Muscle myostatin signalling is enhanced in experimental cancer cachexia.

BACKGROUND/AIMS: Myostatin belongs to the transforming growth factor-beta superfamily and negatively regulates skeletal muscle mass. Its deletion induces muscle overgrowth, while, on the contrary, its overexpression or systemic administration cause muscle atrophy. The present study was aimed at investigating whether muscle depletion as occurring in an experimental model of cancer cachexia, the rat bearing the Yoshida AH-130 hepatoma, is associated with modulations of myostatin signalling and whether the cytokine tumour necrosis factor-alpha may be relevant in this regard. MATERIALS AND METHODS: Protein levels of myostatin, follistatin (myostatin endogenous inhibitor) and the activin receptor type IIB have been evaluated in the gastrocnemius of tumour-bearing rats by Western blotting. Circulating myostatin and follistatin in tumour hosts were evaluated by immunoprecipitation, while the DNA-binding activity of the SMAD transcription factors was determined by electrophoretic-mobility shift assay. RESULTS: In day 4 tumour hosts muscle myostatin levels were comparable to controls, yet follistatin was reduced, and SMAD DNA-binding activity was enhanced. At day 7, both myostatin and follistatin increased in tumour bearers, while SMAD DNA-binding activity was unchanged. To investigate whether tumour necrosis factor-alpha contributed to induce such changes, rats were administered pentoxifylline, an inhibitor of tumour necrosis factor-alpha synthesis that partially corrects muscle depletion in tumour-bearing rats. The drug reduced both myostatin expression and SMAD DNA-binding activity in day 4 tumour hosts and up-regulated follistatin at day 7. CONCLUSIONS: These observations suggest that myostatin pathway should be regarded as a potential therapeutic target in cancer cachexia.

Tumor necrosis factor-alpha mediates changes in tissue protein turnover in a rat cancer cachexia model.

Rats bearing the Yoshida AH-130 ascites hepatoma showed enhanced fractional rates of protein degradation in gastrocnemius muscle, heart, and liver, while fractional synthesis rates were similar to those in non-tumor bearing rats. This hypercatabolic pattern was associated with marked perturbations of the hormonal homeostasis and presence of tumor necrosis factor in the circulation. The daily administration of a goat anti-murine TNF IgG to tumor-bearing rats decreased protein degradation rates in skeletal muscle, heart, and liver as compared with tumor-bearing rats receiving a nonimmune goat IgG. The anti-TNF treatment was also effective in attenuating early perturbations in insulin and corticosterone homeostasis. Although these results suggest that tumor necrosis factor plays a significant role in mediating the changes in protein turnover and hormone levels elicited by tumor growth, the inability of such treatment to prevent a reduction in body weight implies that other mediators or tumor-related events were also involved.

Muscle protein waste in tumor-bearing rats is effectively antagonized by a beta 2-adrenergic agonist (clenbuterol). Role of the ATP-ubiquitin-dependent proteolytic pathway.

Tissue protein hypercatabolism (TPH) is a most important feature in cancer cachexia, particularly with regard to the skeletal muscle. The rat ascites hepatoma Yoshida AH-130 is a very suitable model system for studying the mechanisms involved in the processes that lead to tissue depletion, since it induces in the host a rapid and progressive muscle waste mainly due to TPH (Tessitore, L., G. Bonelli, and F. M. Baccino. 1987. Biochem. J. 241:153-159). Detectable plasma levels of tumor necrosis factor-alpha associated with marked perturbations in the hormonal homeostasis have been shown to concur in forcing metabolism into a catabolic setting (Tessitore, L., P. Costelli, and F. M. Baccino. 1993. Br. J. Cancer. 67:15-23). The present study was directed to investigate if beta 2-adrenergic agonists, which are known to favor skeletal muscle hypertrophy, could effectively antagonize the enhanced muscle protein breakdown in this cancer cachexia model. One such agent, i.e., clenbuterol, indeed largely prevented skeletal muscle waste in AH-130-bearing rats by restoring protein degradative rates close to control values. This normalization of protein breakdown rates was achieved through a decrease of the hyperactivation of the ATP-ubiquitin-dependent proteolytic pathway, as previously demonstrated in our laboratory (Llovera, M., C. García-Martínez, N. Agell, M. Marzábal, F. J. López-Soriano, and J. M. Argilés. 1994. FEBS (Fed. Eur. Biochem. Soc.) Lett. 338:311-318). By contrast, the drug did not exert any measurable effect on various parenchymal organs, nor did it modify the plasma level of corticosterone and insulin, which were increased and decreased, respectively, in the tumor hosts. The present data give new insights into the mechanisms by which clenbuterol exerts its preventive effect on muscle protein waste and seem to warrant the implementation of experimental protocols involving the use of clenbuterol or alike drugs in the treatment of pathological states involving TPH, particularly in skeletal muscle and heart, such as in the present model of cancer cachexia.

Novel investigational drugs mimicking exercise for the treatment of cachexia.

INTRODUCTION: Cachexia is a syndrome characterized by body weight loss, muscle wasting and metabolic abnormalities, that frequently complicates the management of people affected by chronic diseases. No effective therapy is actually available, although several drugs are under clinical evaluation. Altered energy metabolism markedly contributes to the pathogenesis of cachexia; it can be improved by exercise, which is able to both induce anabolism and inhibit catabolism. AREAS COVERED: This review focuses on exercise mimetics and their potential inclusion in combined protocols to treat cachexia. The authors pay with particular reference to the cancer-associated cachexia. EXPERT OPINION: Even though exercise improves muscle phenotype, most patients retain sedentary habits which are quite difficult to disrupt. Moreover, they frequently present with chronic fatigue and comorbidities that reduce exercise tolerance. For these reasons, drugs mimicking exercise could be beneficial to those who are unable to comply with the practice of physical activity. Since some exercise mimetics may exert serious side effects, further investigations should focus on treatments which maintain their effectiveness on muscle phenotype while remaining tolerable at the same time.

Control of cell protein degradation. Changes in activities of lysosomal proteases.

In cycloheximide-treated fibroblasts, the rate of cell proteolysis and the specific activity of cathepsin D show a rapid, concurrent, and proportional decrease over a 48 h period. Cathepsin B1 also decreases, but other lysosomal enzymes show litte or no change in specific activity. These findings are consistent with the hypothesis that the rate of proteolysis in the cell is in part controlled by the specific activities of lysosomal proteases.

Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts.

In quiescent Balb-c mouse 3T3 fibroblasts, the application of whole or dialyzed 10% foetal calf serum elicits a biphasic electrical response, consisting of a transient outward current, flowing through Ca(2+)-activated K+ channels, followed by an inward one, lasting up to 15 min. On the basis of experiments with ion substitutions and blockers, the inward current can be attributed to the opening of cationic channels permeable to Na+ and Ca2+ ions. This current could mediate the calcium influx involved in the sustained elevation of [Ca2+]i that has been observed in many preparations in response to mitogen stimulation and that is involved in triggering cell proliferation.

Cell protein degradation in cultured rat embryo fibroblasts. Suppression by vinblastine of the enhanced proteolysis by serum-deficient media.

Rat embryo fibroblasts grown in Eagle's minimal essential medium with 10% serum were labeled with L-[14C]leucine. After a 24 h cold chase, rates of proteolysis were evaluated by measuring the appearance of trichloroacetic acid-soluble 14C in the media. Cells remaining in minimal essential medium with 10% serum (basal) showed a proteolysis rate of 1% per h, whereas cells placed in minimal essential medium alone (serum-deficient) showed a stimulation of proteolysis to 3--4% per h. This enhanced proteolysis was transitory, occurring only for the first 4--8 h after cells were placed in the serum-deficient media. Vinblastine 10-5 M inhibited the enhanced proteolysis 40% but had no effect on basal proteolysis. Control experiments showed no detectable hydrolysis of extracellular proteins, nor did vinblastine affect the rate of protein synthesis. These data suggest that basal and enhanced proteolysis have at least partially distinct mechanisms in the cell and that only enhanced proteolysis involves microtubules.

Two currents activated by epidermal growth factor in EGFR-T17 fibroblasts.

Application of 10 nM Epidermal Growth Factor (EGF) to single EGFR-T17 fibroblasts induced a marked hyperpolarization that could last for tens of minutes; in many cases the first transient was followed by a series of oscillations of the membrane potential. The outward current responsible for the hyperpolarizing response could be recorded simultaneously to an increase in the intracellular calcium concentration, as measured with the fluorescent indicator fura-2. The conductance was nearly linear in the voltage range from -100 to +50 mV. While the EGF-induced current had many characteristics of a K+ current and was strongly reduced by 50 nM charybdotoxin (ChTx), its reversal potential was apparently more negative than the potassium equilibrium potential (VK). The application of 2 microM ouabain prior to EGF stimulation produced responses that were similar to those obtained without ouabain; however, under these conditions the EGF-induced current showed a reversal potential of -96.6 +/- 3.2 mV, very close to VK. Simultaneous application of both 2 microM ouabain and 50 nM ChTx completely abolished the response. It can be concluded that the response to EGF stimulation in EGFR-T17 cells consists of two components: the first is a current carried through Ca(2+)-activated K+ channels; the second is due to the acceleration of the operation of the Na+/K(+)-ATPase.

Biphasic effects of translational inhibitors on liver tyrosine aminotransferase.

Low doses of cycloheximide or emetine cause rat liver tyrosine aminotransferase activity to rise up to twice the control levels in 2 h. By contrast, in the same interval no changes, or only a slight decrease, are produced by either drug at high dosage. Adrenalectomised animals display the same pattern of response. High doses of either antibiotic virtually afford a complete inhibition of 14C-labelled amino acid incorporation into liver and plasma proteins, whereas no more than a 30% decrease is observed with low doses. When administered in the course of the induction by cortisol, high doses of inhibitor prevent any further change in tyrosine aminotransferase activity, stabilising it at the level already attained; low doses, while slightly affecting the synthetic phase evoked by cortisol, drastically interfere with the deinduction. Six hours after various doses of either inhibitor the tyrosine aminotransferase activity is markedly increased, this late effect being largely dependent on the presence of adrenals. The amino acid incorporating actitivy of the liver may exceed that of controls, as observed particularly after small doses of emetine.

Interleukin-15 mediates reciprocal regulation of adipose and muscle mass: a potential role in body weight control.

Interleukin (IL)-15 is a cytokine which is highly expressed in skeletal muscle. Cell culture studies have indicated that IL-15 may have an important role in muscle fiber growth and anabolism. However, data concerning the metabolic effects of this cytokine in vivo are lacking. In the present study, IL-15 was administered to adult rats for 7 days. While IL-15 did not cause changes in either muscle mass or muscle protein content, it induced significant changes in the fractional rates of both muscle protein synthesis and degradation, with no net changes in protein accumulation. Additionally, IL-15 administration resulted in a 33% decrease in white adipose tissue mass and a 20% decrease in circulating triacylglycerols; this was associated with a 47% lower hepatic lipogenic rate and a 36% lower plasma VLDL triacylglycerol content. The decrease in white fat induced by IL-15 was in adipose tissue. No changes were observed in the rate of lipolysis as a result of cytokine administration. These findings indicate that IL-15 has significant effects on both protein and lipid metabolism, and suggest that this cytokine may participate in reciprocal regulation of muscle and adipose tissue mass.

Rapid and extensive lethal action of clofibrate on hepatoma cells in vitro.

Clofibrate, for a long time in use as a hypolipidemic drug, is a well known peroxisomal proliferator (PP) and hepatocarcinogen in rodents. We show here that in vitro 1 mM clofibrate induces a rapid and massive death of rat AH-130 hepatoma cells. Cell death was prominent already after 4 h of treatment, with a characteristic ;apoptotic' pattern by conventional microscopy. This was further supported by the pronounced chromatin condensation detectable on 4',6-diamine-2'-phenylindole dihydrochloride (DAPI) staining, the clearcut internucleosomal DNA fragmentation on agarose-gel electrophoresis (ladder pattern), and the accumulation of markedly hypochromic cells observed in flow cytometric DNA histograms. Consistently with the apoptotic features of the process, some parameters commonly used to detect cell death, such as plasma membrane permeabilization to trypan blue or propidium iodide, lack of mitochondrial retention of rhodamine 123, or extracellular release of lactate dehydrogenase, were all virtually negative. However, these same parameters became markedly positive after 24 h of treatment, which was suggestive for the occurrence of ;secondary' necrosis among AH-130 cells. By a combination of flow cytometric parameters, after 4 h on 1 mM clofibrate only 41% of the AH-130 cells could still be categorized as viable (i.e., non-apoptotic and non-necrotic), while 46% of cells appeared apoptotic and 13% necrotic. At 24 h, 67% of cells were necrotic, 20% apoptotic and only 13% non-apoptotic and non-necrotic. Apoptosis was also extensive in AH-130 cells treated with another PP such as nafenopin at 1 mM concentration and in human hepatoma HepG2 cells treated with clofibrate. By contrast, clofibrate did not cause apoptosis on primary rat hepatocyte cultures. These observations indicate that: (i) apart from their well-known cell growth-promoting action, PPs such as clofibrate or nafenopin may exert a substantial cytotoxic action on targets such as the AH-130 or HepG2 hepatoma cells; (ii) this cell death evolves from an initial 'apoptotic' to an eventual ;necrotic' pattern; (iii) detection of cell death requires the adoption of a full panel of tests, adequate to cover the whole evolving death pattern, while such tests may even be substantially misleading whenever applied individually; (iv) the cytotoxicity of clofibrate and similar agents on normal and, particularly, tumoural cells may deserve careful reevaluation.

Mice lacking TNFalpha receptors 1 and 2 are resistant to death and fulminant liver injury induced by agonistic anti-Fas antibody.

The liver is particularly susceptible to Fas-mediated cytotoxicity. Mice given an adequate parenteral dose of agonistic anti-Fas antibody (aFas) or of FasL are known to develop a devastating liver injury and to die in a few hours. The present work shows that mice lacking TNFR1 and TNFR2 (R(-)) both survive a single dose of aFas, otherwise rapidly lethal, and develop a mild form of hepatic damage, compared to the much more severe liver injury that in a few hours strikes wild-type mice (R(+)), eventually involving increased activity of proteases of different families (caspase 3-, 8-, and 9-like, calpains, cathepsin B). Neither the overall tissue levels of Fas and FasL nor Fas expression at the hepatocyte surface are altered in the liver of R(-) animals. The DNA-binding activity of the NF-kappaB transcription factor is enhanced after aFas treatment, but much more markedly in R(-) than in R(+) mice. Bcl2, while unchanged in untreated animals, is markedly upregulated in R(-) but not in R(+) mice challenged with aFas. The requirement of a normal TNFR1/TNFR2 phenotype for full deployment of the general and liver-specific aFas toxicity in mice most likely implies that treatment with aFas in some way results in activation of the TNFalpha-TNFRs system and that this activation synergizes with Fas-mediated signals in causing the fulminant liver injury and the animal death. The precise cellular and molecular details underlying this interplay between Fas- and TNFRs-mediated signaling systems in the general and liver-specific aFas toxicity largely remain to be clarified.

Skeletal muscle wasting in tumor-bearing rats is associated with MyoD down-regulation.

Cachexia is a syndrome characterized by profound skeletal muscle wasting that frequently complicates malignancies. A number of studies indicate that protein hypercatabolism, largely mediated by classical hormones and cytokines, is the major component of muscle depletion. Impaired regeneration has been suggested to contribute to the reduction of muscle size. In particular, it has been shown that the expression of MyoD, a muscle-specific transcription factor, is down-regulated by cytokines such as TNFalpha and IFNgamma in a NF-kappaB-dependent posttranscriptional manner. The present study investigated whether modulations of the transcription factor MyoD are associated with the onset of muscle wasting in a well established model of cancer cachexia. Rats bearing the Yoshida AH-130 hepatoma develop a condition of muscle protein hypercatabolism, largely dependent on TNFalpha bioactivity. In the gastrocnemius of these animals the expression of MyoD was markedly reduced, paralleling the decrease of muscle weight. This pattern is associated with increased nuclear translocation of AP-1, while DNA-binding assays did not detect any change in NF-kappaB activity. This is the first observation demonstrating that muscle depletion in tumor-bearing rats is associated with a down-regulation of MyoD levels. Although the underlying mechanisms remain to be clarified, this change is compatible with the hypothesis that a reduced expression of molecules involved in the regulation of the regenerative response may concur to muscle wasting in cancer cachexia.

In vitro and in vivo conditional sensitization of hepatocellular carcinoma cells to TNF-induced apoptosis by taxol.

High mortality among hepatocellular carcinoma (HCC) patients reflects both late diagnosis and low curability, due to pharmacoresistance. Taxol (TAX) is toxic for many human HCC-derived cell lines, yet its clinical efficacy on HCCs is poor. Combining TAX with other drugs appears a promising possibility to overcome such refractoriness. We analyzed whether combining tumor necrosis factor (TNF) with TAX would improve their toxicity. Human HCC-derived cell lines were treated with TAX or TNF, alone or combined. Apoptosis was assessed by morphology and flow-cytometry. Several pro- and anti-apoptotic molecules were evaluated by western blotting and/or enzymatic assay. After a 24 hour treatment, TNF was ineffective and TAX modestly cytotoxic, whereas HCC cells were conditionally sensitized to TNF by TAX. Indeed some relevant parameters were shifted to a prodeath setting: TNF-receptor 1 was increased, SOCS3, c-FLIP and pSTAT3 were markedly downregulated. These observations provide a significant clue to critically improve the drug susceptibility of HCC cells by combining 2 agents, TAX and TNF. The sequential application of TAX at a low dosage followed by TNF for only a short time triggered a strong apoptotic response. Of interest, prior TAX administration could also sensitize to TNF-induced apoptosis in the Yoshida AH-130 hepatoma transplanted in mice. Therefore, scrutinizing the possibility to develop similar combination drug regimens in suitable preclinical models seems highly advisable.

Role of mitogen-induced calcium influx in the control of the cell cycle in Balb-c 3T3 fibroblasts.

The role of mitogen-activated calcium influx from the extracellular medium in the control of cell proliferation was studied in Balb-c 3T3 fibroblasts. Stimulation of serum-deprived, quiescent cells with 10% foetal calf serum (FCS) induced a long-lasting (up to 70 min) elevation of intracellular free calcium concentration ([Ca2+]i). Both the sustained [Ca2+]i increase and the related inward current, described in a previous paper [Lovisolo D. Munaron L. Baccino FM. Bonelli G. (1992) Potassium and calcium currents activated by foetal calf serum in Balb-c 3T3 fibroblasts. Biochim. Biophys. Acta, 1104, 73-82], could be abolished either by chelation of extracellular calcium with EGTA or by SK&F 96365, an imidazole derivative that can block receptor-activated calcium channels. The effect of the abolition of these ionic signals on FCS-induced proliferation was investigated by adding either EGTA or SK&F 96365 to the culture medium during the first hours of stimulation of quiescent cells with 10% FCS. As measured after 24 h, a 22% inhibition of growth was observed when SK&F 96365 was added for the first hour, and stronger inhibitions, up to 56%, were obtained by adding the blocker for the first 2 or 4 h. Similar effects were observed with addition of 3 mM EGTA, though the inhibition was less marked for the 4 h treatment. By contrast, incubation with either substance in the next 4 h of serum stimulation did not influence cell growth, except for a slight inhibition observed when SK&F 96365 was applied from the 4th to the 8th hour. The reduction in growth resulting from the abolition of the early calcium influx was paralleled by an accumulation of cells in the G2/M phase. Both growth inhibition and G2/M accumulation were reversible, since after further 24 h in 10% FCS cells had fully recovered the exponential growth. These data indicate that the early calcium influx seen in response to mitogen stimulation develops on a timescale long enough to play a significant role in cell cycle progression, and that its block in the early G1 phase can lead to a reduction of proliferation by arresting cells in later stages of the cycle.

Effect of 4-Hydroxynonenal on cell cycle progression and expression of differentiation-associated antigens in HL-60 cells.

4-Hydroxynonenal (HNE) is a highly reactive aldehyde produced by lipid peroxidation of cellular membranes that inhibits growth and induces differentiation in HL-60 cells. Its mechanisms of action were investigated by analyzing the cell cycle distribution and the appearance of differentiated phenotypes in HL-60 cells. Data obtained by exposing cells to DMSO for 7.5 h (same time as for HNE treatment) or for the whole length of the experiments (5 d) were used for comparison. HNE induced a marked increase in the proportion of G0/G1 cells after 1 and 2 d. The brief DMSO treatment did not affect the distribution, whereas continuous exposure led to a progressive accumulation of cells in G0/G1 (maximal at day 5). The proportion of phagocytic cells gradually increased in HNE-treated and DMSO long-exposed cultures from day 2 and peaked at day 5 (35 and 63%, respectively), whereas the effect of the brief DMSO treatment was negligible. The expression of CD11b and CD67 increased in cells treated with HNE or continuously exposed to DMSO, whereas CD36 was expressed at low levels on both treatments. These results indicate that the pathway of the granulocytic differentiation induced by HNE in HL-60 cells differs from that of DMSO: with HNE, growth inhibition precedes the onset of differentiation, whereas in DMSO-treated cells the two processes are chronologically associated.

Decreased expression of the high-mobility group protein T160 by antisense RNA impairs the growth of mouse fibroblasts.

The T160 protein belongs to the HMG-1 box protein family and preferentially binds to non-B-DNA conformations with no sequence specificity. Its exact role has yet to be defined, though it seems to participate in processes involving DNA, such as replication, transcription and recombination. We have used an antisense RNA strategy to investigate its role in cell growth and proliferation. T160 expression is strongly suppressed by stable introduction of an antisense construct into NIH3T3 cells, and this decrease is accompanied by substantial changes in the growth properties of the stable transfectants. Impaired growth of T160- cells was mainly related to two mechanisms: i) decreased rates of cell proliferation at normal serum concentration; and ii) occurrence of cell death by apoptosis at low serum concentration, as demonstrated by both flow cytometry and microscopy. The finding that decreased T160 availability affects cell proliferation, provides further evidence of its involvement in a basic cell function, such as DNA replication.

Lack of effect of eicosapentaenoic acid in preventing cancer cachexia and inhibiting tumor growth.

It has been recently reported that a diet enriched in n-3 polyunsaturated fatty acids reduces the growth of different kinds of tumors as well as the host tissue hypercatabolic state frequently associated. The rat ascites hepatoma Yoshida AH-130 is a fast growing tumor that causes a rapid and progressive body weight loss in the host and tissue waste associated with a hypercatabolic condition. Plasma levels of classical hormones and humoral mediators (prostaglandin E2 and tumor necrosis factor-alpha) are early perturbed after tumor transplantation (Tessitore, L., Costelli, P. and Baccino, F.M. (1993) Humoral mediation for cachexia in tumour-bearing rats. Br. J. Cancer, 67, 16-23). Enhanced protein degradation rates and alteration of lipoprotein lipase activity mainly account for the wasting of protein and adipose mass, respectively. However, the daily intragastric administration of eicosapentaenoic acid (1.5 g/kg body wt) to AH-130 bearing rats was completely ineffective either in preventing tissue waste or in reducing tumor growth. The low degree of differentiation and the high growth rate of the AH0130 hepatoma probably account for this lack of effect.

Aliphatic aldehydes inhibit the proliferative response of human peripheral blood lymphocytes to phytohemagglutinin and alloantigens.

The effect of methyl glyoxal (MG) and various 4-hydroxyalkenals on the response of human peripheral blood lymphocytes (PBL) to phytohemagglutinin (PHA) or allogeneic cells has been investigated. Pretreatment of PBL with aldehydes significantly reduced the percentage of blast-transformed cells and [3H]thymidine incorporation into DNA in both PHA- and alloantigen-stimulated cultures, hydroxyalkenals being more effective than MG. Further experiments showed that these aldehydes also affected the proliferation of pre-activated lymphocytes. The percentage of blasts as well as [3H]thymidine incorporation into DNA were significantly decreased when the aldehydes were added until 72 h after application of the mitogenic stimulus.

Effect of two aliphatic aldehydes, methylglyoxal and 4-hydroxypentenal, on the growth of Yoshida ascites hepatoma AH-130.

The influence of a ketoaldehyde, methylglyoxal (MG), and a hydroxyalkenal, 4-hydroxypentenal (HPE), on the growth of a highly-deviated tumour has been investigated. MG and HPE, administered intraperitoneally, strongly depressed in rats the proliferative activity of the Yoshida ascites hepatoma AH-130, reducing its mitotic and labelling indices as well as the proportion of cycling cells (growth fraction). Monitoring the effects on the cell cycle by the labelled mitoses method showed that the percentage of labelled mitoses was markedly lowered after either aldehyde, which is indicative for a blocking effect in the S phase. In addition, the mean cell cycle time was slightly prolonged by MG, probably due to accumulation of cells in G1, whereas HPE delayed the first mitotic peak and increased the mean DNA synthetic period without modifying the overall cycle time. The effects of HPE on the cell cycle were prevented by pretreatment with polyamines. Repeated doses of MG significantly increased the fraction of tumour-bearing rats surviving at 90 days ('indefinite' survivors) as well as the survival time of those which succumbed, implying that the carcinostatic effect of MG persisted over several cell cycles. By contrast, HPE did not significantly modify the survival of AH-130-bearing rats, suggesting that its influence on tumour growth was rapidly reversible.