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Aromatic hydrocarbon receptor interaction with the retinoblastoma protein potentiates repression of E2F-dependent
A Puga1, S J Barnes, T P Dalton
1Center for Environmental Genetics, Department of Environmental Health, University of Cincinnati Medical Center, Cincinnati, Ohio 45267-0056, USA. Alvaro.Puga@UC.EDU
Abstract:
Polyhalogenated aromatic hydrocarbons, of which 2,3,7, 8-tetrachloro-p-dioxin (TCDD) is the prototype compound, elicit a variety of toxic, teratogenic, and carcinogenic responses in exposed animals and in humans. In cultured cells, TCDD shows marked effects on the regulation of cell cycle progression, including thymocyte apoptosis, induction of keratinocyte proliferation and terminal differentiation, and inhibition of estrogen-dependent proliferation in breast cancer cells. The presence of an LXCXE domain in the dioxin aromatic hydrocarbon receptor (AHR), suggested that the effects of TCDD on cell cycle regulation might be mediated by protein-protein interactions between AHR and the retinoblastoma protein (RB). Using the yeast two-hybrid system, AHR and RB were in fact shown to bind to each other. In vitro pull-down experiments with truncated AHR peptides indicated that at least two separate AHR domains form independent complexes with hypophosphorylated RB. Coimmunoprecipitation of whole cell lysates from human breast carcinoma MCF-7 cells, which express both proteins endogenously, revealed that AHR associates with RB in vivo only after receptor transformation and nuclear translocation. However, the AHR nuclear translocator and transcriptional heterodimerization partner, is not required for (nor is it a part of) the AHR.RB complexes detected in vitro. Ectopic expression of AHR and RB in human osteosarcoma SAOS-2 cells, which lack endogenous expression of both proteins, showed that AHR synergizes with RB to repress E2F-dependent transcription and to induce cell cycle arrest. Furthermore, AHR partly blocked T-antigen-mediated reversal of RB-dependent transcriptional repression. These results uncover a potential function for the AHR in cell cycle regulation and suggest that this function may be that of serving as an environmental sensor that signals cell cycle arrest when cells are exposed to certain environmental toxicants.
Insights
The aromatic hydrocarbon receptor (AHR) interacts with the retinoblastoma protein (RB) to regulate cell cycle progression. This interaction, triggered by environmental toxicants like TCDD, signals cell cycle arrest.
Area of Science:
- Toxicology
- Molecular Biology
- Cell Cycle Regulation
Background:
- Polyhalogenated aromatic hydrocarbons, such as 2,3,7,8-tetrachloro-p-dioxin (TCDD), induce toxic, teratogenic, and carcinogenic effects.
- TCDD impacts cell cycle progression in various cell types, including apoptosis, proliferation, and differentiation.
- The dioxin aromatic hydrocarbon receptor (AHR) contains an LXCXE domain, suggesting a role in protein-protein interactions affecting cell cycle regulation.
Purpose of the Study:
- To investigate the potential interaction between the AHR and the retinoblastoma protein (RB) in mediating TCDD's effects on cell cycle regulation.
- To elucidate the mechanism by which AHR influences cell cycle progression in response to environmental toxicants.
Main Methods:
- Yeast two-hybrid system to assess AHR-RB binding.
- In vitro pull-down assays using truncated AHR peptides.
- Coimmunoprecipitation from human breast carcinoma MCF-7 cells.
- Ectopic expression of AHR and RB in human osteosarcoma SAOS-2 cells.
Main Results:
- AHR and RB were shown to bind to each other using the yeast two-hybrid system.
- AHR forms independent complexes with hypophosphorylated RB through at least two separate domains.
- AHR associates with RB in vivo in MCF-7 cells upon receptor transformation and nuclear translocation.
- AHR and RB synergistically repress E2F-dependent transcription and induce cell cycle arrest in SAOS-2 cells.
- AHR partially inhibited T-antigen-mediated reversal of RB-dependent transcriptional repression.
Conclusions:
- The AHR directly interacts with RB, influencing cell cycle control.
- This AHR-RB interaction plays a role in mediating the cellular response to environmental toxicants.
- The AHR may function as an environmental sensor, signaling cell cycle arrest upon exposure to certain toxicants.