Cell cycle inhibition has yet to offer a generally effective approach to cancer treatment, but a full evaluation of different combinations of cell cycle inhibitors has yet to be evaluated. Cyclin A2, a core component of the cell cycle, is often aberrantly expressed in cancer where it may impact cell proliferation. In this study, we investigated the role of cyclin A2 in tumorigenesis using a conditional genetic knockout mouse model. Cyclin A2 deletion in oncogene-transformed MEFs suppressed tumor formation in immunocompromised mice. These findings were confirmed in mice with cyclin A2-deficient hepatocytes, where a delay in liver tumor formation was observed. Since cyclin A2 acts in complex with Cdk2 in the cell cycle, we explored a hypothesized role for Cdk2 dysregulation in this effect through conditional deletions of both genes. In oncogene-transformed MEFs lacking both genes, tumor formation was strongly suppressed in a manner associated with decreased proliferation, premature senescence and error-prone recovery from serum deprivation after immortalization. Whereas loss of cyclin A2 led to a compensatory increase in Cdk1 activity, this did not occur with loss of both Cdk2 and cyclin A2. Our work offers a rationale to explore combinations of Cdk1 and Cdk2 inhibitors as a general approach in cancer therapy.

The identification of new members in the Cdk and cyclin families, functions for many of which are still emerging, has added new facets to the cell cycle regulatory network. With roles extending beyond the classical regulation of cell cycle progression, these new players are involved in diverse processes such as transcription, neuronal function, and ion transport. Members closely related to Cdks and cyclins such as the Speedy/RINGO proteins offer fresh insights and hope for filling in the missing gaps in our understanding of cell division. This chapter will present a broad outlook on the cell cycle and its key regulators with special emphasis on the less-studied members and their emerging roles.

The meiotic functions of Emi2, an inhibitor of the APC/C complex, have been best characterized in oocytes where it mediates metaphase II arrest as a component of the cytostatic factor. We generated knockout mice to determine the in vivo functions of Emi2-in particular, its functions in the testis, where Emi2 is expressed at high levels. Male and female Emi2 knockout mice are viable but sterile, indicating that Emi2 is essential for meiosis but dispensable for embryonic development and mitotic cell divisions. We found that, besides regulating cell-cycle arrest in mouse eggs, Emi2 is essential for meiosis I progression in spermatocytes. In the absence of Emi2, spermatocytes arrest in early diplotene of prophase I. This arrest is associated with decreased Cdk1 activity and was partially rescued by a knockin mouse model of elevated Cdk1 activity. Additionally, we detected expression of Emi2 in spermatids and sperm, suggesting potential post-meiotic functions for Emi2.

To assess the contribution of complement to respiratory immunity in the context of a natural bacterial infection, we used mice genetically deficient in complement components and the murine pathogen Bordetella bronchiseptica. Complement component C3 was not required for the control of bacterial infection or for the generation of infection‐induced protective immunity. However, C3‐deficient (C3–/–) mice were severely defective, compared to wild type, in vaccine‐induced protective immunity. Adoptively transferred immune serum from convalescent wild‐type or C3–/– animals rapidly cleared B. bronchiseptica from the lungs of wild‐type mice but did not affect its growth in C3–/– mice, indicating that the defect is not in the generation of protective immunity, but in its function. Immune serum was effective in C5‐deficient mice but had little effect in the lungs of mice lacking either Fcγ receptors (FcγR) or CR3, suggesting bacterial clearance is not via direct complement‐mediated lysis. Together, these data indicate that complement is required for antibody‐mediated clearance of Bordetella and suggest the mechanism involves C3 opsonization of bacteria for phagocytosis that is both CR3‐ and FcγR‐dependent.

Peroxisome proliferator-activated receptor-alpha (PPARalpha) belongs to the nuclear receptor (NR) family of transcription factors and regulates lipid and glucose metabolism. Like other NRs, the regulation of gene expression by PPARalpha depends on cofactor recruitment to the transcription complex and multiple protein-protein interactions. In this study, Murine Double Minute 2 (MDM2), an E3 ubiquitin ligase, is identified as a PPARalpha-interacting protein that regulates PPARalpha transcriptional activity. MDM2 modulated the transcriptional activity of PPARalpha and PPARbeta/delta, but not PPARgamma in reporter assays. Knockdown of MDM2 by small interfering RNA in rat hepatoma cells inhibited ligand-induced mRNA levels of several PPARalpha target genes involved in lipid metabolism. MDM2 associated with PPARalpha on target gene promoters, and this association increased in response to Wy14,643 treatment. MDM2 interacted with PPARalpha and this interaction occurred with the A/B domain of PPARalpha. Coexpression of MDM2 increased PPARalpha ubiquitination and the E3 ubiquitin ligase activity of MDM2 affected PPARalpha protein expression and transcriptional activity. MDM2 expression was decreased in response to clofibrate in wild-type (WT), but not in PPARalpha null mice, indicating a PPARalpha-dependent regulation. These studies identify a role for MDM2 in regulating PPARalpha-mediated pathways of lipid metabolism.