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ABSTRACT: Antitumor ribonucleases are small (10-28 kDa) basic proteins. They were found among members of both, ribonuclease A and T1 superfamilies. Their cytotoxic properties are conferred by enzymatic activity, i.e., the ability to catalyze cleavages of phosphodiester bonds in RNA. They bind to negatively charged cell membrane, enter cells by endocytosis and translocate to cytosol where they evade mammalian protein ribonuclease inhibitor and degrade RNA. Here, we discuss structures, functions and mechanisms of antitumor activity of several cytotoxic ribonucleases with particular emphasis to the amphibian Onconase, the only enzyme of this class that reached clinical trials. Onconase is the smallest, very stable, less catalytically efficient and more cytotoxic than most RNase A homologues. Its cytostatic, cytotoxic and anticancer effects were extensively studied. It targets tRNA, rRNA, mRNA as well as the non-coding RNA (microRNAs). Numerous cancer lines are sensitive to Onconase; their treatment with 10-100 nM enzyme leads to suppression of cell cycle progression, predominantly through G(1), followed by apoptosis or cell senescence. Onconase also has anticancer properties in animal models. Many effects of this enzyme are consistent with the microRNAs, one of its critical targets. Onconase sensitizes cells to a variety of anticancer modalities and this property is of particular interest, suggesting its application as an adjunct to chemotherapy or radiotherapy in treatment of different tumors. Cytotoxic RNases as exemplified by Onconase represent a new class of antitumor agents, with an entirely different mechanism of action than the drugs currently used in the clinic. Further studies on animal models including human tumors grafted on severe combined immunodefficient (SCID) mice and clinical trials are needed to explore clinical potential of cytotoxic RNases.
European journal of pharmacology 10/2009; 625(1-3):181-9. · 2.59 Impact Factor
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ABSTRACT: Onconase (Onc), a ribonuclease from oocytes of Northern Leopard frogs (Rana pipiens) is cytostatic and cytotoxic to a variety of tumor lines in vitro, inhibits growth of tumors in animal in vivo models and enhances sensitivity of tumor cells to a number of other cytotoxic agents with diverse mechanism of action. In Phase III clinical trials Onc demonstrated significant efficacy in patients with malignant mesothelioma that failed prior chemotherapy. We previously postulated that the antitumor activity of Onc and the observed synergisms with other antitumor modalities at least in part may be mediated by targeting RNA interference (RNAi). In the present study we observed that the silencing of the glyceraldehyde 3-phosphate dehydrogenase (GAPDH) gene in human lung adenocarcinoma A549 cells by siRNA was effectively prevented by Onc. While transfection of cells with GAPDH siRNA reduced expression of this protein by nearly 70%, the expression was restored in the cells exposed to 0.8 muM Onc for 48 or 72 h. The data thus provide evidence that one of the targets of Onc is siRNA, likely within the RNA-induced silencing complex (RISC). In light of the findings that microRNAs are involved in tumor pathogenesis as well as in enhancing cell resistance to anticancer therapy the present data may provide explanation for both, the antitumor Onc activity and its propensity to enhance effectiveness of cytotoxic drugs.
Cell cycle (Georgetown, Tex.) 11/2008; 7(20):3258-61. · 5.36 Impact Factor
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ABSTRACT: Onconase (Onc), a ribonuclease from oocytes or early embryos of Northern Leopard frog (Rana pipiens), is cytostatic and cytotoxic to a variety of tumor lines in vitro, inhibits growth of tumors in animal in vivo models and is currently in Phase IIIb clinical trials for malignant mesothelioma where it displays antitumor activity with minor overall toxicity to the patient. One of the characteristic features of Onc is a synergism with a variety of other antitumor modalities. Cepharanthine (Cep), a biscoclaurine alkaloid from Stephania cepharantha Hayata, is widely used in Japan to treat variety of ailments. It also shows low toxicity to patients. The aim of the present study was to assess the interaction of these two drugs on different tumor cell lines. When human promyelocytic leukemia HL-60, histiomonocytic lymphoma U937, multiple myeloma RPMI-8228, prostate carcinoma DU 145 and prostate adenocarcinoma LNCaP cells were exposed to relatively low concentrations of Onc or Cep their growth rates were somewhat suppressed but the cells were still able to proliferate. Cell growth, however, was totally abolished in each of these cell lines when treated with Onc and Cep combined. The frequency of apoptosis was also many-fold higher in cultures treated with a combination of Onc and Cep than in respective cultures treated with Onc or Cep alone. The mechanism of the observed synergism is unclear but it may be associated with the Onc activity in targeting microRNAs and/or NFkappaB and Cep activity also targeting NFkappaB. The data suggest that the combination of these two drugs, that individually express a low toxic profile, may have strong antitumor potential.
Cancer biology & therapy 05/2008; 7(7):1104-8. · 2.64 Impact Factor
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ABSTRACT: Onconase (Onc), is a novel amphibian cytotoxic ribonuclease with antitumor activity, and is currently in a confirmatory phase III clinical trial for the treatment of malignant mesothelioma. It was recently reported that Rana pipiens oocytes contain still another ribonuclease, named Amphinase (Amph). Amph shows 38-40% amino acid sequence identity with onconase, presents as four variants varying between themselves from 87-99% in amino acid sequence identity and has a molecular mass approximately 13,000. In the present study we describe the effects of Amph on growth of several tumor cell lines. All four variants demonstrated cytostatic and cytotoxic activity against human promyelocytic HL-60-, Jurkat T-cell- and U-937 monocytic leukemia cells. The pattern of Amph activity to certain extent resembled that of Onc. Thus, cell proliferation was suppressed at 0.5-10.0 mug/ml (40-80 nM) Amph concentration with distinct accumulation of cells in G(1) phase of the cell cycle. In addition, the cells were undergoing apoptosis, which manifested by DNA fragmentation (presence of "sub-G1" cells, TUNEL-positivity), caspases and serine proteases activation as well as activation of transglutaminase. The cytostatic and cytotoxic effects of Amph required its ribonuclease activity: the enzymatically inactive Amph-2 having histidine at the active site alkylated was ineffective. The effectiveness and cell cycle specificity was generally similar for all four Amph variants and at the equimolar concentrations was somewhat more pronounced than that of Onc. The observed cytostatic and cytotoxic activity of Amph against tumor cell lines suggests that similar to Onc this cytotoxic ribonuclease may have antitumor activity and find an application in clinical oncology.
Cell cycle (Georgetown, Tex.) 01/2008; 6(24):3097-102. · 5.36 Impact Factor
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ABSTRACT: Onconase (ONC) is a cytotoxic ribonuclease of the pancreatic ribonuclease A superfamily isolated from oocytes or early embryos of the Northern leopard frog (Rana pipiens). It shows anticancer activity and currently is in Phase IIIb clinical trial for unresectable malignant mesothelioma. We generated several variants of ONC possessing mutations in selected structural regions of the molecule that have altered ribonucleolytic activity and/or conformational stability. The relationship between the stability and ribonucleolytic activity of these variants and their cytostatic and cytotoxic properties was investigated on several tumor cell lines. Similar to ONC, all variants were inducing reproductive cell death detected by reduced clonogenicity. The surviving cells proliferated at reduced rates as reflected by diminished size of colonies and prolongation of G(0/1) phase of the cell cycle. Some cells were undergoing apoptosis. The cytotoxic and cytostatic effects of ONC and its variants were predominantly determined by their catalytic activity rather than by conformational stability.
Cancer biology & therapy 09/2007; 6(8):1233-9. · 2.64 Impact Factor
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ABSTRACT: Onconase (ONC), (ranpirnase) a cytotoxic ribonuclease isolated from amphibian oocytes and early embryos targeting tumor cells in vitro and in vivo, is currently in a confirmatory Phase IIIb clinical trial for unresectable malignant mesothelioma where it demonstrates antitumor activity with relatively minor overall toxicity to patients. Since hyperthermia has been shown to be synergistic with certain antitumor modalities, the aim of the present study was to explore whether the cytotoxic effects of ONC can be enhanced under conditions of mild hyperthermia. Treatment of human lymphoblastoid TK6 cells with 2 or 5 microg/ml of ONC at 40 degrees C for 24 or 48 h led to 64-200% enhancement in incidence of apoptosis assessed by frequency of cells showing the presence of activated (cleaved) caspase-3 or activated serine proteases, compared to treatment at 37.5 degrees C. The incidence of apoptosis at 40 degrees C in the absence of ONC was unchanged compared to 37.5 degrees C, for up to 48 h. Although at 41 degrees C in absence of ONC the incidence of apoptosis was elevated compared to 37 degrees C the cytotoxicity of ONC was further enhanced and the overall pro-apoptotic effect was above the level of additive effects of ONC plus that of 41 degrees C-hyperthermia. While the mechanism of the observed enhancement of ONC cytotoxicity is currently under investigation, the findings suggest that a combination of ONC and mild hyperthermia should be explored to increase effectiveness of ONC in cancer treatment.
International Journal of Oncology 05/2007; 30(4):841-7. · 2.40 Impact Factor
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ABSTRACT: An electronic radio frequency (RF) microchip, the microtransponder (MTP), has been developed as a platform for assays in the fields of genomics and proteomics. Upon activation by light, each MTP provides a unique RF identification (ID) signal that matches a chip to the specific biological material attached to it. The MTP is powered by a photocell and has an antenna that transmits the signal. The aim of the present study was to explore utility of MTPs as a platform for cell growth in cytotoxicity assays.
The MCF-7, MCF-116, A549, or T-24 cells growing on MTPs placed in petri dishes or slide chambers were cultured untreated or exposed to antitumor drugs topotecan, mitoxantrone, or onconase for up to 4 days. Their attachment to- and growth on- MTPs was assessed by fluorescence microscopy and laser scanning cytometry (LSC) and compared with growth on the dish surface in the MTP neighborhood. The MTPs were fixed in ethanol, stained with propidium iodide (PI), and interrogated in flow in the instrument capable to rapidly (up to 103 MTPs/s) identify their ID signal and measure fluorescence.
The cells plated on MTPs exhibited similar attachment properties to those plated in culture dishes. When measured by LSC, they had similar mitotic activity, growth rate, and cell cycle distributions as the cells adhering to the culture dish in the neighborhood of MTPs. The fluorescence intensity of MTPs provided information about the cell number per MTP, which made it possible to assess cell growth rate and monitor the cytostatic/cytotoxic effects of the tested drugs.
The MTP-based system holds promise for the multiplexed cell assays in which numerous different cell lines can be screened for their growth rate or sensitivity while exposed to particular agents in the same vessel. Other advantages of the system are the rapidity of the screening and a very large number of ID codes. Because many cell lines/types can be assayed in a single dish, the system also offers cost savings on tissue culture reagents.
Cytometry Part A 12/2006; 69(11):1097-105. · 3.73 Impact Factor
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ABSTRACT: Trans arachidonic acid isomers (trans-AA) constitute a new group of trans fatty acids (trans-FA) generated in vivo via endogenous cis-trans isomerization stimulated by the NO2 radical. Because both NO2 and trans-FA have been implicated as causative factors in cancer, we studied the effect of the trans-AA isomers on proliferation and viability of human promyelocytic (HL-60) cells. The four trans arachidonic (trans-AA) acid isomers synthesized by us have been presently tested with respect to their competence to affect the proliferation and viability of human promyeolocytic HL-60 cells in culture. The data demonstrate that one of the isomers, 5,6-trans-AA, showed distinct activity by targeting cell progression through the cell cycle and inducing apoptosis. The effects were time- and concentration-dependent: the cytostatic effect of 5E-AA was observed at 10 microM following 72 h of treatment. This effect was manifested as a perturbation of cell progression through G1 phase, indicating the 'on' activation of the G1 checkpoint as evidenced by the flow- and laser scanning-cytometry techniques. Apoptotic cells were identified by comparison of their morphology, DNA fragmentation, caspase activation and collapse of mitochondrial potential with control cells. These observations suggested that 5E-AA induced a mitochondrial pathway of apoptosis. There was no evidence of cell-cycle phase specificity in induction of apoptosis by 5E-AA, as the cells showing highly fragmented DNA or caspase-3 activation were distributed in all phases of the cycle. The data suggest that 5E-AA may have at least two targets: one that is cell-cycle specific and associated with the observed arrest in the G1 phase and another, unrelated to the cell cycle, which is responsible for triggering apoptosis indiscriminately, regardless of cycle phase I.
International Journal of Oncology 12/2005; 27(5):1177-85. · 2.40 Impact Factor
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ABSTRACT: Onconase (Ranpirnase), a novel ribonuclease isolated from Rana pipiens oocytes, was reported to suppress cancer cell growth in vitro, reduce tumor size in animals, and augment cytotoxicity of several chemotherapeutic agents. Since onconase is currently in phase III clinical trials tested in treatment of mesothelioma, much emphasis has been placed on the mechanism of its anti-tumor activity. Previous studies have shown that onconase-responsive cells become arrested at the G1/S checkpoint of the cell cycle and also undergo apoptosis. A proposed mechanism for these effects is that the enzymatic activity of onconase targets cellular RNAs, in particular tRNA, with an accompanying inhibition of protein synthesis. In the present study, we have investigated the time- and dose-dependent effects of onconase on growth of Jurkat SN acute T-lymphocytic leukemia cells. Significant suppression of cell proliferation became evident after 72 and 96 h of treatment, and was most pronounced at the highest concentration (10 microg/ml; 8.3x10(-7) M) of onconase. This reduction of cell proliferation, however, was not accompanied by measurable changes in distribution of cells at different phases of the cell cycle, but was paralleled by the induction of apoptosis, as assayed by flow cytometry, and with a modest decrease in the expression of a cell cycle regulatory retinoblastoma protein (Rb). Further biochemical analysis revealed that growth suppression was closely coordinated with a down-regulation in the steady state and subcellular distribution of NF-kappaB, a transcription factor known to be functionally associated with cell survival. The reduction in expression of NF-kappaB by onconase appeared to coincide or even precede growth suppression, suggesting a causal relationship. To further test the hypothesis that cellular localization and expression of NF-kappaB may be critical to cellular response to onconase, we also studied the growth effects of onconase in Jurkat-BalphaM cells, which, unlike the parent SN T cells, contain a stably transfected dominant-negative IkappaB gene. Growth suppression by onconase in BalphaM cells was more pronounced and occurred earlier compared to SN cells, although still did not affect changes in cell cycle phase distribution. Contrary to expectation, however, diminution in NF-kappaB expression by onconase was even more pronounced in BalphaM cells, suggesting that this transcription factor, while presumably prevented from dissociation from its inhibitory protein IkappaB in these cells, is even more efficiently targeted for degradation by onconase. These results implicate NF-kappaB and its turnover as important determinants in the anti-proliferative/apoptotic effects of onconase in acute T-lymphocytic leukemia cells.
International Journal of Oncology 01/2005; 25(6):1745-52. · 2.40 Impact Factor
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ABSTRACT: One of the hallmarks of apoptosis is activation of tissue transglutaminase (Tgase; also called transglutaminase type 2 [TGase 2]). Its activation causes cross-linking of cytoplasmic proteins, making them insoluble and presumably less immunogenic. Several biochemical and cytochemical methods to detect activity of TGase 2 exist, but none has been adapted for multiparameter flow or image cytometry.
Apoptosis of HL-60 or U-937 leukemic cells was induced by camptothecin, tumor necrosis factor alpha, hyperthermia, or the cytotoxic RNase onconase. Two different approaches to detect TGase 2 activation were developed: (a) the unfixed cells were treated with 4',6'-diamidino-2-phenylindole, and sulforhodamine 101 in solutions of nonionic detergents; (b) the TGase 2 substrate fluoresceinated polyamine cadaverine (F-CDV) was administered into the cultures for several hours before cell harvesting. The cells were then fixed and their DNA counterstained with propidium. Cellular fluorescence was measured by flow or laser scanning cytometry.
(a) Exposure of nonapoptotic cells to detergents caused their full lysis, resulting in preparation of isolated nuclei devoid of cytoplasm. Conversely, the cross-linking of cytoplasmic protein by activated TGase 2 in apoptotic cells provided resistance to detergents: the nuclei or nuclear (chromatin) fragments of apoptotic cells remained attached to the cytoplasmic protein, embedded within the proteinaceous "shell." Such cells were identified by their high protein content: intensity of fluorescence after staining with the protein fluorochrome sulforhodamine 101 was markedly higher than that of isolated nuclei. (b) Activation of TGase 2 was also detected by virtue of intense cell labeling with fluoresceinated polyamine cadaverine. Interestingly, in many cells apoptosis progressed without evidence of activation of TGase 2, suggesting that this event may not be a prerequisite for completion of apoptosis.
Activation of TGase 2 can be detected simply by cell resistance to detergents or in situ reactivity with F-CDV. Both methods allow one to correlate activation of TGase 2 with the cell cycle position. However, because activation of TGase 2 is not always detected during apoptosis, the lack of the activation cannot be considered a marker of nonapoptotic cells. Hence, an apoptotic index based solely on TGase 2 activation may underestimate incidence of apoptosis.
Cytometry 11/2002; 49(2):83-9.
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ABSTRACT: Onconase (ONC) is a ribonuclease isolated from amphibian oocytes that is cytostatic and cytotoxic to numerous tumor lines. ONC shows in vivo anti-tumor activity in mouse tumor models and is currently in Phase III clinical trials. Previous studies indicated that ONC induces apoptosis of the target cells most likely along the mitochondrial pathway involving caspase-9 as the initiator caspase. We have recently developed an approach to detect the activation of serine (Ser) proteases during apoptosis. The method is based on affinity labeling of Ser protease active centers with fluorochrome-tagged inhibitors. The aim of the present study was to reveal whether Ser proteases are activated during apoptosis induced by ONC. Human leukemic HL-60 cells were treated with ONC for up to 72 h and then exposed to 5(6)-carboxyfluoresceinyl-L-phenylalanylchloromethyl ketone (FFCK) or 5(6)-carboxyfluoresceinyl-L-leucylchloromethyl ketone (FLCK), the fluorescing green reagents reactive with active centers of the chymotrypsin-like enzymes that cleave proteins at the Phe (FFCK) or Leu (FLCK) site. Activation of caspases was assayed in the same cells using sulforhodamine-labeled (fluorescing red) pan-caspases inhibitor (SR-VAD-FMK). Administration of 1.67 microM ONC into cultures of HL-60 cells led to the appearance of cells that bound SR-VAD-FMK as well as FFCK and FLCK. Most labeled cells had features characteristic of apoptosis. We interpret the binding of these ligands, which was irreversible and withstood cell fixation, as revealing activation of caspases and chymotrypsin-like Ser proteases. Because the induction of binding of each of the three ligands occurred at approximately the same time, the data suggest that during apoptosis caspases and Ser proteases may transactivate each other. The intercellular and subcellular pattern of binding SR-VAD-FMK vs FFCK or vs FLCK was different indicating a variability in abundance and localization of these enzymes within individual apoptotic cells. The FFCK- and FLCK-reactive proteins were of similar molecular mass, approximately 59 and approximately 57 kDa, respectively.
Experimental Cell Research 09/2002; 278(1):61-71. · 3.58 Impact Factor
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ABSTRACT: Recently, accumulated statistical data indicate the protective effect of caffeine consumption against several types of cancer diseases. There are also reports about protective effect of caffeine and other xanthines against tumors induced by polycyclic aromatic hydrocarbons. One of the explanations is based on biological activation of such carcinogens by cytochromes that are also known for metabolism of caffeine. However, there is also numerous data indicating reverse effect on cytotoxicity of anticancer drugs that inhibit the action of topoisomerase I (e.g. Camptothecin or Topotecan) and topoisomerase II inhibitors (e.g. Doxorubicin, Mitoxantrone or mAMSA). In this work we tested the hypothesis that the caffeine protective effect is the result of sequestering of aromatic mutagens by formation of stacking (pi-pi) complexes. As the models for the study we have chosen two well-known mutagens, that do not require metabolical activation: quinacrine mustard(QM, aromatic, heterocyclic nitrogen mustard) and mechlorethamine (NM2, aliphatic nitrogen mustard). The flow cytometry study of these agents' action on the cell cycle of HL-60 cells indicated that caffeine prevents the cytotoxic action of QM, but not that of NM2. The formations of stacking complexes of QM with caffeine were confirmed by light absorption, calorimetric measurements and by molecular modeling calculation. Using the statistical thermodynamics calculations we calculated the "neighborhood" association constant (K(AC)=59+/-2M(-1)) and enthalpy change (DeltaH(0')=-116cal mol(-1)); the favorable entropy change of complex formation (DeltaS(0')=7.72cal mol(-1)K(-1), due to release of several water molecules, associated with components in the process of complex formation). The Gibbs' free energy change of QM-CAF formation is DeltaG(0')=-2.41kcal mol(-1). We were unable to detect any interaction between NM2 and caffeine either by spectroscopic or calorimetric measurement. In order to establish, whether the intercalation of QM plays any role in cytotoxic effect we tested, as a control, non-alkylatiatig, but also intercalating QM derivative-quinacrine (Q). The later had no cytostatic effect on HL-60 cell even at there order of higher concentration than QM or NM2 but, similar to QM forms (which we demonstrated) stacking complexes with caffeine (K(AC)=75+/-3M(-1)). These results strongly indicate, that the attenuating effect of caffeine on cytotoxic or mutagenic effects of some mutagens, is not the results of metabolic processes in the cells, but simply the physicochemical process of sequestering of aromatic molecules (potential carcinogens or mutagens) by formation of stacking complexes with them. The caffeine may then act as the "interceptor" of potential carcinogens (especially in the upper part of digesting track where its concentration can reach the concentration of mM level). There is, however, no indication either in the literature or in our experiments that xanthines can reverse the damage to nucleic acids when the damage to DNA has already occurred.
Biochemical Pharmacology 03/2002; 63(4):625-34. · 4.70 Impact Factor
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ABSTRACT: Background
Various attributes of nucleoli, including abundance of the nucleolar product (rRNA), correlate with cell-proliferative status and are useful markers for tumor diagnosis and prognosis. However, there is a paucity of methods that can quantitatively probe nucleolus. The aim of the present study was to utilize the morphometric capacity of the laser scanning cytometer (LSC) to analyze nucleoli and measure expression of the nucleolar protein nucleolin (NCL) in individual cells and correlate it with their state of proliferation.Materials and Methods
Human lymphocytes were mitogenically stimulated, and at different time points their nucleoli were detected immunocytochemically using NCL Ab. The frequency of nucleoli per nucleus, their area, and the level of expression of NCL, separately in the nuclear and nucleolar compartments, were estimated in relation to the G0 to G1 transition and the cell cycle progression.ResultsDuring the first 24 h of stimulation, when the cells underwent G0 to G1 transition, their RNA content was increased nearly 8-fold, the level of NCL per nucleus also increased 8-fold, the NCL per nucleolus increased 12-fold, nucleolear area increased 3-fold, and NCL/nucleolar area increased nearly 4-fold. During the subsequent 24–48 h of stimulation, when cells were progressing through S, G2, and M and reentering the next cycle, the number of nucleoli per nucleus was increased and a massive translocation of NCL from nucleoli to nucleoplasm was observed; its overall level per nucleus, however, still remained high, at 6-fold above of that of G0 cells.Conclusions
While high expression of NCL in the nucleolar compartment correlates with the rate of rRNA accumulation in the cell and is a sensitive marker of the G0 to G1 transition, the cells progressing through the remainder of the cycle are better distinguished from G0 cells by high overall level of NCL within the nucleus. Such an analysis, when applied to tumors, may be helpful in obtaining the quantitative parameters related to the kinetic status of the tumor-cell population and tumor prognosis. The capability of LSC to measure the protein translocation between nucleolus and nucleoplasm can be used to study the function and regulatory mechanisms of other proteins that reside in these compartments. Cytometry 45:206–213, 2001. © 2001 Wiley-Liss, Inc.
Cytometry 10/2001; 45(3):206 - 213.
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ABSTRACT: Cyclins are key components of the cell cycle progression machinery. They activate their partner cyclin-dependent kinases (CDKs) and possibly target them to respective substrate proteins within the cell. CDK-mediated phosphorylation of specific sets of proteins drives the cell through particular phases or checkpoints of the cell cycle. During unperturbed growth of normal cells, the timing of expression of several cyclins is discontinuous, occurring at discrete and well-defined periods of the cell cycle. Immunocytochemical detection of cyclins in relation to cell cycle position (DNA content) by multiparameter flow cytometry has provided a new approach to cell cycle studies. This approach, like no other method, can be used to detect the unscheduled expression of cyclins, namely, the presentation of G1 cyclins by cells in G2/M and of G2/M cyclins by G1 cells, without the need for cell synchronization. Such unscheduled expression of cyclins B1 and A was seen when cell cycle progression was halted, e.g., after synchronization at the G1/S boundary by inhibitors of DNA replication. The unscheduled expression of cyclins B1 or E, but not of A, was also observed in some tumor cell lines even when their growth was unperturbed. Likewise, whereas the expression of cyclins D1 or D3 in nontumor cells was restricted to an early section of G1, the presentation of these proteins in many tumor cell lines also was seen during S and G2/M. This suggests that the partner kinase CDK4 (which upon activation by D-type cyclins phosphorylates pRB committing the cell to enter S) is perpetually active throughout the cell cycle in these tumor lines. Expression of cyclin D also may serve to discriminate G0 vs. G1 cells and, as an activation marker, to identify the mitogenically stimulated cells entering the cell cycle. Differences in cyclin expression make it possible to discriminate between cells having the same DNA content but residing at different phases such as in G2 vs. M or G2/M of a lower DNA ploidy vs. G1 cells of a higher ploidy. The expression of cyclins D, E, A and B1 provides new cell cycle landmarks that can be used to subdivide the cell cycle into several distinct subcompartments. The point of cell cycle arrest by many antitumor agents can be estimated with better accuracy in relation to these compartments compared to the traditional subdivision into four cell cycle phases. The latter applications, however, pertain only to normal cells or to tumor cells whose phenotype is characterized by scheduled expression of cyclins. As sensitive and specific indicators of the cell's proliferative potential, the cyclins, in particular D-type cyclins, are expected to be key prognostic markers in neoplasia. © 1996 Wiley-Liss, Inc.
Cytometry 08/1996; 25(1):1 - 13.
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ABSTRACT: Several cytotoxic ribonucleases (CRs), homologs of the pancreatic RNase A, have been isolated from amphibian oocytes or embryos. Of them, onconase (Onc), the CR that shows antitumor properties and is in phase III clinical trials, was the most extensively researched. Degradation of tRNA by Onc internalized into cells that leads to inhibition of protein synthesis is considered the mechanism of its cytotoxicity. Several findings, however, cannot be explained by nonspecific decline in protein synthesis alone and suggest additional or alternative mechanism(s). We postulate therefore that miRNAs and/or RNA interference (RNAi) may also be targets of CRs. The following arguments support this postulate: (A) miRNAs and siRNAs appear to be unprotected by proteins and therefore, as tRNA, accessible and degradable by CRs; (B) Onc has preferred cleavage sites on tRNAs: their cleavage may generate segments of dsRNA that interfere with translation. Analogous to Dicer, thus, small RNAs with interfering properties may be generated by CRs within the cell; (C) CRs are abundant in oocytes and during embryonic development; their role there is unknown. Since cells undergo perpetual differentiation during embryogenesis it is likely that the function of CRs is to provide additional level of regulation of gene expression via the mechanisms listed in (A) and/or (B).
Cell cycle (Georgetown, Tex.) 2(1):22-4. · 5.36 Impact Factor
