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

The histone deacetylase inhibitor MS-275 interacts synergistically with fludarabine to induce apoptosis in human leukemia cells

Interactions between the novel benzamide histone deacetylase (HDAC) inhibitor MS-275 and fludarabine were examined in lymphoid and myeloid human leukemia cells in relation to mitochondrial injury, signal transduction events, and apoptosis. Prior exposure of Jurkat lymphoblastic leukemia cells to a marginally toxic concentration of MS-275 (e.g., 500 nM) for 24 h sharply increased mitochondrial injury, caspase activation, and apoptosis in response to a minimally toxic concentration of fludarabine (500 nM), resulting in highly synergistic antileukemic interactions and loss of clonogenic survival. Simultaneous exposure to MS-275 and fludarabine also led to synergistic effects, but these were not as pronounced as observed with sequential treatment. Similar interactions were noted in the case of (a) other human leukemia cell lines (e.g., U937, CCRF-CEM); (b) other HDAC inhibitors (e.g., sodium butyrate); and (c) other nucleoside analogues (e.g., 1-beta-D-arabinofuranosylcytosine, gemcitabine). Potentiation of fludarabine lethality by MS-275 was associated with acetylation of histones H3 and H4, down-regulation of the antiapoptotic proteins XIAP and Mcl-1, enhanced cytosolic release of proapoptotic mitochondrial proteins (e.g., cytochrome c, Smac/DIABLO, and apoptosis-inducing factor), and caspase activation. It was also accompanied by the caspase-dependent down-regulation of p27(KIP1), cyclins A, E, and D(1), and cleavage and diminished phosphorylation of retinoblastoma protein. However, increased lethality of the combination was not associated with enhanced fludarabine triphosphate formation or DNA incorporation and occurred despite a slight reduction in the S-phase fraction. Prior exposure to MS-275 attenuated fludarabine-mediated activation of MEK1/2, extracellular signal-regulated kinase, and Akt, and enhanced c-Jun NH(2)-terminal kinase phosphorylation; furthermore, inducible expression of constitutively active MEK1/2 or Akt significantly diminished MS-275/fludarabine-induced lethality. Combined exposure of cells to MS-275 and fludarabine was associated with a significant increase in generation of reactive oxygen species; moreover, both the increase in reactive oxygen species and apoptosis were largely attenuated by coadministration of the free radical scavenger L-N-acetylcysteine. Finally, prior administration of MS-275 markedly potentiated fludarabine-mediated generation of the proapoptotic lipid second messenger ceramide. Taken together, these findings indicate that the HDAC inhibitor MS-275 induces multiple perturbations in signal transduction, survival, and cell cycle regulatory pathways that lower the threshold for fludarabine-mediated mitochondrial injury and apoptosis in human leukemia cells. They also provide insights into possible mechanisms by which novel, clinically relevant HDAC inhibitors might be used to enhance the antileukemic activity of established nucleoside analogues such as fludarabine.

Expression and purification of the recombinant diphtheria fusion toxin DT388IL3 for phase I clinical trials

A genetically engineered fusion toxin targeted to acute myeloid leukemia (AML) blasts was designed with the first 388 amino acid residues of diphtheria toxin with an H-M linker fused to human interleukin-3. The cDNA was subcloned in the pRK bacterial expression plasmid and used to transform BLR (DE3) Escherichia coli. A single transformed colony was grown in Superbroth with ampicillin; bacteria were centrifuged at an OD(650) of 1.3; master cell bank aliquots of bacteria in 30% glycerol/Superbroth were frozen and stored at -80 degrees C. Master cell bank bacteria were diluted 1500-fold into Superbroth and recombinant protein was induced with 1 mM IPTG at an OD(650) of 0.6. After two additional hours of fermentation, inclusion bodies were isolated, washed, and denatured in guanidine hydrochloride and dithioerythritol. Recombinant protein was refolded by diluted 100-fold in cold buffer with arginine and oxidized glutathione. After dialysis, purified protein was obtained after anion-exchange, size exclusion on FPLC, and polymyxin B affinity chromatography. The final material was filter sterilized, aseptically vialed, and stored at -80 degrees C. Seventy-five 3-L bacterial culture preparations were made and pooled for the AT-1 batch (568 mL) and twenty-four 3-L bacterial culture preparations were made and pooled for the AT-2 batch (169 mL). The final product was characterized by Coomassie Plus protein assay, Coomassie-stained SDS-PAGE, limulus amebocyte lysate endotoxin assay, human AML TF/H-ras cell cytotoxicity assay, sterility, tandem mass spectroscopy, IL3 receptor binding affinity, ADP ribosylation activity, inhibition of normal human CFU-GM, disulfide bond analysis, immunoblots, peptide mapping, stability, HPLC TSK3000, N-terminal sequencing, E. coli DNA contamination, C57BL/6 mouse toxicity, cynomolgus monkey toxicity, and immunohistochemistry. Yields were 25.7+/-5.6 mg/L bacterial culture of denatured fusion toxin. After refolding and chromatography, final yields were 20+/-11% or 5 mg/L. Vialed product was sterile. Batches were in 0.25 M sodium chloride/5 mM Tris, pH 8, and had protein concentrations of 1.8-1.9 mg/mL. Purity by SDS-PAGE was 99+/-1%. Aggregates by HPLC were <1 %. Potency revealed a 48 h IC(50) of 6-8 pM on TF/H-ras cells. Endotoxin levels were 1 eu/mg. The remaining chemical and biologic assays confirmed the purity, composition, and functional activities of the molecule. The LD(10) in mice was 250 microg/kg/day every other day for six doses. The MTD in monkeys was 60 microg/kg/day every other day for six doses. Drug did not react with tested frozen human tissue sections by immunohistochemistry. There was no evidence of loss of solubility, proteolysis aggregation, or loss of potency over 6 months at -80 and -20 degrees C. Further, the drug was stable at 4 and 25 degrees C in the plastic syringe and administration tubing for 24 h and at 37 degrees C in human serum for 24 h. The synthesis of this protein drug should be useful for production for clinical phase I/II clinical trials and may be suitable for other diphtheria fusion toxins indicated for clinical development.

Stimulation of hTAFII68 (NTD)-mediated transactivation by v-Src

The three genes hTAF(II)68, EWS, and TLS (called the TET family) encode related RNA binding proteins containing an RNA recognition motif and three glycine-, arginine-, and proline-rich regions in the C-terminus and a degenerated repeat containing the consensus sequence Ser-Tyr-Gly-Gln-Ser in the N-terminus. In many human cancers, the N-terminal portion of hTAF(II)68, EWS, or TLS is fused to the DNA binding domain of one of several transcription factors including Fli-1, ERG, ETV1, E1AF, WT1, ATF-1, CHOP, or TEC. We have recognized the presence of several potential tyrosine phosphorylation sites within the amino-terminal domain of hTAF(II)68 and have investigated the potential effects of cytoplasmic signaling on hTAF(II)68 function. Herein, we find that hTAF(II)68 is phosphorylated on tyrosine residue(s) by ectopic expression of v-Src protein tyrosine kinase in vitro and in vivo. The hTAF(II)68 protein can associated with the SH3 domains of several cell signaling proteins, including v-Src protein tyrosine kinase. We also document that full-length v-Src can stimulate hTAF(II)68-mediated transcriptional activation, whereas deletion mutants of v-Src are unable to exert this effect. In addition, cellular Src activity appears important for hTAF(II)68 function since hTAF(II)68-mediated transactivation is reduced in a dose-dependent fashion by ectopic overexpression of a dominant-negative mutant of Src. Taken together, our results suggest that the biological activities of hTAF(II)68 are linked to the cytoplasmic Src signal transduction pathway.

Conjugated linoleic acid induces human adipocyte delipidation: autocrine/paracrine regulation of MEK/ERK signaling by adipocytokines

Dietary conjugated linoleic acid (CLA) reduces body fat in animals and some humans. Here we show that trans-10, cis-12 CLA, but not cis-9, trans-11 CLA, when added to cultures of stromal vascular cells containing newly differentiated human adipocytes, caused a time-dependent decrease in triglyceride content, insulin-stimulated glucose and fatty acid uptake, incorporation into lipid, and oxidation compared with controls. In parallel, gene expression of peroxisome proliferator-activated receptor-gamma and many of its downstream targets were diminished by trans-10, cis-12 CLA, whereas leptin gene expression was increased. Prior to changes in gene expression and metabolism, trans-10, cis-12 CLA caused a robust and sustained activation of mitogen-activated protein kinase kinase/extracellular signal-related kinase (MEK/ERK) signaling. Furthermore, the trans-10, cis-12 CLA-mediated activation of MEK/ERK could be attenuated by pretreatment with U0126 and pertussis toxin. In parallel, pretreatment with U0126 blocked the ability of trans-10, cis-12 CLA to alter gene expression and attenuate glucose and fatty acid uptake of the cultures. Intriguingly, the induction by CLA of MEK/ERK signaling was linked to hypersecretion of adipocytokines interleukin-6 and interleukin-8. Collectively, these data demonstrate for the first time that trans-10, cis-12 CLA decreases the triglyceride content of newly differentiated human adipocytes by inducing MEK/ERK signaling through the autocrine/paracrine actions of interleukins-6 and 8.

MAPK signaling pathways modulate IL-1β expression in human keratinocytes

The signaling pathways that modulate IL-1beta expression in human keratinocytes have not been well defined. We have previously shown that TCDD-stimulated AhR-dependent IL-1beta expression in human keratinocytes is due to posttranscriptional regulation involving mRNA stabilization. Since TCDD activates a variety of cellular signaling pathways such as PKC, JNK, and ERK, we investigated these pathways to determine their roles in TCDD-stimulated IL-1beta expression in the human keratinocyte cell line SCC-12F. In this study, we used specific signaling inhibitors to show that ERK and JNK, but not transglutaminase, PKC, or p38, signaling modulate IL-1beta expression. In addition, we show that ERK is constitutively active and unaffected by TCDD treatment and differentiation, while the JNK signaling pathway is modulated by TCDD in an AhR-dependent manner. Thus, both the ERK and JNK MAPK pathways are necessary for IL-1beta expression in TCDD-stimulated human keratinocytes, however, they act at different levels to modulate IL-1beta expression.

Downmodulation of ERK protein kinase activity inhibits VEGF secretion by human myeloma cells and myeloma-induced angiogenesis

The mitogen-activated protein (MAP) cascade leading to the activation of extracellular signal-regulated kinases 1/2 (ERK1/2) is critical for regulating myeloma cell growth; however, the relationship of ERK1/2 activity with vascular endothelial growth factor (VEGF) production and the effects of its downmodulation in myeloma cells are not elucidated. We found that the treatment with MAP/ERK kinase 1 (MEK1) inhibitors PD98059 or PD184352 produced a reduction of phosphorylated ERK1/2 (p-ERK1/2) levels in myeloma cells of more than 80% and prevented the increase of p-ERK1/2 induced by interleukin-6 (IL-6). MEK1 inhibitors also induced a significant inhibition of myeloma cell proliferation and blunted the stimulatory effect induced by IL-6. A significant inhibition of basal VEGF secretion by myeloma cells as well as a suppression of the stimulatory effect of IL-6 on VEGF was observed by either PD98059 or PD184352. Moreover, we also found that the PI3K kinase inhibitors, but not p38 MAPK inhibitors, reduced VEGF secretion by myeloma cells and increase the inhibitory effect of MEK1 inhibitors. In an 'in vitro' model of angiogenesis, we found that MEK1 inhibitors impair vessel formation induced by myeloma cells and restored by VEGF treatment, suggesting that the downmodulation of ERK1/2 activity reduces myeloma-induced angiogenesis by inhibiting VEGF secretion.

The structure of the MAPK scaffold, MP1, bound to its partner, p14. A complex with a critical role in endosomal map kinase signaling

Scaffold proteins of the mitogen-activated protein kinase (MAPK) pathway have been proposed to form an active signaling module and enhance the specificity of the transduced signal. Here, we report a 2-A resolution structure of the MAPK scaffold protein MP1 in a complex with its partner protein, p14, that localizes the complex to late endosomes. The structures of these two proteins are remarkably similar, with a five-stranded beta-sheet flanked on either side by a total of three helices. The proteins form a heterodimer in solution and interact mainly through the edge beta-strand in each protein to generate a 10-stranded beta-sheet core. Both proteins also share structural similarity with the amino-terminal regulatory domains of the membrane trafficking proteins, sec22b and Ykt6p, as well as with sedlin (a component of a Golgi-associated membrane-trafficking complex) and the sigma2 and amino-terminal portion of the mu2 subunits of the clathrin adaptor complex AP2. Because neither MP1 nor p14 have been implicated in membrane traffic, we propose that the similar protein folds allow these relatively small proteins to be involved in multiple and simultaneous protein-protein interactions. Mapping of highly conserved, surface-exposed residues on MP1 and p14 provided insight into the potential sites of binding of the signaling kinases MEK1 and ERK1 to this complex, as well as the areas potentially involved in other protein-protein interactions.

Does the dual-specificity MAPK phosphatase Pyst2-L lead a monogamous relationship with the Erk2 protein?

The dual-specificity phosphatase Pyst2-L was found to be highly expressed in leukocytes derived from AML and ALL patients as well as in certain other solid tumors and lymphoblastoid cell lines. Recently, by use of the 5'-RNA ligation-mediated rapid amplification of cDNA ends (5'-RLM-RACE) technique, we sequenced and cloned the entire open reading frame (ORF) of Pyst2-L. In the present study we determined the effect of exogenous overexpression on Erk1/2 phosphorylation. It was demonstrated that overexpression of this phosphatase in HEK293 cells reduced the basal levels of phospho-Erk1/2 as compared to the same cells transfected with the wild-type vector. This reduction was concomitant with a growth retardation of the Pyst2-L-transfected cells. Treating Pyst2-L transfected cells with known activators of the MAPK signaling cascade such as TPA or stimulating them by serum, it was demonstrated that the up regulation of phospho-Erk1/2, caused by these activators, was only partially suppressed by the over expression of the Pyst2-L phosphatase in these cells. These results together with our previous ones showing that the TPA-induced up regulation of Pyst2-L mRNA was only partially inhibited by the use of a specific Mek1/2 inhibitor, lead us to ask whether the Pyst2-L phosphatase has a monogamous relationship with the Erk2 protein. To answer this question, we employed the pull-down method and showed that in addition to phospho-Erk1/2, recombinant Pyst2-L binds the phospho-JNK protein. These findings may raise new perspectives regarding the role played by this phosphatase in malignant cells and in activation processes.

Diphtheria toxin fused to variant interleukin-3 provides enhanced binding to the interleukin-3 receptor and more potent leukemia cell cytotoxicity

Chemoresistance is a common cause of treatment failure in patients with acute myeloid leukemia (AML). We generated a diphtheria toxin (DT) fusion protein composed of the catalytic and translocation domains of DT (DT388) fused to interleukin-3 (IL-3). IL-3 receptors (IL-3R) are overexpressed on blasts from many AML patients. DT388IL-3 showed cytotoxicity to leukemic blasts in vitro and in vivo and minimal damage to normal tissues in nonhuman primate models. However, only a fraction of patient leukemic samples were sensitive to the agent. To enhance the potency and specificity of the DT388IL-3 molecule, we constructed variants with altered residues in the IL-3 moiety. Two of these variants, DT388IL-3[K116W] and DT388IL-3[Delta125-133], were produced and partially purified from Escherichia coli with excellent yields. They showed enhanced binding to the human IL-3R and greater cytotoxicity to human leukemia cell lines relative to wild-type DT388IL-3. Interestingly, the results support a previously hypothesized model for interaction of the C-terminal residues of IL-3 with a hydrophobic patch on the alpha-subunit of IL-3R. Rational modification of the targeting domain based on structural analysis can produce a fusion toxin with increased ability to kill tumor cells. One or both of these variant fusion proteins merit further development for therapy of chemotherapy refractory AML.

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.

Signal transduction pathways that inhibit hepatitis B virus replication

The replication of hepatitis B virus (HBV) in hepatocytes is strongly inhibited in response to IFN-alpha/beta and IFN-gamma. Although it has been previously demonstrated that IFN-alpha/beta eliminates HBV RNA-containing capsids from the cell in a proteasome-dependent manner, the precise cellular pathway that mediates this antiviral effect has not been identified. Because IFN-induced signal transduction involves kinase-mediated activation of gene expression, we used an immortalized hepatocyte cell line that replicates HBV in an IFN-sensitive manner to investigate the role of cellular kinase activity and the cellular transcription and translation machinery in the antiviral effect. Our results indicate that Janus kinase activity is required for the antiviral effect of IFN against HBV, but that phosphatidylinositol 3-kinase, cyclin-dependent kinase, mitogen-activated protein kinase, and NF-kappaB activity are not. Additionally, we found that inhibitors of cellular transcription and translation completely abolish the antiviral effect, which also appears to require cellular kinase activity downstream of signal transduction and gene expression. Collectively, these results identify IFN-regulated pathways that interrupt the HBV replication cycle by eliminating viral RNA-containing capsids from the cell, and they provide direction for discovery of the terminal effector molecules that ultimately mediate this antiviral effect.

Overexpression of MEKK3 confers resistance to apoptosis through activation of NFkappaB

Many cancers have constitutively activated NFkappaB, the elevation of which contributes to cancer cell resistance to chemotherapeutic agent-induced apoptosis. Although mitogen-activated protein kinase/extracellular-regulated kinase kinase kinase-3 (MEKK3) has been shown to participate in the activation of NFkappaB, its relations to apoptosis and cancer are unclear. In this study, we established cell model systems to examine whether stable expression of MEKK3 could lead to increased NFkappaB activity and confer resistance to apoptosis. In addition, we investigated in breast and ovarian cancers whether MEKK3 expression may be altered and correlated with aberrant NFkappaB activity. We show that stable cell lines overexpressing MEKK3 not only had elevated levels of NFkappaB binding activity but also were more responsive to cytokine stimulation. These stable cells showed 2-4-fold higher basal expression of Bcl-2 and xIAP than the parental cells. Consistent with this increased expression of cell survival genes, MEKK3 stable cells showed reduced activation of caspases 3 and 8 and poly(ADP-ribose) polymerase cleavage and dramatically increased resistance to apoptosis induced by tumor necrosis factor-related apoptosis-inducing ligand, doxorubicin, daunorubicin, camptothecin, and paclitaxel. Intriguingly, analysis of human breast and ovarian cancers showed that a significant fraction of these samples have elevated MEKK3 protein levels with corresponding increases in NFkappaB binding activities. Thus, our results established that elevated expression of MEKK3 appears to be a frequent occurrence in breast and ovarian cancers and that overexpression of MEKK3 in cells leads to increased NFkappaB activity and increased expression of cell survival factors and ultimately contributes to their resistance to apoptosis. As such, MEKK3 may serve as a therapeutic target to control cancer cell resistance to cytokine- or drug-induced apoptosis.

Interleukin-3 receptor in acute leukemia

Recent studies indicate that abnormalities of the interleukin-3 receptor (IL-3R) are frequently observed in acute myeloid leukemias (AMLs) and may contribute to the proliferative advantage of leukemic blasts. This review analyzes the evidences indicating that the IL-3R represents one of the target molecules involved in the stimulation of proliferation of AMLs, and the overexpression of the IL-3Ralpha chain may represent one of the mechanisms contributing to the development of a highly malignant leukemic phenotype. Furthermore, there is evidence that the IL-3Ralpha is a marker of leukemic stem cells, at variance with normal stem cells that are IL-3Ralpha-. Finally, the IL-3R may represent an important target for the development of new antileukemic drugs.

Induction of apoptosis in human leukemia cells by the tyrosine kinase inhibitor adaphostin proceeds through a RAF-1/MEK/ERK- and AKT-dependent process

Effects of the tyrphostin tyrosine kinase inhibitor adaphostin (NSC 680410) have been examined in human leukemia cells (Jurkat, U937) in relation to mitochondrial events, apoptosis, and perturbations in signaling and cell cycle regulatory events. Exposure of cells to adaphostin concentrations > or =0.75 microM for intervals > or =6 h resulted in a pronounced release of cytochrome c and AIF, activation of caspase-9, -8, and -3, and apoptosis. These events were accompanied by the caspase-independent downregulation of Raf-1, inactivation of MEK1/2, ERK, Akt, p70S6K, dephosphorylation of GSK-3, and activation of c-Jun-N-terminal kinase (JNK) and p38 MAPK. Adaphostin also induced cleavage and dephosphorylation of pRb on CDK2- and CDK4-specific sites, as well as the caspase-dependent downregulation of cyclin D1. Inducible expression of a constitutively active MEK1 construct markedly diminished adaphostin-induced cytochrome c and AIF release, JNK activation, and apoptosis in Jurkat cells. Ectopic expression of Raf-1 or constitutively activated (myristolated) Akt also significantly attenuated adaphostin-induced apoptosis, but protection was less than that conferred by enforced activation of MEK. Lastly, antioxidants (e.g., L-N-acetylcysteine; L-NAC) opposed adaphostin-mediated mitochondrial dysfunction, Raf-1/MEK/ERK downregulation, JNK activation, and apoptosis. However, in contrast to L-NAC, enforced activation of MEK failed to block adaphostin-mediated ROS generation. Together, these findings demonstrate that the tyrphostin adaphostin induces multiple perturbations in signal transduction pathways in human leukemia cells, particularly inactivation of the cytoprotective Raf-1/MEK/ERK and Akt cascades, that culminate in mitochondrial injury, caspase activation, and apoptosis. They also suggest that adaphostin-related oxidative stress acts upstream of perturbations in these signaling pathways to trigger the cell death process.

Regulation of SPIN90 Phosphorylation and Interaction with Nck by ERK and Cell Adhesion

SPIN90 is a widely expressed Nck-binding protein that contains one Src homology 3 (SH3) domain, three Pro-rich motifs, and a serine/threonine-rich region, and is known to participate in sarcomere assembly during cardiac myocyte differentiation. We used in vitro binding assays and yeast two-hybrid screening analysis to identify Nck, betaPIX, Wiscott-Aldrich syndrome protein (WASP), and ERK1 as SPIN90-binding proteins. It appears that betaPIX, WASP, and SPIN90 form a complex that interacts with Nck in a manner dependent upon cell adhesion to extracellular matrix. The betaPIX.WASP.SPIN90.Nck interaction was abolished in suspended and cytochalasin D-treated cells, but was recovered when cells were replated on fibronectin-coated dishes. The SPIN90.betaPIX.WASP complex was stable, even in suspended cells, suggesting SPIN90 serves as an adaptor molecule to recruit other proteins to Nck at focal adhesions. In addition, we found that overexpression of the SPIN90 SH3 domain or Pro-rich region, respectively, abolished SPIN90.Nck and SPIN90.betaPIX interactions, resulting in detachment of cells from extracellular matrix. SPIN90 was phosphorylated by ERK1, which was, itself, activated by cell adhesion and platelet-derived growth factor. Such phosphorylation of SPIN90 likely promotes the interaction of the SPIN90.betaPIX.WASP complex and Nck. It thus appears that the interaction of the betaPIX.WASP.SPIN90 complex with Nck is crucial for stable cell adhesion and can be dynamically modulated by SPIN90 phosphorylation that is dependent on cell adhesion and ERK activation.

Pharmacogenetic candidate genes for melanoma

The incidence of melanoma is rising at an alarming rate and has become an important public health concern. If detected early, melanoma carries an excellent prognosis after appropriate surgical resection. Unfortunately, advanced melanoma has a poor prognosis and is notoriously resistant to radiation and chemotherapy. The relative resistance of melanoma to a wide-range of chemotherapeutic agents and high toxicity of current therapies has prompted a search for effective alternative treatments that would improve prognosis and limit side effects. Advances in molecular genetics are revealing in increasing detail the mechanisms responsible for the development of melanoma. Hopefully, elucidation of these pathways will provide a means of screening high-risk individuals and allow new drug development for prevention and treatment by identification of specific pharmacological targets. This review will summarize the genetics of melanoma with the goal of providing insights into potential pharmacogenetic candidate genes.

Treatment by design in leukemia, a meeting report, Philadelphia, Pennsylvania, December 2002

Novel approaches have been designed to treat leukemia based on our understanding of the genetic and biochemical lesions present in different malignancies. This meeting report summarizes some of the recent advances in leukemia treatment. Based on the discoveries of cellular oncogenes, chromosomal translocations, monoclonal antibodies, multidrug resistance pumps, signal transduction pathways, genomics/proteonomic approaches to clinical diagnosis and mutations in biochemical pathways, clinicians and basic scientists have been able to identify the particular genetic mutations and signal transduction pathways involved as well as design more appropriate treatments for the leukemia patient. This meeting report discusses these exciting new therapies and the results obtained from ongoing clinical trials. Furthermore, rational approaches to treat complications of tumor lysis syndrome by administration of the recombinant urate oxidase protein, also known as rasburicase, which corrects the biochemical defect present in humans, were discussed. Clearly, over the past 25 years, molecular biology and biotechnology has provided the hematologist/oncologist novel bullets in their arsenal that will allow treatment by design in leukemia.

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.

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.