p19Ink4d Is a Tumor Suppressor and Controls Pituitary Anterior Lobe Cell Proliferation

Pituitary tumors develop in about one-quarter of the population, and most arise from the anterior lobe (AL). The pituitary gland is particularly sensitive to genetic alteration of genes involved in the cyclin-dependent kinase (CDK) inhibitor (CKI)–CDK-reti-noblastoma protein (Rb) pathway. Mice heterozygous for the Rb mutation develop pituitary tumors, with about 20 % arising from the AL. Perplexingly, none of the CKI-deficient mice reported thus far develop pituitary AL tumors. In this study, we show that deletion of p19Ink4d (p19), a CKI gene, in mice results in spontaneous development of tumors in multiple organs and tissues. Specifically, more than one-half of the mutant mice developed pituitary hyperplasia or tumors predominantly in the AL. Tumor development is associated with increased cell proliferation and enhanced activity of Cdk4 and Cdk6 and phosphorylation of Rb protein. Though Cdk4 is indispensable for postnatal pituitary cell proliferation, it is not required for the hyperproliferative pitu-itary phenotype caused by p19 loss. Loss of p19 phosphorylates Rb in Cdk4/ pituitary AL cells and mouse embryonic fibro-blasts (MEFs) and rescues their proliferation defects, at least partially, through the activation of Cdk6. These results provide the first genetic evidence that p19 is a tumor suppressor and the major CKI gene that controls pituitary AL cell proliferation. The retinoblastoma protein (Rb) family includes three structur-ally related nuclear phosphoproteins, pRB, p107, and p130, which when hypophosphorylated, negatively regulate the activity of E2F transcription factors to prevent S-phase entry. In mamma-lian cells, extracellular mitogens induce the expression of D-typ