Low Expression of the Cell Cycle Inhibitor p27Kip1 in Normal Corticotroph Cells, Corticotroph Tumors, and Malignant Pituitary Tumors

Abstract

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.

p27Kip1-deficient mice exhibit accelerated growth hormone-releasing hormone (GHRH)-induced somatotrope proliferation and adenoma formation

“…This is in sharp contrast to the classic de®nition of a tumor suppressor gene in which elimination of both alleles (either by mutation or somatic loss) is required for tumor formation (Knudson, 1971). The present study, taken together with previous reports of diminished p27 protein levels in human pituitary adenomas Bamberger et al, 1999a;Lidhar et al, 1999), indicates a reduction in p27 levels can contribute to The stimulatory action of GHRH on pituitary GH secretion and somatotrope proliferation is thought to be mediated through ligand/receptor activation of the guanine nucleotide binding protein, G Sa , which in turn activates the adenylyl cyclase-cAMP intracellular signal transduction pathway (Harwood et al, 1984;Bilezikjian et al, 1987). This is supported by the fact that mice expressing the rat GH promoter-driven, cholera toxin transgene (rGH-CT), which constitutively activates G Sa , have enlarged pituitaries due to somatotrope hyperplasia (Burton et al, 1991).…”

“…Although p27 protein levels clearly¯uctuate during progression through the cell cycle, p27 mRNA levels remain relatively constant (Hengst and Reed, 1996). Likewise, in the majority of human neoplasias, including pituitary adenomas, p27 mRNA levels do not di er from those observed in normal tissue, while p27 protein levels are reduced (Tanaka et al, 1997;Takeuchi et al, 1998;Dahia et al, 1998;Lloyd et al, 1997;Jin et al, 1997;Bamberger et al, 1999a;Lidhar et al, 1999;Tsihlias et al, 1999). Taken together these observations suggest that the primary regulation of p27 levels is post-transcriptional and that a defect in this process, which might include a decrease in p27 protein synthesis or an increase in p27 protein degradation, may contribute to neoplastic transformation.…”

“…p27 gene mutations are rarely found in human pituitary tumors (Ikeda et al, 1997;Tanaka et al, 1997;Takeuchi et al, 1998;Dahia et al, 1998) and p27 mRNA levels are reported to be normal in many pituitary tumor subtypes (Tanaka et al, 1997;Takeuchi et al, 1998;Dahia et al, 1998). However pituitary adenomas (LH/FSH, TSH, ACTH, GH/PRL and nonfunctioning) have lower levels of p27 protein compared to normal pituitary tissue Jin et al, 1997;Bamberger et al, 1999a;Lidhar et al, 1999). This later observation suggests that a partial reduction in p27 levels, possibly due to abnormalities in post-transcriptional processing of the p27 gene, may be su cient to initiate or contribute to the progression of anterior pituitary tumor formation.…”

Prostatic hyperplasia in acromegaly, a myth or reality: a case–control study

“…The eighth question refers to the patient's perceived quality of life. The first seven questions of the IPSS are identical to the questions appearing on the American Urological Association (AUA) Symptom Index, which currently categorizes symptoms as follows: mild (symptom score %7), moderate (symptom score range [8][9][10][11][12][13][14][15][16][17][18][19], and severe (symptom score range 20-35) (10,11). IPSS has been previously validated in our population (Santosh Kumar and SK Singh, unpublished data).…”

15 YEARS OF PARAGANGLIOMA: The association of pituitary adenomas and phaeochromocytomas or paragangliomas

“…Kip1 (Pellegata et al 2006), a tumour suppressor previously implicated in pituitary tumorigenesis in knockout mice (Nakayama et al 1996) and known to be downregulated in human pituitary adenomas (Lidhar et al 1999, Korbonits et al 2002. A pathogenic truncating mutation in the human orthologue CDKN1B was identified in a 48-year-old woman with a personal history of acromegaly and primary hyperparathyroidism and a family history of renal Printed in Great Britain angiomyolipoma in a confirmed mutation carrier (Pellegata et al 2006).…”