Role of p53 in the cellular responsiveness/resistance to glucocorticoid hormone-mediated growth arrest in a rat glioma model.
Instituto de Química, Universidade de São Paulo, São Paulo, SP
AIM: The anti-proliferative potential of glucocorticoid hormones (GCs) has been explored in therapeutics as immunosuppressors and anti-inflammatory and anti-tumoral agents. To better understand the molecular mechanisms underlying these anti-proliferative effects of glucocorticoids, we have been investigating the role of the p53 tumor suppressor gene product, a hallmark in growth control, in the cellular responsiveness/resistance to GCs. METHODS: We have used the ST1 and P7 cell lines (variants of the C6 rat glioma cells, which we have previously isolated) which are, respectively, hyper-responsive and resistant to GCs. Hormonal treatment leads ST1 cells to a dramatic tumoral to normal phenotypic reversion, by accumulation in a G0/G1 state. The p53 protein levels of control (untreated) and hydrocortisone (Hy) treated cells were analyzed by immunoprecipitation. The DNA-binding and transactivation activity of p53 were examined by, respectively, EMSA and gene-reporter assays. The transcript levels of three p53 target-genes, namely: thrombospondin-1 (TSP-1), cyclin G and p21, were analyzed by Northern blot, and cyclin G and p21 protein levels by Western blotting. RESULTS: An enhancement of both p53 DNA-binding and transactivation activity in response to Hy treatment was observed in the GC-responsive ST1 cells, whereas no modulation occurred in the GC-resistant P7 cells. The p53 protein level was not altered during the hormonal treatment of both cell lines. The Hy-mediated p53 activation in ST1 cells was accompanied by an increase of cyclin G and p21 mRNA and protein levels and by higher TSP-1 mRNA levels. On the other hand, when the transcript levels of TSP-1 and cyclin G were analyzed in P7 cells, no significant induction was observed upon Hy treatment. CONCLUSIONS: The results suggest that the GC-mediated p53 activation, which occurs in the absence of modulation of p53 protein levels, may be required for the anti-proliferative responsiveness to glucocorticoids. Elucidation of the mechanisms underlying this p53 activation may lead to better understanding of glioma tumor progression and resistance to GC chemotherapy.
Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in poster session 896 (Tumor suppressor genes).