G1/S phosphorylation of the retinoblastoma protein is associated with an altered affinity for the nuclear compartment

pRb-dependent cyclin D3 protein stabilization is required for myogenic differentiation

The expression of retinoblastoma (pRb) and cyclin D3 proteins is highly induced during the process of skeletal myoblast differentiation. We have previously shown that cyclin D3 is nearly totally associated with hypophosphorylated pRb in differentiated myotubes, whereas Rb-/- myocytes fail to accumulate the cyclin D3 protein despite normal induction of cyclin D3 mRNA. Here we report that pRb promotes cyclin D3 protein accumulation in differentiating myoblasts by preventing cyclin D3 degradation. We show that cyclin D3 displays rapid turnover in proliferating myoblasts, which is positively regulated through glycogen synthase kinase 3beta (GSK-3beta)-mediated phosphorylation of cyclin D3 on Thr-283. We describe a novel interaction between pRb and cyclin D3 that maps to the C terminus of pRb and to a region of cyclin D3 proximal to the Thr-283 residue and provide evidence that the pRb-cyclin D3 complex formation in terminally differentiated myotubes hinders the access of GSK-3beta to cyclin D3, thus inhibiting Thr-283 phosphorylation. Interestingly, we observed that the ectopic expression of a stabilized cyclin D3 mutant in C2 myoblasts enhances muscle-specific gene expression; conversely, cyclin D3-null embryonic fibroblasts display impaired MyoD-induced myogenic differentiation. These results indicate that the pRb-dependent accumulation of cyclin D3 is functionally relevant to the process of skeletal muscle cell differentiation.

A microtubule-facilitated nuclear import pathway for cancer regulatory proteins

Nuclear protein import is dependent on specific targeting signals within cargo proteins recognized by importins (IMPs) that mediate translocation through the nuclear pore. Recent evidence, however, implicates a role for the microtubule (MT) network in facilitating nuclear import of the cancer regulatory proteins parathyroid hormone-related protein (PTHrP) and p53 tumor suppressor. Here we assess the extent to which MT and actin integrity may be generally required for nuclear protein import for the first time. We examine 10 nuclear-localizing proteins with diverse IMP-dependent nuclear import pathways, our results indicating that the cytoskeleton does not have a general mechanistic role in nuclear localization sequence-dependent nuclear protein import. Of the proteins examined, only the p110(Rb) tumor suppressor protein Rb, together with p53 and PTHrP, was found to require MT integrity for optimal nuclear import. Fluorescence recovery after photobleaching experiments indicated that the MT-dependent nuclear transport pathway increases both the rate and extent of Rb nuclear import but does not affect Rb nuclear export. Dynamitin overexpression experiments implicate the MT motor dynein in the import process. The results indicate that, additional to IMP/diffusion-dependent processes, certain cancer regulatory proteins utilize an MT-enhanced pathway for accelerated nuclear import that is presumably required for their nuclear functions.

Cooperation between p27 and p107 during endochondral ossification suggests a genetic pathway controlled by p27 and p130

Pocket proteins and cyclin-dependent kinase (CDK) inhibitors negatively regulate cell proliferation and can promote differentiation. However, which members of these gene families, which cell type they interact in, and what they do to promote differentiation in that cell type during mouse development are largely unknown. To identify the cell types in which p107 and p27 interact, we generated compound mutant mice. These mice were null for p107 and had a deletion in p27 that prevented its binding to cyclin-CDK complexes. Although a fraction of these animals survived into adulthood and looked similar to single p27 mutant mice, a larger number of animals died at birth or within a few weeks thereafter. These animals displayed defects in chondrocyte maturation and endochondral bone formation. Proliferation of chondrocytes was increased, and ectopic ossification was observed. Uncommitted mouse embryo fibroblasts could be induced into the chondrocytic lineage ex vivo, but these cells failed to mature normally. These results demonstrate that p27 carries out overlapping functions with p107 in controlling cell cycle exit during chondrocyte maturation. The phenotypic similarities between p107(-/-) p27(D51/D51) and p107(-/-) p130(-/-) mice and the cells derived from them suggest that p27 and p130 act in an analogous pathway during chondrocyte maturation.

Nucleoplasmic lamins and their interaction partners, LAP2alpha, Rb, and BAF, in transcriptional regulation

Lamins are major structural components of the nuclear envelope in multicellular eukaryotes. Particularly A-type lamins are also located in the nucleoplasm, likely involving a specific binding partner, lamina-associated polypeptide 2alpha (LAP2alpha). LAP2alpha-lamins A/C complexes in the nucleoplasm have been implicated in the regulation of gene expression by various means. They bind chromatin proteins and chromatin modifying enzymes, and can thus participate in epigenetic control pathways. Furthermore, binding of lamins A/C complexes to specific transcription factors and repressors may directly affect their transcriptional activity. LAP2alpha-lamins A/C also regulate retinoblastoma protein and influence cell cycle progression and differentiation, which could have important implications for molecular mechanisms of laminopathic diseases, linked to lamins A/C mutations.

Stabilization of p73 by nuclear IkappaB kinase-alpha mediates cisplatin-induced apoptosis

In response to DNA damage, p53 and its homolog p73 have a function antagonistic to NF-kappaB in deciding cell fate. Here, we show for the first time that p73, but not p53, is stabilized by physical interaction with nuclear IkappaB kinase (IKK)-alpha to enhance cisplatin (CDDP)-induced apoptosis. CDDP caused a significant increase in the amounts of nuclear IKK-alpha and p73alpha in human osteosarcoma-derived U2OS cells. Ectopic expression of IKK-alpha prolonged the half-life of p73 by inhibiting its ubiquitination and thereby enhancing its transactivation and pro-apoptotic activities. Consistent with these results, small interfering RNA-mediated knockdown of endogenous IKK-alpha inhibited the CDDP-mediated accumulation of p73alpha. The kinase-deficient mutant form of IKK-alpha interacted with p73alpha, but failed to stabilize it. Furthermore, CDDP-mediated accumulation of endogenous p73alpha was not detected in mouse embryonic fibroblasts (MEFs) prepared from IKK-alpha-deficient mice, and CDDP sensitivity was significantly decreased in IKK-alpha-deficient MEFs compared with wild-type MEFs. Thus, our results strongly suggest that the nuclear IKK-alpha-mediated accumulation of p73alpha is one of the novel molecular mechanisms to induce apoptotic cell death in response to CDDP, which may be particularly important in killing tumor cells with p53 mutation.

Nucleocytoplasmic shuttling of the retinoblastoma tumor suppressor protein via Cdk phosphorylation-dependent nuclear export

The retinoblastoma (RB) tumor suppressor protein is a negative regulator of cell proliferation that is functionally inactivated in the majority of human tumors. Elevated Cdk activity via RB pathway mutations is observed in virtually every human cancer. Thus, Cdk inhibitors have tremendous promise as anticancer agents although detailed mechanistic knowledge of their effects on RB function is needed to harness their full potential. Here, we illustrate a novel function for Cdks in regulating the subcellular localization of RB. We present evidence of significant cytoplasmic mislocalization of ordinarily nuclear RB in cells harboring Cdk4 mutations. Our findings uncover a novel mechanism to circumvent RB-mediated growth suppression by altered nucleocytoplasmic trafficking via the Exportin1 pathway. Cytoplasmically mislocalized RB could be efficiently confined to the nucleus by inhibiting the Exportin1 pathway, reducing Cdk activity, or mutating the Cdk-dependent phosphorylation sites in RB that result in loss of RB-Exportin1 association. Thus RB-mediated tumor suppression can be subverted by phosphorylation-dependent enhancement of nuclear export. These results support the notion that tumor cells can modulate the protein transport machinery thereby making the protein transport process a viable therapeutic target.

Limited redundancy in phosphorylation of retinoblastoma tumor suppressor protein by cyclin-dependent kinases in acute lymphoblastic leukemia

Cyclin-dependent kinases (CDKs) successively phosphorylate the retinoblastoma protein (RB) at the restriction point in G1 phase. Hyperphosphorylation results in functional inactivation of RB, activation of the E2F transcriptional program, and entry of cells into S phase. RB unphosphorylated at serine 608 has growth suppressive activity. Phosphorylation of serines 608/612 inhibits binding of E2F-1 to RB. In Nalm-6 acute lymphoblastic leukemia extracts, serine 608 is phosphorylated by CDK4/6 complexes but not by CDK2. We reasoned that phosphorylation of serines 608/612 by redundant CDKs could accelerate phospho group formation and determined which G1 CDK contributes to serine 612 phosphorylation. Here, we report that CDK4 complexes from Nalm-6 extracts phosphorylated in vitro the CDK2-preferred serine 612, which was inhibited by p16INK4a, and fascaplysin. In contrast, serine 780 and serine 795 were efficiently phosphorylated by CDK4 but not by CDK2. The data suggest that the redundancy in phosphorylation of RB by CDK2 and CDK4 in Nalm-6 extracts is limited. Serine 612 phosphorylation by CDK4 also occurred in extracts of childhood acute lymphoblastic leukemia cells but not in extracts of mobilized CD34+ hemopoietic progenitor cells. This phenomenon could contribute to the commitment of childhood acute lymphocytic leukemia cells to proliferate and explain their refractoriness to differentiation-inducing agents.

Mechanism by which avenanthramide-c, a polyphenol of oats, blocks cell cycle progression in vascular smooth muscle cells

Previously, we reported that avenanthramide-c (Avn-c), one of the major avenanthramides, polyphenols of oats, inhibited the serum-induced proliferation of vascular smooth muscle cells (SMC), which is an important process in the initiation and development of atherosclerosis. In the present study, we further investigated its cell cycle inhibitory mechanism. Rat embryonic aortic smooth muscle cell line A10 was used in this study. Flow cytometry analysis revealed that treatment of A10 cells with 80 muM Avn-c arrested the cell cycle in G1 phase as indicated by an increase in the number of cells in G1 phase and a decrease in the number of cells in S phase. This cell cycle arrest was associated with a decrease in the phosphorylation of retinoblastoma protein (pRb), whose hyperphosphorylation is a hallmark of the G1 to S transition in the cell cycle. The inhibition of pRb phosphorylation with Avn-c was accompanied by a decrease in cyclin D1 expression and an increase in cyclin-dependent kinase inhibitor p21cip1 expression, without significant changes in p27kip1 expression. Furthermore, Avn-c treatment increased the expression level and stability of p53 protein, which could account for the increase of p21cip1 expression. Our results demonstrate for the first time that Avn-c, which is a unique polyphenol found in oats, arrests SMC proliferation at G1 phase by upregulating the p53-p21cip1 pathway and inhibiting pRB phosphorylation. This inhibitory effect of Avn-c on SMC proliferation is an additional indication for the potential health benefit of oat consumption in the prevention of coronary heart disease beyond its known effect through lowering blood cholesterol.

Cell cycle-controlled interaction of nucleolin with the retinoblastoma protein and cancerous cell transformation

Retinoblastoma protein (Rb) is a multifunctional tumor suppressor, frequently inactivated in certain types of human cancer. Nucleolin is an abundant multifunctional phosphoprotein of proliferating and cancerous cells, recently identified as cell cycle-regulated transcription activator, controlling expression of human papillomavirus type 18 (HPV18) oncogenes in cervical cancer. Here we find that nucleolin is associated with Rb in intact cells in the G1 phase of the cell cycle, and the complex formation is mediated by the growth-inhibitory domain of Rb. Association with Rb inhibits the DNA binding function of nucleolin and in consequence the interaction of nucleolin with the HPV18 enhancer, resulting in Rb-mediated repression of the HPV18 oncogenes. The intracellular distribution of nucleolin in epithelial cells is Rb-dependent, and an altered nucleolin localization in human cancerous tissues results from a loss of Rb. Our findings suggest that deregulated nucleolin activity due to a loss of Rb contributes to tumor development in malignant diseases, thus providing further insights into the molecular network for the Rb-mediated tumor suppression.

Nuclear Lamins: Laminopathies and Their Role in Premature Ageing

It has been demonstrated that nuclear lamins are important proteins in maintaining cellular as well as nuclear integrity, and in maintaining chromatin organization in the nucleus. Moreover, there is growing evidence that lamins play a prominent role in transcriptional control. The family of laminopathies is a fast-growing group of diseases caused by abnormalities in the structure or processing of the lamin A/C ( LMNA) gene. Mutations or incorrect processing cause more than a dozen different inherited diseases, ranging from striated muscular diseases, via fat- and peripheral nerve cell diseases, to progeria. This broad spectrum of diseases can only be explained if the responsible A-type lamin proteins perform multiple functions in normal cells. This review gives an overview of current knowledge on lamin structure and function and all known diseases associated with LMNA abnormalities. Based on the knowledge of the different functions of A-type lamins and associated proteins, explanations for the observed phenotypes are postulated. It is concluded that lamins seem to be key players in, among others, controlling the process of cellular ageing, since disturbance in lamin protein structure gives rise to several forms of premature ageing.

CDK2 catalytic activity and loss of nuclear tethering of retinoblastoma protein in childhood acute lymphoblastic leukemia

Acute lymphoblastic leukemia (ALL) is the most common childhood cancer. During cell cycle progression from the mid- to late G1 phase, mammalian cells traverse the restriction point, a transition from mitogen dependence to mitogen independence, regulated by retinoblastoma protein (Rb). Different cyclin-dependent kinases (CDKs) sequentially phosphorylate and inactivate Rb, which is associated by a change in Rb's nuclear affinity and activation of E2F transcription. Here, we show by in vitro kinase assays that ALL extracts contained CDK2 catalytic activity. When liberation of Rb from cell nuclei was tested by immune precipitation of differential cell extractions and Western blot analysis, little Rb was associated with the nuclear compartment. Together with the immunocytochemical analysis at a single cell level that Rb was phosphorylated at serine 612 and threonine 821, sites known to be phosphorylated by CDK2, the data indicated the presence of CDK2 catalytic activity and loss of Rb's nuclear affinity in ALL cells. We conclude that most ALL cells reside at or beyond the restriction point. This could explain the resistance of ALL cells to differentiation induction.

Nuclear tumor suppressors in space and time

Numerous studies have identified key binding partners and functional activities of nuclear tumor-suppressor proteins such as the retinoblastoma protein, p53 and BRCA1. Historically, less attention has been given to the subnuclear locations of these proteins. Here, we describe several recent studies that promote the view that regulated association with subcompartments of the nucleus is inherent to tumor-suppressor function.

The retinoblastoma protein--from bench to bedside

The retinoblastoma tumour suppressor protein (Rb) has come a long way since its initial discovery in 1986. Encoded by the first candidate tumour suppressor gene it has emerged a versatile and context-dependent modulator of cell behaviour. Its activity is managed by signalling networks sensing intra- and extracellular cues. These cues are relayed to hold or permit inactivation of Rb by phosphorylation. Loss or mutation of the retinoblastoma gene is rare in sporadic cancers but defects in the pathways that license inactivation of Rb are found in the majority of them, suggesting that loss of Rb control is central to tumour development and arguing that its reinstatement might reverse tumour formation. Furthermore, mouse models with engineered defects in the Rb-phosphorylating kinases provide evidence that moderation of Rb inactivation may be a strategy for the prevention of tumour formation. The rationale behind these arguments, their underlying molecular concepts and strategies towards therapeutic application will be discussed.

Single cell analysis of G1 check points-the relationship between the restriction point and phosphorylation of pRb

Single cell analysis allows high resolution investigation of temporal relationships between transition events in G1. It has been suggested that phosphorylation of the retinoblastoma tumor suppressor protein (pRb) is the molecular mechanism behind passage through the restriction point (R). We performed a detailed single cell study of the temporal relationship between R and pRb phosphorylation in human fibroblasts using time lapse video-microscopy combined with immunocytochemistry. Four principally different criteria for pRb phosphorylation were used, namely (i) phosphorylation of residues Ser795 and Ser780, (ii) degree of pRb-association with the nuclear structure, a property that is closely related with pRb phosphorylation status, (iii) release of the transcription factor E2F-1 from pRb, and (iv) accumulation of cyclin E, which is dependent on phosphorylation of pRb. The analyses of individual cells revealed that passage through R preceded phosphorylation of pRb, which occurs in a gradually increasing proportion of cells in late G1. Our data clearly suggest that pRb phosphorylation is not the molecular mechanism behind the passage through R. The restriction point and phosphorylation of pRb thus seem to represent two separate check point in G1.

Vitamin A regulates proliferation and apoptosis of human T- and B-cells

Vitamin A is known to protect against infections, but it is not established how vitamin A metabolites stimulate the immune system. We have investigated the effects of physiological levels of retinoic acid on the function of normal human T- and B-cells. Surprisingly, we found that the proliferation of B-cells was inhibited by retinoids, and that this was due to rapid inhibition of the cell cycle machinery regulating G(1)-to-S transition. In contrast, the proliferation of T-cells was enhanced by physiological levels of retinoic acid, and the effect was due to induction of IL-2 (interleukin 2). The 'non-death-receptor'-mediated apoptosis of normal T-cells induced by prolonged (but single) stimulation of the cells was also prevented by retinoid acid, and also this effect was mediated via enhanced production of IL-2. The induction of IL-2 was at the transcriptional level, and all the effects of vitamin A on both B-and T-cells were mediated via the nuclear retinoic acid receptors (RARs), and not retinoid X receptors (RXRs).

c-Myc suppresses p21WAF1/CIP1 expression during estrogen signaling and antiestrogen resistance in human breast cancer cells

Estrogen rapidly induces expression of the proto-oncogene c-myc. c-Myc is required for estrogen-stimulated proliferation of breast cancer cells, and deregulated c-Myc expression has been implicated in antiestrogen resistance. In this report, we investigate the mechanism(s) by which c-Myc mediates estrogen-stimulated proliferation and contributes to cell cycle progression in the presence of antiestrogen. The MCF-7 cell line is a model of estrogen-dependent, antiestrogen-sensitive human breast cancer. Using stable MCF-7 derivatives with inducible c-Myc expression, we demonstrated that in antiestrogen-treated cells, the elevated mRNA and protein levels of p21(WAF1/CIP1), a cell cycle inhibitor, decreased upon either c-Myc induction or estrogen treatment. Expression of p21 blocked c-Myc-mediated cell cycle progression in the presence of antiestrogen, suggesting that the decrease in p21 is necessary for this process. Using RNA interference to suppress c-Myc expression, we further established that c-Myc is required for estrogen-mediated decreases in p21(WAF1/CIP1). Finally, we observed that neither c-Myc nor p21(WAF1/CIP1) is regulated by estrogen or antiestrogen in an antiestrogen-resistant MCF-7 derivative. The p21 levels in the antiestrogen-resistant cells increased when c-Myc expression was suppressed, suggesting that loss of p21 regulation was a consequence of constitutive c-Myc expression. Together, these studies implicate p21(WAF1/CIP1) as an important target of c-Myc in breast cancer cells and provide a link between estrogen, c-Myc, and the cell cycle machinery. They further suggest that aberrant c-Myc expression, which is frequently observed in human breast cancers, can contribute to antiestrogen resistance by altering p21(WAF1/CIP1) regulation.

Sequestration of pRb by cyclin D3 causes intranuclear reorganization of lamin A/C during muscle cell differentiation

The A-type lamins that localize in nuclear domains termed lamin speckles are reorganized and antigenically masked specifically during myoblast differentiation. This rearrangement was observed to be linked to the myogenic program as lamin speckles, stained with monoclonal antibody (mAb) LA-2H10, were reorganized in MyoD-transfected fibroblasts induced to transdifferentiate to muscle cells. In C2C12 myoblasts, speckles were reorganized early during differentiation in cyclin D3-expressing cells. Ectopic cyclin D3 induced lamin reorganization in C2C12 myoblasts but not in other cell types. Experiments with adenovirus E1A protein that can bind to and segregate the retinoblastoma protein (pRb) indicated that pRb was essential for the cyclin D3-mediated reorganization of lamin speckles. Cyclin D3-expressing myoblasts displayed site-specific reduction of pRb phosphorylation. Furthermore, disruption of lamin structures by overexpression of lamins inhibited expression of the muscle regulatory factor myogenin. Our results suggest that the reorganization of internal lamins in muscle cells is mediated by key regulators of the muscle differentiation program.

Rubella virus and birth defects: Molecular insights into the viral teratogenesis at the cellular level

BACKGROUND

In utero rubella virus (RV) infection of a fetus can result in birth defects that are often collectively referred to as congenital rubella syndrome (CRS). In extreme cases, fetal death can occur. In spite of the availability of a safe and effective vaccine against rubella, recent worldwide estimates are that more than 100,000 infants are born with CRS annually.

RECENT PROGRESS

Recently, several significant findings in the field of cell biology, as well as in the RV replication and virus-cell interactions, have originated from the authors' laboratory, and other researchers have provided insights into RV teratogenesis. It has been shown that 1) an RV protein induces cell-cycle arrest by generating a subpopulation of tetraploid nuclei (i.e., 4N DNA) cells, perhaps representative of the tetraploid state following S phase in the cell cycle, due to its interaction with citron-K kinase (CK); 2) RV infection induces apoptosis in cell culture, and 3) CK functional perturbations lead to tetraploidy, followed by apoptosis, in specific cell types.

CONCLUSIONS

Based on several similarities between known RV-associated fetal and cellular manifestations and CK deficiency-associated phenotypes, it is reasonable to postulate that P90-CK interaction in RV-infected cells interferes with CK function and induces cell-cycle arrest following S phase in a subpopulation, perhaps representative of tetraploid stage, which could lead to subsequent apoptosis in RV infection. Taking all these observations to the fetal organogenesis level, it is plausible that P90-CK interaction could perhaps be one of the initial steps in RV infection-induced apoptosis-associated fetal birth defects in utero.

An extracellular signal-regulated kinase 1- and 2-dependent program of chromatin trafficking of c-Fos and Fra-1 is required for cyclin D1 expression during cell cycle reentry

Mitogens activate cell signaling and gene expression cascades that culminate in expression of cyclin D1 during the G(0)-to-G(1) transition of the cell cycle. Using cell cycle arrest in response to oxidative stress, we have delineated a dynamic program of chromatin trafficking of c-Fos and Fra-1 required for cyclin D1 expression during cell cycle reentry. In serum-stimulated lung epithelial cells, c-Fos was expressed, recruited to chromatin, phosphorylated at extracellular signal-regulated kinase 1- and 2 (ERK1,2)-dependent sites, and degraded prior to prolonged recruitment of Fra-1 to chromatin. Immunostaining showed that expression of nuclear c-Fos and that of cyclin D1 are mutually exclusive, whereas nuclear Fra-1 and cyclin D1 are coexpressed as cells traverse G(1). Oxidative stress prolonged the accumulation of phospho-ERK1,2 and phospho-c-Fos on chromatin, inhibited entry of Fra-1 into the nucleus, and blocked cyclin D1 expression. After induction of the immediate-early gene response in the presence of oxidative stress, inhibition of ERK1,2 signaling promoted degradation of c-Fos, recruitment of Fra-1 to chromatin, and expression of cyclin D1. Our data indicate that termination of nuclear ERK1,2 signaling is required for an exchange of Fra-1 for c-Fos on chromatin and initiation of cyclin D1 expression at the G(0)-to-G(1) transition of the cell cycle.

A-type lamins regulate retinoblastoma protein function by promoting subnuclear localization and preventing proteasomal degradation

The retinoblastoma protein (pRB) is a critical regulator of cell proliferation and differentiation and an important tumor suppressor. In the G(1) phase of the cell cycle, pRB localizes to perinucleolar sites associated with lamin A/C intranuclear foci. Here, we examine pRB function in cells lacking lamin A/C, finding that pRB levels are dramatically decreased and that the remaining pRB is mislocalized. We demonstrate that A-type lamins protect pRB from proteasomal degradation. Both pRB levels and localization are restored upon reintroduction of lamin A. Lmna(-/-) cells resemble Rb(-/-) cells, exhibiting altered cell-cycle properties and reduced capacity to undergo cell-cycle arrest in response to DNA damage. These findings establish a functional link between a core nuclear structural component and an important cell-cycle regulator. They further raise the possibility that altered pRB function may be a contributing factor in dystrophic syndromes arising from LMNA mutation.

MCM4 shares homology to a replication/DNA-binding domain in CTF and is contacted by pRb

pRb limits the initiation of DNA replication. By immunoblotting of in vitro kinase assays we show that Cdk4,6-cyclin D contributed to pRb-phosphorylation at Ser-608, which was necessary to release pRb from chromatin in Nalm-6 cells, demonstrated by immunoprecipitations of differential cell extractions. A new binding protein of under-phosphorylated pRb, MCM4, was identified using LexA-Rb(561-660) in the two-hybrid assay. Sequence analysis revealed a novel conserved motif in MCM4's COOH-terminus with homology to the DNA-binding/viral replication activation domain of CTF/NF-I indicating their implication in the same process, and suggesting a model that CTF/NF-I stimulated binding of MCM4 to DNA, thereby to under-phosphorylated pRb. Accordingly, the pRb-MCM4-CTF/NF-I complex was immunodetected in Nalm-6 cells. Moreover, the motif was also in XPD, pointing to a collaboration of CTF/NF-I with XPD in nucleotide excision repair and in basal transcription and with MCM4 in the assembly of MCM complexes in which pRb specifically contacted MCM4.

Cellular senescence requires CDK5 repression of Rac1 activity

Cellular senescence is a tumor-suppressive process characterized by an irreversible cell cycle exit, a unique morphology, and expression of senescence-associated beta-galactosidase (SA-beta-Gal). We report here a role for CDK5 in induction of senescent cytoskeletal changes. CDK5 activation is upregulated in senescing cells. The increased activity of CDK5 further reduces GTPase Rac1 activity and Pak activation. The repression of the activity of the GTPase Rac1 by CDK5 is required for expression of the senescent phenotype. CDK5 regulation of Rac1 activity is necessary for actin polymerization accompanying senescent morphology in response to expression of pRb, activated Ras, or continuous passage. Inhibition of CDK5 attenuates SA-beta-Gal expression and blocks actin polymerization. These results point to a unique, nonneuronal role for CDK5 in regulation of Rac1 activity in senescence, illuminating the mechanisms underlying induction of senescence and the senescent shape change.

Transcriptional regulation by the retinoblastoma protein

The retinoblastoma protein (RB) plays a key role in the control of cell proliferation and mediates the terminal differentiation of certain cell types. Increasing evidence suggests that RB functions by contacting and modifying the behaviour of transcription factors. RB can form complexes with E2F and MyoD in vivo, and complexes with a number of other transcription factors have also been demonstrated in vitro. The interaction of regulatory transcription factors with RB may be explained by sequence similarity between RB and two general transcription factors: TBP and TFIIB. Here I review the evidence for a role of RB in the regulation of transcription and highlight some of the likely mechanisms of RB function.

Active localization of the retinoblastoma protein in chromatin and its response to S phase DNA damage

The Rb protein suppresses development of an abnormal state of endoreduplication arising after S phase DNA damage. In diploid, S phase cells, the activity of protein phosphatase 2A (PP2A) licenses the stable association of un(der)phosphorylated Rb with chromatin. After damage, chromatin-associated pRb is attracted to certain chromosomal replication initiation sites in the order in which they normally fire. Like S phase DNA damage in Rb(-/-) cells, specific interruption of PP2A function in irradiated, S phase wt cells also elicited a state of endoreduplication. Thus, PP2A normally licenses the recruitment of Rb to chromatin sites in S phase from which, after DNA damage, it relocalizes to selected replication control sites and suppresses abnormal, postdamage rereplicative activity.

Retinoblastoma tumor suppressor: analyses of dynamic behavior in living cells reveal multiple modes of regulation

The retinoblastoma tumor suppressor, RB, assembles multiprotein complexes to mediate cell cycle inhibition. Although many RB binding partners have been suggested to underlie these functions, the validity of these interactions on the behavior of RB complexes in living cells has not been investigated. Here, we studied the dynamic behavior of RB by using green fluorescent protein-RB fusion proteins. Although these proteins were universally nuclear, phosphorylation or oncoprotein binding mediated their active exclusion from the nucleolus. In vivo imaging approaches revealed that RB exists in dynamic equilibrium between a highly mobile and a slower diffusing species, and genetic lesions associated with tumorigenesis increased the fraction of RB in a highly mobile state. The RB complexes dictating cell cycle arrest were surprisingly dynamic and harbored a relatively short residence time on chromatin. In contrast, this rapid exchange was attenuated in cells that are hypersensitive to RB, suggesting that responsiveness may inversely correlate with mobility. The stability of RB dynamics within the cell was additionally modified by the presence and function of critical corepressors. Last, the RB-assembled complexes present in living cells were primarily associated with E2F binding sites in chromatin. In contrast to RB, E2F1 consistently maintained a stable association with E2F sites regardless of cell type. Together, these results elucidate the kinetic framework of RB tumor suppressor action in transcriptional repression and cell cycle regulation.

p21-dependent inhibition of colon cancer cell growth by mevastatin is independent of inhibition of G1 cyclin-dependent kinases

Mevastatin arrested HCT116 colon cancer cells at the G1/S transition and increased cellular levels of p21CIP1/WAF1. p21-deficient colon cancer cells continued to proliferate in the presence of mevastatin. Although p21 was necessary for the G1/S block, the G1 cyclin-dependent kinases (Cdks) cyclin E-Cdk2 and cyclin D-Cdk4 remained active. Despite the activity of the G1 Cdks the retinoblastoma protein was hypophosphorylated due to unknown mechanisms that were dependent on the p21 protein. The resulting decrease in cyclin A mRNA and protein led to a decrease in the activity of cyclin A-Cdk2. Therefore, although p21 was required for the G1/S arrest of HCT116 colon cancer cells by mevastatin, its mode of action was more complicated than the simple formation of a physical complex with cyclin-Cdk2. This mechanism of inhibition is different from that seen in prostate cancer cells (Ukomadu, C., and Dutta, A. (2003) J. Biol. Chem. 278, 4840-4846) where the activating phosphorylation of cyclin E-Cdk2 is suppressed and p21 is not required, suggesting the existence of cell line-specific differences in the mechanism by which statins arrest the cell cycle.

c-Abl tyrosine kinase selectively regulates p73 nuclear matrix association

p73 is a structural and functional homologue of the p53 tumor-suppressor protein. Like p53, p73 is activated in response to DNA-damaging insults to induce cell cycle arrest or apoptosis. Under these conditions p73 is tyrosine-phosphorylated by c-Abl, a prerequisite modification for p73 to elicit cell death in fibroblasts. In this study we report that in response to ionizing radiation, p73 undergoes nuclear redistribution and becomes associated with the nuclear matrix. This association is c-Abl-dependent because it was not observed in cells that are defective in c-Abl kinase activation. Moreover, STI-571, a specific c-Abl kinase inhibitor, is sufficient to block significantly p73 alpha nuclear matrix association. The observed c-Abl dependence of nuclear matrix association was recapitulated in the heterologous baculovirus system. Under these conditions p73 alpha but not p53 is specifically tyrosine-phosphorylated by c-Abl. Moreover, the phosphorylated p73 alpha is predominantly found in association with the nuclear matrix. Thus, in response to ionizing radiation p73 is modified in a c-Abl-dependent manner and undergoes nuclear redistribution and translocates to associate with the nuclear matrix. Our data describe a novel mechanism of p73 regulation.

Inhibition of cell proliferation and induction of apoptosis by novel tetravalent peptides inhibiting DNA binding of E2F

We have isolated several peptides from random peptide phage display libraries that specifically recognize the cell cycle regulatory transcription factor E2F and inhibit DNA binding of E2F/DP heterodimers (E2F-1, E2F-2, E2F-3, E2F-4 or E2F-5, and DP-1). The inhibitory efficiency could be strongly enhanced by generating branched tetravalent molecules. To analyse the biological consequences of peptide-mediated E2F inhibition, we fused two of these branched molecules to a cell-penetrating peptide derived from the HTV-Tat protein. Incubation of human tumor cells with these branched Tat-containing peptides led to an inhibition of cell proliferation and induction of apoptosis. These results provide new insights into the function of E2F and further validate E2F as a potential therapeutic target in proliferative diseases.

Lack of inverse dose-rate effect and binding of the retinoblastoma gene product in the nucleus of human cancer T-47D cells arrested in G2 by ionizing radiation

PURPOSE

To investigate the radiosensitivity of human breast cancer cells, T-47D, irradiated with low dose-rates and to study activation of the retinoblastoma gene product in the G1 and G2 phases during irradiation.

MATERIALS AND METHODS

Cells were irradiated with (60)Co gamma-rays with dose-rates of 0.37 and 0.94 Gy h(-1). Cell survival was measured as the ability of cells to form colonies. Cells were extracted, fixed and stained for simultaneous measurements of nuclear-bound pRB content and DNA content. Cell nuclei were stained with monoclonal antibody PMG3-245 and Hoechst 33258 was used for additional staining of DNA. Two-parametric flow cytometry measurements of pRB and DNA content were performed using a FACSTAR(PLUS) flow cytometer.

RESULTS

It was observed that irradiated cells were arrested in G2. No increase in radiation sensitivity was observed when the cells accumulated in G2. Irradiation of cells at both 0.37 and 0.94 Gy h(-1) resulted in exponential dose-survival curves with nearly equal alpha values, i.e. the same radiosensitivity. However, the retinoblastoma gene product was bound in the nucleus, i.e. hypophosphorylated, in about 15% of the cells arrested in G2.

CONCLUSIONS

T47-D cells accumulate in G2 during low dose irradiation, but no inverse dose-rate effect, i.e. a more efficient inactivation of cells at lower than at higher dose-rates, was observed. A population of arrested G2 cells has pRB protein bound in the nucleus, and pRB therefore could play a role in protecting cells against radiation-induced cell death in G2.

Transcriptional regulation of the ornithine decarboxylase gene by c-Myc/Max/Mad network and retinoblastoma protein interacting with c-Myc

c-Myc is an oncogenic transcription factor involved in the regulation of cell proliferation, differentiation and apoptosis. The direct targets of c-Myc mediating these various processes are slowly being unravelled. This study indicates that the ornithine decarboxylase (ODC) gene is a physiological transcriptional target of c-Myc in association with induction of cell proliferation and transformation, but not with induction of apoptosis. In addition to the two conserved CACGTG c-Myc-binding sites in the first intron, the CATGTG motif in the 5'-flanking region of the murine odc is also shown to be a functional c-Myc response element. odc is thus a c-Myc target with three binding sites a distance apart. Transient transfection studies with different c-Myc, Max and Mad constructs in COS-7 cells showed that the balance between c-Myc/Max, Max/Max and Max/Mad complexes is crucial for the regulation, resulting in either transactivation or transrepression of an ODC-CAT reporter gene. Transcription of both ODC-CAT and endogenous odc was strongly induced in HeLa cells expressing tetracycline-regulated c-Myc, concomitant with c-Myc promoting the S-phase entry of the cells. Transformation of NIH3T3 cells by c-Ha-ras-(Val12) oncogene was reversed by expression of transcriptionally inactive c-Myc, which was associated with repression of ODC-CAT expression. Further, the c-Myc-induced transactivation of ODC-CAT in COS-7 cells was suppressed by co-expression of the retinoblastoma tumor suppresser pRb, evidently as a result of pRb directly or indirectly interacting with c-Myc. Importantly, the endogenous c-Myc and pRb proteins were also found to associate in Colo 320HSR cells under physiological conditions. These results suggest that c-Myc and pRb can interact in vivo, and may in part control some aspects of cell proliferation and transformation through modulation of odc expression.

Expression of the retinoblastoma family of tumor suppressors during murine embryonic orofacial development

OBJECTIVE

To elucidate the role of the retinoblastoma (Rb) family of tumor suppressors and growth regulators in transforming growth factor beta (TGFbeta)-mediated embryonic palatal growth and morphogenesis.

DESIGN

The spatio-temporal expression patterns of the RB1, RB2/p130, and p107 tumor suppressor genes, their gene products (pRb, p130 and p107) and phosphoforms were examined in the developing murine secondary palate utilizing reverse transcriptase polymerase chain reaction (RT-PCR) and immunoblot/immunolocalization analyses with phospho-specific antibodies.

RESULTS

The RB1, RB2/p130, and p107 tumor suppressor genes and their gene products (pRb, p130, and p107) were differentially expressed in embryonic palatal tissue during the critical period of secondary palate development [gestational days (GD) 12-14]. Both hyper- (115 kDa) and hypo-phosphorylated (110 kDa) forms of pRb were expressed, with a notable transient decrease in expression on GD 13. Functional (hypo-phosphorylated) forms of pRB predominated during the critical period of palatogenesis. As opposed to pRb expression, p130 expression was transiently elevated on GD 13 in the embryonic palate, and functional (hypo-phosphorylated) forms were expressed at exceedingly low levels. p107 levels gradually declined over the course of palatogenesis. This diminution in expression, however, was accompanied by a transition to more functional (hypo-phosphorylated) forms of the p107 protein. When compared with Rb expression patterns in the whole embryo/fetus, each of the three Rb proteins exhibited unique and specific temporal patterns of expression in the developing palate. RT-PCR analyses of RB1, RB2/p130, and p107 mRNA expression in embryonic palatal tissue revealed patterns of expression which paralleled steady-state protein levels of pRb, p130, and p107 in the developing tissue. Immunolocalization of the Rb proteins demonstrated ubiquitous expression of pRb, p130, and p107 in embryonic palate mesenchyme and epithelium during GD 12-14 of development with intense nuclear staining of the Rbs in palate epithelial cells on days 12 and 13 of gestation.

CONCLUSION

Changing patterns of pRb, p130, and p107 protein expression and phosphorylation were evident in the embryonic secondary palate during the course of palatal ontogenesis. As the function of all three Rb proteins is regulated primarily through their phosphorylation, this suggests significant variation in Rb protein functionality during the course of palate development. Additional evidence from our laboratory (manuscript in preparation) indicates that the TGFbetas, key regulators of palatal growth and morphogenesis, are able to differentially modulate phosphorylation of the Rb proteins in this developing tissue, and hence the TGFbetas may regulate the function(s) of the Rb proteins during palatal ontogenesis. Although functionality of the Rb proteins is known to be regulated primarily through post-translational modification (i.e. phopsphorylation), comparison of RB1, RB2/p130, and p107 steady-state mRNA levels with pRb, p130, and p107 steady-state protein levels in the developing palate levels suggests a significant degree of regulation at the transcriptional level. Differential patterns of Rb expression and phosphorylation in the developing palate suggest that pRb, p107, and p130 may each play unique roles in various aspects of growth, morphogenesis and cellular differentiation during palatal ontogenesis. Studies directed at elucidating the precise cellular role(s) of pRb, p107, and p130 during palate development, and at TGFbeta regulation of Rb expression/phosphorylation are ongoing in our laboratory.

Lamin A/C binding protein LAP2alpha is required for nuclear anchorage of retinoblastoma protein

The phosphorylation-dependent anchorage of retinoblastoma protein Rb in the nucleus is essential for its function. We show that its pocket C domain is both necessary and sufficient for nuclear anchorage by transiently expressing green fluorescent protein (GFP) chimeras of Rb fragments in tissue culture cells and by extracting the cells with hypotonic solutions. Solid phase binding assays using glutathione S-transferase-fusion of Rb pockets A, B, and C revealed a direct association of lamin C exclusively to pocket C. Lamina-associated polypeptide (LAP) 2alpha, a binding partner of lamins A/C, bound strongly to pocket C and weakly to pocket B. When LAP2alpha was immunoprecipitated from soluble nuclear fractions, lamins A/C and hypophosphorylated Rb were coprecipitated efficiently. Similarly, immunoprecipitation of expressed GFP-Rb fragments by using anti-GFP antibodies coprecipitated LAP2alpha, provided that pocket C was present in the GFP chimeras. On redistribution of endogenous lamin A/C and LAP2alpha into nuclear aggregates by overexpressing dominant negative lamin mutants in tissue culture cells, Rb was also sequestered into these aggregates. In primary skin fibroblasts, LAP2alpha is expressed in a growth-dependent manner. Anchorage of hypophosphorylated Rb in the nucleus was weakened significantly in the absence of LAP2alpha. Together, these data suggest that hypophosphorylated Rb is anchored in the nucleus by the interaction of pocket C with LAP2alpha-lamin A/C complexes.

The spatio-temporal organization of DNA replication sites is identical in primary, immortalized and transformed mammalian cells

We investigated the organization of DNA replication sites in primary (young or presenescent), immortalized and transformed mammalian cells. Four different methods were used to visualize replication sites: in vivo pulse-labeling with 5-bromo-2'-deoxyuridine (BrdU), followed by either acid depurination, or incubation in nuclease cocktail to expose single-stranded BrdU-substituted DNA regions for immunolabeling; biotin-dUTP labeling of nascent DNA by run-on replication within intact nuclei and staining with fluorescent streptavidin; and, finally, immunolabeling of the replication fork proteins PCNA and RPA. All methods produced identical results, demonstrating no fundamental differences in the spatio-temporal organization of replication patterns between primary, immortal or transformed mammalian cells. In addition, we did not detect a spatial coincidence between the early firing replicons and nuclear lamin proteins, the retinoblastoma protein or the nucleolus in primary human and rodent cells. The retinoblastoma protein does not colocalize in vivo with members of the Mcm family of proteins (Mcm2, 3 and 7) at any point of the cell cycle and neither in the chromatin-bound nor in the soluble nucleoplasmic fraction. These results argue against a direct role for the retinoblastoma or nuclear lamin proteins in mammalian DNA synthesis under normal physiological conditions.

Proliferation-dependent expression and phosphorylation of pRB in B cell non-Hodgkin's lymphomas: dependence on RB1 copy number

Some studies have suggested that a significant fraction of non-Hodgkin's lymphomas (NHL) do not express pRB protein, possibly due to deletions of RB1. We examined RB1/centromere 17 copy number by fluorescent in situ hybridisation, and pRB expression/phosphorylation by immunohistochemistry (IHC) and immunoblotting (IB) in 66 cases of B cell NHL. Thirteen cases had lost one RB1 copy relative to centromere 17 copy number and total DNA content. Case 458/88 had no RB1 copies. pRB levels were heterogeneous as assessed by IB (0.04-1.12 relative units), but all tumours, except for case 458/88, expressed pRB localised to the nucleus in >75% of the tumour cells by IHC. The fraction of phosphorylated pRB was correlated with pRB expression (r(2)= 0.56, P < 0.001). The 14 cases with loss of RB1 had lower pRB expression (median 0.25) than those without (median 0.48, P < 0.001), but a correlation with S phase fraction (r(2) = 0.43, P < 0.001; previously published data for tumour-specific S phase and apoptotic fractions) indicated that the variation in pRB expression was due to differences in proliferative activity. Furthermore, the regression lines for pRB expression vs S phase fraction were not different for the cases with or without loss of one RB1 copy (P = 0.5). Cases 154/88 (one RB1 copy) and 258/88 (two RB1 copies), in addition to case 458/88, had low expression of (hypophosphorylated) pRB (0.04, 0.08 and 0.04), despite their high S phase fractions (21%, 17% and 21%). There was no association between pRB expression/RB1 copy number and apoptotic fraction. Neither pRB expression nor loss of RB1 had prognostic value, but cases 154/88, 258/88, and 458/88 had short survival times (5, 3 and 46 months, respectively) compared to the others (median survival: 44 months, P = 0.03). It is suggested that pRB expression and function are normal in 63 of 66 NHL cases, including 12 of 13 lymphomas with loss of one RB1 allele.

Further characterization of BC3H1 myogenic cells reveals lack of p53 activity and underexpression of several p53 regulated and extracellular matrix-associated gene products

To catalog factors that may contribute to the completion of myogenesis, we have been looking for molecular differences between BC3H1 and C2C12 cells. Cells of the BC3H1 tumor line, though myogenic, are nonfusing, and withdraw from the cell cycle only reversibly, whereas cells of the C2C12 line fuse, differentiate terminally, and express several muscle-specific gene products that BC3H1 cells do not. Relative to C2C12 cells, BC3H1 cells underaccumulated cyclin-dependent kinase inhibitor p21 and underaccumulated transcripts for p21, GADD45, CDO, decorin, osteopontin, H19, fibronectin, and thrombospondin-1 (tsp-1). Levels of accumulation of H19, tsp-1, and larger isoforms of fibronectin messenger ribonucleic acid (mRNA) were found to increase in response to expression of myogenic regulatory factors as shown by their accumulation in differentiated myogenically converted 10T1/2 cells but not in 10T1/2 fibroblasts. BC3H1s accumulated a temperature-insensitive, geldanamycin-sensitive, misfolded form of p53 incapable of transactivating a p53 responsive reporter, consistent with underexpression of p21, GADD45, and tsp-1. BC3H1 and C2C12 cells were similar with respect to upregulation of p27 protein, downregulation of mitogen-activated protein kinase phosphatase-1 (MKP-1) protein, upregulation of retinoblastoma (Rb) mRNA, and nuclear localization of hypophosphorylated Rb. Cells of both lines expressed the muscle-specific 1b isoform of MEF2D. Although nonfusing in the short term, after more than 18 d in differentiation medium, some cultures of BC3H1 cells formed viable multinucleated cells in which the nuclei did not reinitiate synthesis of DNA in response to serum. Our findings suggest participation of tsp-1 and specific isoforms of fibronectin in myogenesis and suggest additional avenues of research in myogenesis and oncogenesis.

Phosphorylation-dependent migration of retinoblastoma protein into the nucleolus triggered by binding to nucleophosmin/B23

Underphosphorylated retinoblastoma (Rb) protein inhibits progression around the cell cycle by binding to transcription factors like E2F; subsequent hyperphosphorylation of Rb protein releases E2F from the complex so that it can then drive the cell into S phase. We immunolocalized Rb protein in human cells during the cell cycle. Rb protein translocated into nucleoli after DNA replication completed, and the nucleolar Rb was shown to be in the hyperphosphorylated form by immunoblotting. This form, but not its underphosphorylated counterpart, interacted with the nucleolar protein nucleophosmin/B23. The two formed a salt-resistant complex in vitro, and the two could be immunoprecipitated together from nucleolar extracts. These results suggest that hyperphosphorylated Rb protein is imported into nucleoli late in S or G2 phase with nucleophosmin/B23. Analysis of the nucleolar location of Rb protein using various deletion mutants tagged with the green fluorescent protein implicated pocket A of Rb protein as the region responsible for nucleolar targeting; this region also interacted with nucleophosmin/B23. Nucleolar translocation of Rb mutant was inhibited by introducing nucleophosmin/B23 antisense oligomer. These results suggest that nucleolar translocation of Rb protein is promoted by the binding with nucleophosmin/B23 via the pocket A region.

Human OGG1 undergoes serine phosphorylation and associates with the nuclear matrix and mitotic chromatin in vivo

OGG1 is the major DNA glycosylase in human cells for removal of 7,8 dihydro-8-oxoguanine (8-oxoG), one of the most frequent endogenous base lesions formed in the DNA of aerobic organisms. During replication, 8-oxoG will frequently mispair with adenine, thus forming G:C --> T:A transversions, a common somatic mutation associated with human cancers. In the present study, we have constructed a stable transfectant cell line expressing hOGG1 fused at the C-terminal end to green fluorescent protein (GFP) and investigated the cellular distribution of the fusion protein by fluorescence analysis. It is shown that hOGG1 is preferentially associated with chromatin and the nuclear matrix during interphase and becomes associated with the condensed chromatin during mitosis. Chromatin-bound hOGG1 was found to be phosphorylated on a serine residue in vivo as revealed by staining with an anti-phosphoserine-specific antibody. Chromatin-associated hOGG1 was co-precipitated with an antibody against protein kinase C (PKC), suggesting that PKC is responsible for the phosphorylation event. Both purified and nuclear matrix-associated hOGG1 were shown to be substrates for PKC-mediated phosphorylation in vitro. This appears to be the first demonstration of a post-translational modification of hOGG1 in vivo.

Rb localization and phosphorylation kinetics correlate with the cellular phenotype of cultured breast adenocarcinoma cells

Retinoblastoma protein (Rb) expression has been correlated with state of differentiation, proliferation rate, and metastatic potential in breast adenocarcinomas and established cell lines. These observations, based on immunoreactivity of total Rb rather than hypophosphorylated protein, do not address the relationship between functional Rb and indicators of an aggressive transformed cellular phenotype. We hypothesized that the distribution of functional Rb and the kinetics of Rb phosphorylation would differ between cell lines representing immortalized mammary epithelium (MCF10A), differentiated nonmetastatic mammary adenocarcinoma (MCF-7), and poorly differentiated, highly metastatic mammary adenocarcinoma (MDA-MB-231) and that these differences would be informative of the cellular phenotype. Direct immunofluorescence microscopy was used to compare qualitatively the subcellular localization of total and hypophosphorylated Rb protein in synchronized and asynchronous cells. This technique was also used to quantitatively assess the amounts of hypophosphorylated Rb throughout the cell cycle in these representative cell lines. Total Rb stained more prominently than hypophosphorylated Rb in the nucleus of all asynchronous cells. Rb phosphorylation was more rapid in MCF-7 cells than in MCF10A cells, whereas Rb dephosphorylation appeared deregulated in MDA-MB-231 cells. We conclude that assessment of hypophosphorylated Rb may be more useful than assessment of total Rb for the evaluation of transformed breast adenocarcinoma phenotypes.

Subcellular localization of beta-catenin is regulated by cell density

It is generally accepted that subcellular distribution of beta-catenin regulates its function. Membrane-bound beta-catenin mediates cell-cell adhesion, whereas elevation of the cytoplasmic and nuclear pool of the protein is associated with an oncogenic function. Although the role of beta-catenin in transformed cells is relatively well characterized, little is known about its importance in proliferation and cell-cycle control of nontransformed epithelial cells. Using different approaches we show that in human keratinocytes (HaCaT) beta-catenin is distributed throughout the cells in subconfluent, proliferating cultures. In contrast, beta-catenin is nearly exclusively located at the plasma membrane in confluent, contact-inhibited cells. Hence, we demonstrate for the first time that beta-catenin is translocated from the cytoplasm to the plasma membrane in response to high cell density. We conclude that beta-catenin plays an important role in proliferation and mediating contact-inhibition by changing intracellular localization.

Retinoblastoma protein partners

Studies of the retinoblastoma gene (Rb) have shown that its protein product (pRb) acts to restrict cell proliferation, inhibit apoptosis, and promote cell differentiation. The frequent mutation of the Rb gene, and the functional inactivation of pRb in tumor cells, have spurred interest in the mechanism of pRb action. Recently, much attention has focused on pRb's role in the regulation of the E2F transcription factor. However, biochemical studies have suggested that E2F is only one of many pRb-targets and, to date, at least 110 cellular proteins have been reported to associate with pRb. The plethora of pRb-binding proteins raises several important questions. How many functions does pRb possess, which of these functions are important for development, and which contribute to tumor suppression? The goal of this review is to summarize the current literature of pRb-associated proteins.

Nucleocytoplasmic shuttling of p130/RBL2: novel regulatory mechanism

The retinoblastoma-related pocket proteins pRb, p107, and p130 are implicated in the control of cell proliferation, differentiation, and transformation. The function of pocket proteins is in part mediated by their ability to inhibit specific E2F transcription factors. The transcriptional activity of E2Fs is controlled by alteration of their nucleocytoplasmic localization during the cell cycle. p130 was observed to shuttle between the nucleus and cytoplasm in a heterokaryon fusion assay, suggesting the presence of nuclear and cytoplasmic localization signals. Two independent nuclear localization signals (NLS) that could target reporter proteins to the nucleus in transient transfection and microinjection experiments were identified in the C terminus of p130. In addition to the C-terminal NLS, the intact pocket domain of p130 itself was sufficient for nuclear translocation. Moreover, an additional functional NLS was mapped within the unique Loop region of p130. An N-terminal domain that conferred cytoplasmic localization was identified. Removal of the entire N terminus did not affect the ability of p130 to interact with E2F and to induce growth arrest. A model suggesting that the activity of pRb family members can be regulated by intracellular trafficking of the proteins is proposed.

p53 binds the nuclear matrix in normal cells: binding involves the proline-rich domain of p53 and increases following genotoxic stress

The tumour suppressor p53 is a multifunctional protein important for the maintenance of genomic integrity. It is able to form molecular complexes with different DNA targets and also with cellular proteins involved in DNA transcription and DNA repair. In mammalian cells the biochemical processing of DNA occurs on a nuclear sub-structure termed the nuclear matrix. Previously Deppert and co-workers have identified p53 in association with the nuclear matrix in viral- and non-viral transformed cell lines. In the present study we demonstrate, for the first time, that p53 is bound to the nuclear matrix in primary cultures of normal mammalian cells and that this binding increases following DNA damage. Analysis of cell lines expressing structural mutants of p53 revealed that association with the nuclear matrix is independent of the tertiary and quaternary structure of p53. However, the proline-rich domain towards the N-terminus of p53 (residues 67 to 98) appeared important for binding to the nuclear matrix. This was demonstrated by TET-ON regulated expression of p53-derived constructs in p53(-/-) murine embryonic fibroblasts (MEF p53(-/-)). The proline-rich domain of p53 has potential for SH3 protein-protein interaction, and has a role in p53-mediated apoptosis and possibly base excision repair of DNA damage. We discuss our observations in relation to the ability of p53 to facilitate DNA repair and also review evidence indicating that matrix-bound p53 in SV40-transformed cells may facilitate the transforming potential of SV40 large T antigen.

Subnuclear dynamics and transcription factor function

At a simplistic level, the nucleus can be thought of as singular organelle with a nuclear envelope designed to isolate the biochemical reactions required for gene transcription and DNA replication from the cytoplasm. It has become increasingly clear, however, that many higher levels of organization exist within the nucleus. A functional consequence of this organization is that nuclear processes that include transcription, RNA processing, and DNA synthesis are isolated to specific intranuclear domains to ensure efficiency. With the advent of GFP technologies and increasingly sophisticated instrumentation, we have continued to dissect the relationship between organization and function, in particular using live cells and ligand-dependent steroid receptors as a model system. These new opportunities have provided further insight into receptor function and the dependence upon intranuclear dynamics that take place within minutes of hormone addition. J. Cell. Biochem. Suppl. 35:99-106, 2000.

Human chromatid cohesin component hRad21 is phosphorylated in M phase and associated with metaphase centromeres

Sister chromatids duplicated in S phase are connected with each other during G(2) and M phase until the onset of anaphase. This chromatid cohesion is essential for correct segregation of genetic material to daughter cells. Recently, understanding of the molecular mechanisms governing chromatid cohesion in yeast has been greatly advanced, whereas these processes in mammalian cells remain unclear. We report here biochemical and cytological analyses of human Rad21, a homologue of the yeast cohesin subunit, Scc1p/Mcd1p. hRad21 is a nuclear phosphorylated protein. Its abundance does not change during the cell cycle, and it becomes hyperyphosphorylated in M phase. Most hRad21 is not associated with chromatin when the nuclear envelope breakdown takes place in prophase. However, a detailed analysis of the spread chromosomes indicated that hRad21 remains associated with prometaphase-like chromosomes along their entire lengths. The mitotic chromatin-bound hRad21 becomes dissociated in a highly regulated manner because hRad21 remains specifically at the centromeres but disappears from the arm regions on metaphase-like chromosomes. Interestingly, hRad21 at the metaphase centromeres appears to be present at the inner pairing domain where the two sister chromatids are supposed to be in intimate contact. These results suggest that hRad21 has a critical role in chromatid cohesion in human mitotic cells.

Expression of individual lamins in basal cell carcinomas of the skin

In this study we used a unique collection of type specific anti-lamin antibodies to study lamin expression patterns in normal human skin and in skin derived from patients with basal cell carcinomas (BCCs). Lamin expression in serial sections from frozen tissue samples was investigated by single and double indirect immunofluorescence. In normal skin, lamin A was expressed in dermal fibroblasts and in suprabasal epithelial cells but was absent from all basal epithelial cells. Lamin C was expressed in dermal fibroblasts, suprabasal epithelial cells and a majority of basal epithelial cells. However, lamin C was not expressed in quiescent basal epithelial cells. Lamin B1 was expressed in all epithelial cells but was not expressed in dermal fibroblasts. Finally, lamin B2 was expressed in all epithelial cells but was not expressed in dermal fibroblasts. Finally, lamin B2 was expressed in all cell types in normal skin. Lamin expression was also investigated in a collection of 16 BCCs taken from a variety of body sites. Based upon patterns of lamin expression the BCCs were classified into four groups: A-negative (10/16 tumours), C-negative (5/16 tumours), A/C-negative (1/16 tumours) and A/B2-negative (1/16 tumours). Lamin expression was also compared to cell proliferation index by staining serial sections with the proliferation marker Ki67. 9/10 of the lamin A negative tumours were highly proliferative, whereas 4/5 of the lamin C negative tumours were slow growing. Thus as a general rule absence of lamin A was correlated with rapid growth within the tumour, while absence of lamin C was correlated with slow growth within the tumour. Our data supports the hypothesis that lamin A has a negative influence on cell proliferation and its down regulation may be a requisite of tumour progression.

Integration of the pRB and p53 cell cycle control pathways

Two fundamental molecular pathways, the pRB and p53 pathways, regulate cell growth and cell death. The importance of these pathways in cellular growth control is underscored by the observation that members of these pathways are found mutated in all human cancers. These two pathways have typically been studied and described independently. However, as we discuss here, recent data have revealed an intimate molecular and genetic interaction between the p53 and pRB pathways.

Characterization of SWI/SNF protein expression in human breast cancer cell lines and other malignancies

Organization of genomic DNA into chromatin aids in the regulation of gene expression by limiting access to transcriptional machinery. The SWI/SNF family of complexes, which are conserved from yeast to humans, are ATP-dependent chromatin-remodeling enzymes required for the transcription of a number of genes in yeast. In humans, the gene encoding the BAF47/hSNF5 subunit of the complex, located at 22q11.2, has been found to be mutated in a number of human tumors including rhabdoid, rhabdomyosarcoma, chronic myeloid leukemia, and CNS tumors such as medulloblastomas and choroid plexus carcinomas. In addition, loss of heterozygosity (LOH) has been reported for the BAF47 region in breast and liver cancer. LOH has also been reported in breast and ovarian cancer within 17q12-25, a gene-rich area including BRCA1, BAF60B, and BAF57. Interestingly, the gene encoding the BAF155/hSWI3 subunit of the complex maps to 3p21-p23, an area of chromosomal deletion seen in a number of human adenocarcinomas including breast, kidney, pancreas, and ovary. To look for abnormalities in these proteins as well as the SWI/SNF complex in general, we have determined the protein status of core human SWI/SNF components BAF170, BAF155, BAF57, BAF53a, and BAF47 in 21 breast cell lines. The complex status in other human tumor cell lines of various tissue types was also examined. We also determined the protein status of the human SWI2 homologues, hBRM/SWI2alpha and BRG1/SWI2beta as well as two other proteins found in human SWI/SNF complexes, BAF180 and BAF250. In this study, we identified the first cell line negative for the BAF57 protein as well as a pancreatic carcinoma cell line negative for both the BRG-1 and hBRM proteins.

DNA damage-dependent nuclear dynamics of the Mre11 complex

The Mre11 complex has been implicated in diverse aspects of the cellular response to DNA damage. We used in situ fractionation of human fibroblasts to carry out cytologic analysis of Mre11 complex proteins in the double-strand break (DSB) response. In situ fractionation removes most nucleoplasmic protein, permitting immunofluorescent localization of proteins that become more avidly bound to nuclear structures after induction of DNA damage. We found that a fraction of the Mre11 complex was bound to promyelocyte leukemia protein bodies in undamaged cells. Within 10 min after gamma irradiation, nuclear retention of the Mre11 complex in small granular foci was observed and persisted until 2 h postirradiation. In light of the previous demonstration that the Mre11 complex associated with ionizing radiation (IR)-induced DSBs, we infer that the protein retained under these conditions was associated with DNA damage. We also observed increased retention of Rad51 following IR treatment, although IR induced Rad51 foci were distinct from Mre11 foci. The ATM kinase, which phosphorylates Nbs1 during activation of the S-phase checkpoint, was not required for the Mre11 complex to associate with DNA damage. These data suggest that the functions of the Mre11 complex in the DSB response are implicitly dependent upon its ability to detect DNA damage.

Disruption of the actin cytoskeleton leads to inhibition of mitogen-induced cyclin E expression, Cdk2 phosphorylation, and nuclear accumulation of the retinoblastoma protein-related p107 protein

The actin cytoskeleton has been found to be required for mitogen-stimulated cells to passage through the cell cycle checkpoint. Here we show that selective disruption of the actin cytoskeleton by dihydrocytochalasin B (H(2)CB) blocked the mitogenic effect in normal Swiss 3T3 cells, leading to cell cycle arrest at mid to late G(1) phase. Cells treated with H(2)CB remain tightly attached to the substratum and respond to mitogen-induced MAP kinase activation. Upon cytoskeleton disruption, however, growth factors fail to induce hyperphosphorylation of the retinoblastoma protein (pRb) and the pRb-related p107. While cyclin D1 induction and cdk4-associated kinase activity are not affected, induction of cyclin E expression and activation of cyclin E-cdk2 complexes are greatly inhibited in growth-stimulated cells treated with H(2)CB. The inhibition of cyclin E expression appears to be mediated at least in part at the RNA level and the inhibition of cdk2 kinase activity is also attributed to the decrease in cdk2 phosphorylation and proper subcellular localization. The expression patterns of cdk inhibitors p21 and p27 are similar in both untreated and H(2)CB-treated cells upon serum stimulation. In addition, the changes in subcellular localization of pRb and p107 appear to be linked to their phosphorylation states and disruption of normal actin structure affects nuclear migration of p107 during G(1)-to-S progression. Taken together, our results suggest that the actin cytoskeleton-dependent G(1) arrest is linked to the cyclin-cdk pathway. We hypothesize that normal actin structure may be important for proper localization of certain G(1) regulators, consequently modulating specific cyclin and kinase expression.