TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) causes an increase in protein tyrosine kinase activities at an early stage of poisoning in vivo in rat hepatocyte membranes

Growth factors, cytokines and their receptors as downstream targets of arylhydrocarbon receptor (AhR) signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant, which causes a variety of severe health effects, e.g. immunosuppression, hepatotoxicity, and carcinogenesis. The main mediator of TCDD toxicity is the arylhydrocarbon receptor (AhR), which, upon activation, translocates into the nucleus and enforces gene expression. Since most of the pleiotropic effects caused by TCDD are associated with alterations in cell growth and differentiation, the analysis of the interference of the AhR with factors controlling these cellular functions seems to be a promising target regarding the prevention and treatment of chemical-provoked diseases. Cell growth and differentiation are regulated by numerous growth factors and cytokines. These multifunctional peptides promote or inhibit cell growth and regulate differentiation and other cellular processes, depending on cell-type and developmental stage. They are involved in the regulation of a broad range of physiological processes, including immune response, hematopoiesis, neurogenesis, and tissue remodeling. The complex network of growth factors and cytokines is accurately regulated and disturbances of this system are associated with adverse health effects. The molecular mechanisms by which the AhR interferes with this signaling network are multifaceted and the physiological consequences of this cross-talk are quite enigmatic. The investigation of this complex interaction is an exciting task, especially with respect to the recently described non-genomic and/or ligand-independent activities of AhR. Therefore, we summarize the current knowledge about the interaction of the AhR with three cytokine-/growth factor-related signal transducers -- the epidermal growth factor (EGF) family, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta) -- with regard to pathophysiological findings.

Effect of in vitro administered 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on DNA-binding activities of nuclear transcription factors in NIH-3T3 mouse fibroblasts

TCDD is a very toxic environmental contaminant which is known to cause a variety of toxic symptoms in many species. Because of a myriad of biochemical changes TCDD is known to induce in many test animals, it has been difficult to pinpoint the causative event common to all those symptoms in different species. One of the research avenues we have been following is identification of the pattern of TCDD-induced changes in DNA binding characteristics of nuclear transcription factors (NTFs), each of which has the property to trigger a set of coordinated changes in many gene expressions. Since in our previous work we studied animals affected by TCDD in vivo using gel mobility shift assay approached with32P labeled oligonucleotide probes, we examined in the current study the possibility whether we could establish an equivalent in vitro system in NIH-3T3 mouse fibroblast cells so as to be able to learn the similarities and the dissimilarities of TCDD-induced responses of NTFs between in vitro and in vivo. The results indicated that, for a large part, this in vitro test system could reasonably reproduce the pattern of changes occurring in vivo at the early stages of TCDD's action in terms of induced changes in binding of thes NTSs to DNA. The key features were TCDD induced upregulation of NTFs binding to the response elements for AP-1, dioxin (DRE) and T3 (thyroid hormone) and down-regulation of those to response elements (REs) for c-Myc, Sp-1 and retinoic acid receptor (RARE). However, the time course required the changes in DNA binding activity was much shorter in vitro. To study the basic cause for such changes in NTF binding, we studied the effects of exogenously added EGF, forskolin, TPA (12-0-tetradecanoylphorbol acetate) and TNFalpha on the expression of TCDD's action on some of these NTFs. The results showed that these agents indeed greatly influence the outcome. The most influential agents were TNFalpha, forskolin and EGF. These results indicate that this in vitro cell model is useful in simulating the action of TCDD with respect to its basic action on NTFs.

Activation of mitogen-activated protein kinases (MAPKs) by aromatic hydrocarbons: role in the regulation of aryl hydrocarbon receptor (AHR) function

The aromatic hydrocarbon (Ah) receptor (AHR) is the only known cellular receptor of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and of many other widespread environmental contaminants that cause diverse toxic effects in animals and humans. Most, if not all, the biological effects of TCDD are mediated by the activation of AHR, which is a ligand-activated transcription factor required for ligand-induced expression of several detoxification genes, including those encoding for cytochrome P450 enzymes CYP1A1, CYP1A2, and CYP1B1. Environmental agents also activate several mitogen-activated protein kinase (MAPK) pathways, believed to modulate transcription factor function and to regulate gene expression. However, the contribution to TCDD toxicity resulting from cross-talk between AHR and MAPK pathways has yet to be determined. In this study, we show that TCDD and other AHR ligands induced the immediate activation of the extracellular signal-regulated kinases and the Jun N-terminal kinases, but not the p38 MAPKs. MAPK activation by TCDD did not require the AHR, since it occurred equally well in AHR-negative CV-1 cells and in Ahr (-/-) mouse embryonic fibroblasts as in AHR-positive cells. Distinct from serum factors and the tumor promoter TPA-induced MAPKs, which resulted in transcriptional activation of ELK or c-JUN, TCDD-stimulated MAPKs were critical for the induction of AHR-dependent gene transcription and CYP1A1 expression. These data indicate that AHR ligands elicit AHR-independent non-genomic events that are essential for AHR activation and function.

Increased MAPK expression and activity in primary human hepatocellular carcinoma

We investigated the expression and activity of mitogen-activated protein kinase (MAPK) in human hepatocellular carcinoma (HCC). MAPK expression was determined in five human tumors and five normal tissues (adjacent non-neoplastic liver) by Western blotting using specific antisera raised against four MAPK pathway intermediates: Erk-1, Erk-2 (extracellular-signal regulated kinases), Mek-1 and Mek-2 (mitogen activated protein kinase kinases). There was a significant increase in Erk-1, Erk-2, Mek-1 and Mek-2 expression in particulate and cytosolic fractions prepared from tumor specimens as compared with the adjacent normal control tissues. The functional activity of both membrane and cytosolic Erk-2, determined by phosphorylation of myelin basic protein (MBP), was significantly increased in tumor specimens as compared to normal (membrane: 321%+/-50%, p<0.05; and cytosol: 597%+/-233%, p<0.05 percent of normal tissue). These data demonstrate for the first time a significant increase in MAPK expression and functional activity in human HCC. Because of the important role that the MAPK pathway plays in cellular growth and differentiation, overexpression of MAPK may be of critical importance to the formation and maintenance of human hepatocellular carcinoma.

Growth-related signaling in vascular smooth muscle cells is deregulated by TCDD during the G0/G1 transition

Experiments have been conducted to examine the impact of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on growth-related signaling in vascular smooth muscle cells (SMCs). A 40% reduction of peak DNA synthesis was observed in SMCs only when TCDD was added during the G0/G1 transition of the cell cycle. Enhanced phosphorylation of several endogenous proteins during this period was coincident with increased tyrosine kinase activity as early as 15 min following TCDD challenge. No changes in protein phosphorylation status occurred in cells treated with TCDD during the G1/S transition or during S phase. Cotreatment of quiescent SMCs with 10 nM TCDD and serum for 3 h reduced serum-inducible binding activity to a 12-O-tetradecanoyl phorbol 13-acetate responsive element (TRE) by approximately 40%. No alterations of constitutive TRE binding were observed in quiescent SMCs treated with TCDD for up to 5 h. These data show that mitogen-related signaling in vascular SMCs is modulated by TCDD selectively during the G0/G1 transition, and these effects influence the growth behavior of these cells.

Differences in binding of epidermal growth factor to liver membranes of TCDD-resistant and TCDD-sensitive rats after a single dose of TCDD

Epidermal growth factor (EGF) receptor has been implied as having a role in certain actions of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). After a single dose of TCDD, the receptor has been shown to be downregulated in several tissues including the liver. Two rat substrains, the Han/Wistar (Kuopio; H/W) rat and the Long-Evans (Turku AB; L-E) rat exhibit over a 1000-fold difference in their sensitivity to the lethal effect of TCDD. This large sensitivity difference was utilized in the current study to investigate whether or not a correlation exists between TCDD lethality and biochemical endpoints related to the hepatic EGF receptor. In the TCDD-sensitive L-E strain both the B(max) of the EGF receptor and the receptor protein as measured by Western blots, decreased dose and time dependently. Ten days after a lethal dose of TCDD (50 μg/kg), the downregulation was 80%. In the resistant H/W strain, two non-lethal doses were used (50 and 500 μg/kg), since the lethal dose is not known. These doses caused a downregulation already at 4 days after dosing, but no further decrease by day 10. The activity of phosphoenolpyruvate carboxykinase (PEPCK, the main gluconeogenetic enzyme in the liver and a proposed target of TCDD) decreased in H/W rats at least to the same extent as in L-E rats at both 4 and 10 days. It is concluded that EGF receptor downregulation is different in the two rat strains studied, despite the fact that a classical Ah receptor-regulated response (CYP1A1 induction) is similar. The results demonstrate that downregulation of the EGF receptor by TCDD is strain-dependent as well as dose- and time-dependent.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant in several animal species. The purpose of this study was to determine if TCDD affected the activity of adenosine deaminase (ADA), a purine metabolizing enzyme that is vital to the proper functioning of the immune system. The effect of TCDD on ADA activity was studied in various tissues of male Balb/c mice (a TCDD-responsive strain) and DBA/2 mice (a less-responsive strain). Of the tissues examined after administration of TCDD in vivo (115 micrograms/kg, i.p.), ADA activity was found to be significantly reduced in thymic and splenic tissues of Balb/c mice at 24 hours postadministration. The enzyme activity in these affected tissues remained consistently low through 10 days postadministration. Such an effect of TCDD was both dose and time related in the thymic tissue of Balb/c mice. In contrast, no appreciable alterations in ADA activity were evident in any of the tissues of DBA/2 mice at any of the sampling intervals, indicating that such an effect of TCDD is likely to be mediated through the Ah receptor. This in vivo effect of TCDD on thymic ADA activity was also reproducible in situ where isolated whole thymuses were directly incubated with 10 nM TCDD. In this model, TCDD's effects on ADA activity were antagonized by known protein kinase or phosphorylation inhibitors such as quercetin, genistein, tyrphostin, and neomycin. These results indicate that the effect of TCDD on ADA activity in the thymus may be related to its property to elevate protein kinase activities in this tissue. ADA activity was also reduced in 3T3 cells that were treated with 10 nM TCDD in a low (1%) serum media. In contrast, 25 ng/mL epidermal growth factor (EGF) under such conditions consistently stimulated ADA activity. Interestingly, EGF at a similar concentration failed to elicit a stimulatory effect on ADA activity when cells were pretreated with TCDD. The property of TCDD to lower ADA activity under in vivo, in situ, as well as in vitro conditions appears to be largely related to its action to modulate protein phosphorylation activities.

Significance of TCDD-induced changes in protein phosphorylation in the adipocyte of male guinea pigs

The chemical TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) at very low concentrations has been found to cause a rapid rise in protein phosphorylation activities in the extranuclear fraction (i.e., cytosol and cellular membranes) of the adipose tissue from male guinea pigs. The effect occurred both in vivo and when TCDD was added to adipose in tissue culture (in situ). To study the cause for such a TCDD-induced biochemical change, isolated adipose tissues were treated with TCDD in conjunction with various diagnostic agents known to affect protein synthesis (actinomycin D and cycloheximide) and protein phosphorylation activities (genistein). The stimulatory effect of TCDD on protein phosphorylation was not totally abolished by actinomycin D; however, cycloheximide and genistein partially suppressed the effect of TCDD. Such an action of TCDD was accompanied by a quick rise in ras GTP binding activity as well as a rise in phosphorylation of nuclear c-myc protein product within 10 minutes after TCDD addition, indicating that TCDD is likely to activate the signal transduction pathway for growth factors. In view of the absence of an inhibitory action of actinomycin D and the short time required for TCDD to induce these changes, we have formulated a working hypothesis that stimulation of protein phosphorylation activities by TCDD may not be mediated by a transcriptional process. To test this possibility, an extranuclear fraction was obtained from the homogenate of adipose tissue, and the effect of TCDD was studied in vitro cell- and nucleus-free conditions. It was found that TCDD was capable of activating protein phosphorylation activity under cell free conditions even in the absence of nuclei. Such an action of TCDD is dependent on the action of the Ah receptor.

Biphasic modulation of protein kinase C (PKC) activity by polychlorinated dibenzo-p-dioxins (PCDDs) in serum-deprived rat aortic smooth muscle cells

Previous studies in this laboratory have shown that benzo(a)pyrene (BaP) modulates protein kinase C (PKC)-mediated phosphorylation of aortic smooth muscle cell (SMC) proteins. This observation is consistent with the ability of other aromatic hydrocarbons (AHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), to modulate kinase activities in cells of hepatic, testicular, and thymic origin. Because all these chemicals share the ability to bind the aryl hydrocarbon receptor (AhR), the present studies were conducted to determine if changes in PKC activity by AHs conform with established structure-activity relationships. Experiments were conducted to examine the effects of TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF), and 2,8-dichlorodibenzodioxin (DCDD) on the phosphorylation of exogenous histone type-III under basal and PKC-activating conditions. These congeners exhibit both high (TCDD and TCDF) and low (DCDD) AhR agonist activities. Measurements of kinase activity were conducted in the cytosolic and particulate fractions of growth-arrested (i.e., serum-deprived) cultured rat aortic SMCs incubated with 10 nM TCDD, TCDF, and DCDD for 0.5, 12, or 24 hours. No changes in basal kinase activity were induced by these chemicals at any of the times tested. Significant decreases in cytosolic and particulate PKC activity relative to controls were observed upon exposure of SMCs for 0.5 hours to 10 nM TCDD, TCDF, and DCDD. In contrast, SMCs exposed to TCDD and TCDF for 12 hours exhibited a significant increase in PKC activity in both cytosolic and particulate fractions. The PKC activity in cells exposed to DCDD for 12 hours was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

2,3,7,8-Tetrachlorodibenzo-P-dioxin induced alterations in protein phosphorylation in guinea pig adipose tissue

An in situ (explant tissue culture) model has been developed to study the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), hormones, and growth factors either alone or in combination. In our model system, the effect of TCDD on protein phosphorylation was greatly affected by the presence or the absence of externally added D-glucose. In the presence of a physiologically relevant level of glucose (13.3 mM), TCDD clearly stimulated protein phosphorylation as in the case of in vivo data. However, in the absence of D-glucose, TCDD clearly inhibited protein phosphorylation. On the other hand, TCDD reduced the glucose uptake activity in isolated adipose tissue either in the presence or absence of D-glucose (13.3 mM). Therefore, the TCDD-induced reduction of glucose transport does not appear to be related directly to the simultaneous rise in protein phosphorylation. For comparison, several agents which are known to affect protein phosphorylation were tested. These hormonal agents generally affected the TCDD-untreated adipose tissues in the manner expected from their known actions, indicating that this in situ model is an adequate system to study their independent actions. The TCDD-treated adipose tissue samples showed only mild or insignificant response to these hormonal stimuli. In terms of the changes in the pattern of protein phosphorylation activities, the action of TCDD appeared to resemble that of EGF and T3. Since under in situ conditions no agents such as EGF or T3 can be expected to be present, the observed TCDD-induced changes are suggestive of the basic intracellular changes in cellular activities. The types of TCDD-induced protein kinases appear to be protein tyrosine kinases and protein kinase C.

Protein tyrosine phosphorylation as an indicator of 2,3,7,8-tetrachloro-p-dioxin exposure in vivo and in vitro

A dose-dependent increase in tyrosine phosphorylation of five hepatic intracellular proteins with approximate molecular weights of 17, 21, 27, 29, and 34 kDa was seen 24 h after administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to C57BL/6J female mice. The ED50 values for tyrosylphosphorylation of these five proteins, respectively, were 0.26, 0.21, 0.26, 0.31, and 0.38 micrograms TCDD/Kg. TCDD induction of 7-ethoxyresorufin O-deethylase activity (EROD) was characterized by an ED50 of 2.5 micrograms/Kg. An eighteen h exposure of a human lymphoblastoma cell line (X3) to TCDD increased tyrosylphosphorylation status of ten proteins with approximate molecular weights of 16, 17, 24, 26, 27, 32, 33, 34, 35, and 36 kDa in a dose-dependent manner. The EC50 values for these TCDD-dependent tyrosylphosphorylation ranged from 0.01 to 0.07 nM TCDD. EROD induction by TCDD in X3 cells exhibited an EC50 of 0.14 nM. These data indicate that TCDD alters intracellular protein tyrosine phosphorylation and these changes are more sensitive biological indicators of TCDD exposure than induction of EROD.

TCDD causes stimulation of c-ras expression in the hepatic plasma membranes in vivo and in vitro

A series of in vivo and in vitro experiments were conducted to determine the effects of 2, 3, 7, 8-tetrachlorodibenzo-p-dioxin (TCDD) administered on the expression of c-ras. Differences in c-ras expression between control and TCDD treated groups were determined by immunoassay of p21ras protein, or indirectly measured by the specific binding of 3H-GTP to hepatic plasma membrane preparations. Intraperitoneal injection of sublethal doses of TCDD significantly elevated (P less than 0.05, Student t test) levels of hepatic p21ras protein in Sprague-Dawley rats and TCDD sensitive C57BL/6J mice. Such an increase occurred at an early stage of poisoning in the C57BL/6J mice. The earliest increase was detectable 6 hr after dosing, and the difference became statistically significant by 12 and 24 hr after dosing. In contrast, TCDD tolerant DBA/2J mice had only a marginal increase in hepatic p21ras protein which did not become statistically significant even at 24 hr host-dosing. TCDD evoked increases in hepatic p21ras protein of C57BL/6J mice were accompanied by the increase in the specific binding of GTP to hepatic plasma membranes. Column chromatography of solubilized rat hepatic membrane proteins on sephadex G-50 showed TCDD administration increased levels of a 3H-GTP binding protein with MW of approximately 21 Kd. 3H-GTP binding in total hepatic membranes was also elevated (P less than 0.05, Fisher PLSD multiple comparison test) 6 hr and 24 hr after dosing of C57BL/6J mice, but as expected the effect of TCDD was not as conspicuous as that found in the plasma membrane. TCDD treatment increased levels of a 21 Kd protein found in the in vitro translation products of RNA purified from guinea pig liver. This protein was identified as a c-ras protein based upon its ability to bind GTP, precipitation by a polyclonal antibody against the rasHa and Ki proteins and subsequent SDS-PAGE which showed a single protein band of approximately 21 Kd.

A two-channel hypothesis for regulation of cell division and differentiation

We propose the hypothesis that extracellular regulation of cell division and differentiation acts through just two communications channels. These channels consist of a series of redundant components: extracellular messenger hormones; these hormones' receptors; cytoplasmic proteins activated by the hormone-receptor complex; and trans-activating nuclear regulatory proteins. One channel, here labeled "D" ("differentiate"), includes transforming growth factor-beta as one of its hormones; the other, labeled "G" ("growth") includes epidermal growth factor. We postulate that signal reception occurs as a result of competition between different actuating proteins for equilibrium-controlled binding to critical DNA sites. Stem cells commit to mitosis when some high proportion of critical sites is occupied by actuating proteins of the G class, and to terminal differentiation when a high proportion is occupied by "D" actuators. Intermediate occupancy can either lead to division into one differentiated and one stem cell, or to maintenance of cells in the reference state, quiescence. Equilibrium control of binding implies that critical site occupancy will be proportional to the relative concentrations of "D" and "G" actuating proteins in the nuclear fluid. These concentrations depend on the external hormone concentrations, the numbers of receptors on the cell membrane, and the coefficients of the rate-determining steps between internalization of the hormone-receptor complexes and activation of the actuating proteins. All of these quantities can be affected by various factors, including endocrine hormones. This model is consistent with most reported behavior of various growth factors, interferons, etc, toward a variety of cells in culture. It predicts that under otherwise constant conditions, high relative concentrations of a D-hormone (e.g. transforming growth factor-beta) will induce commitment to terminal differentiation, while high relative concentrations of a "G" hormone (e.g. epidermal growth factor) will induce mitosis. We have seen no report of an experiment which adequately tests this prediction. The model implies that cancer causing mutations are those which increase the relative intensity of the "G" signal; this can occur via changes in components of either channel. Such mutant cells should be both more likely to divide and less likely to differentiate than normal stem cells. Conversely, mutations which increase relative sensitivity to the "D" signal during embryonal development can lead to premature differentiation, cessation of growth, and structural abnormalities (terata).

Effects of TCDD on the EGF receptor of XB mouse keratinizing epithelial cells

TCDD was found to cause a marked inhibition of 125I-epidermal growth factor (EGF) binding to its receptor on the cell surface of XB mouse keratinizing epithelial cells (XB cells) cultured in vitro. The EC50 concentration was estimated to be on the order of 3 x 10(-11) M 24 hours after TCDD administration. As early as 12 hours after the addition of 10(-9) M of TCDD, XB cells showed signs of a decline in 125I-EGF binding levels. The level of such EGF receptor downregulation reached a maximum at 24 hours, continued until day 2, but completely recovered by day 3. This was accompanied by a rise in protein kinase activities, particularly those of the protein tyrosine kinases during the initial period of 6-24 hours. To test the hypothesis that the EGF receptors of the cells, by showing TCDD-induced symptoms of downregulation, actually are being activated and triggering EGF-like signals, we examined the effects of both TCDD and exogenously added EGF on cell morphology, colony formation degree of keratinization, the pattern of activation of protein kinases and de novo protein synthesis, and EGF receptor phosphorylation. Based on the similarity of cell responses to these between TCDD- and EGF-treated cells, we concluded that TCDD, directly or indirectly, causes activation of the EGF receptor. In contrast, 12-O-tetradencanoylphorbol-13-acetate (TPA), which is known to downregulate EGF receptors by blocking their protein tyrosine kinase, produced dissimilar end results. The balance of evidence support the notion that the action of TCDD in this cell line is tightly coupled to the activation of the EGF receptor and that one of the key consequences of such a biochemical change is that it signals these cells to commit to terminal differentiation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes increases in expression of c-erb-A and levels of protein-tyrosine kinases in selected tissues of responsive mouse strains

2,3,7,8,-Tetrachlorodibenzo-p-dioxin (TCDD) administered in vivo causes drastic reduction in the weight of the mouse thymus at low doses (e.g., 30 micrograms/kg single i.p. injection), the reduction becoming statistically significant after 2 days. To understand the cause for such thymic involution TCDD-evoked changes in various biochemical parameters in this tissue were examined. The most noticeable change was observed in the increased activity of specific protein-tyrosine kinases and protein kinase C and an increased level of p21ras-associated binding of [3H]GTP. Since no significant change was observed with cAMP-stimulated protein kinases and cAMP levels, the above changes appear to be a selective effect on these special classes of proteins. As a result of a time sequence study it has become apparent that the rise in protein-tyrosine kinase activities becomes significant within 24 hr, whereas the rise in protein kinase C does not become significant until 48 hr. Among protein-tyrosine kinases, pp60c-src and probably pp561skT were found to be significantly elevated by TCDD treatment. In view of similarities between TCDD and thyroid hormones in causing thymic involution, the levels of c-erb-A expression were assessed in the liver by using avian 32P-labeled v-erb-A probe and RNA transfer blot hybridization technique. The results clearly indicate that TCDD has the property to elevate levels of mRNA bearing homology to v-erb-A. Such changes in c-erb-A expression and protein-tyrosine kinase occurred only in TCDD-susceptible (responsive) strains but not in tolerant (nonresponsive) strains of mice at the dose tested. Based on such observations a hypothesis has been proposed that TCDD owes its potency to its ability to stimulate the expression of one of a family of DNAs bearing homology to v-erb-A and that one of the major consequences of such an action is stimulation of various tyrosine kinases.

2,3,7,8-Tetrachlorodibenzo-p-dioxin causes an increase in protein kinases associated with epidermal growth factor receptor in the hepatic plasma membrane

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), administered to male rats at a single intraperitoneal (IP) injection dose of 25 micrograms/kg causes down-regulation of epidermal growth factor (EGF) receptor in the plasma membrane of rat liver which starts after two days and continues throughout the experimental period (20 days). Using monoclonal antibody to EGF receptor, it was determined that TCDD-caused EFG receptor down-regulation in the rat liver was accompanied by increased protein kinase activity. Such an increase in the protein kinase activity involves, at least in part, an activation of protein tyrosine kinase. Examination of serum samples from control and treated rats revealed no detectable difference in the level of EGF itself or EGF receptor-reacting substances (eg, hormones and other growth factors). In vivo TCDD caused early eye opening and tooth eruption and poor body weight gain and hair growth in mouse neonates similar to those observed with exogenously administered EGF. The results indicate that such EGF receptor-mediated effect of TCDD has some toxocilogical significance in vivo. Although TCDD causes significant reduction in [125I]-EGF binding in the hepatic plasma membrane in susceptible strains of mice, it has only modest effects in tolerant strains. The results are consistent with the idea that the action of TCDD on the EGF receptor is mediated through the cytosolic/nuclear TCDD receptor, which is known to be regulated by the Ah locus.