Targeted disruption of CDK4 delays cell cycle entry with enhanced p27(Kip1) activity

"The Octet": Eight Protein Kinases that Control Mammalian DNA Replication

Development of a fertilized human egg into an average sized adult requires about 29 trillion cell divisions, thereby producing enough DNA to stretch to the Sun and back 200 times (DePamphilis and Bell, 2011)! Even more amazing is the fact that throughout these mitotic cell cycles, the human genome is duplicated once and only once each time a cell divides. If a cell accidentally begins to re-replicate its nuclear DNA prior to cell division, checkpoint pathways trigger apoptosis. And yet, some cells are developmentally programmed to respond to environmental cues by switching from mitotic cell cycles to endocycles, a process in which multiple S phases occur in the absence of either mitosis or cytokinesis. Endocycles allow production of viable, differentiated, polyploid cells that no longer proliferate. What is surprising is that among the 516 (Manning et al., 2002) to 557 (BioMart web site) protein kinases encoded by the human genome, only eight regulate nuclear DNA replication directly. These are Cdk1, Cdk2, Cdk4, Cdk6, Cdk7, Cdc7, Checkpoint kinase-1 (Chk1), and Checkpoint kinase-2. Even more remarkable is the fact that only four of these enzymes (Cdk1, Cdk7, Cdc7, and Chk1) are essential for mammalian development. Here we describe how these protein kinases determine when DNA replication occurs during mitotic cell cycles, how mammalian cells switch from mitotic cell cycles to endocycles, and how cancer cells can be selectively targeted for destruction by inducing them to begin a second S phase before mitosis is complete.

Metallofullerol nanoparticles with low toxicity inhibit tumor growth by induction of G0/G1 arrest

AIMS

[Gd@C(82)(OH)(22)](n) is a new type of nanoparticle with potent antineoplastic activity and low toxicity compared with traditional drugs. In this study, we explored, for the first time, the effect of [Gd@C(82)(OH)(22)](n) on the cell cycle using human breast cancer MCF-7 and human umbilical vein endothelial ECV304 cell lines by flow cytometry.

METHODS

Cell viability was assessed through CCK-8 assay, and MCF-7 tumor-bearing mice were examined after 2 weeks of treatment with [Gd@C(82)(OH)(22)](n). Cell cycle-related gene expression was detected by microarray and confirmed by real-time PCR and RNAi.

RESULTS

Cell viability studies confirmed that [Gd@C(82)(OH)(22)](n) inhibits breast cancer effectively with very low toxicity. Flow cytometric data and microarray results reveal that [Gd@C(82)(OH)(22)](n) mediates G0/G1 arrest in both cell lines by regulating the expression of several genes, such as cyclin D2, cyclin E and CDK4, among others, in the related cell cycle.

CONCLUSION

Results further demonstrated that [Gd@C(82)(OH)(22)](n) could inhibit tumor growth by inducing tumor cell and vein endothelial cell G0/G1 arrest, which may explain the low toxicity of [Gd@C(82)(OH)(22)](n).

Type 2 diabetes and polymorphisms on chromosome 9p21: a meta-analysis

BACKGROUND AND AIMS

Genome-wide association studies found some variants on chromosome 9p21 associated with type 2 diabetes (T2D). We performed a meta-analysis to estimate strength, accuracy and feature of the association of polymorphisms in 9p21 with T2D.

METHODS AND RESULTS

Articles were retrieved screening electronic databases and cross references. Twenty-two publications were identified, for a total of 38,455 T2D patients and 60,516 controls. Twenty-one studies investigated the role of the SNP rs10811661; in some studies three additional SNPs (rs564398, rs10757278, rs1333040) were genotyped. Population attributable risk (PAR) was computed as: risk allele frequency∗(OR-1)/OR, using the per-allele odds ratio (OR). The risk allele (T) of rs10811661 was associated with T2D in most of the studies. In meta-analysis the overall per-allele OR was 1.24 (95% CI: 1.21-1.27; P < 10(-15)), with no difference according to ethnicity (P = 0.45), and low heterogeneity (P = 0.040) across studies partly explained by sample size. Modeling of inheritance suggested an additive effect of the T allele. PAR of T2D related to this polymorphism was 15% for Caucasians and 13% for Asians. The overall odds ratio for the T allele of the SNP rs564398 was 1.08 (95% CI: 1.05-1.12; PAR = 6%). The other SNPs showed negligible associations.

CONCLUSIONS

This meta-analysis provides accurate and comprehensive estimates of the association of some genetic variants at chromosome 9p21 and T2D. A relatively small but significant role of the T allele of the rs10811661 SNP in increasing by 21-27% the risk of T2D in an additive way was apparent.

Role of key regulators of the cell cycle in maintenance of hematopoietic stem cells

BACKGROUND

Hematopoietic stem cells (HSCs) are characterized by pluripotentiality and self-renewal ability. To maintain a supply of mature blood cells and to avoid HSC exhaustion during the life span of an organism, most HSCs remain quiescent, with only a limited number entering the cell cycle.

SCOPE OF REVIEW

The molecular mechanisms by which quiescence is maintained in HSCs are addressed, with recent genetic studies having provided important insight into the relation between the cell cycle activity and stemness of HSCs.

MAJOR CONCLUSIONS

The cell cycle is tightly regulated in HSCs by complex factors. Key regulators of the cell cycle in other cell types-including cyclins, cyclin-dependent kinases (CDKs), the retinoblastoma protein family, the transcription factor E2F, and CDK inhibitors-also contribute to such regulation in HSCs. Most, but not all, of these regulators are necessary for maintenance of HSCs, with abnormal activation or suppression of the cell cycle resulting in HSC exhaustion. The cell cycle in HSCs is also regulated by external factors such as cytokines produced by niche cells as well as by the ubiquitin-proteasome pathway.

GENERAL SIGNIFICANCE

Studies of the cell cycle in HSCs may shed light on the pathogenesis of hematopoietic disorders, serve as a basis for the development of new therapeutic strategies for such disorders, prove useful for the expansion of HSCs in vitro as a possible replacement for blood transfusion, and provide insight into stem cell biology in general. This article is part of a Special Issue entitled Biochemistry of Stem Cells.

Thyroid hormone limits postnatal Sertoli cell proliferation in vivo by activation of its alpha1 isoform receptor (TRalpha1) present in these cells and by regulation of Cdk4/JunD/c-myc mRNA levels in mice

Hypo- and hyperthyroidism alter testicular functions in the young. Among T3 receptors, TRalpha1 is ubiquitous, and its previously described knockout leads to an increase in testis weight and sperm production. We tested, for the first time, the hypothesis that TRalpha1-dependent regulation of Sertoli cell (SC) proliferation was directly regulated by TRalpha1 present in these cells. Thus, after crossing with the AMH-Cre line, we generated and analyzed a new line that expressed a dominant-negative TRalpha1 isoform (TRalpha(AMI)) in SCs only. So-called TRalpha(AMI)-SC (TRalpha(AMI/+) Cre(+)) mice exhibited similar phenotypic features to the knockout line: heavier testicular weight and higher sperm reserve, in comparison with their adequate controls (TRalpha(AMI/+) Cre(-)). SC density increased significantly as a result of a higher proliferative index at ages Postnatal Day (P) 0 and P3. When explants of control testes were cultured (at age P3), a significant decrease in the proliferation of SCs was observed in response to an excess of T3. This response was not observed in the TRalpha(AMI)-SC and knockout lines. Finally, when TRalpha(AMI) is present in SCs, the phenotype observed is similar to that of the knockout line. This study demonstrates that T3 limits postnatal SC proliferation by activation of TRalpha1 present in these cells. Moreover, quantitative RT-PCR provided evidence that regulation of the Cdk4/JunD/c-myc pathway was involved in this negative control.

Why minimal is not optimal: driving the mammalian cell cycle--and drug discovery--with a physiologic CDK control network

Progression through the eukaryotic cell division cycle is governed by the activity of cyclin-dependent kinases (CDKs). For a CDK to become active it must (1) bind a positive regulatory subunit (cyclin) and (2) be phosphorylated on its activation (T) loop. In metazoans, multiple CDK catalytic subunits, each with a distinct set of preferred cyclin partners, regulate the cell cycle, but it has been difficult to assign functions to individual CDKs in vivo. Biochemical analyses and experiments with dominant-negative alleles suggested that specific CDK/cyclin complexes regulate different events, but genetic loss of interphase CDKs (Cdk2, -4 and -6), alone or in combination, did not block proliferation of cells in culture. These knockout and knockdown studies suggested redundancy or plasticity built into the CDK network but did not address whether there was true redundancy in normal cells with a full complement of CDKs. Here, we discuss recent work that took a chemical-genetic approach to reveal that the activity of a genetically non-essential CDK, Cdk2, is required for cell proliferation when normal cyclin pairing is maintained. These results have implications for the systems-level organization of the cell cycle, for regulation of the restriction point and G 1/S transition and for efforts to target Cdk2 therapeutically in human cancers.

Enhanced beta cell proliferation in mice overexpressing a constitutively active form of Akt and one allele of p21Cip

AIMS/HYPOTHESIS

The ability of pancreatic beta cells to proliferate is critical both for normal tissue maintenance and in conditions where there is an increased demand for insulin. Protein kinase B(Akt) plays a major role in promoting proliferation in many cell types, including the insulin-producing beta cells. We have previously reported that mice overexpressing a constitutively active form of Akt(caAkt (Tg)) show enhanced beta cell proliferation that is associated with increased protein levels of cyclin D1, cyclin D2 and cyclin-dependent kinase inhibitor 1A (p21(Cip)). In the present study, we sought to assess the mechanisms responsible for augmented p21(Cip) levels in caAkt(Tg) mice and test the role of p21(Cip) in the proliferative responses induced by activation of Akt signalling.

METHODS

To gain a greater understanding of the relationship between Akt and p21(Cip), we evaluated the mechanisms involved in the modulation of p2(Cip) by Akt and the in vivo role of reduced p21(Cip) in proliferative responses induced by Akt.

RESULTS

Our experiments showed that Akt signalling regulates p21(Cip) transcription and protein stability. caAkt(Tg) /p21(Cip+/-) mice exhibited fasting and fed hypoglycaemia as well as hyperinsulinaemia when compared with caAkt(Tg) mice. Glucose tolerance tests revealed improved glucose tolerance in caAkt(Tg)/p21(Cip+/-) mice compared with caAkt (Tg). These changes resulted from increased proliferation, survival and beta cell mass in caAkt(Tg)/p21(Cip+/-) compared with caAkt(Tg) mice.

CONCLUSIONS/INTERPRETATION

Our data indicate that increased p21(Cip) levels in caAkt(Tg) mice act as a compensatory brake, protecting beta cells from unrestrained proliferation. These studies imply that p21(Cip) could play important roles in the adaptive responses of beta cells to proliferate in conditions such as in insulin resistance.

Cycling or not cycling: cell cycle regulatory molecules and adult neurogenesis

The adult brain most probably reaches its highest degree of plasticity with the lifelong generation and integration of new neurons in the hippocampus and olfactory system. Neural precursor cells (NPCs) residing both in the subgranular zone of the dentate gyrus and in the subventricular zone of the lateral ventricles continuously generate neurons that populate the dentate gyrus and the olfactory bulb, respectively. The regulation of NPC proliferation in the adult brain has been widely investigated in the past few years. Yet, the intrinsic cell cycle machinery underlying NPC proliferation remains largely unexplored. In this review, we discuss the cell cycle components that are involved in the regulation of NPC proliferation in both neurogenic areas of the adult brain.

Reduced expression of the LRP16 gene in mouse insulinoma (MIN6) cells exerts multiple effects on insulin content, proliferation and apoptosis

This study assessed the effects of leukemia-related protein 16 (LRP16) on the regulation of pancreatic functions in mouse insulinoma (MIN6) cells. Cells with down-regulated expression of LRP16 were obtained by a shRNA interference strategy. Insulin content and glucose-stimulated insulin secretion (GSIS) were examined by radioimmunoassay. Western blotting was applied to detect protein expression. Glucose-stimulated sub-cellular localization of PDX-1 was immunocytochemically determined. Cell proliferation and apoptosis were detected by flow cytometry. Our results showed that LRP16 regulated insulin content in MIN6 cells by controlling expression of insulin and insulin transcription factors. LRP16 gene silence in MIN6 cells led to reduced cell proliferation and increased apoptosis. The observation of phosphorylation of serine-473 Akt and the localization of PDX-1 to the nucleus under glucose-stimulation exhibited that LRP16 was a component mediating Akt signaling in MIN6 cells. These results suggest that LRP16 plays a key role in maintaining pancreatic β-cell functions and may help us to understand the protective effects of estrogen on the functions of pancreatic β-cells.

Agaricus bisporus lectins mediates islet β-cell proliferation through regulation of cell cycle proteins

This study was designed to determine the therapeutic effect of Agaricus bisporus lectins (ABL) by the regeneration of β-cells in mice following 70% partial pancreatectomy (PPx), and to explore the mechanisms of ABL-induced β-cell proliferation. Adult C57BL/6J mice were subjected to a 70% PPx operation or a sham operation, and mice received 10 mg/kg body weight of ABL or saline immediately after surgery. Blood glucose concentrations and insulin secretion levels were measured. To determine the growth rates of β-cells and duct cells, immunohistological analysis of pancreatic tissues was performed. Key cell cycle proteins and β-cell specific genes were measured by realtime polymerase chain reaction, Western blotting and immunohistological staining. In this study, a significant decrease in blood glucose concentrations, increase in glucose tolerance and expanded β-cell mass were observed in the ABL-treated mice. At the same time, after ABL treatment, increased β-cell proliferation rates were observed. Further studies on the expression of cyclin D1, cyclin D2 and Cdk4 demonstrated that these genes were significantly up-regulated in the ABL-treated mice. Meanwhile, Cdk4 activity was also enhanced. Moreover, the expression of PDX-1 (pancreatic and duodenal homeobox 1), Ngn3 (neurogenin 3), insulin, GLUT-1 (glucose transporter 1) and glucokinase was also increased in the ABL-treated mice. These findings demonstrate that ABL administration could partially reverse the impaired β-cell growth potential by regulating cell cycle proteins. Induction of islet β-cell proliferation by ABL suggests the therapeutic potential in preventing and/or treating diabetes.

Cyclin-dependent kinase 1 (Cdk1) is essential for cell division and suppression of DNA re-replication but not for liver regeneration

Cyclin-dependent kinase 1 (Cdk1) is an archetypical kinase and a central regulator that drives cells through G2 phase and mitosis. Knockouts of Cdk2, Cdk3, Cdk4, or Cdk6 have resulted in viable mice, but the in vivo functions of Cdk1 have not been fully explored in mammals. Here we have generated a conditional-knockout mouse model to study the functions of Cdk1 in vivo. Ablation of Cdk1 leads to arrest of embryonic development around the blastocyst stage. Interestingly, liver-specific deletion of Cdk1 is well tolerated, and liver regeneration after partial hepatectomy is not impaired, indicating that regeneration can be driven by cell growth without cell division. The loss of Cdk1 does not affect S phase progression but results in DNA re-replication because of an increase in Cdk2/cyclin A2 activity. Unlike other Cdks, loss of Cdk1 in the liver confers complete resistance against tumorigenesis induced by activated Ras and silencing of p53.

A defect of the INK4-Cdk4 checkpoint and Myc collaborate in blastoid mantle cell lymphoma-like lymphoma formation in mice

Mantle cell lymphoma (MCL) is a B-cell malignancy characterized by a monoclonal proliferation of lymphocytes with the co-expression of CD5 and CD43, but not of CD23. Typical MCL is associated with overexpression of cyclin D1, and blastoid MCL variants are associated with Myc (alias c-myc) translocations. In this study, we developed a murine model of MCL-like lymphoma by crossing Cdk4(R24C) mice with Myc-3'RR transgenic mice. The Cdk4(R24C) mouse is a knockin strain that expresses a Cdk4 protein that is resistant to inhibition by p16(INK4a) as well as other INK4 family members. Ablation of INK4 control on Cdk4 does not affect lymphomagenesis, B-cell maturation, and functions in Cdk4(R24C) mice. Additionally, B cells were normal in numbers, cell cycle activity, mitogen responsiveness, and Ig synthesis in response to activation. By contrast, breeding Cdk4(R24C) mice with Myc-3'RR transgenic mice prone to develop aggressive Burkitt lymphoma-like lymphoma (CD19(+)IgM(+)IgD(+) cells) leads to the development of clonal blastoid MCL-like lymphoma (CD19(+)IgM(+)CD5(+)CD43(+)CD23(-) cells) in Myc/Cdk4(R24C) mice. Western blot analysis revealed high amounts of Cdk4/cyclin D1 complexes as the main hallmark of these lymphomas. These results indicate that although silent in nonmalignant B cells, a defect in the INK4-Cdk4 checkpoint can participate in lymphomagenesis in conjunction with additional alterations of cell cycle control, a situation that might be reminiscent of the development of human blastoid MCL.

Multiple roles of cyclin-dependent kinase 4/6 inhibitors in cancer therapy

BACKGROUND

Cyclin-dependent kinases (CDKs) regulate cell proliferation and coordinate the cell cycle checkpoint response to DNA damage. Although inhibitors with varying selectivity to specific CDK family members have been developed, selective CDK4/6 inhibitors have emerged as the most attractive antineoplastic agents because of the importance of CDK4/6 activity in regulating cell proliferation and the toxic effects associated with inhibition of other CDKs (eg, CDK1 and CDK2).

METHODS

FVB/N wild-type mice (n = 13) were used to evaluate carboplatin-induced myelosuppression in bone marrow by complete blood cell counts after treatment with the CDK4/6 inhibitor PD0332991. Genetically engineered murine models of retinoblastoma (Rb)-competent (MMTV-c-neu) and Rb-incompetent (C3-TAg) breast cancer (n = 16 MMTV-c-neu mice in the carboplatin plus vehicle control group, n = 17 MMTV-c-neu mice in the carboplatin plus PD0332991 group, n = 17 C3-TAg mice in the carboplatin plus vehicle control group, and n = 14 C3-TAg mice in the carboplatin plus PD0332991 group) were used to investigate the antitumor activity of PD0332991 alone or in combination with chemotherapy. All statistical tests were two-sided.

RESULTS

Coadministration of PD0332991 with carboplatin compared with carboplatin alone in FVB/N wild-type mice increased hematocrit (51.2% vs 33.5%, difference = 17.7%, 95% confidence interval [CI] = -26.7% to -8.6%, P < .001), platelet counts (1321 vs 758.5 thousand cells per μL, difference = 562.5 thousand cells per μL, 95% CI = -902.8 to -222.6, P = .002), myeloid cells (granulocytes and monocytes; 3.1 vs 1.6 thousand cells per μL, difference = 1.5 thousand cells per μL, 95% CI = -2.23 to -0.67, P < .001), and lymphocytes (7.9 vs 5.4 thousand cells per μL, difference = 2.5 thousand cells per μL, 95% CI = -4.75 to -0.18, P = .02). Daily administration of PD0332991 exhibited antitumor activity in MMTV-c-neu mice as a single agent. However, the combination of carboplatin plus PD0332991 decreased antitumor activity compared with carboplatin alone in Rb-competent mice (mean percent change in tumor volume at day 21 = -52.6% vs 3.7% for carboplatin and carboplatin plus PD0332991, respectively, difference = 56.3%, 95% CI = -109.0% to -3.6%, P = .04). In contrast, Rb-deficient tumors in C3-Tag mice were resistant to PD0332991, and coadministration of PD0332991 plus carboplatin had no effect on in vivo tumor growth (mean percent change in tumor volume at day 21 = 118.8% and 109.1% for carboplatin and carboplatin plus PD0332991, respectively, difference = 9.7%, 95% CI = -183.5% to 202.9%, P = .92). Finally, in tumor-bearing mice, coadministration of PD0332991 with carboplatin provided statistically significant protection of platelets (P = .04).

CONCLUSION

We believe that the present data support a possible role for CDK4/6 inhibitors in a majority of patients with advanced cancer: to either inhibit tumor growth in CDK4/6-dependent tumors or ameliorate the dose-limiting toxicities of chemotherapy in CDK4/6-indepdendent tumors. Our data also suggest CDK4/6 inhibitors should not be combined with DNA-damaging therapies, such as carboplatin, to treat tumors that require CDK4/6 activity for proliferation.

A simple matter of life and death-the trials of postnatal Beta-cell mass regulation

Pancreatic beta-cells, which secrete the hormone insulin, are the key arbiters of glucose homeostasis. Defective beta-cell numbers and/or function underlie essentially all major forms of diabetes and must be restored if diabetes is to be cured. Thus, the identification of the molecular regulators of beta-cell mass and a better understanding of the processes of beta-cell differentiation and proliferation may provide further insight for the development of new therapeutic targets for diabetes. This review will focus on the principal hormones and nutrients, as well as downstream signalling pathways regulating beta-cell mass in the adult. Furthermore, we will also address more recently appreciated regulators of beta-cell mass, such as microRNAs.

Regulation of EGFR protein stability by the HECT-type ubiquitin ligase SMURF2

Epidermal growth factor receptor (EGFR) is overexpressed in a variety of epithelial tumors and is considered to be an important therapeutic target. Although gene amplification is responsible for EGFR overexpression in certain human malignancies including lung and head and neck cancers, additional molecular mechanisms are likely. Here, we report a novel interaction of EGFR with an HECT-type ubiquitin ligase SMURF2, which can ubiquitinate, but stabilize EGFR by protecting it from c-Cbl-mediated degradation. Conversely, small interfering RNA (siRNA)-mediated knockdown of SMURF2 destabilized EGFR, induced an autophagic response and reduced the clonogenic survival of EGFR-expressing cancer cell lines, with minimal effects on EGFR-negative cancer cells, normal fibroblasts, and normal epithelial cells. UMSCC74B head and neck squamous cancer cells, which form aggressive tumors in nude mice, significantly lost in vivo tumor-forming ability on siRNA-mediated SMURF2 knockdown. Gene expression microarray data from 443 lung adenocarcinoma patients, and tissue microarray data from 67 such patients, showed a strong correlation of expression between EGFR and SMURF2 at the messenger RNA and protein levels, respectively. Our findings suggest that SMURF2-mediated protective ubiquitination of EGFR may be responsible for EGFR overexpression in certain tumors and support targeting SMURF2-EGFR interaction as a novel therapeutic approach in treating EGFR-addicted tumors.

Cdk6-dependent regulation of G(1) length controls adult neurogenesis

The presence of neurogenic precursors in the adult mammalian brain is now widely accepted, but the mechanisms coupling their proliferation with the onset of neuronal differentiation remain unknown. Here, we unravel the major contribution of the G(1) regulator cyclin-dependent kinase 6 (Cdk6) to adult neurogenesis. We found that Cdk6 was essential for cell proliferation within the dentate gyrus of the hippocampus and the subventricular zone of the lateral ventricles. Specifically, Cdk6 deficiency prevents the expansion of neuronally committed precursors by lengthening G(1) phase duration, reducing concomitantly the production of newborn neurons. Altogether, our data support G(1) length as an essential regulator of the switch between proliferation and neuronal differentiation in the adult brain and Cdk6 as one intrinsic key molecular regulator of this process.

Physiological relevance of cell cycle kinases

The basic biology of the cell division cycle and its control by protein kinases was originally studied through genetic and biochemical studies in yeast and other model organisms. The major regulatory mechanisms identified in this pioneer work are conserved in mammals. However, recent studies in different cell types or genetic models are now providing a new perspective on the function of these major cell cycle regulators in different tissues. Here, we review the physiological relevance of mammalian cell cycle kinases such as cyclin-dependent kinases (Cdks), Aurora and Polo-like kinases, and mitotic checkpoint regulators (Bub1, BubR1, and Mps1) as well as other less-studied enzymes such as Cdc7, Nek proteins, or Mastl and their implications in development, tissue homeostasis, and human disease. Among these functions, the control of self-renewal or asymmetric cell division in stem/progenitor cells and the ability to regenerate injured tissues is a central issue in current research. In addition, many of these proteins play previously unexpected roles in metabolism, cardiovascular function, or neuron biology. The modulation of their enzymatic activity may therefore have multiple therapeutic benefits in human disease.

Established and novel Cdk/cyclin complexes regulating the cell cycle and development

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.

Cyclin D1 and cdk4 mediate development of neurologically destructive oligodendroglioma

Although the molecular changes that characterize gliomas have been studied, the pathogenesis of tumor development remains unclear. p21 contributes to gliomagenesis by stabilizing cyclin D1-cdk4 kinase complexes, suggesting that cyclin D1 and cdk4 may also be required for glial tumor development. In this study, we used a mouse model to attempt to confirm this hypothesis, finding that cyclin D1 and cdk4 played active roles in not only the tumor but also the tumor microenvironment. Loss of cdk4 blocked tumor development, but loss of cyclin D1 did not prevent gliomas from developing. Instead, loss of cyclin D1 impeded progression to higher stages of malignancy. Enforcing expression of cyclin D1 was insufficient to correct the progression defect observed in cyclin D1-deficient animals. In contrast, restoration of cdk4 in the cdk4-deficient animals restored cell proliferation and tumor formation, although at lower tumor grades. Notably, the failure of tumors in the cyclin D1- and cdk4-deficient animals to progress to higher grades was correlated with a failure to fully activate microglia in the tumor microenvironment. Moreover, when platelet-derived growth factor-transformed glial cells were engrafted orthotopically into the mice, the tumors that formed progressed to high grades in wild-type mice but not cyclin D1-deficient animals. Together, our findings establish that the cyclin D1-cdk4 axis is not only critical in glial tumor cells but also in stromal-derived cells in the surrounding tumor microenvironment that are vital to sustain tumor outgrowth.

Expression of arf tumor suppressor in spermatogonia facilitates meiotic progression in male germ cells

The mammalian Cdkn2a (Ink4a-Arf) locus encodes two tumor suppressor proteins (p16^Ink4a and p19^Arf) that respectively enforce the anti-proliferative functions of the retinoblastoma protein (Rb) and the p53 transcription factor in response to oncogenic stress. Although p19^Arf is not normally detected in tissues of young adult mice, a notable exception occurs in the male germ line, where Arf is expressed in spermatogonia, but not in meiotic spermatocytes arising from them. Unlike other contexts in which the induction of Arf potently inhibits cell proliferation, expression of p19^Arf in spermatogonia does not interfere with mitotic cell division. Instead, inactivation of Arf triggers germ cell–autonomous, p53-dependent apoptosis of primary spermatocytes in late meiotic prophase, resulting in reduced sperm production. Arf deficiency also causes premature, elevated, and persistent accumulation of the phosphorylated histone variant H2AX, reduces numbers of chromosome-associated complexes of Rad51 and Dmc1 recombinases during meiotic prophase, and yields incompletely synapsed autosomes during pachynema. Inactivation of Ink4a increases the fraction of spermatogonia in S-phase and restores sperm numbers in Ink4a-Arf doubly deficient mice but does not abrogate γ-H2AX accumulation in spermatocytes or p53-dependent apoptosis resulting from Arf inactivation. Thus, as opposed to its canonical role as a tumor suppressor in inducing p53-dependent senescence or apoptosis, Arf expression in spermatogonia instead initiates a salutary feed-forward program that prevents p53-dependent apoptosis, contributing to the survival of meiotic male germ cells.

The intimately linked Arf and Ink4a genes, encoded in part by overlapping reading frames within the Cdkn2a locus, are induced by oncogenic stress, activating the p53 and Rb tumor suppressors, respectively, to inhibit proliferation of incipient cancer cells. As such, expression of the p19^Arf and p16^Ink4a proteins is undetected in most normal mouse tissues. However, p19^Arf is physiologically expressed in mitotically dividing spermatogonia, the progenitor cells that differentiate to form meiotic spermatocytes in which Arf expression is extinguished. We show that, instead of provoking cell cycle arrest or death, Arf expression in spermatogonia facilitates survival of their meiotic progeny, ensuring production of normal numbers of mature sperm. When Arf is ablated, meiotic defects ensue, along with p53-dependent cell death of spermatocytes, indicating an unexpected role of p53 in monitoring meiotic progression. Surprisingly, it is the absence of p19^Arf rather than its induction that enforces p53 expression in this setting. Co-inactivation of Ink4a compensates for Arf loss by fueling proliferation of spermatogonial progenitors, but does not correct meiotic defects triggered by Arf loss. Although the Arf and Ink4a tumor suppressors are expected to restrain cellular self-renewal, Arf plays an unexpected role in male germ cells by facilitating their proper meiotic progression.

Cdk2 loss accelerates precursor differentiation and remyelination in the adult central nervous system

Cdk2 is not essential for oligodendrocyte maturation and myelination during development, but in response to demyelination, it is required for oligodendrocyte precursor cell proliferation (OPC), and its loss accelerates OPC differentiation and remyelination.

The specific functions of intrinsic regulators of oligodendrocyte progenitor cell (OPC) division are poorly understood. Type 2 cyclin-dependent kinase (Cdk2) controls cell cycle progression of OPCs, but whether it acts during myelination and repair of demyelinating lesions remains unexplored. Here, we took advantage of a viable Cdk2^−/− mutant mouse to investigate the function of this cell cycle regulator in OPC proliferation and differentiation in normal and pathological conditions. During central nervous system (CNS) development, Cdk2 loss does not affect OPC cell cycle, oligodendrocyte cell numbers, or myelination. However, in response to CNS demyelination, it clearly alters adult OPC renewal, cell cycle exit, and differentiation. Importantly, Cdk2 loss accelerates CNS remyelination of demyelinated axons. Thus, Cdk2 is dispensable for myelination but is important for adult OPC renewal, and could be one of the underlying mechanisms that drive adult progenitors to differentiate and thus regenerate myelin.

Cyclin D as a therapeutic target in cancer

Cyclin D1, and to a lesser extent the other D-type cyclins, is frequently deregulated in cancer and is a biomarker of cancer phenotype and disease progression. The ability of these cyclins to activate the cyclin-dependent kinases (CDKs) CDK4 and CDK6 is the most extensively documented mechanism for their oncogenic actions and provides an attractive therapeutic target. Is this an effective means of targeting the cyclin D oncogenes, and how might the patient subgroups that are most likely to benefit be identified?

Regulation of cyclin-dependent kinase 4 translation through CUG-binding protein 1 and microRNA-222 by polyamines

The amino acid-derived polyamines are organic cations that are essential for growth in all mammalian cells, but their exact roles at the molecular level remain largely unknown. Here we provide evidence that polyamines promote the translation of cyclin-dependent kinase 4 (CDK4) by the action of CUG-binding protein 1 (CUGBP1) and microRNA-222 (miR-222) in intestinal epithelial cells. Both CUGBP1 and miR-222 were found to bind the CDK4 mRNA coding region and 3'-untranslated region and repressed CDK4 translation synergistically. Depletion of cellular polyamines increased cytoplasmic CUGBP1 abundance and miR-222 levels, induced their associations with the CDK4 mRNA, and inhibited CDK4 translation, whereas increasing the levels of cellular polyamines decreased CDK4 mRNA interaction with CUGBP1 and miR-222, in turn inducing CDK4 expression. Polyamine-deficient cells exhibited an increased colocalization of tagged CDK4 mRNA with processing bodies; this colocalization was abolished by silencing CUGBP1 and miR-222. Together, our findings indicate that polyamine-regulated CUGBP1 and miR-222 modulate CDK4 translation at least in part by altering the recruitment of CDK4 mRNA to processing bodies.

Cdc2: a monopotent or pluripotent CDK?

Cell cycle progression is controlled by both extracellular and intracellular signalling molecules. It has been generally believed that cdc2/CDK1 only control G(2)-M transition in mammalian and many other higher eukaryotic cells. Accumulating evidence shows that cdc2 not only promotes G(2)-M transition but is also capable of regulating G(1) progress and G(1)-S transition via association with multiple interphase cyclins; cdc2 activity can be inhibited by p21 and p27, two traditional G(1) CDK inhibitors. In addition, cdc2-cyclin B controls pronuclear union in interphase fertilized eggs. These data suggest that cdc2 may be a pluripotent CDK. Although mechanisms responsible for the multiple functions of cdc2 remain to be further investigated, interactions of cdc2 with pRb and with several important transcription factors may provide a clue to the pluripotent role of cdc2.

Association between type 2 diabetes and CDKN2A/B: a meta-analysis study

Cyclin-dependent kinase inhibitor-2A/B (CDKN2A/B) has been reported as a candidate gene of type 2 diabetes (T2D) based on its chromosomal position and its important role in β-cell function and regeneration. However, studies to date have reported inconsistent findings regarding the association between T2D and CDKN2A/B. To clarify this inconsistence, we conducted a meta-analysis based on alleles and genotypes prevalence of rs10811661 and rs564398 in CDKN2A/B. The PubMed, EMBASE, and Medline databases were systematically reviewed for studies published between January, 2006, and November, 2010. A total of 35 reports were collected, among of them only 16 studies (including 24,407 cases and 33,937 controls) match the inclusion criteria and were selected for the statistical test. In the meta-analysis of published data, our results suggest that the rs10811661 T allele (OR 1.28, 95% CI 1.21-1.36, P < 1 × 10(-5)) and TT genotype (OR 1.32, 95% CI 1.22-1.43, P < 1 × 10(-5)) of CDKN2A/B were associated with type 2 diabetes respectively, but rs564398 was not (for allele only: OR 0.96, 95% CI 0.88-1.05, P = 0.35). The association between rs10811661 T allele and T2D was observed both in Asia (P < 1 × 10(-4)) and Europe ethnicity groups (P = 0.002). This meta-analysis yielded evidence that rs10811661 of CDKN2A/B confers risk for T2D. Larger studies with mixed ethnicity subjects are required to validate our findings.

Inhibitor of cyclin-dependent kinase (CDK) interacting with cyclin A1 (INCA1) regulates proliferation and is repressed by oncogenic signaling

The cell cycle is driven by the kinase activity of cyclin·cyclin-dependent kinase (CDK) complexes, which is negatively regulated by CDK inhibitor proteins. Recently, we identified INCA1 as an interaction partner and a substrate of cyclin A1 in complex with CDK2. On a functional level, we identified a novel cyclin-binding site in the INCA1 protein. INCA1 inhibited CDK2 activity and cell proliferation. The inhibitory effects depended on the cyclin-interacting domain. Mitogenic and oncogenic signals suppressed INCA1 expression, whereas it was induced by cell cycle arrest. We established a deletional mouse model that showed increased CDK2 activity in spleen with altered spleen architecture in Inca1(-/-) mice. Inca1(-/-) embryonic fibroblasts showed an increase in the fraction of S-phase cells. Furthermore, blasts from acute lymphoid leukemia and acute myeloid leukemia patients expressed significantly reduced INCA1 levels highlighting its relevance for growth control in vivo. Taken together, this study identifies a novel CDK inhibitor with reduced expression in acute myeloid and lymphoid leukemia. The molecular events that control the cell cycle occur in a sequential process to ensure a tight regulation, which is important for the survival of a cell and includes the detection and repair of genetic damage and the prevention of uncontrolled cell division.

CDK-mediated regulation of cell functions via c-Jun phosphorylation and AP-1 activation

Cyclin-dependent kinases (CDKs) and their targets have been primarily associated with regulation of cell-cycle progression. Here we identify c-Jun, a transcription factor involved in the regulation of a broad spectrum of cellular functions, as a newly recognized CDK substrate. Using immune cells from mouse and human, and several complementary in vitro and in vivo approaches including dominant negative protein expression, pharmacologic inhibitors, kinase assays and CDK4 deficient cells, we demonstrate the ability of CDK4 to phosphorylate c-Jun. Additionally, the activity of AP-1, a ubiquitous transcription factor containing phosphorylated c-Jun as a subunit, was inhibited by abrogating CDK4. Surprisingly, the regulation of c-Jun phosphorylation by CDK4 occurred in non-dividing cells, indicating that this pathway is utilized for cell functions that are independent of proliferation. Our studies identify a new substrate for CDK4 and suggest a mechanism by which CDKs can regulate multiple cellular activation functions, not all of which are directly associated with cell cycle progression. These findings point to additional roles of CDKs in cell signaling and reveal potential implications for therapeutic manipulations of this kinase pathway.

The Cdk4-E2f1 pathway regulates early pancreas development by targeting Pdx1+ progenitors and Ngn3+ endocrine precursors

Cell division and cell differentiation are intricately regulated processes vital to organ development. Cyclin-dependent kinases (Cdks) are master regulators of the cell cycle that orchestrate the cell division and differentiation programs. Cdk1 is essential to drive cell division and is required for the first embryonic divisions, whereas Cdks 2, 4 and 6 are dispensable for organogenesis but vital for tissue-specific cell development. Here, we illustrate an important role for Cdk4 in regulating early pancreas development. Pancreatic development involves extensive morphogenesis, proliferation and differentiation of the epithelium to give rise to the distinct cell lineages of the adult pancreas. The cell cycle molecules that specify lineage commitment within the early pancreas are unknown. We show that Cdk4 and its downstream transcription factor E2f1 regulate mouse pancreas development prior to and during the secondary transition. Cdk4 deficiency reduces embryonic pancreas size owing to impaired mesenchyme development and fewer Pdx1(+) pancreatic progenitor cells. Expression of activated Cdk4(R24C) kinase leads to increased Nkx2.2(+) and Nkx6.1(+) cells and a rise in the number and proliferation of Ngn3(+) endocrine precursors, resulting in expansion of the β cell lineage. We show that E2f1 binds and activates the Ngn3 promoter to modulate Ngn3 expression levels in the embryonic pancreas in a Cdk4-dependent manner. These results suggest that Cdk4 promotes β cell development by directing E2f1-mediated activation of Ngn3 and increasing the pool of endocrine precursors, and identify Cdk4 as an important regulator of early pancreas development that modulates the proliferation potential of pancreatic progenitors and endocrine precursors.

Glucose tolerance, insulin sensitivity and insulin release in European non-diabetic carriers of a polymorphism upstream of CDKN2A and CDKN2B

AIMS/HYPOTHESIS

The aim of this study was to investigate the association of the rs10811661 polymorphism near the CDKN2B/CDKN2A genes with glucose tolerance, insulin sensitivity and insulin release in three samples of white people with European ancestry.

METHODS

Sample 1 comprised 845 non-diabetic offspring of type 2 diabetes patients recruited in five European centres participating in the EUGENE2 study. Samples 2 and 3 comprised, respectively, 864 and 524 Italian non-diabetic participants. All individuals underwent an OGTT. Screening for the rs10811661 polymorphism was performed using a TaqMan allelic discrimination assay.

RESULTS

The rs10811661 polymorphism did not show a significant association with age, BMI and insulin sensitivity. Participants carrying the TT genotype showed a significant reduction in insulin release, measured by an OGTT-derived index, compared with carriers of the C allele, in the three samples. When these results were pooled with those of three published studies, and meta-analysed with a random-effects model, the T allele was significantly associated with reduced insulin secretion (-35.09 [95% CI 14.68-55.52], p = 0.0008 for CC+CT vs TT; and -29.45 [95% CI 9.51-49.38], p = 0.0038, for the additive model). In addition, in our three samples, participants carrying the TT genotype exhibited an increased risk for impaired glucose tolerance (IGT) compared with carriers of the C allele (OR 1.55 [95% CI 1.20-1.95] for the meta-analysis of the three samples).

CONCLUSIONS/INTERPRETATION

Our data, together with the meta-analysis of previously published studies, show that the rs10811661 polymorphism is associated with impaired insulin release and IGT, suggesting that this variant may contribute to type 2 diabetes by affecting beta cell function.

Δ40 Isoform of p53 controls β-cell proliferation and glucose homeostasis in mice

OBJECTIVE

Investigating the dynamics of pancreatic β-cell mass is critical for developing strategies to treat both type 1 and type 2 diabetes. p53, a key regulator of the cell cycle and apoptosis, has mostly been a focus of investigation as a tumor suppressor. Although p53 alternative transcripts can modulate p53 activity, their functions are not fully understood. We hypothesized that β-cell proliferation and glucose homeostasis were controlled by Δ40p53, a p53 isoform lacking the transactivation domain of the full-length protein that modulates total p53 activity and regulates organ size and life span in mice.

RESEARCH DESIGN AND METHODS

We phenotyped metabolic parameters in Δ40p53 transgenic (p44tg) mice and used quantitative RT-PCR, Western blotting, and immunohistochemistry to examine β-cell proliferation.

RESULTS

Transgenic mice with an ectopic p53 gene encoding Δ40p53 developed hypoinsulinemia and glucose intolerance by 3 months of age, which worsened in older mice and led to overt diabetes and premature death from ∼14 months of age. Consistent with a dramatic decrease in β-cell mass and reduced β-cell proliferation, lower expression of cyclin D2 and pancreatic duodenal homeobox-1, two key regulators of proliferation, was observed, whereas expression of the cell cycle inhibitor p21, a p53 target gene, was increased.

CONCLUSIONS

These data indicate a significant and novel role for Δ40p53 in β-cell proliferation with implications for the development of age-dependent diabetes.

RB regulates pancreas development by stabilizing Pdx1

RB is a key substrate of Cdks and an important regulator of the mammalian cell cycle. RB either represses E2Fs that promote cell proliferation or enhances the activity of cell-specific factors that promote differentiation, although the mechanism that facilitates this dual interaction is unclear. Here, we demonstrate that RB associates with and stabilizes pancreatic duodenal homeobox-1 (Pdx-1) that is essential for embryonic pancreas development and adult β-cell function. Interestingly, Pdx-1 utilizes a conserved RB-interaction motif (RIM) that is also present in E2Fs. Point mutations within the RIM reduce RB-Pdx-1 complex formation, destabilize Pdx-1 and promote its proteasomal degradation. Glucose regulates RB and Pdx-1 levels, RB/Pdx-1 complex formation and Pdx-1 degradation. RB occupies the promoters of β-cell-specific genes, and knockdown of RB results in reduced expression of Pdx-1 and its target genes. Further, RB-deficiency in vivo results in reduced pancreas size due to decreased proliferation of Pdx-1(+) pancreatic progenitors, increased apoptosis and aberrant expression of regulators of pancreatic development. These results demonstrate an unanticipated regulatory mechanism for pancreatic development and β-cell function, which involves RB-mediated stabilization of the pancreas-specific transcription factor Pdx-1.

Cycling through metabolism

Since the discovery of cyclins, the role of cell cycle regulators in the control of cell proliferation has been extensively studied. It is clear that proliferation requires an adapted metabolic response of the cells; hence the regulation of cell cycle must be linked to metabolic control. While at a much slower pace, the impact that the activities of cell cycle regulators such as cyclins, cyclin dependent kinases or E2F factor, transcription factor have on cell metabolism are also being uncovered. Here we will focus on recent data implicating cell cycle regulators in metabolic control, with particular attention to studies performed using mouse models. Furthermore, we will discuss the possible relevance of these findings in the context of metabolic disorders such as obesity or diabetes.

Unexpected reduction of skin tumorigenesis on expression of cyclin-dependent kinase 6 in mouse epidermis

Cyclin-dependent kinases (CDKs) 4 and 6 are important regulators of the G(1) phase of the cell cycle, share 71% amino acid identity, and are expressed ubiquitously. As a result, it was assumed that each of these kinases plays a redundant role regulating normal and neoplastic proliferation. In previous reports, we have described the effects of CDK4 expression in transgenic mice, including the development of epidermal hyperplasia and increased malignant progression to squamous cell carcinoma. To study the role of CDK6 in epithelial growth and tumorigenesis, we generated transgenic mice carrying the CDK6 gene under the keratin 5 promoter (K5CDK6). Similar to K5CDK4 mice, epidermal proliferation increased substantially in K5CDK6 mice; however, no hyperplasia was observed. CDK6 overexpression also triggered keratinocyte apoptosis in interfollicular and follicular epidermis as a compensatory mechanism to override aberrant proliferation. Unexpectedly, CDK6 overexpression results in decreased skin tumor development compared with wild-type siblings. The inhibition in skin tumorigenesis was similar to that previously reported in K5-cyclin D3 mice. Furthermore, biochemical analysis of the K5CDK6 epidermis showed preferential complex formation between CDK6 and cyclin D3, suggesting that this particular complex plays an important role in tumor restraint. These studies provide in vivo evidence that CDK4 and CDK6 play a similar role as a mediator of keratinocyte proliferation but differ in apoptosis activation and skin tumor development.

Role of Cdk4 in lymphocyte function and allergen response

We recently described a new adhesion pathway in lymphocytes that is dependent on Cyclin-dependent kinase (Cdk) 4 activity and mediates lymphocyte interactions with endothelial matrix. We showed that Cdk4(-/-) mice had impaired recruitment of lymphocytes following bleomycin model of acute lung injury. In this study, we characterized the development and function of hematopoietic cells in Cdk4(-/-) mice and assessed the response of Cdk4(-/-) mice to allergen challenge. Cdk4(-/-) mice had hypoplastic thymuses with decreased total thymocyte cell numbers and increased CD4/CD8 double negative cells. Cdk4(-/-) bone marrow (BM) chimeric mice showed similar findings. Thymocytes from either Cdk4(-/-) or Cdk4(-/-) BM chimeric mice proliferated equally well as wild type controls in response to IL-2 activation. However Cdk4(-/-) thymocytes had decreased adhesion to both endothelial cell matrix and fibronectin compared to wildtype (WT) controls, whereas Cdk4(-/-) and WT splenocytes had similar adhesion. When Cdk4(-/-) BM chimeric mice and wild type BM chimeric mice were sensitized and challenged by intranasal administration of ovalbumin, we found no differences in allergic responses in the lung and airways between the two groups, as measured by inflammatory cell infiltrate, airway hyperreactivity, IgE levels and cytokine levels. In summary, we show that Cdk4 plays a previously unrecognized role in thymocyte maturation and adhesion, but is not required for thymocyte proliferation. In addition, Cdk4 is not required for lymphocyte trafficking to the lung following allergen sensitization and challenge.

A novel pyrazolo[1,5-a]pyrimidine is a potent inhibitor of cyclin-dependent protein kinases 1, 2, and 9, which demonstrates antitumor effects in human tumor xenografts following oral administration

Cyclin-dependent protein kinases (CDKs) are central to the appropriate regulation of cell proliferation, apoptosis, and gene expression. Abnormalities in CDK activity and regulation are common features of cancer, making CDK family members attractive targets for the development of anticancer drugs. Here, we report the identification of a pyrazolo[1,5-a]pyrimidine derived compound, 4k (BS-194), as a selective and potent CDK inhibitor, which inhibits CDK2, CDK1, CDK5, CDK7, and CDK9 (IC₅₀= 3, 30, 30, 250, and 90 nmol/L, respectively). Cell-based studies showed inhibition of the phosphorylation of CDK substrates, Rb and the RNA polymerase II C-terminal domain, down-regulation of cyclins A, E, and D1, and cell cycle block in the S and G₂/M phases. Consistent with these findings, 4k demonstrated potent antiproliferative activity in 60 cancer cell lines tested (mean GI₅₀= 280 nmol/L). Pharmacokinetic studies showed that 4k is orally bioavailable, with an elimination half-life of 178 min following oral dosing in mice. When administered at a concentration of 25 mg/kg orally, 4k inhibited human tumor xenografts and suppressed CDK substrate phosphorylation. These findings identify 4k as a novel, potent CDK selective inhibitor with potential for oral delivery in cancer patients.

A synthetic lethal interaction between K-Ras oncogenes and Cdk4 unveils a therapeutic strategy for non-small cell lung carcinoma

We have unveiled a synthetic lethal interaction between K-Ras oncogenes and Cdk4 in a mouse tumor model that closely recapitulates human non-small cell lung carcinoma (NSCLC). Ablation of Cdk4, but not Cdk2 or Cdk6, induces an immediate senescence response only in lung cells that express an endogenous K-Ras oncogene. No such response occurs in lungs expressing a single Cdk4 allele or in other K-Ras-expressing tissues. More importantly, targeting Cdk4 alleles in advanced tumors detectable by computed tomography scanning also induces senescence and prevents tumor progression. These observations suggest that robust and selective pharmacological inhibition of Cdk4 may provide therapeutic benefit for NSCLC patients carrying K-RAS oncogenes.

Cyclin D2 is essential for the compensatory beta-cell hyperplastic response to insulin resistance in rodents

OBJECTIVE

A major determinant of the progression from insulin resistance to the development of overt type 2 diabetes is a failure to mount an appropriate compensatory beta-cell hyperplastic response to maintain normoglycemia. We undertook the present study to directly explore the significance of the cell cycle protein cyclin D2 in the expansion of beta-cell mass in two different models of insulin resistance.

RESEARCH DESIGN AND METHODS

We created compound knockouts by crossing mice deficient in cyclin D2 (D2KO) with either the insulin receptor substrate 1 knockout (IRS1KO) mice or the insulin receptor liver-specific knockout mice (LIRKO), neither of which develops overt diabetes on its own because of robust compensatory beta-cell hyperplasia. We phenotyped the double knockouts and used RT-qPCR and immunohistochemistry to examine beta-cell mass.

RESULTS

Both compound knockouts, D2KO/LIRKO and D2KO/IRS1KO, exhibited insulin resistance and hyperinsulinemia and an absence of compensatory beta-cell hyperplasia. However, the diabetic D2KO/LIRKO group rapidly succumbed early compared with a relatively normal lifespan in the glucose-intolerant D2KO/IRS1KO mice.

CONCLUSIONS

This study provides direct genetic evidence that cyclin D2 is essential for the expansion of beta-cell mass in response to a spectrum of insulin resistance and points to the cell-cycle protein as a potential therapeutic target that can be harnessed for preventing and curing type 2 diabetes.

Nucleophosmin phosphorylation by v-cyclin-CDK6 controls KSHV latency

Nucleophosmin (NPM) is a multifunctional nuclear phosphoprotein and a histone chaperone implicated in chromatin organization and transcription control. Oncogenic Kaposi's sarcoma herpesvirus (KSHV) is the etiological agent of Kaposi's sarcoma, primary effusion lymphoma (PEL) and multicentric Castleman disease (MCD). In the infected host cell KSHV displays two modes of infection, the latency and productive viral replication phases, involving extensive viral DNA replication and gene expression. A sustained balance between latency and reactivation to the productive infection state is essential for viral persistence and KSHV pathogenesis. Our study demonstrates that the KSHV v-cyclin and cellular CDK6 kinase phosphorylate NPM on threonine 199 (Thr199) in de novo and naturally KSHV-infected cells and that NPM is phosphorylated to the same site in primary KS tumors. Furthermore, v-cyclin-mediated phosphorylation of NPM engages the interaction between NPM and the latency-associated nuclear antigen LANA, a KSHV-encoded repressor of viral lytic replication. Strikingly, depletion of NPM in PEL cells leads to viral reactivation, and production of new infectious virus particles. Moreover, the phosphorylation of NPM negatively correlates with the level of spontaneous viral reactivation in PEL cells. This work demonstrates that NPM is a critical regulator of KSHV latency via functional interactions with v-cyclin and LANA.

Latency is the predominant mode of viral persistence in KS and PEL tumors, and has a fundamental impact on KSHV tumorigenesis. Establishment and maintenance of latency involves a number of viral and cellular factors. This study provides a novel functional link between LANA and v-cyclin by showing that phosphorylation of nucleophosmin (NPM) by the v-cyclin-CDK6 kinase complex supports its interaction with LANA, and thus enables the transcriptional silencing of KSHV lytic genes needed for latency. These findings indicate that KSHV has evolved mechanisms to utilize host proteins for maintaining the latency, and underscores the role of NPM as a regulator of not only mammalian transcription but also of viral transcription. Taken together, our data suggests that a cellular protein, NPM, is a critical factor for the latency of this oncogenic human virus, and may thus represent an attractive novel target for intervention.

Normal proliferation and tumorigenesis but impaired pancreatic function in mice lacking the cell cycle regulator sei1

Sei1 is a positive regulator of proliferation that promotes the assembly of Cdk4-cyclin D complexes and enhances the transcriptional activity of E2f1. The potential oncogenic role of Sei1 is further suggested by its overexpression in various types of human cancers. To study the role of Sei1, we have generated a mouse line deficient for this gene. Sei1-null fibroblasts did not show abnormalities regarding proliferation or susceptibility to neoplastic transformation, nor did we observe defects on Cdk4 complexes or E2f activity. Sei1-null mice were viable, did not present overt pathologies, had a normal lifespan, and had a normal susceptibility to spontaneous and chemically-induced cancer. Pancreatic insulin-producing cells are known to be particularly sensitive to Cdk4-cyclin D and E2f activities, and we have observed that Sei1 is highly expressed in pancreatic islets compared to other tissues. Interestingly, Sei1-null mice present lower number of islets, decreased beta-cell area, impaired insulin secretion, and glucose intolerance. These defects were associated to nuclear accumulation of the cell-cycle inhibitors p21(Cip1) and p27(Kip1) in islet cells. We conclude that Sei1 plays an important role in pancreatic beta-cells, which supports a functional link between Sei1 and the core cell cycle regulators specifically in the context of the pancreas.

Cdk4 regulates recruitment of quiescent beta-cells and ductal epithelial progenitors to reconstitute beta-cell mass

Insulin-producing pancreatic islet β cells (β-cells) are destroyed, severely depleted or functionally impaired in diabetes. Therefore, replacing functional β-cell mass would advance clinical diabetes management. We have previously demonstrated the importance of Cdk4 in regulating β-cell mass. Cdk4-deficient mice display β-cell hypoplasia and develop diabetes, whereas β-cell hyperplasia is observed in mice expressing an active Cdk4R24C kinase. While β-cell replication appears to be the primary mechanism responsible for β-cell mass increase, considerable evidence also supports a contribution from the pancreatic ductal epithelium in generation of new β-cells. Further, while it is believed that majority of β-cells are in a state of ‘dormancy’, it is unclear if and to what extent the quiescent cells can be coaxed to participate in the β-cell regenerative response. Here, we address these queries using a model of partial pancreatectomy (PX) in Cdk4 mutant mice. To investigate the kinetics of the regeneration process precisely, we performed DNA analog-based lineage-tracing studies followed by mathematical modeling. Within a week after PX, we observed considerable proliferation of islet β-cells and ductal epithelial cells. Interestingly, the mathematical model showed that recruitment of quiescent cells into the active cell cycle promotes β-cell mass reconstitution in the Cdk4R24C pancreas. Moreover, within 24–48 hours post-PX, ductal epithelial cells expressing the transcription factor Pdx-1 dramatically increased. We also detected insulin-positive cells in the ductal epithelium along with a significant increase of islet-like cell clusters in the Cdk4R24C pancreas. We conclude that Cdk4 not only promotes β-cell replication, but also facilitates the activation of β-cell progenitors in the ductal epithelium. In addition, we show that Cdk4 controls β-cell mass by recruiting quiescent cells to enter the cell cycle. Comparing the contribution of cell proliferation and islet-like clusters to the total increase in insulin-positive cells suggests a hitherto uncharacterized large non-proliferative contribution.

Absence of p53-dependent apoptosis combined with nonhomologous end-joining deficiency leads to a severe diabetic phenotype in mice

OBJECTIVE

Regulation of pancreatic beta-cell mass is essential to preserve sufficient insulin levels for the maintenance of glucose homeostasis. Previously, we reported that DNA double-strand breaks (DSBs) resulting from nonhomologous end-joining (NHEJ) deficiency induce apoptosis and, when combined with p53 deficiency, progressed rapidly into lymphomagenesis in mice. Combination of NHEJ deficiency with a hypomorphic mutation, p53R172P, leads to the abrogation of apoptosis, upregulation of p21, and senescence in precursor lymphocytes. This was sufficient to prevent tumorigenesis. However, these mutant mice succumb to severe diabetes and die at an early age. The aim of this study was to determine the pathogenesis of diabetes in these mutant mice.

RESEARCH DESIGN AND METHODS

We analyzed the morphology of the pancreatic islets and the function, proliferation rate, and senescence of beta-cells. We also profiled DNA damage and p53 and p21 expression in the pancreas.

RESULTS

NHEJ-p53R172P mutant mice succumb to diabetes at 3-5 months of age. These mice show a progressive decrease in pancreatic islet mass that is independent of apoptosis and innate immunity. We observed an accumulation of DNA damage, accompanied with increased levels of p53 and p21, a significant decrease in beta-cell proliferation, and cellular senescence in the mutant pancreatic islets.

CONCLUSIONS

Combined DSBs with an absence of p53-dependent apoptosis activate p53-dependent senescence, which leads to a diminished beta-cell self-replication, massive depletion of the pancreatic islets, and severe diabetes. This is a model that connects impaired DNA repair and accumulative DNA damage, a common phenotype in aging individuals, to the onset of diabetes.

Molecular regulation of pancreas development in zebrafish

The pancreas is a vertebrate-specific organ of endodermal origin which is responsible for production of digestive enzymes and hormones involved in regulating glucose homeostasis, in particular insulin, deficiency of which results in diabetes. Basic research on the genetic and molecular pathways regulating pancreas formation and function has gained major importance for the development of regenerative medical approaches aimed at improving diabetes treatment. Among the different model organisms that are currently used to elucidate the basic pathways of pancreas development and regeneration, the zebrafish is distinguished by its unique opportunities to combine genetic and pharmacological approaches with sophisticated live-imaging methodology, and by its ability to regenerate the pancreas within a short time. Here we review current perspectives and present methods for studying two important processes contributing to pancreas development and regeneration, namely cell migration via time-lapse micropscopy and cell proliferation via incorporation of nucleotide analog EdU, with a focus on the insulin-producing beta cells of the islet.

Coming full circle: cyclin-dependent kinases as anti-cancer drug targets

Because the normal control of cell proliferation is disturbed in cancer, the cyclin-dependent kinases (CDKs) that trigger DNA synthesis and mitosis have been popular targets for inhibition with small molecules, but the jury is still out on whether this will be an effective anti-tumor strategy. There is debate about which of the multiple CDKs active during the mammalian cell cycle might be good targets, reflecting fundamental confusion about what, precisely, those different CDKs really do. In the classical view, based largely on their activation timing in cycling cells, different CDKs are specialized to perform discrete functions during distinct cell-cycle intervals. A revisionist model has emerged in which all functions essential to cell division can be performed by a single catalytic subunit, based on the ability of cells to proliferate and animals to survive when individual CDKs are removed by gene deletion or depleted by RNA interference. That those situations in no way resemble ones in which CDKs are inhibited pharmacologically is often overlooked or downplayed. A more nuanced - and accurate - picture is now coming into view, thanks to recent studies that reveal kinetically distinct pathways of activation for closely related CDKs and CDK-specific roles in the temporal control of S phase. The basic question of whether CDKs can be effectively targeted in cancer has yet to be answered but can now be addressed in chemical-genetic model systems that approximate the situation - still hypothetical - of truly selective CDK inhibition in vivo.

Cdk2 and Cdk4 regulate the centrosome cycle and are critical mediators of centrosome amplification in p53-null cells

The two mitotic centrosomes direct spindle bipolarity to maintain euploidy. Centrosome amplification-the acquisition of > or =3 centrosomes-generates multipolar mitoses, aneuploidy, and chromosome instability to promote cancer biogenesis. While much evidence suggests that Cdk2 is the major conductor of the centrosome cycle and that it mediates centrosome amplification induced by various altered tumor suppressors, the role played by Cdk4 in a normal or deregulated centrosome cycle is unknown. Using a gene knockout approach, we report that Cdk2 and Cdk4 are critical to the centrosome cycle, since centrosome separation and duplication are premature in Cdk2(-)(/)(-) mouse embryonic fibroblasts (MEFs) and are compromised in Cdk4(-)(/)(-) MEFs. Additionally, ablation of Cdk4 or Cdk2 abrogates centrosome amplification and chromosome instability in p53-null MEFs. Absence of Cdk2 or Cdk4 prevents centrosome amplification by abrogating excessive centriole duplication. Furthermore, hyperactive Cdk2 and Cdk4 deregulate the licensing of the centrosome duplication cycle in p53-null cells by hyperphosphorylating nucleophosmin (NPM) at Thr199, as evidenced by observations that ablation of Cdk2, Cdk4, or both Cdk2 and Cdk4 abrogates that excessive phosphorylation. Since a mutant form of NPM lacking the G(1) Cdk phosphorylation site (NPM(T199A)) prevents centrosome amplification to the same extent as ablation of Cdk2 or Cdk4, we conclude that the Cdk2/Cdk4/NPM pathway is a major guardian of centrosome dysfunction and genomic integrity.

The mammalian ovary from genesis to revelation

Two major functions of the mammalian ovary are the production of germ cells (oocytes), which allow continuation of the species, and the generation of bioactive molecules, primarily steroids (mainly estrogens and progestins) and peptide growth factors, which are critical for ovarian function, regulation of the hypothalamic-pituitary-ovarian axis, and development of secondary sex characteristics. The female germline is created during embryogenesis when the precursors of primordial germ cells differentiate from somatic lineages of the embryo and take a unique route to reach the urogenital ridge. This undifferentiated gonad will differentiate along a female pathway, and the newly formed oocytes will proliferate and subsequently enter meiosis. At this point, the oocyte has two alternative fates: die, a common destiny of millions of oocytes, or be fertilized, a fate of at most approximately 100 oocytes, depending on the species. At every step from germline development and ovary formation to oogenesis and ovarian development and differentiation, there are coordinated interactions of hundreds of proteins and small RNAs. These studies have helped reproductive biologists to understand not only the normal functioning of the ovary but also the pathophysiology and genetics of diseases such as infertility and ovarian cancer. Over the last two decades, parallel progress has been made in the assisted reproductive technology clinic including better hormonal preparations, prenatal genetic testing, and optimal oocyte and embryo analysis and cryopreservation. Clearly, we have learned much about the mammalian ovary and manipulating its most important cargo, the oocyte, since the birth of Louise Brown over 30 yr ago.

Cyclin D2 protein stability is regulated in pancreatic beta-cells

The molecular determinants of beta-cell mass expansion remain poorly understood. Cyclin D2 is the major D-type cyclin expressed in beta-cells, essential for adult beta-cell growth. We hypothesized that cyclin D2 could be actively regulated in beta-cells, which could allow mitogenic stimuli to influence beta-cell expansion. Cyclin D2 protein was sharply increased after partial pancreatectomy, but cyclin D2 mRNA was unchanged, suggesting posttranscriptional regulatory mechanisms influence cyclin D2 expression in beta-cells. Consistent with this hypothesis, cyclin D2 protein stability is powerfully regulated in fibroblasts. Threonine 280 of cyclin D2 is phosphorylated, and this residue critically limits D2 stability. We derived transgenic (tg) mice with threonine 280 of cyclin D2 mutated to alanine (T280A) or wild-type cyclin D2 under the control of the insulin promoter. Cyclin D2 T280A protein was expressed at much higher levels than wild-type cyclin D2 protein in beta-cells, despite equivalent expression of tg mRNAs. Cyclin D2 T280A tg mice exhibited a constitutively nuclear cyclin D2 localization in beta-cells, and increased cyclin D2 stability in islets. Interestingly, threonine 280-mutant cyclin D2 tg mice had greatly reduced beta-cell apoptosis, with suppressed expression of proapoptotic genes. Suppressed beta-cell apoptosis in threonine 280-mutant cyclin D2 tg mice resulted in greatly increased beta-cell area in aged mice. Taken together, these data indicate that cyclin D2 is regulated by protein stability in pancreatic beta-cells, that signals that act upon threonine 280 limit cyclin D2 stability in beta-cells, and that threonine 280-mutant cyclin D2 overexpression prolongs beta-cell survival and augments beta-cell mass expansion.

Akt and PTEN: beta-cell mass and pancreas plasticity

The capacity of pancreatic beta-cells to adapt to insulin resistance is crucial for glucose homeostasis and is a factor in the development of type 2 diabetes. The insulin receptor substrate (insulin receptor 2/phosphoinositide 3-kinase [PI3K]) pathway plays a crucial part in regulating beta-cell mass and function. The serine-threonine kinase Akt, also known as protein kinase B, is one of the major downstream targets of the PI3K pathway and is negatively regulated by phosphatase and tensin homologue deleted on chromosome 10. This Akt signaling pathway has recently been implicated in cell-cycle progression and survival of pancreatic beta-cells. Understanding the mechanisms that link Akt to modulation of beta-cell mass, function and plasticity will positively affect treatment of human diabetes.

Cell cycle kinases as therapeutic targets for cancer

Several families of protein kinases orchestrate the complex events that drive the cell cycle, and their activity is frequently deregulated in hyperproliferative cancer cells. Although several molecules that inhibit cell cycle kinases have been developed and clinically screened as potential anticancer agents, none of these has been approved for commercial use and an effective strategy to specifically control malignant cell proliferation has yet to be established. However, recent genetic and biochemical studies have provided information about the requirement for certain cell cycle kinases by specific tumours and specialized tissue types. Here, we discuss the potential and limitations of established cell cycle kinases as targets in anticancer drug discovery as well as novel strategies for the design of new agents.