Table of contents

Phenotypic effects of CDK inhibitor mutations

p21CIP1/WAF1 is a CDK inhibitor regulated by the tumor suppressor p53 and is hypothesized to mediate G1 arrest. p53 has been suggested to derive anti-oncogenic properties from this relationship. Mice lacking p21CIP1/WAF1 develop normally and (unlike p53-/- mice) did not develop spontaneous malignancies during 7 months of observation. Nonetheless, p21-/- embryonic fibroblasts are significantly deficient in their ability to arrest in G1 in response to DNA damage and nucleotide pool perturbation. p21-/- cells also exhibit a significant growth alteration in vitro, achieving a saturation density as high as that observed in p53-/- cells. In contrast, other aspects of p53 function, such as thymocytic apoptosis and the mitotic spindle checkpoint, appear normal. These results establish the role of p21CIP1/WAF1 in the G1 checkpoint, but suggest that the anti-apoptotic and the anti-oncogenic effects of p53 are more complex (Deng, 1995).

Targeted disruption of the murine p27(Kip1) gene caused a gene dose-dependent increase in animal size without other gross morphologic abnormalities. All tissues are enlarged and contain more cells, although endocrine abnormalities are not evident. Thymic hyperplasia is associated with increased T lymphocyte proliferation, and T cells show enhanced IL-2 responsiveness in vitro. Thus, p27 deficiency may cause a cell-autonomous defect resulting in enhanced proliferation in response to mitogens. In the spleen, the absence of p27 selectively enhances proliferation of hematopoietic progenitor cells. p27 deletion, like deletion of the Rb gene, is unique in causing neoplastic growth of the pituitary pars intermedia, suggesting that p27 and Rb function in the same regulatory pathway. The absence of p27 also causes an ovulatory defect and female sterility. Maturation of secondary ovarian follicles into corpora lutea, which express high levels of p27, is markedly impaired (Fero, 1996). However, increased growth occurs without an increase in the amounts of either growth hormone or IGF-I. Luteal cell differentiation is impaired, and a disordered estrus cycle is detected. These results reflect a disturbance of the hypothalamic-pituitary-ovarian axis. The phenotypes of these mice suggest that loss of p27 causes an alteration in cell proliferation that can lead to specific endocrine dysfunction (Kiyokawa, 1996).

Mice lacking p27(Kip1) have been created by gene targeting in embryonic stem cells. These mice are larger than the control animals, with thymus, pituitary, and adrenal glands and gonadal organs exhibiting striking enlargement. CDK2 activity is elevated about 10-fold in p27(-/-) thymocytes. Development of ovarian follicles seems to be impaired, resulting in female sterility. Similar to mice with the Rb mutation, the p27(-/-) mice often develop pituitary tumors spontaneously. The retinas of the mutant mice show a disturbed organization of the normal cellular layer pattern. These findings indicate that p27(Kip1) acts to regulate the growth of a variety of cells. Unexpectedly, the cell cycle arrest mediated by TGFbeta, rapamycin, or contact inhibition remains intact in p27(-/-) cells, suggesting that p27(Kip1) is not required in these pathways (Nakayama, 1996).

The commitment of cells to replicate and divide correlates with the activation of cyclin-dependent kinases and the inactivation of Rb, the product of the retinoblastoma tumor suppressor gene. Rb is a target of the cyclin-dependent kinases and, when phosphorylated, is inactivated. Biochemical studies exploring the nature of the relationship between cyclin-dependent kinase inhibitors and Rb have supported the hypothesis that these proteins are on a linear pathway regulating commitment. This relationship has been studied genetically by examining the phenotype of Rb+/-p27-/- mice. Tumors arise from the intermediate lobe cells of the pituitary gland in p27-/- mice, as well as in Rb+/- mice after loss of the remaining wild-type allele of Rb. Using these mouse models, the genetic interaction between Rb and p27 was examined. The development of pituitary tumors in Rb+/- mice correlates with a reduction in p27 mRNA and protein expression. To determine whether the loss of p27 is an indirect consequence of tumor formation or a contributing factor to the development of this tumor, the phenotype of Rb+/-p27-/- mice was analyzed. These mice develop pituitary adenocarcinoma with loss of the remaining wild-type allele of Rb and a high-grade thyroid C cell carcinoma that is more aggressive than the disease in either Rb+/- or p27-/- mice. Importantly, both pituitary and thyroid tumors are detected earlier in the Rb+/-p27-/- mice. It is therefore proposed that Rb and p27 cooperate to suppress tumor development by integrating different regulatory signals (Park, 1999).

p27Kip is a candidate human tumor-suppressor protein, because it is able to inhibit cyclin-dependent kinases and block cell proliferation. Abnormally low levels of the p27 protein are frequently found in human carcinomas, and these low levels correlate directly with both histological aggressiveness and patient mortality. However, it has not been possible to establish a causal link between p27 and tumor suppression, because only rare instances of homozygous inactivating mutations of the p27 gene have been found in human tumors. Thus, p27Kip1 does not fulfil Knudson's 'two-mutation' criterion for a tumor-suppressor gene. Both p27 nullizygous and p27 heterozygous mice are predisposed to tumors in multiple tissues when challenged with gamma-irradiation or a chemical carcinogen. Therefore p27 is a multiple-tissue tumor suppressor in mice. Molecular analyses of tumors in p27 heterozygous mice show that the remaining wild-type allele is neither mutated nor silenced. Hence, p27 is haplo-insufficient for tumor suppression. The assumption that null mutations in tumor-suppressor genes are recessive excludes those genes that exhibit haplo-insufficiency (Fero, 1998).

p57Kip2 is a paternally imprinted gene that encodes a potent inhibitor of several cyclin/Cdk complexes. p57Kip2 is expressed primarily in terminally differentiated cells; it associates with G1 Cdks, and can cause cell cycle arrest in G1 phase. At embryonic day 10.5, high p57Kip2 immunoreactivity is observed in newly differentiated neurons that have finished their last cell cycle and are in the process of migrating out of the ventricular zone. High levels of protein are also observed in the somites and in muscle cells of the developing heart. Later in embryogenesis, the protein is found in additional tissues and structures. To investigate the role of p57Kip2 in vivo, the p57Kip2 gene was ablated by homologous recombination in ES cells and mice were generated devoid of p57Kip2 expression. Most p57Kip2 null mice die after birth and display severe developmental defects with varying degrees of penetrance. As expected, heterozygous mice that inherit a maternal, but not a paternal, targeted allele exhibit similar deficiencies and neonatal death. Developmental defects of p57Kip2 mutant mice include cleft palate and gastrointestinal abnormalities ranging from an inflated GI tract to loss of the jejunum and ileum. These tissues display a significant increase of apoptotic cells in the absence of p57Kip2. Most p57Kip2 mutant mice have short limbs, a defect attributable to abnormal endochondral ossification caused by delayed cell cycle exit during chondrocyte differentiation. A similar defect has been observed in mice lacking p107 and p130, thus suggesting that p57Kip2 might be an upstream regulator of these Rb-related proteins. The p57Kip2 locus has been implicated in the Beckwith-Wiedemann syndrome and in the development of sporadic Wilms' tumors and lung carcinomas. To date, no neoplastic development has been observed in the knockout mice even in those p57Kip2 mutant mice that have survived for >5 months of age. These findings indicate that p57Kip2 has an important role during mouse development that cannot be compensated by other Cdk inhibitors (Yan, 1997).

INK4 and CIP/KIP are two distinct families of cyclin-dependent kinase (CDK) inhibitors implicated in mediating a wide range of cell growth control signals. p18(INK4c)-deficient mice develop gigantism and widespread organomegaly. In these mice, the pituitary gland, spleen, and thymus are disproportionately enlarged and hyperplastic. T and B lymphocytes develop normally in p18-deficient mice, but both exhibit increased cellularity and a higher proliferative rate upon mitogenic stimulation. Loss of p18, like that of p27, but not other CDK inhibitor genes, leads to a gradual progression from intermediate lobe pituitary hyperplasia in young mice to an adenoma by 10 months of age with a nearly complete penetrance. Mice lacking both p18 and p27, like mice chimeric for Rb deficiency, invariably die from pituitary adenomas by 3 months. Hence, p18 and p27 mediate two separate pathways to collaboratively suppress pituitary tumorigenesis, likely by controlling the function of Rb (Franklin, 1998).

Ataxia telangiectasia (AT) is an autosomal recessive disorder characterized by growth retardation, cerebellar ataxia, oculocutaneous telangiectasias, and a high incidence of lymphomas and leukemias. In addition, AT patients are sensitive to ionizing radiation. Atm-deficient mice recapitulate most of the AT phenotype. p21, an inhibitor of cyclin-dependent kinases, has been implicated in cellular senescence and response to gamma-radiation-induced DNA damage. To study the role of p21 in ATM-mediated signal transduction pathways, the combined effect of the genetic loss of atm and p21 on growth control, radiation sensitivity, and tumorigenesis were examined. p21 modifies the in vitro senescent response seen in AT fibroblasts. It is a downstream effector of ATM-mediated growth control. However, loss of p21 in the context of an atm-deficient mouse leads to a delay in thymic lymphomagenesis and an increase in acute radiation sensitivity in vivo (the latter principally because of effects on the gut epithelium). Modification of these two crucial aspects of the ATM phenotype can be related to an apparent increase in spontaneous apoptosis seen in tumor cells and in the irradiated intestinal epithelium of mice doubly null for atm and p21. Thus, loss of p21 seems to contribute to tumor suppression by a mechanism that operates via a sensitized apoptotic response. These results have implications for cancer therapy in general and AT patients in particular (Wang, 1997).

Disruption of the mouse Atm gene, whose human counterpart is consistently mutated in ataxia-telangiectasia (A-T) patients, creates an A-T mouse model exhibiting most of the A-T-related systematic and cellular defects. While ATM plays a major role in signaling the p53 response to DNA strand break damage, Atm-/- p53(-/-) mice develop lymphomas earlier than either Atm-/- or p53(-/-) mice, indicating that mutations in these two genes lead to synergy in tumorigenesis. The cell cycle G1/S checkpoint is abolished in Atm-/- p53(-/-) mouse embryonic fibroblasts (MEFs) following gamma-irradiation, suggesting that the partial G1 cell cycle arrest in Atm-/- cells following gamma-irradiation is due to the residual p53 response in these cells. In addition, the Atm-/- p21(-/-) MEFs are more severely defective in their cell cycle G1 arrest following gamma-irradiation than Atm-/- and p21(-/-) MEFs. The Atm-/- MEFs exhibit multiple cellular proliferative defects in culture; an increased constitutive level of p21 in these cells might account for these cellular proliferation defects. Consistent with this notion, Atm-/- p21(-/-) MEFs proliferate similarly to wild type MEFs and exhibit no premature senescence. These cellular proliferative defects are also rescued in Atm-/- p53(-/-) MEFs; little p21 can be detected in these cells, indicating that the abnormal p21 protein level in Atm-/- cells is also p53 dependent and leads to the cellular proliferative defects in these cells. However, the p21 mRNA level in Atm-/- MEFs is lower than that in Atm+/+ MEFs, suggesting that the higher level of constitutive p21 protein in Atm-/- MEFs is likely due to increased stability of the p21 protein (Xu, 1998).

The ability of cyclin-dependent kinases (CDKs) to promote cell proliferation is opposed by cyclin-dependent kinase inhibitors (CKIs), proteins that bind tightly to cyclin-CDK complexes and block the phosphorylation of exogenous substrates. Mice with targeted CKI gene deletions have only subtle proliferative abnormalities, however, and cells prepared from these mice seem remarkably normal when grown in vitro. One explanation may be the operation of compensatory pathways that control CDK activity and cell proliferation when normal pathways are inactivated. Mice lacking the CKIs p21(Cip1) and p27(Kip1) were used to investigate this issue, specifically with respect to CDK regulation by mitogens. p27 is the major inhibitor of Cdk2 activity in mitogen-starved wild-type murine embryonic fibroblasts (MEFs). Nevertheless, inactivation of the cyclin E-Cdk2 complex in response to mitogen starvation occurs normally in MEFs that have a homozygous deletion of the p27 gene. Moreover, CDK regulation by mitogens is also not affected by the absence of both p27 and p21. A titratable Cdk2 inhibitor compensates for the absence of both CKIs, and this inhibitor is identified as p130, a protein related to the retinoblastoma gene product Rb. Thus, cyclin E-Cdk2 kinase activity cannot be inhibited by mitogen starvation of MEFs that lack both p27 and p130. In addition, cell types that naturally express low amounts of p130, such as T lymphocytes, are completely dependent on p27 for regulation of the cyclin E-Cdk2 complex by mitogens. It is concluded that inhibition of Cdk2 activity in mitogen-starved fibroblasts is usually performed by the CKI p27, and to a minor extent by p21. Remarkably p130, a protein in the Rb family that is not related to either p21 or p27, will directly substitute for the CKIs and restore normal CDK regulation by mitogens in cells lacking both p27 and p21. p130 has the 'RxL' (Arg-X-Lys) motif, which is present in other cyclin-binding proteins and is required for CDK inhibition by members of the p21/p27 family. It is not clear, however, whether or not this motif is required for p130 to inhibit cyclin E-Ck2 in vitro, because the motif is absent from the amino-terminal inhibitory fragment of p130. Hence, the mode of inhibition of p130 may not mimic the one employed by the p21/p27 proteins. This compensatory use of p130 may be important in settings in which CKIs are not expressed at standard levels, as is the case in many human tumors (Coats, 1999).

The widely prevailing view that the cyclin-dependent kinase inhibitors (CKIs) are solely negative regulators of cyclin-dependent kinases (CDKs) is challenged here by observations that normal up-regulation of cyclin D-CDK4 in mitogen-stimulated fibroblasts depends redundantly upon p21(Cip1) and p27(Kip1). Primary mouse embryonic fibroblasts that lack genes encoding both p21 and p27 fail to assemble detectable amounts of cyclin D-CDK complexes, express cyclin D proteins at much reduced levels, and are unable to efficiently direct cyclin D proteins to the cell nucleus. Restoration of CKI function reverses all three defects and thereby restores cyclin D activity to normal physiological levels. In the absence of both CKIs, the severe reduction in cyclin D-dependent kinase activity is well tolerated and has no overt effects on the cell cycle (Cheng, 1999).

In many tissues, progenitor cells permanently withdraw from the cell cycle prior to commitment toward a differentiated phenotype. In the oligodendrocyte lineage a counting mechanism has been proposed, linking the number of cell divisions to growth arrest and differentiation. A direct prediction of this model is that an increase in the number of cell divisions would result in a delayed onset of differentiation. Since the cell cycle inhibitor p27Kip1 is an essential component of the machinery leading to oligodendrocyte progenitor growth arrest, the temporal relationship has been examined between cell cycle withdrawal and expression of late differentiation markers in vivo, in mice carrying a targeted deletion in the p27Kip1 gene. Using bromodeoxyuridine to label proliferating cells, quaking (QKI) to identify embryonic glial progenitors, NG2 to identify neonatal oligodendrocyte progenitors, and myelin basic protein to label differentiated oligodendrocytes, an increased number of proliferating QKI- and NG2-positive cells have been found in germinal zones of p27Kip1-/- mice at the peak of gliogenesis. However, no delay is observed in these mice in the appearance of the late differentiation marker myelin basic protein in the developing corpus callosum and cerebellum. Significantly, a decrease in cyclin E levels is observed in the brain of p27Kip1 null mice coincident with oligodendrocyte growth arrest. It is concluded that two distinct modalities of growth arrest occur in the oligodendrocyte lineage: a p27Kip1-dependent mechanism of growth arrest affecting proliferation in early phases of gliogenesis, and a p27Kip1-independent event leading to withdrawal from the cell cycle and differentiation (Casaccia-Bonnefil, 1999).

Developing cardiac myocytes divide a limited number of times before they stop and terminally differentiate, but the mechanism that stops their division is unknown. To help study the stopping mechanism, conditions were defined under which embryonic rat cardiac myocytes cultured in serum-free medium proliferate and exit the cell cycle on a schedule that closely resembles that seen in vivo. The culture medium contains FGF-1 and FGF-2, which stimulate cell proliferation, and thyroid hormone, which seems to be necessary for stable cell-cycle exit. Time-lapse video recording shows that the cells within a clone tend to divide a similar number of times before they stop, whereas cells in different clones divide a variable number of times before they stop. Cells cultured at 33 degrees C divide more slowly but stop dividing at around the same time as cells cultured at 37 degrees C, having undergone fewer divisions. Together, these findings suggest that an intrinsic timer helps control when cardiac myocytes withdraw from the cell cycle and that the timer does not operate by simply counting cell divisions. Evidence is provided that the cyclin-dependent kinase inhibitors p18 and p27 may be part of the timer and that thyroid hormone may help developing cardiac myocytes stably withdraw from the cell cycle (Burton, 1999).

Cardiac myocytes isolated from E14 p27-/- mice proliferate more than cells isolated from p27+/+ littermates. Moreover, p27-/- cardiac myocytes tend to form larger clones in cultures than do p27+/+ cardiac myocytes (50% of clones being five cells or greater at 7 days compared to only 10% of p27+/+ clones). Although p27 seems to play a part in stopping the cell-cycle in developing cardiac myocytes, the amount of p27 in these cells does not seem to increase in p27+/+ cells, as they proliferate in culture, as assessed by Western blotting. When the cells were immunostained for p27 and examined in the confocal microscope, however, the ratio of nuclear to cytoplasmic p27 was found to progressively increase as the cells proliferate -- from 1.1:1 after 2 days to 3.2:1 after 6 days in cultures . This finding raises the possibility that the accumulation of p27 in the nucleus may help the cells withdraw from the cell cycle. Although p27-/- cardiac myocytes proliferate more than wild-type cells in serum-free medium containing FGF-1 and -2 plus T3, when such cells are switched to medium containing 10% FCS after 7 days, they do not proliferate further, suggesting that p27 is not required for stable cell-cycle exit (Burton, 1999).

The cyclin-dependent kinase (CDK) inhibitors p21Cip1 and p27Kip1 are induced in response to anti-proliferative stimuli and block G1/S-phase progression through the inhibition of CDK2. Although the cyclin E-CDK2 pathway is often deregulated in tumors, the relative contribution of p21Cip1 and p27Kip1 to tumorigenesis is still unclear. The MYC transcription factor is an important regulator of the G1/S transition and its expression is frequently altered in tumors. It has been suggested that p27Kip1 is a crucial G1 target of MYC. In mice, deficiency for p27Kip1 but not p21Cip1 results in decreased survival to retrovirally-induced lymphomagenesis. Importantly, in such p27Kip1 deficient lymphomas an increased frequency of Myc activation is observed. p27Kip1 deficiency also collaborates with MYC overexpression in transgenic lymphoma models. Thus, in vivo, the capacity of MYC to promote tumor growth is fully retained and even enhanced upon p27Kip1 loss. In lymphocytes, MYC overexpression and p27Kip1 deficiency independently stimulate CDK2 activity and augment the fraction of cells in S phase, in support of their distinct roles in tumorigenesis (Martins, 2002).

The Xenopus p27Xic1 gene encodes a cyclin dependent kinase (CDK) inhibitor of the Cip/Kip family. p27Xic1 is expressed in the cells of the neural plate as they become post-mitotic. To investigate whether p27Xic1 is necessary for cell cycle exit and/or neuronal differentiation, antisense morpholino oligos (MO) were used to knockdown the protein levels in vivo. For such knockdown studies, Xenopus tropicalis is a better model system than Xenopus laevis, since it has a diploid genome. Indeed, while X. laevis has two p27Xic1 paralogs, p27Xic1 and p28Kix1, only one ortholog is found in X. tropicalis, equidistant from the X. laevis genes. The X. tropicalis p27Xic1 is expressed in a similar pattern to the X. laevis gene. Depletion of p27Xic1 in X. tropicalis causes an increase in proliferation and a suppression of the neuronal differentiation marker, N-tubulin. At the same time, an increase is found in the expression of ElrC, a marker of cells as they undergo a transition from proliferation to differentiation. It is concluded that p27Xic1 is necessary for cells to exit the cell cycle and differentiate: in its absence, cells accumulate in a progenitor state. The expression of p27Xic1 in the embryo is regionalized but the transcriptional regulation of p27Xic1 is not well understood. A p27Xic1 genomic clone has been isolated and a 5' region capable of driving reporter gene expression specifically in the neural tube and the eye is reported (Carruthers, 2003)

p21 loss compromises the relative quiescence of forebrain stem cell proliferation leading to exhaustion of their proliferation capacity

Adult stem cells in various tissues are relatively quiescent. The cell cycle inhibitor p21cip1/waf1 (p21) has been shown to be important for maintaining hematopoietic stem cell quiescence and self-renewal. The role of p21 in the regulation of adult mammalian forebrain neural stem cells (NSCs) was examined. It was found that p21-/- mice between post-natal age 60-240 d have more NSCs than wild-type (+/+) controls due to higher proliferation rates of p21-/- NSCs. Thereafter, NSCs in p21-/- mice decline and are reduced in number at 16 mo relative to p21+/+ mice. Similarly, both p21-/- and p21+/+ NSCs display self-renewal in vitro; however, p21-/- NSCs display limited in vitro self-renewal (surviving a few passages, then exhausting). Thus, p21 contributes to adult NSC relative quiescence, which is proposed to be necessary for the life-long maintenance of NSC self-renewal because NSCs may be limited to a finite number of divisions (Kippin, 2005).

Following NSC expansion in young adults, p21-/- mice display an accelerated reduction of NSCs at 480 d, thus demonstrating a decrement in the in vivo longevity of NSCs. Furthermore, in vitro isolated p21-/- NSCs display initially increased expansion followed by a progressive reduction in self-renewal and eventual exhaustion (i.e., loss of neurosphere-forming ability) in vitro. Importantly, the finding that the neurosphere-forming cells that eventually undergo exhaustion are initially capable of high levels of self-renewal and are multipotential, demonstrates that these spheres are not generated from progenitor cells because progenitor-derived spheres lack self-renewal and are not capable of forming neurons. Together, these findings indicate that the p21 loss results in NSC self-renewal exhaustion, as a consequence of prior increased division either in vivo or in vitro. Thus, the increase in NSC division induced by p21 loss has allowed the demonstration of NSC proliferative exhaustion; essentially, NSCs do not have an unlimited self-renewal capacity in the absence of transformation (Kippin, 2005).

Decreased NSC self-renewal also is associated with a progressive increase in cell cycle time (i.e., slower proliferation rate), indicating that NSC exhaustion may be mediated by proliferative senescence. Cell division in p21-/- NSCs may lead to senescence by several distinct mechanisms. (1) Several p21-independent cell senescence pathways have been elucidated, including cell cycle arrest via the p16INK4a-retinoblastoma tumor suppressor protein pathway and the PTEN-p27Kip1 pathway. Moreover, cell expansion results in the progressive up-regulation of p16INKa and p27Kip1; thus, the absence of p21 may lead to NSC senescence (due to prior increased proliferation) mediated by increased expression of other cell cycle inhibitors. However, using immunocytochemistry, expression of the putative senescence marker, beta-galactosidase was not detected in p21-/- NSCs undergoing exhaustion (Kippin, 2005).

(2) Decreased NSC self-renewal after Notch pathway mutations has been associated previously with increased differentiation into neurons and glia, suggesting terminal symmetric division (i.e., producing two progenitors) as a mechanism of NSC loss. However, p21-/- NSCs do not yield more neurons or glia when neurospheres were differentiated in vitro from either primary cultures or from subsequent passages. Thus, it is unlikely that terminal symmetric divisions into two progenitor cells and later differentiation contribute to the decline in p21-/- NSCs seen in vivo or in vitro (Kippin, 2005).

(3) Decreased cell viability may mediate p21-/- NSC exhaustion. p21 directs p53-mediated processes toward senescence over apoptosis, with p21 loss increasing apoptosis. Cell division also increases p53 levels. Thus, increased NSC division in the absence of p21 may elevate p53 and result in increased NSC death. However, no increase in cell death accompanied loss of self-renewal in p21-/- NSC cultures, indicating that increased apoptosis does not underlie NSC exhaustion. Furthermore, early in vitro passaging of p21-/- NSCs from young adult mice produce enlarged neurospheres, demonstrating that an increased rate of proliferation does not bias p21-/- cells unequivocally toward apoptosis. Similarly during embryonic development, p21 does not appear to regulate NSC expansion and NSC proliferation rate is at near maximum levels, yet the numbers of NSCs are increased in young adult p21-/- mice relative to p21+/+ mice. Additionally, NSCs are highly resistant to apoptosis because of low expression of pro-apoptotic genes. Together, these findings suggest a bias toward cell death is unlikely to explain the loss in NSC longevity (Kippin, 2005).

(4) Cell division progressively decreases telomere length, and telomere shortening is linked to exhaustion of proliferative capacity. The increased NSC proliferation in the absence of p21 observed in the present experiments may increase the rate of telomere shortening. Despite the presence of extremely long telomeres in mouse cells, serial transplantations of mouse hematopoietic stem cells have revealed that telomere shortening may play a role in the senescence of these stem cells. Similarly, loss of telomerase function in TERC-deficient mice that are undergoing germline senescence is associated with reduced proliferation in adult-derived neurospheres, and importantly, the reduction in proliferation results in cell cycle arrest, but not cell death. Accordingly, reductions in telomere length could potentially account for the effects of p21 loss on NSCs (Kippin, 2005).

In summary, p21 loss results in decreased cell cycle times leading to an initial post-natal expansion of NSCs, demonstrating that p21 plays a role in the maintenance of relative quiescence in adult NSCs. The increase in NSC proliferation induced by p21 loss results in more total cumulative (i.e., both symmetric and asymmetric) cell divisions, leading to impairment of long-term self-renewal and eventual exhaustion of NSCs in aging mice. Thus, a major implication of the present study is that adult multipotential NSCs have extensive, but finite, self-renewal capacity, and a population of these cells lasts the lifetime of an organism because their proliferation is tightly regulated (i.e., they are relatively quiescent). A similar role for p21 has been demonstrated in hematopoietic stem cell regulation during serial repopulation. Thus, stem cells in at least two tissues share the common properties of both finite proliferation capacity and p21-regulated relative quiescence as a mechanism to ensure maintenance of the stem cell niche throughout the life span. Furthermore, NSC self-renewal decrements result in a reduction in the production of new neurons in the olfactory bulb of aging mice (Kippin, 2005).

The proliferation-quiescence decision is controlled by a bifurcation in CDK2 activity at mitotic exit

Tissue homeostasis in metazoans is regulated by transitions of cells between quiescence and proliferation. The hallmark of proliferating populations is progression through the cell cycle, which is driven by cyclin-dependent kinase (CDK) activity. This study introduced a live-cell sensor for CDK2 activity in mammalian cultured cells. It was unexpectedly found that proliferating cells bifurcate into two populations as they exit mitosis. Many cells immediately commit to the next cell cycle by building up CDK2 activity from an intermediate level, while other cells lack CDK2 activity and enter a transient state of quiescence. This bifurcation is directly controlled by the CDK inhibitor p21 and is regulated by mitogens during a restriction window at the end of the previous cell cycle. Thus, cells decide at the end of mitosis to either start the next cell cycle by immediately building up CDK2 activity or to enter a transient G0-like state by suppressing CDK2 activity (Spencer, 2013).

The restriction point was originally defined as a point in late G1 after which cells would continue through the cell cycle even if mitogens were withdrawn. It has also been proposed that commitment to the next cell cycle is made at the end of the preceding cycle. The current study argues that these apparently contradictory models can be explained by taking into account the two types of cell behaviors that this study has observed within the same cell population. The results argue that cells integrate mitogenic and potentially other inputs during a restriction window at the end of the previous cell cycle (R1) to regulate the bifurcation into the intermediate level of CDK2 (CDK2inc) or or low level of CDK2 (CDK2low) state upon completion of mitosis. Only the cells in the CDK2low state experience a second restriction window (R2) in which mitogens are needed to re-enter the cell cycle and build up CDK2 activity. Given the different fractions of CDK2low cells that were found in the cell types that were tested, the relative importance of R1 versus R2 will vary depending on the cell type, strength of mitogen stimuli and other conditions. Although the two restriction windows R1 and R2 cover different phases of the cell cycle, they reflect an underlying principle that cells require the continued presence of mitogens for several hours before they commit to building up CDK2 activity. Finally, the relationship between CDK2 activity and cell-cycle commitment that is described in this study may be integral to other cell fate decisions such as the senescence of somatic cells, the differentiation of stem cells, or the progression of cancer cells (Spencer, 2013).

CDK inhibitors and senescence

The cyclin-dependent kinase (CDK) inhibitor p21 was identified initially as a gene induced in senescent cells and itself has been shown to cause permanent growth arrest/senescence. Reactive oxygen species (ROS), a byproduct of oxidative processes, can also induce an irreversible growth arrest similar to senescence. p21 increases intracellular levels of ROS both in normal fibroblasts and in p53-negative cancer cells. N-acetyl-L-cysteine, an ROS inhibitor, rescues p21-induced senescence, showing that ROS elevation is necessary for induction of the permanent growth arrest phenotype. p16Ink4a, a CDK4- and CDK6-specific inhibitor, fails to increase ROS levels, and cell cycle arrest induced by p16 is reversible following its down-regulation, demonstrating the specificity of this p21 effect. A p21 mutant that lacks the ability to bind proliferating cell nuclear antigen (PCNA) retains the ability to induce both ROS and permanent growth arrest. All of these findings establish that p21 mediates senescence by a mechanism involving ROS accumulation, which does not require either its PCNA binding or the CDK inhibitory functions shared with p16 (Macip, 2002).

Adenovirus E1A-associated p400 belongs to the SWI2/SNF2 family of chromatin remodeling proteins and is a component of the p53-p21WAF1/CIP1/sid1 pathway, regulating the p21 transcription and senescence induction program. Acute depletion of p400 expression by shRNA (short hairpin RNA) synthesis led to premature senescence of untransformed human fibroblasts, whose features include G1 arrest, p21 induction, senescence-associated heterochromatic foci (SAHF), and beta-gal staining. Importantly, p400shRNA-induced premature senescence phenotypes were rescued by coexpression of p53-shRNA or p21-shRNA. Furthermore, p400 complex colocalizes with p53 on the p21 promoter. These data suggest that the p400 complex inhibits p53 -->p21 transcription and the development of premature senescence (Chan, 2005).

Cellular senescence has been hypothesized to serve as a barrier against tumor development. The ability to overcome senescence is a prerequisite for tumor formation. Both E1A and c-Myc up-regulate p400 expression. Furthermore, p400 is required for E1A-dependent apoptosis. Given the results reported above, one wonders whether E1A or c-Myc, by up-regulating p400 expression, override p53/p21-dependent cell cycle arrest, an outcome that has been suggested to sensitize cells to p53-dependent apoptosis. If so, one would argue that c-Myc and/or E1A do not inactivate p400. Rather, one wonders whether c-Myc/p400 or E1A/p400 complexes exhibit a qualitative alteration in p400 function or even its activation. Since senescence is suspected of serving as a major component of tumor suppression and is associated, in at least one setting, with suppression of p400 complex function, it is conceivable that up-regulation of p400 expression/function by certain oncogenes (and, quite possibly, its binding to the N-terminal region of E1A) contributes to neoplastic transformation. An analysis of whether p400 function is deregulated in certain human tumors now seems timely (Chan, 2005).

CDK inhibitors and transformation

Transformation by oncogenic Ras requires the function of the Rho family GTPases. Ras-transformed cells have elevated levels of RhoA-GTP, which functions to inhibit the expression of the cell cycle inhibitor p21/Waf1. These high levels of Rho-GTP are not a direct consequence of Ras signaling but are selected for in response to sustained ERK-MAP kinase signaling. While the elevated levels of Rho-GTP control the level of p21/Waf, they no longer regulate the formation of actin stress fibers in transformed cells. The sustained ERK-MAP kinase signaling resulting from transformation by oncogenic Ras down-regulates ROCK1 and Rho-kinase, two Rho effectors required for actin stress fiber formation. The repression of Rho-dependent stress fiber formation by ERK-MAP kinase signaling contributes to the increased motility of Ras-transformed fibroblasts. Overexpression of the ROCK target LIM kinase restores actin stress fibers and inhibits the motility of Ras-transformed fibroblasts. A model is proposed in which Ras and Rho signaling pathways cross-talk to promote signaling pathways favoring transformation (Sahai, 2001).

Expression of p21/Waf1 following mitogenic stimulation is dependent on the ERK-MAP kinase pathway. In the control of cell cycle progression, p21/Waf1 has a dual role: it serves as an assembly factor for active complexes of D-type cyclins and their cyclin-dependent kinases (CDKs) and is an inhibitor of CDK2. In studies employing transient assays of Ras-driven proliferation, the absence of signaling through RhoA results in the Ras-driven ERK-MAP kinase pathway, inducing levels of p21/Waf1 that are inhibitory to cell cycle progression. signaling through RhoA is required to suppress growth inhibitory levels of p21/Waf1 in Ras-transformed Swiss-3T3 cells and in two human colorectal cancer cell lines. In these human cell lines, inhibition of the Raf/MEK/ERK and PI-3-kinase Ras effector pathways does not affect Rho-GTP levels, thus supporting the model that Rho activity is determined by selection; however, the levels of Rho-GTP in these cells compared with untransformed cells could not be determined due to the lack of genotypically matched controls. It is proposed that cells with high levels of Rho activity are selected for because they counteract the high levels of p21/Waf1 induced by oncogenic Ras and proliferate, while cells with low Rho activity remain growth arrested by high p21/Waf1 levels. Interestingly, the Ras-transformed Swiss-3T3 cells have higher levels of p21/Waf1 than parental non-transformed cells. However, these cells proliferate presumably because the levels of p21/Waf1 are such that they enable the assembly of active cyclin D-CDK complexes rather than inhibit CDK2. It is proposed that the elevated levels of Rho-GTP in the transformed cells set a threshold level of p21/Waf1 that is compatible with proliferation (Sahai, 2001).

The cell cycle inhibitor p27Kip1 also has cyclin-cyclin-dependent kinase (CDK)-independent functions. To investigate the significance of these functions in vivo, a knock-in mouse was generated in which four amino acid substitutions in the cdkn1b gene product (p27Kip1) prevent its interaction with cyclins and CDKs (p27CK-). In striking contrast to complete deletion of the cdkn1b gene, which causes spontaneous tumorigenesis only in the pituitary, the p27CK- protein dominantly caused hyperplastic lesions and tumors in multiple organs, including the lung, retina, pituitary, ovary, adrenals, spleen, and lymphomas. Moreover, the high incidence of spontaneous tumors in the lung and retina was associated with amplification of stem/progenitor cell populations. Therefore, independently of its role as a CDK inhibitor, p27Kip1 promotes stem cell expansion and functioned as a dominant oncogene in vivo. Thus, the p27CK- mouse unveils a dual role for p27 during tumorigenesis: It is a tumor suppressor by virtue of its cyclin-CDK regulatory function, and also an oncogene through a cyclin-CDK-independent function. This may explain why the cdkn1b gene is rarely inactivated in human tumors, and the p27CK- mouse in which the tumor suppressor function is lost but the cyclin-CDK-independent (oncogenic) function is maintained may represent a more faithful model for the widespread role of p27 misregulation in human cancers than the p27 null (Besson, 2007).

CDK function in the cytoplasm to regulate cytoskeletal dynamics

p21(Cip1/WAF1) has cell cycle inhibitory activity by binding to and inhibiting both cyclin/Cdk kinases and proliferating cell nuclear antigen. p21(Cip1/WAF1) is induced in the cytoplasm during the course of differentiation of chick retinal precursor cells and N1E-115 cells. Ectopic expression of p21(Cip1/WAF1) lacking the nuclear localization signal in N1E-115 cells and NIH3T3 cells affects the formation of actin structures, characteristic of inactivation of Rho. p21(Cip1/WAF1) forms a complex with Rho-kinase and inhibits its activity in vitro and in vivo. Neurite outgrowth and branching from the hippocampal neurons are promoted if p21(Cip1/WAF1) is expressed abundantly in the cytoplasm. These results suggest that cytoplasmic p21(Cip1/WAF1) may contribute to the developmental process of the newborn neurons that extend axons and dendrites into target regions (Tanaka, 2002).

Accumulating evidence suggests that p21(Cip1) located in the cytoplasm might play a role in promoting transformation and tumor progression. Oncogenic H-RasV12 contributes to the loss of actin stress fibers by inducing cytoplasmic localization of p21(Cip1), which uncouples Rho-GTP from stress fiber formation by inhibiting Rho kinase (ROCK). Concomitant with the loss of stress fibers in Ras-transformed cells, there is a decrease in the phosphorylation level of cofilin, which is indicative of a compromised ROCK/LIMK/cofilin pathway. Inhibition of MEK in Ras-transformed NIH3T3 results in restoration of actin stress fibers accompanied by a loss of cytoplasmic p21(Cip1), and increased phosphorylation of cofilin. Ectopic expression of cytoplasmic but not nuclear p21(Cip1) in Ras-transformed cells is effective in preventing stress fibers from being restored upon MEK inhibition and inhibits phosphorylation of cofilin. p21(Cip1) forms a complex with ROCK in Ras-transformed cells in vivo. Furthermore, inhibition of the PI 3-kinase pathway results in loss of p21(Cip1) expression, accompanied by restoration of phosphocofilin, that is not accompanied by stress fiber formation. These results suggest that restoration of cofilin phosphorylation in Ras-transformed cells is necessary but not sufficient for stress fiber formation. These findings define a novel mechanism for coupling cytoplasmic p21(Cip1) to the control of actin polymerization by compromising the Rho/ROCK/LIMK/cofilin pathway by oncogenic Ras. These studies suggest that localization of p21(Cip1) to the cytoplasm in transformed cells contributes to pathways that favor not only cell proliferation, but also cell motility, thereby contributing to invasion and metastasis (Lee, 2003).

p57Kip2 is the only cyclin-dependent kinase (Cdk) inhibitor shown to be essential for mouse embryogenesis. The fact suggests that p57 has a specific role that cannot be compensated by other Cdk inhibitors. LIM-kinase 1 (LIMK-1) is a downstream effector of the Rho family of GTPases that phosphorylates and inactivates an actin depolymerization factor, cofilin, to induce the formation of actin fiber. p57 is shown to regulate actin dynamics by binding and translocating LIMK-1 from the cytoplasm into the nucleus, which in turn results in a reorganization of actin fiber. The central region of p57, a unique feature among the Cdk inhibitors, and the N-terminal region of LIMK-1, which contains the LIM domains, are essential for the interaction. Expression of p57, but not p27Kip1 or a p57 mutant with a deletion in the central region, induces marked reorganization of actin filament and a translocation of LIMK-1. These findings indicate p57 may act as a key regulator in embryogenesis by bearing two distinct functions, the regulation of cell cycle through binding to Cdks and the regulation of actin dynamics through binding to LIMK-1, both of which should be important in developmental procedure (Yokoo, 2003).

Table of contents

dacapo: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | References

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