On the biochemical mechanism of tumorigenesis in mouse skin

Tumor promotion and depletion of protein kinase C in epidermal JB6 cells

Promotion of JB6 epidermal cells to anchorage-independent growth requires exposure to TPA for greater than 4 days. Over a similar time span, a practically complete loss of enzymic and immunoreactive proteinkinase C (PKC) equivalents was observed at greater than 10 nM TPA. Promotion did not appear to require (transient) activation of PKC since PKC inhibitors H7 and HA1004 did not prevent but enhanced colony formation in soft agar at concentrations greater than IC50-values. The efficacy of the inhibitors in vivo was shown by their ability to suppress PKC-induced transcription of c-fos gen. PKC inhibitors that interfered with cell proliferation at lower concentrations than those required for PKC inhibition (sphingosine, staurosporin, sangivamycin, trifluoperazine) did not stimulate anchorage-independent growth. As H7 as well as HA1004 were able to promote JB6 cells in the complete absence of TPA, and induced neither depletion nor processing of PKC we postulate that depletion/inactivation rather than activation of PKC correlates with the promotion of epidermal JB6 cells to anchorage-independent growth.

Antioxidants and multistage carcinogenesis in mouse skin

The two-step initiation-promotion protocol for the induction of skin tumors in mice is a convenient model to elucidate what molecular events are involved in the multistage process of carcinogenesis and how they can be modulated. The current theories concerning the mechanisms of skin tumor initiation, stages 1 and 2 of tumor promotion, and tumor progression are reviewed. Because chemical carcinogens and tumor promoters may, directly or indirectly, generate reactive oxygen species (ROS) and because various antioxidants inhibit effectively some of the biochemical and biological events linked to tumor initiation, promotion and/or progression, it is conceivable that different sequences and levels of free radical-induced macromolecule damage may contribute to the evolution of the epidermal target cells from the preneoplastic stage to the malignant stage.

Posttranslational modifications of the cytochrome P-450 monooxygenase system

Two forms of enzymatic posttranslational modifications of the monooxygenase system are described: modification by phosphatase and modification by protein kinase. Phosphatase treatment of microsomes isolated from phenobarbital-pretreated rabbits and rats caused a marked decrease of monooxygenase activity which was paralleled by a comparable decrease of NADPH-cytochrome P-450 reductase activity while the second essential component of the system, cytochrome P-450, remained unaltered. Thus phosphatase attacks monooxygenase via reductase. Protein kinases showed the opposite preference; while cytochrome P-450 was phosphorylated, NADPH-cytochrome P-450 reductase was not. Thus the kinase affects monooxygenase via cytochrome P-450. The phosphorylation of cytochrome P-450 turned out to be a specific reaction observed only with certain cytochrome P-450 isoenzymes and certain protein kinases.

Three stage carcinogenesis in mouse skin--recent results and present status of an advanced model system of chemical carcinogenesis

In the three stage model of carcinogenesis in mouse skin, to a certain compartment of normal cells new and adverse biological properties are imposed by the initiator DMBA yielding 'potential tumor cells,' or 'initiated cells.' These initiated cells exhibit a selective advantage of growth over surrounding normal cells if exposed to DTE promoters. The new properties may result from a genomic mutation associated with an increase of density, affinity or cooperativity of certain membrane receptors as compared to normal cells. Complete promoters, such as TPA, but not incomplete promoters such as RPA may impose a second additional genomic or epigenomic insult onto initiated cells ('conversion'). As a consequence, initiated cells become ready to respond to unspecific mitogenic stimuli ('propagation'), as provided by hyperplasiogenic agents. The initiation/promotion model of mouse skin presents new and attractive possibilities to apply the oncogene approach for further in-depth analysis of the molecular mechanisms of its stages. It remains to be seen to what extent molecular events demonstrated causative for the stages in the mouse skin model are applicable also to stages in multistage models of chemical carcinogenesis operational in other target tissues.

Carcinogen metabolizing enzymes in nude mice

Hepatic and cutaneous microsomal hydroxylating and conjugating enzymes from female and male NMRI nu/nu mice were analyzed, and the response of these enzymes to repeated exposures with 3-methylcholanthrene were studied. Sex differences were observed in basal activities of hydroxylating enzymes. These differences were not the same in the liver as in the skin and were reversed in female and male. Activity ratios of hepatic and cutaneous hydroxylating enzymes were between 40-200. The ratios between hydroxylating and conjugating enzymes were much lower in the skin than in liver. Furthermore the ratios were depending on hydroxylating enzymes. 3-Methylcholanthrene treatment increased both hydroxylating and conjugating enzymes in the liver and in the skin. Again, there were sex differences in the induction pattern, and also the induction in the liver was unrelated to that in the skin. There was no correlation in induction between hydroxylating and conjugating enzymes.

Effects of inducers on the regio- and stereoselective metabolism of benzo[a]pyrene by mouse tissue microsomes

The metabolism of benzo[a]pyrene (BP) by microsomal fractions of the skin, lungs and liver of the mouse, and the effects on this process of pretreatment with the xenobiotics phenobarbital (PB) and 3-methylcholanthrene (3-MC) were examined. Differences between the untreated tissues were found both in terms of the total amounts of diol recovered and in the relative proportions of the individual diols extracted following incubation. Induction with PB or 3-MC significantly altered the profiles of metabolic diols obtained with epidermal and hepatic microsomes compared with their respective controls. Pulmonary microsomes showed similar trends to those obtained with liver microsomes but these were not statistically significant. The optical purity of the BP-7,8-diol that was formed by each microsomal type was examined by direct resolution of the enantiomers on HPLC using a chiral stationary phase. In each case the (-)-7R,8R-enantiomer predominated. Pretreatment with 3-MC significantly decreased the optical purity of BP-7,8-diol recovered from incubations with skin microsomes, but significantly increased the optical purity of the diol extracted from incubations with lung and liver microsomes. In addition to the diols, an unidentified BP metabolite was found that eluted between BP-9,10- and 4,5-diol on a reverse-phase high-performance liquid chromatography (HPLC) system and which represented a major product in extracts of incubations of BP with both induced and uninduced skin and lung microsomal fractions.

Monooxygenase activity of human epithelial cells (HeLa): comparison of the response to treatment by arylhydrocarbons and diterpene esters

Arylhydrocarbons are inducers of monooxygenase activity in epithelial cells in culture (Hela) whereas diterpene esters are not. This is one more example of a specific biochemical difference - here in gene activation - exhibited by tumor initiators and tumor promoters of the classes of compounds assayed.

Two-stage skin carcinogenesis by systemic initiation of pregnant mice with 7,12-dimethylbenz(a)anthracene during gestation days 6-20 and postnatal promotion of the F 1-generation with the phorbol ester 12-tetradecanoylphorbol-13-acetate

The DMBA-TPA-mediated two-stage skin carcinogenesis experiment was modified in that pregnant mice were systemically treated once during pregnancy with the carcinogen. Intragastric application times were fetal days 6, 8, and 10-20. A control group of pregnant mice received repeated doses (from days 8-20) of sesame oil, which was used as a solvent for DMBA. At the age of 12 weeks, the offspring of the control group were divided into two groups, one of which was left completely untreated, the other received TPA applications over 26 weeks. The 12-week old F 1-progeny of each transplacentally initiated group was also divided into subgroups, which either received no further treatment (subgroups A) or were promoted with TPA (subgroups B). Neither the F 1-animals of the two control groups nor that of the transplacentally initiated but postnatally not promoted subgroups 6 A-20 A developed skin tumors. The same holds true for the TPA-promoted offsprings of mother animals which had received DMBA at days 6 and 8 of gestation. Skin tumor development after TPA promotion was first observed in animals of subgroup 10 B. Thereafter, tumor rates and tumor yields increased and latency periods decreased progressively in the B-subgroups with the postponement of initiation to later fetal periods. Day 19 of prenatal development proved to be the most sensitive period to transplacental initiation, whereas initiation at day 20 led to a significant decrease in tumor rate and yield. The capability to initiate skin tumors and the extent of initiation can be correlated to both the organogenesis of the epidermis and its proliferative rate in utero.

Mutagenicity of benzo[a]pyrene 7,8-dihydrodiol and 7,12-dimethylbenz[a]anthracene 3,4-dihydrodiol in S. typhimurium mediated by microsomes from rat liver and mouse skin

The mutagenic activities of trans-7,8-dihydro-7,8-dihydroxybenzo[a]-pyrene (BP 7,8-diol) and of trans-3,4-dihydroxy-7,12-dimethylbenz[a]-anthracene (DMBA 3,4-diol) towards S. typhimurium TA100 were measured in assays that were carried out on a micro-scale in liquid medium in the presence of microsomal fractions prepared from mouse skin or rat liver. In the presence of an NADPH-generating system, microsomal enzymes converted both diols into mutagens that were probably the respective 'bay-region' diol-epoxides. The rate of the enzyme-catalysed conversion of the BP 7,8-diol into mutagens by microsomal preparations from mouse epidermis was similar to that occurring with microsomes from rat liver. Pretreatment of mice by the topical application of benz[a]anthracene (BA) or 7,12-dimethylbenz[a]-anthracene (DMBA) increased the mutagenic activity of BP 7,8-diol mediated by mouse skin microsomal preparations by 2-fold and this was paralleled by a 4-fold increase in epidermal aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH) activity. The results are discussed in relation to the high susceptibility of mouse skin to polycyclic aromatic hydrocarbon (PAH) carcinogenesis.

Stimulation of mammalian epoxide hydrase activity by flavones

1. A series of flavones have been examined for the ability to stimulate the activity in vitro of rat liver microsomal epoxide hydrase. Flavone and 7,8-benzoflavone both stimulate epoxide hydrase activity in vitro with styrene oxide as the substrate. The stimulation by flavone itself was several times that of 7,8-benzoflavone. 2. When tested in vitro under the same conditions, the stimulation by 7,8-benzoflavone was significantly greater than the stimulation by metyrapone, a recognized stimulator of the activity of epoxide hydrase in vitro. The stimulation by 6,7-benzoflavone was barely detectable; the 5,6-benzoflavone isomer did not stimulate epoxide hydrase activity. 3. Exposure of rate to either 7,8- or 5,6-benzoflavone for 4 days before killing did not increase the epoxide hydrase specific activity in the subsequently isolated microsomes. Exposure to flavone, however, did induce epoxide hydrase activity. 4. The possible relevance of these observations to the effects of benzoflavone upon arylhydrocarbon-induced tumours is discussed.

Tumour initiation in mouse skin by 7,12-dimethylbenz[a]anthracene: irrelevance of systemic activation

The effect was investigated of 7,8-benzoflavone (BF) on tumour initiation by 7,12-dimethylbenz[a]anthracene (DMBA) applied either intragastrically or topically. A single topical application of BF drastically decreased tumour initiation via both routes. This decrease was in both cases inversely proportional to the interval between administration of DMBA and BF. The data suggest that the decisive step of metabolic activation of DMBA is mediated by cytochrome P-448 dependent monooxygenase(s) located within the epidermis.