The cyclin/cyclin-dependent kinase (cdk) inhibitor p27kip1 is thought to be responsible for the onset and maintenance of the quiescent state. It is possible, however, that cells respond differently to p27kip1 in different conditions, and using a BALB/c-3T3 cell line (termed p27-47) that inducibly expresses high levels of this protein, we show that the effect of p27kip1 on cell cycle traverse is determined by cell density. We found that ectopic expression of p27kip1 blocked the proliferation of p27-47 cells at high density but had little effect on the growth of cells at low density whether exponentially cycling or stimulated from quiescence. Regardless of cell density, the activities of cdk4 and cdk2 were markedly repressed by p27kip1 expression, as was the cdk4-dependent dissociation of E2F4/p130 complexes. Infection of cells with SV40, a DNA tumor virus known to abrogate formation of p130- and Rb-containing complexes, allowed dense cultures to proliferate in the presence of supraphysiological amounts of p27kip1 but did not stimulate cell cycle traverse when cultures were cotreated with the potent cdk2 inhibitor roscovitine. Our data suggest that residual levels of cyclin/cdk activity persist in p27kip1-expressing p27-47 cells and are sufficient for the growth of low-density cells and of high-density cells infected with SV40, and that effective disruption of p130 and/or Rb complexes is obligatory for the proliferation of high-density cultures.
The p57 CDKi integrates stress signals into cell-cycle progression to promote cell survival upon stress
The stress-activated protein kinase p38 phosphorylates p57/Kip2, resulting in enhanced CDK2 inhibition and a cell-cycle delay that helps cells to survive under stress.
The p57Kip2 cyclin-dependent kinase inhibitor (CDKi) has been implicated in embryogenesis, stem-cell senescence and pathologies, but little is known of its role in cell cycle control. Here, we show that p57Kip2 is targeted by the p38 stress-activated protein kinase (SAPK). Phosphorylation of p57Kip2 at T143 by p38 enhances its association with and inhibition of Cdk2, which results in cell-cycle delay upon stress. Genetic inactivation of the SAPK or the CDKi abolishes cell-cycle delay upon osmostress and results in decreased cell viability. Oxidative stress and ionomycin also induce p38-mediated phosphorylation of p57 and cells lacking p38 or p57 display reduced viability to these stresses. Therefore, cell survival to various stresses depends on p57 phosphorylation by p38 that inhibits CDK activity. Together, these findings provide a novel molecular mechanism by which cells can delay cell cycle progression to maximize cell survival upon stress.
cell cycle; cell stress; cell survival; p38 SAPK; p57 CDKi
p27kip1 is a cyclin-dependent kinase inhibitor that regulates progression from G1 into S phase. Aberrations in cell cycle control are often observed in tumors and might even be necessary in tumor development. Recent reports showed that low p27kip1 expression is associated with poor prognosis in several tumors and leukemia. To investigate the expression of p27kip1 in malignant lymphomas and elucidate the role of p27kip1 as a possible prognostic indicator, the authors performed an immunohistochemical staining of p27kip1 correlated with Ki-67 labelling index and clinical parameters. p27kip1 expression was reduced variably in most malignant lymphomas and inversely correlated with Ki-67 labelling index (p=0.0151). Regarding chemotherapeutic response, p271kip1 expression in the complete remission group showed statistically significant difference in expression compared to the progressive disease group (p=0.0021). There were significant differences in survival between cases with low and high p27kip1 expression (p=0.0071). In a multivariate Cox analysis, p27kip1 expression was independent prognostic factors as well as other known prognostic factors including age, grade, stage and chemotherapeutic response. In conclusion, the study suggests that reduced expression of p27kip1 protein may play a role in the pathogenesis and biologically aggressive behavior of malignant lymphomas.
Cell cycle inhibitors, such as the cyclin-dependent kinase (Cdk) inhibitor proteins and retinoblastoma (Rb) family members, control exit from the cell cycle during the development of a variety of terminally differentiated tissues. It is unclear whether sustained expression of these proteins is required to prevent cell cycle re-entry in quiescent and terminally differentiated cells. The organ of Corti (cochlear sensory epithelium) and pars intermedia (intermediate lobe of the pituitary) are two tissues that share the characteristic of ongoing cell division in mice lacking either the p27Kip1 Cdk inhibitor, Ink4 proteins or Rb. Here, we use tamoxifen-inducible mouse models to delete p27Kip1 in postnatal animals and show this is sufficient to induce proliferation in both the organ of Corti and pars intermedia. Thus, these tissues remain sensitive to the presence of p27Kip1 even after their developmental exit from the cell cycle. The neonatal cochlea displayed heightened sensitivity to changes in p27Kip1 expression, with a proliferative response higher than that of constitutive null mice. In adults, the proliferative response was reduced but was accompanied by increased cell survival. In contrast, re-establishment of normal p27Kip1 expression in animals with established pituitary tumors, in an inducible “knock-on” model, led to cessation of pituitary tumor growth, indicating the cells had maintained their susceptibility to p27-mediated growth suppression. Although restoration of p27Kip1 did not induce apoptosis, it did lead to resolution of pathological features and normalization of gene expression. Our data underscore the importance of p27Kip1 expression in the maintenance of cellular quiescence and terminal differentiation.
proliferation; cell cycle; p27; Cdk inhibitor; auditory; cochlea; pituitary
To investigate how overexpression of p27KIP1, a downstream effector of TGF-beta and a universal cyclin-dependent kinase (CDK) inhibitor could influence the malignant phenotype of malignant human brain tumor cells, an adenovirus vector system was used to transfer the human p27KIP1 gene (Adp27KIP1) into the human astrocytoma cell line, U-373MG. Inhibition of CDK activity in Adp27KIP1-infected cells was indicated by inhibition of [3H]thymidine incorporation, an increase in cell doubling time and by cell cycle arrest in G1. Notably, ectopic overexpression of p27KIP1 was associated with a marked decrease in the accumulation of aneuploid cells. Diminished malignant potential of Adp27KIP1-infected cells was manifested by the loss of anchorage-independent growth in soft agar and by the inability to induce tumorgenesis in a xenograft model. These studies suggest that p27KIP1 is a tumor suppressor gene and supports the use of Adp27KIP1 for gene therapy of human brain tumors.
We describe the design of a potent and selective peptidomimetic inhibitor of geranylgeranyltransferase I (GGTI), GGTI-2418, and its methyl ester GGTI-2417, which increases the levels of the cyclin-dependent kinase (Cdk) inhibitor p27Kip1 and induces breast tumor regression in vivo. Experiments with p27Kip1 small interfering RNA in breast cancer cells and p27Kip1 null murine embryonic fibroblasts demonstrate that the ability of GGTI-2417 to induce cell death requires p27Kip1. GGTI-2417 inhibits the Cdk2-mediated phosphorylation of p27Kip1 at Thr187 and accumulates p27Kip1 in the nucleus. In nude mouse xenografts, GGTI-2418 suppresses the growth of human breast tumors. Furthermore, in ErbB2 transgenic mice, GGTI-2418 increases p27Kip1 and induces significant regression of breast tumors. We conclude that GGTIs' antitumor activity is, at least in part, due to inhibiting Cdk2-dependent p27Kip1 phosphorylation at Thr187 and accumulating nuclear p27Kip1. Thus, GGTI treatment might improve the poor prognosis of breast cancer patients with low nuclear p27Kip1 levels.
p27Kip1 is a cell cycle inhibitor that prevents cyclin dependent kinase (CDK)/cyclin complexes from phosphorylating their targets. p27Kip1 is a known tumor suppressor, as the germline loss of p27Kip1 results in sporadic pituitary formation in aged rodents, and its presence in human cancers is indicative of a poor prognosis. In addition to its role in cancer, loss of p27Kip1 results in regenerative phenotypes in some tissues and maintenance of stem cell pluripotency, suggesting that p27Kip1 inhibitors could be beneficial for tissue regeneration. Because p27Kip1 is an intrinsically disordered protein, identifying direct inhibitors of the p27Kip1 protein is difficult. Therefore, we pursued a high-throughput screening strategy to identify novel p27Kip1 transcriptional inhibitors. We utilized a luciferase reporter plasmid driven by the p27Kip1 promoter to transiently transfect HeLa cells and used cyclohexamide as a positive control for non-specific inhibition. We screened a “bioactive” library consisting of 8,904 (4,359 unique) compounds, of which 830 are Food and Drug Administration (FDA) approved. From this screen, we successfully identified 111 primary hits with inhibitory effect against the promoter of p27Kip1. These hits were further refined using a battery of secondary screens. Here we report four novel p27Kip1 transcriptional inhibitors, and further demonstrate that our most potent hit compound (IC50 = 200 nM) Alsterpaullone 2-cyanoethyl, inhibits p27Kip1 transcription by preventing FoxO3a from binding to the p27Kip1 promoter. This screen represents one of the first attempts to identify inhibitors of p27Kip1 and may prove useful for future tissue regeneration studies.
The cyclin-dependent kinase inhibitor p27Kip1 is essential for proper control of cell cycle progression. The levels of p27Kip1 are regulated by several mechanisms including transcriptional and translational controls. In order to delineate the molecular details of these regulatory mechanisms it is important to identify the transcription initiation site within the p27Kip1 gene, thereby defining the promoter region of the gene and the 5'-untranslated region of the p27Kip1 mRNA. Although several previous studies have attempted to map p27Kip1 transcription start sites, the results vary widely for both the mouse and human genes. In addition, even though the mouse and human p27Kip1 gene sequences are very highly conserved, the reported start sites are notably different.
In this report, using a method that identifies capped ends of mRNA molecules together with RNase protection assays, we demonstrate that p27Kip1 transcription is initiated predominantly from a single site which is conserved in the human and mouse genes. Initiation at this site produces a 5'-untranslated region of 472 nucleotides in the human p27Kip1 mRNA and 502 nucleotides in the mouse p27Kip1 mRNA. In addition, several minor transcription start sites were identified for both the mouse and human genes.
These results demonstrate that the major transcription initiation sites in the mouse and human p27Kip1 genes are conserved and that the 5'-UTR of the p27Kip1 mRNA is much longer than generally believed. It will be important to consider these findings when designing experiments to identify elements that are involved in regulating the cellular levels of p27Kip1.
p27Kip1 is an inhibitor of the cyclin-dependent kinases and it plays an inhibitory role in the progression of cell cycle through G1 phase. To investigate the mechanism of cell cycle inhibition by p27Kip1, we constructed a cell line that inducibly expresses p27Kip1 upon addition of isopropyl-1-thio-beta-D-galactopyranoside in the culture medium. Isopropyl-1-thio-beta-D-galactopyranoside-induced expression of p27Kip1 in these cells causes a specific reduction in the expression of the E2F-regulated genes such as cyclin E, cyclin A, and dihydrofolate reductase. The reduction in the expression of these genes correlates with the p27Kip1-induced accumulation of the repressor complexes of the E2F family of factors (E2Fs). Our previous studies indicated that p21WAF1 could disrupt the interaction between cyclin/cyclin-dependent kinase 2 (cdk2) and the E2F repressor complexes E2F-p130 and E2F-p107. We show that p27Kip1, like p21WAF1, disrupts cyclin/cdk2-containing complexes of E2F-p130 leading to the accumulation of the E2F-p130 complexes, which is found in growth-arrested cells. In transient transfection assays, expression of p27Kip1 specifically inhibits transcription of a promoter containing E2F-binding sites. Mutants of p27Kip1 harboring changes in the cyclin- and cdk2-binding motifs are deficient in inhibiting transcription from the E2F sites containing reporter gene. Moreover, these mutants of p27Kip1 are also impaired in disrupting the interaction between cyclin/cdk2 and the repressor complexes of E2Fs. Taken together, these observations suggest that p27Kip1 reduces expression of the E2F-regulated genes by generating repressor complexes of E2Fs. Furthermore, the results also demonstrate that p27Kip1 inhibits expression of cyclin A and cyclin E, which are critical for progression through the G1-S phases.
Cell cycle inhibition of neural stem and progenitor cells is critical for maintaining the stability of central nervous system in adults, but it may represent a significant hurdle for neural regeneration after injury. We have previously demonstrated that the cyclin-dependent kinase inhibitor (CKI) p21cip1/waf1 (p21) maintains the quiescence of neural stem-like cells under cerebral ischemia, as similarly shown for the hematopoietic stem cells. Here, we report the distinct role of another CKI member, p27kip1 (p27) in neural progenitor cells (NPCs) from adult brain (subventricular zone and hippocampal subgranular zone) under both homeostatic and ischemic conditions. The basal level of NPC proliferation in the p27−/− mice was higher than that in p27+/+ mice. Upon ischemia, the overall proliferation of NPCs continued to be higher in p27−/− mice than that in p27 +/+ mice. Moreover, the increase of NPC proliferation in p27−/− mice remained until 2 weeks after ischemia whereas it resumed back to the basal level in p27+/+ mice. As a result, newly generated neuronal cells in the granular layer of p27−/− brain were more abundant compared to p27 +/+ controls. These new data demonstrate that p27 functions as a distinct inhibitor for NPC proliferation under homeostatic as well as ischemic conditions.
Mounting evidence indicates cyclin-dependent kinase (CDK) inhibitors (CKIs) of the Cip/Kip family, including p57Kip2 and p27Kip1, control not only cell cycle exit but also corticogenesis. Nevertheless, distinct activities of p57Kip2 remain poorly defined. Using in vivo and culture approaches, we show p57Kip2 overexpression at E14.5–15.5 elicits precursor cell cycle exit, promotes transition from proliferation to neuronal differentiation, and enhances process outgrowth, while opposite effects occur in p57Kip2-deficient precursors. Studies at later ages indicate p57Kip2 overexpression also induces precocious glial differentiation, suggesting stage-dependent effects. In embryonic cortex, p57Kip2 overexpression advances cell radial migration and alters postnatal laminar positioning. While both CKIs induce differentiation, p57Kip2 was twice as effective as p27Kip1 in inducing neuronal differentiation and was not permissive to astrogliogenic effects of ciliary neurotrophic factor, suggesting that the CKIs differentially modulate cell fate decisions. At molecular levels, although highly conserved N-terminal regions of both CKIs elicit cycle withdrawal and differentiation, the C-terminal region of p57Kip2 alone inhibits in vivo migration. Furthermore, p57Kip2 effects on neurogenesis and gliogenesis require the N-terminal cyclin/CDK binding/inhibitory domains, while previous p27Kip1 studies report cell cycle-independent functions. These observations suggest p57Kip2 coordinates multiple stages of corticogenesis and exhibits distinct and common activities compared with related family member p27Kip1.
gliogenesis; in utero electroporation; neurite outgrowth; neurogenesis; transfection
BACKGROUND: Low levels of the cyclin-dependent kinase inhibitor p27Kip1 are associated with poor prognosis in cancer. It is unclear whether this is related strictly to p27Kip1-mediated cell cycle inhibition or to other, possibly extranuclear, roles of this protein. In this study, we examined p27Kip1 expression in quiescent and activated lymphocytes. T-cell membranes have been shown to possess sphingolipid and cholesterol-rich microdomains that are insoluble in non-ionic detergents. These "rafts" provide a scaffold for signaling proteins. Signal transduction coincides with coalescence of these microdomains into larger complexes. METHODS: Localization of p27Kip1 was studied by electron and confocal microscopy. Association of p27Kip1 with membrane microdomains in unstimulated and stimulated lymphocytes was determined using Western blots analysis of isolated membranes variably treated with detergents. RESULTS: We demonstrated that p27Kip1 was present in clusters associated with the plasma membrane in normal lymphocytes. The solubility profile of p27Kip1 in isolated membranes indicated that it was localized to raft structures. When lymphocytes were stimulated, however, p27Kip1 was excluded from aggregated raft complexes. CONCLUSIONS: This study identifies, for the first time, the localization of p27 within a membrane microdomain associated with signaling. Because some cell surface signaling complexes lose p27Kip1 upon cellular activation, p27Kip1 may play a functional role in modulating membrane signaling.
Purpose: The cyclin-dependent kinase (Cdk) inhibitor p27Kip1 may be involved in regulating re-entry of residual hepatocytes into the cell cycle upon loss of liver tissue by partial hepatectomy (PH). As yet, changes in Kip1 expression during the initial period following PH are not well-characterized. We investigated immediate changes in Kip1 mRNA and protein levels as well as changes in Kip1 phosphorylation in liver tissue within the relevant time window between surgery and the onset of DNA synthesis at 10–12 h.
Methods: We used real-time PCR, quantitative Western blotting, and immune histochemistry on tissue samples of adult rats obtained during or between 2 and 10 h after surgical removal of two thirds of the liver to analyze Kip1 mRNA or protein levels, respectively, or to quantify nuclear expression of Kip1.
Results: Kip1 mRNA was down-regulated within 4 h after PH by 60% and remained unchanged thereafter up to 10 h. With a lag phase of 2–3 h, Kip1-protein was down-regulated to a level of 40% of the control. The level of Thr187-phosphorylated Kip1 started to increase at 4 h and reached a maximum level at 8–10 h after PH. Kip1 immunoreactivity was observed in 30% of the hepatocytes before PH. Within 6–8 h after PH, more than half of the hepatocytes lost nuclear Kip1 signals. Kip1-specific micro-RNAs (miRNA221, miRNA222) were not changed upon PH.
Conclusions: A portion of hepatocytes in adult rats constitutively express Kip1 and down-regulate Kip1 immediately upon PH. This response involves transcriptional processes (loss of Kip1 mRNA) as well as accelerated degradation of existing protein (increase in pThr187-phosphorylation mediating polyubiquitinylation and proteasomal degradation of Kip1). Kip1 down-regulation occurs precisely within the intervall between surgery and onset of DNA synthesis which supports the hypothesis that it mediates activation of G0/0S-phase Cdk/cyclin-complexes and re-entry of hepatocytes into the cell cycle.
cell cycle regulator; cyclin-dependent kinase inhibitor; Kip1; compensatory growth; liver regeneration; rat hepatocytes; cell proliferation
HER2/neu oncogene is frequently deregulated in cancers, and the (PI3K)-Akt signaling is one of the major pathways in mediating HER2/neu oncogenic signal. p57Kip2, an inhibitor of cyclin-depependent kinases, is pivotal in regulating cell cycle progression, but its upstream regulators remain unclear. Here we show that the HER2-Akt axis is linked to p57Kip2 regulation, and that Akt is a negative regulator of p57Kip2. Ectopic expression of Akt can decrease the expression of p57Kip2, while Akt inhibition leads to p57Kip2 stabilization. Mechanistic studies show that Akt interacts with p57Kip2 and causes cytoplasmic localization of p57Kip2. Akt phosphorylates p57 on Ser 282 or Thr310. Akt activity results in destabilization of p57 by accelerating turnover rate of p57 and enhancing p57 ubiquitination. Importantly, the negative impact of HER2/Akt on p57 stability contributes to HER2-mediated cell proliferation, transformational activity and tumorigenicity. p57 restoration can attenuate these defects caused by HER2. Significantly, Kaplan-Meier analysis of tumor samples demonstrate that in tumors where HER2 expression was observed, high expression levels of p57Kip2 were associated with better overall survival. These data suggest that HER2/Akt is an important negative regulator of p57Kip2, and that p57 restoration in HER2-overexpressing cells can reduce breast tumor growth. Our findings indicate the applicability of employing p57 regulation as a therapeutic intervention in HER2-overexpressing cancers.
Akt; cell cycle; HER2; p57 Kip2; phosphorylation; subcellular localizaiton
Cell cycle progression is regulated by the combined action of cyclins, cyclin-dependent kinases (CDKs), and CDK-inhibitors (CDKi), which are negative cell cycle regulators. p27KIP1 is a CDKi key in cell cycle regulation, whose degradation is required for G1/S transition. In spite of the absence of p27KIP1 expression in proliferating lymphocytes, some aggressive B-cell lymphomas have been reported to show an anomalous p27KIP1 staining. We analysed p27KIP1 expression in a series of Diffuse Large B-cell Lymphoma (DLBCL), correlating it with the proliferative index and clinical outcome, to characterize the implications of this anomalous staining in lymphomagenesis in greater depth. For the above mentioned purposes, an immunohistochemical technique in paraffin-embedded tissues was employed, using commercially available antibodies, in a series of 133 patients with known clinical outcomes. Statistical analysis was performed in order to ascertain which clinical and molecular variables may influence outcome, in terms of disease-free survival (DFS) and overall survival (OS). The relationships between p27KIP1 and MIB-1 (Ki-67) were also tested. An abnormally high expression of p27KIP1 was found in lymphomas of this type. The overall correlation between p27KIP1 and MIB-1 showed there to be no significant relationship between these two parameters, this differing from observations in reactive lymphoid and other tissues. Analysis of the clinical relevance of these findings showed that a high level of p27KIP1 expression in this type of tumour is an adverse prognostic marker, in both univariate and multivariate analysis. These results show that there is abnormal p27KIP1 expression in DLBCL, with adverse clinical significance, suggesting that this anomalous p27KIP1 protein may be rendered non-functional through interaction with other cell cycle regulator proteins. © 1999 Cancer Research Campaign
p27KIP1; MIB1; CDK inhibitors; diffuse large B-cell lymphoma; survival analysis
To understand how cellular differentiation is coupled to withdrawal from the cell cycle, we have focused on two negative regulators of the cell cycle, the MYC antagonist MAD1 and the cyclin-dependent kinase inhibitor p27KIP1. Generation of Mad1/p27KIP1 double-null mice revealed a number of synthetic effects between the null alleles of Mad1 and p27KIP1, including embryonic lethality, increased proliferation, and impaired differentiation of granulocyte precursors. Furthermore, with granulocyte cell lines derived from the Mad1/p27KIP1 double-null mice, we observed constitutive Myc expression and cyclin E-CDK2 kinase activity as well as impaired differentiation following treatment with an inducer of differentiation. By contrast, similar treatment of granulocytes from Mad1 or p27KIP1 single-null mice resulted in differentiation accompanied by downregulation of both Myc expression and cyclin E-CDK2 kinase activity. In the double-null granulocytic cells, addition of a CDK2 inhibitor in the presence of differentiation inducer was sufficient to restore differentiation and reduce Myc levels. We conclude that Mad1 and p27KIP1 operate, at least in part, by distinct mechanisms to downregulate CDK2 activity and Myc expression in order to promote cell cycle exit during differentiation.
Hypoxia induces the proliferation of pulmonary arterial smooth muscle cell (PASMC) in vivo and in vitro, and prostacyclin analogues are thought to inhibit the growth of PASMC. Previous studies suggest that p27kip1, a kind of cyclin-dependent kinase inhibitor, play an important role in the smooth muscle cell proliferation. However, the mechanism of hypoxia and the subcellular interactions between p27kip1 and prostacyclin analogues in human pulmonary arterial smooth muscle cell (HPASMC) are not fully understood.
We investigated the role of p27kip1 in the ability of Beraprost sodium (BPS; a stable prostacyclin analogue) to inhibit the proliferation of HPASMC during hypoxia. To clarify the biological effects of hypoxic air exposure and BPS on HPASMC, the cells were cultured in a hypoxic chamber under various oxygen concentrations (0.1–21%). Thereafter, DNA synthesis was measured as bromodeoxyuridine (BrdU) incorporation, the cell cycle was analyzed by flow cytometry with propidium iodide staining. The p27kip1 mRNA and protein expression and it's stability was measured by real-time RT-PCR and Western blotting. Further, we assessed the role of p27kip1 in HPASMC proliferation using p27kip1 gene knockdown using small interfering RNA (siRNA) transfection.
Although severe hypoxia (0.1% oxygen) suppressed the proliferation of serum-stimulated HPASMC, moderate hypoxia (2% oxygen) enhanced proliferation in accordance with enhanced p27kip1 protein degradation, whereas BPS suppressed HPASMC proliferation under both hypoxic and normoxic conditions by suppressing p27kip1 degradation with intracellular cAMP-elevation. The 8-bromo-cyclic adenosine monophosphate (8-Br-cAMP), a cAMP analogue, had similar action as BPS in the regulation of p27kip1. Moderate hypoxia did not affect the stability of p27kip1 protein expression, but PDGF, known as major hypoxia-induced growth factors, significantly decreased p27kip1 protein stability. We also demonstrated that BPS and 8-Br-cAMP suppressed HPASMC proliferation under both hypoxic and normoxic conditions by blocking p27kip1 mRNA degradation. Furthermore, p27kip1 gene silencing partially attenuated the effects of BPS and partially restored hypoxia-induced proliferation.
Our study suggests that moderate hypoxia induces HPASMC proliferation, which is partially dependent of p27kip1 down-regulation probably via the induction of growth factors such as PDGF, and BPS inhibits both the cell proliferation and p27kip1 mRNA degradation through cAMP pathway.
The cortactin oncoprotein is frequently overexpressed in head and neck squamous cell carcinoma (HNSCC), often due to amplification of the encoding gene (CTTN). While cortactin overexpression enhances invasive potential, recent research indicates that it also promotes cell proliferation, but how cortactin regulates the cell cycle machinery is unclear. In this article we report that stable short hairpin RNA-mediated cortactin knockdown in the 11q13-amplified cell line FaDu led to increased expression of the Cip/Kip cyclin-dependent kinase inhibitors (CDKIs) p21WAF1/Cip1, p27Kip1, and p57Kip2 and inhibition of S-phase entry. These effects were associated with increased binding of p21WAF1/Cip1 and p27Kip1 to cyclin D1- and E1-containing complexes and decreased retinoblastoma protein phosphorylation. Cortactin regulated expression of p21WAF1/Cip1 and p27Kip1 at the transcriptional and posttranscriptional levels, respectively. The direct roles of p21WAF1/Cip1, p27Kip1, and p57Kip2 downstream of cortactin were confirmed by the transient knockdown of each CDKI by specific small interfering RNAs, which led to partial rescue of cell cycle progression. Interestingly, FaDu cells with reduced cortactin levels also exhibited a significant diminution in RhoA expression and activity, together with decreased expression of Skp2, a critical component of the SCF ubiquitin ligase that targets p27Kip1 and p57Kip2 for degradation. Transient knockdown of RhoA in FaDu cells decreased expression of Skp2, enhanced the level of Cip/Kip CDKIs, and attenuated S-phase entry. These findings identify a novel mechanism for regulation of proliferation in 11q13-amplified HNSCC cells, in which overexpressed cortactin acts via RhoA to decrease expression of Cip/Kip CDKIs, and highlight Skp2 as a downstream effector for RhoA in this process.
The Cyclin-dependent kinase inhibitor 1B (p27Kip1) is a key protein in the decision between proliferation and cell cycle exit. Quiescent cells show nuclear p27Kip1, but this protein is exported to the cytoplasm in response to proliferating signals. We recently reported that catalase treatment increases the levels of p27Kip1 in vitro and in vivo in a murine model. In order to characterize and broaden these findings, we evaluated the regulation of p27Kip1 by hydrogen peroxide (H2O2) in human melanoma cells and melanocytes. We observed a high percentage of p27Kip1 positive nuclei in melanoma cells overexpressing or treated with exogenous catalase, while non-treated controls showed a cytoplasmic localization of p27Kip1. Then we studied the levels of p27Kip1 phosphorylated (p27p) at serine 10 (S10) and at threonine 198 (T198) because phosphorylation at these sites enables nuclear exportation of this protein, leading to accumulation and stabilization of p27pT198 in the cytoplasm. We demonstrated by western blot a decrease in p27pS10 and p27pT198 levels in response to H2O2 removal in melanoma cells, associated with nuclear p27Kip1. Melanocytes also exhibited nuclear p27Kip1 and lower levels of p27pS10 and p27pT198 than melanoma cells, which showed cytoplasmic p27Kip1. We also showed that the addition of H2O2 (0.1 µM) to melanoma cells arrested in G1 by serum starvation induces proliferation and increases the levels of p27pS10 and p27pT198 leading to cytoplasmic localization of p27Kip1. Nuclear localization and post-translational modifications of p27Kip1 were also demonstrated by catalase treatment of colorectal carcinoma and neuroblastoma cells, extending our findings to these other human cancer types. In conclusion, we showed in the present work that H2O2 scavenging prevents nuclear exportation of p27Kip1, allowing cell cycle arrest, suggesting that cancer cells take advantage of their intrinsic pro-oxidant state to favor cytoplasmic localization of p27Kip1.
CD8 T cells exhibit dynamic alterations in proliferation and apoptosis during various phases of the CD8 T cell response, but the mechanisms that regulate cellular proliferation from the standpoint of CD8 T cell memory are not well defined. The cyclin-dependent kinase inhibitor p27Kip1 functions as a negative regulator of the cell cycle in various cell types including T cells and it has been implicated in regulating cellular processes including differentiation, transcription and migration. Here, we investigated whether p27Kip1 regulates CD8 T cell memory by T cell-intrinsic or T cell-extrinsic mechanisms, by conditional ablation of p27Kip1 in T cells or non-T cells. Studies of T cell responses to an acute viral infection show that p27Kip1 negatively regulates the proliferation of CD8 T cells by T cell-intrinsic mechanisms. However, the enhanced proliferation of CD8 T cells induced by T cell-specific p27Kip1 deficiency minimally affects the primary expansion or the magnitude of CD8 T cell memory. Unexpectedly, p27Kip1 ablation in non-T cells markedly augmented the number of high quality memory CD8 T cells by enhancing the accumulation of memory precursor effector cells without increasing their proliferation. Further studies show that p27Kip1 deficiency in immunizing DCs fail to enhance CD8 T cell memory. Nevertheless, we have delineated the T cell-intrinsic, anti-proliferative activities of p27Kip1 in CD8 T cells from its role as a factor in non-T cells that restricts the development of CD8 T cell memory. These findings have implications in vaccine development and understanding the mechanisms that maintain T cell homeostasis.
CD8 T cells; Cell cycle; Memory; p27Kip1; Proliferation
p57 (Kip2, cyclin-dependent kinase inhibitor 1C), often found downregulated in cancer, is reported to hold tumor suppressor properties. Originally described as a cyclin-dependent kinase (cdk) inhibitor, p57KIP2 has since been shown to influence other cellular processes, beyond cell cycle regulation, including cell death and cell migration. Inhibition of cell migration by p57KIP2 is attributed to the stabilization of the actin cytoskeleton through the activation of LIM domain kinase-1 (LIMK-1). Furthermore, p57KIP2 is able to enhance mitochondrial-mediated apoptosis. Here, we report that the cell death promoting effect of p57KIP2 is linked to its effect on the actin cytoskeleton. Indeed, whereas Jasplakinolide, an actin cytoskeleton-stabilizing agent, mimicked p57KIP2's pro-apoptotic effect, destabilizing the actin cytoskeleton with cytochalsin D reversed p57KIP2's pro-apoptotic function. Conversely, LIMK-1, the enzyme mediating p57KIP2's effect on the actin cytoskeleton, was required for p57KIP2's death promoting effect. Finally, p57KIP2-mediated stabilization of the actin cytoskeleton was associated with the displacement of hexokinase-1, an inhibitor of the mitochondrial voltage-dependent anion channel, from the mitochondria, providing a possible mechanism for the promotion of the mitochondrial apoptotic cell death pathway. Altogether, our findings link together two tumor suppressor properties of p57KIP2, by showing that the promotion of cell death by p57KIP2 requires its actin cytoskeleton stabilization function.
p57KIP2; cell migration; cancer; cytoskeleton
The expression levels of the p21Cip1 family CDK inhibitors (CKIs), p21Cip1, p27Kip1 and p57Kip2, play a pivotal role in the precise regulation of cyclin-dependent kinase (CDK) activity, which is instrumental to proper cell cycle progression. The stabilities of p21Cip1, p27Kip1 and p57Kip2 are all tightly and differentially regulated by ubiquitylation and proteasome-mediated degradation during various stages of the cell cycle, either in steady state or in response to extracellular stimuli, which often elicit site-specific phosphorylation of CKIs triggering their degradation.
phosphorylation; ubiquitylation; proteasome; p21Cip1; p27Kip1; p57Kip2
The cyclin-dependent kinase inhibitors p15INK4b and p57KIP2 are important regulators of the cell cycle, and their abnormal expression has been detected in various tumors. However, little is known about the role of p15INK4b and p57KIP2 in the pathogenesis of vulvar carcinoma, and the prognostic impact is still unknown. In our current study, we examined the expression of p15INK4b and p57KIP2 in a large series of vulvar squamous cell carcinomas to elucidate the prognostic impact.
Expression of p15INK4b and p57KIP2 were examined in 297 vulvar squamous cell carcinomas using immunohistochemistry. Both uni- and multivariate analysis of prognostic factors were performed, and correlations with clinicopathologic parameters were examined.
Compared to the high levels of p15INK4b and p57KIP2 in normal vulvar squamous epithelium, low levels of p15INK4b and p57KIP2 were found in 82% and 44% of vulvar carcinomas, respectively. Low levels of p15INK4b and p57KIP2 correlated significantly with malignant features, including large tumor diameter (p = 0.03 and p = 0.001, respectively) and increased invasiveness (p = 0.003 and p = 0.04, respectively). Although p15INK4b and p57KIP2 levels could not be identified as prognostic markers, combined analysis of p14ARF/p15INK4b/p16INK4a showed that patients whose tumors expressed low levels of two or three of these INK4 proteins had a worse prognosis than those with only low levels of one or no protein (univariate analysis p = 0.02). The independent prognostic significance of these INK4 proteins was confirmed by multivariate analysis (p = 0.008).
We show for the first time that p15INK4b and p57KIP2 may be involved in the progression of vulvar carcinomas and the combined p14ARF/p15INK4b/p16INK4a status was a statistically independent prognostic factor.
p27Kip1 levels increase in many cells as they leave the cell cycle and begin to differentiate. The increase in p27Kip1 levels generally precedes the expression of differentiation-specific genes. Previous studies from our laboratory showed that the overexpression of p27Kip1 enhances myelin basic protein (MBP) promoter activity. This activation is specific to p27Kip1. Additionally, inhibition of cyclin-dependent kinase activity alone is not sufficient to increase MBP expression. In this study, we focused on understanding how p27Kip1 can activate gene transcription by using the MBP gene in oligodendrocytes as a model. We show that the enhancement of MBP promoter activity by p27Kip1 is mediated by a proximal region of the MBP promoter that contains a conserved GC box binding sequence. This sequence binds transcription factors Sp1 and Sp3. Increased expression of p27Kip1 increases the level of Sp1 promoter binding to the GC box but does not change the level of Sp3 binding. The binding of Sp1 to this element activates the MBP promoter. p27Kip1 leads to increased Sp1 binding through a decrease in Sp1 protein turnover. Enhancement of MBP promoter activity by an increase in the level of p27Kip1 involves a novel mechanism that is mediated through the stabilization and binding of transcription factor Sp1.
Endoreduplication is an unusual form of cell cycle in which rounds of DNA synthesis repeat in the absence of intervening mitoses. How G1/S cyclin-dependent kinase (Cdk) activity is regulated during the mammalian endocycle is poorly understood. We show here that expression of the G1/S Cdk inhibitor p57Kip2 is induced coincidentally with the transition to the endocycle in trophoblast giant cells. Kip2 mRNA is constitutively expressed during subsequent endocycles, but the protein level fluctuates. In trophoblast giant cells synchronized for the first few endocycles, the p57Kip2 protein accumulates only at the end of S-phase and then rapidly disappears a few hours before the onset of the next S-phase. The protein becomes stabilized by mutation of a C-terminal Cdk phosphorylation site. As a consequence, introduction of this stable form of p57Kip2 into giant cells blocks S-phase entry. These data imply that p57Kip2 is subject to phosphorylation-dependent turnover. Surprisingly, although this occurs in endoreduplicating giant cells, p57Kip2 is stable when ectopically expressed in proliferating trophoblast cells, indicating that these cells lack the mechanism for protein targeting and/or degradation. These data show that the appearance of p57Kip2 punctuates the completion of DNA replication, whereas its turnover is subsequently required to initiate the next round of endoreduplication in trophoblast giant cells. Cyclical expression of a Cdk inhibitor, by terminating G1/S Cdk activity, may help promote the resetting of DNA replication machinery.