PKCeta is localized in the Golgi, ER and nuclear envelope and translocates to the nuclear envelope upon PMA activation and serum-starvation: C1b domain and the pseudosubstrate containing fragment target PKCeta to the Golgi and the nuclear envelope

Protein kinase C (PKC) represents a family of serin/threonine kinases, playing a central role in the regulation of cell growth, differentiation and transformation. These enzymes differ in their primary structure, biochemical properties, tissue distribution and subcellular localization. The specific cellular functions of PKC isoforms are largely controlled by their localization. PKCeta, a member of the novel subfamily, is expressed predominantly in epithelial tissues. However, not much is known with respect to its mechanism of activation and regulation. Our recent studies suggest its role in cell cycle control. Here we show that PKCeta is localized at the Golgi apparatus, ER and the nuclear envelope. Furthermore, using GFP-fusion proteins of the different functional domains of PKCeta we deciphered the specific structural domains of the protein responsible for its apparent localization. We show that the cysteine-rich repeat C1b is responsible for its Golgi localization, while for its presence at the ER/nuclear envelope the pseudosubstrate containing fragment coupled to the C1 domain is required. In response to short-term activation by PMA we show translocation of PKCeta to the plasma membrane and the nuclear envelope. We demonstrate that the C1b is sufficient for its translocation to the plasma membrane. Interestingly, accumulation of PKCeta at the nuclear envelope also occurred in response to serum-starvation. It should be noted that interaction of PKCeta with the cyclin E/Cdk2 complex at the perinuclear region was recently reported by us in response to serum-starvation. Thus, our studies demonstrate translocation of PKCeta to the nuclear envelope, and suggest that the spatial regulation of PKCeta could be important for its cellular functions including effects on cell cycle control and involvement in tumor promotion.     

Cyclin E2, a Novel G1 Cyclin That Binds Cdk2 and Is Aberrantly Expressed in Human Cancers

A novel cyclin gene was discovered by searching an expressed sequence tag database with a cyclin box profile. The human cyclin E2 gene encodes a 404-amino-acid protein that is most closely related to cyclin E. Cyclin E2 associates with Cdk2 in a functional kinase complex that is inhibited by both p27^Kip1 and p21^Cip1. The catalytic activity associated with cyclin E2 complexes is cell cycle regulated and peaks at the G₁/S transition. Overexpression of cyclin E2 in mammalian cells accelerates G₁, demonstrating that cyclin E2 may be rate limiting for G₁ progression. Unlike cyclin E1, which is expressed in most proliferating normal and tumor cells, cyclin E2 levels were low to undetectable in nontransformed cells and increased significantly in tumor-derived cells. The discovery of a novel second cyclin E family member suggests that multiple unique cyclin E-CDK complexes regulate cell cycle progression.     

Functional Interaction between the Bovine Papillomavirus Virus Type 1 Replicative Helicase E1 and Cyclin E-Cdk2

We have found that the replicative helicase E1 of bovine papillomavirus type 1 (BPV-1) interacts with a key cell cycle regulator of S phase, the cyclin E-Cdk2 kinase. The E1 helicase, which interacts with cyclin E and not with Cdk2, presents the highest affinity for catalytically active kinase complexes. In addition, E1, cyclin E, and Cdk2 expressed in Xenopus egg extracts are quantitatively coimmunoprecipitated from crude extracts by either anti-Cdk2 or anti-E1 antibodies. E1 protein is also a substrate of the cyclin E-Cdk2 kinase in vitro. Using the viral components required for in vitro BPV-1 replication and free-membrane cytosol from Xenopus eggs, we show that efficient replication of BPV plasmids is dependent on the addition of E1-cyclin E-Cdk2 complexes. Thus, the BPV initiator of replication and cyclin E-Cdk2 are likely to function together as a protein complex which may be the key to the cell cycle regulation of papillomavirus replication.     

Phosphorylation independent activation of human cyclin-dependent kinase 2 by cyclin A in vitro.

p33cdk2 is a serine-threonine protein kinase that associates with cyclins A, D, and E and has been implicated in the control of the G1/S transition in mammalian cells. Recent evidence indicates that cyclin-dependent kinase 2 (Cdk2), like its homolog Cdc2, requires cyclin binding and phosphorylation (of threonine-160) for activation in vivo. However, the extent to which mechanistic details of the activation process are conserved between Cdc2 and Cdk2 is unknown. We have developed bacterial expression and purification systems for Cdk2 and cyclin A that allow mechanistic studies of the activation process to be performed in the absence of cell extracts. Recombinant Cdk2 is essentially inactive as a histone H1 kinase (< 4 x 10(-5) pmol phosphate transferred.min-1 x microgram-1 Cdk2). However, in the presence of equimolar cyclin A, the specific activity is approximately 16 pmol.mon-1 x microgram-1, 4 x 10(5)-fold higher than Cdk2 alone. Mutation of T160 in Cdk2 to either alanine or glutamic acid had little impact on the specific activity of the Cdk2/cyclin A complex: the activity of Cdk2T160E was indistinguishable from Cdk2, whereas that of Cdk2T160A was reduced by five-fold. To determine if the Cdk2/cyclin A complex could be activated further by phosphorylation of T160, complexes were treated with Cdc2 activating kinase (CAK), purified approximately 12,000-fold from Xenopus eggs. This treatment resulted in an 80-fold increase in specific activity. This specific activity is comparable with that of the Cdc2/cyclin B complex after complete activation by CAK (approximately 1600 pmol.mon-1 x microgram-1). Neither Cdk2T160A/cyclin A nor Cdk2T160E/cyclin A complexes were activated further by treatment with CAK. In striking contrast with cyclin A, cyclin B did not directly activate Cdk2. However, both Cdk2/cyclin A and Cdk2/cyclin B complexes display similar activity after activation by CAK. For the Cdk2/cyclin A complex, both cyclin binding and phosphorylation contribute significantly to activation, although the energetic contribution of cyclin A binding is greater than that of T160 phosphorylation by approximately 5 kcal/mol. The potential significance of direct activation of Cdk2 by cyclins with respect to regulation of cell cycle progression is discussed.     

Retinoic Acid-Mediated G1 Arrest Is Associated with Induction of p27 and Inhibition of Cyclin-Dependent Kinase 3 in Human Lung Squamous Carcinoma CH27 Cells

Retinoids are promising agents for the prevention and treatment of several human malignancies including lung cancer. In this study, the effect of retinoic acid (RA) on cell growth and the mechanism of growth modulation were examined in human lung squamous carcinoma CH27 cells. Here we report that RA mediated the dose- and time-dependent growth arrest in G1 phase, accompanied by the up-regulation of p27^Kip1 and the down-regulation of the cyclin-dependent kinase 3 (Cdk3) and p21^CIP1/Waf1 proteins. Furthermore, RA-induced growth arrest of CH27 cells was also associated with increased retinoic acid receptor β (RARβ) and reduced c-Myc expression. However, RA had no effect on the levels of cyclins A, D1, D3, E, or H, or on Cdk2, Cdk4, Cdk5, CDk6, Cdk7, p16^Ink4A, p15^Ink4B, p53, or pRb proteins in CH27 cells. Evaluation of the kinase activity of cyclin–Cdk complexes showed that RA increases p27^Kip1 expression in CH27 cells leading to markedly reduced cyclin A/Cdk2 kinase activity and slightly reduced cyclin E/Cdk2 kinase activity, with no effect on cyclin D/Cdk4 and cyclin D/Cdk6 activities. Moreover, coincident with the decrease in kinase activity was a drastic increase in cyclin A-bound p27^Kip1. These results suggest that increases in the levels of p27^Kip1 and its binding to cyclin A, as well as reduction of Cdk3 protein expression, are strong candidates for the cell cycle regulator that prevents the entry into the S phase in RA-treated CH27 cells, with prolongation of G1 phase and inhibition of DNA synthesis.     

Expression of PKCeta in NIH-3T3 cells promotes production of the pro-inflammatory cytokine interleukin-6

Protein kinase C encodes a family of enzymes implicated in cellular differentiation, growth control and tumor promotion. The generation and characterization of NIH-3T3 cells which stably overexpress the PKCeta isoform has been previously described by us. In these cells, overexpression of PKCeta altered the expression of specific cell cycle regulators and promoted differentiation [20]. Since PKC has been implicated in the regulation of gene expression, including that of various cytokines, we examined the production of several cytokines in these cells. We report here that out of the major pro-inflammatory cytokines examined, IL-1alpha, IL-1beta, TNF-alpha and IL-6, only IL-6 was generated and secreted in PKCeta -expressing cells without any additional inducer in serum-supplemented cultures (10% FCS). IL-6 was not detected in the control cell line, transfected with the same vector, but lacking the cDNA coding for PKCeta. Moreover, the production of IL-6 on serum stimulation correlated with the levels of PKCeta expressed in these cells. This implies that factors in the serum activate PKCeta and induce IL-6 production. We have examined several growth factors and cytokines for their ability to induce IL-6 production in our PKCeta-expressing cells. Among the growth factors tested (EGF, PDGF, FGF, insulin, IGF-1 and IL-1), PDGF and FGF were the most potent IL-6 inducers. The effects of FGF and PDGF on IL-6 production were blocked in the presence of PKC inhibitors. We also examined the signaling pathways that mediate production of IL-6 in PKCeta-expressing cells. Using specific inhibitors of the MAPK pathway, we have shown a role for ERK and p38 MAPK in FGF- and serum-stimulated IL-6 production, but only for p38 MAPK in PDGF-stimulated IL-6 production. Our studies provide evidence that PDGF and FGF can serve as upstream regulators of PKCeta and that PKCeta is involved in the expression of IL-6. This suggests that inhibition of PKC may provide a basis for the development of drugs for the treatment of disorders in which IL-6 is pathologically involved.     

Cyclin D1 Promotes Cell Cycle Progression through Enhancing NDR1/2 Kinase Activity Independent of Cyclin-dependent Kinase 4

Cyclin/cyclin-dependent kinases (Cdks) are critical protein kinases in regulating cell cycle progression. Among them, cyclin D1/Cdk4 exerts its function mainly in the G₁ phase. By using the tandem affinity purification tag approach, we identified a set of proteins interacting with Cdk4, including NDR1/2. Interestingly, confirming the interactions between NDR1/2 and cyclin D1/Cdk4, we observed that NDR1/2 interacted with cyclin D1 independent of Cdk4, but NDR1/2 and cyclin D1/Cdk4 did not phosphorylate each other. In addition, we found that NDR1/2 did not affect the kinase activity of cyclin D1/Cdk4 upon phosphorylation of GST-Rb. However, cyclin D1 but not Cdk4 promoted the kinase activity of NDR1/2. We also demonstrated that cyclin D1 K112E, which could not bind Cdk4, enhanced the kinase activity of NDR1/2. To test whether cyclin D1 promotes G₁/S transition though enhancing NDR1/2 kinase activity, we performed flow cytometry analysis using cyclin D1 and cyclin D1 K112E Tet-On inducible cell lines. The data show that both cyclin D1 and cyclin D1 K112E promoted G₁/S transition. Importantly, knockdown of NDR1/2 almost completely abolished the function of cyclin D1 K112E in promoting G₁/S transition. Consistently, we found that the protein level of p21 was reduced in cells overexpressing cyclin D1 K112E but not when NDR1/2 was knocked down. Taken together, these results reveal a novel function of cyclin D1 in promoting cell cycle progression by enhancing NDR kinase activity independent of Cdk4.     

Cyclin D3 and c-MYC control glucocorticoid-induced cell cycle arrest but not apoptosis in lymphoblastic leukemia cells

Glucocorticoids (GC) induce cell cycle arrest and apoptosis in lymphoblastic leukemia cells. To investigate cell cycle effects of GC in the absence of obscuring apoptotic events, we used human CCRF-CEM leukemia cells protected from cell death by transgenic bcl-2. GC treatment arrested these cells in the G1 phase of the cell cycle due to repression of cyclin D3 and c-myc. Cyclin E and Cdk2 protein levels remained high, but the kinase complex was inactive due to increased levels of bound p27(Kip1). Conditional expression of cyclin D3 and/or c-myc was sufficient to prevent GC-induced G1 arrest and p27(Kip1) accumulation but, importantly, did not interfere with the induction of apoptosis. The combined data suggest that repression of both, c-myc and cyclin D3, is necessary to arrest human leukemia cells in the G1 phase of the cell division cycle, but that neither one is required for GC-induced apoptosis.     

Molecular basis for the specificity of p27 toward cyclin-dependent kinases that regulate cell division

The cyclin-dependent kinase inhibitors (CKIs) bind to and directly regulate the catalytic activity of cyclin-dependent kinase (Cdk)/cyclin complexes involved in cell cycle control and do not regulate other, closely related Cdks. We showed previously that the CKI, p27, binds to Cdk2/cyclin A though a sequential mechanism that involves folding-on-binding. The first step in the kinetic mechanism is interaction of a small, highly dynamic domain of p27 (domain 1) with the cyclin subunit of the Cdk2/cyclin A complex, followed by much slower binding of a more lengthy and less flexible domain (domain 2) to Cdk2. The second step requires folding of domain 2 into the kinase inhibitory conformation. Rapid binding of p27 domain 1 to cyclin A tethers the inhibitor to the binary Cdk2/cyclin A complex, which reduces the entropic barrier associated with slow binding of domain 2 to the catalytic subunit. We show here that p27/cyclin interactions are an important determinant of p27 specificity towards cell cycle Cdks. We used surface plasmon resonance, limited proteolysis, mass spectrometry, and NMR spectroscopy to study the interaction of p27 with Cdk2/cyclin A, and with another Cdk complex, Cdk5/p25, that is involved in neurodegeneration. Importantly, Cdk5/p35 (the parent complex of Cdk5/p25) is not regulated by p27 in neurons. Our results show that p27 binds to Cdk5 and Cdk2 with similar, slow kinetics. However, p27 fails to interact with p25 within the Cdk5/p25 complex, which we believe prevents formation of a kinetically trapped, inhibited p27/Cdk5/p25 complex in vivo. The helical topology of p25 is very similar to that of cyclin A. However, p25 lacks the MRAIL sequence in one helix that, in the cell cycle cyclins, mediates specific interactions with domain 1 of p21 and p27. Our results strongly suggest that p21 and p27, related Cdk inhibitors, select their cell cycle regulatory Cdk targets by binding specifically to the cyclin subunit of these Cdk/cyclin complexes as a first step in a sequential, folding-on-binding mechanism.     

Early events in DNA replication require cyclin E and are blocked by p21CIP1

Using immunodepletion of cyclin E and the inhibitor protein p21WAF/CIP1, we demonstrate that the cyclin E protein, in association with Cdk2, is required for the elongation phase of replication on single-stranded substrates. Although cyclin E/Cdk2 is likely to be the major target by which p21 inhibits the initiation of sperm DNA replication, p21 can inhibit single-stranded replication through a mechanism dependent on PCNA. While the cyclin E/Cdk2 complex appears to have a role in the initiation of DNA replication, another Cdk kinase, possibly cyclin A/Cdk, may be involved in a later step controlling the switch from initiation to elongation. The provision of a large maternal pool of cyclin E protein shows that regulators of replication are constitutively present, which explains the lack of a protein synthesis requirement for replication in the early embryonic cell cycle.     

c-Myc Regulates Cyclin D-Cdk4 and -Cdk6 Activity but Affects Cell Cycle Progression at Multiple Independent Points

c-myc is a cellular proto-oncogene associated with a variety of human cancers and is strongly implicated in the control of cellular proliferation, programmed cell death, and differentiation. We have previously reported the first isolation of a c-myc-null cell line. Loss of c-Myc causes a profound growth defect manifested by the lengthening of both the G1 and G2 phases of the cell cycle. To gain a clearer understanding of the role of c-Myc in cellular proliferation, we have performed a comprehensive analysis of the components that regulate cell cycle progression. The largest defect observed in c-myc-/- cells is a 12-fold reduction in the activity of cyclin D1-Cdk4 and -Cdk6 complexes during the G0-to-S transition. Downstream events, such as activation of cyclin E-Cdk2 and cyclin A-Cdk2 complexes, are delayed and reduced in magnitude. However, it is clear that c-Myc affects the cell cycle at multiple independent points, because restoration of the Cdk4 and -6 defect does not significantly increase growth rate. In exponentially cycling cells the absence of c-Myc reduces coordinately the activities of all cyclin-cyclin-dependent kinase complexes. An analysis of cyclin-dependent kinase complex regulators revealed increased expression of p27(KIP1) and decreased expression of Cdk7 in c-myc-/- cells. We propose that c-Myc functions as a crucial link in the coordinate adjustment of growth rate to environmental conditions.     

Retinoic acid-mediated G1 arrest is associated with induction of p27(Kip1) and inhibition of cyclin-dependent kinase 3 in human lung squamous carcinoma CH27 cells

Retinoids are promising agents for the prevention and treatment of several human malignancies including lung cancer. In this study, the effect of retinoic acid (RA) on cell growth and the mechanism of growth modulation were examined in human lung squamous carcinoma CH27 cells. Here we report that RA mediated the dose- and time-dependent growth arrest in G1 phase, accompanied by the up-regulation of p27(Kip1) and the down-regulation of the cyclin-dependent kinase 3 (Cdk3) and p21(CIP1/Waf1) proteins. Furthermore, RA-induced growth arrest of CH27 cells was also associated with increased retinoic acid receptor beta (RARbeta) and reduced c-Myc expression. However, RA had no effect on the levels of cyclins A, D1, D3, E, or H, or on Cdk2, Cdk4, Cdk5, CDk6, Cdk7, p16(Ink4A), p15(Ink4B), p53, or pRb proteins in CH27 cells. Evaluation of the kinase activity of cyclin-Cdk complexes showed that RA increases p27(Kip1) expression in CH27 cells leading to markedly reduced cyclin A/Cdk2 kinase activity and slightly reduced cyclin E/Cdk2 kinase activity, with no effect on cyclin D/Cdk4 and cyclin D/Cdk6 activities. Moreover, coincident with the decrease in kinase activity was a drastic increase in cyclin A-bound p27(Kip1). These results suggest that increases in the levels of p27(Kip1) and its binding to cyclin A, as well as reduction of Cdk3 protein expression, are strong candidates for the cell cycle regulator that prevents the entry into the S phase in RA-treated CH27 cells, with prolongation of G1 phase and inhibition of DNA synthesis.     

Raf-Induced Cell Cycle Progression in Human TF-1 Hematopoietic Cells

Ras/Raf/MEK/ERK is a crucial pathway regulating cell cycle progression, apoptosis, and drug resistance. The Ras oncogene is frequently mutated in human cancer, which can result in the activation of the downstream Raf/MEK/ERK cascade leading to cell cycle progression in the absence of a growth stimulus. Raf-induced proliferation has been observed in hematopoietic cells. However, the mechanisms by which Raf affects cell cycle progression are not well described. To investigate the importance of Raf/MEK/ERK signaling in human hematopoietic cell growth, the effects of three different Raf genes, A-Raf, B-Raf and Raf-1, on cell cycle progression and regulatory gene expression were examined in TF-1 cells transformed to grow in response to b-estradiol-regulated DRaf:ER genes. Raf activation increased the expression of cyclin A, cyclin D, cyclin E, and p21Cip1, which are associated with G1 progression. Activated DRaf-1:ER and DA-Raf:ER but not DB-Raf:ER increased Cdk2 and Cdk4 kinase activity. The regulatory role of p16Ink4a, a potent Cdk4 kinase inhibitor, on the kinase activity of Cdk2 and Cdk4 was also examined. Raf induced p16Ink4a suppressor but this did not eliminate Cdk4 kinase activity. These results indicate that human hematopoietic cells transformed to grow in response to activated Raf can be used to elucidate the mechanisms by which various cell cycle regulatory molecules effect cell cycle progression. Furthermore, the differences that the various Raf isoforms have on Cdk4 activity and other cell cycle regulatory molecules can be determined in these cells.

Key Words:

Cell cycle, Raf, p21Cip1, p27Kip1, Cyclins, Cdks, Hematopoietic cells     

Cyclin E constrains Cdk5 activity to regulate synaptic plasticity and memory formation

Cyclin E is a component of the core cell cycle machinery, and it drives cell proliferation by regulating entry and progression of cells through the DNA synthesis phase. Cyclin E expression is normally restricted to proliferating cells. However, high levels of cyclin E are expressed in the adult brain. The function of cyclin E in quiescent, postmitotic nervous system remains unknown. Here we use a combination of in?vivo quantitative proteomics and analyses of cyclin E knockout mice to demonstrate that in terminally differentiated neurons cyclin E forms complexes with Cdk5 and controls synapse function by restraining Cdk5 activity. Ablation of cyclin E led to?a decreased number of synapses, reduced number and volume of dendritic spines, and resulted in impaired synaptic plasticity and memory formation in cyclin E-deficient animals. These results reveal a cell cycle-independent role for a core cell cycle protein, cyclin E, in synapse function and memory.     

Ras induces anchorage-independent growth by subverting multiple adhesion-regulated cell cycle events.

Anchorage-independent growth is a hallmark of transformed cells, but little is known of the molecular mechanisms that underlie this phenomenon. We describe here studies of cell cycle control of anchorage-independent growth induced by the ras oncogene, with the use of a somatic cell mutant fibroblast line (ER-1-2) that is specifically defective in oncogene-mediated, anchorage-independent growth. Control, nontransformed PKC3-F4 cells and ER-1-2 cells cannot proliferate in semisolid medium. Three important cell cycle events are dependent on adhesion of these cells to a substratum: phosphorylation of the retinoblastoma protein, pRB; cyclin E-dependent kinase activity; and cyclin A expression. PKC3-F4 cells that express ras (PKC3-F4/ras cells) proliferate in nonadherent cultures, and each of these three events occurs in the absence of adhesion in PKC3-F4/ras cells. Thus, ras can override the adhesion requirement of cellular functions that are necessary for cell cycle progression. ER-1-2 cells that express ras (ER-1-2/ras cells) possess hyperphosphorylated forms of pRB and cyclin E-dependent kinase activity in the absence of adhesion but remain adhesion dependent for expression of cyclin A. The adhesion dependence of pRB phosphorylation and cyclin E-dependent kinase activity is therefore dissociable from the adhesion dependence of cyclin A expression. Furthermore, ectopic expression of cyclin A is sufficient to rescue anchorage-independent growth of ER-1-2/ras cells but does not induce anchorage-independent growth of PKC3-F4 or ER-1-2 cells. However, like pRB phosphorylation and cyclin E-dependent kinase activity, the kinase activity associated with ectopically expressed cyclin A is dependent on cell adhesion, and this dependence is overcome by ras. Thus, the induction of anchorage-independent growth by ras may involve multiple signals that lead to both expression of cyclin A and activation of G1 cyclin-dependent kinase activities in the absence of cell adhesion.     

Cdk2 knockout mice are viable

BACKGROUND: Cyclin-dependent kinases (Cdks) and their cyclin regulatory subunits control cell growth and division. Cdk2/cyclin E complexes are thought to be required because they phosphorylate the retinoblastoma protein and drive cells through the G1/S transition into the S phase of the cell cycle. In addition, Cdk2 associates with cyclin A, which itself is essential for cell proliferation during early embryonic development. RESULTS: In order to study the functions of Cdk2 in vivo, we generated Cdk2 knockout mice. Surprisingly, these mice are viable, and therefore Cdk2 is not an essential gene in the mouse. However, Cdk2 is required for germ cell development; both male and female Cdk2(-/-) mice are sterile. Immunoprecipitates of cyclin E1 complexes from Cdk2(-/-) spleen extracts displayed no activity toward histone H1. Cyclin A2 complexes were active in primary mouse embryonic fibroblasts (MEFs), embryo extracts and in spleen extracts from young animals. In contrast, there was little cyclin A2 kinase activity in immortalized MEFs and spleen extracts from adult animals. Cdk2(-/-) MEFs proliferate but enter delayed into S phase. Ectopic expression of Cdk2 in Cdk2(-/-) MEFs rescued the delayed entry into S phase. CONCLUSIONS: Although Cdk2 is not an essential gene in the mouse, it is required for germ cell development and meiosis. Loss of Cdk2 affects the timing of S phase, suggesting that Cdk2 is involved in regulating progression through the mitotic cell cycle.