Synergy between Raf and BCL2 in abrogating the cytokine dependency of hematopoietic cells

Drug-resistance in doxorubicin-resistant FL5.12 hematopoietic cells: elevated MDR1, drug efflux and side-population positive and decreased BCL2-family member expression

Chemotherapeutic drug treatment can result in the emergence of drug-resistant cells. By culturing an interleukin-3 (IL-3)-dependent cell line, FL5.12 cells in the presence of the chemotherapeutic drug doxorubicin, we isolated FL/Doxo cells which are multi-drug resistant. Increased levels of drug efflux were detected in FL/Doxo cells which could be inhibited by the MDR1 inhibitor verapamil but not by the MRP1 inhibitor MK571. The effects of TP53 and MEK1 were examined by infection of FL/Doxo cells with retroviruses encoding either a dominant negative TP-53 gene (FL/Doxo+ TP53 (DN) or a constitutively-activated MEK-1 gene (FL/Doxo + MEK1 (CA). Elevated MDR1 but not MRP1 mRNA transcripts were detected by quantitative RT-PCR in the drug-resistant cells while transcripts encoding anti-apoptotic genes such as: BCL2, BCLXL and MCL1 were observed at higher levels in the drug-sensitive FL5.12 cells. The percentage of cells that were side-population positive was increased in the drug-resistant cells compared to the parental line. Drug-resistance and side-positive population cells have been associated with cancer stem cells (CSC). Our studies suggest mechanisms which could allow the targeting of these molecules to prevent drug-resistance.

The role of slingshot-1L (SSH1L) in the differentiation of human bone marrow mesenchymal stem cells into cardiomyocyte-like cells

Adult cardiomyocytes (CMs) have very limited capacity to regenerate. Therefore, there is a great interest in developing strategies to treat infarcted CMs that are able to regenerate cardiac tissue and promote revascularization of infarcted zones in the heart. Recently, stem cell transplantation has been proposed to replace infarcted CMs and to restore the function of the affected tissue. This area of research has become very active in recent years due to the huge clinical need to improve the efficacy of currently available therapies. Slingshot (SSH) is a family of protein phosphatases, which can specifically dephosphorylate and reactivate cofilin and inhibit the polymerization of actin filaments and actively involved in cytoskeleton rearrangement. In this study, we found that SSH1L promoted morphology changes of microfilaments during differentiation but was inhibited by the inhibitors of actin polymerization such as cytochalasin D. Overexpression of SSH1L could promote cardiac-specific protein and genes expression. 5-Aza can induce the differentiation of hMSCs into cardiomyocyte-like cells in vitro. We also observed that SSH1L efficiently promotes hMSCs differentiation into cardiomyocyte-like cells through regulation and rearrangement of cytoskeleton. Our work provides evidence that supports the positive role of SSH1L in the mechanism of stem cell differentiation into cardiomyocyte-like cells.

Roles of the Ras/Raf/MEK/ERK pathway in leukemia therapy

The Ras/Raf/mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK) pathway is often implicated in sensitivity and resistance to leukemia therapy. Dysregulated signaling through the Ras/Raf/MEK/ERK pathway is often the result of genetic alterations in critical components in this pathway as well as mutations at upstream growth factor receptors. Unrestricted leukemia proliferation and decreased sensitivity to apoptotic-inducing agents and chemoresistance are typically associated with activation of pro-survival pathways. Mutations in this pathway and upstream signaling molecules can alter sensitivity to small molecule inhibitors targeting components of this cascade as well as to inhibitors targeting other key pathways (for example, phosphatidylinositol 3 kinase (PI3K)/phosphatase and tensin homologue deleted on chromosome 10 (PTEN)/Akt/mammalian target of rapamycin (mTOR)) activated in leukemia. Similarly, PI3K mutations can result in resistance to inhibitors targeting the Ras/Raf/MEK/ERK pathway, indicating important interaction points between the pathways (cross-talk). Furthermore, the Ras/Raf/MEK/ERK pathway can be activated by chemotherapeutic drugs commonly used in leukemia therapy. This review discusses the mechanisms by which abnormal expression of the Ras/Raf/MEK/ERK pathway can contribute to drug resistance as well as resistance to targeted leukemia therapy. Controlling the expression of this pathway could improve leukemia therapy and ameliorate human health.

Blockade of mitogen-activated protein kinase/extracellular signal-regulated kinase kinase and murine double minute synergistically induces Apoptosis in acute myeloid leukemia via BH3-only proteins Puma and Bim

Molecular aberrations of the Ras/Raf/mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK and/or Murine double minute (MDM2)/p53 signaling pathways have been reported in 80% and 50% of primary acute myeloid leukemia (AML) samples and confer poor outcome. In this study, antileukemic effects of combined MEK inhibition by AZD6244 and nongenotoxic p53 activation by MDM2 antagonist Nutlin-3a were investigated. Simultaneous blockade of MEK and MDM2 signaling by AZD6244 and Nutlin-3a triggered synergistic proapoptotic responses in AML cell lines [combination index (CI) = 0.06 +/- 0.03 and 0.43 +/- 0.03 in OCI/AML3 and MOLM13 cells, respectively] and in primary AML cells (CI = 0.52 +/- 0.01). Mechanistically, the combination upregulated levels of BH3-only proteins Puma and Bim, in part via transcriptional upregulation of the FOXO3a transcription factor. Suppression of Puma and Bim by short interfering RNA rescued OCI/AML3 cells from AZD/Nutlin-induced apoptosis. These results strongly indicate the therapeutic potential of combined MEK/MDM2 blockade in AML and implicate Puma and Bim as major regulators of AML cell survival.

Results of a Phase I trial of sorafenib (BAY 43-9006) in combination with doxorubicin in patients with refractory solid tumors

BACKGROUND

Sorafenib (BAY 43-9006), a novel, oral multi-kinase inhibitor, blocks serine/threonine and receptor tyrosine kinases in the tumor and vasculature. Sorafenib demonstrated single-agent activity in Phase I studies, and was tolerated and inhibited tumor growth in combination with doxorubicin in preclinical studies. This Phase I dose-escalation study determined the safety, pharmacokinetics and efficacy of sorafenib plus doxorubicin.

PATIENTS AND METHODS

Thirty-four patients with refractory, solid tumors received doxorubicin 60 mg/m(2) on Day 1 of 3-week cycles, and oral sorafenib from Day 4 of Cycle 1 at 100, 200 or 400 mg bid.

RESULTS

Common drug-related adverse events were neutropenia (56%), hand-foot skin reaction (44%), stomatitis (32%), and diarrhea (32%). The maximum tolerated dose was not reached. One patient with pleural mesothelioma achieved a partial response (modified WHO criteria) and remained on therapy for 39.7 weeks. Fifteen patients (48%) achieved stable disease for >/=12 weeks. Doxorubicin exposure increased moderately with sorafenib 400 mg bid. The pharmacokinetics of sorafenib and doxorubicinol were not affected.

CONCLUSION

Sorafenib 400 mg bid plus doxorubicin 60 mg/m(2) was well tolerated. The increased doxorubicin exposure with sorafenib 400 mg bid did not result in significantly increased toxicity; low patient numbers make the clinical significance of this unclear. These promising efficacy results justify further clinical investigation.

Development of a Conditional In vivo Model to Evaluate the Efficacy of Small Molecule Inhibitors for the Treatment of Raf-Transformed Hematopoietic Cells

Conditionally active forms of the Raf proteins (Raf-1, B-Raf, and A-Raf) were created by ligating NH2-terminal truncated activated forms (Delta) to the estrogen receptor (ER) hormone-binding domain resulting in estradiol-regulated constructs (DeltaRaf:ER). These different Raf:ER oncoproteins were introduced into the murine FDC-P1 hematopoietic cell line, and cells that grew in response to the three DeltaRaf:ER oncoproteins were isolated. The ability of FDC-P1, DeltaRaf-1:ER, DeltaA-Raf:ER, and DeltaB-Raf:ER cells to form tumors in severe combined immunodeficient mice was compared. Mice injected with DeltaRaf:ER cells were implanted with beta-estradiol pellets to induce the DeltaRaf:ER oncoprotein. Cytokine-dependent parental cell lines did not form tumors. Implantation of beta-estradiol pellets into mice injected with DeltaRaf:ER cells significantly accelerated tumor onset and tumor size. The recovered DeltaRaf:ER cells displayed induction of extracellular signal-regulated kinase (ERK) in response to beta-estradiol stimulation, indicating that they had retained conditional activation of ERK even when passed through a severe combined immunodeficient mouse. The DeltaRaf:ER cells were very sensitive to induction of apoptosis by the mitogen-activated protein/ERK kinase (MEK) 1 inhibitor CI1040 whereas parental cells were much less affected, demonstrating that the MEK1 may be useful in eliminating Ras/Raf/MEK-transformed cells. Furthermore, the effects of in vivo administration of the MEK1 inhibitor were evaluated and this inhibitor was observed to suppress the tumorigenicity of the injected cells. This DeltaRaf:ER system can serve as a preclinical model to evaluate the effects of signal transduction inhibitors which target the Raf and MEK proteins.

Phosphatidylinositol 3'-kinase activation leads to multidrug resistance protein-1 expression and subsequent chemoresistance in advanced prostate cancer cells

The development of androgen-independent growth in advanced carcinoma of the prostate (CaP) is associated with poor prognosis and few therapeutic options. Chemotherapeutic drugs offer the afflicted patient palliative benefits, but these are short-lived because of the chemoresistant nature of hormone-refractory prostate cancer. Given the high percentage of CaP patients with mutations in the PTEN tumor suppressor gene, we sought to determine the involvement of the phosphatidylinositol 3'-kinase (PI3K) cascade in the development of CaP drug resistance. PTEN-negative PC3 cells were observed to have increased resistance to both doxorubicin and paclitaxel when compared with PTEN-positive DU145 cells. Furthermore, modulation of PI3K activity with the use of constitutively active and dominant-negative inhibitors was found to affect the ability to CaP cells to respond to chemotoxic treatments. Additionally, inhibition of PI3K with a small molecular weight inhibitor (LY294002) was able to potentiate the antineoplastic activity of both doxorubicin and paclitaxel in CaP cells. Interestingly, multidrug resistance protein-1 (MRP-1) expression, but not MDR-1 (p-glycoprotein), was observed to be induced as a consequence of PI3K activation in these cell types. Inhibition of MRP-1 expression via siRNA was observed to synergistically sensitize CaP cells to chemotoxic drugs while having no appreciable effect on cell growth in the absence of these compounds. Taken together, these data suggest that PI3K activation can lead to the development of chemoresistant cells in prostatic carcinomas through the up-regulation of MRP-1. Thus, inhibition of PI3K activity with concomitant administration of chemotoxic compounds may prove beneficial in preventing the development of drug resistance in patients with hormone-refractory prostate cancer.

JAK/STAT, Raf/MEK/ERK, PI3K/Akt and BCR-ABL in cell cycle progression and leukemogenesis

The roles of the JAK/STAT, Raf/MEK/ERK and PI3K/Akt signal transduction pathways and the BCR-ABL oncoprotein in leukemogenesis and their importance in the regulation of cell cycle progression and apoptosis are discussed in this review. These pathways have evolved regulatory proteins, which serve to limit their proliferative and antiapoptotic effects. Small molecular weight cell membrane-permeable drugs that target these pathways have been developed for leukemia therapy. One such example is imatinib mesylate, which targets the BCR-ABL kinase as well as a few structurally related kinases. This drug has proven to be effective in the treatment of CML patients. However, leukemic cells have evolved mechanisms to become resistant to this drug. A means to combat drug resistance is to target other prominent signaling components involved in the pathway or to inhibit BCR-ABL by other mechanisms. Treatment of imatinib-resistant leukemia cells with drugs that target Ras (farnysyl transferase inhibitors) or with the protein destabilizer geldanamycin has proven to be a means to inhibit the growth of resistant cells. This review will tie together three important signal transduction pathways involved in the regulation of hematopoietic cell growth and indicate how their expression is dysregulated by the BCR-ABL oncoprotein.

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.

Therapeutic efficacy of prenylation inhibitors in the treatment of myeloid leukemia

Farnesyltransferase inhibitors (FTIs) represent a new class of anticancer agents that specifically target post-translational farnesylation of various proteins that mediate several cellular processes such as signal transduction, growth, differentiation, angiogenesis and apoptosis. These compounds were originally designed to block oncogenic RAS-induced tumor growth by impeding RAS localization to the membrane, but it is now evident that FTIs also affect processing of several other proteins. The need for novel therapies in myeloid leukemia is underscored by the high rate of treatment failure due to high incidences of relapse- and treatment-related toxicities. As RAS deregulation is important in the pathogenesis of myeloid leukemias, targeting of RAS signaling may provide a new therapeutic strategy. Several FTIs (eg BMS-214662, L-778,123, R-115777 and SCH66336) have entered phase I and phase II clinical trials in myeloid leukemias. This review discusses recent clinical results, potential combination therapies, mechanisms of resistance and the clinical challenges of toxicities associated with prenylation inhibitors.

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.

Bax expression correlates with cellular drug sensitivity to doxorubicin, cyclophosphamide and chlorambucil but not fludarabine, cladribine or corticosteroids in B cell chronic lymphocytic leukemia

In B-CLL, non-proliferating B cells accumulate due to defective apoptosis. Cytotoxic therapies trigger apoptosis and deregulation of apoptotic pathways contributes to chemoresistance. Loss of the apoptosis-promoting Bax has been implicated in resistance to cytotoxic therapy. We therefore evaluated ex vivo drug sensitivity of CLL, producing chemoresponse data which are prognostic indicators for B-CLL, in particular in the case of purine nucleoside analogs. To analyze the underlying mechanisms of drug resistance, we compared endogenous Bax and Bcl-2 expression to ex vivo response to eight drugs, and to survival in 39 B-CLL patients. We found that reduced Bax levels correlated well with ex vivo resistance to traditional B-CLL therapies - anthracyclines, alkylating agents and vincristine (all P < 0.04). Surprisingly, no such relationship was observed for the purine nucleoside analogs or corticosteroids (all P > 0.5). Mutational analysis of p53 could not explain the loss of Bax protein expression. Levels of Bcl-2 were not associated with sensitivity to any drug. In contrast to the ex vivo data, neither Bax or Bcl-2 expression nor doxorubicin sensitivity were associated with increased survival whereas sensitivity to fludarabine correlated with better overall survival (P = 0.031). These findings suggest that the resistance to purine nucleoside analogs and corticosteroids in B-CLL is due to inactivation of pathways different from those activated by anthracyclines, vinca alkaloids and alkylating agents and may be the molecular rationale for the efficacy of purine analogs in this disease.

Hsp-90-associated oncoproteins: multiple targets of geldanamycin and its analogs

Geldanamycin (GA), herbimycin A and radicicol bind heat-shock protein-90 (Hsp90) and destabilize its client proteins including v-Src, Bcr-Abl, Raf-1, ErbB2, some growth factor receptors and steroid receptors. Thus, Hsp90-active agents induce ubiquitination and proteasomal degradation of numerous oncoproteins. Depending on the cellular context, HSP90-active agents cause growth arrest, differentiation and apoptosis, or can prevent apoptosis. HSP-active agents are undergoing clinical trials. Like targets of most chemotherapeutics, Hsp90 is not a cancer-specific protein. By attacking a nonspecific target, HSP-90-active compounds still may preferentially kill certain tumor cells. How can this be achieved? How can therapeutic potentials be exploited? This article starts the discussion.

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.

Enhanced ability of daniplestim and myelopoietin-1 to suppress apoptosis in human hematopoietic cells

Modified and chimeric cytokines have been developed to aid in the recovery of hematopoietic precursor cells after myeloablative chemotherapy. The interleukin-3 (IL-3) receptor agonist, daniplestim, binds to the IL-3 receptor-alpha subunit with 60-fold greater affinity and induces cell proliferation and colony-forming unit formation 10- to 22-fold better than native IL-3. A chimeric cytokine, myelopoietin-1, composed of daniplestim and a G-CSF receptor agonist binds both the IL-3 and G-CSF receptors. While the in vivo effects of daniplestim and myelopoietin-1 are well described, the mechanisms by which they stimulate growth are not well understood. We have investigated the effects of daniplestim and myelopoietin-1 on the prevention of apoptosis in two human hematopoietic cell lines, OCI-AML.5 and AML 193. Daniplestim and myelopoietin-1 prevented apoptosis to a greater degree than native recombinant IL-3 or G-CSF as determined by annexin V/propidium iodide binding and TUNEL assays. Daniplestim and myelopoietin-1 promoted the maintenance of the mitochondrial membrane potential better than native IL-3 or G-CSF. These cytokines promoted a lower redox potential as higher levels of free radicals were detected after cytokine treatment than in cytokine-deprived cells implying increased respiration. These results indicate that daniplestim and myelopoietin-1 are able to prevent apoptosis in hematopoietic cells more effectively than native IL-3 and G-CSF. These effects of daniplestim and myelopoietin-1 may contribute to their effective ability to repopulate hematopoietic precursor cells after chemotherapy.

Ceramide regulates cellular homeostasis via diverse stress signaling pathways

The sphingolipid ceramide is an important second signal molecule that regulates diverse signaling pathways involving apoptosis, cell senescence, the cell cycle, and differentiation. For the most part, ceramide's effects are antagonistic to growth and survival. Interestingly, ceramide and the pro-growth agonist, diacylglycerol (DAG) appear to be regulated simultaneously but in opposite directions in the sphingomyelin cycle. While ceramide stimulates signal transduction pathways that are associated with cell death or at least are inhibitory to cell growth (eg stress-activated protein kinase, SAPK, pathways), DAG activates the classical and novel isoforms of the protein kinase C (PKC) family. These PKC isoforms are associated with cell growth and cell survival. Furthermore, DAG activation of PKC stimulates other signal transduction pathways that support cell proliferation (eg mitogen-activated protein kinase, MAPK, pathways). Thus, ceramide and DAG generation may serve to monitor cellular homeostasis by inducing pro-death or pro-growth pathways, respectively. The production of ceramide is emerging as a fixture of programmed cell death. Ceramide levels are elevated in response to diverse stress challenges including chemotherapeutic drug treatment, irradiation, or treatment with pro-death ligands such as tumor necrosis factor alpha, TNF alpha. Consistent with this notion, ceramide itself is a potent apoptogenic agent. Ceramide activates stress-activated protein kinases like c-jun N-terminal kinase (JNK) and thus affects transcription pathways involving c-jun. Ceramide activates protein phosphatases such as protein phosphatase 1 (PP1) and protein phosphatase 2 (PP2A). Ceramide activation of protein phosphatases has been shown to promote inactivation of a number of pro-growth cellular regulators including the kinases PKC alpha and Akt, Bcl2 and the retinoblastoma protein. A new role has recently emerged for ceramide in the regulation of protein synthesis. Ceramide-induced activation of double-stranded RNA-dependent protein kinase (PKR), a protein kinase important in anti-viral host defense mechanisms and recently implicated in cellular stress pathways, results in the inhibition of protein synthesis as a prelude to cell death. Taken together, these properties of ceramide suggest that this important second-signal molecule may have useful properties as an anti-neoplastic agent. Thus, strategies to promote ceramide metabolism or use of ceramide analogs directly may one day become useful in the treatment of diseases like leukemia.

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

To investigate the functions of the different Raf genes in hematopoietic cell proliferation, the capacities of beta-estradiol-regulated Delta Raf:ER genes to induce cell cycle regulatory gene expression and cell cycle progression in FDC-P1 cells were examined. Raf activation increased the expression of Cdk2, Cdk4, cyclin A, cyclin D, cyclin E, p21(Cip1) and c-Myc and decreased the expression of p27(Kip1) which are associated with G(1) progression. However only the cell clones with moderate Raf activation, i.e. FD/Delta Raf-1:ER and FD/Delta A-Raf:ER, successfully underwent cell proliferation. The cell clones with the highest Delta Raf activity, FD/Delta B-Raf:ER, underwent apoptosis before cell proliferation. p21(Cip1) induced by Raf activation specifically bound with Cdk4/cyclin D complexes but not Cdk2/cyclin E complexes and this binding was associated with the increased Cdk4 activity. However, no binding of p27(Kip1) with either Cdk2/cyclin E or Cdk4/cyclin D was observed. Thus Raf mediated growth was associated with elevated p21(Cip1) expression, which may specifically bind with and activate Cdk4/cyclin D complexes and with decreased p27(Kip1) expression.

Suppression of apoptosis: role in cell growth and neoplasia

A cell is a potentially dangerous thing. In unicellular organisms, cells divide and multiply in a manner that is chiefly determined by the availability of nutritional substrates. In a multicellular organism, each cell has a distinct growth potential that is designed to subsume a role in the function of the whole body. Departure from this path to one of uncontrolled cellular proliferation leads to cancer. For this reason, evolution has endowed cells with an elaborate set of systems that cause errant cells to self-destruct. This process of cell suicide is known as apoptosis or programmed cell death and it plays a crucial role in the growth of both normal and malignant cells. In this review, we describe the mechanisms whereby programmed cell death is induced and executed. In particular, we concentrate on how anti-apoptotic signals generated by cytokines promote cell survival and how these signal transduction pathways may be involved in the pathogenesis of neoplasia. Understanding how these processes contribute to tumorigenesis may suggest new therapeutic options.

Unwinding the loop of Bcl-2 phosphorylation

Recent evidence indicates that anti-apoptotic functions of BcI-2 can be regulated by its phosphorylation. According to the 'mitotic arrest-induced' model, multi-site phosphorylation of the BcI-2 loop domain is followed by cell death. In contrast, in cytokine-dependent cell lines, cytokines mediate phosphorylation of BcI-2 on S70, preventing apoptosis. As discussed in this review, these models are not mutually exclusive but reflect different cellular contexts. During mitotic arrest, signal transduction is unique and is fundamentally different from classical mitogenic signaling, since the nucleus membrane is dissolved, gene expression is reduced, and numerous kinases and regulatory proteins are hyperphosphorylated. Hyperphosphorylation of BcI-2 mediated by paclitaxel and other microtubule-active drugs is strictly dependent on targeting microtubules that in turn cause mitotic arrest. In addition to serine-70 (S70), microtubule-active agents promote phosphorylation of S87 and threonine-69 (T69), inactivating BcI-2. A major obstacle for identification of the mitotic BcI-2 kinase(s) is that inhibition of putative kinase(s) by any means (dominant-negative mutants, antisense oligonucleotides, pharmacological agents) may arrest cycle, preventing mitosis and BcI-2 phosphorylation. The role of BcI-2 phosphorylation in cell death is discussed.

Kinases: positive and negative regulators of apoptosis

Cells sense and respond to extracellular factors via receptors on the cell surface that trigger intracellular signaling pathways. The signals received by the receptors on hematopoietic cells often determine if the cell proliferates, survives or undergoes apoptosis. Apoptosis can be induced by almost any cytotoxic stimuli. These stimuli may be an absence of signals arising from cellular receptors, stimulation of specific ligand receptors on the cell surface, chemotherapeutic agents, and ionizing radiation or oxygen radicals, as well as a number of other factors. Cellular kinases and phosphatases participate in signaling cascades that influence this process. We review the ability of the calmodulin-dependent-kinases, I-kappaB kinases, PI3-kinases, Jakkinases, PKC, PKA, and MAP kinase signaling pathways (Erk, Jnk, and p38), to influence the apoptotic process. In addition, we discuss the cross-talk that exists between signaling cascades that are pro-apoptotic and anti-apoptotic.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.

Effects of deregulated Raf activation on integrin, cytokine-receptor expression and the induction of apoptosis in hematopoietic cells

The effects of deregulated Raf activation on the growth and differentiation of hematopoietic cells were investigated. The cytokine-dependent murine myeloid FDC-P1 and human erythroleukemic TF-1 cell lines were transformed to grow in response to deregulated Raf expression in the absence of exogenous cytokines. The conditionally active Raf proteins were regulated by beta-estradiol as cDNAs containing the Raf catalytic, but lacking negative-regulatory domains, were ligated to the hormone binding domain of the estrogen receptor (deltaRaf:ER). Continuous deltaRaf expression prevented apoptosis in the absence of exogenous cytokines and altered the morphology of the FD/deltaRaf:ER cells as they grew in large aggregated masses (>100 cells) whereas the parental cytokine-dependent FDC-P1 cells grew in smaller grape-like clusters (< 10 cells). FD/deltaRaf-1:ER cells growing in response to Raf activation displayed decreased levels of the Mac-2 and Mac-3 molecules on their cell surface. In contrast, when these cells were cultured in IL-3, higher levels of these adhesion molecules were detected. Expression of activated Raf oncoproteins also abrogated cytokine dependency and prevented apoptosis of TF-1 cells. Moreover, the differentiation status of these Raf-responsive cells was more immature upon Raf activation as culture with the differentiation-inducing agent phorbol 12 myristate 13-acetate (PMA) and beta-estradiol resulted in decreased levels of the CD11b and CD18 integrin molecules on the cell surface. In contrast when the Raf-responsive cells were induced to differentiate with PMA and GM-CSF, in the absence of deltaRaf:ER activation, increased levels of the CD11b and CD18 molecules were detected. Retinoic acid (RA) inhibited 3H-thymidine incorporation in response to GM-CSF. Interestingly, Raf activation counterbalanced the inhibition of DNA synthesis caused by RA but not PMA. Thus deregulated Raf expression can alter cytokine dependency, integrin expression and the stage of differentiation. These Raf-responsive cell lines will be useful in elucidating the roles of the MAP kinase cascade on hematopoietic cell differentiation and malignant transformation.