Alternative pathway of transcriptional induction of p21WAF1/Cip1 by cyclosporine A in p53-deficient human glioblastoma cells

The cyclin-dependent kinase inhibitor p21WAF1/CIP1, a critical regulator of the cell cycle, is mainly regulated by p53 tumour suppressor at the transcriptional level. Restoration of p21WAF1/Cip1 expression in p53-deficient malignant cells suppress tumour growth. Cyclosporine A (CsA) affects proliferation and survival of cultured malignant glioma cells and impairs growth of experimental gliomas. CsA induced p21WAF1/Cip1 expression de novo in human glioblastoma cells with p53 deficiency. We demonstrate that transcriptional activation of p21WAF1/Cip1 expression correlated with induction of ERK1/2 and c-Jun phosphorylation in CsA-treated glioblastoma cells. Pre-treatment with ERK pathway inhibitors or overexpression of dominant-negative mutants MKK1, ERK2 and c-Jun reduced activation of the p21WAF1/Cip1 promoter. Overexpression of tethered AP-1 dimers containing c-Jun was sufficient to activate the truncated − 200 bp p21WAF1/Cip1 promoter, which does not contain p53 binding sites. Chromatin immunoprecipitation revealed that P-c-Jun is bound to the proximal part of p21WAF1/Cip1 promoter in CsA-treated glioblastoma cells. It suggests that CsA activates p53-independent, transcriptional activation p21WAF1/Cip1 expression, mediated by ERK/c-Jun/AP-1 signaling pathway.