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

The retinoblastoma gene: a prototypic and multifunctional tumor suppressor

Genome instability has been implicated in the generation of multiple somatic mutations that underlie cancer. Germline mutation in the retinoblastoma (RB) gene leads to tumor formation in both human and experimental animal models, and reintroduction of wild-type RB is able to suppress neoplastic phenotypes. Rb governs the passage of cells through the G1 phase-restriction point and this control is lost in most cancer cells. Rb has also been shown to promote terminal differentiation and prevent cell cycle reentry. Recent studies implicate Rb in mitotic progression, faithful chromosome segregation, checkpoint control, and chromatin remodeling, suggesting that Rb may function in the maintenance of genome integrity. It is likely that Rb suppresses tumor formation by virtue of its multiple biological activities. A single protein capable of performing multiple antioncogenic functions may be a common characteristic of other tumor suppressors including p53 and BRCA1/2.

Altered regulation of cell cycle machinery involved in interleukin-1-induced G(1) and G(2) phase growth arrest of A375S2 human melanoma cells

Interleukin-1 (IL-1) inhibits the growth of A375S2 human melanoma cells by arresting them at G(1) and G(2) phases of the cell cycle. The arrests are preceded by a rapid decrease in kinase activities of cyclin E-Cdk2 and cyclin B1-Cdc2, which are critical for G(1)-S and G(2)-M progression, respectively. IL-1 quickly enhances the protein expression of the CDK inhibitor p21(cip1). The induced p21 binds preferentially to cyclin E-Cdk2, and the increase in p21 binding parallels the decrease in cyclin E-Cdk2 activity. Thus, p21 is likely to be responsible for the inhibition of cyclin E-Cdk2 activity and G(1) arrest. Coinciding with the decrease in cyclin B1-Cdc2 activity, there is an increase in tyrosine phosphorylation of Cdc2, suggesting that an increase in the inactive Tyr-15-phosphorylated form of Cdc2 is involved in the decrease in cyclin B1-Cdc2 activity and G(2) arrest. Furthermore, we found that IL-1 causes rapid dephosphorylation of p107, but not of pRb or p130, while the total protein levels of p130 are increased. Thus, IL-1 may exert its growth-arresting effects via p107 and p130 pathways rather than through pRb.

Update on genetic events in the pathogenesis of melanoma

Melanoma is the most common fatal malignancy among young adults, and its incidence and mortality continue to increase at an alarming rate. Epidemiologic studies have clearly demonstrated roles for genetic predisposition and sun exposure in melanoma development. In the past few years, substantial information has been added to the body of evidence suggesting that inherited and somatic genetic events contribute to the pathogenesis of melanoma. This review focuses on recent advances in the understanding of the genetic events, particularly aberration of cell cycle control and transcriptional control mechanisms, implicated in the pathogenesis of melanoma.

Recessive oncogenes: current status

Cell growth is under the control of a variety of positive and negative signals. An imbalance of such signals results in deregulation of cell behavior. Recessive oncogenes or tumor suppressor genes, opposite to dominant oncogenes, encode important cellular proteins which could function as negative regulators of the cell cycle, i.e., cell cycle brakes. Inactivation of recessive oncogenes, by allelic deletion, loss of expression, mutation, or functional inactivation by interacting with oncogene products of DNA tumor viruses or with amplified cellular binding proteins, will lead to uncontrolled cell growth or tumor formation. Besides the classic suppressor genes such as the p53 and RB, a growing number of novel tumor suppressor genes have been identified in recent years. While some tumor suppressor genes have been found to be important for the development of a large number of human malignancies (e.g., the p53 gene), others are more tumor type-specific (e.g., the NF-1 gene). Many human cancer types showed abnormalities of multiple tumor suppressor genes, offering strong support to the concept that tumorigenesis and progression result from an accumulation of multiple genetic alterations. In this review, we will begin with an overview (gene, transcript, protein and mechanisms of action) of the tumor suppressor genes (the RB, p53, DCC, APC, MCC, WT1, VHL, MST1, and BRCA1 genes) identified to date and then discuss the specific involvement of tumor suppressor genes in human malignancies including prostate cancer. Various chromosomal regions which potentially may contain tumor suppressor genes also will be reviewed.

Transforming growth factor: signal transduction pathways, cell cycle mediation, and effects on hematopoiesis

Transforming growth factor-beta (TGF-beta) is a potent growth inhibitor of various cell types including hematopoietic cells. Two receptors, TGFbetaRI and TGFbetaRII, govern the interaction between the cell and the TGF-beta ligand. Primary binding of the ligand occurs with the RII receptor, promoting formation of a heterodimer with RI and activation of signaling. This induces transient association of Smad proteins with the receptors. Smad 3 and 4 may be involved in the TGF-beta-induced G(1) arrest. TGF-beta(1) down-regulates G(1) and G(2) cyclin-dependent kinases (cdks) and cyclins in terms of both kinase activity and protein amount. TGF- beta (1) also inhibits phosphorylation of the product of the retinoblastoma tumor suppressor gene (pRb) at multiple serine and threonine residues in human myeloid leukemia cells. The underphosphorylated pRb associates with transcription factor E2F-4 in G(1) phase, whereas the phosphorylated pRb mainly binds to E2F-1 and E2F-3. Because TGF-beta(1) up-regulates p130(pRb family member)/E2F-4 complex formation and down-regulates p107(pRb family member)/E2F-4 complex formation, with E2F-4 levels remaining constant, these results suggest that E2F-4 is switched from p107 to pRb and p130 when cells exit from the cell cycle and arrest in G(1) by the action of TGF-beta(1). The "cdk inhibitor" p27 is both a positive and a negative regulator of TGF-beta(1)-mediated cell cycle control. Although TGF-beta(1) has been reported to be a selected inhibitor of normal primitive hematopoietic stem cells, TGF-beta inhibits both primitive and more differentiated myeloid leukemia cell lines.

E2F4 is exported from the nucleus in a CRM1-dependent manner

E2F is a family of transcription factors required for normal cell cycle control and for cell cycle arrest in G1. E2F4 is the most abundant E2F protein in many cell types. In quiescent cells, it is localized to the nucleus, where it is bound to the retinoblastoma-related protein p130. During entry into the cell cycle, the protein disappears from the nucleus and appears in the cytoplasm. The mechanism by which this change occurs has, in the past, been unclear. We have found that E2F4 is actively exported from the nucleus and that leptomycin B, a specific inhibitor of nuclear export, inhibits this process. E2F4 export is mediated by two hydrophobic export sequences, mutations in either of which result in export failure. Individual export mutants of E2F4, but not a mutant with inactivation of both export signals, can be efficiently excluded from the nucleus by forced coexpression of the nuclear export receptor CRM1. Similarly, CRM1 overexpression can prevent cell cycle arrest induced by the cyclin kinase inhibitor p16(INK4a), an E2F4-dependent process. Taken together, these data suggest that nuclear export contributes to the regulation of E2F4 function, including its ability to regulate exit from G1 in association with a suitable pocket protein.

Prognostic implications of p27 and cyclin E protein contents in malignant lymphomas

The G1/S transition in the cell cycle is one of the checkpoints that can be deregulated in tumor development potentially causing increased proliferation and impaired capacity to arrest genetically damaged cells. The balance between activating and inhibitory molecules acting in the check point area seems to be critical and overexpression of cyclins and/or downregulation of the cdk inhibitors have been observed in many malignancies including lymphomas. In this review we have focused on two of the interplayers in the G1/S transition namely cyclin E and p27, and present the current knowledge of aberrations affecting these proteins in lymphomas as well as associations with clinico-pathological data including survival.

A murine tumor progression model for pancreatic cancer recapitulating the genetic alterations of the human disease

This study describes a tumor progression model for ductal pancreatic cancer in mice overexpressing TGF-alpha. Activation of Ras and Erk causes induction of cyclin D1-Cdk4 without increase of cyclin E or PCNA in ductal lesions. Thus, TGF-alpha is able to promote progression throughout G1, but not S phase. Crossbreeding with p53 null mice accelerates tumor development in TGF-alpha transgenic mice dramatically. In tumors developing in these mice, biallelic deletion of Ink4a/Arf or LOH of the Smad4 locus is found suggesting that loci in addition to p53 are involved in antitumor activities. We conclude that these genetic events are critical for pancreatic tumor formation in mice. This model recapitulates pathomorphological features and genetic alterations of the human disease.

HTLV-I Tax related dysfunction of cell cycle regulators and oncogenesis of adult T cell leukemia

HTLV-I is causually related to the oncogenesis of adult T cell leukemia (ATL). However, the precise mechanism of HTLV-I oncogenesis is unclear. HTLV-I Tax protein functions as an activator of various cellular genes, including IL-2, IL-2 receptor-alpha, and c-fos through the activation of nuclear transfer factors such as NF-kappaB and SRF, and also potently activates trascription of viral genes through CREB/ATF sites in the viral LTR. However, Tax activation of HTLV-I infected T cells through the above pathways induces polyclonal proliferation of the cells in vitro; Tax however may function only transiently in the immediate post-infection period following infection in vivo. The long latent period of 60 years from infection to onset of disease suggests other mechanisms for ATL oncogenesis. Recent studies suggest that the malignant transformation of ATL is a multi-hit phenomena, suggesting that discrete genetic events are responsible for ATL oncogenesis. These genetic events could be responsible for the different stages of ATL: smoldering, chronic, lymphoma, and acute type, p16 and p53 genes are important negative regulators of the cell cycle and are often found to be mutated in neoplasms. Recent studies including ours demonstrated a high frequency of alteration of these two genes in primary ATL cells. Furthermore, alteration of the two genes is associated with acute but not chronic type ATL. In addition, p16 gene alteration is linked to the growth rate of ATL cells, suggesting that the alteration of these cell cycle regulatory genes may be related to progression from smoldering or chronic to acute or lymphoma type ATL. Tax may be involved in mutagenesis of these genes through suppression of DNA-beta polymerase gene expression during the process from latent period to acute/lymphoma type. Once transformation occurs, activation of the pathway between Tax and the three nuclear transfer factors, NF-kappaB, SRF, and CREB/ATF, contributes to establish the aggressive manifestations of acute/lymphoma type ATL cells.

Loss of p27Kip1 expression is a strong independent prognostic factor of reduced survival in N0 gastric carcinomas

BACKGROUND

p27KiP1 is a cyclin-dependent kinase inhibitor and is a potential tumor suppressor gene. Reduced expression of p27Kip1 is a powerful negative prognostic marker in primary lung, breast, colon, bladder, and prostate carcinomas. In the current study, the prognostic value of p27Kip1 in gastric cancer was evaluated and compared with other histopathologic parameters and p53 expression.

METHODS

p27Kip1 and p53 protein expression were determined by immunohistochemistry in 96 gastric carcinomas. The tumors were from a low incidence population and were selected for the absence of lymph node involvement.

RESULTS

Reduced expression of p27KiP1 (< or = 50% positive cells) and nuclear p53 accumulation (> 30% positive cells) were observed in 67 (69.8%) and 9 (9%) tumors, respectively, and were not related to either the pT category or tumor histology. Kaplan-Meier analyses revealed a significant impact on survival by p27Kip1 (P = 0.0001 by log rank test), p53 (P < 0.0001) expression, and the pT category (P < 0.0001). On multivariate analysis, reduced p27Kip1 protein expression was the strongest independent predictor of reduced survival (P = 0.005; relative risk = 3.348) out weighing the pT category (P = 0.010; relative risk = 2.257) and p53 overexpression (P = 0.016; relative risk = 2.618).

CONCLUSIONS

These data indicated that immunohistochemical detection of p27Kip1 could help to identify gastric carcinoma patients who are at high risk of death, even in the absence of lymph node involvement, and who might benefit from an adjuvant treatment following surgery.

Frequent loss of 1p32 region but no mutation of the p18 tumor suppressor gene in meningiomas

After chromosome 22 and NF2 inactivation, the loss of chromosome 1p is one of the most frequent abnormalities encountered in meningiomas. However the putative tumor suppressor gene located on 1p inactivated in meningiomas has still to be identified. We screened 68 meningiomas for LOH on chromosome 22 and 1. We found 34 LOH on the NF2 region on chromosome 22 (50%) and 19 LOH on 1p (28%), 16 being associated with loss of chromosome 22. Partial deletions delimited a candidate region located between D1S234 and D1S2797. The p18INK4C tumor suppressor gene, a member of the genes family coding for inhibitors of cyclin-dependent kinases, is located in this region. To determine whether p18 is involved in development of meningiomas, we performed a mutation analysis of the p18 gene and a search for homozygous deletion in the 19 meningiomas with 1p loss. Sequencing analysis of the p18 gene revealed one polymorphism, but no somatic mutations and no homozygous deletions were found. These results confirm that the loss of chromosome 1p32 is a frequent feature in meningiomas, however the p18 tumor suppressor gene which is located in this region, does not seem to be involved.

The Raf signal transduction cascade as a target for chemotherapeutic intervention in growth factor-responsive tumors

This review focuses on the Ras-Raf-mitogen-activated protein kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) signal transduction pathway and the consequences of its unregulation in the development of cancer. The roles of some of the cell membrane receptors involved in the activation of this pathway, the G-protein Ras, the Raf, MEK and ERK kinases, the phosphatases that regulate these kinases, as well as the downstream transcription factors that become activated, are discussed. The roles of the Ras-Raf-MEK-ERK pathway in the regulation of apoptosis and cell cycle progression are also analyzed. In addition, potential targets for pharmacological intervention in growth factor-responsive cells are evaluated.

5-Aza-2'-deoxycytidine leads to down-regulation of aberrant p16INK4A RNA transcripts and restores the functional retinoblastoma protein pathway in hepatocellular carcinoma cell lines

The inactivation of the cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitor p16INK4A may be caused by gene deletion, mutation or promoter hypermethylation. We have previously reported that p16INK4A in hepatocellular carcinoma (HCC) tissues and cell lines is inactivated predominantly by promoter hypermethylation rather than genomic aberrations. In the present experiments, we have studied the effects of the demethylating agent, 5-aza-2'-deoxycytidine (5-AZA/decitabine), on the expression of aberrant p16INK4A RNA transcripts and the CDK-retinoblastoma gene pathway in HCC cell lines with p16INK4A promoter hypermethylation. The expression of aberrant p16INK4A RNA transcripts was down-regulated and p16INK4A protein was strongly re-expressed in the HCC cell lines, SNU 354, 398, 423 and 475 after 5-AZA/decitabine treatment for 5 days. The re-expressed p16INK4A was functional, because it bound to and inhibited CDK4 kinase activity, and increased the concentrations of the hypophosphorylated form of retinoblastoma protein (pRB) in cells with a wild type RB gene. Moreover, treatment with the demethylating agent led not only to G1 cell cycle arrest, but also to the increased expression of the senescence-associated marker beta-galactosidase. This up-regulation of p16INK4A mRNA and protein correlated with demethylation of the p16INK4A promoter, and with the down-regulation or disappearance of aberrant p16INK4A transcripts. These results suggest that the aberrant p16INK4A RNA transcript can be transcribed from the methylated p16INK4A gene, and endogenous reactivation of functional p16INK4A mRNA by a demethylating agent can restore the pRB pathway in HCC, and foster the terminal differentiation of the malignant cells. Therefore, demethylating agents, such as 5-AZA/decitabine, may have potential in the treatment of HCC.

Inhibition of vascular smooth muscle cell proliferation by sodium salicylate mediated by upregulation of p21(Waf1) and p27(Kip1)

BACKGROUND

Salicylates may have direct vascular effects by mechanisms that are independent of platelet inhibition.

METHODS AND RESULTS

We investigated the effect of salicylates on vascular smooth muscle cell (SMC) proliferation in response to platelet-derived growth factor (PDGF) in vitro. Salicylate concentrations of 5 and 10 mmol/L inhibited serum- or PDGF-induced SMC cell count and [(3)H]thymidine incorporation by 62% and 81%, respectively. There was no evidence of cellular toxicity or apoptosis as determined by trypan blue exclusion and FACS analyses. Because cell cycle progression is regulated by hyperphosphorylation of the retinoblastoma (Rb) protein, we examined the effects of salicylate on Rb hyperphosphorylation. Treatment with salicylate, but not indomethacin, inhibited nuclear factor-kappaB activation and completely abolished Rb hyperphosphorylation in PDGF-treated SMCs. This effect was associated with a decrease in cyclin-dependent kinase (Cdk)-2 and, to a lesser extent, Cdk-6, but not Cdk-4 activity, without changes in Cdk-2, -4, and -6 and cyclin D and E protein levels. Because Cdk-2 activity is regulated by the Cdk inhibitors p21(Waf1) and p27(Kip1), we studied the effects of salicylate on p21(Waf1) and p27(Kip1) expression. Treatment with salicylate prevented PDGF-induced downregulation of p21(Waf1) and p27(Kip1) but not of the Cdk-4/-6 inhibitor p16(Ink4).

CONCLUSIONS

These findings indicate that high doses of salicylates inhibit SMC proliferation by cell cycle arrest at the G(1)-S phase and suggest a beneficial role for high-dose salicylates in the treatment of vascular proliferative disorders.

The expression of p18INK4 and p27kip1 cyclin-dependent kinase inhibitors is regulated differently during human B cell differentiation

Cell cycle progression is under the control of cyclin-dependent kinases (cdks), the activity of which is dependent on the expression of specific cdk inhibitors. In this paper we report that the two cdk inhibitors, p27(Kip1) and p18(INK4c), are differently expressed and control different steps of human B lymphocyte activation. Resting B cells contain large amounts of p27(Kip1) and no p18(INK4c). In vitro stimulation by Staphylococcus aureus Cowan 1 strain or CD40 ligand associated with IL-10 and IL-2 induces a rapid decrease in p27(Kip1) expression combined with cell cycle entry and progression. In contrast, in vitro Ig production correlates with specific expression of p18(INK4c) and early G(1) arrest. This G(1) arrest is associated with inhibition of cyclin D3/cdk6-mediated retinoblastoma protein phosphorylation by p18(INK4c). A similar contrasting pattern of p18(INK4c) and p27(Kip1) expression is observed both in B cells activated in vivo and in various leukemic cells. Expression of p18(INK4c) was also detected in various Ig-secreting cell lines in which both maximum Ig secretion and specific p18(INK4c) expression were observed during the G(1) phase. Our study shows that p27(Kip1) and p18(INK4c) have different roles in B cell activation; p27(Kip1) is involved in the control of cell cycle entry, and p18(INK4c) is involved in the subsequent early G(1) arrest necessary for terminal B lymphocyte differentiation.

Regulation of CDK7-carboxyl-terminal domain kinase activity by the tumor suppressor p16(INK4A) contributes to cell cycle regulation

The eukaryotic cell cycle is regulated by cyclin-dependent kinases (CDKs). CDK4 and CDK6, which are activated by D-type cyclins during the G(1) phase of the cell cycle, are thought to be responsible for phosphorylation of the retinoblastoma gene product (pRb). The tumor suppressor p16(INK4A) inhibits phosphorylation of pRb by CDK4 and CDK6 and can thereby block cell cycle progression at the G(1)/S boundary. Phosphorylation of the carboxyl-terminal domain (CTD) of the large subunit of RNA polymerase II by general transcription factor TFIIH is believed to be an important regulatory event in transcription. TFIIH contains a CDK7 kinase subunit and phosphorylates the CTD. We have previously shown that p16(INK4A) inhibits phosphorylation of the CTD by TFIIH. Here we report that the ability of p16(INK4A) to inhibit CDK7-CTD kinase contributes to the capacity to induce cell cycle arrest. These results suggest that p16(INK4A) may regulate cell cycle progression by inhibiting not only CDK4-pRb kinase activity but also by modulating CDK7-CTD kinase activity. Regulation of CDK7-CTD kinase activity by p16(INK4A) thus may represent an alternative pathway for controlling cell cycle progression.

P18 tumor suppressor gene and progression of oligodendrogliomas to anaplasia

P18INK4C is a good candidate to be the tumor suppressor gene involved in oligodendrogliomas on 1p32. Loss of heterozygosity on 1p, mutation(s), homozygous deletion(s), and expression of p18 in 30 oligodendroglial tumors were investigated. Loss of heterozygosity on 1p was found in 15 tumors. A p18 mutation was found at an recurrence of an anaplastic oligodendroglioma, but not in the primary, low-grade tumor. No homozygous deletions were found and p18 was expressed in all cases. These results show that p18 alteration is involved in tumor progression in a subset of oligodendrogliomas.

Complex regulation of CDKs and G1 arrest during the granulocytic differentiation of human myeloblastic leukemia ML-1 cells

We previously reported that all-trans retinoic acid (ATRA) and granulocyte-macrophage colony-stimulating factor (GM-CSF) synergistically induced granulocytic differentiation in human myeloblastic leukemia ML-1 cells. The combination of these agents also suppressed DNA-synthesis. In the present study, we investigated the suppression of cyclin dependent kinase (CDK) activities resulting in G1 arrest in differentiated ML-1 cells. We show that treatment of ML-1 cells with ATRA plus GMCSF results in G1 arrest and suppression of CDK activities. Protein levels of the G1 CDKs were essentially unchanged during this time. However, we observed an increase in CDK2-bound p27 and CDK4-bound p18, and a decrease in CDK6-bound cyclin D3. These results suggest that complex regulation of CDKs play a key role in G1 arrest of ML-1 after treatment with ATRA and GM-CSF. We also showed that an increase in CDK2-bound p27 and CDK4-bound p18 are caused by treatment with ATRA and a decrease in CDK6-bound cyclin D3 is induced synergistically by treatment with both reagents. Furthermore, we propose that the changes in binding of p18 and cyclin D3 to CDKs are due to changes at the protein expression level and that the increase in p27 binding to CDK2 is due to a novel mechanism.

Cell cycle activation by c-myc in a burkitt lymphoma model cell line

The product of the proto-oncogene c-myc (myc) is a potent activator of cell proliferation. In Burkitt lymphoma (BL), a human B-cell tumor, myc is consistently found to be transcriptionally activated by chromosomal translocation. The mechanisms by which myc promotes cell cycle progression in B-cells is not known. As a model for myc activation in BL cells, we have established a human EBV-EBNA1 positive B-cell line, P493-6, in which myc is expressed under the control of a tetracycline regulated promoter. If the expression of myc is switched off, P493-6 cells arrest in G0/G1 in the presence of serum. Re-expression of myc activates the cell cycle without inducing apoptosis. myc triggers the expression of cyclin D2, cyclin E and Cdk4, followed by the activation of cyclin E-associated kinase and hyper-phosphorylation of Rb. The transcription factor E2F-1 is expressed in proliferating and arrested cells at constant levels. The Cdk inhibitors p16, p21, p27 and p57 are expressed at low or not detectable levels in proliferating cells and are not induced after repression of myc. Ectopic expression of p16 inhibits cell cycle progression. These data suggest that myc triggers proliferation of P493-6 cells by promoting the expression of a set of cell cycle activators but not by inactivating cell cycle inhibitors.

Cell cycle activation by c-myc in a burkitt lymphoma model cell line

The product of the proto-oncogene c-myc (myc) is a potent activator of cell proliferation. In Burkitt lymphoma (BL), a human B-cell tumor, myc is consistently found to be transcriptionally activated by chromosomal translocation. The mechanisms by which myc promotes cell cycle progression in B-cells is not known. As a model for myc activation in BL cells, we have established a human EBV-EBNA1 positive B-cell line, P493-6, in which myc is expressed under the control of a tetracycline regulated promoter. If the expression of myc is switched off, P493-6 cells arrest in G0/G1 in the presence of serum. Re-expression of myc activates the cell cycle without inducing apoptosis. myc triggers the expression of cyclin D2, cyclin E and Cdk4, followed by the activation of cyclin E-associated kinase and hyper-phosphorylation of Rb. The transcription factor E2F-1 is expressed in proliferating and arrested cells at constant levels. The Cdk inhibitors p16, p21, p27 and p57 are expressed at low or not detectable levels in proliferating cells and are not induced after repression of myc. Ectopic expression of p16 inhibits cell cycle progression. These data suggest that myc triggers proliferation of P493-6 cells by promoting the expression of a set of cell cycle activators but not by inactivating cell cycle inhibitors.

Drosophila Cdk4 is required for normal growth and is dispensable for cell cycle progression

Complexes of D-type cyclins and cdk4 or 6 are thought to govern progression through the G(1) phase of the cell cycle. In Drosophila, single genes for Cyclin D and Cdk4 have been identified, simplifying genetic analysis. Here, we show that Drosophila Cdk4 interacts with Cyclin D and the Rb homolog RBF as expected, but is not absolutely essential. Flies homozygous for null mutations develop to the adult stage and are fertile, although only to a very limited degree. Overexpression of inactive mutant Cdk4, which is able to bind Cyclin D, does not enhance the Cdk4 mutant phenotype, confirming the absence of additional Cyclin D-dependent cdks. Our results indicate, therefore, that progression into and through the cell cycle can occur in the absence of Cdk4. However, the growth of cells and of the organism is reduced in Cdk4 mutants, indicating a role of D-type cyclin-dependent protein kinases in the modulation of growth rates.

Bcr-Abl kinase down-regulates cyclin-dependent kinase inhibitor p27 in human and murine cell lines

Chronic myeloid leukemia (CML) is a malignant stem cell disease characterized by an expansion of myeloid progenitor cells expressing the constitutively activated Bcr-Abl kinase. This oncogenic event causes a deregulation of apoptosis and cell cycle progression. Although the molecular mechanisms protecting from apoptosis in CML cells are well characterized, the cell cycle regulatory event is poorly understood. An inhibitor of the cyclin-dependent kinases, p27, plays a central role in the regulation of growth factor dependent proliferation of hematopoietic cells. Therefore, we have analyzed the influence of Bcr-Abl in the regulation of p27 expression in various hematopoietic cell systems. An active Bcr-Abl kinase causes down-regulation of p27 expression in murine Ba/F3 cells and human M07 cells. Bcr-Abl blocks up-regulation of p27 after growth factor withdrawal and serum reduction. In addition, p27 induction by transforming growth factor-beta (TGF-β) is completely blocked in Bcr-Abl positive M07/p210 cells. This deregulation is directly mediated by the activity of the Bcr-Abl kinase. A Bcr-Abl kinase inhibitor completely abolishes p27 down-regulation by Bcr-Abl in both Ba/F3 cells transfected either with a constitutively active Bcr-Abl or with a temperature sensitive mutant. The down-regulation of p27 by Bcr-Abl depends on proteasomal degradation and can be blocked by lactacystin. Overexpression of wild-type p27 partially antagonizes Bcr-Abl–induced proliferation in Ba/F3 cells. We conclude that Bcr-Abl promotes cell cycle progression and activation of cyclin-dependent kinases by interfering with the regulation of the cell cycle inhibitory protein p27.

Bcr-Abl kinase down-regulates cyclin-dependent kinase inhibitor p27 in human and murine cell lines

Chronic myeloid leukemia (CML) is a malignant stem cell disease characterized by an expansion of myeloid progenitor cells expressing the constitutively activated Bcr-Abl kinase. This oncogenic event causes a deregulation of apoptosis and cell cycle progression. Although the molecular mechanisms protecting from apoptosis in CML cells are well characterized, the cell cycle regulatory event is poorly understood. An inhibitor of the cyclin-dependent kinases, p27, plays a central role in the regulation of growth factor dependent proliferation of hematopoietic cells. Therefore, we have analyzed the influence of Bcr-Abl in the regulation of p27 expression in various hematopoietic cell systems. An active Bcr-Abl kinase causes down-regulation of p27 expression in murine Ba/F3 cells and human M07 cells. Bcr-Abl blocks up-regulation of p27 after growth factor withdrawal and serum reduction. In addition, p27 induction by transforming growth factor-beta (TGF-β) is completely blocked in Bcr-Abl positive M07/p210 cells. This deregulation is directly mediated by the activity of the Bcr-Abl kinase. A Bcr-Abl kinase inhibitor completely abolishes p27 down-regulation by Bcr-Abl in both Ba/F3 cells transfected either with a constitutively active Bcr-Abl or with a temperature sensitive mutant. The down-regulation of p27 by Bcr-Abl depends on proteasomal degradation and can be blocked by lactacystin. Overexpression of wild-type p27 partially antagonizes Bcr-Abl–induced proliferation in Ba/F3 cells. We conclude that Bcr-Abl promotes cell cycle progression and activation of cyclin-dependent kinases by interfering with the regulation of the cell cycle inhibitory protein p27.

E2F4 and E2F5 play an essential role in pocket protein-mediated G1 control

E2F transcription factors are major regulators of cell proliferation. The diversity of the E2F family suggests that individual members perform distinct functions in cell cycle control. E2F4 and E2F5 constitute a defined subset of the family. Until now, there has been little understanding of their individual biochemical and biological functions. Here, we report that simultaneous inactivation of E2F4 and E2F5 in mice results in neonatal lethality, suggesting that they perform overlapping functions during mouse development. Embryonic fibroblasts isolated from these mice proliferated normally and reentered from Go with normal kinetics compared to wild-type cells. However, they failed to arrest in G1 in response to p16INK4a. Thus, E2F4 and E2F5 are dispensable for cell cycle progression but necessary for pocket protein-mediated G1 arrest of cycling cells.

Retinoblastoma: the disease, gene and protein provide critical leads to understand cancer

Retinoblastoma has contributed much to the understanding of cancer. The protein product of the RB gene, pRB, is a multifaceted regulator of transcription which controls the cell cycle, differentiation and apoptosis in normal development of specific tissues. Elucidating the mechanisms in which pRB plays a critical role will enable novel therapies and strategies for prevention, not only for retinoblastoma, but for cancer in general.

Establishment of irreversible growth arrest in myogenic differentiation requires the RB LXCXE-binding function

The crystal structure of the A-B domain of RB has defined the binding pocket for the LXCXE peptide motif. Using the crystal structure as a guide, we have inactivated the LXCXE-binding pocket by replacing N757 with Phe [to obtain RB(N757F)]. RB(N757F) does not bind to viral oncoproteins but retains the ability to bind and inhibit E2F. RB(N757F) is less effective than the wild-type RB [RB(WT)] in repressing E2F-regulated transcription, and its repression activity is not affected by trichostatin A, an inhibitor of histone deacetylases. However, RB(N757F) is as effective as RB(WT) in suppressing cell growth. Interestingly, RB(N757F) cannot establish an irreversible growth arrest in differentiated myocytes. Differentiated myocytes with RB(WT) become refractory to serum. By contrast, differentiated myocytes with RB(N757F) undergo DNA synthesis and phosphorylate RB(N757F) in response to serum, despite a high level of p21Cip1 expression. Mutation of the phosphorylation sites in RB(N757F) rescued its defect and allowed myocytes to permanently withdraw from the cell cycle. These results demonstrate that it is possible to inactivate the LXCXE-binding pocket without compromising the overall integrity of RB. Moreover, the LXCXE-binding pocket is dispensable for the intrinsic growth suppression function of RB. However, the LXCXE-binding function is essential for RB to establish the serum-refractory state in differentiated myocytes.

Overexpression of p27(KIP1) induced cell cycle arrest in G(1) phase and subsequent apoptosis in HCC-9204 cell line

AIM:We have previously reported that inducible over-expression of Bak may prolong cell cycle in G(1) phase and lead to apoptosis in HCC-9204 cells. This study is to investigate whether p27(KIP1) plays an important role in this process.METHODS:In order to elucidate the exact function of p27(KIP1) in this process, a zinc inducible p27(KIP1) stable transfectant and transient p27(KIP1)-GFP fusion transfectant were constructed. The effects of inducible p27(KIP1) on cell growth, cell cycle arrest and apoptosis were examined in the mock, control pMD vector, and pMD-KIP1 transfected HCC-9204 cells.RESULTS:This p27(KIP1)-GFP transfectant may transiently express the fusion gene. The cell growth was reduced by 35% at 48 h of p27(KIP1) induction with zinc treatment as determined by trypan blue exclusion assay. These differences remained the same after 72h of p27(KIP1) expression. p27(KIP1) caused cell cycle arrest after 24 h of induction, with 40% increase in G(1) population. Prolonged p27(KIP1) expression in this cell line induced apoptotic cell death reflected by TUNEL assay. Fourty-eight h and 72 h of p27(KIP1) expression showed a characteristic DNA ladder on agarose gel electrophoresis.CONCLUSION:Bak may induce cell cycle arrest in G(1) phase through upregulating expression of p27(KIP1) and subsequently lead to apoptosis in HCC-9204 cells. The p27(KIP1) -GFP fusion protein can be transiently expre-ssed in HCC-9204 cells. The inducible p27(KIP1)-expressing cell line provides a model to assess p27(KIP1) function.

P53 but not p16INK4a induces growth arrest in retinoblastoma-deficient hepatocellular carcinoma cells

BACKGROUND/AIM

Both p16INK4a and p53 proteins are negative regulators of the cell cycle. In human hepatocellular carcinomas (HCC), the loss of function of p53, retinoblastoma (pRb) and pl6INK4a genes by different mechanisms has been largely documented, but their hepatocellular effects are poorly known. We compared the growth-inhibitory effects of p16INK4a and p53 proteins in Hep3B cell line-derived clones.

METHODS

Cells were transfected with inducible p16INK4a and p53 expression vectors, and stable clones were analyzed for transgene expression by Western blotting and immunoperoxidase staining. Effects on cell growth were analyzed by in vitro growth assay, thymidine incorporation and flow cytometry. Biochemical effects of p53 were tested by Northern blotting of p21Cip1 transcripts and by Western blotting of p21Cip1, mdm-2, bax, cyclin-dependent kinase 2 and cyclin E proteins. The pRb protein was studied by Western blotting and immunoprecipitation assays.

RESULTS

The induction of p16INK4a protein expression did not affect in vitro growth of cells. In contrast, p53 protein in its wild-type conformation provoked a growth arrest accompanied by transactivation of p21Cip1 gene and accumulation of p21Cip1, bax and mdm-2 proteins. p53-induced growth arrest was due to a cell cycle arrest at the G1/S transition, probably mediated by p21Cip1 protein, which inhibits cyclin-dependent kinase 2/cyclin E complexes.

CONCLUSIONS

The lack of detectable pRb protein and resistance of cells to p16TNK4a strongly suggest that p53 is able to arrest the growth of HCC cells by a mechanism independent of "p53-retinoblastoma pathway". These findings are applicable to HCC with abberrations of both p53 and pRb genes, and may not represent the universal effects of p53 in hepatic cells.

Association of the cyclin-dependent kinases and 14-3-3 sigma negatively regulates cell cycle progression

14-3-3 sigma, implicated in cell cycle arrest by p53, was cloned by expression cloning through cyclin-dependent kinase 2 (CDK2) association. 14-3-3 sigma shares cyclin-CDK2 binding motifs with different cell cycle regulators, including p107, p130, p21(CIP1), p27(KIP1), and p57(KIP2), and is associated with cyclin.CDK complexes in vitro and in vivo. Overexpression of 14-3-3 sigma obstructs cell cycle entry by inhibiting cyclin-CDK activity in many breast cancer cell lines. Overexpression of 14-3-3 sigma can also inhibit cell proliferation and prevent anchorage-independent growth of these cell lines. These findings define 14-3-3 sigma as a negative regulator of the cell cycle progression and suggest that it has an important function in preventing breast tumor cell growth.

Cell cycle and transcriptional control of human myeloid leukemic cells by transforming growth factor beta

TGFbeta1 is a potent growth inhibitor of both primitive and more differentiated human myeloid leukemic cells. The extent of the growth inhibitory response to TGFbeta varies with cell type, and is not linked to stages of differentiation of cell lines. Downregulation of multiple cell cycle-regulatory molecules is a dominant event in TGFbeta1-mediated growth inhibition of human MV4-11 myeloid leukemia cells. Both G1-phase and G2-phase cyclins and cdks participate in the regulation of TGFbeta1-mediated growth inhibition of MV4-11 cells. By both depressing cdk2 synthesis and up-regulating cyclin E-associated p27, TGFbeta1 may magnify its inhibitory efficiency. TGFbeta1 also rapidly inhibits phosphorylation of pRb at several serine and threonine residues. The underphosphorylated pRb associates with E2F-4 in G1 phase, whereas the phosphorylated pRb mainly binds to E2F-1 and E2F-3 in proliferating MV4-11 cells. Since TGFbeta1 upregulates p130/E2F-4 complex formation and downregulates p107/E2F-4 complex formation, with E2F-4 levels remaining constant, our results suggest that E2F-4 is switched from p107 to pRb and p130 when cells exit from the cell cycle and arrest in G1 by TGFbeta1. In summary, TGFbeta1 inhibits growth of human myeloid leukemic cells through multiple pathways, whereas the "cdk inhibitor" p27 is both a positive and negative regulator.

Cell-anchorage, cell cytoskeleton, and Rho-GTPase family in regulation of cell cycle progression

It has been well known that cell-anchorage and the cell cytoskeleton are deeply involved in the regulation of cell proliferation and cell cycle. However, the precise molecular mechanism involved in cell-anchorage and the cell cytoskeleton have remained be to elucidated. The recent great volume of information regarding cell cycle regulators such as cyclin, cyclin-dependent kinases (CDKs) and CDK inhibitors (CKI) has facilitated the understanding of the cell cycle in mammalian cells. In this review, we will focus on these cell cycle regulators to discuss the regulation of cell proliferation controlled by cell-anchorage and the cytoskeleton, and especially the roles of Rho family GTPases.

The GADD45 inhibition of Cdc2 kinase correlates with GADD45-mediated growth suppression

Cell cycle growth arrest is an important cellular response to genotoxic stress. Gadd45, a p53-regulated stress protein, plays an important role in the cell cycle G(2)-M checkpoint following exposure to certain types of DNA-damaging agents such as UV radiation and methylmethane sulfonate. Recent findings indicate that Gadd45 interacts with Cdc2 protein and inhibits Cdc2 kinase activity. In the present study, a series of Myc-tagged Gadd45 deletion mutants and a Gadd45 overlapping peptide library were used to define the Gadd45 domains that are involved in the interaction of Gadd45 with Cdc2. Both in vitro and in vivo studies indicate that the interaction of Gadd45 with Cdc2 involves a central region of the Gadd45 protein (amino acids 65-84). The Cdc2-binding domain of Gadd45 is also required for Gadd45 inhibition of Cdc2 kinase activity. Sequence analysis of the central Gadd45 region reveals no homology to inhibitory motifs of known cyclin-dependent kinase inhibitors, indicating that the Cdc2-binding and -inhibitory domains on Gadd45 are a novel motif. The peptide containing the Cdc2-binding domain (amino acids 65-84) disrupted the Cdc2-cyclin B1 protein complex, suggesting that dissociation of this complex results from a direct interaction between the Gadd45 and Cdc2 proteins. GADD45-induced cell cycle G(2)-M arrest was abolished when its Cdc2 binding motif was disrupted. Importantly, a short term survival assay demonstrated that GADD45-induced cell cycle G(2)-M arrest correlates with GADD45-mediated growth suppression. These findings indicate that the cell cycle G(2)-M growth arrest mediated by GADD45 is one of the major mechanisms by which GADD45 suppresses cell growth.

Ubiquitin/proteasome-mediated degradation of p19INK4d determines its periodic expression during the cell cycle

Assembly and activity of the proto-oncogenic cyclin D/CDK4(6) complexes, the major driving force of G1 phase progression, is negatively regulated by a family of INK4 CDK inhibitors p16INK4a, p15INK4b, p18INK4c, and p19INK4d. Expression of the INK4 family members is controlled at the transcriptional level, through differential response to environmental and intracellular signals such as cytokines, oncogenic overload, or cellular senescence. Here we show that the periodic oscillation of the p19INK4d protein during the cell cycle is determined by the ubiquitin/proteasome-dependent mechanism, allowing the protein abundance to follow the changes in its mRNA expression. Within the INK4 family, this regulatory mode appears restricted to p19INK4d whose ubiquitination was dependent on the integrity of lysine 62, and binding to CDK4. These results highlight unexpected differences among the INK4 inhibitors, and suggest how p19INK4d may help regulate the rate of cyclin D/CDK4(6) complex formation, and thereby timely progression through the mammalian cell division cycle. Oncogene (2000) 19, 2870 - 2876

Tumor suppressor INK4: refinement of p16INK4A structure and determination of p15INK4B structure by comparative modeling and NMR data

Within the tumor suppressor protein INK4 (inhibitor of cyclin-dependent kinase 4) family, p15INK4B is the smallest and the only one whose structure has not been determined previously, probably due to the protein's conformational flexibility and instability. In this work, multidimensional NMR studies were performed on this protein. The first tertiary structure was built by comparative modeling with p16INK4A as the template, followed by restrained energy minimization with NMR constraints (NOE and H-bonds). For this purpose, the solution structure of pl6INK4A, whose quality was also limited by similar problems, was refined with additional NMR experiments conducted on an 800 MHz spectrometer and by structure-based iterative NOE assignments. The nonhelical regions showed major improvement with root-mean-square deviation (RMSD) improved from 1.23 to 0.68 A for backbone heavy atoms. The completion of p15INK4B coupled with refinement of p16INK4A made it possible to compare the structures of the four INK4 members in depth, and to compare the structures of p16INK4A in the free form and in the p16INK4A-CDK6 complex. This is an important step toward a comprehensive understanding of the precise functional roles of each INK4 member.

Aging and longevity. A paradigm of complementation between homeostatic mechanisms and genetic control?

Aging is a universal and inevitable phenomenon that affects nearly all animal species. It can be considered the product of an interaction between genetic, environmental, and lifestyle factors, which in turn influence longevity that varies between and within species. It has been proposed not only that the aging process is under genetic control, but that it can also be considered a result of the failure of homeostasis due to the accumulation of damage. This review article discusses these issues, focusing on the function of genes that associate with aging and longevity, as well as on the molecular mechanisms that control cell survival and maintenance during aging.

Melanoma genetics: an update with focus on the CDKN2A(p16)/ARF tumor suppressors

Investigative interest in atypical nevi and familial melanoma has contributed to the identification of several candidate melanoma loci within the human genome. Molecular defects in both tumor suppressor genes and oncogenes have been pathogenically linked to melanoma in recent studies. Of the loci currently characterized, the major gene resides on chromosome 9p and encodes a tumor suppressor designated p16. This gene, which is also known as CDKN2A, is either mutated or deleted in a large majority of melanoma cell lines, as well as in many uncultured melanoma cells and in the germline of melanoma kindreds. A novel aspect of the p16 locus is that it encodes not just one but two separate gene products that are transcribed in alternative reading frames. Both products function as negative regulators of cell cycle progression. The p16 protein itself executes its effects by competitively inhibiting cyclin-dependent kinase 4, which is a factor necessary for cellular progression through a major regulatory transition of the cell division cycle. Inherited and acquired deletions or point mutations in the p16 gene increase the likelihood that potentially mutagenic DNA damage will escape repair before cell division. Notably, the second product of the locus, ARF (for alternative reading frame), regulates cell growth through independent effects on the p53 pathway. Although there is little evidence that ARF by itself is involved in the pathogenesis of melanoma, deletions at the p16 locus disable two separate pathways that control cell growth. These recent advances open up the possibility of genetic testing for melanoma susceptibility in the setting of familial melanoma and suggest novel therapeutic strategies for melanoma based on gene therapy or small molecule mimicry targeted to the correction of defects in the p16 regulatory pathway. (J Am Acad Dermatol 2000;42:705-22.)

LEARNING OBJECTIVE

At the conclusion of this learning activity, participants should be familiar with the historical aspects of melanoma genetics and should have a greater understanding of the CDKN2A(p16)/ARF tumor suppressor genes.

Induction of G(1) cell cycle arrest and p27(KIP1) increase by panaxydol isolated from Panax ginseng

Polyacetylenic compounds of Panax ginseng roots have been shown to inhibit growth of several human malignant tumor cell lines. Panaxydol is known to be one of the cytotoxic polyacetylenic compounds of P. ginseng. In this study, we first showed that panaxydol decreased markedly the proliferation, and to a lesser extent, the number of cells in a human melanoma cell line, SK-MEL-1. Next, the effect of panaxydol on cell cycle progression and its mechanism of action were investigated. Cell cycle analysis revealed that panaxydol inhibited cell cycle progression of a human malignant melanoma cell line, SK-MEL-1, at G(1)-S transition. At the same time, panaxydol increased the protein expression of p27(KIP1) as early as 1 hr after treatment. Cyclin-dependent kinase 2 (Cdk2) activity was decreased in a dose-dependent manner after 24 hr of panaxydol treatment. Protein levels of p21(WAF1), p16(INK4a), p53, pRb (retinoblastoma protein), and E2F-1 were not changed. It was also found that cycloheximide reversed the growth inhibition induced by panaxydol and partially abrogated the increase in p27(KIP1) expression. These results indicate that panaxydol induces G(1) cell cycle arrest by decreasing Cdk2 activity and up-regulating p27(KIP1) protein expression.

Divergent expression of cyclin-dependent kinase inhibitors (CKI) and p14ARF/p16 beta in non-Hodgkin's lymphomas and chronic lymphocytic leukemia

Chronic B-cell lymphocytic leukaemia (CLL) and low-grade B-cell Non Hodgkin's lymphomas (Lg-NHL) are characterized by slow accumulation of neoplastic cells arrested in the G0/G1 phase of the cell cycle. In contrast, proliferation rates are high in aggressive B-cell lymphomas (Hg-NHL). Divergent expression of cyclin-dependent kinase inhibitors (CKI) in the cell cycle may contribute to these differences. We analysed CLL as well as low and high grade B-cell NHL for expression of G1-specific and universal CKI by competitive RT-PCR and immunostaining. p16(INK4A) expression was low in all types of neoplasms. Highest p14(ARF) /p16 beta expression levels were found in normal lymphocytes. Expression of this CKI was significantly lower in CLL, but still higher in CLL than in the lymphomas (median 27 vs. 3 mRNA transcripts x 10(3), p = 0.0001). p14(ARF) /p16 beta immunostaining correlated with mRNA expression. Highest p21 mRNA levels were found in CLL, but three of four CLL with abundant p21 mRNA production were negative on immunostaining. High grade lymphomas showed markedly decreased p21 expression (3.9 in Hg-NHL vs. 12 in Lg-NHL and 29 in CLL; values expressed as mRNA transcripts x 10(3), p < 0.009). mRNA and protein expression of p27 was considerably higher in CLL than in the lymphomas. Differential CKI expression in various B-cell neoplasias may provide important biological markers, if not the molecular underpinning of their different cell cycle kinetics. Targeted interference with such genes governing cell cycle control in lymphoid neoplasia may pave the way towards new treatment strategies.

Stat3-dependent induction of p19INK4D by IL-10 contributes to inhibition of macrophage proliferation

We have previously reported that IL-10 inhibits proliferation of normal bone marrow-derived macrophages and of the monocyte/macrophage cell line J774. Activation of Stat3 was shown to be necessary and sufficient to mediate inhibition of proliferation. To investigate further the mechanism of growth arrest, we examined the effect of IL-10 on expression of cell cycle inhibitors. We found that IL-10 treatment increases expression of the cyclin-dependent kinase inhibitors p19INK4D and p21CIP1 in macrophages. IL-10 cannot induce p19INK4D expression or block proliferation when Stat3 signaling is blocked by a dominant negative Stat3 or a mutant IL-10Ralpha which does not recruit Stat3 in J774 cells, whereas p21CIP1 induction is not affected. An inducibly active Stat3 (coumermycin-dimerizable Stat3-Gyrase B), which suppresses J774 cell proliferation, also induced p19INK4D expression. Sequencing of the murine p19INK4D promoter revealed two candidate Stat3 binding sites, and IL-10 treatment activated a reporter gene controlled by this promoter. These data suggest that Stat3-dependent induction of p19INK4D mediates inhibition of proliferation. Enforced expression of murine p19INK4D cDNA J774 cells significantly reduced their proliferation. Use of antisense p19INK4D and analysis of p19INK4D-deficient macrophages confirmed that p19INK4D is required for optimal inhibition of proliferation by IL-10, and indicated that additional IL-10 signaling events contribute to this response. These data indicate that Stat3-dependent induction of p19INK4D and Stat3-independent induction of p21CIP1 are important components of the mechanism by which IL-10 blocks proliferation in macrophages.

Cell-cycle inhibitors: three families united by a common cause

In the cellular program leading to DNA synthesis, signals that drive cells into S-phase converge at the level of CDK activity. The products of at least three different gene families, Ink4, Cip/Kip and the pRb pocket-protein family, suppress S-phase entry. Ink4 proteins act by antagonizing the formation and activation of cyclin D-CDK4 complexes, of which the ultimate downstream target as related to S-phase entry appears to be pRb. Cip/Kip inhibitors impinge upon that pathway by inhibiting CDK2 kinases that participate in the inactivation of pRb and, like cyclin E, may also have roles independent of pRb. How the activities of these three classes of proteins are coordinated remains obscure. In recent years, development of mouse models has accelerated the elucidation of this complex network, showing roles that are sometimes cooperative and sometimes overlapping. We will discuss the interrelationships between Cip/Kip inhibitors and the components of the pRb pathway, and how their activities ultimately regulate cell proliferation.

Conserved region 2 of adenovirus E1A has a function distinct from pRb binding required to prevent cell cycle arrest by p16INK4a or p27Kip1

Ectopic expression of the CDK inhibitors (CKIs) p16INK4a and p27Kip1 in Rat1 fibroblasts induces dephosphorylation and activation of Retinoblastoma-family proteins (pRb, p107 and p130), their association with E2F proteins, and cell cycle arrest in G1. The growth-inhibitory action of p16, in particular, is believed to be mediated essentially via pRb activation. The 12S E1A protein of human Adenovirus 5 associates with pRb-family proteins via residues in its Conserved Regions (CR) 1 and 2, in particular through the motif LXCXE in CR2. These interactions are required for E1A to prevent G1 arrest upon co-expression of CKIs. We show here that mutating either of two conserved motifs adjacent to LXCXE in CR2, GFP and SDDEDEE, also impairs the ability of E1A to overcome G1 arrest by p16 or p27. Strikingly, however, these mutations affect neither the association of E1A with pRb, p07 and p130, nor its ability to derepress E2F-1 transcriptional activity in transient transfection assays. One of the EIA mutants, however, is defective in derepressing several endogenous E2F target genes in the presence of p16 or p27. Thus, CR2 possesses an essential function besides pRb-binding. We speculate that this function might be required for the full derepression of E2F-regulated genes in their natural chromatin context.

Cooperation of p27(Kip1) and p18(INK4c) in progestin-mediated cell cycle arrest in T-47D breast cancer cells

The steroid hormone progesterone regulates proliferation and differentiation in the mammary gland and uterus by cell cycle phase-specific actions. The long-term effect of progestins on T-47D breast cancer cells is inhibition of cellular proliferation. This is accompanied by decreased G(1) cyclin-dependent kinase (CDK) activities, redistribution of the CDK inhibitor p27(Kip1) among these CDK complexes, and alterations in the elution profile of cyclin E-Cdk2 upon gel filtration chromatography, such that high-molecular-weight complexes predominate. This study aimed to determine the relative contribution of CDK inhibitors to these events. Following progestin treatment, the majority of cyclin E- and D-CDK complexes were bound to p27(Kip1) and few were bound to p21(Cip1). In vitro, recombinant His(6)-p27 could quantitatively reproduce the effects on cyclin E-Cdk2 kinase activity and the shift in molecular weight observed following progestin treatment. In contrast, cyclin D-Cdk4 was not inhibited by His(6)-p27 in vitro or p27(Kip1) in vivo. However, an increase in the expression of the Cdk4/6 inhibitor p18(INK4c) and its extensive association with Cdk4 and Cdk6 were apparent following progestin treatment. Recombinant p18(INK4c) led to the reassortment of cyclin-CDK-CDK inhibitor complexes in vitro, with consequent decrease in cyclin E-Cdk2 activity. These results suggest a concerted model of progestin action whereby p27(Kip1) and p18(INK4c) cooperate to inhibit cyclin E-Cdk2 and Cdk4. Since similar models have been developed for growth inhibition by transforming growth factor beta and during adipogenesis, interaction between the Cip/Kip and INK4 families of inhibitors may be a common theme in physiological growth arrest and differentiation.

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

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

Distinct versus redundant properties among members of the INK4 family of cyclin-dependent kinase inhibitors

p16(INK4a), p15(INK4b), p18(INK4c) and p19(INK4d) comprise a family of cyclin-dependent kinase inhibitors and tumor suppressors. We report that the INK4 proteins share the ability to arrest cells in G1, and interact with CDK4 or CDK6 with similar avidity. In contrast, only p18 and particularly p19 are phosphorylated in vivo, and each of the human INK4 proteins shows unique expression patterns dependent on cell and tissue type, and differentiation stage. Thus, the INK4 proteins harbor redundant as well as non-overlapping properties, suggesting distinct regulatory modes, and diverse roles for the individual INK4 family members in cell cycle control, cellular differentiation, and multistep oncogenesis.

Aberrant p15 promoter methylation in adult and childhood acute leukemias of nearly all morphologic subtypes: potential prognostic implications

We prospectively analyzed p15 and p16 promoter methylation patterns using methylation-specific polymerase chain reaction (PCR) in patients with adult and childhood acute leukemias and studied the association of methylation patterns with chromosomal abnormalities and prognostic variables. In nearly all French-American-British leukemia subtypes, we found p15methylation in bone marrow or peripheral blood cells from 58% (46/79) of patients with acute myeloid leukemia (AML), acute lymphoblastic leukemia (ALL), or acute biphenotypic leukemia (ABL). An identical alteration was detected in blood plasma from 11 of 12 of these patients (92%). We also demonstrated for the first time concomitant p16and p15 methylation in 22% (8/37) of adults with AML or ALL, exclusively in those with M2, M4, or L2 subtypes. According to cytogenetic data from 35 patients with ALL, AML, or ABL, 82% (14/17) of those with unmethylated p15 alleles had normal karyotypes or hyperdiploidies associated with a favorable prognosis. Conversely, 44% (8/18) of patients with p15 methylation had chromosomal translocations, inversions, or deletions, suggesting an interplay of these abnormalities with p15 methylation. As a prognostic marker for disease monitoring, p15 methylation appears to be more widely applicable than BCR-ABL, AF4-MLL, andAML1-ETO transcripts, which were detectable in only 8% (4/48) of patients by reverse transcriptase-PCR. Thirty-nine of 43 blood samples (91%) sequentially collected from 12 patients with AML, ALL, or ABL showed p15 methylation status in excellent concordance with morphologic disease stage. Early detection of p15methylation at apparent remission or its acquisition during follow-up may prove valuable for predicting relapse. Overall survival of patients with p15 methylation was notably shortened among 38 adults with AML and 12 adults with ALL. Aberrant p15 methylation may have important prognostic implications for clinical monitoring and risk assessment.

Activation of the human p27(Kip1) promoter by IFNalpha 2b

p27(Kip1) is one of the key regulatory proteins in cell cycle through inhibition of pRB phosphorylation by suppression of the activity of several cyclin/Cdk complexes. The expression of p27(Kip1) has been shown to be controlled by a posttranslational mechanism, although vitamin D(3) and neuronal differentiation can also induce its mRNA. Recently, the p27(Kip1) promoter was isolated and sequenced from a human leukocyte genomic library. In this report, we demonstrate that IFNalpha 2b, activates the human p27(Kip1) promoter-driven luciferase reporter gene in transient expression assays in H82 cells. This induction might involve two IRF 1-like binding sites present in the p27(Kip1) promoter. To our knowledge this is the first report on the direct activation of the human p27(Kip1) promoter by IFNalpha 2b.

Human PC4 is a substrate-specific inhibitor of RNA polymerase II phosphorylation

The activity of cyclin-dependent protein kinases (cdks) is physiologically regulated by phosphorylation, association with the specific cyclin subunits, and repression by specific cdk inhibitors. All three physiological regulatory mechanisms are specific for one or more cdks, but none is known to be substrate specific. In contrast, synthetic cdk peptide inhibitors that specifically inhibit cdk phosphorylation of only some substrates, "aptamers," have been described. Here, we show that PC4, a naturally occurring transcriptional coactivator, competitively inhibits cdk-1, -2, and -7-mediated phosphorylation of the largest subunit of RNA polymerase II (RNAPII), but it does not inhibit phosphorylation of other substrates of the same kinases. Interestingly, the phosphorylated form of PC4 is devoid of kinase inhibitory activity. We also show that wild-type PC4 but not the kinase inhibitory-deficient mutant of PC4 represses transcription in vivo. Our results point to a novel role for PC4 as a specific inhibitor of RNAPII phosphorylation.

Regulation of the G1 phase of the mammalian cell cycle

In any multi-cellular organism, the balance between cell division and cell death maintains a constant cell number. Both cell division cycle and cell death are highly regulated events. Whether the cell will proceed through the cycle or not, depends upon whether the conditions required at the checkpoints during the cycle are fulfilled. In higher eucaryotic cells, such as mammalian cells, signals that arrest the cycle usually act at a G1 checkpoint. Cells that pass this restriction point are committed to complete the cycle. Regulation of the G1 phase of the cell cycle is extremely complex and involves many different families of proteins such as retinoblastoma family, cyclin dependent kinases, cyclins, and cyclin kinase inhibitors.