Expression analysis of cyclins in pituitary adenomas and the normal pituitary gland

The molecular events involved in pituitary tumour development are still poorly understood. The cyclins play an important role in the control of the cell cycle during cell proliferation and over-expression of the cyclins has been shown in many different tumour types. The aim of this study was to investigate whether, in comparison to the normal gland, ectopic expression of cyclins occurs in pituitary tumours, and whether differences in cyclin expression are seen with different pituitary tumour types or in association with different tumour behaviour. In contrast to work on cyclin D there are no published data on cyclin B, A and E in human pituitary tumours.

Sixty-seven surgically removed pituitary tumours and 10 specimens of normal human anterior gland were studied using immunohistochemistry to detect the nuclear expression of cyclin A, B, D and E. The microvascular density (as a measure of angiogenesis), Ki-67 labelling index (to assess cell proliferation) and bcl-2 expression had previously been investigated in this cohort.

All tumours studied contained cells that immunostained positively for cyclin A, B, D and E. However the proportion of positive cells in each tumour type was different. In contrast, there were no cyclin D positive cells in the normal anterior pituitary gland studied, and labelling indices (LI) for cyclins A, B and E were significantly lower in the normal gland than in pituitary adenomas. The cyclin LIs for A, B, D and E were significantly higher in macroadenomas when compared to microadenomas. Non-functioning pituitary tumours (NFA) generally showed the highest cyclin LI. In particular, both recurrent and nonrecurrent NFA showed significantly higher cyclin D LI than other tumours. The ratio of cells expressing cyclin B compared to those expressing cyclin A was significantly higher in functionless tumours that regrew when compared to NFAs that did not (P<0.05). Cyclin D LI and the overall Ki-67 LI as a measure of cell proliferation were related (R2 = 11.4, P = 0.0033) and bcl-2 positive tumours had significantly higher cyclin D LI compared with bcl-2 negative tumours. There was a weak relationship between angiogenesis and the relative proportion of cells expressing D when compared to those in S phase (D/A ratio) (r2 = 10.5, P = 0.02).

We have demonstrated that ectopic expression of cyclin D and over-expression of cyclins A, B and E, regulating different stages of the cell cycle is common in pituitary adenomas. In addition, cyclin expression was related to size and to pituitary tumour regrowth. The differences between functionless tumours that regrow and those that do not, may be due to reduced bcl-2 expression, increased cell proliferation, more cells at the G2/M stage (B/A ratio) and reduced cell differentiation with more aggressive subsequent tumour behaviour. Cyclin D expression and cell proliferation were related indicating that the cells entering the cycle become 'committed' to cell cycle progression. There was no relative over-expression of individual cyclins, and therefore no evidence of relative increase in cell cycle phase, indicating that the increased cyclin expression is more likely to be due to constant mitogenic stimulation rather than cell cycle regulatory failure. Although nuclear cyclin expression is a good marker of tumour growth and aggressive behaviour, the growth signal that leads to cyclin expression remains to be identified.