Mdm2 and the p53 pathway in human pituitary adenomas

Studies on pituitary tumours have failed to identify mutations in the tumour suppressor gene p53 suggesting that the protein identified is wild type. p21(WAF--1) is a downstream effector of p53 which promotes growth arrest. Mdm2 (mouse double minute) is a protein induced by wild type p53 and forms an autoregulatory feedback loop suppressing wild type p53 activity. The purpose of this study was to examine a group of pituitary tumours for expression of p53 and its two downstream effector proteins p21(WAF--1) and mdm2 and to compare this with their radiological invasive status and proliferative potential as assessed by Ki-67 expression.

Sixty-nine tumours removed at transsphenoidal surgery were examined by immunocytochemistry using antibodies against p53, p21(WAF--1), mdm2 and Ki-67 (MIB-1). The invasive status of the tumours was determined from the preoperative CT/MRI scans.

p53 was expressed in 42 of 69 (61%) pituitary adenomas but there was no relationship with either pituitary tumour invasive status (P = 0.71) or volume (P = 0.33). p53 expression correlated, however, with the proliferative state of the tumours as assessed by the MIB-1 labelling index (P = 0.0065). Invasive tumours had a higher growth fraction than non-invasive ones (P = 0.027). p21(WAF--1) was expressed in the nuclei of 58/69 (84%) pituitary adenomas and its expression correlated with that of p53 (r = 0.26, P = 0.03). Mdm2 was expressed in the cytoplasm of 46/69 (67%) tumours and this correlated with the nuclear staining for p53 (P = 0.022) while nuclear staining was seen in 32/69 (46%) tumours but this did not correlate significantly with nuclear p53 staining (P = 0.096).

These results suggest that p53, p21(WAF--1) and mdm2 are all expressed in pituitary tumours suggesting that the p53 protein detected by immunocytochemistry is wild type. Expression of p53 is associated with tumours which have a higher proliferative status. The p53 activity is probably the result of upstream signals of local stresses mediated through either genetic change, cytokines, hypoxia or hormonal factors. Our results suggest, however, that the downstream pathway mediated through the activities of p21(WAF--1) and mdm2 may be dysfunctional in these tumours.