p53-Dependent and -independent functions of the Arf tumor suppressor.

The Ink4a-Arf locus encodes two closely wedded tumor suppressor proteins (p16(Ink4a) and p19(Arf)) that inhibit cell proliferation by activating Rb and p53, respectively. With few exceptions, the Arf gene is repressed during mouse embryonic development, thereby helping to limit p53 expression during organogenesis. However, in adult mice, sustained hyperproliferative signals conveyed by somatically activated oncogenes can induce Arf gene expression and trigger a p53 response that eliminates incipient cancer cells. Disruption of this tumor surveillance pathway predisposes to cancer, and inactivation of INK4a- ARF by deletion, silencing, or mutation has been frequently observed in many forms of human cancer. Although it is accepted that much of Arf's tumor-suppressive activity is mediated by p53, more recent genetic evidence has pointed to additional p53- independent functions of Arf, including its ability to inhibit gene expression by a number of other transcription factors. Surprisingly, the enforced expression of Arf in mammalian cells promotes the sumoylation of several Arf-interacting proteins, implying that Arf has an associated catalytic activity. We speculate that transcriptional down-regulation in response to Arf-induced sumoylation may account for Arf's p53-independent functions.

Control of spermatogenesis in mice by the cyclin D-dependent kinase inhibitors p18(Ink4c) and p19(Ink4d).

Male mice lacking both the Ink4c and Ink4d genes, which encode two inhibitors of D-type cyclin-dependent kinases (Cdks), are infertile, whereas female fecundity is unaffected. Both p18(Ink4c) and p19(Ink4d) are expressed in the seminiferous tubules of postnatal wild-type mice, being largely confined to postmitotic spermatocytes undergoing meiosis. Their combined loss is associated with the delayed exit of spermatogonia from the mitotic cell cycle, leading to the retarded appearance of meiotic cells that do not properly differentiate and instead undergo apoptosis at an increased frequency. As a result, mice lacking both Ink4c and Ink4d produce few mature sperm, and the residual spermatozoa have reduced motility and decreased viability. Whether or not Ink4d is present, animals lacking Ink4c develop hyperplasia of interstitial testicular Leydig cells, which produce reduced levels of testosterone. The anterior pituitary of fertile mice lacking Ink4c or infertile mice doubly deficient for Ink4c and Ink4d produces normal levels of luteinizing hormone (LH). Therefore, the failure of Leydig cells to produce testosterone is not secondary to defects in LH production, and reduced testosterone levels do not account for infertility in the doubly deficient strain. By contrast, Ink4d-null or double-null mice produce elevated levels of follicle-stimulating hormone (FSH). Because Ink4d-null mice are fertile, increased FSH production by the anterior pituitary is also unlikely to contribute to the sterility observed in Ink4c/Ink4d double-null males. Our data indicate that p18(Ink4c) and p19(Ink4d) are essential for male fertility. These two Cdk inhibitors collaborate in regulating spermatogenesis, helping to ensure mitotic exit and the normal meiotic maturation of spermatocytes.