0000000000011255
AUTHOR
Dagmar Faust
2,3,7,8-Tetrachlorodibenzo-p-dioxin-Dependent Release from Contact Inhibition in WB-F344 Cells: Involvement of Cyclin A
2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is the most potent tumor promoter ever tested in rodents. Although it is known that most of TCDD actions are mediated by binding to the aryl hydrocarbon receptor (AhR), the mechanisms leading to tumor promotion still remain to be elucidated. Loss of contact inhibition is one characteristic hallmark in tumorigenesis. In rat liver epithelial WB-F344 cells, TCDD induces a release from contact inhibition, which is manifested by a twofold increase in cell number when TCDD (1 nM for 48 h) is added to confluent cells in the presence of serum, but not when given to exponentially growing or subconfluent, serum-deprived WB-F344 cells. Loss of G1 arrest was a…
The aryl hydrocarbon receptor-dependent deregulation of cell cycle control induced by polycyclic aromatic hydrocarbons in rat liver epithelial cells
Disruption of cell proliferation control by polycyclic aromatic hydrocarbons (PAHs) may contribute to their carcinogenicity. We investigated role of the aryl hydrocarbon receptor (AhR) in disruption of contact inhibition in rat liver epithelial WB-F344 'stem-like' cells, induced by the weakly mutagenic benz[a]anthracene (BaA), benzo[b]fluoranthene (BbF) and by the strongly mutagenic benzo[a]pyrene (BaP). There were significant differences between the effects of BaA and BbF, and those of the strongly genotoxic BaP. Both BaA and BbF increased percentage of cells entering S-phase and cell numbers, associated with an increased expression of Cyclin A and Cyclin A/cdk2 complex activity. Their eff…
TCDD deregulates contact inhibition in rat liver oval cells via Ah receptor, JunD and cyclin A.
The aryl hydrocarbon receptor (AhR) is a transcription factor involved in physiological processes, but also mediates most, if not all, toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Activation of the AhR by TCDD leads to its dimerization with aryl hydrocarbon nuclear translocator (ARNT) and transcriptional activation of several phase I and II metabolizing enzymes. However, this classical signalling pathway so far failed to explain the pleiotropic hazardous effects of TCDD, such as developmental toxicity and tumour promotion. Thus, there is an urgent need to define genetic programmes orchestrated by AhR to unravel its role in physiology and toxicology. Here we show that TCDD …
Aryl hydrocarbon receptor-dependent cell cycle arrest in isolated mouse oval cells
The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor, which mediates toxic responses to environmental pollutants, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds. Besides its well known role in induction of xenobiotic metabolizing enzymes, for instance CYP1A1, the AhR is also involved in tumor promotion in rodents although the underlying mechanisms are still poorly understood. Additionally, the AhR is known to regulate cellular proliferation, which might result in either inhibition or stimulation of proliferation depending on the cell-type studied. Potential targets in hepatocarcinogenesis are liver oval (stem/progenitor) cells. In the pres…
p38α MAPK is required for contact inhibition
Proliferation of nontransformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Despite its generally accepted importance for cell cycle control, knowledge about the intracellular signalling pathways involved in contact inhibition is scarce. In the present work we show that p38alpha mitogen-activated protein kinase (MAPK) is involved in the growth-inhibitory signalling cascade of contact inhibition in fibroblasts. p38alpha activity is increased in confluent cultures of human fibroblasts compared to proliferating cultures. Time course studies show a sustained activation of p38alpha in response to cell-cell contacts in contrast to a transient activation …
TCDD induces c-jun expression via a novel Ah (dioxin) receptor-mediated p38–MAPK-dependent pathway
The aryl hydrocarbon receptor (AhR) has a fundamental role during postnatal liver development and is essential for mediating dioxin toxicity. However, the genetic programs mediating, both, the toxic and physiological effects downstream of the transcription factor AhR are in major parts unknown. We have identified the proto-oncogene c-jun as a novel target gene of AhR. Induction of c-jun depends on activation of p38-mitogen-activated protein kinase (MAPK) by an AhR-dependent mechanism. None of the kinases that are known to phosphorylate p38-MAPK is activated by AhR. Neither the dephosphorylation rate of p38-MAPK is reduced. Furthermore, increased p38-MAPK phosphorylation in response to dioxi…
Involvement of protein kinase Cdelta in contact-dependent inhibition of growth in human and murine fibroblasts.
There is evidence that protein kinase C delta (PKCdelta) is a tumor suppressor, although its physiological role has not been elucidated so far. Since important anti-proliferative signals are mediated by cell-cell contacts we studied whether PKCdelta is involved in contact-dependent inhibition of growth in human (FH109) and murine (NIH3T3) fibroblasts. Cell-cell contacts were imitated by the addition of glutardialdehyde-fixed cells to sparsely seeded fibroblasts. Downregulation of the PKC isoforms alpha, delta, epsilon, and mu after prolonged treatment with 12-O-tetradecanoylphorbol-13-acetate (TPA, 0.1 microM) resulted in a significant release from contact-inhibition in FH109 cells. Bryosta…
Regulation of ERK1/2 activity upon contact inhibition in fibroblasts.
Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Despite its generally accepted importance for maintaining tissue homeostasis knowledge about the underlying molecular mechanisms of contact inhibition is still scarce. Since the MAPK ERK1/2 plays a pivotal role in the control of proliferation, we investigated regulation of ERK1/2 phosphorylation which is downregulated in confluent NIH3T3 cultures. We found a decrease in upstream signaling including phosphorylation of the growth factor receptor adaptor protein ShcA and the MAPK kinase MEK1/2 in confluent compared to exponentially growing cultures whereas involvement of ERK1/2 phosphatases in ERK1/2 inact…
Subcellular Localization of β-Catenin Is Regulated by Cell Density
It is generally accepted that subcellular distribution of beta-catenin regulates its function. Membrane-bound beta-catenin mediates cell-cell adhesion, whereas elevation of the cytoplasmic and nuclear pool of the protein is associated with an oncogenic function. Although the role of beta-catenin in transformed cells is relatively well characterized, little is known about its importance in proliferation and cell-cycle control of nontransformed epithelial cells. Using different approaches we show that in human keratinocytes (HaCaT) beta-catenin is distributed throughout the cells in subconfluent, proliferating cultures. In contrast, beta-catenin is nearly exclusively located at the plasma mem…
TCDD-dependent downregulation of gamma-catenin in rat liver epithelial cells (WB-F344).
TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) is the most potent tumor promoter ever tested in rodents. Although it is known that most of the effects of TCDD are mediated by binding to the aryl hydrocarbon receptor (AHR), the mechanisms leading to tumor promotion still remain to be elucidated. Loss of contact-inhibition is a characteristic hallmark in tumorigenesis. In WB-F344 cells, TCDD induces a release from contact-inhibition manifested by a 2- to 3-fold increase in DNA-synthesis and the emergence of foci when TCDD (1 nM) is given to confluent cells. We focussed our interest on potential cell membrane proteins mediating contact-inhibition in WB-F344 cells, namely E-cadherin, alpha,- beta,-…
Involvement of the transcription factor FoxM1 in contact inhibition
Contact inhibition is a crucial mechanism regulating proliferation in vitro and in vivo. Although it is generally accepted that contact inhibition plays a pivotal role in maintaining tissue homeostasis, the molecular mechanisms of contact inhibition are still not fully understood. FoxM1 is known as a proliferation-associated transcription factor and is upregulated in many cancer types. Vice versa, anti-proliferative signals, such as TGF-β and differentiation signals decrease FoxM1 expression. Here we investigated the role of FoxM1 in contact inhibition in fibroblasts. We show that protein expression of FoxM1 is severely and rapidly downregulated upon contact inhibition, probably by inhibiti…
The transcriptional programme of contact-inhibition.
Proliferation of non-transformed cells is regulated by cell-cell contacts, which are referred to as contact-inhibition. Vice versa, transformed cells are characterised by a loss of contact-inhibition. Despite its generally accepted importance for cell-cycle control, little is known about the intracellular signalling pathways involved in contact-inhibition. Unravelling the molecular mechanisms of contact-inhibition and its loss during tumourigenesis will be an important step towards the identification of novel target genes in tumour diagnosis and treatment. To better understand the underlying molecular mechanisms we identified the transcriptional programme of contact-inhibition in NIH3T3 fib…