The cell cycle is regulated by a number of inhibitors, including p27Kip1 (p27), which belongs to the kip1 family. By binding to the cyclin/cyclin-dependent kinase complexes, it regulates progression of G1 to S phase in the cell cycle. It has been reported that p27 knockout mice develop multiorgan hyperplasia and intermediate lobe pituitary tumors secreting ACTH. Previously, we and others have been unable to show any consistent change in messenger RNA expression or genomic mutations for p27 in human corticotroph adenomas. However, dysregulation at the protein level has been reported in nonendocrine tumors, and we, therefore, investigated the expression of p27 in a range of benign and metastatic pituitary tumors. We studied a total of 107 pituitaries, including normal pituitary (n = 20), Cushing's disease (n = 21), acromegaly (n = 19), nonfunctioning adenomas (n = 18), prolactinomas (n = 7), TSH-omas (n = 2), FSH-omas (n = 6), aggressive tumors showing invasiveness and recurrence (n = 9), and metastatic pituitary carcinomas (n = 5). Using standard immunohistochemical techniques with a highly specific monoclonal antibody, p27 expression was determined quantitatively as the percentage of cells showing strongly positive, weak, or negative staining. In each sample, approximately 500 cells were analyzed. We also analyzed normal pituitaries using double-labeling for p27 and each of the pituitary hormones to characterize the expression of p27 in each cell type. p27 was expressed in normal pituitary cells; in tumors expressing GH, prolactin, TSH, and FSH; and in aggressive tumors, but markedly reduced expression of p27 was seen in corticotroph tumors and pituitary carcinomas. In the normal pituitary, somatotroph, lactotroph, and thyrotroph cells showed strong p27 staining, whereas normal corticotroph cells showed a much lower level of p27 staining (P < 0.001). Somatotroph, lactotroph, gonadotroph, and thyrotroph adenomas showed a lower level of p27 expression compared with normal somatotrophs (P = 0.02), lactotrophs (P = 0.03), gonadotrophs (P = 0.01), and thyrotrophs, respectively, whereas the lower level of p27 expression present in normal corticotrophs virtually disappeared in corticotroph adenomas (P = 0.001). We conclude that pituitary adenomas show a lower level of p27 protein expression than the normal cells from which they are derived, with malignant transformation leading to complete loss of p27 immunoreactivity. Corticotrophs are quite different to other pituitary cell types in terms of p27 immunoreactivity because both normal and tumorous corticotrophs have low p27 staining, and we speculate that this may relate to their inherent control mechanisms.

The cyclin-dependent kinase inhibitor p27(Kip1) (p27) plays a pivotal role in controlling cell proliferation during development and tumorigenesis. p27 has been implicated in pituitary tumorigenesis in studies of knockout mice and in analyses of human pituitary tumor samples. In this study, we further explored the role of p27 in human pituitary tumors by measuring levels of phosphorylated p27 (P-p27), and also Jun activation domain-binding protein 1 (Jab1), which is thought to facilitate the phosphorylation and degradation of p27, in normal pituitary tissue (n = 21), pituitary adenomas (n = 75), and pituitary carcinomas (n = 10). The amount of p27 protein in corticotroph adenomas and pituitary carcinomas was much lower than that in normal pituitary tissue or other types of pituitary adenoma. Nuclear P-p27 protein levels were significantly decreased in the adenomas, compared with the normals, and were much lower in the carcinomas, compared with either normal pituitary tissue or pituitary adenomas. However, P-p27 levels in corticotroph adenomas were similar to normal pituitary tissue, thus demonstrating a greatly increased ratio of P-p27 to p27 specifically in corticotroph tumors. No difference was found in Jab1 protein levels in either corticotroph tumors or other pituitary adenomas, compared with normal tissue, but there was a small but significant increase in Jab1 levels in carcinomas. Corticotroph and metastatic tumors both showed a significantly higher Ki-67 labeling index than normal pituitary or other types of pituitary adenomas, and in general the Ki-67 labeling index was negatively correlated with p27 nuclear staining. The amount of p27 and Jab1 mRNA was positively correlated in all pituitary samples studied but did not correlate with the changes in immunostaining. Our findings suggest that in corticotroph tumors there is an accentuated phosphorylation of p27 into P-p27, possibly related to increased cyclin E expression, whereas both p27 and P-p27 are subject to increased degradation in pituitary carcinomas. Such variations in phosphorylation may play a role in pituitary tumorigenesis, but modulation of Jab1 is unlikely to be important in the pathogenesis of pituitary adenomas.

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