allowing this active cyclin-cdk complex to phosphorylate Rb pocket proteins efficiently and push cells through the G₁/S checkpoint. These two distinct functional and physical activities of p21/waf1 suggest that RNA tumor viruses manipulate the G₁/S checkpoint by deregulating cyclin and cdk complexes.

Trihydrophobin 1 Is a New Negative Regulator of A-Raf Kinase

Our previous work indicated that instead of binding to B-Raf or C-Raf, trihydrophobin 1 (TH1) specifically binds to A-Raf kinase both in vitro and in vivo. In this work, we investigated its function further. Using confocal microscopy, we found that TH1 colocalizes with A-Raf, which confirms our former results. The region of TH1 responsible for the interaction with A-Raf is mapped to amino acids 1-372. Coimmunoprecipitation experiments demonstrate that TH1 is associated with A-Raf in both quiescent and serum-stimulated cells. Wild type A-Raf binds increasingly to TH1 when it is activated by serum and/or upstream oncogenic Ras/Src compared with that of "kinase-dead" A-Raf. The latter can still bind to TH1 under the same experimental condition. The binding pattern of A-Raf implies that this interaction is mediated in part by the A-Raf kinase activity. As indicated by Raf protein kinase assays, TH1 inhibits A-Raf kinase, whereas neither B-Raf nor C-Raf kinase activity is influenced. Furthermore, we observed that TH1 inhibited cell cycle progression in TH1 stably transfected 7721 cells compared with mock cells, and flow cell cytometry analysis suggested that the TH1 stably transfected 7721 cells were G(0)/G(1) phase-arrested. Taken together, our data provide a clue to understanding the cellular function of TH1 on Raf isoform-specific regulation.

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.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Effects of the RAF/MEK/ERK and PI3K/AKT signal transduction pathways on the abrogation of cytokine-dependence and prevention of apoptosis in hematopoietic cells

The Raf/MEK/ERK kinase cascade is pivotal in transmitting signals from membrane receptors to transcription factors that control gene expression culminating in the regulation of cell cycle progression. This cascade can prevent cell death through ERK2 and p90(Rsk) activation and phosphorylation of apoptotic and cell cycle regulatory proteins. The PI3K/Akt kinase cascade also controls apoptosis and can phosphorylate many apoptotic and cell cycle regulatory proteins. These pathways are interwoven as Akt can phosphorylate Raf and result in its inactivation, and Raf can be required for the antiapoptotic effects of Akt. In this study, the effects of activated Raf (Raf-1, A-Raf and B-Raf) and PI3K/Akt proteins on abrogation of cytokine dependence in FL5.12 hematopoietic cells were examined. Activated Raf, PI3K or Akt expression, by themselves, did not readily relieve cytokine dependence. The presence of activated Raf and PI3K/Akt increased the isolation of factor-independent cells from 400- to 2500-fold depending upon the particular combination examined. The individual effects of activated Raf and Akt on proliferation, apoptosis and autocrine growth factor synthesis were further examined with hormone-inducible constructs (Delta Raf-1:AR and Delta Akt:ER*(Myr(+)). Activation of either Raf or Akt hindered cell death; however, both proliferation and maximal synthesis of autocrine cytokines were dependent upon activation of both signaling pathways. The effects of small molecular weight inhibitors on DNA synthesis and cytokine gene expression were also examined. The PI3K inhibitor, LY294002, inhibited growth and cytokine gene expression. This effect could be synergistically increased by addition of the MEK inhibitor UO126. These cells will be useful in elucidating the interactions between Raf/MEK/ERK and PI3K/Akt cascades in proliferation, apoptosis, and leukemogenesis, as well as evaluating the efficacy of signal transduction inhibitors that target these cascades.

Involvement of PI3K/Akt pathway in cell cycle progression, apoptosis, and neoplastic transformation: a target for cancer chemotherapy

The PI3K/Akt signal transduction cascade has been investigated extensively for its roles in oncogenic transformation. Initial studies implicated both PI3K and Akt in prevention of apoptosis. However, more recent evidence has also associated this pathway with regulation of cell cycle progression. Uncovering the signaling network spanning from extracellular environment to the nucleus should illuminate biochemical events contributing to malignant transformation. Here, we discuss PI3K/Akt-mediated signal transduction including its mechanisms of activation, signal transducing molecules, and effects on gene expression that contribute to tumorigenesis. Effects of PI3K/Akt signaling on important proteins controlling cellular proliferation are emphasized. These targets include cyclins, cyclin-dependent kinases, and cyclin-dependent kinase inhibitors. Furthermore, strategies used to inhibit the PI3K/Akt pathway are presented. The potential for cancer treatment with agents inhibiting this pathway is also addressed.

Autocrine human growth hormone inhibits placental transforming growth factor-beta gene transcription to prevent apoptosis and allow cell cycle progression of human mammary carcinoma cells

Multiple cellular effects of human growth hormone (hGH) are mediated by an indirect mechanism requiring transcriptional activation of genes encoding protein effector molecules such as insulin-like growth factor-1. Such protein effector molecules then act directly to mediate the cellular functions of hGH. We report here that autocrine hGH production by mammary carcinoma cells specifically results in the transcriptional repression of the p53-regulated placental transforming growth factor-beta (PTGF-beta) gene. Transcriptional repression of the PTGF-beta gene does not require the p53-binding sites in the PTGF-beta promoter, and autocrine hGH also desensitized the response of the PTGF-beta promoter to p53 overexpression. Transcriptional repression of the PTGF-beta gene is accompanied by consequent decreases in its protein product, Smad-mediated transcription, and its cellular effects that include cell cycle arrest and apoptosis. PTGF-beta specifically inhibited the autocrine hGH-stimulated expression of cyclin D1 required for autocrine hGH-stimulated mammary carcinoma cell cycle progression. Thus, one mechanism by which autocrine hGH promotes an increase in mammary carcinoma cell number is by transcriptional repression of protein effector molecules that promote cell cycle arrest and apoptosis. Such transcriptional repression of negative regulatory factors, such as PTGF-beta, may also be requisite for direct stimulation of mammary carcinoma cell mitogenesis by hGH.

The Raf/MEK/ERK signal transduction cascade as a target for chemotherapeutic intervention in leukemia

The Raf/MEK/ERK (MAPK) signal transduction cascade is a vital mediator of a number of cellular fates including growth, proliferation and survival, among others. The focus of this review centers on the MAPK signal transduction pathway, its mechanisms of activation, downstream mediators of signaling, and the transcription factors that ultimately alter gene expression. Furthermore, negative regulators of this cascade, including phosphatases, are discussed with an emphasis placed upon chemotherapeutic intervention at various points along the pathway. In addition, mounting evidence suggests that the PI3K/Akt pathway may play a role in the effects elicited via MAPK signaling; as such, potential interactions and their possible cellular ramifications are discussed.