P21 Cip1 Expression Research Articles

ErbB2 overexpression in human breast carcinoma is correlated with p21Cip1 up‐regulation and tyrosine‐15 hyperphosphorylation of p34Cdc2

Clinical investigations have shown that in patients with breast carcinoma, tumors that overexpress the erb-B2 gene are less responsive to certain chemotherapy regimens compared with tumors that express low levels of ErbB2, suggesting that ErbB2 overexpression may be used as a marker for poor response to chemotherapy in patients with breast carcinoma. The combination of cyclophosphamide, methotrexate, and 5-fluorouracil (CMF) is one of the most widely used chemotherapy regimens in patients with breast carcinoma. Patients who have ErbB2-overexpressing breast carcinomas have poorer responses to CMF compared with patients who have breast carcinomas with low ErbB2 expression. ErbB2-overexpressing breast tumor cells are resistant to taxol-induced apoptotic cell death. The underlying molecular mechanism is that ErbB2 inhibits p34(Cdc2) activation, which is required for taxol-induced apoptosis, by up-regulating p21(Cip1) and by hyperphosphorylating p34(Cdc2) on tyrosine-15. However, the relation between ErbB2, p21(Cip1), and p34(Cdc2) in patients with breast carcinoma remains elusive. The contribution of these molecular alterations to ErbB2-mediated CMF resistance has not been examined. Formalin-fixed, paraffin-embedded, 5 microm thick tissue sections from 107 patients with invasive breast carcinoma were immunostained using specific antibodies against ErbB2, p21(Cip1), and phosphorylated tyrosine (Tyr)-15 of p34(Cdc2). Ninety-four of 107 patients were treated with the CMF regimen. In situ hybridization of p21(Cip1)mRNA also was performed in 20 of the sections described above. ErbB2 expression levels, p21(Cip1) expression levels, and phosphorylation status on Tyr15 of p34(Cdc2) were analyzed for correlations with clinicopathologic parameters for the 107 patients and for correlations with disease-free survival (DFS) in the 94 patients who were treated with the CMF regimen. Among 94 patients with breast carcinoma who were treated with CMF, it was found that ErbB2 overexpression was associated significantly with poor DFS (P < 0.01). Patients who had higher p21(Cip1) expression had worse DFS compared with patients who had low p21(Cip1) expression (P = 0.02). However, no significant correlation was found between p34(Cdc2)-Tyr15 phosphorylation and DFS (P > 0.05). It is noteworthy that p21(Cip1) expression and p34(Cdc2)-Tyr15 phosphorylation were correlated significantly and positively with ErbB2 expression (P < 0.01). The current study suggests that p21(Cip1) expression, but not p34(Cdc2)-Tyr15 phosphorylation, may play a role in ErbB2-mediated CMF resistance, which may contribute to the poor survival of patients with ErbB2-overexpressing breast carcinomas who were treated on the CMF regimen. In addition, ErbB2 overexpression was correlated with p21(Cip1) up-regulation and with increased p34(Cdc2)-Tyr15 phosphorylation in breast tumors.

Effects of overexpressingp14 ARF on the apoptosis in human melanoma cells irradiated with γ-ray

Tumor suppressor ARF can induce cell cycle arrest or apoptosis by activating p53. In order to explore the molecular mechanism of the induction of apoptosis byp14ARF, a human melanoma cell model overexpressingp14ARF was constructed. Present study indicated that in the cells overexpressingp14ARF, p53 was accumulated in nucleus while it dispersed in cytosol in the control cells. Irradiated with γ-ray, overexpressingp14ARF promoted the apoptosis of A375 cells, triggered Smac release from mitochondria to cytosol, and increased the expression of p53, Bax, Caspase-3, Caspase-9, p21cip1 and p27kip1. However, the protein level of Bcl-2 and phospho-ERK was down-regulated. These results suggested a possible mechanism of p14ARF in promotion of apoptosis.

Inhibition of liver trans-sulphuration pathway by propargylglycine mimics gene expression changes found in the mammary gland of weaned lactating rats: role of glutathione.

In the lactating mammary gland, weaning produces mitochondrial cytochrome c release and nuclear DNA fragmentation, as determined by gel electrophoresis. This is followed by a significant decrease in lactation. Weaning for 2 h produces an early induction of the tumour suppressor/transcription factor p53, whereas the oncoprotein c-Jun and c-Jun N-terminal kinase are elevated after 24 h of weaning when compared with controls. The expression of p21(cip1) and p27(kip1), cyclin-dependent kinase inhibitors, was significantly higher in weaned rats when compared with control lactating rats. All the changes mentioned above also happen in the lactating mammary gland when propargylglycine, an inhibitor of the liver trans-sulphuration pathway, is administered. This effect is partially reversed by N -acetylcysteine administration. The administration of buthionine sulphoximine, an irreversible inhibitor of gamma-glutamylcysteine synthetase, to lactating rats produces a decrease in GSH levels and changes in protein concentrations and gene transcripts similar to those in rats with impaired trans-sulphuration pathway. These data suggest that the inter-tissue flux of GSH is an important mechanism of L-cysteine delivery to the lactating mammary gland and emphasize the importance of this physiological event in maintaining the gene expression required to sustain lactation.

Evaluation of the cyclin-dependent kinase inhibitor p21Cip1 in epithelial ovarian tumors of low malignant potential and adenocarcinomas.

In view of the controversial information on the significance of the cyclin-dependent kinase inhibitor p21Cip1 in ovarian cancer, we conducted a retrospective investigation to clarify the relationships of this protein to proliferation rate, clinicopathological variables and prognosis of epithelial ovarian tumors. Paraffin-embedded tissue from 43 ovarian tumors of low malignant potential (LMP) and 82 primary ovarian adenocarcinomas were stained immunohistochemically for p21Cip1, p53 protein and Ki-67 antigen (a marker of cell proliferation). p21Cip1 levels were significantly higher in LMP tumors (p<0.001) as well as in early stage adenocarcinomas (p=0.021) and those associated with minimal residual disease (p=0.008). However, no relationship existed between p21Cip1 expression and the proliferation rate of adenocarcinomas or LMP tumors. In the vast majority of LMP tumors p21Cip1 expression was not accompanied by p53 accumulation. This p21Cip1-positive/p53-negative phenotype prevailed in the early stage (p=0.026), lower grade (p=0.018) adenocarcinomas as well as in those left with minimal residual disease (p=0.059). In patients with lower grade adenocarcinomas, decreased p21Cip1 expression was adversely related to poor overall survival on its own (p=0.0500) and when combined with p53 protein overexpression (p=0.0323). In multivariate analysis, only the stage remained as the independent predictor of survival. Decreased p21Cip1 expression is related to several indicators of aggressiveness in ovarian adenocarcinomas and seems to be differentially regulated in LMP tumors and adenocarcinomas. On the contrary, deregulation of p21Cip1 expression does not seem to participate in the pathogenesis of LMP tumors. Furthermore, although p21Cip1 alone or combined with p53 is of prognostic significance in lower grade adenocarcinomas, it does not appear to add to the information gained from traditional prognosticators.

P21Cip1 Gene Expression Is Modulated by Egr1: A NOVEL REGULATORY MECHANISM INVOLVED IN THE RESVERATROL ANTIPROLIFERATIVE EFFECT

Epidemiological observations indicate that resveratrol, a natural antioxidant stilbene, exerts cardioprotective and chemopreventive effects. Moreover, the molecule induces in vitro cell growth inhibition and differentiation. Using human erythroleukemic K562 cells as model system, we demonstrated that resveratrol induces a remarkable gamma-globin synthesis, the erythroid differentiation being linked to impairment of cell proliferation, increased p21Cip1 expression and inhibition of cdk2 activity. The up-regulation of p21Cip1 transcription is prevented by cycloheximide, indicating the requirement of intermediate protein(s), which, in turn, regulate gene expression. The quantitative analysis of some transcription factors involved in the erythroid lineage, namely GATA-1, GATA-2, and Egr1, indicated that resveratrol selectively up-regulates Egr1 by an Erk1/2-dependent mechanism. The presence of an Egr1 consensus sequence in the p21Cip1 promoter suggested the hypothesis that this transcription factor directly regulates the expression of the cdk inhibitor. Transfection studies with deleted gene promoter constructs, as well as EMSA, pull-down, and chromatin immunoprecipitation experiments substantiated this view, demonstrating that Egr1 binds in vitro and in vivo to the identified consensus sequence of the p21Cip1 promoter. Moreover, an Egr1 phosphorothioate antisense hinders p21Cip1 accumulation and the antiproliferative effects of resveratrol. In conclusion, this is the first demonstration that Egr1 controls p21Cip1 expression by directly interacting with a specific sequence on its gene promoter. The identified regulatory mechanism also contributes to the clarification of the complex chemopreventive and antiproliferative properties of resveratrol.

Effect of 20- epi-1α,25-dihydroxyvitamin D 3 on the proliferation of human neuroblastoma: role of cell cycle regulators and the Myc–Id2 pathway

The antiproliferative effects of 1α,25-dihydroxyvitamin D 3 [1,25(OH) 2D 3] and its epimer, 20- epi-1α,25-dihydroxyvitamin D 3 [20- epi-1,25(OH) 2D 3], in six human neuroblastoma (NB) cell lines (SH-SY5Y, NB69, SK-N-AS, IMR5, CHP134, and NGP) were investigated. We determined the ability of 1,25(OH) 2D 3 and 20- epi-1,25(OH) 2D 3 to influence cell viability by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, cell proliferation by bromodeoxyuridine (BrdU) incorporation, and their antineoplastic effect on colony formation in a soft agar assay. A concentration-dependent decrease in cell viability, inhibition of DNA synthesis, and suppression of clonal proliferation was observed with both compounds. 20- epi-1,25(OH) 2D 3 was more potent in suppressing the proliferation of all six NB cell lines. To understand the mechanisms of action, we examined the effect of 20- epi-1,25(OH) 2D 3 on the Myc–Id2 cell proliferative network and also on key regulators of the cell cycle. For the first time, we show that 20- epi-1,25(OH) 2D 3 down-regulated Myc and Id2 expression by western blot analysis. Semi-quantitative reverse transcription–polymerase chain reaction (RT–PCR) analysis revealed that 20- epi-1,25(OH) 2D 3 induced the expression of retinoic acid receptor-β and p21 Cip1, and down-regulated the expression of cyclin D1 resulting in decreased phosphorylation of retinoblastoma protein (pRB). In sum, we show that 20- epi-1,25(OH) 2D 3 exerts strong antiproliferative effects by regulating key growth control networks (Myc–Id2–pRB) in NB cells.

Effects of regulatory domains of specific isoforms of protein kinase C on growth control and apoptosis in MCF‐7 breast cancer cells

Protein kinase C (PKC) is a multigene family consisting of at least 11 isoforms that play key roles in growth control and tumorigenesis. To understand the roles of specific isoforms of PKC in breast cancer, we generated derivatives of the human breast cancer cell line MCF-7 that stably overexpress dominant negative mutants (REG) of PKC-alpha, -epsilon, or -zeta, which encode only the regulatory domains of the respective isoforms. When stimulated to re-enter the cell cycle after serum starvation, the MCF-7/PKC-alpha-REG cell line exhibited enhanced cell-cycle progression in comparison to the control cell line. These cells also showed increased sensitivity to growth inhibition and induction of apoptosis in response to various cytotoxic stimuli, including serum starvation, tamoxifen, and gamma-radiation. Western blot analysis indicated that the MCF-7/PKC-alpha-REG cell line displayed marked decreases in the levels of the cyclin-dependent kinase inhibitor p21CIP1 and the anti-apoptotic protein bcl-2. Similar, but less striking, effects were seen in the MCF-7/PKC-epsilon-REG cell line, and the MCF-7/PKC-zeta-REG cell line showed minimal changes, when compared to the control cells. Taken together, these results suggest that the endogenous PKC-alpha in MCF-7 cells plays a critical role in regulating cell-cycle control and apoptosis, in part through upregulating the expression of p21CIP1 and bcl-2. Therefore, inhibitors of PKC-alpha may potentiate the activity of cytotoxic agents in the therapy of breast cancer.

V-Jun stimulates both cdk2 kinase activity and G1/S progression via transcriptional repression of p21 CIP1.

Previous studies have shown that the viral Jun (v-Jun) oncoprotein induces marked alterations in cell cycle control, which are associated with, and may be caused by, increased cdk2 kinase activity. Since p21 CIP1 is an important regulator of cdk2, we investigated whether aberrant expression of this cyclin-dependent kinase inhibitor might contribute to cell cycle deregulation by v-Jun. We find that the basal levels of p21 CIP1 mRNA and protein expression are greatly reduced in chick embryo fibroblasts (CEF) transformed by v-Jun, and that v-Jun blocks the increases in p21 CIP1 expression that normally accompany growth inhibition induced by serum deprivation or confluency in untransformed CEF. Importantly, ectopic expression of p21 CIP1 in v-Jun-transformed CEF inhibits both cdk2 kinase activity and cell cycle progression, indicating that these alterations in p21 CIP1 expression are likely to be functionally significant for growth deregulation. We also investigated the mechanism through which v-Jun disturbs p21 CIP1 expression and the possible involvement of a known p21 CIP1 regulator, p53, as an intermediate in this process. This analysis revealed that repression is mediated primarily at the level of p21 CIP1 gene transcription, however the mechanism is complex; both p53-dependent and -independent mechanisms contribute as judged by analysis of p21 CIP1 promoter mutants and other assays of p53 transcriptional activity.

The selective tyrosine kinase inhibitor, STI571, inhibits growth of anaplastic thyroid cancer cells.

To establish a molecular targeting therapy for anaplastic thyroid carcinomas, we studied the effect of the specific tyrosine kinase inhibitor, STI571, on anaplastic thyroid cancer cell lines highly expressing c-ABL ARO (mutated p53) and FRO (undetectable p53). These lines showed marked inhibition of cell growth after treatment with STI571. In contrast, the growth of papillary thyroid cancer cell lines that harbor wild-type p53 and have low levels of c-ABL was not affected by STI571. Fluorescent-activated cell sorting analysis revealed that STI571 treatment increased the fraction of FRO and ARO cells in S and G(2)/M phases, respectively, indicating induction of S and G(2)/M transition arrest. These changes were accompanied by inhibition of c-ABL phosphorylation/activation and increased expression of p21(cip1) in FRO and p27(kip1) in both FRO and ARO cells. Treatment with STI571 also led to reduction of cyclin A, B1, and CDC2 levels. The growth of FRO cells implanted into immunocompromised mice was significantly inhibited by STI571. Taken together, these results suggest that selective suppression of c-ABL activity by STI571 may represent a potential anticancer strategy for p53-mutated undifferentiated thyroid carcinomas.

Myc represses differentiation-induced p21CIP1 expression via Miz-1-dependent interaction with the p21 core promoter.

Inhibition of cellular differentiation is one of the well-known biological activities of c-Myc-family proteins. We show here that Myc represses differentiation-induced expression of the cyclin-dependent kinase (CDK) inhibitor p21CIP1 (CDKN1A, p21), known to play an important role in cell fate decisions during growth and differentiation, in hematopoietic cells. Our results demonstrate that the c-Myc-responsive region is situated in the p21 core promoter. c-Myc binds to this region in vitro and in vivo through interaction with the initiator-binding Zn-finger transcription factor Miz-1, which associates directly with the promoter. Association of Myc with the promoter in vivo correlates inversely with p21 expression. Using mutants of c-Myc with impaired binding to Miz-1, our results further show that repression of p21 promoter/reporters as well as the endogenous p21 gene by Myc depends on interaction with Miz-1. Expression of Miz-1 increases during hematopoietic differentiation and Miz-1 activates the p21 promoter under conditions of low Myc levels, indicating a positive role for free Miz-1 in this process. In conclusion, repression of differentiation-induced p21 expression through Miz-1 may be an important mechanism by which Myc blocks differentiation.

Carbon monoxide suppresses arteriosclerotic lesions associated with chronic graft rejection and with balloon injury

Carbon monoxide (CO), one of the products of heme oxygenase action on heme, prevents arteriosclerotic lesions that occur following aorta transplantation; pre-exposure to 250 parts per million of CO for 1 hour before injury suppresses stenosis after carotid balloon injury in rats as well as in mice. The protective effect of CO is associated with a profound inhibition of graft leukocyte infiltration/activation as well as with inhibition of smooth muscle cell proliferation. The anti-proliferative effect of CO in vitro requires the activation of guanylate cyclase, the generation of cGMP, the activation of p38 mitogen-activated protein kinases and the expression of the cell cycle inhibitor p21Cip1. These findings demonstrate a protective role for CO in vascular injury and support its use as a therapeutic agent.

Restoration of p53 gene function in 12-O-tetradecanoylphorbor 13-acetate-resistant human leukemia K562/TPA cells

The human leukemia K562 cell line does not express wild-type p53 protein. Due to the loss of one p53 allele and an insertion mutation in exon 5 of the other allele resulting in a frameshift mutation, K562 cells express a truncated p53 protein of 148 amino acids. A human leukemia phorbol ester-resistant subline, K562/TPA, is cross-resistant to some anticancer agents. A remarkable difference in cell cycle progression at G1/S phase was observed in the synchronised K562/TPA cells as compared with K562 cells. Southern blot and DNA sequence analysis revealed no mutation in exon 5 of the p53 gene in K562/TPA cells. p21Cip1 expression was also restored in K562/TPA cells confirming that the reversal of this p53 gene mutation restored wild-type p53 function in these cells. This is a unique report describing reversal of p53 gene mutation by drugs. This was associated with the expression of wild-type p53 mRNA and protein in K562/TPA cells. The K562/TPA cell line may provide a very useful tool for the investigation of the relationship between p53 status and chemosensitization.

Alteration of mesodermal cell differentiation by EWS/FLI-1, the oncogene implicated in Ewing's sarcoma.

The chimeric fusion gene EWS/FLI-1 is detected in most cases of Ewing's sarcoma (ES), the second most common malignant bone tumor of childhood. Although 80% of ES tumors develop in skeletal sites, the remainder can arise in almost any soft tissue location. The lineage of the cell developing the EWS/FLI-1 gene fusion has not been fully characterized but is generally considered to be of either mesenchymal or neural crest origin. To study this oncogene in a conceptually relevant target cell, EWS/FLI-1 was introduced into the murine cell line C2C12, a myoblast cell line capable of differentiation into muscle, bone, or fat. In this cellular context, EWS/FLI-1 profoundly inhibited the myogenic differentiation program. The block in C2C12 myogenic differentiation required the nuclear localization and DNA-binding functions of EWS/FLI-1 and was mediated by transcriptional and posttranscriptional suppression of the myogenic transcription factors MyoD and myogenin. Interestingly, C2C12-EWS/FLI-1 cells constitutively expressed alkaline phosphatase, a bone lineage marker, and were alkaline phosphatase positive by histochemistry but showed no other evidence of bone lineage commitment. Consistent with recent findings in human ES tumor cell lines, C2C12-EWS/FLI-1 cells constitutively expressed cyclin D1 and demonstrated decreased expression of the cell cycle regulator p21(cip1), even under differentiation conditions and at confluent density. This C2C12-EWS/FLI-1 cell model may assist in the identification of novel differentially expressed genes relevant to ES and provide further insight into the cell(s) of origin developing ES-associated genetic fusions.

Anti-proliferative effects of 20-epi-vitamin-D3 analogue, KH1060 in human neuroblastoma: induction of RAR-β and p21Cip1

We determined the in vitro biological activities of 1α, 25-dihdroxyvitamin D3 (1,25-D3) and its analogue, 20-epi-22-oxa-24a, 26a, 27a-trihomo-1 α, 25 (OH)2 vitamin D3 (KH1060) in six human neuroblastoma (NB) cell lines (SH-SY5Y, NB69, SK-N-AS, IMR5, CHP-134, NGP). The ability of these compounds to inhibit cell growth and DNA synthesis was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and BrdU incorporation, respectively. The induction of cell death was monitored by caspase-3 activity. Their antineoplastic effect was assessed by clonal proliferation in soft agar. KH1060 was more effective than 1,25 D3 in inhibiting cell growth and DNA synthesis. The IC-50 (inhibition of 50% cell viability) indicated that KH1060 was about 10–20-fold more potent than 1,25 D3. This growth inhibition was also accompanied by induction of caspase-3 activity, indicating that these compounds induce cell death in a caspase-dependent fashion. Moreover, KH1060 exerted potent antineoplastic activity by suppressing the clonal proliferation of the six NB cells. For the first time we demonstrate that KH1060 induces the expression of retinoic acid receptor-β and p21Cip1 suggesting that these proteins in part mediate the growth inhibitory effects. Taken together, all the six NB cells were more susceptible to growth inhibition by KH1060 than 1,25-D3, suggesting its possible use in NB to potentiate the action of retinoids, which are in clinical use for this disease.

G1/S cell cycle arrest provoked in human T cells by antibody to CD26.

CD26 is T cell costimulatory molecule with dipeptidyl peptidase IV (DPPIV) enzyme activity located in its extracellular region. The expression of CD26 is enhanced after activation of T cells, while it is preferentially expressed on a subset of CD4+ memory T cells in the resting state. In this paper, we demonstrate that binding of the soluble anti-CD26 monoclonal antibody (mAb) 1F7 inhibits human T-cell growth and proliferation in both CD26-transfected Jurkat T-cell lines and human T-cell clones by inducing G1/S arrest, which is associated with enhancement of p21Cip1 expression. This effect depends on the DPPIV enzyme activity of the CD26 molecule. Moreover, we show that expression of p21Cip1 after treatment with the anti-CD26 mAb 1F7 appears to be induced through activation of extracellular signal-regulated kinase (ERK) pathway. These data thus suggest that anti-CD26 treatment may have potential use in the clinical setting involving activated T cell dysregulation, including autoimmune disorders and graft-vs.-host disease.

Growth/differentiation factor-5 induces growth arrest and apoptosis in mouse B lineage cells with modulation by Smad

Bone morphogenetic proteins, including growth/differentiation factor-5 (GDF-5), are multifunctional cytokines. Recent studies of intracellular signal transduction mechanisms for the transforming growth factor-β superfamily have focused on Smad proteins. However, scant attention has been given to the mechanism by which GDF-5 exerts its negative growth effect on immunological competent cells. In the present study, we demonstrated that GDF-5 induced cell cycle arrest in the G1 phase before the appearance of apoptosis in mouse B cell hybridoma HS-72 cells, while the ectopic expression of Smad6 and Smad7 in HS-72 cells suppressed the GDF-5-induced G1 cell cycle arrest by abolishing the expression of p21 CIP-1/WAF-1 and hypophosphorylation of retinoblastoma protein. Moreover, we found that Smad6 and Smad7 suppressed GDF-5-induced apoptosis in HS-72 cells. These findings indicated that Smad6 and Smad7 exhibit inhibitory effects toward GDF-5-mediated signaling in B lineage cells.

Myc suppression of the p21(Cip1) Cdk inhibitor influences the outcome of the p53 response to DNA damage.

Activation of the tumour suppressor p53 by DNA damage induces either cell cycle arrest or apoptotic cell death. The cytostatic effect of p53 is mediated by transcriptional activation of the cyclin-dependent kinase (CDK) inhibitor p21(Cip1), whereas the apoptotic effect is mediated by transcriptional activation of mediators including PUMA and PIG3 (ref. 2). What determines the choice between cytostasis and apoptosis is not clear. Here we show that the transcription factor Myc is a principal determinant of this choice. Myc is directly recruited to the p21(Cip1) promoter by the DNA-binding protein Miz-1. This interaction blocks p21(Cip1) induction by p53 and other activators. As a result Myc switches, from cytostatic to apoptotic, the p53-dependent response of colon cancer cells to DNA damage. Myc does not modify the ability of p53 to bind to the p21(Cip1) or PUMA promoters, but selectively inhibits bound p53 from activating p21(Cip1) transcription. By inhibiting p21(Cip1) expression Myc favours the initiation of apoptosis, thereby influencing the outcome of a p53 response in favour of cell death.

Raf-1-induced growth arrest in human mammary epithelial cells is p16-independent and is overcome in immortal cells during conversion.

Using an estrogen-inducible retroviral system, we demonstrate that oncogenic Raf-1 induces growth arrest and morphological changes in finite lifespan human mammary epithelial cells (HMEC). This arrest does not rely on expression of the cyclin-dependent kinase inhibitor (CKI) p16(INK4a), nor on changes in expression of the CKIs p21(Cip1), p14(ARF), p27(Kip1) or p57(Kip2). The Raf-induced arrest is independent of viral oncogene mediated inactivation of p53 and pRB, or c-myc overexpression. Flow cytometric analysis demonstrates that cells arrest in both G1 and G2. The Raf-induced arrest is mitigated or eliminated in some immortally transformed HMEC. Immortal HMEC that have both overcome replicative senescence and undergone the recently described conversion process maintain growth in the presence of transduced oncogenic Raf-1; they also gain EGF-independent growth and a low frequency of anchorage-independent growth. However, HMEC that have overcome replicative senescence but have not undergone conversion and HMEC immortalized by transduction with the catalytic subunit of telomerase, hTERT, remain severely growth arrested. These results indicate that the molecular mechanisms responsible for the Raf-1-induced growth arrest may vary among different finite lifespan cell types, and that in HMEC, this mechanism is altered during the conversion process, rather than as a direct consequence of overcoming senescence or expressing hTERT.

The ability of antigen, but not interleukin-2, to promote n-butyrate-induced T helper 1 cell anergy is associated with increased expression and altered association patterns of cyclin-dependent kinase inhibitors.

The ability of the cell cycle inhibitor n-butyrate to induce T helper 1 (Th1) cell anergy is dependent upon its ability to block the cell cycle progression of activated Th1 cells in G1. Results reported here show that although both interleukin (IL)-2 and antigen (Ag) push Th1 cells into G1 where they are blocked by n-butyrate, only the Ag-activated Th1 cells demonstrate functional anergy once the n-butyrate has been removed from the culture. Because n-butyrate-induced Th1 cell anergy has been linked to increased expression of the cyclin-dependent kinase inhibitors p21Cip1 and p27Kip1, mechanistic experiments focused on the role of these inhibitors. It was found that when Th1 cells were reincubated in Ag-stimulated secondary cultures, the Th1 cells previously exposed to Ag and n-butyrate (anergic Th1 cells) demonstrated a cumulative increase in p21Cip1 and p27Kip1 when compared with Th1 cells previously exposed to recombinant (r)IL-2 and n-butyrate (non-anergic Th1 cells). p27Kip1 in the anergic Th1 cells from the secondary cultures was associated with cyclin-dependent kinases (cdks). In contrast, p21Cip1 in the anergic Th1 cells, although present at high levels, did not associate significantly with cdks, suggesting that p21Cip1 may target some other protein in the anergic Th1 cells. Taken together, these findings suggest that Th1 cell exposure to Ag and n-butyrate, rather than IL-2 and n-butyrate, is needed to induce the cumulative increase in p21Cip1 and p27Kip1 that is associated with the proliferative unresponsiveness in anergic Th1 cells. In addition, p21Cip1 may inhibit proliferation in the anergic Th1 cells by some mechanism other than suppression of cdks that is unique to the induction of Th1 cell anergy.

UCN-01 (7-Hydroxystauorsporine) Blocks PMA-Induced Maturation and Reciprocally Promotes Apoptosis in Human Myelomonocytic Leukemia Cells (U937)

Interactions between the protein kinase inhibitor UCN-01 and the PKC activator phorbol ester (PMA) have been examined in relation to differentiation and apoptosis in human myelomonocytic leukemia cells (U937). Coadministratation of 100 nM UCN-01 with a low concentration of PMA e.g., 2 nM, inhibited rather than promoted differentiation, reflected by reduced surface expression of the monocytic maturation marker CD11b and diminished cell adherence. Instead, administration of UCN-01 with PMA led to a marked increase in mitochondrial injury (e.g, cytochrome c release), activation of caspases-3 and -8, Bid cleavage, PARP degradation, and apoptosis, accompanied by a substantial reduction in viability and clonogenic survival. These phenomena were associated with multiple perturbations in cell cycle regulatory events, including abrogation of p21CIP1 induction, p27KIP1 cleavage, down-regulation of cyclin D1, dephosphorylation (activation) of p34cdc2, and degradation of underphosphorylated pRb. Potentiation of PMA-mediated apoptosis was partially mimicked by caffeine suggesting the involvement of Chk1 in the potentiation of apoptosis. Induction of cell death by UCN-01 and PMA was increased in cells stably expressing a p21CIP1 mRNA antisense construct, suggesting that p21CIP1 expression may protect cells from the lethal effects of this drug combination. Finally, ectopic expression of a Bcl-2 but not dominant-negative caspase-8 protected cells from UCN-01/PMA-mediated apoptosis, suggesting the lethal effects of this combination primarily involves the mitochondrial rather than the TNF-related extrinsic apoptotic pathway. Taken together, these findings suggest that UCN-01 disrupts a variety of cell cycle events in leukemic cells exposed to the maturation-inducing agent PMA, causing cells to engage an apoptotic rather than a differentiation-related program. Key Words:PMA, UCN-01, Differentiation, Apoptosis