Effects of raf kinase inhibitor protein expression on suppression of prostate cancer metastasis

An Orthotopic Model of Human Osteosarcoma Growth and Spontaneous Pulmonary Metastasis

Osteosarcoma is the most common primary malignancy of bone and patients often develop pulmonary metastases. In order to investigate the pathogenesis of human osteosarcoma, there is a great need to develop a clinically relevant animal model. Here we report the development of an osteosarcoma animal model using three related human osteosarcoma lines, the parental TE-85 and two derivative lines MNNG/HOS and 143B. In vitro characterization demonstrated that the 143B line had the greatest cell migration and the least cell adhesion activities among the three lines. The 143B line also exhibited the greatest ability for anchorage independent growth. When GFP-tagged osteosarcoma cells were injected into the proximal tibia of athymic mice, we found that 143B cells were highly tumorigenic and metastatic, and MNNG/HOS cells were tumorigenic but significantly less metastatic. TE85 cells were neither tumorigenic nor metastatic. The number of pulmonary metastases was found 50-fold higher in 143B injected animals than that in MNNG/HOS injected mice. No pulmonary metastases were detected in TE85 injected animals for up to 8 weeks. Primary tumors formed by MNNG/HOS and 143B cells could be visualized by whole body fluorescence imaging, while the pulmonary metastases were visualized on the necropsied samples. The GFP tagged 143B cells (and to a lesser extent, MNNG/HOS cells) were readily recovered from lung metastases. This clinically relevant model of human osteosarcoma provides varying degrees of tumor growth at the primary site and metastatic potential. Thus, this orthotopic model should be a valuable tool to investigate factors that promote or inhibit osteosarcoma growth and/or metastasis.

A single amino acid converts a repressor to an activator of flowering

Homologous proteins occurring through gene duplication may give rise to novel functions through mutations affecting protein sequence or expression. Comparison of such homologues allows insight into how morphological traits evolve. However, it is often unclear which changes are key to determining new functions. To address these ideas, we have studied a system where two homologues have evolved clear and opposite functions in controlling a major developmental switch. In plants, flowering is a major developmental transition that is critical to reproductive success. Arabidopsis phosphatidylethanolamine-binding protein homologues TERMINAL FLOWER 1 (TFL1) and FLOWERING LOCUS T (FT) are key controllers of flowering, determining when and where flowers are made, but as opposing functions: TFL1 is a repressor, FT is an activator. We have uncovered a striking molecular basis for how these homologous proteins have diverged. Although <60% identical, we have shown that swapping a single amino acid is sufficient to convert TFL1 to FT function and vice versa. Therefore, these key residues may have strongly contributed to the selection of these important functions over plant evolution. Further, our results suggest that TFL1 and FT are highly conserved in biochemical function and that they act as repressors or activators of flowering through discrimination of structurally related interactors by a single residue.

The biology of a prostate cancer metastasis suppressor protein: Raf kinase inhibitor protein

Raf kinase inhibitor protein (RKIP) was originally identified as a protein that bound membrane phospholipids and was named phosphatidylethanolamine binding protein-2 (PEBP-2). RKIP was than identified as a protein that bound Raf and blocked its ability to phosphorylate MEK, thus earning its new name of RKIP. Subsequent to identification of its role in the Raf:MEK pathway, RKIP has been demonstrated to regulate several other signaling pathways including G-protein signaling and NF-kappaB signaling. Its involvement in several signaling pathways has engendered RKIP to contribute to several physiological processes including membrane biosynthesis, spermatogenesis, neural development, and apoptosis. RKIP is expressed in many tissues including brain, lung, and liver and thus, dysregulation of RKIP expression or function has potential to contribute to pathophysiology in these tissues. Loss of RKIP expression in prostate cancer cells confers a metastatic phenotype on them. Additionally, restoration of RKIP expression in a metastatic prostate cancer cell line does not effect primary tumor growth, but it does inhibit prostate cancer metastasis. These parameters identify RKIP as a metastasis suppressor gene. In this review, the biology and pathophysiology of RKIP is described.

Raf kinase inhibitory protein regulates Raf-1 but not B-Raf kinase activation

Raf kinase inhibitory protein (RKIP; also known as phosphatidylethanolamine-binding protein or PEBP) is a modulator of the Raf/MAPK signaling cascade and a suppressor of metastatic cancer. Here, we show that RKIP inhibits MAPK by regulating Raf-1 activation; specifically, RKIP acts subsequent to Raf-1 membrane recruitment, prevents association of Raf-1 and p21-activated kinase (PAK), and blocks phosphorylation of the Raf-1 kinase domain by PAK and Src family kinases. Mutation of the PAK and Src phosphorylation sites on Raf-1 to aspartate, a phosphate mimic, prevented RKIP association with or inhibition of Raf-1 signaling. Interestingly, although RKIP can interact with B-Raf, RKIP depletion had no effect on activation of B-Raf. Because c-Raf-1 and B-Raf are both required for maximal MAPK stimulation by epidermal growth factor in neuronal and epithelial cell lines, we determined whether RKIP significantly affects MAPK signaling. In fact, RKIP depletion increased not only the amplitude but also the sensitivity of MAPK and DNA synthesis to epidermal growth factor stimulation by up to an order of magnitude. These results indicate that selective modulation of c-Raf-1 but not B-Raf activation by RKIP can limit the dynamic range of the MAPK signaling response to growth factors and may play a critical role in growth and development.

Cellular and molecular signal transduction pathways modulated by rituximab (rituxan, anti-CD20 mAb) in non-Hodgkin's lymphoma: implications in chemosensitization and therapeutic intervention

The clinical application of rituximab (chimeric mouse anti-human CD20 mAb, Rituxan, IDEC-C2B8), alone and/or combined with chemotherapy, has significantly ameliorated the treatment outcome of patients with relapsed and refractory low-grade or follicular non-Hodgkin's lymphoma (NHL). The exact in vivo mechanisms of action of rituximab are not fully understood, although antibody-dependent cell-mediated cytotoxicity (ADCC), complement-dependent cytotoxicity (CDC), and apoptosis have been suggested. We have proposed that modifications of the cellular signaling pathways by rituximab may be crucial for its clinical response. The B-cell restricted cell surface phosphoprotein CD20 is involved in many cellular signaling events including proliferation, activation, differentiation, and apoptosis upon crosslinking. Monomeric rituximab chemosensitizes drug-resistant NHL cells via selective downregulation of antiapoptotic factors through the type II mitochondrial apoptotic pathway. Several signaling pathways are affected by rituximab which are implicated in the underlying molecular mechanisms of chemosensitization. ARL (acquired immunodeficiency syndrome (AIDS)-related lymphoma) and non-ARL cell lines have been examined as in vitro model systems. In ARL, rituximab diminishes the activity of the p38MAPK signaling pathway resulting in inhibition of the interleukin (IL)-10/IL-10R autocrine/paracrine cytokine autoregulatory loop leading to the inhibition of constitutive STAT-3 activity and subsequent downregulation of Bcl-2 expression leading to chemosensitization. Rituximab upregulates Raf-1 kinase inhibitor protein (RKIP) expression in non-ARL cells. Through physical association with Raf-1 and nuclear factor kappaB (NF-kappa B)-inducing kinase (NIK), RKIP negatively regulates two major survival pathways, namely, the extracellular signal-regulated kinase1/2 (ERK1/2) and the NF-kappa B pathways, respectively. Downmodulation of the ERK1/2 and NF-kappa B pathways inhibits the transcriptional activity of AP-1 and NF-kappa B transcription factors, respectively, both of which lead to the downregulation of Bcl-(xL) (Bcl-2 related gene (long alternatively spliced variant of Bcl-x gene)) transcription and expression and sensitization to drug-induced apoptosis. Bcl-(xL)-overexpressing cells corroborated the pivotal role of Bcl-(xL) in chemosensitization. The specificity of rituximab-mediated signaling and functional effects were corroborated by the use of specific pharmacological inhibitors. Many patients do not respond and/or relapse and the mechanisms of unresponsiveness are unknown. Rituximab-resistant B-NHL clones were generated to investigate the acquired resistance to rituximab-mediated signaling, and chemosensitization. Resistant clones display different phenotypic, genetic and functional properties compared to wild-type cells. This review summarizes the data highlighting a novel role of rituximab as a signal-inducing antibody and as a chemosensitizing agent through negative regulation of major survival pathways. Studies presented herein also reveal several intracellular targets modified by rituximab, which can be exploited for therapeutic and prognostic purposes in the treatment of patients with rituximab- and drug-refractory NHL.

Rituximab (Chimeric Anti-CD20 Monoclonal Antibody) Inhibits the Constitutive Nuclear Factor-κB Signaling Pathway in Non-Hodgkin's Lymphoma B-Cell Lines: Role in Sensitization to Chemotherapeutic Drug-induced Apoptosis

The chimeric anti-CD20 antibody rituximab (Rituxan, IDEC-C2B8) is widely used in the clinical treatment of patients with non-Hodgkin's lymphoma (NHL). Rituximab sensitizes NHL B-cell lines to drug-induced apoptosis via down-regulation of Bcl-xL expression. We hypothesized that the mechanism by which rituximab down-regulates Bcl-xL may be, in part, due to inhibition of constitutive nuclear factor-κB (NF-κB) activity that regulates Bcl-xL expression. This hypothesis was tested in CD20+ drug-resistant Ramos (Bcl-2−/Bcl-xL+) and Daudi (Bcl-2+/Bcl-xL+) cell lines. Rituximab decreased the phosphorylation of NF-κB-inducing kinase, IκB kinase, and IκB-α, diminished IKK kinase activity, and decreased NF-κB DNA binding activity. These events occurred with similar kinetics and were observed 3 to 6 hours post-rituximab treatment. Rituximab significantly up-regulated Raf-1 kinase inhibitor protein expression, thus interrupting the NF-κB signaling pathway concomitant with Bcl-xL and Bfl-1/A1 down-regulation. The role of NF-κB in the regulation of Bcl-xL transcription was shown using promoter reporter assays in which deletion of the two-tandem NF-κB binding sites in the upstream promoter region significantly reduced the luciferase activity. This was further corroborated by using IκB superrepressor cells and by NF-κB–specific inhibitors. The direct role of Bcl-xL in drug resistance was assessed by using Bcl-xL–overexpressing cells, which exhibited higher drug resistance that was partially reversed by rituximab. Rituximab-mediated inhibition of the NF-κB signaling pathway and chemosensitization was corroborated by the use of specific inhibitors. These findings reveal a novel pathway mediated by rituximab through Raf-1 kinase inhibitor protein induction that negatively regulates the constitutive NF-κB pathway and chemosensitization of the NHL B-cells.

Tfs1p, a member of the PEBP family, inhibits the Ira2p but not the Ira1p Ras GTPase-activating protein in Saccharomyces cerevisiae

Ras proteins are guanine nucleotide-binding proteins that are highly conserved among eukaryotes. They are involved in signal transduction pathways and are tightly regulated by two sets of antagonistic proteins: GTPase-activating proteins (GAPs) inhibit Ras proteins, whereas guanine exchange factors activate them. In this work, we describe Tfs1p, the first physiological inhibitor of a Ras GAP, Ira2p, in Saccharomyces cerevisiae. TFS1 is a multicopy suppressor of the cdc25-1 mutation in yeast and corresponds to the so-called Ic CPY cytoplasmic inhibitor. Moreover, Tfs1p belongs to the phosphatidylethanolamine-binding protein (PEBP) family, one member of which is RKIP, a kinase and serine protease inhibitor and a metastasis inhibitor in prostate cancer. In this work, the results of (i) a two-hybrid screen of a yeast genomic library, (ii) glutathione S-transferase pulldown experiments, (iii) multicopy suppressor tests of cdc25-1 mutants, and (iv) stress resistance tests to evaluate the activation level of Ras demonstrate that Tfs1p interacts with and inhibits Ira2p. We further show that the conserved ligand-binding pocket of Tfs1-the hallmark of the PEBP family-is important for its inhibitory activity.

p42/p44 Mitogen-Activated Protein Kinase Signal Transduction Pathway: A Novel Target for the Treatment of Hormone-Resistant Prostate Cancer?

Prostate cancer is the second leading cause of cancer deaths in men. Conventional therapies produce a high rate of cure for patients with localized prostate cancer, but there is no cure once the disease has spread beyond the prostate. Androgen withdrawal remains the only treatment for these men with clinically advanced disease; however, most of these men, who initially respond to hormone ablation therapy, fail and the disease progresses. There is at present no effective treatment for hormone-independent prostate cancer. Several lines of evidence suggest a role of p42/p44 mitogen-activated protein kinase (p42/p44 MAP kinase) signal transduction pathways in prostate cancer. At the molecular level, a variety of genetic alterations lead to an epigenetic mechanism by which a feedback autocrine loop between membrane receptors and associated ligands serves as an essential component of the growth, proliferation, and metastasis of prostate cancer at an advanced and androgen-independent stage. Peptide growth factors are known to exert their effects by a complex array of mechanisms primarily mediated by the p42/p44 MAP kinase signal transduction pathway. Thus, we hypothesized that MAP kinase signal transduction pathways could serve as new and novel targets in prostate cancer therapy. In this article we provide an overview of the role played by MAP kinase signal transduction in the prostate.

Metastasis suppressor genes: a role for raf kinase inhibitor protein (RKIP)

The metastatic cascade is a complicated process that involves many steps from gain of the metastatic phenotype in the primary tumor cells through establishment of macroscopic tumor at the distant target organ. A group of genes, termed metastasis suppressor genes (MSG), encode for proteins that inhibit various steps of the metastatic cascade. Accordingly, loss of MSG promotes the metastatic phenotype. Although several MSG have been identified, the mechanisms through which they enhance metastasis are not clearly defined. Gene array analysis of a low metastatic LNCaP prostate cancer cell line compared to its highly metastatic derivative C4-2B prostate cancer cell line revealed decreased expression of raf kinase inhibitor protein (RKIP) in the C4-2B cell line. RKIP blocks the activation of several signaling pathways including MEK, G-proteins and NFkappaB. Immunohistochemical analysis of prostate cancer primary tumors and metastases revealed that RKIP protein expression was decreased in metastases. Restoration of RKIP expression in the C4-2B cell line diminished metastasis in a murine model. These results demonstrate that RKIP is a MSG. Loss of RKIP enhanced both angiogenesis and vascular invasion, and protected against apoptosis. These findings suggest that targeting the RKIP pathway may diminish the metastatic cascade. However, challenges exist as to the best method to target RKIP expression. Restoration of RKIP expression in all cancer cells in vivo is challenging. A plausible strategy is to use small molecules that target proteins in signaling pathways that are dysregulated due to loss of RKIP.

Raf kinase inhibitory protein inhibits beta-cell proliferation

BACKGROUND

Raf-1 kinase inhibitory protein (RKIP) was recently identified as a physiologic endogenous inhibitor of the extracellular signal-regulated kinase (ERK) pathway. The expression and role of RKIP within the pancreas are unknown.

METHODS

RKIP expression in normal pancreas and human insulinomas was examined by using paraffin-embedded sections. Co-localization of RKIP within islet cell subtypes was performed by using double immunofluorescence staining with antibodies directed toward RKIP and endocrine markers. To examine the role of RKIP in beta-cell proliferation, stable expression of sense (ss) and antisense (as) RKIP was established in HIT-T15 beta cells. The effect of RKIP on the ERK-signaling pathway in beta cells was determined by Western blotting with the use of phospho-specific antibodies directed against mitogen-activated protein kinase kinase (MEK) and ERK. The role of RKIP in beta-cell proliferation was assessed by using MTS assay and FACS analysis.

RESULTS

RKIP was expressed only within pancreatic islet cells. Immunofluorescent double staining revealed that RKIP was expressed in most beta cells and a subset of pancreatic polypeptide-expressing cells. Based on the known function of RKIP, we hypothesized that RKIP expression would be downregulated in insulinomas: 8 of 9 human insulinomas demonstrated no RKIP staining, with decreased expression in 1 of 9 insulinomas. Studies using asRKIP and ssRKIP demonstrated that RKIP blocked activation of MEK and ERK by Raf-1 in beta cells. We also showed that RKIP inhibited beta-cell proliferation by altering cell cycle distribution, rather than by promoting apoptosis.

CONCLUSIONS

RKIP is important in beta-cell proliferation, and its downregulation may play a role in islet neoplasia.

Inhibition of the Raf-MEK1/2-ERK1/2 signaling pathway, Bcl-xL down-regulation, and chemosensitization of non-Hodgkin's lymphoma B cells by Rituximab

Rituximab (Rituxan, IDEC-C2B8) has been shown to sensitize non-Hodgkin's lymphoma (NHL) cell lines to chemotherapeutic drug-induced apoptosis. Rituximab treatment of Bcl-2-deficient Ramos cells and Bcl-2-expressing Daudi cells selectively decreases Bcl-(xL) expression and sensitizes the cells to paclitaxel-induced apoptosis. This study delineates the signaling pathway involved in rituximab-mediated Bcl-(xL) down-regulation in Ramos and Daudi NHL B cells. We hypothesized that rituximab may interfere with the extracellular signal-regulated kinase (ERK) 1/2 pathway, leading to decreased Bcl-(xL) expression. Rituximab (20 microg/mL) inhibited the kinase activity of mitogen-activated protein kinase kinase (MEK) 1/2 and reduced the phosphorylation of the components of the ERK1/2 pathway (Raf-1, MEK1/2, and ERK1/2) and decreased activator protein-1 DNA binding activity and Bcl-(xL) gene expression. These events occurred with similar kinetics and were observed 3 to 6 hours after rituximab treatment. Rituximab-mediated effects were corroborated by using specific inhibitors of the ERK1/2 pathway, which also reduced Bcl-(xL) levels and sensitized the NHL B cells to paclitaxel-induced apoptosis. Previous findings implicated a negative regulatory role of the Raf-1 kinase inhibitor protein (RKIP) on the ERK1/2 pathway. Rituximab treatment of NHL B cells significantly up-regulated RKIP expression, thus interrupting the ERK1/2 signaling pathway through the physical association between Raf-1 and RKIP, which was concomitant with Bcl-(xL) down-regulation. These novel findings reveal a signaling pathway triggered by rituximab, whereby rituximab-mediated up-regulation of RKIP adversely regulates the activity of the ERK1/2 pathway, Bcl-(xL) expression, and subsequent chemosensitization of drug-refractory NHL B cells. The significance of these findings is discussed.

The role of Raf kinase inhibitor protein (RKIP) in health and disease

Raf kinase inhibitor protein (RKIP) is a member of the phosphatidylethanolamine-binding protein (PEBP) family. RKIP plays a pivotal modulatory role in several protein kinase signaling cascades. RKIP binds inhibits Raf-1-mediated phosphorylation of MEK through binding to Raf-1. Protein kinase C (PKC) phosphorylates RKIP, resulting in release of Raf-1 and activation of MEK and ERK. The phosphorylated RKIP binds to and inhibits G-protein-coupled receptor kinase, resulting in sustained G-protein signaling. The regulatory role that RKIP has in cell signaling is reflected in its role in physiology and pathophysiology. RKIP is involved in neural development, cardiac function and spermatogenesis and appears to have serine protease activity. In addition to its roles in physiology, dysregulated RKIP expression has the potential to contribute to pathophysiological processes including Alzheimer's disease and diabetic nephropathy. RKIP has been shown to fit the criteria of being a metastasis suppressor gene, including having decreased expression in prostate cancer metastases and restoring RKIP expression in a prostate cancer cell line diminishes metastasis in a murine model. Clearly, RKIP has multiple molecular and cellular functions. In this review, RKIP's molecular roles in intracellular signaling, its physiological functions and its role in disease are described.

Reduction in Raf kinase inhibitor protein expression is associated with increased Ras-extracellular signal-regulated kinase signaling in melanoma cell lines

Mutations in the Raf signaling pathway are known to play a pivotal role in the progression of malignant melanoma. In this study, we provide evidence that the Raf-1 kinase inhibitory protein (RKIP) and its effects on Raf-1-mediated activation of mitogen-activated protein/extracellular signal-regulated kinase kinase are important for the metastatic potential of malignant melanoma. Screening nine melanoma cell lines at mRNA and protein levels, we detected significant down-regulation of RKIP expression in comparison with normal melanocytes. Loss of RKIP expression in transformed cells in vivo was confirmed in immunohistochemical analyses demonstrating reduction of RKIP expression already in primary melanoma and even stronger down-regulation or complete loss in melanoma metastases. Stable transfection of the melanoma cell line Mel Im with an RKIP expression plasmid blocked the Raf kinase pathway, resulting in down-regulation of extracellular signal-regulated kinase 1/2 and activator protein 1 activity. In very good agreement with the in vivo finding that down-regulation of RKIP expression is most obvious in melanoma metastasis, overexpression of RKIP in the highly invasive Mel Im cell line leads to a significant inhibition of invasiveness in vitro. Taken together, our results suggest that loss of RKIP in malignant melanoma contributes to enhanced invasiveness of transformed cells and therefore to progression of the disease.

Metastasis suppressor genes: signal transduction, cross-talk and the potential for modulating the behavior of metastatic cells

In the past decade, research from various disciplines has stimulated a re-evaluation of our ideas of how cancers metastasize. Two important findings have been fundamental to this re-evaluation: that cancer cells are subject to growth regulation at the secondary site and that a specific class of proteins suppresses the metastatic phenotype. These proteins are encoded by metastasis suppressor genes, which are operationally defined as genes that suppress in vivo metastasis without inhibiting primary tumor growth when transfected into metastatic cell lines and injected into experimental animals. Recent biochemical studies have shown that certain metastasis suppressor proteins participate in highly conserved signal transduction cascades that mediate cellular responses to growth factors, cytokines and cellular stresses. Further elucidation of the biochemical foundations of these pathways coupled with strong in vivo studies should give us insight into the mechanisms of cancer metastasis, and may hold important implications for the future of cancer staging and therapy, using both existing and novel modalities.

Mitogen Activated Protein kinase signal transduction pathways in the prostate

The biochemistry of the mitogen activated protein kinases ERK, JNK, and p38 have been studied in prostate physiology in an attempt to elucidate novel mechanisms and pathways for the treatment of prostatic disease. We reviewed articles examining mitogen-activated protein kinases using prostate tissue or cell lines. As with other tissue types, these signaling modules are links/transmitters for important pathways in prostate cells that can result in cellular survival or apoptosis. While the activation of the ERK pathway appears to primarily result in survival, the roles of JNK and p38 are less clear. Manipulation of these pathways could have important implications for the treatment of prostate cancer and benign prostatic hypertrophy.

Critique of laparoscopic lymphadenectomy in genitourinary oncology

Regional lymphadenectomy is prognostic and selectively therapeutic in urologic oncology. The role of lymphadenectomy continues to be defined with the evolving multimodal management of genitourinary malignancies. Laparoscopy is playing a greater role in the management of genitourinary malignancies and thus, it is germane to critique the role of laparoscopic lymphadenectomy in the management of these tumors. Review of the literature suggests that laparoscopic pelvic lymphadenectomy is feasible with nodal yields commensurate to those in open published series. Although laparoscopic retroperitoneal lymph node dissection for nonseminomatous germ cell tumor is feasible, the technique and efficacy of this procedure require further investigation.

Constitutive activation of the Ras-Raf signaling pathway in metastatic melanoma is associated with poor prognosis

Background

Genes of the Raf family encode kinases that are regulated by Ras and mediate cellular responses to growth signals. Recently, it was shown that activating mutations of BRaf are found with high frequency in human melanomas. The Ras family member most often mutated in melanoma is NRas.

Methods

The constitutive activation of the Ras/Raf signaling pathway suggests an impact on the clinical course of the tumor. To address this notion, we analyzed tumor DNA from 114 primary cutaneous melanomas and of 86 metastatic lesions obtained from 174 patients for mutations in BRaf (exons 15 and 11) and NRas (exons 1 and 2) by direct sequencing of PCR products and correlated these results with the clinical course.

Results

In 57.5% of the tumors either BRaf or NRas were mutated with a higher incidence in metastatic (66.3%) than in primary lesions (50.9%). Although the majority of BRaf mutations affected codon 599, almost 15% of mutations at this position were different from the well-described exchange from valine to glutamic acid. These mutations (V599R and V599K) also displayed increased kinase and transforming activity. Surprisingly, the additional BRaf variants D593V, G465R and G465E showed a complete loss of activity in the in vitro kinase assay; however, cells overexpressing these mutants displayed increased Erk phosphorylation. The correlation of mutational status and clinical course revealed that the presence of BRaf/NRas mutations in primary tumors did not negatively impact progression free or overall survival. In contrast, however, for metastatic lesions the presence of BRAF/NRAS mutations was associated with a significantly poorer prognosis, i.e. a shortened survival.

Conclusion

We demonstrate a high – albeit lower than initially anticipated – frequency of activating BRaf mutations in melanoma in the largest series of directly analyzed tumors reported to date. Notably, the clinical course of patients harboring activating BRaf mutations in metastatic melanoma was significantly affected by the presence of a constitutive BRaf activation in these.

RKIP sensitizes prostate and breast cancer cells to drug-induced apoptosis

Cancer cells are more susceptible to chemotherapeutic agent-induced apoptosis than their normal counterparts. Although it has been demonstrated that the increased sensitivity results from deregulation of oncoproteins during cancer development (Evan, G. I., and Vousden, K. H. (2001) Nature 411, 342-348; Green, D. R., and Evan, G. I. (2002) Cancer Cell 1, 19-30), little is known about the signaling pathways leading to changes in the apoptotic threshold in cancer cells. Here we show that low RKIP expression levels in tumorigenic human prostate and breast cancer cells are rapidly induced upon chemotherapeutic drug treatment, sensitizing the cells to apoptosis. We show that the maximal RKIP expression correlates perfectly with the onset of apoptosis. In cancer cells resistant to DNA-damaging agents, treatment with the drugs does not up-regulate RKIP expression. However, ectopic expression of RKIP resensitizes DNA-damaging agent-resistant cells to undergo apoptosis. This sensitization can be reversed by up-regulation of survival pathways. Down-regulation of endogenous RKIP by expression of antisense and small interfering RNA (siRNA) confers resistance on sensitive cancer cells to anticancer drug-induced apoptosis. Our studies suggest that RKIP may represent a novel effector of signal transduction pathways leading to apoptosis and a prognostic marker of the pathogenesis of human cancer cells and tumors after treatment with clinically relevant chemotherapeutic drugs.

A New Look at an Old Problem: The Survival and Organ-Specific Growth of Metastases

Despite improvements in cancer detection and therapy, metastatic disease is largely incurable. Recent research indicates that tumor cells disseminate widely early in the process of pathogenesis, and that the survival and proliferation of these cells--and thus the development of metastases--depend on interactions between the disseminated cells and their particular microenvironment. Proliferative signals and the inhibition of proapoptotic responses are both critically involved in the development of clinically significant metastases. Identification of the underlying signaling cascades may provide additional targets for antimetastatic therapy.

Raf-1 Kinase Inhibitor Protein: Structure, Function, Regulation of Cell Signaling, and Pivotal Role in Apoptosis

The acquisition of resistance to conventional therapies such as radiation and chemotherapeutic drugs remains the major obstacle in the successful treatment of cancer patients. Tumor cells acquire resistance to apoptotic stimuli and it has been demonstrated that conventional therapies exert their cytotoxic activities primarily by inducing apoptosis in the cells. Resistance to radiation and chemotherapeutic drugs has led to the development of immunotherapy and gene therapy approaches with the intent of overcoming resistance to drugs and radiation as well as enhancing the specificity to eliminate tumor cells. However, cytotoxic lymphocytes primarily kill by apoptosis and, therefore, drug-resistant tumor cells may also be cross-resistant to immunotherapy. To evade apoptosis, tumor cells have adopted various mechanisms that interfere with the apoptotic signaling pathways and promote constitutive activation of cellular proliferation and survival pathways. Thus, modifications of the antiapoptotic genes in cancer cells are warranted for the effectiveness of conventional therapies as well as novel immunotherapeutic approaches. Such modifications will avert the resistant phenotype of the tumor cells and will render them susceptible to apoptosis. Current studies, both in vitro and preclinically in vivo, have been aimed at the modification and regulation of expression of apoptosis-related gene products and their activities. A novel protein designated Raf-1 kinase inhibitor protein (RKIP) has been partially characterized. RKIP is a member of the phosphatidylethanolamine-binding protein family. RKIP has been shown to disrupt the Raf-1-MEK1/2 [mitogen-activated protein kinase-ERK (extracellular signal-regulated kinase) kinase-1/2]-ERK1/2 and NF-kappaB signaling pathways, via physical interaction with Raf-1-MEK1/2 and NF-kappaB-inducing kinase or transforming growth factor beta-activated kinase-1, respectively, thereby abrogating the survival and antiapoptotic properties of these signaling pathways. In addition, RKIP has been shown to act as a signal modifier that enhances receptor signaling by inhibiting G protein-coupled receptor kinase-2. By regulating cell signaling, growth, and survival through its expression and activity, RKIP is considered to play a pivotal role in cancer, regulating apoptosis induced by drugs or immune-mediated stimuli. Overexpression of RKIP sensitizes tumor cells to chemotherapeutic drug-induced apoptosis. Also, induction of RKIP by drugs or anti-receptor antibodies sensitizes cancer cells to drug-induced apoptosis. In this review, we discuss the discovery, structure, function, and significance of RKIP in cancer.