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

• Published in: Oncogene Vol. 19; no. 15; pp. 1875 - 1884
• Main Authors: TEIXEIRA, L. T, KIYOKAWA, H, PENG, X. D, CHRISTOV, K. T, FROHMAN, L. A, KINEMAN, R. D
• Format: Journal Article
• Language: English
• Published: Basingstoke Nature Publishing 06.04.2000; Nature Publishing Group
• Subjects: Adenoma; Adenoma - genetics; Age; Age Factors; Animals; Biological and medical sciences; Brain tumors; Cell cycle; Cell Cycle Proteins; Cell physiology; Cell transformation and carcinogenesis. Action of oncogenes and antioncogenes; Cross-breeding; Cyclin-Dependent Kinase Inhibitor p27; Cyclin-Dependent Kinases - antagonists & inhibitors; Fundamental and applied biological sciences. Psychology; Genes; Genes, Tumor Suppressor; Genetic aspects; Growth hormone-releasing hormone; Growth Hormone-Releasing Hormone - pharmacology; Growth hormones; Humans; Hyperplasia; Hypotheses; Kinases; Liver - pathology; Metallothionein; Mice; Mice, Transgenic; Microtubule-Associated Proteins - genetics; Microtubule-Associated Proteins - metabolism; Molecular and cellular biology; mRNA; Ovaries; p27 gene; p53 Protein; Physiological aspects; Pituitary; Pituitary Gland - pathology; Pituitary Neoplasms - genetics; Risk factors; Somatotropin; Somatotropin releasing hormone; Spleen; Spleen - pathology; Thymus gland; Transgenic mice; Tumor Suppressor Proteins; Tumors; United States; United States > US; Illinois; Chicago Illinois; Cell proliferation; Nucleotide sequence; Enzyme; Induction; Somatotropin releasing factor; Transferases; Hyperplasia; Rodentia; Transgenic animal; Cyclin; Adenoma; Gene expression; Carcinogenesis; Signal transduction; Vertebrata; Insulin like growth factor; Mammalia; Mutagenesis; Mouse; Protein kinase; Molecular complex; Benign neoplasm
• DOI: 10.1038/sj.onc.1203490
• ISSN: 0950-9232

p27Kip1 (p27) controls cell cycle progression by binding to and inhibiting the activity of cyclin dependent kinases. Disruption of the p27 gene in mice (p27-/-) results in increased body growth with a disproportionate enlargement of the spleen, thymus, testis, ovary and pituitary. The increase in pituitary size is due to selective hyperplasia of the intermediate lobe (IL) while the anterior lobe (AL) is not overtly affected. p27 heterozygous mice (p27+/-), as well as p27-/- mice, are hypersensitive to radiation- and chemical-induced tumors compared to wildtype (p27+/+) littermates. Therefore, unlike classical tumor suppressors, only a reduction in p27 levels is necessary to predispose tissues to secondary tumor promoters. Consistent with these studies is the fact that the p27 gene sequence and mRNA levels appear normal in human pituitary adenomas while p27 protein levels are decreased. Therefore, a reduction in p27 levels could be sufficient to sensitize pituitary cells to tumorigenic factors. To test this hypothesis, metallothionein promoter-driven, human growth hormone-releasing hormone (MT-hGHRH) transgenic mice, that exhibit somatotrope hyperplasia before 9 months of age and subsequent adenoma formation with 30 - 40% penetrance, were crossbred with p27+/- mice for two successive generations to produce p27+/+, p27+/- and p27-/- mice that expressed the hGHRH transgene. At 10 - 12 weeks of age, p27-/- and p27+/+, hGHRH mice were larger than their p27+/+ littermates and displayed characteristic hyperplasia of the IL and AL, respectively. Expression of the hGHRH transgene in both p27+/- and p27-/- mice selectively expanded the population of somatotropes within the AL, where pituitaries of p27+/-, hGHRH and p27-/-, hGHRH mice were two- and fivefold larger than p27+/+, hGHRH pituitaries, respectively. There was also a synergistic effect of hGHRH transgene expression and p27-deficiency on liver, spleen and ovarian growth. At 6 - 8 months of age, 83% of p27+/-, hGHRH mice displayed macroscopic AL adenomas (>100 mg), while all pituitaries from p27+/+, hGHRH mice remained hyperplastic (<20 mg). In contrast to the dramatic effects of p27-deficiency on hGHRH-induced organ growth, elimination of p53, by crossbreeding MT-hGHRH mice to p53-deficient mice, did not augment the hyperplastic/tumorigenic effects of hGHRH transgene expression. Taken together these results demonstrate that a reduction in p27 expression is sufficient to sensitize somatotropes to the proliferative actions of excess GHRH, resulting in the earlier appearance and increased penetrance of hGHRH-induced pituitary tumors.