Antiproliferative effect of L-NAME on rat vascular smooth muscle cells

The nitric oxide synthase (NOS) inhibitor L-NAME may have growth inhibitory effects in vivo. We investigated in vitro the potential growth inhibitory effects of three different NOS inhibitors: L-NAME (1 mM), LNMMA (1 mM) and aminoguanidine (0.5 mM), on fetal bovine serum (FBS) and platelet derived growth factor (PDGF-BB)-stimulated growth in cultured vascular smooth muscle cells (VSMCs). [3H]-thymidine incorporation into rat mesenteric VSMCs was measured as an index of VSMCs proliferation (DNA synthesis) and activation of extracellular signal regulated kinase (ERK1/2), a major signaling event in cell growth, was measured by western blot assay. PDGF-BB (0-5 ng/mL) and FBS (0-5%) increased [3H]-thymidine incorporation in a dose-dependent manner up to 6-10 fold. L-NAME significantly reduced PDGF-BB (5 ng/ml) and FBS (5%) stimulated DNA synthesis by 46% and 38% respectively. The increase of [3H]-thymidine incorporation induced by PDGF-BB and FBS was unaltered by L-NMMA. In contrast, aminoguanidine induced an increase in FBS and PDGF-BB-stimulated [3H]-thymidine incorporation of 64% and 34% respectively above cells not exposed to aminoguanidine. ERK1/2 phosphorylation induced by PDGF-BB and FBS was not affected by pre-treatment with L-NAME or aminoguanidine. In conclusion, NOS inhibitors differentially influence DNA synthesis in VSMCs: L-NAME inhibits FBS and PDGF-BB-stimulated cellular proliferation whereas aminoguanidine accentuates FBS and PDGF-BB-stimulated VSMCs proliferation. These phenomena are independent of the ERK1/2 pathway. The growth inhibitory effects of L-NAME may be related to differences in properties from other NOS inhibitors, and independent of its ability to inhibit NOS.     

Sequential study on the influence of adriamycin on cell proliferation and ultrastructure of cultured mammary tumor cells

The time-dependent cytocidal and growth inhibitory effects of Adriamycin (ADM) on monolayer cultures of 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumor cells were analyzed. The inhibitory effect on cell proliferation was assessed by colony formation in soft agar. Growth inhibition and [3H]thymidine labeling indices clearly demonstrate a dose-dependent antimitotic and cytotoxic effect of the drug. At low concentrations (10(-9)-10(-8) M), 90-100% of cells survived 24-hr exposure. At a higher concentration (10(-5) M), 75-80% of cells survived after 8-hr exposure; by 72 hr only 20-30% of the cells remained. Autoradiographic examination of the pulse-labeled cultures demonstrated no change in the proportion of cells in S-phase during the first 4 hr of treatment. Subsequently DNA synthesis was completely abolished and remained inhibited for the duration of the experiment (72 hr). Clonogenic assay revealed a complete arrest of growth in cells exposed to 10(-5) M ADM and greater than 60% inhibition of cell proliferation at 10(-7) M. Ultrastructural changes were not observed in cells during the first 4 hr of treatment; however, after 8 hr most surviving cells exhibited alterations in nuclear chromatin. The surviving cells showed mitochondrial degeneration, myelin body formation, and vacuolization of the endoplasmic reticulum. This study shows the potential usefulness of the primary culture system in drug evaluation. In addition, serial observation of the effects of ADM revealed a cell subpopulation of the primary culture with differential sensitivity to the drug.     

Interferon inhibition of thymidine incorporation into DNA through effects on thymidine transport and uptake

Replenishment of medium after 72 hr of growth of HeLa-S3 cells in dense suspension cultures increased [³H]-thymidine uptake into cells and incorporation into DNA, with the levels reaching a peak ～ 12 hr following medium change; β interferon inhibits the enhanced uptake of [³H]-thymidine and labeling of DNA in a dose-dependent manner. Some reduction in these processes is observed at a concentration as low as 1 u/ml, and ～ 75% inhibition at 640 u/ml. Kinetic analysis has revealed that the rate of labeling of the acid-soluble pool with [³H]-thymidine, measured either at 22°C, or 37°C, is reduced in interferon-treated (640 u/ml, 24 hr) HeLa-S3 cells. At 22°C, the initial rate of thymidine transport at a high (500 μM) thymidine concentration, determined within the first 30 sec of [³H]-thymidine addition was depressed by 44% in interferon-treated HeLa cells. At 37°C, labeled precursors accumulate in acid-soluble material for ～ 8 min after the addition of [³H]-thymidine, after which an apparent equilibrium level is attained. At this temperature, the rate of thymidine uptake and the apparent equilibrium level attained were depressed by 70% in interferon-treated HeLa cells. The reduced incorporation of [³H]-thymidine into DNA in interferon-treated HeLa-S3 cells can be largely explained by interferon inhibition of thymidine transport and phosphorylation.     

Cellular test for RNA-synthesis in rat bone marrow and its use in testing cryoprotective and toxic agents]

A system for the measurement of the RNA-synthesis of bone marrow cells of the rat has been developed and the incorporation of [3H]-uridine into the cellular RNA has been standardized with respect to the time of incubation, the concentration of [3H]-uridine and the number of cells. A plateau of the incorporation of [3H]-uridine into the RNA is reached after 20 min of incubation and is interpreted as the expression of a steady state in synthesis and degradation of the cellular RNA. A constant labelling of the RNA is reached above 8.3 with 10(-6)M [3H]-uridine. The optimal cell number in the 500 mul standard assay is 4 with 10(6). Actinomycin D inhibits the RNA-synthesis to 94% in a concentration of 1.2 with 10(2) mug/ml. The cryoprotectants dimethylsulfoxide, polyethylene-oxide and glycerol and the potential haematotoxic substances dichlorodiphenyltrichloroethane and gamma-hexane were tested in this system. 5% dimethylsulfoxide and 10% polyethylen-oxide in Eagle's-medium with ethylendiamintetra-acetate do not influence the RNA-synthesis. 5% glycerol reduces the incorporation of [3H]-uridine into the cellular RNA to about 30%.     

Action of cortisol on the proliferation of rainbow trout fibroblasts

The effect of cortisol on the proliferation of the rainbow trout fibroblast cell line, RTG-2, was examined in synchronous and asynchronous cultures. When the transition from G1 to S was synchronized by restoring serum to serum-deprived cultures, the addition of cortisol at the time of serum restoration delayed the entry of cells into S phase. However, if cortisol was added 24 h after serum restoration, at the G1/S transition point, the subsequent peak of DNA synthesis was unaffected. In asynchronous cultures cortisol inhibited [3H]-thymidine and [3H]-uridine but not [3H]-leucine incorporation into acid-insoluble material. If the exogenous nucleoside concentration was raised, [3H]-thymidine but not [3H]-uridine incorporation continued to be inhibited by cortisol. This suggested that cortisol's effect on [3H]-thymidine incorporation reflected a change in entry into S phase and not just on thymidine uptake and metabolism. Cortisol inhibited the proliferation of RTG-2 in asynchronous cultures. At 1000 ng/ml of cortisol a reduction in cell number became apparent before the RTG-2 cultures were confluent, whereas at 100 ng/ml the reduction only became evident in confluent cultures. The synthetic antiglucocorticoid, RU 486, which acts at the level of the corticosteroid receptor, blocked the growth inhibition by cortisol. These results suggest that cortisol regulates rainbow trout fibroblast proliferation via the corticosteroid receptor and that the G1/S transition is one point at which this regulation occurs.     

Angiotensin II stimulation of the rat pituitary tumoral cell proliferation in vitro.

The effect of angiotensin II (AT II) on proliferation of rat pituitary tumoral cells was investigated in vitro. The tumoral cells were isolated from the prolactin-secreting pituitary tumors induced by stilboestrol implantation. The incorporation of [3H]-thymidine into DNA was used as an index of cell proliferation. It was found that AT II significantly enhanced the [3H]-thymidine incorporation into pituitary tumoral cells in the concentrations of 10(-10) and 10(-8) M. The stimulatory effect disappeared at the concentration of 10(-6) M. The possible involvement of pituitary renin-angiotensin system in pituitary tumorigenesis was discussed.     

Alterations of MCF-7 human breast cancer cell after prostaglandins PGA1 and PGF2 alpha treatment

Treatment of monolayer cultures of MCF-7 cells with prostaglandins PGA1 and PGF2 alpha inhibited cell proliferation, reduced the rate of labeled precursor incorporation into DNA, RNA, and protein, and induced morphological changes in a dose-dependent manner. The rate of [3H]thymidine incorporation was increased by PGA1 at 10(-10)-10(-8) M, while a sharp decrease was observed at 10(-6)-10(-4) M (p less than 0.05 and p less than 0.005). PGF2 alpha inhibited [3H]thymidine incorporation at all concentrations tested. Similar results were obtained for [3H]uridine incorporation with both PGs. PGA1 inhibited [3H]leucine incorporation at 10(-4) M, but increased incorporation at 10(-10)-10(-6) M. At the ultrastructural level, neither PG induced morphological alterations at 10(-12)-10(-8) M. However, at 10(-6)-10(-4) M both PGA1 and PGF2 alpha diminished the number and size of cell surface projections; some cells appeared to completely lack microvilli. Disorganization of mitochondrial cristae and increased electron density of the matrix were also evident.     

Inhibitory effect of a long-acting somatostatin analogue on EGF-stimulated cell proliferation in Capan-2 cells.

Numerous studies have reported diverse effects of gut-derived regulatory peptides on growth of the normal pancreas, pancreatic neoplasms induced experimentally in animals, and pancreatic cancer cell lines, but the results of these investigations are rather controversial. The stimulatory effect of epidermal growth factor (EGF) on cell proliferation of pancreatic cell lines is well established. Whether this action can be modulated by somatostatin is not clear. Furthermore, it is not certain whether another regulatory peptide, cholecystokinin (CCK), affects the proliferation of these cells. In the present study we investigated the presence of CCK-A and CCK-B, as well as somatostatin-2 (SSTR2) receptors by RT-PCR, and studied the actions of EGF, CCK and octreotide on DNA synthesis in the human pancreatic adenocarcinoma cell line Capan-2. Octreotide, a long-acting somatostatin analogue was used as somatostatin agonist. Cells were cultured in RPMI-1640 medium. They were incubated in serum free medium containing 0.2% BSA in the absence (control) or the presence of the peptides. [3H]-thymidine incorporation into DNA was measured after 48 h of incubation. By means of RT-PCR analysis we were able to demonstrate SSTR2 expression, but not CCK-A or CCK-B receptor mRNA in Capan-2 cells. DNA synthesis evaluated by [3H]-thymidine incorporation was found to be increased by 45.2 +/- 5.6% in response to EGF (10(-8) M) and decreased by 11.7 +/- 2.6% to octreotide (10(-8) M) compared to controls (P < 0.01). The increase in [3H]-thymidine incorporation was significantly lower when EGF treatment was combined with octreotide administration (10.1 +/- 2.5% over control). In the concentration range of 10(-11)-10(-8) M, CCK did not alter significantly the incorporation of [3H]-thymidine into DNA in Capan-2 cells. In conclusion, these data support a role for EGF as a growth factor for the human pancreatic cancer cell Capan-2. Somatostatin may play an important role in regulating cell proliferation in Capan-2 cells both under basal, and growth factor-stimulated conditions. Our results suggest, however, that CCK receptors are not expressed, and CCK does not affect cell proliferation in this transformed pancreatic cell line.     

Short-period hypoxia increases mouse embryonic stem cell proliferation through cooperation of arachidonic acid and PI3K/Akt signalling pathways

Hypoxia plays important roles in some early stages of mammalian embryonic development and in various physiological functions. This study examined the effect of arachidonic acid on short-period hypoxia-induced regulation of G(1) phase cell-cycle progression and inter-relationships among possible signalling molecules in mouse embryonic stem cells. Hypoxia increased the level of hypoxia-inducible factor-1alpha (HIF-1alpha) expression and H2O2 generation in a time-dependent manner. In addition, hypoxia increased the levels of cell-cycle regulatory proteins (cyclin D(1), cyclin E, cyclin-dependent kinase 2 (CDK2) and CDK4). Maximum increases in the level of these proteins and retinoblastoma phosphorylation were observed after 12-24 h of exposure to hypoxic conditions, and then decreased. Alternatively, the level of the CDK inhibitors, p21(Cip1) and p27(Kip1) were decreased. These results were consistent with the results of [3H]-thymidine incorporation and cell counting. Hypoxia also increased the level of [3H]-arachidonic acid release and inhibition of cPLA(2) reduced hypoxia-induced increase in levels of the cell-cycle regulatory proteins and [3H]-thymidine incorporation. The level of cyclooxygenase-2 (COX-2) was also increased by hypoxia and inhibition of COX-2 decreased the levels of cell-cycle regulatory proteins and [3H]-thymidine incorporation. Indeed, the percentage of cells in S phase, levels of cell cycle regulatory proteins, and [3H]-thymidine incorporation were further increased in hypoxic conditions with arachidonic acid treatment compared to normoxic conditions. Hypoxia-induced Akt and mitogen-activated protein kinase (MAPK) phosphorylation was inhibited by vitamin C (antioxidant, 10(-3) M). In addition, hypoxia-induced increase of cell-cycle regulatory protein expression and [(3)H]-thymidine incorporation were attenuated by LY294002 (PI3K inhibitor, 10(-6) M), Akt inhibitor (10(-6) M), rapamycin (mTOR inhibitor, 10(-9) M), PD98059 (p44/42 inhibitor, 10(-5) M), and SB203580 (p38 MAPK inhibitor, 10(-6) M). Furthermore, hypoxia-induced increase of [(3)H]-arachidonic acid release was blocked by PD98059 or SB203580, but not by LY294002 or Akt inhibitor. In conclusion, arachidonic acid up-regulates short time-period hypoxia-induced G(1) phase cyclins D(1) and E, and CDK 2 and 4, in mouse embryonic stem cells through the cooperation of PI3K/Akt/mTOR, MAPK and cPLA(2)-mediated signal pathways.     

Inhibition of proliferation of MCF-7 breast cancer cells by a blocker of Ca2+-permeable channel

In MCF-7 breast cancer cells, insulin-like growth factor-1 (IGF-1) increased the calcium-permeability of the cells by activating a voltage-independent calcium-permeable channel. IGF-1 also induced oscillatory elevation of cytoplasmic free calcium concentration in these cells. An anti-allergic compound, tranilast, reduced the calcium-permeability augmented by IGF-1 in a dose-dependent manner and blocked the oscillatory elevation of cytoplasmic free calcium concentration. Tranilast did not affect early intracellular signals activated by IGF-1, including receptor autophosphorylation, activations of Ras, mitogen-activated protein kinase and phosphatidylinositol 3-kinase. Tranilast inhibited increases in [³H]-thymidine incorporation, DNA content and cell number induced by IGF-1. The ID₅₀ for [³H]-thymidine incorporation and DNA content were about 10 μM. The inhibitory effect of tranilast was reversible, and cell viability was not affected. Treatment with tranilast increased the number of cells in the G₁ phase suggesting that this compound induced G₁ arrest. Tranilast also reduced the phosphorylation of the retinoblastoma protein. These results indicate that tranilast inhibits the IGF-1-induced cell growth in MCF-7 cells by blocking calcium entry.     

Mitotic activity and nucleic acid precursor incorporation in denervated and innervated limb stumps of axolotl larvae.

Mitotic activity and DNA and RNA precursor incorporation were compared in innervated regenerating limbs and in denervated, non-regenerating limbs on days 8 and 9 post-amputation. Innervated limbs had well-developed cone stage blastemas which showed high cellular mitotic indices and H3-thymidine labeling indices of 0.40-0.50 and H3-uridine labeling indices of 0.50-0.75. In contrast, denervated limbs showed dedifferentiated cells distally under thickened wound epithelia, but essentially no mitotic activity and no blastema formation. These dedifferentiated cells showed lower levels of H3-thymidine (0.10 index) and H3-uridine (0.50) incorporation than regenerating limbs. Labeling indices of wound epithelia are also compared.     

Effects of chloramphenicol on plant cells: Potential as a selectable marker for transformation studies

The growth of tobacco and carrot cells in suspension cultures and on solidified medium was strongly inhibited by the antibiotic chloramphenicol. This effect was irreversible. The drug had no effect on [³H]-uridine incorporation but inhibited protein synthesis and induced striking morphological alterations in intoxicated cells. O₂ consumption was also reduced in treated cells. Tobacco and carrot cells were unable to inactivate chloramphenicol by enzymatic acetylation.     

Polyamine-mediated inhibition of in-vitro cell proliferation is not due to acrolein

The influence of acrolein or spermine on the viability and growth of phytohaemagglutinin-stimulated rat thymic lymphocytes in cultures supplemented with foetal calf serum have been investigated. Acrolein (greater than 20 microM) was cytotoxic; spermine had little effect on viability, but inhibited [3H]TdR incorporation at low concentrations (approximately 10 microM). Cells treated with greater than 8 microM acrolein 3 hr before stimulation exhibited irreversible inhibition of protein synthesis, whereas 50 microM spermine had no effect, even when cells were treated for 24 hr before stimulation. However, addition of 25 microM spermine after stimulation did inhibit both [3H]-uridine incorporation and protein synthesis: this was reversible if cells were freed of polyamine within 4 hr, but not if washed after 24 hr. These results show that, contrary to several previous reports, in-vitro inhibition of cell proliferation by spermine is not due to the formation and action of acrolein.     

Inhibitory effect of bombesin and SMS 201-995 on DNA synthesis in the rat thyroid lobes incubated in vitro

The effects of 4-h incubation in the presence of bombesin on the incorporation of [3H]-thymidine into DNA of the rat thyroid lobes, collected from animals treated in vivo with a long-acting somatostatin analog (SMS 201-995) or with 0.9% NaCl, were investigated. It was shown that not only in vivo injections of SMS 201-995, but also, unexpectedly, in vitro incubation with bombesin inhibited [3H]-thymidine incorporation. The two examined substances did not reveal any additive action in their inhibitory effects on the thyroid growth.     

Autoradiographic study of RNA synthesis in adrenal cells of the young rat in primary culture

Primary cultures of young rat adrenal cells were used in a radioautographic study of the effect of ACTH on RNA synthesis. Control and ACTH-stimulated cultures were incubated with 5.6 [3H]-uridine for 30 min and chased for 15 and 30 min. Labelling over nucleolar and extranucleolar areas was always lower in the ACTH-stimulated animals at all experimental times. At 30 min chase-time a relatively high labelling of nuclei in both experimental conditions suggest the slow metabolism of RNA in adrenal. The results confirm the inhibitor effect of adrenocorticotrophin on [3H]-uridine incorporation.     

Aurintricarboxylic acid (ATA) and DNA synthesis. II. Effect of ATA on structure and function of the crypts of the small intestine.

ATA affects only slightly DNA synthesis of continuously replicating cells. A single injection of the drug reduces the incorporation of (3H)-thymidine into DNA of crypt cells to only 62% of the control. The effect on DNA synthesis is preceded by a slight inhibition of protein synthesis, and by a partial decrease in the number of dividing cells. On the contrary, the incorporation of (3H)-uridine into RNA was enhanced. Electron microscopic studies revealed no cytologic abnormalities in ATA-treated animals. In view of the fact that ATA at the same concentration inhibits DNA synthesis of growth stimulated cells to 100% (Novi, 1976), it was suggested that the drug may become an useful tool in inducing a preferential inhibition of growth stimulation.     

Effects of Cl- channel blockers on endothelin-1-induced proliferation of rat vascular smooth muscle cells

The effects of Cl- channel blockers on endothelin-1 (ET-1)-induced proliferation of rat aortic vascular smooth muscle cells (VSMC) were examined. We found ET-1 concentration-dependently increased cell count and [3H]-thymidine incorporation into VSMC, with EC50 values of 24.8 and 11.4 nM, respectively. Both nifedipine and SK&F96365 inhibited 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC with the maximal inhibitory concentrations of 1 and 10 microM, respectively. DIDS inhibited 10 nM ET-1-induced increase in cell count and [3H]-thymidine incorporation into VSMC in a concentration-dependent manner, whereas other Cl- channel blockers including IAA-94, NPPB, DPC, SITS and furosemide did not produce these effects. 3 microM DIDS reduced 10 nM ET-1-induced sustained increase in cytoplasmic Ca2+ concentration ([Ca2+]) by 52%. Pretreatment of VSMC with 1 microM nifedipine completely inhibited the DIDS effect on 10 nM ET-1-induced [3H]-thymidine incorporation into VSMC and sustained increase in [Ca2+]i, whereas pretreatment with 10 microM SK&F96365 did not completely block these effects of DIDS. DIDS did not affect ET-1-induced Ca2+ release and 30 mM KCl-induced increase in [Ca2+]i. Our data suggest that DIDS-sensitive Cl- channels mediate VSMC proliferation induced by ET-1 by mechanisms related to membrane depolarization and Ca2+ influx through voltage-dependent Ca2+ channels.     

Effect of the neuropeptide substance P on the rat bone marrow-derived osteogenic cells in vitro

Substance P containing, thin, sensory nerve fibres have been demonstrated in bone and bone marrow. However the role of substance P in bone tissue is not fully understood. We investigated the effects of substance P on the growth and development of rat bone marrow-derived osteogenic cells in vitro. To examine this, the marrow-derived osteogenic cells were treated from 3rd to 6th day of subculture with substance P at concentrations 10(-10), 10(-9) and 10(-8)M. [(3)H]-thymidine, L-2,3-[(3)H]-proline incorporation, protein accumulation, alkaline phosphatase activity, and calcium deposition were measured in cultures. Substance P slightly stimulated [(3)H]-thymidine incorporation at 10(-10) M. Protein accumulation and L-2,3-[(3)H]-proline incorporation were enhanced in a dose dependent manner. Simultaneous application of spantide, a substance P receptor antagonist, could not block substance P-induced L-2,3-[(3)H]-proline incorporation probably because of statistically significant effect of spantide itself. Calcium deposits were significantly lower (about 30%) in cultures treated with SP. This effect was probably due in part by the fall in alkaline phosphatase activity which in substance P treated cultures was decreased about 17%. Our results indicate that substance P could be one of the factors modulating bone metabolism.     

Nucleic acid synthesis in cancerous cells under the effect of gnidilatimonoein from Daphne mucronata

Cytotoxicity evaluation of gnidilatimonoein, the most active isolated diterpene ester from Daphne mucronata [Sadeghi H, Mianabadi M, Yazdanparast R, (2002) Journal of Tropical. Medicinal Plant1 3: 169-173], revealed the strong antiproliferative activity among several different human cancer cell lines (K562, CCRF-CEM, HL-60 and MOLT-4 leukemia cell lines, LNCaP-FGC-10 a prostate cancer cell line) and a mouse BALB/C fibrosarcoma cell line (WEHI-164). Using flow cytometry technique, it was found that treatment of the most responsive cells (K562) with gnidilatimonoein inhibited the progression of cells through G1 phase by almost 15% compared to the untreated cells. The population of the treated cells in the S and G2 phases also reduced by 8.3% and 5.4%, respectively. Based on the extent of [3H]-thymidine and [3H]-uridine incorporation into DNA and RNA, respectively, the major metabolic effects of gnidilatimonoein were found to be mainly on DNA and to a less extent on RNA synthesis. Additionally, the activity of inosine-5'-monophosphate dehydrogenase (IMPDH), under the effects of genidilatimonoein, was reduced in the treated cells by 44%. These data strongly suggest that the purine biosynthetic pathway is significantly affected by gnidilatimonoein.     

Regulation of T-cell activation and T-cell growth factor (TCGF) production by hydrogen peroxide

Activated macrophages are known to release a variety of immunoregulatory substances including the low-molecular-weight substances hydrogen peroxide and lactate. We report here that lactate but not hydrogen peroxide is capable of supporting a substantial production of T-cell growth factor (TCGF) in cultures of accessory cell-depleted splenic T-cell populations after stimulation with concanavalin A. Hydrogen peroxide and its biosynthetic precursor superoxide anion (O2-) mediate, however, a strong augmentation of the TCGF production by accessory cell-depleted T-cell populations in the presence of lactate. Lactate inhibits the incorporation of [3H]thymidine in short-term cultures (18-26 hr) of accessory cell-depleted T cells. This confirms the rule that (optimal) production of T-cell growth factor requires a growth inhibitory signal. Concentrations of hydrogen peroxide which augment TCGF production most effectively (i.e., 1 X 10(-5) M) do not inhibit the incorporation of [3H]thymidine; and higher concentrations (3 X 10(-5)-1 X 10(-4) M) of hydrogen peroxide inhibit both the production of TCGF and the incorporation of [3H]thymidine. In agreement with the augmenting effect of hydrogen peroxide on TCGF production, it was observed that the proliferative response in mixed lymphocyte cultures is suppressed by catalase and augmented by 1 X 10(-5) M H2O2. Proliferative and cytotoxic responses in mixed lymphocyte cultures with an external source of interleukin 2 (IL-2) in contrast, are not augmented by 1 X 10(-5) M H2O2. The relatively high concentration of 1 X 10(-4) M hydrogen peroxide was found to inhibit the proliferative responses in mixed lymphocyte cultures with or without external IL-2 but not the cytotoxic response in the presence of IL-2. This indicates that CTL precursor cells may be relatively resistant against H2O2.