Caspases are essential components of the mammalian cell death machinery. Here we test the hypothesis that Caspase 9 (Casp9) is a critical upstream activator of caspases through gene targeting in mice. The majority of Casp9 knockout mice die perinatally with a markedly enlarged and malformed cerebrum caused by reduced apoptosis during brain development. Casp9 deletion prevents activation of Casp3 in embryonic brains in vivo, and Casp9-deficient thymocytes show resistance to a subset of apoptotic stimuli, including absence of Casp3-like cleavage and delayed DNA fragmentation. Moreover, the cytochrome c-mediated cleavage of Casp3 is absent in the cytosolic extracts of Casp9-deficient cells but is restored after addition of in vitro-translated Casp9. Together, these results indicate that Casp9 is a critical upstream activator of the caspase cascade in vivo.

The interleukin-1beta (IL-1beta) converting enzyme (ICE) processes the inactive IL-1beta precursor to the proinflammatory cytokine. ICE was also shown to cleave the precursor of interferon-gamma inducing factor (IGIF) at the authentic processing site with high efficiency, thereby activating IGIF and facilitating its export. Lipopolysaccharide-activated ICE-deficient (ICE-/-) Kupffer cells synthesized the IGIF precursor but failed to process it into the active form. Interferon-gamma and IGIF were diminished in the sera of ICE-/- mice exposed to Propionibacterium acnes and lipopolysaccharide. The lack of multiple proinflammatory cytokines in ICE-/- mice may account for their protection from septic shock.

We have examined the role of phosphorylation in the regulation of human cyclin‐dependent kinase‐2 (CDK2), a protein closely related to the cell cycle regulatory kinase CDC2. We find that CDK2 from HeLa cells contains three major tryptic phosphopeptides. Analysis of site‐directed mutant proteins, expressed by transient transfection of COS cells, demonstrates that the two major phosphorylation sites are Tyr15 (Y15) and Thr160 (T160). Additional phosphorylation probably occurs on Thr14 (T14). Replacement of T160 with alanine abolishes the kinase activity of CDK2, indicating that phosphorylation at this site (as in CDC2) is required for kinase activity. Mutation of Y15 and T14 stimulates kinase activity, demonstrating that phosphorylation at these sites (as in CDC2) is inhibitory. Similarly, CDK2 is activated in vitro by dephosphorylation of Y15 and T14 by the phosphatase CDC25. Analysis of HeLa cells synchronized at various cell cycle stages indicates that CDK2 phosphorylation on T160 increases during S phase and G2, when CDK2 is most active. Phosphorylation on the inhibitory sites T14 and Y15 is also maximal during S phase and G2. Thus, the activity of a subpopulation of CDK2 molecules is inhibited at a time in the cell cycle when overall CDK2 activity is increased.

IL-18 is synthesized as a precursor molecule without a signal peptide but requires the IL-1beta converting enzyme (ICE, caspase-1) for cleavage into a mature peptide. Human precursor IL-18 was expressed, purified, and cleaved by ICE into a 18-kD mature form. Mature IL-18 induced IL-8, macrophage inflammatory protein-1alpha, and monocyte chemotactic protein-1 in human peripheral blood mononuclear cells in the absence of any co-stimuli. Blocking IL-1 with IL-1 receptor antagonist resulted in a 50% reduction in IL-8. Neutralization of TNF with TNF binding protein resulted in a 66% reduction in IL-1beta, an 80% reduction of IL-8, and an 88% reduction in mean TNFalpha mRNA. In purified CD14+ cells but not CD3+/CD4+, IL-18 induced gene expression and synthesis of IL-8 and IL-1beta. TNFalpha production was induced in the non-CD14+ population and there was no induction of TNFbeta by IL-18. In purified natural killer cells, IL-18 induced IL-8 that was also inhibited by TNF binding protein. IL-18 did not induce antiinflammatory cytokines, IL-1Ra, or IL-10, although IL-18 induction of TNFalpha was inhibited by IL-10. In the presence of IFNgamma, IL-18-induced TNFalpha was enhanced and there was an increase in the mature form of IL-1beta. We conclude that IL-18 possesses proinflammatory properties by direct stimulation of gene expression and synthesis of TNFalpha from CD3+/CD4+ and natural killer cells with subsequent production of IL-1beta and IL-8 from the CD14+ population.

The major events of the cell division cycle are triggered by periodic changes in the activity of cyclin-dependent protein kinases (CDKs). In mammals, the members of the CDK family include CDK2 and CDC2, which are thought to be involved in the control of DNA replication and mitosis, respectively. The protein kinase activity of these enzymes is controlled by a complex array of mechanisms. Activation of the CDK catalytic subunit requires association with a positive regulatory subunit (cyclin) and phosphorylation (at Thr 160 in CDK2). This activated complex can be inhibited by additional phosphorylation at Thr 14 and Tyr 15. Here we report the identification of a new mechanism for the regulation of CDK2 activity. We find that CDK2/cyclin complexes in mouse fibroblasts associate tightly with a 20K protein (CAP20). Complexes containing CAP20 were isolated from cell lysates and found to have negligible kinase activity, indicating that CAP20 association in vivo may inhibit CDK2 activity. We purified CAP20 from 3T3 cells and found that low concentrations of the protein completely inhibit the kinase activity of CDK2 in vitro. Thus CAP20 represents a new negative regulatory subunit that inhibits the activity of CDK2/cyclin complexes in mammalian cells.