Crystal structures of the anticancer clinical candidates R115777 (Tipifarnib) and BMS-214662 complexed with protein farnesyltransferase suggest a mechanism of FTI selectivity

The search for new cancer therapeutics has identified protein farnesyltransferase (FTase) as a promising drug target. This enzyme attaches isoprenoid lipids to signal transduction proteins involved in growth and differentiation. The two FTase inhibitors (FTIs), R115777 (tipifarnib/Zarnestra) and BMS-214662, have undergone evaluation as cancer therapeutics in phase I and II clinical trials. R115777 has been evaluated in phase III clinical trials and shows indications for the treatment of blood and breast malignancies. Here we present crystal structures of R115777 and BMS-214662 complexed with mammalian FTase. These structures illustrate the molecular mechanism of inhibition and selectivity toward FTase over the related enzyme, protein geranylgeranyltransferase type I (GGTase-I). These results, combined with previous biochemical and structural analyses, identify features of FTase that could be exploited to modulate inhibitor potency and specificity and should aid in the continued development of FTIs as therapeutics for the treatment of cancer and parasitic infections.     

Lipid posttranslational modifications. Farnesyl transferase inhibitors

Some proteins undergo posttranslational modification by the addition of an isoprenyl lipid (farnesyl- or geranylgeranyl-isoprenoid) to a cysteine residue proximal to the C terminus. Protein isoprenylation promotes membrane association and contributes to protein-protein interactions. Farnesylated proteins include small GTPases, tyrosine phosphatases, nuclear lamina, cochaperones, and centromere-associated proteins. Prenylation is required for the transforming activity of Ras. Because of the high frequency of Ras mutations in cancer, farnesyl transferase inhibitors (FTIs) were investigated as a means to antagonize Ras function. Evaluation of FTIs led to the finding that both K- and N-Ras are alternatively modified by geranylgeranyl prenyltransferase-1 in FTI-treated cells. Geranylgeranylated forms of Ras retain the ability to associate with the plasma membrane and activate substrates. Despite this, FTIs are effective at inhibiting the growth of human tumor cells in vitro, suggesting that activity is dependent on blocking the farnesylation of other proteins. FTIs also inhibit the in vivo growth of human tumor xenografts and sensitize these models to chemotherapeutics, most notably taxanes. Several FTIs have entered clinical trials for various cancer indications. In some clinical settings, primarily hematologic malignancies, FTIs have displayed evidence of single-agent activity. Clinical studies in progress are exploring the antitumor activity of FTIs as single agents and in combination. This review will summarize the basic biology of FTIs, their antitumor activity in preclinical models, and the current status of clinical studies with these agents.     

Inhibitors of farnesyl protein transferase and MEK1,2 induce apoptosis in fibroblasts transformed with farnesylated but not geranylgeranylated H-Ras

Farnesyl protein transferase inhibitors (FTIs) reverse the transformed phenotype of fibroblasts expressing activated H-Ras and block anchorage-independent growth and tumorigenesis of tumor cell lines independent of their Ras mutational status. FTIs induce significant tumor regression accompanied by apoptosis in several transgenic mouse tumor models. FTI treatment of tumor cells in vitro is proapoptotic under certain cell culture conditions. Induction of apoptosis by FTIs in vitro generally requires a second death-promoting signal. To better understand FTI-induced apoptosis we analyzed the effect of SCH 66336, a tricyclic FTI, on apoptosis of Ras-transformed Rat2 fibroblasts. Treatment of H-Ras-CVLS-transformed fibroblasts with MEK1,2 inhibitors provides a pharmacological second signal to enhance FTI-induced apoptosis. Simultaneous treatment of these cells with a MEK1,2 inhibitor markedly enhanced caspase-3 activity and the apoptotic response to SCH 66336. The combination treatment resulted in a more complete and sustained inhibition of MAPK pathway activity than observed with either drug alone. Surprisingly, after treatment with either agent alone or in combination, no apoptotic response was observed in Rat2 cells transformed with a geranylgeranylated form of H-Ras (H-Ras-CVLL). Differences were also observed when SCH 66336 treatment was combined with forced suspension growth or serum withdrawal, in that an increase in drug-induced apoptosis was observed in H-Ras-CVLS-transformed Rat2 cells but not H-Ras-CVLL-transformed Rat2 cells. The lack of apoptotic effect of SCH 66336 and MEK inhibitor, alone or in combination, in H-Ras-CVLL-transformed cells suggests a difference in the reliance of cells transformed with farnesylated and geranylgeranylated forms of H-Ras on the MAPK signal transduction cascade for survival. K-Ras-transformed cells underwent apoptosis upon MEK1,2 inhibition but not in response to SCH 66336 treatment. The apoptotic response induced by MEK1,2 inhibitors is much greater in magnitude in H-Ras-transformed cells than in K-Ras-transformed cells, also pointing to differences in pathway utilization and/or dependence for these two Ras isoforms.     

Pro-tumorigenic function of autophagy in mammary oncogenesis

Farnesyltransferase inhibitors (FTIs) were designed to block the action of Ras oncoproteins which depend on posttranslational modification by adding a farnesyl isoprenoid membrane anchor. However, off-target actions are believed to account for most of their antitumor activity. We recently reported the induction of autophagy in cancer cells in a dose-dependent manner by FTIs. We observed similar results of autophagy in a panel of tumor cell lines for the three FTIs tested. Therefore, the induction of autophagy is very likely a pharmacological class effect of inhibition of farnesyltransferase. In this addendum, we discuss the possible mechanisms underlying the induction of autophagy by FTIs, including reactive oxygen species-, DNA damage- and Ras-mediated pathways as alternatives to Rheb-mediated regulation of mTOR and autophagy.     

Farnesyltransferase inhibitors define a role for RhoB in controlling neoplastic pathophysiology

A long-standing goal in cancer research is to identify cellular functions that have selective roles in regulating neoplastic pathophysiology. Farnesyl-transferase inhibitors (FTIs) are a novel class of cancer chemotherapeutics which have little effect on normal cell physiology but which inhibit or reverse malignant cell phenotypes. FTIs were originally developed as a strategy to inhibit oncogenic Ras, the activity of which depends upon posttranslational farnesylation. However, recent work indicates the antineoplastic effects of FTIs are not linked to Ras inhibition but instead to alteration of RhoB, a small GTPase of the Rho family of cytoskeletal regulators that controls trafficking of cell surface receptors. Rho proteins integrate signals from integrins and cytokine receptors with cell shape via the actin cytoskeleton. A connection between FTIs and Rho alteration is interesting given that histological differences have long been used to define clinical cancer. RhoB is dispensable for normal cell growth and differentiation in mice. Thus, research into the antineoplastic effects of FTIs has led to the identification of a function(s) that is unnecessary for normal cell physiology but crucial for controlling malignant phenotypes.     

Inhibition of farnesyltransferase: a rational approach to treat cancer?

This article presents in brief the development of farnesyltransferase inhibitors (FTIs) and their preclinical and clinical status. In this review the mechanism of action of FTIs is discussed and their selectivity issue towards tumor cells is also addressed. The significant efficacy of FTIs as single or combined agents in preclinical studies stands in contrast with only moderate effects in Clinical Phase II-III studies. This suggests that there is a need to further explore and understand the complex mechanism of action of FTIs and their interaction with cytotoxic agents.     

组蛋白去乙酰化酶抑制剂诱导肿瘤细胞凋亡的机制

组蛋白去乙酰化酶抑制剂（HDACi）是一类新的化疗药物,能够有效抑制组蛋白去乙酰化酶的活性,促进组蛋白及非组蛋白的乙酰化修饰,在转录和翻译后修饰水平调控肿瘤靶蛋白及凋亡相关蛋白的表达和降解,活化凋亡信号通路,诱导肿瘤细胞凋亡。HDACi抑制抗氧化蛋白的表达,提高细胞内活性氧的水平,引起细胞的氧化损伤。因此,氧化损伤诱导的细胞凋亡也是HDACi杀伤肿瘤细胞的重要机制。HDACi诱导细胞凋亡机制的发现将进一步促进HDACi在临床治疗中的应用。     

Preventing farnesylation of the dynein adaptor Spindly contributes to the mitotic defects caused by farnesyltransferase inhibitors

The clinical interest in farnesyltransferase inhibitors (FTIs) makes it important to understand how these compounds affect cellular processes involving farnesylated proteins. Mitotic abnormalities observed after treatment with FTIs have so far been attributed to defects in the farnesylation of the outer kinetochore proteins CENP-E and CENP-F, which are involved in chromosome congression and spindle assembly checkpoint signaling. Here we identify the cytoplasmic dynein adaptor Spindly as an additional component of the outer kinetochore that is modified by farnesyltransferase (FTase). We show that farnesylation of Spindly is essential for its localization, and thus for the proper localization of dynein and its cofactor dynactin, to prometaphase kinetochores and that Spindly kinetochore recruitment is more severely affected by FTase inhibition than kinetochore recruitment of CENP-E and CENP-F. Molecular replacement experiments show that both Spindly and CENP-E farnesylation are required for efficient chromosome congression. The identification of Spindly as a new mitotic substrate of FTase provides insight into the causes of the mitotic phenotypes observed with FTase inhibitors.     

Farnesyl transferase inhibitors: one target may be found in another

The fact that proteins such as Ras require farnesylation to induce malignant transformation prompted many investigators to design farnesyl transferase inhibitors (FTI) as novel anticancer drugs. FTIs inhibit the growth of ras transformed cells in vitro and induce tumor regression in ras dependent tumor in vivo. Moreover, FTIs inhibit tumor progression in human tumor xenograft models. Currently, FTIs are undergoing phase I and II trials in various cancer types. They show impressive antitumour efficacy and they lack toxicity. Despite these promising results, the development of such molecules in hindered by the absence of appropriate clinical endpoints and of surrogate biological markers. Indeed, it seems likely that Ras is not the critical target of FTIs and that inhibition of the farnesylation of proteins such as RhoB, might also contribute to the observed antitumour properties. Identification of targets that underlie their biological effect is essential in order to predict and evaluate their efficacy.     

Divergence in Signaling Pathways Involved in Promotion of Cell Viability Mediated by bFGF,NGF, and EGF in PC12 Cells

We employed a series of inhibitors of intracellular cascade to disclose the precise molecular mechanisms by which basic fibroblast growth factor (bFGF) promotes viability of PC12 cells and compared with nerve growth factor (NGF) and epidermal growth factor (EGF). The MEK 1 and 2 inhibitors, U0126 and PD98059, significantly suppressed cell viability mediated by bFGF in a dose-dependent manner, and to a greater extent compared with EGF and NGF. The degree of MEK dependency for growth factor-mediated cell viability was estimated to be in the order of bFGF, EGF, and NGF. Rapamycin strongly inhibited the effect of NGF on cell viability, compared with bFGF and EGF. The mechanisms of action of NGF-mediated cell viability may depend largely on p70 S6 kinase-related signal transduction pathways comparing to bFGF and EGF. The present findings suggest that different signal transduction systems may be involved in the molecular mechanisms by which bFGF, NGF, and EGF mediate cell viability.     

Role of protein phosphorylation in neuronal signal transduction

Protein phosphorylation is involved in the regulation of a wide variety of physiological processes in the nervous system. Studies in which purified protein kinases or kinase inhibitors have been microinjected into defined cells while a specific response is monitored have demonstrated that protein phosphorylation is both necessary and sufficient to mediate responses of excitable cells to extracellular signals. The precise molecular mechanisms involved in neuronal signal transduction processes can be further elucidated by identification and characterization of the substrate proteins for the various protein kinases. The roles of three such substrate proteins in signal transduction are described in this article: 1) synapsin I, whose phosphorylation increases neurotransmitter release and thereby modulates synaptic transmission presynaptically; 2) the nicotinic acetylcholine receptor, whose phosphorylation increases its rate of desensitization and thereby modulates synaptic transmission postsynaptically; and 3) DARPP-32, whose phosphorylation converts it to a protein phosphatase inhibitor and which thereby may mediate interactions between dopamine and other neurotransmitter systems. The characterization of the large number of additional phosphoproteins that have been found in the nervous system should elucidate many additional molecular mechanisms involved in signal transduction in neurons.     

Therapeutic potential of natural product signal transduction agents

Modern drug discovery embraces a strategy of targeting cellular signal transduction pathways as a means of finding new therapeutic agents. Historically, natural products derived from microorganisms have played an important role as drug leads and clinical candidates under this paradigm. The future drug potential of natural products as signal transduction agents looks promising, as illustrated by two key examples. First, substantial advances have been made in the development of inhibitors based on immunophilin ligand polyketides, which target the TOR-mediated pathways and can modulate processes including cell proliferation and cell-cycle arrest. Second, the discovery of natural product inhibitors of the ubiquitin-proteasome proteolytic signal transduction pathway represents an emerging field. Given these examples, together with the diversity of as yet undiscovered agents, natural product signal transduction agents offer great potential for future drug discovery efforts.     

Cell growth inhibition by farnesyltransferase inhibitors is mediated by gain of geranylgeranylated RhoB

Recent results have shown that the ability of farnesyltransferase inhibitors (FTIs) to inhibit malignant cell transformation and Ras prenylation can be separated. We proposed previously that farnesylated Rho proteins are important targets for alternation by FTIs, based on studies of RhoB (the FTI-Rho hypothesis). Cells treated with FTIs exhibit a loss of farnesylated RhoB but a gain of geranylgeranylated RhoB (RhoB-GG), which is associated with loss of growth-promoting activity. In this study, we tested whether the gain of RhoB-GG elicited by FTI treatment was sufficient to mediate FTI-induced cell growth inhibition. In support of this hypothesis, when expressed in Ras-transformed cells RhoB-GG induced phenotypic reversion, cell growth inhibition, and activation of the cell cycle kinase inhibitor p21WAF1. RhoB-GG did not affect the phenotype or growth of normal cells. These effects were similar to FTI treatment insofar as they were all induced in transformed cells but not in normal cells. RhoB-GG did not promote anoikis of Ras-transformed cells, implying that this response to FTIs involves loss-of-function effects. Our findings corroborate the FTI-Rho hypothesis and demonstrate that gain-of-function effects on Rho are part of the drug mechanism. Gain of RhoB-GG may explain how FTIs inhibit the growth of human tumor cells that lack Ras mutations.     

Pyridone-containing farnesyltransferase inhibitors: synthesis and biological evaluation

Farnesyltransferase inhibitors (FTIs) have been developed as potential anti-cancer agents due to their efficacy in blocking malignant growth in a variety of murine models of human tumors. To that end, we have developed a series of pyridone farnesyltransferase inhibitors with potent in vitro and cellular activity. The synthesis, SAR and biological properties of these compounds will be discussed.     

Livin与恶性肿瘤的关系

livin属于凋亡抑制蛋白基因家族,与恶性肿瘤发生发展关系密切。根据livin基因结构特点,Livin可能参与调控胱蛋白天冬酶信号通路、丝裂原激活蛋白激酶途径,参与Wnt／beta-catenin信号通路。探讨了Livin与恶性黑色素瘤、膀胱癌、肺癌等多种恶性肿瘤的关系,为临床治疗这些疾病提供新的方法和思路。     

Oxidative stress is fundamental to hyperbaric oxygen therapy

The goal of this review is to outline advances addressing the role that reactive species of oxygen and nitrogen play in therapeutic mechanisms of hyperbaric oxygen. The review will be organized around major categories of problems or processes where controlled clinical trials have demonstrated clinical efficacy for hyperbaric oxygen therapy. Reactive species are now recognized to play a major role in cell signal transduction cascades, and the discussion will focus on how hyperbaric oxygen acts through these pathways to mediate wound healing and ameliorate postischemic and inflammatory injuries.     

Direct inhibition of myosin II effectively blocks glioma invasion in the presence of multiple motogens

Anaplastic gliomas, the most common and malignant of primary brain tumors, frequently contain activating mutations and amplifications in promigratory signal transduction pathways. However, targeting these pathways with individual signal transduction inhibitors does not appreciably reduce tumor invasion, because these pathways are redundant; blockade of any one pathway can be overcome by stimulation of another. This implies that a more effective approach would be to target a component at which these pathways converge. In this study, we have investigated whether the molecular motor myosin II represents such a target by examining glioma invasion in a series of increasingly complex models that are sensitive to platelet-derived growth factor, epidermal growth factor, or both. Our results lead to two conclusions. First, malignant glioma cells are stimulated to invade brain through the activation of multiple signaling cascades not accounted for in simple in vitro assays. Second, even though there is a high degree of redundancy in promigratory signaling cascades in gliomas, blocking tumor invasion by directly targeting myosin II remains effective. Our results thus support our hypothesis that myosin II represents a point of convergence for signal transduction pathways that drive glioma invasion and that its inhibition cannot be overcome by other motility mechanisms.     

肿瘤血管生成抑制剂的作用机制研究进展

肿瘤血管生成抑制剂批能破坏或抑制血管生成,有效地阻止肿瘤生长和转移的药物,可分为特异性和非特异性两大类。其作用机制主要有：（１）调控血管形成生长因子;（２）抑制基底膜降解;（３）影响信号转导通路;（４）调控细胞生长周期;（５）调控肿瘤机关基因。本文对其作用机制的进展作一综述。