Growth suppression by p18, a p16INK4/MTS1- and p14INK4B/MTS2-related CDK6 inhibitor, correlates with wild-type pRb function

Regulation of cdk2 activity in endothelial cells that are inhibited from growth by cell contact

Endothelial cells (ECs) are quiescent in normal blood vessels but undergo rapid bursts of proliferation after vascular injury and during angiogenesis. Here we show that the activity of cyclin-dependent kinase-2 (cdk2), a key regulator of the G1 and S phases of the cell cycle, is expressed at high levels in proliferating ECs but at low levels in ECs that are contact-inhibited for growth. Despite these differences in kinase activity, the protein levels of cdk2 and 1 of its activating subunits, cyclin E, are not modulated by these different growth conditions. The cdk inhibitor p27 is highly expressed in contact-inhibited but not proliferating ECs, whereas the level of cyclin A protein is preferentially expressed in proliferating ECs. p27 protein was detected in immunoprecipitable complexes with cdk2 or cyclin E in cultures that were contact-inhibited for growth. The functional significance of the p27 induction was indicated by the detection of a heat-stable cdk2 inhibitory activity that was induced by endothelial cell-cell contact and could be immunodepleted with anti-p27 antibodies. In a confluent EC monolayer, cdk2 kinase activity was activated by a scraping injury that led to cell migration and proliferation. The injury-induced activation of cdk2 coincided with the downregulation of p27 and the induction of cyclin A. These data demonstrate that p27 is induced in confluent cultures of ECs. They also indicate that both p27 induction and cyclin A downregulation contribute to the inhibition of cdk2 and cell proliferation by cell-cell contact in ECs.

Kinase inhibitors in cancer therapy: a look ahead

The most essential kinases involved in cell membrane receptor activation, signal transduction and cell cycle control or programmed cell death and their interconnections are reviewed. In tumours, the genes of many of those kinases are mutated or amplified or the proteins are overexpressed. The use of key kinases offers the possibility to screen in vitro for synthetic small molecule kinase inhibitors. In view of the many interconnections of cellular kinases, their role in preventing or inducing programmed cell death and the possibility that a considerable number of signal transducing proteins are still unknown, cellular test systems are recommended in which the respective key kinase or one of its main partner molecules are overexpressed.

Arrest of G(1)-S progression by the p53-inducible gene PC3 is Rb dependent and relies on the inhibition of cyclin D1 transcription

The p53-inducible gene PC3 (TIS21, BTG2) is endowed with antiproliferative activity. Here we report that expression of PC3 in cycling cells induced accumulation of hypophosphorylated, growth-inhibitory forms of pRb and led to G(1) arrest. This latter was not observed in cells with genetic disruption of the Rb gene, indicating that the PC3-mediated G(1) arrest was Rb dependent. Furthermore, (i) the arrest of G(1)-S transition exerted by PC3 was completely rescued by coexpression of cyclin D1 but not by that of cyclin A or E; (ii) expression of PC3 caused a significant down-regulation of cyclin D1 protein levels, also in Rb-defective cells, accompanied by inhibition of CDK4 activity in vivo; and (iii) the removal from the PC3 molecule of residues 50 to 68, a conserved domain of the PC3/BTG/Tob gene family, which we term GR, led to a loss of the inhibition of proliferation as well as of the down-regulation of cyclin D1 levels. These data point to cyclin D1 down-regulation as the main factor responsible for the growth inhibition by PC3. Such an effect was associated with a decrease of cyclin D1 transcript and of cyclin D1 promoter activity, whereas no effect of PC3 was observed on cyclin D1 protein stability. Taken together, these findings indicate that PC3 impairs G(1)-S transition by inhibiting pRb function in consequence of a reduction of cyclin D1 levels and that PC3 acts, either directly or indirectly, as a transcriptional regulator of cyclin D1.

Differential regulation of p27(Kip1) expression by mitogenic and hypertrophic factors: Involvement of transcriptional and posttranscriptional mechanisms

Platelet-derived growth factor-BB (PDGF-BB) acts as a full mitogen for cultured aortic smooth muscle cells (SMC), promoting DNA synthesis and cell proliferation. In contrast, angiotensin II (Ang II) induces cellular hypertrophy as a result of increased protein synthesis, but is unable to drive cells into S phase. In an effort to understand the molecular basis for this differential growth response, we have examined the downstream effects of PDGF-BB and Ang II on regulators of the cell cycle machinery in rat aortic SMC. Both PDGF-BB and Ang II were found to stimulate the accumulation of G(1) cyclins with similar kinetics. In addition, little difference was observed in the expression level of their catalytic partners, Cdk4 and Cdk2. However, while both factors increased the enzymatic activity of Cdk4, only PDGF-BB stimulated Cdk2 activity in late G(1) phase. The lack of activation of Cdk2 in Ang II-treated cells was causally related to the failure of Ang II to stimulate phosphorylation of the enzyme on threonine and to downregulate p27(Kip1) expression. By contrast, exposure to PDGF-BB resulted in a progressive and dramatic reduction in the level of p27(Kip1) protein. The time course of p27(Kip1) decline was correlated with a reduced rate of synthesis and an increased rate of degradation of the protein. Importantly, the repression of p27(Kip1) synthesis by PDGF-BB was associated with a marked attenuation of Kip1 gene transcription and a corresponding decrease in Kip1 mRNA accumulation. We also show that the failure of Ang II to promote S phase entry is not related to the autocrine production of transforming growth factor-beta1 by aortic SMC. These results identify p27(Kip1) as an important regulator of the phenotypic response of vascular SMC to mitogenic and hypertrophic stimuli.

Molecular analysis of alterations of the p18INK4c gene in human meningiomas

Meningiomas are common primary brain tumours frequently presenting with deleted and/or mutated NF2 gene located on 22q.1p has been reported as the second most commonly deleted chromosomal region in these neoplasms. A new member of the INK4 family of CDK inhibitors, the p18INK4c gene, has recently been mapped to this chromosomal arm. By virtue of its structural and functional similarities with the p16 gene, p18 has been implicated as a tumour suppressor gene in a variety of cancers. In this paper 40 human meningiomas were analysed for loss of heterozygosity (LOH) at the p18 locus, mutations and inactivating methylation of the p18 gene. LOH at D1S193, D1S463 and D1S211 microsatellite marker loci mapped to 1p32 was detected in 13 of 35 (37%), four of 20 (20%), and six of 24 (25%) tumour samples, respectively. One sample presented with homozygous deletion at D1S193. Mutational analysis using single stranded conformational polymorphism (SSCP) and direct sequencing did not detect any missense mutation but revealed a novel silent mutation, G to T, at coding nucleotide 435. Analysis of HgaI, BsaHI, ScrFI and Eco0109I restriction sites of p18 exon 1 revealed absence of inactivating methylation. Immunohistochemistry with p18 monoclonal antibody detected presence of cytoplasmic p18 staining in 21 of 22 examined samples. One sample did not stain and was shown to carry homozygous deletion at D1S193. Despite the high frequency of LOH at 1p32 microsatellite markers, the lack of genetic and epigenetic aberrations in the p18 gene together with the presence of p18 protein in all but one meningioma samples argues against the role of p18 as a tumour suppressor gene important for meningioma development.

Monoclonal antibody probes for p21WAF1/CIP1 and the INK4 family of cyclin-dependent kinase inhibitors

Inhibition of cyclin dependent kinases (cdk) by proteins of two families of cdk inhibitors (CKIs) represents one of the key modes of cell-cycle control. Although not fully understood at present, the functions of the individual members of the Cip/Kip and INK4 families of CKIs have been implicated in fundamental biological processes as diverse as cellular proliferation, responses to genotoxic stress, regulation of cellular differentiation, and senescence. In addition, the seven currently known CKIs qualify as either established or candidate tumor suppressors whose loss or inactivation contribute to molecular pathogenesis of a wide range of tumor types. In this study, we report the isolation and characterization of a panel of 10 mouse monoclonal antibodies (MAbs) that specifically recognize p21WAF1/CIP1 (p21) or the individual members of the INK4 family of CKIs: p15INK4b (p15), p16INK4a (p16), p18INK4c (p18), or p19INK4d (p19). These antibodies are proving to be invaluable molecular probes for analyses of protein abundance, subcellular localization, interacting cellular proteins, and ultimately the function(s) of these cell cycle regulators. Epitopes targeted by the antibodies were mapped by peptide enzyme-linked immunoadsorbent assay (ELISA), and performance of the MAbs assessed in a range of immunochemical techniques. Individual MAbs of our series recognize distinct pools of the respective CKIs, a feature reflected by their differential applicability in immunoblotting, immunoprecipitation, and immunostaining including immunohistochemistry on archival paraffin-embedded tissue sections. Together, these antibodies represent useful reagents to study CKIs in cells and tissues, a set of tools that should help elucidate the physiological roles played by the individual CKIs, and better understand the molecular mechanisms of loss or inactivation of these (candidate) tumor suppressors in human malignancies.

Cdk4 activation is dependent on the subunit rearrangement in the complexes

Although several factors have been implicated in the regulation of Cdk4 activity, little is known regarding the contributions of cyclin-dependent kinase inhibitors (CKIs) in Cdk4 activation in the mid G1 phase. Using a mouse macrophage cell line (Bac1.2F5), we found that most of Cdk4 bound to p15 when cells were in a quiescent state. Following CSF-1 stimulation, Cdk4 bound to cyclin D1 and then to p21, concomitant with the dissociation of p15 from the complexes. The activation of Cdk4 correlated well with p21 binding to the complexes, and the majority of active Cdk4 complexes contained p21. During regeneration of mouse liver after partial hepatectomy, Cdk4 activity coincided precisely with ternary complex formation of cyclin D1/Cdk4/p21. Using the baculovirus expression system, we succeeded in reconstituting a capacity for Cdk4 activation in insect cells, forming an active cyclin D1/Cdk4/p21 ternary complex. Taken together, it is suggested that p21 and cyclin D1 act cooperatively as activators of Cdk4 through the release of CKIs of the INK4 family.

Relationship between Ras pathways and cell cycle control

The ordered execution of the two main events of cellular reproduction, duplication of the genome and cell division, characterize progression through the cell cycle. Cultured cells can be switched between cycling and non-cycling states by alteration of extracellular conditions and the notion that a critical cellular control mechanism presides on this decision, whose temporal location is known as the restriction point, has become the focus for the study of how extracellular mitogenic signalling impinges upon the cell cycle to influence proliferation. This review attempts to cover the disparate pathways of Ras-mediated mitogenic signal transduction that impact upon restriction point control.

HTLV-I Tax and cell cycle progression

Human T-cell leukemia virus type I (HTLV-I) is the etiological agent for adult T-cell leukemia (ATL) and various human myopathies/neuropathies. HTLV-I encodes a 40 kDa phosphoprotein, Tax, which has been implicated in cellular transformation. In similarity with several other oncoproteins such as Myc, Jun, and Fos, Tax is a transcriptional activator. How Tax mechanistically dysregulates the cell cycle remains unclear. Recent findings from us and others have shown that Tax targets key regulators of G1/S and M progression such as p16INK4a, cyclin D1, cyclin D3-cdk, and the mitotic spindle checkpoint apparatus. Thus, Tax influences the progression of cells in various phases of the cell cycle. In this regard, we will discuss three distinct mechanisms through which Tax affects cell-cycling: a) through direct association Tax can abrogate the inhibitory function of p16INK4a on the G1-cdks, b) Tax can also directly influence cyclin D-cdk activities by a protein-protein interaction, and c) Tax targets the HsMAD1 mitotic spindle-assembly checkpoint protein. Through these varied routes, the HTLV-I oncoprotein dysregulates cellular growth controls and engenders a proclivity of cells toward a loss of DNA-damage surveillance.

An analysis of p16 tumour suppressor gene expression in acral lentiginous melanoma

Acral lentiginous melanoma is a particularly aggressive melanocytic lesion but, due to its comparative rarity, biological investigations into the behaviour of this subtype of melanoma are lacking. The activity of the recently described p16 tumour suppressor gene, thought to be the 'familial melanoma gene', was studied in 24 patients with subungual melanoma and 44 patients with plantar melanoma. Lower levels of p16 oncoprotein were demonstrated than that found in other histogenetic types of melanoma. Stratification of patients of all disease stages revealed a poorer survival in patients with low p16 expression (log rank test, chi(2)= 3.9, P = 0. 05). These data suggest that p16 inactivation may play an important role in the development and progression of acral lentiginous melanomas. However, the level of p16 expression was not prognostic since survival analysis on stratification of stage I patients according to p16 level did not reach statistical significance for both survival and disease free interval.

Rux is a cyclin-dependent kinase inhibitor (CKI) specific for mitotic cyclin-Cdk complexes

BACKGROUND

Roughex (Rux) is a cell-cycle regulator that contributes to the establishment and maintenance of the G1 state in the fruit fly Drosophila. Genetic data show that Rux inhibits the S-phase function of the cyclin A (CycA)-cyclin-dependent kinase 1 (Cdk1) complex; in addition, it can prevent the mitotic functions of CycA and CycB when overexpressed. Rux has no homology to known Cdk inhibitors (CKIs), and the molecular mechanism of Rux function is not known.

RESULTS

Rux interacted with CycA and CycB in coprecipitation experiments. Expression of Rux caused nuclear translocation of CycA and CycB, and inhibited Cdk1 but not Cdk2 kinase activity. Cdk1 inhibition by Rux did not rely on inhibitory phosphorylation, disruption of cyclin-Cdk complex formation or changes in subcellular localization. Rux inhibited Cdk1 kinase in two ways: Rux prevented the activating phosphorylation on Cdk1 and also inhibited activated Cdk1 complexes. Surprisingly, Rux had a stimulating effect on CycA-Cdk1 activity when present in low concentrations.

CONCLUSIONS

Rux fulfils all the criteria for a CKI. This is the first description in a multicellular organism of a CKI that specifically inhibits mitotic cyclin-Cdk complexes. This function of Rux is required for the G1 state and male meiosis and could also be involved in mitotic regulation, while the stimulating effect of Rux might assist in any S-phase function of CycA-Cdk1.

p53-independent inhibition of proliferation and p21(WAF1/Cip1)-modulated induction of cell death by the antioxidants N-acetylcysteine and vitamin E

Epidemiological evidence has suggested an association between diets rich in antioxidants and diminished risks of various types of cancer. Proposed mechanisms for protective effects of antioxidants have involved inhibition of free radical-mediated DNA damage. Recent data suggest that antioxidants may prevent or eliminate cancerous cells through their ability to inhibit proliferation or to induce programmed cell death (PCD). To begin to identify cell cycle and cell death regulatory factors involved in antioxidant-induced growth arrest and PCD, we have studied colorectal carcinoma cells (CRCs) that differ in expression of the tumor suppressor protein p53, and of the cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1). The antioxidants, N-acetylcysteine (NAC) and vitamin E either inhibited proliferation in a p53-independent manner without affecting cell viability or induced cell death. Growth arrest was not associated with upregulation of the CDK inhibitors p21(Waf1/Cip1), p18(ink4c) or p16(ink4a), but was associated with a decrease in reactive oxygen species (ROS). In contrast to previous observations, the absence of p21(Waf1/Cip1) increased susceptibility of CRCs to antioxidant-induced PCD. NAC decreased levels of retinoblastoma protein (Rb) phosphorylation in all cells tested, but Rb was cleaved only in cells which underwent NAC-induced death. Although NAC decreased ROS in all cells studied, cell lines in which PCD occurred had higher baseline levels of ROS than cell lines in which proliferation was blocked. These observations suggest that expression of p21(Waf1/Cip1) and basal levels of ROS are important determinants of outcome after antioxidant treatment.

Molecular pathology of soft tissue and bone tumors. A review

OBJECTIVE

To present recent concepts on the molecular pathogenesis of tumors of soft tissue and bone, and on the use of molecular genetic methods, including their significance as diagnostic markers and prognostic indicators.

DATA SOURCES AND STUDY SELECTION

Reports on tumors of bone and/or soft tissue published in the English language literature and observations made using specimens available at the Departments of Pathology at Albany Medical College and Loyola University Medical Center.

DATA EXTRACTION AND SYNTHESIS

Studies on bone and soft tissue tumors containing chromosomal or genetic evaluation were selected for further analysis. Specific chromosomal abnormalities, such as numerical aberrations or translocations with production of fusion genes, were classified according to the tumor of origin. Data were also collected on mutations in tumor suppressor genes, genes coding for growth factors or their receptors, and genes coding for tyrosine kinases. Also noted were mutations of uncertain significance, for which the pathogenic connection between tumor production and mutated gene function is still unclear.

CONCLUSIONS

In general, the mutations reported interfere with the action of peptide growth factors coordinating mesenchyme proliferation and differentiation, although membrane-bound receptors expressing the intracellular signaling modifier, tyrosine kinase activity, have also been involved. Functional types of genes most commonly affected include tumor suppressors, oncogenes, and nuclear transcription factors. Thus, the mutations involved in the pathogenesis of soft tissue and bone tumors have affected multiple genes. Moreover, aberrant fusion gene products may be formed in tumoral tissue and may then act as transcription regulators stimulating cellular proliferation. Cytogenetic studies help at the clinical level by demonstrating aneuploidy and increased ploidy, which may correlate with malignant behavior. Diagnostic tumor-specific chromosomal translocations may be detected with Southern hybridization analysis, polymerase chain reaction, reverse-transcription polymerase chain reaction, or with the fluorescence in situ hybridization technique. Notably, early metastatic disease may be detectable in blood specimens using polymerase chain reaction or reverse-transcription polymerase chain reaction techniques.

Deletions of the INK4A gene occur in malignant peripheral nerve sheath tumors but not in neurofibromas

The INK4A gene, a candidate tumor suppressor gene located on chromosome 9p21, encodes two protein products, p16 and p19(ARF). p16 is a negative cell cycle regulator capable of arresting cells in the G1 phase by inhibiting cyclin-dependent kinases 4 (Cdk4) and 6 (Cdk6), thus preventing pRB phosphorylation. p19(ARF) prevents Mdm2-mediated neutralization of p53. Loss of INK4A is a frequent molecular alteration involved in the genesis of several neoplasms, including tumors of neuroectodermal origin. This study investigated the frequency of INK4A gene alterations in a series of malignant peripheral nerve sheath tumors (MPNSTs) and neurofibromas (NFs). INK4A gene and the p19(ARF)-specific exon 1beta were studied in 11 MPNST samples from 8 patients and 7 neurofibromas. Presence of INK4A deletions was assessed by Southern blotting hybridization and by a multiplex polymerase chain reaction (mPCR). INK4A point mutations were examined by single-strand conformation polymorphism (SSCP) and sequencing. The p16 promoter methylation status was determined by PCR amplification of bisulfite-treated DNA. Homozygous deletions of exon 2, thus affecting both p16 and p19(ARF), were identified in MPNSTs from 4 of 8 patients. Deletions, mutations, or silencing by methylation were not identified in the neurofibromas analyzed. Based on our results, we conclude that INK4A deletions are frequent events in MPNSTs and may participate in tumor progression. Silencing of p16 by methylation, which occurs often in several tumor types, is uncommon in MPNSTs.

Expression of p27(kip) and other cell cycle regulators in malignant peripheral nerve sheath tumors and neurofibromas: the emerging role of p27(kip) in malignant transformation of neurofibromas

There is little information regarding the status of cell cycle regulators in malignant peripheral nerve sheath tumors (MPNSTs) and neurofibromas (NFs). In this study, we investigated patterns of expression of p53 and pRB, cyclin-dependent kinase inhibitors (CKIs) p21 and p27, as well as cyclins D1 and E, in a cohort of 35 well-characterized MPNSTs and 16 NFs. These phenotypes were correlated with proliferative index, as assessed by Ki-67, as well as clinicopathological parameters of poor outcome. p53 nuclear overexpression was found in 10 of 35 (29%) MPNSTs, and it was lacking in NFs (P = 0.02). There were no differences in the patterns of expression of pRB, cyclin D1, and p21 between MPNSTs and NFs. However, p27 nuclear expression was present in most NFs, but it was absent in the majority of MPNSTs, which displayed cytoplasmic staining (P < 0.001). Nuclear cyclin E expression was more pronounced in MPNSTs than in NFs. We observed inverse patterns of expression for nuclear p27 and nuclear cyclin E expression. The staining profiles of cytoplasmic p27 and nuclear cyclin E expression were found to be statistically associated (P = 0.01). High Ki-67 expression was found in 20 of 34 (59%) MPNSTs but was absent in NFs (P < 0.001). Furthermore, detection of cytoplasmic p27 expression was found to be a prognostic factor for poor survival in MPNSTs (P = 0.03, relative risk = 2.4).

p57Kip2, a glucocorticoid-induced inhibitor of cell cycle progression in HeLa cells

Glucocorticoids exert antiproliferative effects on a number of cell types, including the HeLa cervical carcinoma cell line. However, the mechanism responsible for the antiproliferative effect is poorly understood. In this report we have investigated the role of the recently identified cyclin-dependent kinase inhibitor (CDI) p57Kip2 in the antiproliferative effect conferred by glucocorticoids. When HeLa cells were treated with the synthetic glucocorticoid dexamethasone (DEX), the doubling time of exponentially growing cells increased 2-fold. Within 11 h of DEX treatment, this was accompanied by an accumulation of cells in the G1 phase of the cell cycle with a corresponding decreased proportion of cells in the S phase and decreased CDK2 activity. DEX treatment of the HeLa cells dramatically induced the protein and mRNA expression of the CDI p57Kip2. This induction was seen within 4 h of DEX treatment, preceding a major DEX-induced accumulation of cells in the G1 phase. DEX-induced mRNA expression of p57Kip2 did not require de novo protein synthesis, and the transcription of the p57Kip2 gene was increased as determined by a run-on transcription assay. Furthermore, DEX induction of p57Kip2 was not a consequence of the cell cycle arrest, since other growth inhibition signals did not result in strong p57Kip2 induction. Overexpression of p57Kip2 using HeLa cells stably transfected with a tetracycline-inducible vector showed that p57Kip2 is sufficient to reconstitute an antiproliferative effect similar to that seen in DEX-treated cells. Selective p57Kip2 expression by the tetracycline analog doxycycline to levels comparable to those observed on DEX induction resulted in a 1.7-fold increase in the doubling time and a shift of HeLa cells to the G1 phase as well as a decrease in CDK2 activity. Taken together, these results suggest that glucocorticoid treatment directly induces transcription of the p57Kip2 gene and that the p57Kip2 protein is involved in the glucocorticoid-induced antiproliferative effect.

Reduced expression and altered subcellular localization of the cyclin-dependent kinase inhibitor p27(Kip1) in human colon cancer

The p27(Kip1) protein is a negative regulator of the cell cycle and a potential tumor suppressor gene. Reduced expression of the p27(Kip1) protein has been reported in several human tumors and has been associated with higher tumor grade and increased mortality in breast, lung, colon, prostate, bladder, and gastric cancers. On the other hand, increased expression of the p27(Kip1) protein, in the absence of gene mutation, has been observed in primary colon and breast cancers. It was recently suggested that sequestration in the cytoplasm might be an alternative way to inactivate p27(Kip1)-associated inhibitory activity. This study was undertaken to further evaluate p27(Kip1) expression in primary colon tumors and to verify whether differences exist between normal and cancer tissues in terms of subcellular localization of this protein. Both normal and neoplastic tissues expressed variable amounts of the p27(Kip1) protein, as assessed by western blot analyses. Although the mean values were not different between tumor and normal mucosa samples, the expression of total p27(Kip1) was reduced in a subset of tumors. Decreased levels of total p27(Kip1) were associated with high tumor grade (P=0.03) and stage (P=0.04). Moreover, while there was no significant difference in nuclear p27(Kip1), the amount of p27(Kip1) in the cytoplasmic fraction was significantly higher in the tumor samples than in the normal mucosa samples (P=0.0001). These results suggest that p27(Kip1) expression is lost in a subset of colorectal tumors and that alterations in the subcellular localization of this protein might play a role in colon carcinogenesis.

Protein expression of cell cycle regulator, p27Kip1, correlates with histopathological grade of non-Hodgkin's lymphoma

The protein p27Kip1 is one of the cyclin-dependent kinase inhibitors that are known to play important roles in the regulation of cell-cycle progression. Low levels of p27 expression in malignant cells are associated with poor prognosis in patients with breast, lung, colorectal and gastric cancers. To determine the relation of cyclin-dependent kinase inhibitors to histopathological grades of B-cell non-Hodgkin's lymphomas, the expression of p27, cyclin D1 and cyclin E in lymph node tissues was investigated in 56 patients with B-cell non-Hodgkin's lymphomas by western blotting and immunohistochemical techniques. High levels of p27 expression were observed in most lymph node tissue samples (93%) obtained from patients with low grade B-cell non-Hodgkin's lymphomas, while expression was low in lymph node tissue taken from all patients with intermediate and high grade B-cell non-Hodgkin's lymphomas. The difference in p27 expression in lymphoma tissues was significant among the different histopathological grades of B-cell non-Hodgkin's lymphomas (P<0.01). The analysis of the survival time of patients showed that the reduction of p27 expression correlated with poor prognosis. Cyclin D1, showed a high level of expression in mantle cell lymphomas and high grade B-cell non-Hodgkin's lymphomas. Cyclin E showed limited expression in 18 of 31 lymphoma tissues. Both cyclin D1 and E protein expression were not significantly different among the grades of B-cell non-Hodgkin's lymphomas. These results demonstrate that the level of p27 expression in lymphoma tissue is an important parameter in the classification of B-cell non-Hodgkin's lymphomas and in the prediction of prognosis.

Anomalous p27kip1 expression in a subset of malignant gliomas

p27Kip (p27) expression was immunohistochemically investigated in 28 astrocytic tumors, and compared with the cell proliferation index (MIB-1 staining index). Normal rat brains and surgical specimens from human nonneoplastic brain lesions were used as controls. In the rat brains, the astrocytes were exclusively p27-positive. The reactive astrocytes in various disease processes sometimes lacked p27 expression. The distribution of p27-positive cells was uniform in low-grade astrocytomas and heterogeneous in high-grade tumors. Double staining of p27 and MIB-1 showed a reciprocal pattern in most cases. The frequency of p27 expression was inversely correlated with MIB-1 staining index and tumor grade. However, several malignant gliomas showed high p27 expression in spite of high MIB-1 staining indices. In such cases, MIB-1-positive cells were occasionally p27-positive. In this paper we discuss the etiology of the anomalous p27 expression in a subset of malignant gliomas.

Homozygous deletions of the CDKN2C/p18INK4C gene on the short arm of chromosome 1 in anaplastic oligodendrogliomas

Allelic deletions of the short arm of chromosome 1 are common in oligodendrogliomas and have been correlated with chemosensitivity and better prognosis in patients with high-grade oligodendrogliomas. In these tumors, 1p loss is also inversely related to deletions of the CDKN2A gene on 9p, which encodes the key cell cycle regulatory molecule p16INK4A. Because the CDKN2C gene, which encodes the homologous p18INK4C cell cycle regulatory protein, maps to chromosomal band 1p32, CDKN2C is an attractive candidate for the oligodendroglioma suppressor gene on chromosome 1. To evaluate this possibility, we studied 39 high-grade oligodendrogliomas for homozygous deletions and point mutations of the CDKN2C gene, as well as for allelic loss of 1p. Although no mutations were detected in the CDKN2C coding region, two tumors had homozygous deletions that involved CDKN2C. Interestingly, these cases did not have CDKN2A gene deletions. Coupled with the recent report of rare point mutations of CDKN2C in oligodendrogliomas, these findings suggest that CDKN2C inactivation may be oncogenic in a small percentage of human oligodendrogliomas.

MAP kinase cascade is required for p27 downregulation and S phase entry in fibroblasts and epithelial cells

The present report delineates the critical pathway in the G(1) phase involved in downregulation of p27(Kip1), a cyclin-dependent kinase inhibitor, which plays a pivotal role in controlling entry into the S phase of the cell cycle. In resting CCL39 fibroblasts and IEC-6 intestinal epithelial cells, protein levels of p27(Kip1) were elevated but dramatically decreased on serum stimulation, along with hyperphosphorylation of pRb and increased CDK2 activity. In both cell types, expression of ras resulted in an increase of basal and serum-stimulated E2F-dependent transcriptional activity and a reduction in p27(Kip1) protein levels as well. The role of the mitogen-activated protein (MAP) kinase cascade in p27(Kip1) reduction and S phase reentry was reinforced by the blockades of serum-induced E2F-dependent transcriptional activity and p27(Kip1) downregulation with the MKK-1/2 inhibitor PD-98059. In both cell lines, downregulation of p27(Kip1) was associated with a repression of its synthesis, an event mediated by the p42/p44 MAP kinase pathway. Using an antisense approach, we demonstrated that p27(Kip1) may control cell cycle exit in both cell types. These data indicate that activation of the MAP kinase cascade is required for S phase entry and p27(Kip1) downregulation in fibroblasts and epithelial cells.

Low expression of the cell cycle inhibitor p27Kip1 in normal corticotroph cells, corticotroph tumors, and malignant pituitary tumors

The cell cycle is regulated by a number of inhibitors, including p27Kip1 (p27), which belongs to the kip1 family. By binding to the cyclin/cyclin-dependent kinase complexes, it regulates progression of G1 to S phase in the cell cycle. It has been reported that p27 knockout mice develop multiorgan hyperplasia and intermediate lobe pituitary tumors secreting ACTH. Previously, we and others have been unable to show any consistent change in messenger RNA expression or genomic mutations for p27 in human corticotroph adenomas. However, dysregulation at the protein level has been reported in nonendocrine tumors, and we, therefore, investigated the expression of p27 in a range of benign and metastatic pituitary tumors. We studied a total of 107 pituitaries, including normal pituitary (n = 20), Cushing's disease (n = 21), acromegaly (n = 19), nonfunctioning adenomas (n = 18), prolactinomas (n = 7), TSH-omas (n = 2), FSH-omas (n = 6), aggressive tumors showing invasiveness and recurrence (n = 9), and metastatic pituitary carcinomas (n = 5). Using standard immunohistochemical techniques with a highly specific monoclonal antibody, p27 expression was determined quantitatively as the percentage of cells showing strongly positive, weak, or negative staining. In each sample, approximately 500 cells were analyzed. We also analyzed normal pituitaries using double-labeling for p27 and each of the pituitary hormones to characterize the expression of p27 in each cell type. p27 was expressed in normal pituitary cells; in tumors expressing GH, prolactin, TSH, and FSH; and in aggressive tumors, but markedly reduced expression of p27 was seen in corticotroph tumors and pituitary carcinomas. In the normal pituitary, somatotroph, lactotroph, and thyrotroph cells showed strong p27 staining, whereas normal corticotroph cells showed a much lower level of p27 staining (P < 0.001). Somatotroph, lactotroph, gonadotroph, and thyrotroph adenomas showed a lower level of p27 expression compared with normal somatotrophs (P = 0.02), lactotrophs (P = 0.03), gonadotrophs (P = 0.01), and thyrotrophs, respectively, whereas the lower level of p27 expression present in normal corticotrophs virtually disappeared in corticotroph adenomas (P = 0.001). We conclude that pituitary adenomas show a lower level of p27 protein expression than the normal cells from which they are derived, with malignant transformation leading to complete loss of p27 immunoreactivity. Corticotrophs are quite different to other pituitary cell types in terms of p27 immunoreactivity because both normal and tumorous corticotrophs have low p27 staining, and we speculate that this may relate to their inherent control mechanisms.

Comparative structure and characterization of a CDKN2 gene in a Xiphophorus fish melanoma model

We have cloned, sequenced, and characterized the RNA expression properties of a fish CDKN2 gene from Xiphophorus helleri and X. maculatus. This gene, termed CDKN2X, shows a high degree of amino acid sequence similarity to members of the mammalian CDKN2 gene family, which includes the tumor suppressor loci CDKN2A (P16) and CDKN2B (P15). Comparative sequence analysis suggests that fish CDKN2X is similarly related to all four mammalian gene family members, and may represent a descendant of an ancestral prototypic CDKN2 gene. CDKN2X was mapped to a region on autosomal Xiphophorus linkage group V (LG V) known to contain the DIFF gene that acts as a tumor suppressor of melanoma formation in X. helleri/X. maculatus backcross hybrids. Thus, CDKN2X may be a candidate for the tumor suppressor DIFF gene. Here we have sequenced CDKN2X in both Xiphophorus species and have characterized its expression in normal and melanotic tissues within control and backcross hybrid fish. A simultaneous expressional analysis of the Xmrk-2 tyrosine kinase receptor gene, which is strongly implicated in melanomagenesis in this system, was also performed. RT - PCR analyses revealed that both genes were highly expressed in melanomas. For CDKN2X, this result contrasts numerous findings in human tumors including human melanoma in which either CDKN2A (P16) deactivation or LOH was observed.

Rb independent inhibition of cell growth by p15(INK4B)

The INK4 cyclin dependent kinase inhibitors (CDKI), such as p15(INK4B) and p16(INK4A), block cell cycle progression from G to S phase. This is mediated by inhibition of phosphorylation of proteins, including the retinoblastoma susceptibility protein (Rb), by cyclin dependent kinases. Ectopic over-expression of the p16(INK4A) CDKI can inhibit growth of cell lines depending on Rb status. Cell lines lacking Rb, with few exceptions, are resistant to growth inhibition by p16(INK4A). The effects of ectopic over-expression of p15(INK4B) in cell lines with and without wild type Rb were examined by measuring cell recovery. Proliferation was inhibited in cells lacking Rb as well as in cells with wild type Rb expression. Experiments analyzing the effectiveness of chimeric p15(INK4B)/p16(INK4A) proteins indicated that the Rb independent growth inhibition required N-terminal residues of p15(INK4B). Linker insertion mutation of p15(INK4B) showed that the inhibition was dependent on intact ankyrin structures. Double staining flow cytometry found that the growth inhibition correlated with a decrease in cells in G2/M phases of the cell cycle. These findings are consistent with Rb independent inhibition of the progression from G1 to S caused by overexpression of p15(INK4B).

Molecular analysis of selected cell cycle regulatory proteins during aerobic and hypoxic maintenance of human ovarian carcinoma cells

We have previously reported on the development of an in vitro model system for studying the effect of hypoxia on ovarian carcinoma cell proliferation and invasion (Krtolica and Ludlow, 1996). These data indicate that the cell division cycle is reversibly arrested during the G1 phase. Here, we have continued this study to include the proliferation properties of both aerobic and hypoxic human ovarian carcinoma cells at the molecular level. The growth suppressor product of the retinoblastoma susceptibility gene, pRB, appears to be functional in these cells as determined by SV40 T-antigen binding studies. Additional G1-to-S cell cycle regulatory proteins, cyclins D and E, cyclin-dependent kinases (cdks) 4 and 2, and cdk inhibitors p27 and p18, also appear to be intact based on their apparent molecular weights and cell cycle stage-specific abundance. During hypoxia, there is a decrease in abundance of cyclins D and E, with an increase in p27 abundance. cdk4 activity towards pRB and cdk2 activity towards histone H1 are also decreased. Co-precipitation studies revealed an increased amount of p27 complexing with cyclin E-cdk2 during hypoxia than during aerobic cell growth. In addition, pRB-directed phosphatase activity was found to be greater in hypoxic than aerobic cells. Taken together, a model is suggested to explain hypoxia-induced cell cycle arrest in SKA human ovarian carcinoma cells.

Down-regulation of the INK4 family of cyclin-dependent kinase inhibitors by tax protein of HTLV-1 through two distinct mechanisms

Tax oncoprotein of human T-cell leukemia virus type 1 (HTLV-1) affects multiple regulatory processes of infected cells through activation and repression of specific transcription and also through modulation of functions of cell cycle regulators. Previously, we found that Tax binds to p16ink4a, a member of the INK4 family of cyclin-dependent kinase inhibitors, and counteracts its inhibitory activity, resulting in cell cycle progression. In this study, we examined the effects of Tax on other members of the INK4 family and found that Tax can bind to p15ink4b similarly to p16ink4a, but not to p18ink4c and p19ink4d. Tax binding to p15ink4b inactivated its function and restored CDK4 kinase activity. Accordingly, Tax-expressing cells became resistant to p15ink4b-mediated growth arrest induced by TGFbeta. On the other hand, expression of p18ink4c was transcriptionally repressed by Tax through the E-box element of the promoter, which may contribute to the marked reduction of p18ink4c mRNA in HTLV-1-infected T-cells. These observations indicate that Tax suppresses the inhibitory activities of INK4 family members through two independent mechanisms: functional inhibition of two INK4 proteins and repression of expression of another INK4 protein. These effects may play roles in HTLV-1-induced deregulation of the cell cycle, possibly promoting cellular transformation.

Induction of the p16INK4a senescence gene as a new therapeutic strategy for the treatment of rheumatoid arthritis

Synovial tissue affected by rheumatoid arthritis is characterized by proliferation, which leads to irreversible cartilage and bone destruction. Current and experimental treatments have been aimed mainly at correcting the underlying immune abnormalities, but these treatments often prove ineffective in preventing the invasive destruction. We studied the expression of cyclin-dependent kinase inhibitors in rheumatoid synovial cells as a means of suppressing synovial cell proliferation. Synovial cells derived from hypertrophic synovial tissue readily expressed p16INK4a when they were growth-inhibited. This was not seen in other fibroblasts, including those derived from normal and osteoarthritis-affected synovial tissues. In vivo adenoviral gene therapy with the p16INK4a gene efficiently inhibited the pathology in an animal model of rheumatoid arthritis. Thus, the induction of p16INK4a may provide a new approach to the effective treatment of rheumatoid arthritis.

c-Myc regulates cyclin D-Cdk4 and -Cdk6 activity but affects cell cycle progression at multiple independent points

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

p27KIP1 is abnormally expressed in Diffuse Large B-cell Lymphomas and is associated with an adverse clinical outcome

Cell cycle progression is regulated by the combined action of cyclins, cyclin-dependent kinases (CDKs), and CDK-inhibitors (CDKi), which are negative cell cycle regulators. p27KIP1 is a CDKi key in cell cycle regulation, whose degradation is required for G1/S transition. In spite of the absence of p27KIP1 expression in proliferating lymphocytes, some aggressive B-cell lymphomas have been reported to show an anomalous p27KIP1 staining. We analysed p27KIP1 expression in a series of Diffuse Large B-cell Lymphoma (DLBCL), correlating it with the proliferative index and clinical outcome, to characterize the implications of this anomalous staining in lymphomagenesis in greater depth. For the above mentioned purposes, an immunohistochemical technique in paraffin-embedded tissues was employed, using commercially available antibodies, in a series of 133 patients with known clinical outcomes. Statistical analysis was performed in order to ascertain which clinical and molecular variables may influence outcome, in terms of disease-free survival (DFS) and overall survival (OS). The relationships between p27KIP1 and MIB-1 (Ki-67) were also tested. An abnormally high expression of p27KIP1 was found in lymphomas of this type. The overall correlation between p27KIP1 and MIB-1 showed there to be no significant relationship between these two parameters, this differing from observations in reactive lymphoid and other tissues. Analysis of the clinical relevance of these findings showed that a high level of p27KIP1 expression in this type of tumour is an adverse prognostic marker, in both univariate and multivariate analysis. These results show that there is abnormal p27KIP1 expression in DLBCL, with adverse clinical significance, suggesting that this anomalous p27KIP1 protein may be rendered non-functional through interaction with other cell cycle regulator proteins.

Constitutive overexpression of cyclin D1 in human breast epithelial cells does not prevent G1 arrest induced by deprivation of epidermal growth factor

Non-transformed human breast epithelial cell line MCF10A is dependent on exogenous epidermal growth factor (EGF) for continued growth. Complete G1 arrest was rapidly induced following EGF deprivation. The cell cycle arrest was accompanied by increased levels of p27KIP1, a cyclin-dependent kinase inhibitor, and reduced level of cyclin D1. This was associated with strong inhibition of cyclin-dependent kinase 2 and cyclin D1-associated kinase activities. Introduction of exogenous cyclin D1 into MCF10A (MCF10AD1) cells resulted in an accelerated cell growth rate but did not confer colony-forming capacity. Cell cycle arrest was still achieved in MCF10AD1 cells following EGF deprivation. In the great majority of MCF10AD1 clones, accumulation in G1 phase was accompanied by reduced cyclin D1 and increased p27KIP1 protein levels. In two clones where cyclin D remained unchanged during G1 arrest, it was found that more cyclin D1 protein was bound to p27KIP1. The data demonstrate that ectopic expression of cyclin D1 alone could not transform MCF10A cells nor was it sufficient to prevent G1 arrest induced by EGF deprivation.

Viral encoded cyclins

Cyclins are known effectors of cellular proliferation. While originally considered as the product of cellular genes, it is now clear that representatives of this class of proteins can be encoded by certain viruses. One of these viruses is HHV-8, a gamma herpesvirus implicated as a causative agent of Kaposi's Sarcoma and lymphomas in humans. The significance of the virally encoded cyclin proteins in viral propagation is as yet unclear. However, the fact that deregulation of cellular cyclin expression is a known event in tumour development suggests that the virally encoded cyclins could be part of a mechanism utilised by these viruses to induce tumour formation.

Cdkn2a encodes functional variation of p16INK4a but not p19ARF, which confers selection in mouse lung tumorigenesis

The cyclin-dependent kinase inhibitor 2a (Cdkn2a) locus encodes two distinct tumor suppressors, p16INK4a and p19ARF, whose functions interrelate in the regulation of cell proliferation as key components of the retinoblastoma and p53 pathways, respectively. In many types of cancer, alterations of Cdkn2a abrogate the functions of both suppressors, implying that both are integral to the genesis of certain cancer types. While this has been observed in mouse lung adenocarcinogenesis, recent observations also suggested that naturally occurring variation at the Cdkn2a locus is probably operative in the development of these tumors. Firstly, two common haplotypes of mouse Cdkn2a have been identified, each of which encodes cosegregating variants of p16INK4a and p19ARF. The p16INK4a variants differ at amino acids 18 (histidine or proline) and 51 (valine or isoleucine), whereas the p19ARF variants differ only at amino acid 72 (histidine or arginine). Secondly, genetic resistance to lung tumor formation appears to segregate with one particular haplotype, which also is deleted preferentially in lung adenocarcinomas of Cdkn2a heterozygous mice. Here we attempt to explain these observations and to characterize further the roles of p16INK4 and p19ARF in mouse lung tumorigenesis by examining the function and expression of each of the variants of Cdkn2a. Functional analysis showed that the proline 18/isoleucine 51 p16INK4a variant was diminished in cdk6 binding, cdk6 inhibition and NIH/3T3 fibroblast growth suppression compared with the histidine 18/valine 51 variant, whereas both of the p19ARF variants suppressed growth with similar potencies. Also, the different alleles for p16INK4a and p19ARF were transcribed equally in the normal lungs of Cdkn2a heterozygotes, as determined by comparative reverse transcription-polymerase chain reaction-single-stranded conformation polymorphism analysis. These results indicate that strain-specific variation in p16INK4a function is exploited in mouse lung tumorigenesis and strongly implicate a role for p16INK4a in lung cancer predisposition and development.

Cyclins and cell cycle checkpoints

The eucaryotic cell cycle is regulated by the periodic synthesis and destruction of cyclins that associate with and activate cyclin-dependent kinases. Cyclin-dependent kinase inhibitors, such as p21 and p16, also play important roles in cell cycle control by coordinating internal and external signals and impeding proliferation at several key checkpoints. Understanding how these proteins interact to regulate the cell cycle has become increasingly important to researchers and clinicians with the discovery that many of the genes that encode cell cycle regulatory activities are targets for alterations that underlie the development of cancer. Several therapeutic agents, such as DNA-damaging drugs, microtubule inhibitors, antimetabolites, and topoisomerase inhibitors, take advantage of this disruption in normal cell cycle regulation to target checkpoint controls and ultimately induce growth arrest or apoptosis of neoplastic cells. Other therapeutic drugs being developed, such as UCN-01, specifically inhibit cell cycle regulatory proteins.

A cyclin D-Cdk4 activity required for G2 phase cell cycle progression is inhibited in ultraviolet radiation-induced G2 phase delay

Cyclin D-Cdk4 complexes have a demonstrated role in G1 phase, regulating the function of the retinoblastoma susceptibility gene product (Rb). Previously, we have shown that following treatment with low doses of UV radiation, cell lines that express wild-type p16 and Cdk4 responded with a G2 phase cell cycle delay. The UV-responsive lines contained elevated levels of p16 post-treatment, and the accumulation of p16 correlated with the G2 delay. Here we report that in UV-irradiated HeLa and A2058 cells, p16 bound Cdk4 and Cdk6 complexes with increased avidity and inhibited a cyclin D3-Cdk4 complex normally activated in late S/early G2 phase. Activation of this complex was correlated with the caffeine-induced release from the UV-induced G2 delay and a decrease in the level of p16 bound to Cdk4. Finally, overexpression of a dominant-negative mutant of Cdk4 blocked cells in G2 phase. These data indicate that the cyclin D3-Cdk4 activity is necessary for cell cycle progression through G2 phase into mitosis and that the increased binding of p16 blocks this activity and G2 phase progression after UV exposure.

Mysterious liaisons: the relationship between c-Myc and the cell cycle

A large body of physiological evidence shows that either upregulation or downregulation of intracellular c-Myc activity has profound consequences on cell cycle progression. Recent work suggests that c-Myc may stimulate the activity of cyclin E/cyclin-dependent kinase 2 (Cdk2) complexes and antagonize the action of the Cdk inhibitor p27KIP1. Cyclin D/Cdk4/6 complexes have also been implicated as targets of c-Myc activity. However, in spite of considerable effort, the mechanisms by which c-Myc interacts with the intrinsic cyclin/Cdk cell cycle machinery remain undefined.

p21Waf1/Cip1/Sdi1 induces permanent growth arrest with markers of replicative senescence in human tumor cells lacking functional p53

We have shown previously that wild type p53 can rapidly induce replicative senescence in EJ human bladder carcinoma cells lacking functional p53. A major effector of p53 functions is p21Waf1/Cip1/Sdi1, a potent cyclin-dependent kinase inhibitor. p21Waf1/Cip1/Sdi1 has been shown to be involved in both p53 dependent and independent control of cell proliferation, differentiation and death. To directly investigate the effects of p21Waf1/Cip1/Sdi1 in the p53 response observed in EJ tumor cells, we established p21Waf1/Cip1/Sdi1 inducible lines using the tetracycline-regulatable vector system. p21Waf1/Cip1/Sdi1 induction caused irreversible cell cycle arrest in both G1 and G2/M, and diminished Cdk2 kinase activity. In addition, p21Waf1/Cip1/Sdi1 induction led to morphological alterations characteristic of cells undergoing replicative senescence with morphological, biochemical and ultrastructural markers of the senescent phenotype. Furthermore, sustained p21Waf1/Cip1/Sdi1 induction sensitized EJ cells to apoptotic cell death induced by mitomycin C, a cross-linking DNA damaging agent. These findings support the function of p21Waf1/Cip1/Sdi1 as an inducer of replicative senescence and a major mediator of this phenomenon in response to p53. Moreover, our results imply that therapeutic intervention in human cancers might be aimed at sustained elevation of p21Waf1/Cip1/Sdi1 expression.

Cell cycle proteins and the development of oral squamous cell carcinoma

Expression of cell cycle regulatory proteins was evaluated in premalignant and malignant oral epithelial lesions, to test the hypothesis that protein regulation of the cell cycle may be altered in the development of oral squamous cell carcinoma. Archived paraffin-embedded specimens (n = 90) from 25 patients with recurrent or persistent lesions were evaluated in immunohistochemically stained sections for cell cycle regulatory proteins p53, Rb, Cyclin D1, p27, and p21. The cell cycle was also evaluated by expression of nuclear protein Ki 67. Sections were graded semiquantitatively using a 0-3 + scale to indicate the percentage of positively stained cells. The initial histologic diagnosis for 17/25 patients was either focal keratosis, mild dysplasia, or moderate dysplasia; the initial diagnosis for the remaining eight patients ranged from severe dysplasia to moderately differentiated squamous cell carcinoma. Thirty-three of 90 specimens showed positive p53 expression, 11 of which were dysplasias. Eighty-nine of 90 specimens, from all stages of disease, showed positive Rb expression. Twenty-three of 90 specimens showed positive Cyclin D1 expression, typically in the later stages (carcinoma) of a patient's disease. Eighty-four of 90 specimens showed positive p21 expression; while 55 of 90 specimens were positive for p27. In control mucosa, p27 was highly expressed, while Rb and p21 proteins were expressed at relatively low levels; p53 and Cyclin D1 proteins were largely absent. Generally, staining of p53, Rb, p21, and Ki 67 increased with time in serial biopsies, while p27 showed decreased staining with disease progression. These data show that cell cycle regulatory proteins are altered in both premalignant and malignant disease, and that protein phenotypes are heterogeneous. P53 expression is seen early, and Cyclin D1 expression is seen late in the development of oral premalignant and malignant disease. Expression of p53, Rb, p21 and Ki67 increased, while p27 decreased, with disease progression.

Alteration of p16 and p15 genes in human uterine tumours

The roles of the p16 and p15 inhibitor of cyclin-dependent kinase tumour suppressor genes were examined in human uterine cervical and endometrial cancers. p16 mRNA, examined by reverse transcription polymerase chain reaction (RT-PCR), was significantly reduced in five of 19 (26%) cervical and four of 25 (16%) endometrial tumours. Reduced expression of p16 protein, detected by immunohistochemistry, occurred even more frequently, in nine of 33 (27%) cervical and seven of 37 (19%) endometrial tumours. Hypermethylation of a site within the 5'-CpG island of the p16 gene was detected in only one of 32 (3%) cervical tumours and none of 26 endometrial tumours. Homozygous p16 gene deletion, evaluated by differential PCR analysis, was found in four of 40 (10%) cervical tumours and one of 38 (3%) endometrial tumours. Homozygous deletion of p15 was found in three of 40 (8%) cervical tumours and one of 38 (3%) endometrial tumours. PCR-SSCP (single-strand conformation polymorphism) analysis detected point mutations in the p16 gene in six (8%) of 78 uterine tumours (four of 40 (10%) cervical tumours and two of 38 (5%) endometrial tumours). Three were mis-sense mutations, one in codon 74 (CTG-->ATG) and one in codon 129 (ACC-->ATC), both in cervical carcinomas, and the other was in codon 127 (GGG-->GAG) in an endometrial carcinoma. There was one non-sense mutation, in codon 50 (CGA-->TGA), in an endometrial carcinoma. The remaining two were silent somatic cell mutations, both in cervical carcinomas, resulting in no amino acid change. These observations suggest that inactivation of the p16 gene, either by homologous deletion, mutation or loss of expression, occurs in a subset of uterine tumours.

p57(Kip2) is degraded through the proteasome in osteoblasts stimulated to proliferation by transforming growth factor beta1

Cyclin-dependent kinase inhibitory proteins are negative regulators of the cell cycle. Although all the cyclin-dependent kinase inhibitory proteins may be involved in cell cycle control during a differentiation process, only p57(Kip2) is shown to be essential for embryonic development. However, the role of p57 in the control of the cell cycle is poorly understood. Using osteoblasts derived from the calvaria of rat fetus, we show that p57 is accumulated in cells starved by low serum. Cyclin-dependent kinase 2 activity was suppressed in these cells with a significant amount bound to p57. Treatment of the cells with transforming growth factor beta1 dramatically reduced the amount of p57, resulting in an activation of cyclin-dependent kinase 2 activity and the stimulation of cell proliferation. The decrease in p57 was inhibited by treating the cells with proteasome inhibitors, Z-Leu-Leu-Leu-aldehyde or lactacystin, but not with Z-Leu-Leu-aldehyde, which is an inhibitor of calpain, indicating that p57 is degraded through the proteasome pathway. p57 was also shown to be ubiquitinated in vitro. Because transforming growth factor beta1 not only stimulates the growth but also inhibits the differentiation of the cells in this system, our results may suggest a possible involvement of p57 in the control of osteoblastic cell proliferation and differentiation.

Induced Differentiation of U937 Cells by 1,25-dihydroxyvitamin D3 Involves Cell Cycle Arrest in G1 That Is Preceded by a Transient Proliferative Burst and an Increase in Cyclin Expression

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3[1,25(OH)2D3], is a potent inhibitor of cellular proliferation as well as an inducer of differentiation of myeloid leukemic cells to macrophages. We have previously reported that a number of genes are upregulated by 1,25(OH)2D3 during myeloid differentiation, including the cyclin-dependent kinase (CDK) inhibitors p21, p27, 15, and p18, suggesting that cell cycle arrest and differentiation are tightly linked processes. We further explore here the relationship between growth inhibition and differentiation. We report that, upon 1,25(OH)2D3 treatment, U937 cells exhibited an early proliferative burst followed by growth inhibition and subsequent differentiation. Although CDK levels remain constant throughout, this transient increase in proliferation was accompanied by increases in cyclin A, D1, and E protein levels. p21 and p27 levels were also elevated during both the proliferative burst and subsequent inhibition of cell growth. Ectopic overexpression of p21 and/or p27 in U937 cells, in the absence of hormone, resulted in an induction of the expression of monocyte/macrophage-specific markers, whereas overexpression of p15 and p18 had no effect, suggesting that a subset of CDK inhibitors are important for both growth arrest and differentiation and that an early increase in proliferation is somehow a prerequisite for subsequent differentiation. However, no such biphasic behavior was detected in cells that are growth inhibited by 1,25(OH)2D3but do not differentiate, such as MCF-7 cells. Taken together, these results indicate that both growth stimulation and subsequent inhibition precede differentiation and involve induction of both cyclins and p21 and p27, whereas cell cycle arrest of differentiated cells can be achieved simply by elevations in CDK inhibitors.

Induced Differentiation of U937 Cells by 1,25-dihydroxyvitamin D3 Involves Cell Cycle Arrest in G1 That Is Preceded by a Transient Proliferative Burst and an Increase in Cyclin Expression

The hormonal form of vitamin D, 1,25-dihydroxyvitamin D3[1,25(OH)2D3], is a potent inhibitor of cellular proliferation as well as an inducer of differentiation of myeloid leukemic cells to macrophages. We have previously reported that a number of genes are upregulated by 1,25(OH)2D3 during myeloid differentiation, including the cyclin-dependent kinase (CDK) inhibitors p21, p27, 15, and p18, suggesting that cell cycle arrest and differentiation are tightly linked processes. We further explore here the relationship between growth inhibition and differentiation. We report that, upon 1,25(OH)2D3 treatment, U937 cells exhibited an early proliferative burst followed by growth inhibition and subsequent differentiation. Although CDK levels remain constant throughout, this transient increase in proliferation was accompanied by increases in cyclin A, D1, and E protein levels. p21 and p27 levels were also elevated during both the proliferative burst and subsequent inhibition of cell growth. Ectopic overexpression of p21 and/or p27 in U937 cells, in the absence of hormone, resulted in an induction of the expression of monocyte/macrophage-specific markers, whereas overexpression of p15 and p18 had no effect, suggesting that a subset of CDK inhibitors are important for both growth arrest and differentiation and that an early increase in proliferation is somehow a prerequisite for subsequent differentiation. However, no such biphasic behavior was detected in cells that are growth inhibited by 1,25(OH)2D3but do not differentiate, such as MCF-7 cells. Taken together, these results indicate that both growth stimulation and subsequent inhibition precede differentiation and involve induction of both cyclins and p21 and p27, whereas cell cycle arrest of differentiated cells can be achieved simply by elevations in CDK inhibitors.

Active transcriptional repression by the Rb-E2F complex mediates G1 arrest triggered by p16INK4a, TGFbeta, and contact inhibition

Rb inhibits progression from G1 to S phase of the cell cycle. It associates with a number of cellular proteins; however, the nature of these interactions and their relative significance in cell cycle regulation are still unclear. We present evidence that Rb must normally interact with the E2F family of transcription factors to arrest cells in G1, and that this arrest results from active transcriptional repression by the Rb-E2F complex, not from inactivation of E2F. Thus, a major role of E2F in cell cycle regulation is assembly of this repressor complex. We demonstrate that active repression by Rb-E2F mediates the G1 arrest triggered by TGFbeta, p16INK4a, and contact inhibition.

p53 and p16INK4A mutations during the progression of glomus tumor

Glomus tumors are significantly rare tumors of carotid body. The great majority of these tumors are benign in character. Here we present two brothers with hereditary glomus jugulare tumor who had consanguineous parents. Radiotherapy was applied approximately 8 and 10 years ago for treatment in both cases. Eight years later, one of these cases came to our notice due to relapse. The mutation pattern of p53, p57KIP2, p16INK4A and p15NK4B genes which have roles in the cell cycle, was analyzed in tumor samples obtained from the two affected cases in the initial phase and from one of these cases at relapse. The DNA sample obtained from the case in initial diagnosis phase revealed no p53, p57KIP2, p16INK4A or p15INK4B mutation. He is still in remission phase. Despite the lack of p53, p57KIP2, p16INK4A and p15INK4B mutation at initial diagnosis the tumor DNA of the other case in relapse revealed p53 codon 243 (ATG-->ATC; met-->ile) and p16 codon 97 (GAC-->AAC; asp-->asn) missense point mutations. No loss of heterozygosity in p53 and p16INK4A was observed by microsatellite analysis of tumoral tissues in these cases. P53 and p16INK4A mutations observed in relapse phase were in conserved regions of both genes. No previous reports have been published with these mutations in glomus tumor during progression. The mutation observed in this case may due to radiotherapy. In spite of this possibility, the missense point mutations in conserved region of p53 and p16INK4A genes may indicate the role of p53 and p16INK4A in tumor progression of glomus tumors.

The p21(Cip1) and p27(Kip1) CDK 'inhibitors' are essential activators of cyclin D-dependent kinases in murine fibroblasts

The widely prevailing view that the cyclin-dependent kinase inhibitors (CKIs) are solely negative regulators of cyclin-dependent kinases (CDKs) is challenged here by observations that normal up-regulation of cyclin D- CDK4 in mitogen-stimulated fibroblasts depends redundantly upon p21(Cip1) and p27(Kip1). Primary mouse embryonic fibroblasts that lack genes encoding both p21 and p27 fail to assemble detectable amounts of cyclin D-CDK complexes, express cyclin D proteins at much reduced levels, and are unable to efficiently direct cyclin D proteins to the cell nucleus. Restoration of CKI function reverses all three defects and thereby restores cyclin D activity to normal physiological levels. In the absence of both CKIs, the severe reduction in cyclin D-dependent kinase activity was well tolerated and had no overt effects on the cell cycle.

Prognostic implications of expression of the cell cycle inhibitor protein p27Kip1

Mitogenic and growth inhibitory signals influence the activity of a family of cyclin dependent kinases (cdks). p27 is an important cdk inhibitor, acting in G1 to inhibit cyclin-cdks. As negative growth regulators, the cdk inhibitors may function as tumor suppressors. While the p16 gene plays a tumor suppressor role in cancers, p27 gene mutations have been identified only rarely. While high levels of p27 protein are expressed in normal human mammary epithelium, loss of p27 is frequent and is of independent prognostic significance in breast cancers. Low p27 is also a poor prognostic factor in colon, gastric, esophageal, lung, and prostate carcinomas, and enhanced proteasomal degradation may underlie loss of p27 in tumor cells. Loss of p27 has not been significantly correlated with tumor proliferation in a number of studies and may reflect alterations in differentiation and adhesion-dependent growth regulation germane to oncogenesis and tumor progression. Efforts to confirm the prognostic value of p27 are under way in a number of large breast cancer studies. These studies may also indicate whether loss of p27 in association with other traditional or novel markers has greater prognostic potential than each factor alone. p27 immunostaining is inexpensive and reliable and may become part of the routine histopathologic processing of tumors in the near future. Widespread application of p27 in prognostic testing will require greater uniformity in scoring techniques and determination of the cut off levels which distinguish individuals at high and low risk of cancer recurrence and death. Finally, the greatest utility of p27 may lie in the information it sheds on the biology of aberrant growth regulation in breast cancer and the potential to use this in the generation of novel therapeutic strategies.

Overexpression of proliferating cell nuclear antigen in mammalian cells negates growth arrest by serum starvation and cell contact

Proliferating cell nuclear antigen (PCNA) functions as a processivity factor for DNA polymerase delta, and is expressed at high levels in growing normal and tumor cells. To clarify the relationship between cell proliferation and PCNA expression, we generated NIH-3T3 cells that overexpress PCNA and analyzed the phenotype of these cells. The resulting 3T3-PCNA cells, which overexpressed PCNA, were found to proliferate beyond the saturation density of the parental NIH-3T3 cells. Although NIH-3T3 cell proliferation is arrested under serum starvation conditions, 3T3-PCNA cell proliferation is not arrested by serum starvation. The expression levels of cdk2, cdk4 and cdk6 were the same in 3T3-PCNA and NIH-3T3 cells. The activity of cdk4 was identical for both cell types. However, the activity of cdk2 was higher in serum-starved 3T3-PCNA cells than in NIH-3T3 cells, although the expression of cyclin E decreased in both types of cells, suggesting that increases in cdk2 activity are related to negation of growth arrest in 3T3-PCNA cells. These results indicate that increases in PCNA expression lead to the disruption of growth control and may lead to malignant transformation.

Induced expression of p16(INK4a) inhibits both CDK4- and CDK2-associated kinase activity by reassortment of cyclin-CDK-inhibitor complexes

To investigate the mode of action of the p16(INK4a) tumor suppressor protein, we have established U2-OS cells in which the expression of p16(INK4a) can be regulated by addition or removal of isopropyl-beta-D-thiogalactopyranoside. As expected, induction of p16(INK4a) results in a G1 cell cycle arrest by inhibiting phosphorylation of the retinoblastoma protein (pRb) by the cyclin-dependent kinases CDK4 and CDK6. However, induction of p16(INK4a) also causes marked inhibition of CDK2 activity. In the case of cyclin E-CDK2, this is brought about by reassortment of cyclin, CDK, and CDK-inhibitor complexes, particularly those involving p27(KIP1). Size fractionation of the cellular lysates reveals that a substantial proportion of CDK4 participates in active kinase complexes of around 200 kDa. Upon induction of p16(INK4a), this complex is partly dissociated, and the majority of CDK4 is found in lower-molecular-weight fractions consistent with the formation of a binary complex with p16(INK4a). Sequestration of CDK4 by p16(INK4a) allows cyclin D1 to associate increasingly with CDK2, without affecting its interactions with the CIP/KIP inhibitors. Thus, upon the induction of p16(INK4a), p27(KIP1) appears to switch its allegiance from CDK4 to CDK2, and the accompanying reassortment of components leads to the inhibition of cyclin E-CDK2 by p27(KIP1) and p21(CIP1). Significantly, p16(INK4a) itself does not appear to form higher-order complexes, and the overwhelming majority remains either free or forms binary associations with CDK4 and CDK6.