Interleukin-1 induces growth arrest by hypophosphorylation of the retinoblastoma susceptibility gene product RB

Human amniotic epithelial cells inhibit growth of epithelial ovarian cancer cells via TGF‑β1-mediated cell cycle arrest

It is reported that human amniotic epithelial cells (hAECs) endow intrinsic antitumor effects on certain kinds of cancer. This research was designed to evaluate whether hAECs endowed potential anticancer properties on epithelial ovarian cancer (EOC) cells in vivo and in vitro, which has not been reported before. In this study, we established a xenografted BALB/c nude mouse model by subcutaneously co-injecting ovarian cancer cell line, SK-OV-3, and hAECs for 28 days. In ex vivo experiments, CCK‑8 cell viability assay, real-time PCR, cell counting assay, cell cycle analysis and immunohistochemistry (IHC) assay were used to detect the effects of hAEC‑secreted factors on the proliferation and cell cycle progression of EOC cells. A cytokine array was conducted to detect anticancer-related cytokines released from hAECs. Human recombinant TGF‑β1 and TGF‑β1 antibody were used to treat EOC cells and analyzed whether TGF‑β1 contributed to the cell cycle arrest. Results from in vivo and ex vivo experiments showed that hAEC-secreted factors and rhTGF‑β1 decreased proliferation of EOC cells and induced G0/G1 cell cycle arrest in cancer cells, which could be partially reversed by excess TGF‑β1 antibody. These data indicate that hAECs endow potential anticancer properties on epithelial ovarian cancer in vivo and in vitro which is partially mediated by hAEC‑secreted TGF‑β1-induced cell cycle arrest. This study suggests a potential application of hAEC‑based therapy against epithelial ovarian cancer.

E2F1 transcription factor and its impact on growth factor and cytokine signaling

E2F1 is a transcription factor involved in cell cycle regulation and apoptosis. The transactivation capacity of E2F1 is regulated by pRb. In its hypophosphorylated form, pRb binds and inactivates DNA binding and transactivating functions of E2F1. The growth factor stimulation of cells leads to activation of CDKs (cyclin dependent kinases), which in turn phosphorylate Rb and hyperphosphorylated Rb is released from E2F1 or E2F1/DP complex, and free E2F1 can induce transcription of several genes involved in cell cycle entry, induction or inhibition of apoptosis. Thus, growth factors and cytokines generally utilize E2F1 to direct cells to either fate. Furthermore, E2F1 regulates expressions of various cytokines and growth factor receptors, establishing positive or negative feedback mechanisms. This review focuses on the relationship between E2F1 transcription factor and cytokines (IL-1, IL-2, IL-3, IL-6, TGF-beta, G-CSF, LIF), growth factors (EGF, KGF, VEGF, IGF, FGF, PDGF, HGF, NGF), and interferons (IFN-α, IFN-β and IFN-γ).

Contribution of extracellular signal-regulated kinases to the IL-1-induced growth inhibition of human melanoma cells A375

The role of ERK1/2 in the IL-1-induced growth inhibition was investigated using human melanoma A375-6 cells. A selective inhibitor of ERK1/2 pathway, PD98059 and a selective inhibitor of p38MAPK, SB203580 each alone significantly reversed the IL-1-induced growth inhibition of A375-6 cells. Co-treatment with PD98059 and SB203580 completely reversed the IL-1-induced growth inhibition. ERK1/2 was constitutively activated in A375-6 cells, and IL-1 further augmented ERK activation. Antiproliferative effect of IL-1 was attenuated by the expression of dominant negative form of ERK2. IL-1 induced cell cycle arrest in G(0)/G(1) phase, expression of p21 and p27 proteins, and down-regulation of cyclin D/cyclin-dependent kinase (CDK) 2 and CDK4 activities. These effects of IL-1 were reversed by PD98059. PD98059 also reversed the IL-1-induced hypophosphorylation of RB protein (pRB) and down-regulation of E2F activity. These findings demonstrate that ERK1/2 contribute to the IL-1-induced growth inhibition through induction of CDK inhibitors, down-regulation of CDK activity, pRB phosphorylation and E2F activity.

Nitric oxide inactivates the retinoblastoma pathway in chronic inflammation

Patients with chronic inflammatory bowel disease have a high risk of colon cancer. The molecules that initiate and promote colon cancer and the cancer pathways altered remain undefined. Here, using in vitro models and a mouse model of colitis, we show that nitric oxide (NO) species induce retinoblastoma protein (pRb) hyperphosphorylation and inactivation, resulting in increased proliferation through the pRb-E2F1 pathway. NO-driven pRb hyperphosphorylation occurs through soluble guanylyl cyclase/guanosine 3',5'-cyclic monophosphate signaling and is dependent on the mitogen-activated protein kinase/extracellular signal-regulated kinase kinase MEK/ERK and phosphatidylinositol 3-kinase/AKT pathways. Our results reveal a link between NO and pRb inactivation and provide insight into molecules that can be targeted in the prevention of the inflammation-to-cancer sequence.

Interleukin-1 modulation of human placental trophoblast proliferation

During early pregnancy, interleukin-1 (IL-1) is mainly produced and secreted by maternal decidua. Yet, its biological function on placental cells is not well defined. In this study, we employed JAR choriocarcinoma cell line as a model of human placental trophoblast to study the effect of IL-1. Treatment with recombinant human IL-1β resulted in significant inhibition of JAR proliferation (P < .05) paralleled with increased cytotoxicity. The inhibitory effect was blocked by both IL-1 receptor antagonist (IL-1Ra) and antihuman IL-1β monoclonal antibody. Analyzing the mode of action, IL-1β was found to induce cell cycle arrest in the G0/G1 phase and triggered apoptotic cell death. These findings demonstrated that IL-1 regulates human trophoblast growth by induction of cell cycle delay and cell death.

Tumor necrosis factor alpha-dependent drug resistance to purine and pyrimidine analogues in human colon tumor cells mediated through IKK

Development of drug resistance in cancer is one of the main challenges in chemotherapy, and many mechanisms are still unknown. In this study, we show that tumor necrosis factor alpha (TNFalpha) increases postdrug survival from 5-fluoro-2'-deoxyuridine (FdUrd) in two human colon tumor cell lines. This resulted in the development of drug-resistant cells in a TNFalpha-dependent manner. Interestingly, although the drug-resistant cells were selected using FdUrd, they are also resistant to a number of other antimetabolites in the DNA synthesis pathway in a TNFalpha-dependent manner. Only in the drug-resistant cells (p35-colo201) TNFalpha treatment resulted in G(0)-G(1) arrest but not in the parental colo201 and other cell types. Blocking TNFalpha-induced cell cycle arrest sensitized drug-resistant cells to FdUrd. TNFalpha-induced cell cycle arrest required IKK. IKK inhibition by a small molecule inhibitor or by the knockdown of IKKalpha, IKKbeta, or RelA/p65 using siRNA, but not the inhibition of JNK, MEK, p38, or caspase-8 pathways, blocked TNFalpha-induced G(0)-G(1) arrest and restored sensitivity to FdUrd of drug-resistant cells. TNFalpha reduced the transcripts and protein levels of phosphorylated retinoblastoma protein (Rb), Rb, E2F1, and Cdk4 only in drug-resistant p35-colo201 cells. This effect of TNFalpha was reversed by IKK inhibitor, suggesting that TNFalpha-induced cell cycle arrest is probably due to the reduction of Rb, E2F1, and Cdk4. Taken together, this study shows that, in vitro, TNFalpha-induced cell cycle arrest through IKK can provide a mechanism for the development of drug resistance to anti-cancer drugs, purine and pyrimidine analogues.

Proteasome-dependent degradation of cyclin D1 in 1-methyl-4-phenylpyridinium ion (MPP+)-induced cell cycle arrest

1-Methyl-4-phenylpyridinium ion (MPP(+)), an active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, induces cell death and inhibition of cell proliferation in various cells. However, the mechanism whereby MPP(+) inhibits cell proliferation is still unclear. In this study, we found that MPP(+) suppressed the proliferation with accumulation in G(1) phase without inducing cell death in p53-deficient MG63 osteosarcoma cells. MPP(+) induced hypophosphorylation of retinoblastoma protein and rapidly down-regulated the protein but not mRNA levels of cyclin D1 in MG63 cells. The down-regulation of cyclin D1 protein was suppressed by a proteasome inhibitor, MG132. The cyclin D1 down-regulation by MPP(+) was also observed in p53-positive PC12, HeLa S3, and HeLa rho(0) cells, which are a subclone of HeLa S3 lacking mitochondrial DNA. Moreover, MPP(+) dephosphorylated Akt in PC12 cells, which was rescued by the pretreatment with nerve growth factor. In addition, the pretreatment with nerve growth factor or lithium chloride, a glycogen synthase kinase-3beta inhibitor, suppressed the cyclin D1 down-regulation caused by MPP(+). Our results demonstrate that MPP(+) induces cell cycle arrest independently of its mitochondrial toxicity or the p53 status of the target cells, but rather through the proteasome- and phosphatidylinositol 3-Akt-glycogen synthase kinase-3beta-dependent cyclin D1 degradation.

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.

Growth-stimulating effects of tumor necrosis factor-alpha on simian virus 40-transformed human keratinocytes is linked to phosphorylation of retinoblastoma protein

In this report, we show that tumor necrosis factor-alpha (TNF-alpha) has a strong growth-stimulating effect on simian virus 40 (SV40)-transformed keratinocytes, while it exerts a potent growth-inhibitory effect on normal human keratinocytes. Addition of TNF-alpha to SV40-transformed keratinocytes stimulated cell growth by 216, 246, or 248% at 25, 50, or 250 ng/ml after 4 days culture compared with untreated cells, while addition to normal human keratinocytes reduced growth to 65, 38, and 32% at the same concentrations, respectively. Cell cycle analysis by flow cytometry showed that treatment of SV40-transformed keratinocytes with 250 ng/ml TNF-alpha for 48 h increased the S-phase cells from 19.8 +/- 0.3 to 49.7 +/- 2.4%. The percentage of G1/G0 phase cells decreased from 62.6 +/- 1.6 to 32.4 +/- 3.3%. We also demonstrated by immunofluorescent staining that SV40 large T antigen was enhanced by TNF-alpha incubation. Additionally, we examined the hyper and hypo-phosphorylated state of retinoblastoma protein (pRB), the cell cycle regulatory protein. TNF-alpha induced hyperphosphorylated pRB in SV40-transformed keratinocytes. On the other hand, TNF-alpha suppressed it in normal keratinocytes. We hypothesize that the enhancement of SV40 large T antigen and hyperphosphorylated pRB is involved in the mechanism of growth stimulation of SV40-transformed keratinocytes by TNF-alpha.

Negative regulation of Par-4 by oncogenic Ras is essential for cellular transformation

Oncogenic variants of the cellular Ras protein are often associated with different types of human cancers. However, the mechanisms by which oncogenic Ras induces transformation are not fully established. Expression of the transcriptional repressor Par-4 was down-regulated by oncogenic Ras via the Raf-MEK-ERK pathway. Restoration of Par-4 levels by abrogation of the Raf-MEK-ERK pathway with the MEK-inhibitor PD98059 or by ectopic Par-4, that acted to inhibit ERK expression and activation, was sufficient to suppress oncogenic Ras-induced transformation. These findings identify Par-4 as a novel target that has to be down-modulated by oncogenic Ras for successful transformation.

The G1-phase growth-arresting action of interleukin-1 is independent of p53 and p21/WAF1 function

Interleukin-1 (IL-1) causes G1-phase growth arrest of A375-C6 human melanoma cells by hypophosphorylation of the retinoblastoma susceptibility gene product Rb. Because p53 and p21/WAF1 proteins are key components of growth arrest pathways involving Rb hypophosphorylation, we tested the functional role of these two proteins in IL-1 action. Exposure to IL-1 caused induction of both p53 and p21/WAF1 proteins. However, inhibition of p53 function by the K1 mutant of SV40-T antigen or by m175 (Arg to His) dominant-negative mutant of p53 did not result in abrogation of IL-1 action, suggesting that p53 function is not required for growth arrest by IL-1. Studies aimed at testing the role of p21/WAF1 in IL-1 action indicated that IL-1 induced p21/WAF1 expression independently of the p53 status of the cells. However, inhibition of p21/WAF1 expression resulted in only a marginal rescue from the growth-arresting action of IL-1. These findings imply that despite their induction, neither wild-type p53 nor p21 can fully account for the growth arrest by IL-1. Thus, a p53- and p21-independent pathway(s) mediates IL-1 action.

Cooperative interactions between RB and p53 regulate cell proliferation, cell senescence, and apoptosis in human vascular smooth muscle cells from atherosclerotic plaques

Compared with vascular smooth muscle cells (VSMCs) from normal vessels, VSMCs from human atherosclerotic plaques proliferate more slowly, undergo earlier senescence, and demonstrate higher levels of apoptosis in culture. The tumor suppressor genes p105RB (retinoblastoma, acting through the E2F transcription factor family) and p53 regulate cell proliferation, cell senescence, and apoptosis in many cell types. We have therefore determined whether these stable growth properties of plaque VSMCs reflect altered activity of RB and/or p53. VSMCs were derived from coronary atherectomies or from normal coronary arteries from transplant recipients. Compared with normal VSMCs, plaque VSMCs showed a higher ratio of the active (hypophosphorylated) to the inactive (phosphorylated) form of RB and a lower level of E2F transcriptional activity. Cells were stably transfected with retrovirus constructs that inhibited RB or p53 alone or in combination. Suppression of RB alone increased rates of cell proliferation and apoptosis and inhibited cell senescence in normal VSMCs. Suppression of p53 and RB together had similar effects but, additionally, resulted in immortalization of normal VSMC cultures. In contrast, inhibition of RB binding to E2F or ectopic expression of E2F-1 in plaque VSMCs induced massive apoptosis, which required suppression of p53 to rescue cells. Suppression of RB and p53 together increased cell proliferation and delayed senescence but failed to immortalize plaque VSMCs. Inhibition of p53 alone had minimal effects on plaque VSMCs but increased the lifespan of normal VSMCs. We conclude that human plaque VSMCs have slower rates of cell proliferation and earlier senescence than do cells from normal vessels because of a defect in phosphorylation of RB. Furthermore, both disruption of RB/E2F and inhibition of p53 are required for plaque VSMCs to proliferate without apoptosis. This observation may explain the relatively low level of cell proliferation and high level of apoptosis seen in VSMCs in human atherosclerotic plaques.

Growth dependency of a new human pancreatic cancer cell line, YAPC, on autocrine interleukin-1alpha stimulation

We established a new human pancreatic cancer cell line from the malignant ascites of a patient with pancreatic cancer and called it YAPC. Cytogenetic and morphological analysis indicated that this cell line is monoclonal and of human origin. YAPC cells grow in nude mice, resulting in the formation of a tumor with some functional characteristics of the original tumor. The cells secreted a large amount of inflammatory cytokines including interleukin-1alpha(IL-1alpha), IL-6 and IL-8 in the culture medium. Removal of serum from the culture medium did not change the growth rate of YAPC cells, but the removal of the conditioned medium arrested their proliferation under the serum-free conditions. Exogenous IL-1alpha but neither IL-6 nor IL-8 stimulated DNA synthesis of the cells and accelerated the progress of cell cycle from G1 to the S phase. Anti-IL-1alpha antibody prevented growth of the cells in a dose-dependent fashion. In this pancreatic cancer cell line cell growth is dependent on IL-1alpha in an autocrine fashion. This line may be a useful model for studying growth regulation mechanisms of pancreatic cancer.

The restriction point and control of cell proliferation

Research over the past two decades has defined a window of time in the early/mid G1 phase of the cell cycle during which mammalian cells are responsive to extracellular signals. Recent evidence indicates that this period ends with the phosphorylation of the retinoblastoma protein, enabling the cells to pass through the restriction point at the end of mid G1 phase and to commit to completing the remaining phases of the growth cycle.

Expression and function of the leucine zipper protein Par-4 in apoptosis

The prostate apoptosis response-4 (par-4) gene was identified by differential screening for genes that are upregulated when prostate cancer cells are induced to undergo apoptosis. The par-4 gene is induced by apoptotic signals but not by growth-arresting, necrotic, or growth-stimulatory signals. The deduced amino acid sequence of par-4 predicts a protein with a leucine zipper domain at its carboxy terminus. We have recently shown that the Par-4 protein binds, via its leucine zipper domain, to the zinc finger domain of Wilms' tumor protein WT1 (R. W. Johnstone et al., Mol. Cell. Biol. 16:6945-6956, 1996). In experiments aimed at determining the functional role of par-4 in apoptosis, an antisense par-4 oligomer abrogated par-4 expression and activator-driven apoptosis in rat prostate cancer cell line AT-3, suggesting that par-4 is required for apoptosis in these cells. Consistent with a functional role for par-4 in apoptosis, ectopic overexpression of par-4 in prostate cancer cell line PC-3 and melanoma cell line A375-C6 conferred supersensitivity to apoptotic stimuli. Transfection studies with deletion mutants of Par-4 revealed that full-length Par-4, but not mutants that lacked the leucine zipper domain of Par-4, conferred enhanced sensitivity to apoptotic stimuli. Most importantly, ectopic coexpression of the leucine zipper domain of Par-4 inhibited the ability of Par-4 to enhance apoptosis. Finally, ectopic expression of WT1 attenuated apoptosis, and coexpression of Par-4 but not a leucine zipperless mutant of Par-4 rescued the cells from the antiapoptotic effect of WT1. These findings suggest that the leucine zipper domain is required for the Par-4 protein to function in apoptosis.

Retinoblastoma protein in growth suppression and death protection

Puzzling new information indicates an inadequacy in our understanding of the retinoblastoma protein (RB). RB and the transcription factor E2F appear to be collaborators. RB-E2F interaction is necessary but not sufficient for growth suppression. Unbecoming of a tumor suppressor, RB has an active role in antagonizing the death response. How RB integrates its multiple functions into a tumor suppression program is still an open issue.

Cyclin D1 expression is regulated positively by the p42/p44MAPK and negatively by the p38/HOGMAPK pathway

We have previously shown that the persistent activation of p42/p44(MAPK) is required to pass the G1 restriction point in fibroblasts (Pagès, G., Lenormand, P., L'Allemain, G., Chambard, J. C., Meloche, S., and Pouysségur, J. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8319-8323) and postulated that MAPKs control the activation of G1 cyclin-dependent complexes. We examined the mitogen-dependent induction of cyclin D1 expression, one of the earliest cell cycle-related events to occur during the G0/G1 to S-phase transition, as a potential target of MAPK regulation. Effects exerted either by the p42/p44(MAPK) or the p38/HOGMAPK cascade on the regulation of cyclin D1 promoter activity or cyclin D1 expression were compared in CCL39 cells, using a co-transfection procedure. We found that inhibition of the p42/p44(MAPK) signaling by expression of dominant-negative forms of either mitogen-activated protein kinase kinase 1 (MKK1) or p44(MAPK), or by expression of the MAP kinase phosphatase, MKP-1, strongly inhibited expression of a reporter gene driven by the human cyclin D1 promoter as well as the endogenous cyclin D1 protein. Conversely, activation of this signaling pathway by expression of a constitutively active MKK1 mutant dramatically increased cyclin D1 promoter activity and cyclin D1 protein expression, in a growth factor-independent manner. Moreover, the use of a CCL39-derived cell line that stably expresses an inducible chimera of the estrogen receptor fused to a constitutively active Raf-1 mutant (DeltaRaf-1:ER) revealed that in absence of growth factors, activation of the Raf > MKK1 > p42/p44MAPK cascade is sufficient to fully induce cyclin D1. In marked contrast, the p38(MAPK) cascade showed an opposite effect on the regulation of cyclin D1 expression. In cells co-expressing high levels of the p38(MAPK) kinase (MKK3) together with the p38(MAPK), a significant inhibition of mitogen-induced cyclin D1 expression was observed. Furthermore, inhibition of p38(MAPK) activity with the specific inhibitor, SB203580, enhanced cyclin D1 transcription and protein level. Altogether, these results support the notion that MAPK cascades drive specific cell cycle responses to extracellular stimuli, at least in part, through the modulation of cyclin D1 expression and associated cdk activities.