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

Differential effects of 12-O-tetradecanoylphorbol 13-acetate on cytochrome P-450-dependent monooxygenase activities in rat hepatoma cells: induction of P-450I and suppression of P-450II

We have studied the effects of the tumor promoter 12-O-tetradecanoylphorbol 13-acetate (TPA) on cytochrome P-450-dependent monooxygenase activities in several differentiated and dedifferentiated Reuber rat hepatoma cell lines using aryl hydrocarbon (benzo[a]pyrene) hydroxylase (AHH), ethoxyresorufin O-deethylase (EROD), and aldrin epoxidase (AE) as test systems. The following results were obtained: (1) Exposure of cultures to 400 nM TPA for 18-24 h increased AHH activities in the differentiated lines 2sFou, H41IEC3/G- and Fao as well as in the dedifferentiated line 5L, 1.5-2.5-fold. The phorbol ester did not affect AHH activity in the dedifferentiated line H5. (2) EROD, a marker for P-450I, was induced by the phorbol ester to a similar degree as AHH. (3) A monoclonal antibody directed against P-450I strongly inhibited the AHH activity induced by TPA. (4) The onset of AHH or EROD induction by TPA was much later than that elicited by benz[a]anthracene. (6) In contrast to the induction of AHH and EROD, TPA decreased AE activity, a marker for P-450II, by about 50% in all the cell lines containing this monooxygenase activity. (7) The half-maximum-effect concentration of TPA for inducing or suppressing AHH and AE, respectively, was approximately 20 nM. (8) TPA did not interfere with AHH induction by benz[a]anthracene. However, the phorbol ester moderately decreased AHH induction and markedly suppressed AE induction by dexamethasone. The results indicate that TPA simultaneously induces P-450I and suppresses P-450II forms in rat hepatoma cells. P-450I induction by TPA in these cells did not appear to depend on their status of differentiation. Furthermore, the results suggest that the mechanism of P-450I induction by TPA differs from that elicited by polycyclic aromatic hydrocarbons or glucocorticoids.

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.