Biomedical subjects

V Kinzel

Publications and source records attributed to V Kinzel.

At least 73 records · Page 4Linked to original sources

PubMed1988

Phorbol ester-induced G2 delay in HeLa cells analyzed by time lapse photography.

The tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) has been shown previously to mimic X-irradiation in altering cell cycle parameters in Hela cells [Kinzel, V., Richards, J., and Stöhr, M., Science (Wash. DC), 210: 429-431, 1980]. These changes include a delay in G2 phase from which cells recover in the presence of TPA, which suggests an involvement of cellular mediators. In order to obtain information on the onset and the duration of the G2 delay, as well as the onset and rate of recovery, a time-lapse study has been carried out. The analysis of cells in prophase shows that at 10(-6) M and 10(-7) M concentrations, TPA and 12-O-retinoylphorbol-13-acetate (RPA) cause a G2 delay which lasts on the order of 3.5 to 4 h. Below 10(-7) M of RPA and below 10(-8) M of TPA a clear-cut inhibition of HeLa cells in G2 is no longer detectable by this method. These results for a given phorbol ester are dose dependent within a certain range but unlike the case of X-rays are not proportional to dose. Within the dose range studied the recovery rate follows the opposite order. At 10(-6) M TPA and RPA an indication of a parasynchronous burst is observed. At smaller concentrations or with less biological activity of phorbol ester, the cell multiplication rate approaches that of the control or remains even smaller. Possible reasons are discussed. The determination of the transition points seems to indicate that the cellular events inhibited in G2 occur shortly before visible prophase.

Cell Cycle↗

PubMed1987

Circular dichroic investigations of secondary structure in synthetic peptide inhibitors of cAMP-dependent protein kinase: a model for inhibitory potential.

The structure of the inhibitory domain of the inhibitor protein of the cAMP-dependent protein kinase has been assessed by circular dichroism studies of synthetic inhibitory peptides. Using the inhibitory peptide PKI(5-22)amide (Thr5-Thr-Tyr-Ala-Asp-Phe-Ile-Ala-Ser-Gly-Arg-Thr-Gly-Arg-Arg-Asn- Ala-Ile22) [Cheng, H.-C., Kemp, B. E., Pearson, R. B., Smith, A. J., Misconi, L., Van Patten, S. M., & Walsh, D. A. (1986) J. Biol. Chem. 261, 989-992] and shorter peptides of this sequence, it has been estimated that this parent peptide is composed of approximately 30% alpha-helix with the remainder being random coil with one beta-turn. The pseudosubstrate arginine cluster (Arg15-Arg19) is within the suggested region of random coil and beta-turn, representing one critical region of binding recognition by the protein kinase. The alpha-helix region proposed between Thr6 and Ile11 likewise contributes to the full biological potency and specificity of the inhibitor peptide and inhibitor protein. The removal of the two N-terminal threonines, for example, causes both a marked conformational change in the peptide and a diminishment by an order of magnitude of inhibitory activity. It is proposed that this alpha-helix region could serve one of several possibilities, including that it may provide a suitable constraint on the Tyr7 such that the hydroxyl is oriented in a position proximal to the pseudosubstrate domain, and/or may allow the optimal location of other protein kinase recognition signals. These data provide an initial description of some of the structural features of the inhibitor protein that could contribute to its high biological potency.

Amino Acid Sequence↗

PubMed1987

Early changes in the arachidonic acid metabolism of HeLa cells in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) and related compounds.

In order to search for possible mediators involved in the transient radiomimetic effectiveness of TPA and related compounds early changes in the AA metabolism of HeLa cells prelabeled with 1-14C-AA have been analyzed. Maximum release of AA with different concentrations of TPA (3 X 10(-9) to 3 X 10(-5) M) was observed after 2-3 h treatment in the presence of 10% calf serum. Released AA was reincorporated by the cells after that period, a phenomenon which was largely abolished or delayed by cycloheximide. Reincorporation of released AA was observed in the presence of 10% fresh serum as well as with 0.5% BSA, and appears to be due to an induction of responsible enzyme(s) by the phorbol ester. The earliest metabolites of AA produced via the cyclooxygenase such as PGE2 and PGF2 alpha and via lipoxygenases such as 12-, and 15-hydroxyeicosatetraenoic acids appear in small amounts and after later time points. AA release exhibited a pluriphasic dose response to TPA with maxima at 3 X 10(-8) M and greater than or equal to 10(-5) M. Comparative dose response measurements with respect to AA release were established using various promoting skin mitogens which exhibited the following order of potency: TPA greater than teleocidin approximately equal to RPA greater than mezerein much greater than EPA greater than 4-O-Me-TPA. For reasons discussed it appears unlikely that AA, Prostaglandins, or hydroxyeicosatetraenoic acid products play a significant role as mediators of the radiomimetic effects of TPA in G2 of the cell cycle.

Arachidonic Acid↗

PubMed1987

Chromatographic separation of two heterogeneous forms of the catalytic subunit of cyclic AMP-dependent protein kinase holoenzyme type I and type II from striated muscle of different mammalian species.

Electrophoretically homogeneous preparations of catalytic subunit (C) of cAMP-dependent protein kinase isolated according to two different procedures from holoenzyme type I and type II from rabbit and from holoenzyme type II from rat skeletal muscle and from bovine cardiac muscle can be separated on carboxymethyl cellulose or on a Mono S column (Pharmacia) by salt gradient elution into two enzymatically active peaks called A and B, which do not interconvert on rechromatography. Cochromatography of peak A fractions or of peak B fractions derived from both holoenzymes respectively yields single enzyme peaks in each case, thus indicating that both represent different entities, which were named CA and CB. The separate character of both enzyme forms is supported by the fact that CB under all conditions is degraded faster by the C-specific protease (E. Alhanaty et al. (1981) Proc. Natl. Acad. Sci. USA 78, 3492-3495) than CA, a phenomenon which is enhanced in both enzyme forms by substrate (Kemptide). The separation of both subtypes from each other is probably based on differences in isoelectric values (delta pH less than or equal to 0.5 units). The reason for the charge difference is not presently known. CA and CB do not differ significantly in their phosphate content. No differences between CA and CB have been detectable so far with respect to their migration in SDS gels, kinetic behavior regarding both substrates and cosubstrate, pH dependence, inhibition by regulatory subunits of holoenzyme type I (rabbit skeletal muscle) and of type II (bovine cardiac muscle), and inhibition by specific-heat and acid-stable inhibitor-modulator. The peptide pattern of both forms after limited proteolysis exhibits small differences.

Animals↗

PubMed1987

Role of arachidonic acid release in the G2 delay induced by tumor promoter TPA in HeLa cells.

The tumor promoter TPA2 (12-O-tetradecanoylphorbol-13-acetate) has been shown to exhibit a radiomimetic activity on the cell cycle of HeLa cells (V. Kinzel, J. Richards, and M. Stöhr (1980) Science 210, 429). The response includes a delay of cells in G2 phase. The relation between TPA-induced release of arachidonic acid (AA) and the inhibition in G2 phase was studied. Exogenous AA (greater than 10(-4) M; in presence of 10% serum) is shown to delay HeLa cells in G2 and to enhance the effectiveness of TPA in this respect. The inhibition of the TPA-induced AA liberation by fluocinolone acetonide, however, does not influence the TPA-effected G2 delay. The diacylglycerols 1,2-dioctanoyl-glycerol and 1-oleoyl-2-acetylglycerol delay HeLa cells in G2 but without major stimulation of AA liberation. On the basis of the data it is concluded that AA released from HeLa cells due to the action of TPA is not involved in the TPA-induced delay of cells in G2 phase.

Arachidonic Acid↗

PubMed1987

Catalytic and molecular properties of highly purified phosvitin/casein kinase type II from human epithelial cells in culture (HeLa) and relation to ecto protein kinase.

Phosvitin/casein type II kinase was purified from HeLa cell extracts to homogeneity and characterized. The kinase prefers phosvitin over casein (Vmax phosvitin greater than Vmax casein; apparent Km 0.5 microM phosvitin and 3.3 microM casein) and utilizes as cosubstrate ATP (apparent Km 3-4 microM), GTP (apparent Km 4-5 microM) and other purine nucleoside triphosphates, including dATP and dGTP but not pyrimidine nucleoside triphosphates. Enzyme reaction is optimal at pH 6-8 and at 10-25 mM Mg2+.Mg2+ cannot be replaced by, but is antagonized by other divalent metal ions. The kinase is stimulated by polycations (spermine) and monovalent cations (Na+,K+), and is inhibited by fluoride, 2,3-diphosphoglycerate, and low levels of heparin (50% inhibition at 0.1 microgram/ml). The HeLa enzyme is composed of three subunits with Mr of approximately 43,000 (alpha), 38,000 (alpha'), and 28,000 (beta) forming alpha alpha'beta 2 and alpha'2 beta 2 structures with obvious sequence homology of alpha with alpha' but not with beta. Photoaffinity labeling with [alpha-32P]- and [gamma-32P]8-azido-ATP revealed high affinity binding sites on subunits alpha and alpha' but not on subunit beta. The kinase autophosphorylates subunit beta and, much weaker, subunits alpha and alpha'. Ecto protein kinase, detectable only by its enzyme activity but not yet as a protein (J. Biol. Chem. 257, 322-329), was characterized in cell-bound form and in released form, and the released form both with and without prior separation from phosvitin which was employed to induce the kinase release from intact HeLa cells (Proc. Natl. Acad. Sci. U.S.A. 80, 4021-4025). Ratios of phosvitin/casein phosphorylation (greater than 2) and of ATP/GTP utilization (1.5-2.1), inhibition by heparin (50% inhibition at 0.1 microgram/ml), and amino-acid side chains phosphorylated in phosvitin and casein (serine, threonine) are comparable for cell-bound and released form. These properties resemble those of type II kinase as does Mr of released ecto kinase (120-150,000). Consistently, a protein with Mr 125,000 in calf serum and a protein (possibly two) with Mr greater than 300,000 in calf plasma which are selectively phosphorylated by the ecto kinase are also substrates of the type II kinase. Thus, nearly all properties examined of the ecto kinase are characteristic for a type II kinase.

Anions↗

PubMed1987

Substrates for the cell surface protein kinase in blood: a calf serum protein and its precursor in plasma.

A protein in calf serum with molecular mass of 125,000 is selectively phosphorylated by the surface kinase activity of intact tissue culture cells and erythrocytes. The protein, termed pp125, is phosphorylated at serine and threonine residues to a ratio of greater than 1 mol P/mol. The pp125 is an acidic protein (pI 4.4) which also serves as substrate for purified phosvitin/casein kinases but not for cyclic AMP-dependent protein kinases. About 80-fold purification of pp125 was achieved by ion exchange and affinity chromatography. Gel filtration under non-reducing conditions showed that pp125 is part of a complex (Mr 535,000). The pp125 obviously originates from a large plasma protein: the incubation of calf plasma with intact cells in the presence of [gamma-32P]ATP resulted in the labeling of a protein with Mr greater than 300,000 (pp greater than 300). The relationship between pp greater than 300 in plasma and pp125 in serum was demonstrated by cyanogen bromide peptide patterns, and the use of specific anti-serum raised against pp125. Furthermore, it was shown that pp125 is derived from pp greater than 300 during blood clotting.

Animals↗

PubMed1986

Three-stage tumorigenesis in mouse skin: DNA synthesis as a prerequisite for the conversion stage induced by TPA prior to initiation.

Recent evidence shows that stage I of tumor promotion in NMRI-mouse skin induced by a single dose of 12-O-tetradecanoylphorbol-13-acetate (TPA) can be effected not only after initiation by 7,12-dimethylbenz[a]anthracene (DMBA) but also several weeks prior to initiation. In view of this partial inversion of the initiation-promotion sequence we proposed to replace the term 'stage I of promotion' by the term 'conversion'. The convertogenic effectivity of a single dose of TPA applied after DMBA can be suppressed in a rather characteristic and non-toxic fashion by hydroxyurea (HU). In the present study we asked whether HU might also interfere with the converting effectivity of a single dose of TPA given prior to DMBA. NMRI mice received a single dose of TPA 3 weeks prior to initiation by DMBA which was followed by twice weekly application of skin irritant 12-O-retinoylphorbol-13-acetate (RPA) in order to effect stage II of promotion. A single dose of HU given i.p. at different times after TPA was found to interfere with tumor formation exhibiting an almost complete inhibition if administered 18 h after TPA. This inhibition did not prevent a subsequent promotion by repetitive TPA treatment. The data indicate that conversion can be inhibited by HU in the same characteristic fashion regardless of whether TPA was administered after or prior to initiation. The data also support the autonomous character of the conversion process for which epidermal DNA synthesis appears to be obligatory.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed1986

Variation and adaptation of Pseudomonas aeruginosa toxicity to HeLa cells and fibroblasts.

The toxic components of supernatants from Pseudomonas aeruginosa cultures directed against HeLa cells and Staphylococcus aureus were evaluated with the aim of discovering interactions. Supernatants of eight different strains of P. aeruginosa were assayed for cytotoxic activity. All were active against HeLa cells; seven were toxic for S. aureus. On repeated suspension of P. aeruginosa in 0.9% sodium chloride solution, a shift from HeLa cell toxicity to staphylococcal lytic activity occurred along with a change of toxic activity from a high (50,000 +/- 5,000) to a low (8,000 +/- 400) molecular weight (MW) range on gel filtration. Addition of protein to the minimal medium of cultures producing material toxic only for S. aureus reactivated the generation of HeLa cell-toxic material. Cultivation of P. aeruginosa in the presence of HeLa cells and a chloramphenicol supplement produced suppression of the generation of material toxic for S. aureus but facilitated that of HeLa-toxic material of high MW. Adaptation of toxicity against fibroblasts developed only on cocultivation of P. aeruginosa together with S. aureus and in the presence of fibroblasts. Under these conditions a strong lytic activity for S. aureus appeared, even in the presence of chloramphenicol. Chloramphenicol caused the material toxic for fibroblasts to elute at a low MW well separated from that toxic for HeLa cells. In contrast to the high-MW toxic substances, the low-MW material did not induce antibodies after injection into rabbits. This may explain failures of vaccination against P. aeruginosa infection and of serum therapy of homologous sepsis in humans.

Bacterial Toxins↗

PubMed1985

Biosynthesis and membrane topography of the neural cell adhesion molecule L1.

The biosynthesis and membrane topography of the neural cell adhesion molecule L1 have been studied in cerebellar cell cultures by metabolic labeling and immunoprecipitation. Pulse and pulse-chase experiments with [35S]methionine show that L1 is synthesized in its high mol. wt. form, the 200 kd component. The lower mol. wt. components with 40, 80 and 140 K apparent mol. wts. can be generated by proteolysis in intact cellular membranes. Peptide maps generated by protease treatment of L1 isolated from adult mouse brain show that the 80 and 140 kd components are related to the 200 kd component, but not to each other. The 200, 80 and 40 kd components can be biosynthetically phosphorylated. The 140 kd component is not phosphorylated and not released from the surface membrane during tryspinization. The phosphorylated amino acid is serine. In the presence of tunicamycin the 200 kd component is synthesized as a 150 kd protein. Pulse-chase experiments in the presence of tunicamycin indicate that the carbohydrate moieties are predominantly N-glycosidically linked and that the contribution of O-glycosylation is minimal. The carbohydrate moieties are of the complex type as shown by treatment with endoglycosidase H. Since monensin inhibits processing of the carbohydrate moieties, the 200 kd component appears to be transported to the surface membrane via the Golgi apparatus.

Amino Acids↗

PubMed1985

Protein kinase activities in mammalian blood fluid.

Two protein kinase activities, one specific for phosvitin and another specific for histone, were detected in serum and plasma of calf as well as of human blood after precipitation with ammonium sulfate (40%) and chromatography on DEAE-Sephacel. The enzymes were separated by chromatography on phosphocellulose. The histone kinase is not related to the cyclic AMP-dependent protein kinase; it may derive at least partly from damaged cells. The phosvitin kinase activity carries characteristics of the so called casein kinase type II similar to that present at the surface of cells including blood cells.

Adenosine Triphosphate↗

PubMed1985

Partial inversion of the initiation-promotion sequence of multistage tumorigenesis in the skin of NMRI mice.

Alterations in NMRI mouse skin induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate in "stage I of tumor promotion" are slowly reversible, and this reversibility has a half-time of 10 to 12 weeks. The tumor response observed in the course of an initiation-promotion experiment in vivo is independent of whether stage I of promotion occurs before or after initiation. Since the time interval between treatment with the promoter, and subsequent initiation can be extended up to at least 6 weeks, an enhancement of initiation because of promoter-induced cellular DNA synthesis seems to be unlikely. This result may be inconsistent with the two-stage model of tumor promotion because it indicates that in skin the existence of initiated cells is not required for the induction of cellular alterations that are essential for the stage of skin tumorigenesis called stage I of promotion.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed1985

Circular dichroic evidence for an ordered sequence of ligand/binding site interactions in the catalytic reaction of the cAMP-dependent protein kinase.

A limiting requirement for substrate specificity of the cAMP-dependent protein kinase is the presence of one or two basic residues located to the N-terminal side of the target substrate serine. Furthermore, circular dichroic (CD) studies have shown that binding of protein substrate involves a series of at least two independent conformational changes in the enzyme, each of which is initiated by a recognition signal on the substrate protein. The present study attempts to elucidate further the complete sequence of enzyme/ligand interactions by using the synthetic substrate peptide Kemptide and analogues differing from it at crucial points in the sequence: the Ala-peptide, where alanine is substituted for the target serine, and D-Ser-Kemptide, where the target serine is in the D rather than the L configuration. Examination of the effects of binding of these substrates on the intrinsic UV CD of the enzyme and the induced CD in the presence of Blue Dextran has revealed a third step in the substrate/enzyme binding interaction. Although sections of the conformational change at the active site are dependent on the basic subsite and the serine hydroxyl group on the peptide, respectively, the complete conformational change requires that the substrate be bound in random coil conformation. Where this does not occur, the kinetics show that the peptide will not act either as substrate or as inhibitor of the enzyme. Further, the interaction between the serine hydroxyl group and an enzyme tyrosine residue, previously observed, appears to be dependent on the correct orientation as well as the mere presence of the target -OH group.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗

PubMed1985

Mechanistic aspects of the delay in the G2 phase of the cell cycle caused by tumor promoter 12-O-tetradecanoylphorbol-13-acetate in HeLa cells.

In order to gain an insight into the nature of the radiomimetic activity by which the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA) alters cell cycle parameters in HeLa cells, possibilities of modifying the TPA-induced G2 block and recovery from it were studied. TPA-induced G2 blockage was analysed by counting mitotic figures. It was not influenced by hydroxyurea (10(-3) M) thus indicating that it is independent of DNA synthesis. TPA-induced decrease of mitotic activity occurred faster than that caused by cycloheximide (10(-5) M) indicating that the TPA-sensitive transition point in G2 is closer to mitosis than that for cycloheximide. Superoxide dismutase, catalase, alpha-tocopherol, the radioprotector S-(2-aminoethyl)isothiuroniumbromide.HBr (AET), caffeine and indomethacin and eicosatetraynoic acid (ETYA), both inhibitors of oxygenases in the arachidonic acid cascade, were not capable of reducing the TPA-induced G2 response. Under certain conditions small concentrations of AET (10(-8) M) and ETYA (10(-8) M) appeared to improve recovery slightly. Mannitol and sorbitol, however, both hydroxyl radical scavengers at 0.1 M concentration reduced TPA-effectiveness to a large degree (0.1 M D- and L-mannose were ineffective). Dimethylsulfoxide (0.1 M), another hydroxyl radical scavenger, was ineffective.

5,8,11,14-Eicosatetraynoic Acid↗

PubMed1984

Near- and far-ultraviolet circular dichroism of the catalytic subunit of adenosine cyclic 5'-monophosphate dependent protein kinase.

The circular dichroism spectrum of the catalytic subunit of cAMP-dependent protein kinase was measured in the far-UV (190-240 nm) and near-UV (250-300 nm) region. Data from the far-UV spectra were processed with the CONTIN program for estimation of globular protein secondary structure [ Provencher , S. W. (1982) CONTIN (Version 2) User's Manual, European Molecular Biology Laboratory, Heidelberg, West Germany]. The composition of the protein determined by this method was 49 +/- 2% alpha-helix, 20 +/- 4% beta-sheet, and 31 +/- 3% remainder. This composition changes when the protein is allowed to bind Kemptide , a synthetic peptide substrate, with more than half of the disordered portion of the protein taking the form of beta-sheet. A certain portion of the alpha-helical structure also appears to move into a beta-sheet form. The near-UV CD spectrum of catalytic subunit shows changes in aromatic amino acid dichroism associated with substrate binding. These changes can be ascribed with a fair degree of certainty to alterations in the orientation of a tyrosine residue at the surface of the protein. These findings are discussed in terms of previous work on induced dichroism in this enzyme with regard to control mechanisms operating at the active site.

Amino Acid Sequence↗

PubMed1984

Ligand binding site interaction in adenosine cyclic 3',5'-monophosphate dependent protein kinase catalytic subunit: circular dichroic evidence for intramolecular transmission of conformational change.

Blue Dextran has been shown to interact specifically with the nucleotide binding site of the catalytic subunit of cAMP-dependent protein kinase. By observing changes in the induced dichroism associated with the absorbance of the bound chromophore, one can monitor conformational changes in the immediate vicinity of the ATP binding site. With this technique, it has been possible to demonstrate that attachment of ligand at the protein substrate binding site of the enzyme results in a conformational change at the ATP binding site. This alteration takes place in at least two steps, one of which appears to be dependent on the presence of a phosphorylatable hydroxyl group on the substrate and the other being triggered by the "basic subsite" (usually one or more arginine residues) to the N-terminal side of the target serine or threonine. Competition experiments suggest that the change induced results in closure over the substrate protein after the initial electrostatic binding; the movement initiated by the presence of a serine hydroxyl group may also involve interaction with a tyrosine residue at the surface of the ATP binding site.

Adenosine Triphosphate↗

PubMed1984

Suppression of the first stage of phorbol 12-tetradecanoate 13-acetate-effected tumor promotion in mouse skin by nontoxic inhibition of DNA synthesis.

In order to evaluate the significance of epidermal cell proliferation for the first stage of skin tumor promotion, the effect of hydroxyurea (HU), an inhibitor of DNA synthesis, on tumor formation was studied. Mice initiated with 7,12-dimethylbenz[a]anthracene received a single dose of phorbol 12-myristate 13-acetate (PMA) in stage I of promotion, followed by twice weekly application of the irritant skin mitogen phorbol 12-retinoate 13-acetate in stage II. A single dose of HU given intraperitoneally at different times before or after treatment with PMA was found to interfere with tumor formation, exhibiting an almost complete inhibition if administered 18 hr after PMA--i.e., at the time of maximal DNA synthesis. The inhibition of tumor formation by HU in the two-stage promotion experiment did not prevent a subsequent promotion of cells by repetitive PMA treatment. This indicates that the inhibitory effect of HU was due neither to cytotoxicity (killing of initiated cells) nor to an interference with initiation. The data indicate that epidermal DNA synthesis is obligatory for PMA-induced first-stage promotion. The causal relationship between both events remains to be established.

9,10-Dimethyl-1,2-benzanthracene↗

PubMed1984

Interaction of phorbol derivatives with replicating cells.

The significance of the interaction of phorbol derivatives with replicating cells has been studied in HeLa cells and in tumour promotion experiments. Dose-response relationships for the radiomimetic activity of phorbol derivatives in HeLa cells (Kinzel et al., 1980) were obtained by analysis of the degree of transient blockage in G2 phase of the cell cycle. HeLa cells are shown to be one order of magnitude more sensitive to 12-O-tetradecanoylphorbol-13-acetate (TPA) than to equally mitogenic and irritant but much less promoting derivatives. The susceptibility of HeLa cells reflects the promoting capacity of these compounds. Studies on effects of modifiers on TPA-induced G2 blockage indicate that the influence of TPA, even in a single cell cycle phase, may be the result of a 'multiple site' attack. The significance of the interaction of TPA with replicating mouse epidermal cells in tumour promotion has been demonstrated in animal experiments after initiation with 7-12-dimethylbenz[a]anthracene by using a two-stage protocol which effects promotion with a single dose of TPA followed by repeated treatment with 12-O-retinoylphorbol-13-acetate (Fürstenberger et al., 1981). A single dose of the inhibitor of DNA synthesis, hydroxyurea, given to groups of mice at different times before and after treatment with TPA interferes with tumour formation; almost complete inhibition is observed after 18 h. The interaction of TPA and DNA-synthesizing cells seems to be of crucial importance to the first stage of tumour promotion.

9,10-Dimethyl-1,2-benzanthracene↗