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Dexamethasone-mediated protection from drug cytotoxicity: association with p21WAF1/CIP1 protein accumulation?
1Department of Neurology, University of Tübingen, Medical School, Germany.
Abstract:
Dexamethasone (DEX)-mediated inhibition of drug-induced, but not CD95 ligand-induced, apoptosis in malignant glioma cells correlates with wild-type p53 status. Here, we examined mechanisms underlying DEX-mediated protection from apoptosis. DEX did not induce p53 expression in two p53 wild-type cell lines (U87MG, LN-229) and did not alter drug-induced p53 accumulation. Forced expression of temperature-sensitive p53val135 in mutant conformation failed to prevent accumulation of endogenous wild-type p53 but acted in a transdominant negative manner to inhibit p53-mediated, camptothecin-induced p21WAF1/CIP1 expression. p53val135-transfected cells retained responsiveness to DEX at restrictive temperature, suggesting that p53 activity is not required for cytoprotection. Forced expression of wild-type p53val135 abrogated the protective effect of DEX, suggesting redundant cytoprotective effects of DEX and p53. Indeed, DEX induced moderate accumulation of p21WAF1/CIP1 in U87MG, LN-229 and p53 mutant LN-18 cells, but not in p53 mutant LN-308 or T98G cells. LN-18 is also the p53 mutant cell line with the best cytoprotective response to DEX. p21WAF1/CIP1 accumulation occurred in the absence of changes in p21WAF1/CIP1 mRNA expression. Wild-type p53 was not required for this DEX effect since DEX induced p21WAF1/CIP1 accumulation in p53val135-transfected LN-229 cells, too. DEX failed to induce p21WAF1/CIP1 expression or cytoprotection in untransformed rat astrocytes. The same lack of modulation of p21WAF1/CIP1 expression and drug toxicity was observed in p21(+/+), p21(+/-) and p21(-/-) human colon carcinoma cells. Paradoxically, while only p21(+/+) and p21(+/-) mouse embryonic fibroblasts showed enhance p21WAF1/CIP1 levels after exposure to DEX, only p21(-/-) fibroblasts were protected from drug toxicity by DEX. The present study links DEX-mediated protection from cancer chemotherapy to a p53-independent pathway of regulating p21WAF1/CIP1 expression in glioma cells but this effect appears to cell type-specific.
Insights
Dexamethasone protects malignant glioma cells from chemotherapy by a p53-independent pathway that regulates p21 expression. This cytoprotective mechanism is specific to cell type and does not involve wild-type p53 activity.
Area of Science:
- Oncology
- Molecular Biology
- Cell Biology
Background:
- Dexamethasone (DEX) inhibits apoptosis in malignant glioma cells, but the underlying mechanisms are not fully understood.
- The role of wild-type p53 status in DEX-mediated cytoprotection is unclear.
- Previous studies suggest a correlation between DEX-mediated apoptosis inhibition and wild-type p53, but not CD95 ligand-induced apoptosis.
Purpose of the Study:
- To investigate the molecular mechanisms of DEX-mediated protection from apoptosis in malignant glioma cells.
- To determine the involvement of p53 and p21WAF1/CIP1 in DEX-induced cytoprotection.
- To explore the cell type-specificity of DEX-mediated effects on apoptosis and p21 expression.
Main Methods:
- Utilized human glioma cell lines with varying p53 and p21 statuses (wild-type, mutant, knockout).
- Employed gene transfection to express temperature-sensitive p53val135 (mutant and wild-type conformations).
- Assessed apoptosis, p53 and p21WAF1/CIP1 expression (protein and mRNA), and drug toxicity in response to DEX treatment.
Main Results:
- DEX-mediated protection from drug-induced apoptosis in glioma cells occurred independently of wild-type p53 status.
- DEX induced p21WAF1/CIP1 accumulation in a p53-independent manner in some glioma cell lines, but not in astrocytes or colon carcinoma cells.
- The protective effect of DEX against drug toxicity was observed in p21-deficient mouse embryonic fibroblasts, suggesting a complex interplay between p53, p21, and DEX.
Conclusions:
- DEX confers cytoprotection against chemotherapy in malignant glioma cells via a p53-independent pathway regulating p21WAF1/CIP1 expression.
- This DEX-mediated effect on p21 expression and cytoprotection is cell type-specific.
- The findings highlight a novel mechanism of chemoresistance potentially exploitable in glioma treatment strategies.