The cyclin-dependent kinase inhibitor p27Kip1 is localized to the cytosol in Swiss/3T3 cells

Invadopodia-mediated ECM degradation is enhanced in the G1 phase of the cell cycle

The process of tumor cell invasion and metastasis includes assembly of invadopodia, protrusions capable of degrading the extracellular matrix (ECM). The effect of cell cycle progression on invadopodia has not been elucidated. In this study, by using invadopodia and cell cycle fluorescent markers, we show in 2D and 3D cultures, as well as in vivo, that breast carcinoma cells assemble invadopodia and invade into the surrounding ECM preferentially during the G1 phase. The expression (MT1-MMP, also known as MMP14, and cortactin) and localization (Tks5; also known as SH3PXD2A) of invadopodia components are elevated in G1 phase, and cells synchronized in G1 phase exhibit significantly higher ECM degradation compared to the cells synchronized in S phase. The cyclin-dependent kinase inhibitor (CKI) p27^kip1 (also known as CDKN1B) localizes to the sites of invadopodia assembly. Overexpression and stable knockdown of p27^kip1 lead to contrasting effects on invadopodia turnover and ECM degradation. Taken together, these findings suggest that expression of invadopodia components, as well as invadopodia function, are linked to cell cycle progression, and that invadopodia are controlled by cell cycle regulators. Our results caution that this coordination between invasion and cell cycle must be considered when designing effective chemotherapies.

Disease-associated mutations of TDP-43 promote turnover of the protein through the proteasomal pathway

TAR DNA-binding protein (TDP-43) is a major component of most ubiquitin-positive neuronal and glial inclusions of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). A number of missense mutations in the TARDBP gene have been identified in patients with familial and sporadic ALS, as well as familial FTLD with ALS. In the diseased states, TDP-43 proteins exhibit characteristic alterations, including truncation, abnormal phosphorylation, and altered subcellular distribution. However, the mechanisms by which TDP-43 mutations induce neurodegeneration remain unclear at present. In the current study, we analyzed protein turnover and subcellular distribution of wild-type TDP-43 and two disease-associated mutants (G298S and A382T) in human neuroblastoma SH-SY5Y cells stably expressing TDP-43 with a C-terminal tag. Cycloheximide chase experiments revealed more rapid turnover of TDP-43 mutant proteins than their wild-type counterpart. The decrease in the TDP-43 level after cycloheximide treatment was partially recovered upon co-treatment with the proteasome inhibitor, epoxomicin, but not the lysosomotropic agent, chloroquine, suggesting involvement of the proteasomal pathway in TDP-43 degradation. Analysis of the subcellular distribution of TDP-43 revealed predominant localization in the nuclear fraction, whereas the relative level in the cytoplasm remained unaltered in cells expressing either mutant protein, compared with wild-type protein. Our results suggest that higher turnover of disease-associated mutant TDP-43 proteins through the ubiquitin proteasome system is pathogenetically relevant and highlight the significance of proteolysis in the pathogenetic mechanism of TDP-43 proteinopathy.

Ablation of Akt2 induces autophagy through cell cycle arrest, the downregulation of p70S6K, and the deregulation of mitochondria in MDA-MB231 cells

Background

Akt/PKB is a promising anticancer therapeutic target, since abnormally elevated Akt activity is directly correlated to tumor development, progression, poor prognosis and resistance to cancer therapies. Currently, the unique role of each Akt isoform and their relevance to human breast cancer are poorly understood.

Methodology/Principal Findings

We previously found that Akt1, 2 and 3 are localized at specific subcellular compartments (the cytoplasm, mitochondria and nucleus, respectively), raising the possibility that each isoform may have unique functions and employ different regulation mechanisms. By systematically studying Akt-ablated MDA-MB231 breast cancer cells with isoform-specific siRNA, we here show that Akt2 is the most relevant isoform to cell proliferation and survival in our cancer model. Prolonged ablation of Akt2 with siRNA resulted in cell-cycle arrest in G0/G1 by downregulating Cdk2 and cyclin D, and upregulating p27. The analysis of the Akt downstream signaling pathways suggested that Akt2 specifically targets and activates the p70S6K signaling pathway. We also found that Akt2 ablation initially resulted in an increase in the mitochondrial volume concomitantly with the upregulation of PGC-1α, a regulator of mitochondrial biogenesis. Prolonged ablation of Akt2, but not Akt1 or Akt3, eventually led to cell death by autophagy of the mitochondria (i.e., mitophagy).

Conclusions/Significance

Collectively, our data demonstrates that Akt2 augments cell proliferation by facilitating cell cycle progression through the upregulation of the cell cycle engine, and protects a cell from pathological autophagy by modulating mitochondrial homeostasis. Our data, thus, raises the possibility that Akt2 can be an effective anticancer target for the control of (breast) cancer.

Targeted anti-vascular endothelial growth factor receptor-2 therapy leads to short-term and long-term impairment of vascular function and increase in tumor hypoxia

Because antiangiogenic therapies inhibit the growth of new tumor-associated blood vessels, as well as prune newly formed vasculature, they would be expected to reduce the supply of oxygen and thus increase tumor hypoxia. However, it is not clear if antiangiogenic treatments lead only to consistent and sustained increases in hypoxia, or transient decreases in tumor hypoxia along with periods of increased hypoxia. We undertook a detailed analysis of an orthotopically transplanted human breast carcinoma (MDA-MB-231) over a 3-week treatment period using DC101, an anti-vascular endothelial growth factor receptor 2 antibody. We observed consistent reductions in microvascular density, blood flow (measured by high-frequency micro-ultrasound), and perfusion. These effects resulted in an increase in the hypoxic tumor fraction, measured with an exogenous marker, pimonidazole, concurrent with an elevation in hypoxia-inducible factor-1alpha expression, an endogenous marker. The increase in tumor hypoxia was evident within 5 days and remained so throughout the entire course of treatment. Vascular perfusion and flow were impaired at days 2, 5, 7, 8, 14, and 21 after the first injection, but not at 4 hours. A modest increase in the vessel maturation index was detected after the 3-week treatment period, but this was not accompanied by an improvement in vascular function. These results suggest that sustained hypoxia and impairment of vascular function can be two consistent consequences of antiangiogenic drug treatment. The implications of the results are discussed, particularly with respect to how they relate to different theories for the counterintuitive chemosensitizing effects of antiangiogenic drugs, even when hypoxia is increased.

The plant cell cycle

Cell division in plants is controlled by the activity of cyclin-dependent kinase (CDK) complexes. Although this basic mechanism is conserved with all other eukaryotes, plants show novel features of cell-cycle control in the molecules involved and their regulation, including novel CDKs showing strong transcriptional regulation in mitosis. Plant development is characterized by indeterminate growth and reiteration of organogenesis and is therefore intimately associated with cell division. This may explain why plants have a large number of cell-cycle regulators that appear to have overlapping and distinct functions. Here we review the recent considerable progress in understanding how core cell-cycle regulators are involved in integrating and coordinating cell division at the molecular level.

Novel p27(kip1) C-terminal scatter domain mediates Rac-dependent cell migration independent of cell cycle arrest functions

Hepatocyte growth factor (HGF) signaling via its receptor, the proto-oncogene Met, alters cell proliferation and motility and has been associated with tumor metastasis. HGF treatment of HepG2 human hepatocellular carcinoma cells induces cell migration concomitant with increased levels of the p27(kip1) cyclin-cdk inhibitor. HGF signaling resulted in nuclear export of endogenous p27 to the cytoplasm, via Ser-10 phosphorylation, where it colocalized with F-actin. Introduction of transducible p27 protein (TATp27) was sufficient for actin cytoskeletal rearrangement and migration of HepG2 cells. TATp27 mutational analysis identified a novel p27 C-terminal domain required for cell migration, distinct from the N-terminal cyclin-cyclin-dependent kinase (cdk) binding domain. Loss or disruption of the p27 C-terminal domain abolished both actin rearrangement and cell migration. The cell-scattering activity of p27 occurred independently of its cell cycle arrest functions and required cytoplasmic localization of p27 via Ser-10 phosphorylation. Furthermore, Rac GTPase was necessary for p27-dependent migration but alone was insufficient for HepG2 cell migration. These results predicted a migration defect in p27-deficient cells. Indeed, p27-deficient primary fibroblasts failed to migrate, and reconstitution with TATp27 rescued the motility defect. These observations define a novel role for p27 in cell motility that is independent of its function in cell cycle inhibition.

H2O2 induces a transient multi-phase cell cycle arrest in mouse fibroblasts through modulating cyclin D and p21Cip1 expression

To defend against the potential damages induced by reactive oxygen species, proliferating cells enter a transient cell cycle arrest. We treated mouse fibroblasts with H(2)O(2) and found that sublethal doses of H(2)O(2) induced a transient multi-phase cell cycle arrest at the G(1), S, and G(2) phases but not the M phase. Western blot analysis demonstrated that this transient cell cycle arrest is associated with the down-regulation of cyclins D1 and D3 and up-regulation of the CKI p21(Cip1) expression. We also demonstrate that the induction in p21(Cip1) expression by H(2)O(2) is at least partially mediated at the transcriptional level and can occur in the absence of p53 function. Further immunoprecipitation kinase and immunodepletion assays indicated that in response to H(2)O(2) treatment, the down-regulation of cyclin Ds expression are associated with repression of cyclin D-CDK4, whereas the accumulation of p21(Cip1) is responsible for the inhibition of cyclin E and A-CDK2 activity and associated with the down-regulation of cyclin B-CDC2 activity. These data could account for the cell cycle arrest at the G(1), S, and G(2) phases following H(2)O(2) stimulation. Deletion of p21(Cip1), restoration of cyclin D expression, or overexpression of cyclin E alone is insufficient to effectively overcome the cell cycle arrest caused by sublethal doses of H(2)O(2). By contrast, overexpression of the human Herpesvirus 8 K cyclin, which can mimic the function of cyclin D and E, is enough to override this transient cell cycle arrest. On the basis of our findings, we propose a model in which moderate levels of H(2)O(2) induce a transient multi-phase cell cycle arrest at least partially through up-regulation of p21(Cip1) and down-regulation of cyclin D expression.

Differential effects of transforming growth factor on cell cycle regulatory molecules in human myeloid leukemia cells

In this report we have studied the mechanism by which Transforming Growth Factor beta (TGF beta) inhibits growth of human myeloid leukemia cell lines. TGF beta 1 arrested cells in G1 phase and significantly downregulated the expression of cyclin D2, cyclin D3, cdk4, cyclin A, and cdk2. The downregulation of the molecules resulted in approximately 50-90% decrease of the molecule-dependent kinase activity, varying with each molecule. Although treatment of cells with TGF beta 1 up-regulated accumulation of p27(kip1) in both nucleus and cytoplasm, the association of the p27(kip1) with cdk2, cyclin A, cyclin D2, cyclin D3, and cdk4 was markedly down-regulated, suggesting that p27(kip1) is not responsible for the downregulation of the kinase activity. In contrast, TGF beta 1 upregulated cyclin E-associated p27(kip1) with no effect on the expression of cyclin E. p27(kip1)-immunodepletion upregulated cyclin E-dependent kinase activity by more than 10-fold in TGF beta 1-treated cells but not in proliferating cells; whereas immunodepletion of p27(kip1) from cdk2-immunoprecipitates markedly downregulated cdk2 kinase activity in the lysates extracted from both proliferating and TGF beta-treated cells. Consistent with this observation, TGF beta 1 and p27(kip1) antisense cDNA had a synergistic or additive inhibitory effect on cdk2 but not cyclin E-dependent kinase activity. Our data suggest that (1) TGF beta 1-mediated growth inhibition is accomplished through multiple pathways and (2) p27(kip1) has opposing effects on cdk2 and cyclin E activity in response to TGF beta 1.

The p42/p44 mitogen-activated protein kinase activation triggers p27Kip1 degradation independently of CDK2/cyclin E in NIH 3T3 cells

The p42/p44 mitogen-activated protein (MAP) kinase is stimulated by various mitogenic stimuli, and its sustained activation is necessary for cell cycle G(1) progression and G(1)/S transition. G(1) progression and G(1)/S transition also depend on sequential cyclin-dependent kinase (CDK) activation. Here, we demonstrate that MAP kinase inhibition leads to accumulation of the CDK inhibitor p27(Kip1) in NIH 3T3 cells. Blocking the proteasome-dependent degradation of p27(Kip1) impaired this accumulation, suggesting that MAP kinase does not act on p27(Kip1) protein synthesis. In the absence of extracellular signals (growth factors or cell adhesion), genetic activation of MAP kinase decreased the expression of p27(Kip1) as assessed by cotransfection experiments and by immunofluorescence detection. Importantly, MAP kinase activation also decreased the expression of a p27(Kip1) mutant, which cannot be phosphorylated by CDK2, suggesting that MAP kinase-dependent p27(Kip1) regulation is CDK2-independent. Accordingly, expression of dominant-negative CDK2 did not impair the down-regulation of p27(Kip1) induced by MAP kinase activation. These data demonstrate that the MAP kinase pathway regulates p27(Kip1) expression in fibroblasts essentially through a degradation mechanism, independently of p27(Kip1) phosphorylation by CDK2. This strengthens the role of this CDK inhibitor as a key effector of G(1) growth arrest, whose expression can be controlled by extracellular stimuli-dependent signaling pathways.

Minimal requirements for the nuclear localization of p27(Kip1), a cyclin-dependent kinase inhibitor

p27(Kip1) is a cyclin-dependent kinase inhibitor, and its nuclear localization is a prerequisite for it to function as a cell cycle regulator. In the present study, the minimal requirement for the nuclear localization signal (NLS) of p27(Kip1) was determined by analyzing the localization of various mutants of p27(Kip1) tagged with green fluorescent protein (GFP) in HeLa cells and porcine aortic endothelial cells. Wild-type p27(Kip1) exclusively localized into nucleus, while GFP alone localized in both cytosol and nucleus. A comparison of various truncation mutants revealed residues 153-166 to be the minimal region necessary for nuclear localization. However, a fusion of this region to GFP showed cytoplasmic retention in addition to nuclear localization, thus suggesting that some extension flanking this region is required to achieve a full function of NLS. The site-directed mutation of the full-length p27(Kip1) therefore showed that four basic residues (K153, R154, K165, R166), especially R166, play a critical role in the nuclear localization of p27(Kip1).

Density-dependent growth inhibition of fibroblasts ectopically expressing p27(kip1)

The cyclin/cyclin-dependent kinase (cdk) inhibitor p27(kip1) is thought to be responsible for the onset and maintenance of the quiescent state. It is possible, however, that cells respond differently to p27(kip1) 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 p27(kip1) on cell cycle traverse is determined by cell density. We found that ectopic expression of p27(kip1) 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 p27(kip1) 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 p27(kip1) 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 p27(kip1)-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.

Multiple functions of p27(Kip1) and its alterations in tumor cells: a review

Cyclin-dependent kinases (CDKs), together with cyclins, their regulatory subunits, govern cell-cycle progression in eukaryotic cells. p27(Kip1) is a member of a family of CDK inhibitors (CDIs) that bind to cyclin/CDK complexes and arrest cell division. There is considerable evidence that p27(Kip1) plays an important role in multiple fundamental cellular processes, including cell proliferation, cell differentiation, and apoptosis. Moreover, p27(Kip1) is a putative tumor-suppressor gene that appears to play a critical role in the pathogenesis of several human malignancies and its reduced expression has been shown to correlate with poor prognosis in cancer patients. This study reviews current information on the functions of p27(Kip1), its abnormalities found in human tumors, and the possible clinical implications of these findings with respect to the management of cancer patients.