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Cyclin proteolysis as a retinoid cancer prevention mechanism
K H Dragnev1, S J Freemantle, M J Spinella
1Norris Cotton Cancer Center and Department of Medicine, Dartmouth-Hitchcock Medical Center, Lebanon, New Hampshire 03756, USA.
Abstract:
The retinoids, natural and synthetic derivatives of vitamin A, are active in cancer therapy and prevention. Their biological effects are mediated through ligand-dependent interactions with retinoid receptors that associate with specific co-regulators. A better understanding of retinoid chemopreventive mechanisms is needed. Our prior work revealed that all-trans-retinoic acid (RA) prevented tobacco-specific carcinogenic transformation of cultured human bronchial epithelial cells. RA signaled G1 arrest that permitted repair of genomic DNA damage caused by these carcinogens. RA triggered G1 arrest at least partly through proteasome-dependent degradation of cyclin D1. Proteasomal inhibitors blocked RA-mediated cyclin D1 degradation. To confirm that a specific proteolysis pathway was induced by RA-treatment, a degradation assay was established using in vitro translated cyclin D1 and cellular extracts from RA-treated or untreated human bronchial epithelial cells. Incubation of RA-treated but not the control cellular extracts with in vitro translated cyclin D1 led to cyclin degradation. This degradation depended on the PEST domain of cyclin D1, implicating ubiquitination in this retinoid degradation. Retinoid receptor selective agonists demonstrated that retinoic acid receptor (RAR)beta and retinoid X receptor (RXR) but not RARalpha- or RARgamma-dependent pathways signaled this cyclin degradation. Findings were extended to the NT2/D1 human embryonal carcinoma differentiation model where a similar pathway was activated by RA-treatment. To determine whether G1 cyclins were involved directly in bronchial preneoplasia, immunohistochemical expression profiles for cyclins D1 and E were examined. Aberrant expression of these cyclins was frequent in bronchial preneoplasia. Taken together, these findings indicate that ubiquitin-dependent proteolysis of G1 cyclins is a retinoid chemoprevention mechanism. Whether the retinoids represent the optimal agents to activate this pathway is the subject of ongoing work. These findings provide a rationale for combining the retinoids in chemoprevention trials with other agents that do not activate this proteolysis pathway. What is now known about the retinoids as cancer prevention agents will be reviewed. Emphasis is placed on retinoid effects on cell cycle progression at G1.
Insights
Retinoids, vitamin A derivatives, prevent cancer by degrading G1 cyclins via a ubiquitin-dependent proteolysis pathway. This mechanism, involving specific retinoid receptors, offers a rationale for combining retinoids in cancer prevention trials.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- Retinoids, vitamin A derivatives, are known for their roles in cancer therapy and prevention.
- Their mechanisms involve interactions with retinoid receptors and co-regulators, but understanding chemopreventive pathways requires further investigation.
- Previous research indicated all-trans-retinoic acid (RA) prevents carcinogen-induced transformation of human bronchial cells by inducing G1 arrest.
Purpose of the Study:
- To elucidate the specific molecular mechanisms by which retinoids exert their chemopreventive effects.
- To investigate the role of G1 cyclin degradation in retinoid-mediated cancer prevention.
- To identify the specific retinoid receptors involved in signaling this degradation pathway.
Main Methods:
- Established an in vitro degradation assay using in vitro translated cyclin D1 and cellular extracts from RA-treated or untreated human bronchial epithelial cells.
- Utilized proteasomal inhibitors to assess the role of proteasome-dependent degradation.
- Employed retinoid receptor selective agonists (RAR and RXR) to determine receptor-specific signaling pathways.
- Examined immunohistochemical expression profiles of cyclins D1 and E in bronchial preneoplasia.
Main Results:
- Retinoic acid (RA) treatment induced proteasome-dependent degradation of cyclin D1, a key G1 regulator.
- The degradation pathway was confirmed in vitro and was dependent on the PEST domain of cyclin D1, suggesting ubiquitination.
- Retinoic acid receptor (RAR)beta and retinoid X receptor (RXR) signaling pathways, but not RARalpha or RARgamma, mediated this cyclin degradation.
- Aberrant expression of G1 cyclins (D1 and E) was frequently observed in bronchial preneoplasia.
Conclusions:
- Ubiquitin-dependent proteolysis of G1 cyclins represents a novel retinoid chemoprevention mechanism.
- This pathway is activated by specific retinoid receptors (RARbeta and RXR).
- Findings provide a rationale for combining retinoids with other agents in chemoprevention strategies, particularly those that do not activate this proteolysis pathway.