D-type cyclin-dependent kinase activity in mammalian cells

1994 ◽  
Vol 14 (3) ◽  
pp. 2066-2076
Author(s):  
H Matsushime ◽  
D E Quelle ◽  
S A Shurtleff ◽  
M Shibuya ◽  
C J Sherr ◽  
...  
Keyword(s):  
Cyclin D1 ◽  
Mammalian Cells ◽  
Kinase Activity ◽  
Cyclin D3 ◽  
Tween 20 ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Proliferating Cells ◽  
Quiescent Cells ◽  
Serum Stimulation

D-type cyclin-dependent kinase activities have not so far been detected in mammalian cells. Lysis of rodent fibroblasts, mouse macrophages, or myeloid cells with Tween 20 followed by precipitation with antibodies to cyclins D1, D2, and D3 or to their major catalytic partner, cyclin-dependent kinase 4 (cdk4), yielded kinase activities in immune complexes which readily phosphorylated the retinoblastoma protein (pRb) but not histone H1 or casein. Virtually all cyclin D1-dependent kinase activity in proliferating macrophages and fibroblasts could be attributed to cdk4. When quiescent cells were stimulated by growth factors to enter the cell cycle, cyclin D1-dependent kinase activity was first detected in mid G1, reached a maximum near the G1/S transition, and remained elevated in proliferating cells. The rate of appearance of kinase activity during G1 phase lagged significantly behind cyclin induction and correlated with the more delayed accumulation of cdk4 and formation of cyclin D1-cdk4 complexes. Thus, cyclin D1-associated kinase activity was not detected during the G0-to-G1 transition, which occurs within the first few hours following growth factor stimulation. Rodent fibroblasts engineered to constitutively overexpress either cyclin D1 alone or cyclin D3 together with cdk4 exhibited greatly elevated cyclin D-dependent kinase activity, which remained absent in quiescent cells but rose to supraphysiologic levels as cells progressed through G1. Therefore, despite continued enforced overproduction of cyclins and cdk4, the assembly of cyclin D-cdk4 complexes and the appearance of their kinase activities remained dependent upon serum stimulation, indicating that upstream regulators must govern formation of the active enzymes.

scholarly journals D-type cyclin-dependent kinase activity in mammalian cells.

1994 ◽  
Vol 14 (3) ◽  
pp. 2066-2076 ◽  
Author(s):  
H Matsushime ◽  
D E Quelle ◽  
S A Shurtleff ◽  
M Shibuya ◽  
C J Sherr ◽  
...  
Keyword(s):  
Cyclin D1 ◽  
Mammalian Cells ◽  
Kinase Activity ◽  
Cyclin D3 ◽  
Tween 20 ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Proliferating Cells ◽  
Quiescent Cells ◽  
Serum Stimulation

D-type cyclin-dependent kinase activities have not so far been detected in mammalian cells. Lysis of rodent fibroblasts, mouse macrophages, or myeloid cells with Tween 20 followed by precipitation with antibodies to cyclins D1, D2, and D3 or to their major catalytic partner, cyclin-dependent kinase 4 (cdk4), yielded kinase activities in immune complexes which readily phosphorylated the retinoblastoma protein (pRb) but not histone H1 or casein. Virtually all cyclin D1-dependent kinase activity in proliferating macrophages and fibroblasts could be attributed to cdk4. When quiescent cells were stimulated by growth factors to enter the cell cycle, cyclin D1-dependent kinase activity was first detected in mid G1, reached a maximum near the G1/S transition, and remained elevated in proliferating cells. The rate of appearance of kinase activity during G1 phase lagged significantly behind cyclin induction and correlated with the more delayed accumulation of cdk4 and formation of cyclin D1-cdk4 complexes. Thus, cyclin D1-associated kinase activity was not detected during the G0-to-G1 transition, which occurs within the first few hours following growth factor stimulation. Rodent fibroblasts engineered to constitutively overexpress either cyclin D1 alone or cyclin D3 together with cdk4 exhibited greatly elevated cyclin D-dependent kinase activity, which remained absent in quiescent cells but rose to supraphysiologic levels as cells progressed through G1. Therefore, despite continued enforced overproduction of cyclins and cdk4, the assembly of cyclin D-cdk4 complexes and the appearance of their kinase activities remained dependent upon serum stimulation, indicating that upstream regulators must govern formation of the active enzymes.

scholarly journals Cyclin D–Cdk4 is regulated by GATA-1 and required for megakaryocyte growth and polyploidization

Blood ◽  
2007 ◽  
Vol 109 (12) ◽  
pp. 5199-5207 ◽  
Author(s):  
Andrew G. Muntean ◽  
Liyan Pang ◽  
Mortimer Poncz ◽  
Steven F. Dowdy ◽  
Gerd A. Blobel ◽  
...  
Keyword(s):  
Transcription Factors ◽  
Cyclin D1 ◽  
Dna Content ◽  
Kinase Activity ◽  
Cyclin D3 ◽  
Cyclin D ◽  
Wild Type ◽  
Unique Form ◽  
Additional Defect ◽  
Deficient Mice

Abstract Endomitosis is a unique form of cell cycle used by megakaryocytes, in which the latter stages of mitosis are bypassed so that the cell can increase its DNA content and size. Although several transcription factors, including GATA-1 and RUNX-1, have been implicated in this process, the link between transcription factors and polyploidization remains undefined. Here we show that GATA-1–deficient megakaryocytes, which display reduced size and polyploidization, express nearly 10-fold less cyclin D1 and 10-fold increased levels of p16 compared with their wild-type counterparts. We further demonstrate that cyclin D1 is a direct GATA-1 target in megakaryocytes, but not erythroid cells. Restoration of cyclin D1 expression, when accompanied by ectopic overexpression of its partner Cdk4, resulted in a dramatic increase in megakaryocyte size and DNA content. However, terminal differentiation was not rescued. Of note, polyploidization was only modestly reduced in cyclin D1–deficient mice, likely due to compensation by elevated cyclin D3 expression. Finally, consistent with an additional defect conferred by increased levels of p16, inhibition of cyclin D-Cdk4 complexes with a TAT-p16 fusion peptide significantly blocked polyploidization of wild-type megakaryocytes. Together, these data show that GATA-1 controls growth and polyploidization by regulating cyclin D-Cdk4 kinase activity.

scholarly journals p27Kip1 Inhibits Cyclin D-Cyclin-Dependent Kinase 4 by Two Independent Modes

2008 ◽  
Vol 29 (4) ◽  
pp. 986-999 ◽  
Author(s):  
Arpita Ray ◽  
Melissa K. James ◽  
Stéphane Larochelle ◽  
Robert P. Fisher ◽  
Stacy W. Blain
Keyword(s):  
Cell Cycle ◽  
Active Site ◽  
Cell Cycle Progression ◽  
Kinase Activity ◽  
Growth Conditions ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Proliferating Cells ◽  
Cell Cycle Reentry

ABSTRACT Cell cycle progression is regulated by cyclin-dependent kinases (cdk's), which in turn are regulated by their interactions with stoichiometric inhibitors, such as p27Kip1. Although p27 associates with cyclin D-cyclin-dependent kinase 4 (cdk4) constitutively, whether or not it inhibits this complex is dependent on the absence or presence of a specific tyrosine phosphorylation that converts p27 from a bound inhibitor to a bound noninhibitor under different growth conditions. This phosphorylation occurs within the 3-10 helix of p27 and may dislodge the helix from cdk4's active site to allow ATP binding. Here we show that the interaction of nonphosphorylated p27 with cdk4 also prevents the activating phosphorylation of the T-loop by cyclin H-cdk7, the cdk-activating kinase (CAK). Even though the cyclin H-cdk7 complex is present and active in contact-arrested cells, p27's association with cyclin D-cdk4 prevents T-loop phosphorylation. When p27 is tyrosine phosphorylated in proliferating cells or in vitro with the tyrosine Y kinase Abl, phosphorylation of cdk4 by cyclin H-cdk7 is permitted, even without dissociation of p27. This suggests that upon release from the contact-arrested state, a temporal order for the reactivation of inactive p27-cyclin D-cdk4 complexes must exist: p27 must be Y phosphorylated first, directly permitting cyclin H-cdk7 phosphorylation of residue T172 and the consequent restoration of kinase activity. The non-Y-phosphorylated p27-cyclin D-cdk4 complex could be phosphorylated by purified Csk1, a single-subunit CAK from fission yeast, but was still inactive due to p27's occlusion of the active site. Thus, the two modes by which p27 inhibits cyclin D-cdk4 are independent and may reinforce one another to inhibit kinase activity in contact-arrested cells, while maintaining a reservoir of preformed complex that can be activated rapidly upon cell cycle reentry.

scholarly journals Regulation of cyclin D-dependent kinase 4 (cdk4) by cdk4-activating kinase.

1994 ◽  
Vol 14 (4) ◽  
pp. 2713-2721 ◽  
Author(s):  
J Y Kato ◽  
M Matsuoka ◽  
D K Strom ◽  
C J Sherr
Keyword(s):  
Mammalian Cells ◽  
Insect Cells ◽  
G1 Phase ◽  
Sf9 Cells ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Active Protein ◽  
Serum Stimulation ◽  
Rate Limiting

The accumulation of assembled holoenzymes composed of regulatory D-type cyclins and their catalytic partner, cyclin-dependent kinase 4 (cdk4), is rate limiting for progression through the G1 phase of the cell cycle in mammalian fibroblasts. Both the synthesis and assembly of D-type cyclins and cdk4 depend upon serum stimulation, but even when both subunits are ectopically overproduced, they do not assemble into complexes in serum-deprived cells. When coexpressed from baculoviral vectors in intact Sf9 insect cells, cdk4 assembles with D-type cyclins to form active protein kinases. In contrast, recombinant D-type cyclin and cdk4 subunits produced in insect cells or in bacteria do not assemble as efficiently into functional holoenzymes when combined in vitro but can be activated in the presence of lysates obtained from proliferating mammalian cells. Assembly of cyclin D-cdk4 complexes in coinfected Sf9 cells facilitates phosphorylation of cdk4 on threonine 172 by a cdk-activating kinase (CAK). Assembly can proceed in the absence of this modification, but cdk4 mutants which cannot be phosphorylated by CAK remain catalytically inactive. Therefore, formation of the cyclin D-cdk4 complex and phosphorylation of the bound catalytic subunit are independently regulated, and in addition to the requirement for CAK activity, serum stimulation is required to promote assembly of the complexes in mammalian cells.

Regulation of cyclin D-dependent kinase 4 (cdk4) by cdk4-activating kinase

1994 ◽  
Vol 14 (4) ◽  
pp. 2713-2721
Author(s):  
J Y Kato ◽  
M Matsuoka ◽  
D K Strom ◽  
C J Sherr
Keyword(s):  
Mammalian Cells ◽  
Insect Cells ◽  
G1 Phase ◽  
Sf9 Cells ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Active Protein ◽  
Serum Stimulation ◽  
Rate Limiting

The accumulation of assembled holoenzymes composed of regulatory D-type cyclins and their catalytic partner, cyclin-dependent kinase 4 (cdk4), is rate limiting for progression through the G1 phase of the cell cycle in mammalian fibroblasts. Both the synthesis and assembly of D-type cyclins and cdk4 depend upon serum stimulation, but even when both subunits are ectopically overproduced, they do not assemble into complexes in serum-deprived cells. When coexpressed from baculoviral vectors in intact Sf9 insect cells, cdk4 assembles with D-type cyclins to form active protein kinases. In contrast, recombinant D-type cyclin and cdk4 subunits produced in insect cells or in bacteria do not assemble as efficiently into functional holoenzymes when combined in vitro but can be activated in the presence of lysates obtained from proliferating mammalian cells. Assembly of cyclin D-cdk4 complexes in coinfected Sf9 cells facilitates phosphorylation of cdk4 on threonine 172 by a cdk-activating kinase (CAK). Assembly can proceed in the absence of this modification, but cdk4 mutants which cannot be phosphorylated by CAK remain catalytically inactive. Therefore, formation of the cyclin D-cdk4 complex and phosphorylation of the bound catalytic subunit are independently regulated, and in addition to the requirement for CAK activity, serum stimulation is required to promote assembly of the complexes in mammalian cells.

scholarly journals Primase p49 mRNA expression is serum stimulated but does not vary with the cell cycle.

1989 ◽  
Vol 9 (5) ◽  
pp. 1940-1945 ◽  
Author(s):  
B Y Tseng ◽  
C E Prussak ◽  
M T Almazan
Keyword(s):  
Cell Cycle ◽  
Dna Synthesis ◽  
Mammalian Cells ◽  
Cell Cycle Progression ◽  
Small Subunit ◽  
Mrna Levels ◽  
Proliferating Cells ◽  
Restriction Point ◽  
Serum Stimulation ◽  
G1 Cells

Expression of the small-subunit p49 mRNA of primase, the enzyme that synthesizes oligoribonucleotides for initiation of DNA replication, was examined in mouse cells stimulated to proliferate by serum and in growing cells. The level of p49 mRNA increased approximately 10-fold after serum stimulation and preceded synthesis of DNA and histone H3 mRNA by several hours. Expression of p49 mRNA was not sensitive to inhibition by low concentrations of cycloheximide, which suggested that the increase in mRNA occurred before the restriction point control for cell cycle progression described for mammalian cells and was not under its control. p49 mRNA levels were not coupled to DNA synthesis, as observed for the replication-dependent histone genes, since hydroxyurea or aphidicolin had no effect on p49 mRNA levels when added before or during S phase. These inhibitors did have an effect, however, on the stability of p49 mRNA and increased the half-life from 3.5 h to about 20 h, which suggested an interdependence of p49 mRNA degradation and DNA synthesis. When growing cells were examined after separation by centrifugal elutriation, little difference was detected for p49 mRNA levels in different phases of the cell cycle. This was also observed when elutriated G1 cells were allowed to continue growth and then were blocked in M phase with colcemid. Only a small decrease in p49 mRNA occurred, whereas H3 mRNA rapidly decreased, when cells entered G2/M. These results indicate that the level of primase p49 mRNA is not cell cycle regulated but is present constitutively in proliferating cells.

scholarly journals Cyclin D1: A Golden Gene in Cancer, Cardiotoxicity, and Cardioprotection

2021 ◽  
Vol In Press (In Press) ◽  
Author(s):  
Habib Haybar ◽  
Mehhdi Shahrouzian ◽  
Zahra Gatavizadeh ◽  
Najmaldin Saki ◽  
Mahmood Maniati ◽  
...  
Keyword(s):  
Cyclin D1 ◽  
Poor Prognosis ◽  
Therapeutic Response ◽  
Single Injection ◽  
Cdk Inhibitors ◽  
Cyclin D ◽  
Cyclin Dependent Kinase ◽  
Hypertrophic Growth ◽  
Cardiac System ◽  
Treatment Responses

Objective: Cyclin D1 is an essential protein that acts as a mitogenic sensor. In this manuscript, we discuss the importance of cyclin D1 in oncology and cardio-oncology, and we challenge the prognostic and therapeutic response values of cyclin D1 to figure out if it can be a beneficial marker. We also discuss the agents and microRNAs that can be used as a potential therapeutic approach via regulating cyclin D1 expression in oncology and cardio-oncology. Discussion: Clinical significance of cyclin D1 is defined not only in several cancers such as breast cancer, melanoma, and glioblastoma but also in cardiomyocyte regeneration and cardiac hypertrophic growth. Several studies have indicated that the injection of cardiotoxic agents such as doxorubicin (DOX) induces damage to the cardiac system and increases cyclin D expression at single injection, which might be related to DXO-mediated damage in the adult heart. However, cyclin D1 overexpression leads to hypertrophic growth of cardiomyocytes, and cyclin-dependent kinase (CDK)) inhibitors such as p16 do not inhibit the hypertrophic growth of cardiomyocytes. Thus, the reaction is CDK-independent. Conclusions: Cyclin D1 overexpression is positively correlated with tumor progression, treatment response, cardiotoxicity, and poor prognosis. Cyclin D1 expression has an important role in cardiac hypertrophy, and it can be a promising marker in monitoring cardiomyocyte treatment responses, cardioprotection, and cardiotoxicity. Finally, cyclin D1 plays an important role in hypertrophic growth of cardiomyocytes via a novel mechanism. Given all these pieces of evidence, cyclin D1 can be introduced as a favorable biomarker in future cardiology and cardio-oncology.

Anomalous High p27/KIP1 Expression in a Subset of Aggressive B-Cell Lymphomas Is Associated With Cyclin D3 Overexpression. p27/KIP1—Cyclin D3 Colocalization in Tumor Cells

Blood ◽  
1999 ◽  
Vol 94 (2) ◽  
pp. 765-772 ◽  
Author(s):  
Margarita Sánchez-Beato ◽  
Francisca I. Camacho ◽  
Juan C. Martı́nez-Montero ◽  
Ana I. Sáez ◽  
Raquel Villuendas ◽  
...  
Keyword(s):  
B Cell ◽  
Cyclin D1 ◽  
Kinase Inhibitor ◽  
Cyclin E ◽  
Cyclin D3 ◽  
Nuclear Accumulation ◽  
Proliferation Index ◽  
B Cell Lymphomas ◽  
Proliferating Cells ◽  
P27 Kip1

Abstract p27 cyclin-dependent kinase inhibitor downregulation is essential for transition to the S phase of the cell cycle. Thus, proliferating cells in reactive lymphoid tissue show no detectable p27 expression. Nevertheless, anomalous high p27 expression has been shown to be present in a group of aggressive B-cell lymphomas with high proliferation index and adverse clinical outcome. This suggests that abnormally accumulated p27 protein has been rendered functionally inactive. We analyzed the causes of this anomalous presence of p27 in a group of aggressive B-cell lymphomas, including 54 cases of diffuse large B-cell lymphomas and 20 Burkitt’s lymphomas. We simultaneously studied them for p27, cyclin D3, cyclin D2, cyclin D1, and cyclin E expression, because it has been stated that high levels of expression of cyclin D1 or E lead to increased p27 levels in some cell types. A statistically significant association between p27 and cyclin D3 expression was found for the group as a whole. Additionally, when dividing the cases according to the level of expression of cyclin D3 by reactive germinal centers, it was observed that cases with stronger cyclin D3 expression also show higher p27 expression. The relationship between both proteins was also shown at a subcellular level by laser confocal studies, showing that in cases with high expression of both proteins there was a marked colocalization. Additional evidence in favor of p27 sequestration by cyclin D3 was provided by coimmunoprecipitation studies in a Burkitt’s cell line (Raji) showing the existence of cyclin D3/p27 complexes and the absence of CDK2/p27 complexes. These results could support the hypothesis that there are cyclin D3/p27 complexes in a subset of aggressive B-cell lymphomas in which p27 lacks the inhibitory activity found when it is bound to cyclin E/CDK2 complexes. This interaction between both proteins could lead to an abnormal nuclear accumulation, detectable by immunohistochemical techniques.

Cyclin D: CDK4/6 Kinase Activity, Regulated by GATA-1, Is Required for Megakaryocyte Polyploidization.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 780-780
Author(s):  
Andrew G. Muntean ◽  
Liyan Pang ◽  
Mortimer Poncz ◽  
Steve Dowdy ◽  
Gerd Blobel ◽  
...  
Keyword(s):  
Cell Cycle ◽  
Transcription Factors ◽  
Cyclin D1 ◽  
Kinase Activity ◽  
Fetal Liver ◽  
Cyclin D3 ◽  
Luciferase Reporter ◽  
Cell Cycle Regulators ◽  
Wild Type

Abstract Megakaryocytes, which fragment to give rise to platelets, undergo a unique form of cell cycle, termed endomitosis, to become polyploid and terminally differentiate. During this process, cells transverse the cell cycle but the late stages of mitosis are bypassed to lead to accumulation of DNA up to 128N. While the mechanisms of polyploidization in megakaryocytes are poorly understood, a few cell cycle regulators, such as cyclin D3, have been implicated in this process. Hematopoietic transcription factors, including GATA-1 and RUNX1 are also essential for polyploidization, as both GATA1-deficient and RUNX1-null megakaryocytes undergo fewer rounds of endomitosis. Interestingly, GATA-1 deficient megakaryocytes are also smaller than their wild-type counterparts. However, the link between transcription factors and the growth and polyploidization of megakaryocytes has not been established. In our studies to identify key downstream targets of GATA-1 in the megakaryocyte lineage, we discovered that the cell cycle regulators cyclin D1 and p16 were aberrantly expressed in the absence of GATA-1: cyclin D1 expression was reduced nearly 10-fold, while that of p16ink4a was increased 10-fold. Luciferase reporter assays revealed that GATA-1, but not the leukemic isoform GATA-1s, promotes cyclinD1 expression. Consistent with these observations, megakaryocytes that express GATA-1s in place of full-length GATA-1 are smaller than their wild-type counterparts. Chromatin immunoprecipitation studies revealed that GATA-1 is bound to the cyclin D1 promoter in vivo, in primary fetal liver derived megakaryocytes. In contrast, GATA-1 is not associated with the cyclin D1 promoter in erythroid cells, which do not become polyploid. Thus, cyclin D1 is a bona fide GATA-1 target gene in megakaryocytes. To investigate whether restoration of cyclin D1 expression could rescue the polyploidization defect in GATA-1 deficient cells, we infected fetal liver progenitors isolated from GATA-1 knock-down mice with retroviruses harboring the cyclin D1 cDNA (and GFP via an IRES element) or GFP alone. Surprisingly, expression of cyclin D1 did not increase the extent of polyploidization of the GATA-1 deficient megakaryocytes. However, co-overexpression of cyclin D1 and Cdk4 resulted in a dramatic increase in polyploidization. Consistent with the model that cyclinD:Cdk4/6 also regulates cellular metabolism, we observed that the size of the doubly infected cells was also significantly increased. Finally, in support of our model that cyclin D:Cdk4/6 kinase activity is essential for endomitosis, we discovered that introduction of wild-type p16 TAT fusion protein, but not a mutant that fails to interact with Cdk4/6, significantly blocked polyploidization of primary fetal liver derived megakaryocytes. Taken together, our data reveal that the process of endomitosis and cell growth relies heavily on cyclinD:Cdk4/6 kinase activity and that the maturation defects in GATA-1 deficient megakaryocytes are due, in part, to reduced Cyclin D1 and increase p16 expression.

Anomalous High p27/KIP1 Expression in a Subset of Aggressive B-Cell Lymphomas Is Associated With Cyclin D3 Overexpression. p27/KIP1—Cyclin D3 Colocalization in Tumor Cells

Blood ◽  
1999 ◽  
Vol 94 (2) ◽  
pp. 765-772 ◽  
Author(s):  
Margarita Sánchez-Beato ◽  
Francisca I. Camacho ◽  
Juan C. Martı́nez-Montero ◽  
Ana I. Sáez ◽  
Raquel Villuendas ◽  
...  
Keyword(s):  
B Cell ◽  
Cyclin D1 ◽  
Kinase Inhibitor ◽  
Cyclin E ◽  
Cyclin D3 ◽  
Nuclear Accumulation ◽  
Proliferation Index ◽  
B Cell Lymphomas ◽  
Proliferating Cells ◽  
P27 Kip1

p27 cyclin-dependent kinase inhibitor downregulation is essential for transition to the S phase of the cell cycle. Thus, proliferating cells in reactive lymphoid tissue show no detectable p27 expression. Nevertheless, anomalous high p27 expression has been shown to be present in a group of aggressive B-cell lymphomas with high proliferation index and adverse clinical outcome. This suggests that abnormally accumulated p27 protein has been rendered functionally inactive. We analyzed the causes of this anomalous presence of p27 in a group of aggressive B-cell lymphomas, including 54 cases of diffuse large B-cell lymphomas and 20 Burkitt’s lymphomas. We simultaneously studied them for p27, cyclin D3, cyclin D2, cyclin D1, and cyclin E expression, because it has been stated that high levels of expression of cyclin D1 or E lead to increased p27 levels in some cell types. A statistically significant association between p27 and cyclin D3 expression was found for the group as a whole. Additionally, when dividing the cases according to the level of expression of cyclin D3 by reactive germinal centers, it was observed that cases with stronger cyclin D3 expression also show higher p27 expression. The relationship between both proteins was also shown at a subcellular level by laser confocal studies, showing that in cases with high expression of both proteins there was a marked colocalization. Additional evidence in favor of p27 sequestration by cyclin D3 was provided by coimmunoprecipitation studies in a Burkitt’s cell line (Raji) showing the existence of cyclin D3/p27 complexes and the absence of CDK2/p27 complexes. These results could support the hypothesis that there are cyclin D3/p27 complexes in a subset of aggressive B-cell lymphomas in which p27 lacks the inhibitory activity found when it is bound to cyclin E/CDK2 complexes. This interaction between both proteins could lead to an abnormal nuclear accumulation, detectable by immunohistochemical techniques.

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