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Understanding how cyclin-cdk complexes recognize their substrates is a central problem in cell cycle biology. We identified an E2F1-derived eight-residue peptide which blocked the binding of cyclin A and E-cdk2 complexes to E2F1 and p21. Short peptides spanning similar sequences in p107, p130, and p21-like cdk inhibitors likewise bound to cyclin A-cdk2 and cyclin E-cdk2. In addition, these peptides promoted formation of stable cyclin A-cdk2 complexes in vitro but inhibited the phosphorylation of the retinoblastoma protein by cyclin A- but not cyclin B-associated kinases. Mutation of the cyclin-cdk2 binding motifs in p107 and E2F1 likewise prevented their phosphorylation by cyclin A-associated kinases in vitro. The cdk inhibitor p21 was found to contain two functional copies of this recognition motif, as determined by in vitro kinase binding/inhibition assays and in vivo growth suppression assays. Thus, these studies have identified a cyclin A- and E-cdk2 substrate recognition motif. Furthermore, these data suggest that p21-like cdk inhibitors function, at least in part, by blocking the interaction of substrates with cyclin-cdk2 complexes.

Adolescence is the developmental epoch during which children become adults – intellectually, physically, hormonally, and socially. Adolescence is a tumultuous time, full of changes and transformations. The pubertal transition to adulthood involves both gonadal and behavioral maturation. Magnetic resonance imaging studies have discovered that myelinogenesis, required for proper insulation and efficient neurocybernetics, continues from childhood and the brain’s region-specific neurocircuitry remains structurally and functionally vulnerable to impulsive sex, food, and sleep habits. The maturation of the adolescent brain is also influenced by heredity, environment, and sex hormones (estrogen, progesterone, and testosterone), which play a crucial role in myelination. Furthermore, glutamatergic neurotransmission predominates, whereas gamma-aminobutyric acid neurotransmission remains under construction, and this might be responsible for immature and impulsive behavior and neurobehavioral excitement during adolescent life. The adolescent population is highly vulnerable to driving under the influence of alcohol and social maladjustments due to an immature limbic system and prefrontal cortex. Synaptic plasticity and the release of neurotransmitters may also be influenced by environmental neurotoxins and drugs of abuse including cigarettes, caffeine, and alcohol during adolescence. Adolescents may become involved with offensive crimes, irresponsible behavior, unprotected sex, juvenile courts, or even prison. According to a report by the Centers for Disease Control and Prevention, the major cause of death among the teenage population is due to injury and violence related to sex and substance abuse. Prenatal neglect, cigarette smoking, and alcohol consumption may also significantly impact maturation of the adolescent brain. Pharmacological interventions to regulate adolescent behavior have been attempted with limited success. Since several factors, including age, sex, disease, nutritional status, and substance abuse have a significant impact on the maturation of the adolescent brain, we have highlighted the influence of these clinically significant and socially important aspects in this report.

We investigated intra-arterially administered autologous bone marrow mononuclear cells (MNCs) in rats with acute ischemic stroke. Long Evans rats (2 to 3 months or 12 months old) underwent tandem reversible common carotid artery (CCA)/middle cerebral artery (MCA) occlusion (CCAo/MCAo) for 3 h and then 24 h later underwent tibial bone marrow harvest. Ten million or 4 million cells were re-injected by an intra-carotid infusion. Control animals underwent marrow needle insertion and then saline injection into the carotid artery. Animals were assessed on a battery of neurological tests. MNCs in the ischemic brain were tracked using Q-dot nanocrystal labeling. Infarct volume and cytokines in the ischemia-affected brain were analyzed. Cell-treated animals in the younger and older groups showed improvement from 7 to 30 days after stroke compared with vehicle-treated animals. MNCs significantly reduced infarct volume compared with saline. There was a significant reduction in tumor necrosis factor-α, interleukin-1α (IL-1α), IL-β, IL-6, and a significant increase in IL-10 in injured brains harvested from the cell-treated groups compared with saline controls. Labeled MNCs were found in the peri-infarcted area at 1 h and exponentially decreased over the ensuing week after injection. Autologous bone marrow MNCs can be safely harvested from rodents after stroke, migrate to the peri-infarct area, enhance recovery, and modulate the post-ischemic inflammatory response.

Recent studies identified a short peptide motif that serves as a docking site for cyclin͞cyclin-dependent kinase (cdk) 2 complexes . Peptides containing this motif block the phosphorylation of substrates by cyclin A͞cdk2 or cyclin E͞cdk2. Here we report that cell membrane-permeable forms of such peptides preferentially induced transformed cells to undergo apoptosis relative to nontransformed cells. Deregulation of E2F family transcription factors is a common event during transformation and was sufficient to sensitize cells to the cyclin͞cdk2 inhibitory peptides. These results suggest that deregulation of E2F and inhibition of cdk2 are synthetically lethal and provide a rationale for the development of cdk2 antagonists as antineoplastic agents.Certain molecular pathways frequently are altered during human carcinogenesis. Therefore one approach to treating cancer, while minimizing host toxicity, would be to develop drugs that preferentially kill cells in which such pathways are altered.An example of a molecular pathway that is recurrently altered in cancer involves the retinoblastoma tumor suppressor protein (pRB) (1-3). Many tumors lack a wild-type RB-1 allele, thus depriving them of pRB. Furthermore, pRB is negatively regulated by cyclin-dependent kinases (cdks). These cdks are, in turn, negatively regulated by certain cdk inhibitors and positively regulated by certain cyclins. These upstream pRB regulators frequently are altered in tumors that retain a wild-type RB-1 allele. Thus, functional inactivation of pRB, a known inhibitor of cell growth, may be a necessary step in human carcinogenesis.Members of the E2F cell-cycle regulatory transcription factor family are critical downstream targets of pRB. Binding to pRB converts E2F from a transcriptional activator to a potent transcriptional repressor. Consequently, E2F-responsive genes are activated in cancer cells because of loss of pRB͞E2F repressor complexes and liberation of free, transcriptionally active E2F. Of note, at least some E2F family members, including E2F1, are themselves transcribed from E2F-responsive promoters (4-7). Thus, pathological activation of E2F responsive genes can establish a positive feedback loop. Paradoxically, forced activation of E2F-responsive genes, such as through the overproduction of E2F1, can induce both cellular proliferation and cell death (apoptosis) (8-11).E2F family members bind to DNA as heterodimers with members of the DP family. The DNA-binding capability of some of these heterodimers is negatively regulated by cyclin A͞cdk2 (12-15). For example, E2F1, E2F2, and E2F3 each contain a short, collinear cyclin A͞cdk2 binding motif that is required for the timely neutralization of E2F DNA-binding capability as cells traverse and prepare to exit S phase (13,16,17). Mutation of this motif in E2F1 enhances its ability to induce apoptosis (13). In summary, E2F is negatively regulated by both pRB and cyclin A͞cdk2. We therefore reasoned that pharmacologic inhibition of cyclin A͞cdk2 might preferentially kill cells in whic...

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