Interferons and interleukin 6 suppress phosphorylation of the retinoblastoma protein in growth-sensitive hematopoietic cells

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.

ROS mediates interferon gamma induced phosphorylation of Src, through the Raf/ERK pathway, in MCF-7 human breast cancer cell line

Interferon gamma (IFN-ɣ) is a pleiotropic cytokine which plays dual contrasting roles in cancer. Although IFN-ɣ has been clinically used to treat various malignancies, it was recently shown to have protumorigenic activities. Reactive oxygen species (ROS) are overproduced in cancer cells, mainly due to NADPH oxidase activity, which results into several changes in signaling pathways. In this study, we examined IFN-ɣ effect on the phosphorylation levels of key signaling proteins, through ROS production, in the human breast cancer cell line MCF-7. After treatment by IFN-ɣ, results showed a significant increase in the phosphorylation of STAT1, Src, raf, AKT, ERK1/2 and p38 signaling molecules, in a time specific manner. Src and Raf were found to be involved in early stages of IFN-ɣ signaling since their phosphorylation increased very rapidly. Selective inhibition of Src-family kinases resulted in an immediate significant decrease in the phosphorylation status of Raf and ERK1/2, but not p38 and AKT. On the other hand, IFN-ɣ resulted in ROS generation, through H₂O₂ production, whereas pre-treatment with the ROS inhibitor NAC caused ROS inhibition and a significant decrease in the phosphorylation levels of AKT, ERK1/2, p38 and STAT1. Moreover, pretreatment with a selective NOX1 inhibitor resulted in a significant decrease of AKT phosphorylation. Finally, no direct relationship was found between ROS production and calcium mobilization. In summary, IFN-ɣ signaling in MCF-7 cell line is ROS-dependent and follows the Src/Raf/ERK pathway whereas its signaling through the AKT pathway is highly dependent on NOX1.

Salivary IL-6 levels in oral leukoplakia with dysplasia and its clinical relevance to tobacco habits and periodontitis

The development of oral cancer proceeds through discrete molecular changes that are acquired from loss of genomic integrity after continued exposure to environmental risk factors. It is preceded in the majority of cases by clinically evident oral potentially malignant disorders, the most common of which is leukoplakia. Early detection of these oral lesions by screening methods using suitable markers is critical as it mirrors molecular alterations, long before cancer phenotypes are manifested. Assessment of salivary interleukin-6 (IL-6) as a marker of malignant progression was undertaken in patients with leukoplakia having coexisting periodontitis (n = 20), periodontitis patients without leukoplakia (n = 20), and healthy controls (n = 20) by competitive enzyme-linked immunosorbent assay. Results showed elevation of IL-6 levels in leukoplakia with coexisting periodontitis and in periodontitis patients when compared to healthy control (P < 0.001). Within the leukoplakia group, IL-6 level was found to be increased with increase in the severity of dysplasia. The use of tobacco was seen to play a significant role in the elevation of salivary IL-6.The importance of IL-6 as a specific marker for leukoplakia with dysplasia and the role of tobacco as an independent risk factor has been highlighted.

Changes of immunological profiles in patients with chronic myeloid leukemia in the course of treatment

In the previous paper of ours we compared, prior to start any treatment, a number of immunological parameters in 24 chronic myeloid leukemia patients with the same number of healthy subjects matched by age and sex. We found significant differences in the levels of immunoglobulins, the C4 component of complement, the C-reactive protein, interleukin 6, the composition of lymphocyte population and the production of some cytokines by stimulated CD3+ cells. Eleven of these patients were followed longitudinally. After treatment with hydroxyurea, interferon alpha, imatinib mesylate and dasatinib, or various combinations thereof, hematological remission was achieved in all patients and complete cytogenetic remission in nine of them. There was a nearly general tendency towards normalization of the abnormalities observed in the patients at their enrollment.

Activation of NF-kB pathway by virus infection requires Rb expression

The retinoblastoma protein Rb is a tumor suppressor involved in cell cycle control, differentiation, and inhibition of oncogenic transformation. Besides these roles, additional functions in the control of immune response have been suggested. In the present study we investigated the consequences of loss of Rb in viral infection. Here we show that virus replication is increased by the absence of Rb, and that Rb is required for the activation of the NF-kB pathway in response to virus infection. These results reveal a novel role for tumor suppressor Rb in viral infection surveillance and further extend the concept of a link between tumor suppressors and antiviral 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.

The GRIMs: a new interface between cell death regulation and interferon/retinoid induced growth suppression

Cytokines and vitamins play a central role in controlling neoplastic cell growth. The interferon (IFN) family of cytokines regulates antiviral, anti-tumor, antimicrobial, differentiation, and immune responses in mammals. Significant advances have been made with respect to IFN-induced signal transduction pathways and antiviral responses. However, the IFN-induced anti-tumor actions are poorly defined. Although IFNs themselves inhibit tumor growth, combination of IFNs with retinoids (a class of Vitamin A related compounds) strongly potentiates the IFN-regulated anti-tumor action in a number of cell types. To define the molecular mechanisms involved in IFN/retinoid (RA)-induced apoptosis we have employed a genetic approach and identified several critical genes. In this review, I provide the current picture of IFN- RA- and IFN/RA-regulated growth suppressive pathways. In particular, I focus on a novel set of genes, the genes-associated with retinoid-interferon induced mortality (GRIM). GRIMs may be novel types of tumor suppressors, useful as biological response markers and potentially novel targets for drug development.

IFN-beta induces caspase-mediated apoptosis by disrupting mitochondria in human advanced stage colon cancer cell lines

Various human colon cancer cell lines tested in vitro differed significantly in susceptibility to growth inhibition of recombinant human interferon-beta (rHuIFN-beta). Two p53-mutant lines, COH and CC-M2, derived from high-grade colon adenocarcinoma, showed signs of apoptosis after treatment with 250 IU/ml of HuIFN- beta in the culture medium. The similarly p53-mutated HT-29 line from a grade I adenocarcinoma showed no apoptosis, however, and only cell cycle G1/G0 or S phase retardation with 1000 IU/ml HuIFN-beta. After HuIFN-beta exposure, COH and CC-M2 cells showed increased levels of Fas and FasL proteins, alteration of mitochondrial membrane potential, and activation of caspase-9, caspase-8, and caspase-3 in a time-dependent manner. Treatment of COH and CC-M2 cells with anti-FasL antibodies or rFas/Fc fusion protein, however, could not prevent the apoptosis induced by HuIFN-beta. In contrast, cell-permeable specific inhibitors of the three caspases could inhibit the DNA fragmentation and cell death but not the mitochondrial membrane potential changes. Treatment with mitochondria-stabilizing reagents could significantly abrogate the apoptosis and caspase activation induced by HuIFN-beta. These results suggest that in COH and CC-M2 colon cancer cell lines, HuIFN-beta induces apoptosis mainly through mitochondrial membrane alteration and subsequent activation of the caspase cascade pathway, but not by the Fas/FasL interaction or the p53-dependent apoptotic mechanism.

Prostate-specific antitumor activity by probasin promoter-directed p202 expression

p202, an interferon (IFN) inducible protein, arrests cell cycle at G1 phase leading to cell growth retardation. We previously showed that ectopic expression of p202 in human prostate cancer cells renders growth inhibition and suppression of transformation phenotype in vitro. In this report, we showed that prostate cancer cells with stable expression of p202 were less tumorigenic than the parental cells. The antitumor activity of p202 was further demonstrated by an ex vivo treatment of prostate cancer cells with p202 expression vector that showed significant tumor suppression in mouse xenograft model. Importantly, to achieve a prostate-specific antitumor effect by p202, we employed a prostate-specific probasin (ARR2PB) gene promoter to direct p202 expression (ARR2PB-p202) in an androgen receptor (AR)-positive manner. The ARR2PB-p202/liposome complex was systemically administered into mice bearing orthotopic AR-positive prostate tumors. We showed that parenteral administration of an ARR2PB-p202/liposome preparation led to prostate-specific p202 expression and tumor suppression in orthotopic prostate cancer xenograft model. Furthermore, with DNA array technique, we showed that the expression of p202 was accompanied by downregulation of G2/M phase cell-cycle regulators, cyclin B, and p55cdc. Together, our results suggest that p202 suppresses prostate tumor growth, and that a prostate-specific antitumor effect can be achieved by systemic administration of liposome-mediated delivery of ARR2PB-p202.

The JAK/STAT pathway is not sufficient to sustain the antiproliferative response in an interferon-resistant human melanoma cell line

The mechanism of resistance of malignant melanoma to treatment with interferon-alpha is unknown, and currently there is no reliable method of predicting response. Signalling via the JAK/STAT pathway is known to mediate many interferon-regulated events and has been implicated in mediating the antiproliferative response. The objective of this study was to determine whether defects in JAK/STAT signalling may be responsible for interferon resistance. The in vitro response to interferon was determined in a panel of established melanoma cell lines, and the components and functioning of the JAK/STAT pathway were examined in sensitive and resistant cell lines. Two melanoma cell lines, characterized as sensitive (MM418) and resistant (MeWo) to the antiproliferative effect of interferon, were both shown by Western blotting to possess all the protein components of the JAK/STAT pathway, and were shown to be capable of producing functional transcription factors using an electrophoretic mobility shift assay and a ribonuclease protection assay of known interferon-induced genes. In addition, both cell lines had intact antiviral and HLA upregulation responses. These data suggest that there is no defect in the JAK/STAT pathway per se in the MeWo cell line, and that the substantial resistance to interferon must be mediated through components either downstream or additional to this signalling pathway. Others have shown JAK/STAT defects to be responsible for interferon resistance in some melanoma cell lines. However, our results highlight the likely heterogeneity in the mechanisms leading to interferon resistance both in cell lines and tumours, and suggest that a clinical assay based on analysis of components of the JAK/STAT pathway may have only limited use as a predictor of interferon response.

MEK and ERK inhibitors enhance the anti-proliferative effect of interferon-alpha2b

Interferon (IFN)-alpha, initially characterized as an antiviral cytokine, affects several cellular functions. It is used in clinical practice for the treatment of several tumors, including hematopoietic malignancies, due to its antiproliferative effects. To better characterize the molecular mechanism(s) underlying this property, we conducted our studies in purified primary CD4+ T cells stimulated with anti-CD3 and interleukin (IL)-2. Upon treatment with IFN-alpha, the cells were blocked in the G0/G1 phase of the cell cycle and exhibited impaired entry into S phase and reduced proliferation. Moreover, we detected short- and long-term inhibition of extracellular signal-regulated kinase (ERK) and mitogen-activated ERK-regulating kinase (MEK) function, known to control cellular proliferation. The activity of the upstream regulators, Ras and Raf-1, was not affected. Analysis of downstream events controlled by the MEK/ERK pathway showed reduced activity of cyclin-dependent kinase (Cdk)-2 and -4, high levels of the mitotic inhibitors, p21Waf1 and p27Kipl, and decreased cyclin D and E expression. When IFN-alpha was used in combination with MEK and ERK inhibitors, we observed a dose-dependent additive effect in reducing cellular proliferation. Our data demonstrate that IFN-alpha may be associated with other molecules to inhibit cellular growth by targeting the MEK/ERK pathway. This may eventually lead to new clinical strategies to strengthen its anticancer effect.

Characterization of monoclonal antibodies against GRIM-19, a novel IFN-beta and retinoic acid-activated regulator of cell death

A combination of interferon-beta (IFN-beta) and all-trans retinoic acid (IFN/RA) induces tumor cell apoptosis via some unknown mechanisms. Apoptosis is a gene-directed process that limits the proliferation of undesired cells. Several genes are required to regulate cell death in the higher-order animals. Earlier, we employed a gene expression knockout technique to isolate cell death-related genes. A novel gene, the gene associated with retinoid-interferon-induced mortality-19 (GRIM-19), was found to be essential for tumor cell death induced by IFN/RA. Here, we describe the development and characterization of three monoclonal antibodies (mAbs) against GRIM-19. GRIM-19 is present in the nucleus and cytoplasm. Its expression is induced by the IFN/RA combination. We also show that GRIM-19 inhibits the cell-transforming property of viral oncogenic protein viral IFN regulatory factor-1 (vIRF-1) via a physical interaction. mAbs developed in this study should be useful for studying the other physiologic roles of GRIM-19 and serve as a potent tool for studying tumor responses to IFN/RA therapy.

Mutational analysis of human thioredoxin reductase 1. Effects on p53-mediated gene expression and interferon and retinoic acid-induced cell death

The interferon (IFN)-beta and all-trans-retinoic acid combination suppresses tumor growth by inducing apoptosis in several tumor cell lines. A genetic technique permitted the isolation of human thioredoxin reductase (TR) as a critical regulator of IFN/all-trans-retinoic acid-induced cell death. Our recent studies have shown that TR1:thioredoxin 1-regulated cell death is effected in part through the activation of p53-dependent responses. To understand its death regulatory function, we have performed a mutational analysis of TR. Human TR1 has three major structural domains, the FAD binding domain, the NADPH binding domain, and an interface domain (ID). Here, we show that the deletion of the C-terminal interface domain results in a constitutive activation of TR-dependent death responses and promotes p53-dependent gene expression. TR mutant without the ID still retains its dependence on thioredoxin for promoting these responses. Thus, our data suggest that TR-ID acts as a regulatory domain.

The interferon-inducible gene, Ifi204, acquires malignant transformation capability upon mutation at the Rb-binding sites

p204 overexpression in retinoblastoma (Rb)-/- mouse embryo fibroblasts or transfection of p204 mutated at both Rb-binding sites confer growth advantages, resulting in a significantly higher number of foci in a cell focus assay. To investigate the possibility that mutated p204 acquires malignant transformation capability, NIH3T3 cells were stably transfected with the expression vector pRcRSV204 double-mutant (p204dm) harboring both the C-terminal deletion up to amino acid 568 and the point mutation from glutamic acid to lysine at position 427, and analyzed for markers typical of cell immortalization and transformation. We detected a greater abundance of cell colonies in soft agar with p204dm-expressing cells than vector control cells. The p204dm-transfected cells also displayed two other characteristics associated with malignant transformation, i.e. growth under low-serum conditions and formation of tumors in athymic nude mice. Moreover, their telomerase activity was significantly higher than in the vector control cells. It would thus seem that p204, devoid of functional Rb-binding motifs, can become oncogenic.

Biological assays for interferons

Interferons (IFN) are potent biologically active proteins synthesised and secreted by somatic cells of all mammalian species. They have been well characterised, especially those of human origin, with respect to structure, biological activities, and clinical therapeutic effects. While structural differences are known to exist among the IFN species that constitute the "IFN family" and despite the existence of different receptors for type I and type II IFN, all species have been shown to exert a similar spectrum of in vitro biological activities in responsive cells. Principal among the biological activities induced by IFN is antiviral activity, the activity used to originally define IFN. Antiviral activity of IFN is mediated via cell receptors and is dependent on the activation of signalling pathways, the expression of specific gene products, and the development of antiviral mechanisms. Sensitivity of cells to IFN-mediated antiviral activity is variable, and depends on a number of factors including cell type, expression of IFN receptors and downstream effector response elements, effectiveness of antiviral mechanisms, and the type of virus used to infect cells. Nevertheless, by the judicious use of sensitive cell lines in combination with appropriate cytopathic viruses, effective assays to measure the antiviral activity have been developed. Historically, "antiviral assays" (AVA) were the first type of biological assays that were developed to measure the relative activity or potency of IFN preparations. However, the subsequent discoveries of several other biological activities of IFN has opened the way to the development of assays based on one or other of these activities. The latter include inhibition of cell proliferation, regulation of functional cellular activities, regulation of cellular differentiation and immunomodulation. More recently, the cloning of IFN responsive genes has led to the development of "reporter gene assays". In this case, the promoter region of IFN responsive genes is linked with a heterologous reporter gene, for example, firefly luciferase or alkaline phosphatase, and transfected into an IFN-sensitive cell line. Stably transfected cell lines exposed to IFN increase expression of the reporter gene product in direct relation to the dose of IFN, the readout being a measure of this product's enzymic action. The current review aims to give a critical overview of the development, specificity, standardisation and present use of the various biological assay methods now available for the quantification of IFN activity.

Modulation of p53 dependent gene expression and cell death through thioredoxin-thioredoxin reductase by the Interferon-Retinoid combination

We have shown earlier that the IFN-beta and all-trans retinoic acid (RA) combination, but not the single agents, induces death in several tumor cell lines. Employing a genetic technique we have identified several Genes associated with Retinoid-IFN induced Mortality (GRIM). One of the GRIMs was human thioredoxin reductase (TR), a redox enzyme. Since the overexpressed TR augments IFN/RA stimulated cell death, we explored the mechanisms of TR-mediated death. Here we show that TR augments cell death by upregulating the transcriptional activity of p53 tumor suppressor. This process does not involve a physical increase in levels of p53. Using redox inactive mutants of TR and its substrate, thioredoxin (Trx), we demonstrate that IFN/RA-induced regulation of p53 dependent gene expression requires TR and Trx. In contrast-over-expression of wildtype TR or Trx augment the p53 dependent gene expression in response to IFN/RA treatment. Consistent with these results an increased DNA binding activity of p53 was noted in the presence of TR. These studies identify a novel mechanism of p53 mediated cell death regulation involving redox enzymes.

Regulation of interferon and retinoic acid-induced cell death activation through thioredoxin reductase

Interferons (IFNs) and retinoids are potent biological response modifiers. The IFN-beta and all-trans-retinoic acid combination, but not these single agents individually, induces death in several tumor cell lines. To elucidate the molecular basis for these actions, we have employed an antisense knockout approach to identify the gene products that mediate cell death and isolated several genes associated with retinoid-IFN-induced mortality (GRIMs). One of the GRIM cDNAs, GRIM-12, was identical to human thioredoxin reductase (TR). To define the functional relevance of TR to cell death and to define its mechanism of death-modulating functions, we generated mutants of TR and studied their influence on the IFN/RA-induced death regulatory functions of caspases. Wild-type TR activates cell death that was inhibited in the presence of caspase inhibitors or catalytically inactive caspases. A mutant TR, lacking the active site cysteines, inhibits the cell death induced by caspase 8. IFN/all-trans-retinoic acid-induced cytochrome c release from the mitochondrion was promoted in the presence of wild type and was inhibited in the presence of mutant TR. We find that TR modulates the activity of caspase 8 to promote death. This effect is in part caused by the stimulation of death receptor gene expression. These studies identify a new mechanism of cell death regulation by the IFN/all-trans-retinoic acid combination involving redox enzymes.

Inositol hexakisphosphate kinase 2 mediates growth suppressive and apoptotic effects of interferon-beta in ovarian carcinoma cells

Interferons (IFNs) regulate the expression of genes that mediate their antiviral, antitumor, and immunomodulatory actions. We have previously shown that IFN-beta suppresses growth of human ovarian carcinoma xenografts in vivo and induces apoptosis of ovarian carcinoma cells in vitro. To investigate mechanisms of IFN-beta-induced apoptosis we employed an antisense technical knockout approach to identify gene products that mediate cell death and have isolated several regulators of interferon-induced death (RIDs). In this investigation, we have characterized one of the RIDs, RID-2. Sequence analysis revealed that RID-2 was identical to human inositol hexakisphosphate kinase 2 (IP6K2). IP6K2 is post-transcriptionally induced by IFN-beta in ovarian carcinoma cells. A mutant IP6K2 with substitutions in the putative inositol phosphate binding domain abrogates IFN-beta-induced apoptosis. These studies identify a novel function for IP6K2 in cell growth regulation and apoptosis.

Thioredoxin participates in a cell death pathway induced by interferon and retinoid combination

Interferons (IFNs) and retinoids are potent tumor growth suppressors. We have shown earlier that the IFN-beta and all-trans retinoic acid combination, but not the single agents, induces death in several tumor cell lines. Employing a genetic approach we have recently identified several Genes associated with Retinoid-IFN induced Mortality (GRIM) that mediate the cell death effect of IFN/RA combination. One of the GRIMs, GRIM-12, was identical to human thioredoxin reductase (TR), an enzyme that controls intracellular redox state. To define the participants of TR mediated death pathway we have examined the role of thioredoxin (Trx), its downstream substrate, and its influence on IFN/RA-induced death regulation. Inhibition of the thioredoxin expression by antisense RNA suppressed cell death. Similarly, a mutant Trx1 lacking the critical cysteine residues blocked cell death. In contrast, overexpression of wildtype thioredoxin augmented cell death. This effect of Trx1 was in part due to its ability to augment cell death via caspase-8. The redox inactive Trx1 mutant inhibits the cell death induced by caspase-8 but not caspase-3. These studies identify a novel mechanism of cell death regulation by IFN/RA combination involving redox enzymes.

Regulation of cyclin-dependent kinase 2 activity by ceramide

Cyclin-dependent kinases have been implicated in the inactivation of retinoblastoma (Rb) protein and cell cycle progression. Recent studies have demonstrated that the lipid molecule ceramide is able to induce Rb hypophosphorylation leading to growth arrest and cellular senescence. In this study, we examined the underlying mechanisms of Rb hypophosphorylation and cell cycle progression utilizing the antiproliferative molecule ceramide. C6-Ceramide induced a G0/G1 arrest of the cell cycle in WI38 human diploid fibroblasts. Employing immunoprecipitation kinase assays, we found that ceramide specifically inhibited cyclin-dependent kinase CDK2, with a mild effect on CDC2 and significantly less effect on CDK4. The effect of ceramide was specific such that C6-dihydroceramide was not effective. Ceramide did not directly inhibit CDK2 in vitro but caused activation of p21, a major class of CDK-inhibitory proteins, and led to a greater association of p21 to CDK2. Using purified protein phosphatases, we showed that ceramide activated both protein phosphatase 1 and protein phosphatase 2A activities specific for CDK2 in vitro. Further, calyculin A and okadaic acid, both potent protein phosphatase inhibitors, together almost completely reversed the effects of ceramide on CDK2 inhibition. Taken together, these results demonstrate a dual mechanism by which ceramide inhibits the cell cycle. Ceramide causes an increase in p21 association with CDK2 and through activation of protein phosphatases selectively regulates CDK2. These events may lead to activation of Rb protein and subsequent cell cycle arrest.

Interferon-alpha2b reduces phosphorylation and activity of MEK and ERK through a Ras/Raf-independent mechanism

Interferon (IFN)-alpha affects the growth, differentiation and function of various cell types by transducing regulatory signals through the Janus tyrosine kinase/signal transducers of activation and transcription (Jak/STAT) pathway. The signalling pathways employing the mitogen-activated ERK-activating kinase (MEK) and the extracellular-regulated kinase (ERK) are critical in growth factors signalling. Engagement of the receptors, and subsequent stimulation of Ras and Raf, initiates a phosphorylative cascade leading to activation of several proteins among which MEK and ERK play a central role in routing signals critical in controlling cell development, activation and proliferation. We demonstrate here that 24-48 h following treatment of transformed T- and monocytoid cell lines with recombinant human IFN-alpha2b both the phosphorylation and activity of MEK1 and its substrates ERK1/2 were reduced. In contrast, the activities of the upstream molecules Ras and Raf-1 were not affected. No effect on MEK/ERK activity was observed upon short-term exposure (1-30 min) to IFN. The anti-proliferative effect of IFN-alpha was increased by the addition in the culture medium of a specific inhibitor of MEK, namely PD98059. In conclusion, our results indicate that IFN-alpha regulates the activity of the MEK/ERK pathway and consequently modulates cellular proliferation through a Ras/Raf-independent mechanism. Targeting the MEK/ERK pathway may strengthen the IFN-mediated anti-cancer effect.

The interferon-beta and tamoxifen combination induces apoptosis using thioredoxin reductase

Interferons (IFNs) suppress cell growth by inducing cellular genes. The anti-estrogen tamoxifen (Tam), binds to estrogen receptor and inhibits transcription of estrogen stimulated genes. In cells resistant to IFN-induced growth suppression, IFN/Tam combination causes cell death. We previously reported that the combination of IFN-beta and Tam was a more potent growth suppressor of human tumor xenografts than either agent alone. The IFN/Tam combination acts in a manner similar to the IFN/retinoic acid combination. Using a genetic technique, we have recently identified several genes associated with retinoid-IFN-induced mortality (GRIM). One such gene, GRIM-12, was identical to human thioredoxin reductase (TR). In the present study we have examined whether the IFN/Tam combination also requires GRIM-12 for inducing cell death. We report here that GRIM-12 is necessary for mediating the cell death effects of IFN/Tam, and its expression is induced by IFN/Tam at a post-transcriptional stage. Repression of GRIM-12 levels either by antisense expression or by dominant negative inhibitors caused resistance to IFN/Tam induced death and promoted cell growth. Overexpression of GRIM-12 increased IFN/Tam induced apoptosis. Thus, these studies have identified a critical role for GRIM-12 (TR) in apoptosis.

Molecular mechanism of interferon alfa-mediated growth inhibition in human neuroendocrine tumor cells

BACKGROUND & AIMS

Although human neuroendocrine tumors respond to interferon (IFN)-alpha treatment in vivo, the underlying mechanisms of growth inhibition are poorly understood. To characterize the antiproliferative effects at a molecular level, we explored the growth-regulatory action of IFN-alpha in the human neuroendocrine tumor cell lines BON and QGP1.

METHODS

IFN-alpha receptor expression and signal transduction were examined by reverse-transcription polymerase chain reaction, immunoblotting, subcellular fractionation, and transactivation assays. Growth regulation was evaluated by cell numbers, soft agar assays, and cell cycle analysis using flow cytometry. Expression and activity of cell cycle-regulatory molecules were determined by immunoblotting and histone H1-kinase assays.

RESULTS

Both cell lines expressed IFN-alpha receptor mRNA transcripts. Ligand binding initiated phosphorylation of Jak kinases and Stat transcription factors, resulting in Stat activation, nuclear translocation, and transcription from an ISRE-reporter construct. Prolonged IFN-alpha treatment dose-dependently inhibited both anchorage-dependent and -independent growth. Cell cycle analysis of IFN-alpha-treated, unsynchronized cultures revealed an increased S-phase population, which was further substantiated in G(1) synchronized QGP1 cells. IFN-alpha-treated cells entered S phase in parallel to control cultures, but their progress into G(2)/M phase was delayed. Both cellular cyclin B levels and CDC 2 activity were substantially reduced. The extent and time course of this reduction corresponded to the observed S-phase accumulation.

CONCLUSIONS

IFN-alpha directly inhibits growth of human neuroendocrine tumor cells by specifically delaying progression through S phase and into G(2)/M. These cell cycle changes are associated with inhibition of cyclin B expression, resulting in reduced CDC2 activity.

Chinese medicinal herb, Acanthopanax gracilistylus, extract induces cell cycle arrest of human tumor cells in vitro

We investigated the effect of a Chinese medicinal herb, Acanthopanax gracilistylus (AG), extract (E) on the growth of human tumor cell lines in vitro. AGE markedly inhibited the proliferation of several tumor cell lines such as MT-2, Raji, HL-60, TMK-1 and HSC-2. The activity was associated with a protein of 60 kDa, which was purified by gel-filtration chromatography. Cell viability analyses indicated that the treatment with AGE inhibits cell proliferation, but does not induce cell death. The mechanism of AGE-induced inhibition of tumor cell growth involves arrest of the cell cycle at the G(0) / G(1) stage without a direct cytotoxic effect. The cell cycle arrest induced by AGE was accompanied by a decrease of phosphorylated retinoblastoma (Rb) protein. Furthermore, cyclin-dependent kinases 2 and 4 (Cdk2 and Cdk4), which are involved in the phosphorylation of Rb, were also decreased. These results suggest that AGE inhibits tumor cell growth by affecting phosphorylated Rb proteins and Cdks.

IFN-alpha 2b reduces IL-2 production and IL-2 receptor function in primary CD4+ T cells

Initially described as an antiviral cytokine, IFN-alpha has been subsequently shown to affect several cellular functions, including cellular differentiation and proliferation. For these reasons, IFN-alpha is currently used in clinical practice for the treatment of viral infections and malignancies. In this manuscript, we show two novel mechanisms concomitantly responsible for the antiproliferative effect of IFN-alpha. First, long-term treatment with IFN-alpha of primary CD4+ T cells reduced surface expression of CD3 and CD28. These events resulted in decreased phosphorylation of the mitogen-activated extracellular signal-regulated activating kinase and its substrate extracellular signal-regulated kinase, leading to diminished production of IL-2. Second, IFN-alpha treatment of primary CD4+ T cells reduced proliferative response to stimulation in the presence of exogenous IL-2 by markedly decreasing mRNA synthesis and surface expression of CD25 (alpha-chain), a critical component of the IL-2R complex. These results may be relevant for the antitumor effects of IFN-alpha and may help us to better understand its detrimental role in the inhibition of proliferation of the bulk of CD4+ T cells (uninfected cells) in HIV-infected persons, who are known to overproduce IFN-alpha.

Interferon-alpha repressed telomerase along with G1-accumulation of Daudi cells

The implications of telomerase on senescence and human carcinogenesis are widely accepted, but the changes of telomerase activity along with cell cycle modulation by anticancer treatment still remain obscure. In this paper, we issued whether the telomerase activity fluctuated along with cell cycle of cultured cancer cells using the antiproliferative effect of interferon-alpha (IFN-alpha). Daudi Burkitt lymphoma cells, treated with IFN-alpha, showed proliferation inhibition and cell cycle arrest at G1. The telomerase activity at 72 h was repressed to about 20% of control cells. Furthermore, after 72 h IFN-alpha treatment, the cells in G1 phase showed the marked decrease of telomerase activity, while cells in S and G2/M still possessed it. Among expressions of telomerase-related genes, only the catalytic subunit of telomerase (hTERT) decreased from 48 h, while the template RNA component (hTERC) and telomerase-associated protein 1 (TEP-1) were not affected. The downregulation of c-Myc preceded the change of hTERT. Moreover, the analysis of cells treated with IFN-alpha for 24 h revealed that cells in G1-to-S transition mainly expressed high hTERT, while S and G2/M cells had higher level of telomerase activity than that of G1 cells. These results indicate that (i) the expression of hTERT precedes the telomerase activity which is higher in S and G2/M phases than G1 phase, (ii) IFN-alpha repressed the telomerase activity in a cell cycle-dependent manner with the downregulation of hTERT.

Interferon-gamma-mediated inhibition of cyclin A gene transcription is independent of individual cis-acting elements in the cyclin A promoter

Interferons (IFNs) affect cellular functions by altering gene expression. The eukaryotic cell cycle is governed in part by the periodic transcription of cyclin genes, whose protein products associate with and positively regulate the cyclin-dependent kinases. To understand better the growth inhibitory effect of IFN-gamma on vascular smooth muscle cells (VSMCs), we compared the expression and activity of G1 and S phase cyclins in control and IFN-gamma-treated VSMCs. IFN-gamma treatment did not inhibit the G1 cyclins but did decrease cyclin A protein, mRNA, and associated kinase activity by 85, 90, and 90%, respectively. Nuclear run-on and mRNA stability determinations indicated that this decrease was the result of transcriptional inhibition. To investigate the molecular basis of this inhibition, we examined protein-DNA interactions involving the cyclin A promoter. Electromobility shift assays showed little change with IFN-gamma treatment in the binding of nuclear proteins to isolated ATF, NF-Y, and CDE elements. In vivo genomic footprinting indicated that IFN-gamma treatment changed the occupancy of chromosomal NF-Y and CDE sites slightly and did not affect occupancy of the ATF site. In a previous study of transforming growth factor-beta1-mediated inhibition of the cyclin A promoter, we mapped the inhibitory effect to the ATF site; in the present study of IFN-gamma treatment, functional analysis by transient transfection showed that inhibition of the cyclin A promoter persisted despite mutation of the ATF, NF-Y, or CDE elements. We hypothesize that IFN-gamma inhibits cyclin A transcription by modifying co-activators or general transcription factors within the complex that drives transcription of the cyclin A gene.

Transcriptional repression of the E2F-1 gene by interferon-alpha is mediated through induction of E2F-4/pRB and E2F-4/p130 complexes

E2F is a heterodimeric transcription factor composed of one of five E2F subunits (E2F-1 to E2F-5) and a DP subunit. E2F regulates the expression of several growth-promoting genes, and thus, can be a target of antiproliferative action of interferons (IFNs). In this study, we investigated the mechanisms whereby IFN-alpha suppresses transcription of the E2F-1 gene. Transfection studies revealed that E2F-1 promoter was functionally divided into two parts: upstream activation sequences (UAS) and a downstream negative-regulatory element (E2F-binding sites). When cells were proliferating, transcription of the E2F-1 gene was primarily driven by the UAS, while E2F sites were not involved in activation. IFN-alpha markedly reduced E2F-1 promoter activity, but introduction of non-binding mutation at the E2F sites completely abrogated the inhibition. Free E2F4 was found to be the predominant species bound to the E2F sites in proliferating cells. IFN-alpha induced upregulation of E2F-4 along with dephosphorylation of pRB and p130, which resulted in the formation of E2F-4/pRB and E2F-4/p130 complexes on the E2F-1 promoter. These complexes function as transcriptional repressors to inhibit E2F-1 mRNA expression. Our findings indicate that E2F-4 is a critical regulator of E2F-1, which offer an excellent paradigm for understanding functional diversity within the E2F family.

Butyrate augments interferon-alpha-induced S phase accumulation and persistent tyrosine phosphorylation of cdc2 in K562 cells

Interferon-alpha (IFN-alpha) is a clinically useful cytokine for treatment of a variety of cancers, including chronic myelocytic leukaemia (CML). Most CML cells are sensitive to IFN-alpha; however, its biological effects on leukaemic cells are incompletely characterized. Here, we provide evidence that IFN-alpha induces a significant increase in the S phase population in human CML leukaemic cell line, K562, and that the S phase accumulation was augmented by sodium butyrate. In contrast, neither sodium butyrate alone, nor sodium butyrate plus IFN-gamma, affected the cell cycle in K562 cells. These data suggest that the effect of sodium butyrate depended upon IFN-alpha-mediated signalling. The ability of leukaemic cells to exhibit the S phase accumulation after stimulation by IFN-alpha plus sodium butyrate correlated well with persistent tyrosine phosphorylation of cdc2, whereas treatment with IFN-gamma plus sodium butyrate did not affect its phosphorylation levels. Considering that dephosphorylation of cdc2 leads to entry to the M phase, the persistent tyrosine phosphorylation of cdc2 may be associated with the S phase accumulation induced by IFN-alpha and sodium butyrate. In addition, another human CML leukaemic cell line, MEG-01, also showed the S phase accumulation after stimulation with IFN-alpha plus sodium butyrate. Taken together, our studies reveal a novel effect of sodium butyrate on the S phase accumulation and suggest its clinical application for a combination therapy with IFN-alpha, leading to a great improvement of clinical effects of IFN-alpha against CML cells.

Interleukin-6 dependent induction of the cyclin dependent kinase inhibitor p21WAF1/CIP1 is lost during progression of human malignant melanoma

Human melanoma cell lines derived from early stage primary tumors are particularly sensitive to growth arrest induced by interleukin-6 (IL-6). This response is lost in cell lines derived from advanced lesions, a phenomenon which may contribute to tumor aggressiveness. We sought to determine whether resistance to growth inhibition by IL-6 can be explained by oncogenic alterations in cell cycle regulators or relevant components of intracellular signaling. Our results show that IL-6 treatment of early stage melanoma cell lines caused G1 arrest, which could not be explained by changes in levels of G1 cyclins (D1, E), cdks (cdk4, cdk2) or by loss of cyclin/cdk complex formation. Instead, IL-6 caused a marked induction of the cdk inhibitor p21WAF1/CIP1 in three different IL-6 sensitive cell lines, two of which also showed a marked accumulation of the cdk inhibitor p27Kip1. In contrast, IL-6 failed to induce p21WAF1/CIP1 transcript and did not increase p21WAF1/CIP1 or p27kip1 proteins in any of the resistant lines. In fact, of five IL-6 resistant cell lines, only two expressed detectable levels of p21WAF1/CIP1 mRNA and protein, while in three other lines, p21WAF1/CIP1 was undetectable. IL-6 dependent upregulation of p21WAF1/CIP1 was associated with binding of both STAT3 and STAT1 to the p21WAF1/CIP1 promoter. Surprisingly, however, IL-6 stimulated STAT binding to this promoter in both sensitive and resistant cell lines (with one exception), suggesting that gross deregulation of this event is not the unifying cause of the defect in p21WAF1/CIP1 induction in IL-6 resistant cells. In somatic cell hybrids of IL-6 sensitive and resistant cell lines, the resistant phenotype was dominant and IL-6 failed to induce p21WAF1/CIP1. Thus, our results suggest that in early stage human melanoma cells, IL-6 induced growth inhibition involves induction of p21WAF1/CIP1 which is lost in the course of tumor progression presumably as a result of a dominant oncogenic event.

Cell cycle regulation of the double stranded RNA activated protein kinase, PKR

The interferon (IFN)-induced, double stranded RNA (dsRNA)-activated serine/threonine kinase, PKR, is a potent negative regulator of cell growth when overexpressed in yeast or mammalian cells. To determine whether endogenous PKR plays a role in cell growth control, we have investigated the regulation of PKR levels and activity during the cell cycle in human glioblastoma T98G cells. The steady-state level of PKR mRNA in T98G cells was highest in growth arrested cells, dropped sharply within 3 h of serum stimulation then gradually increased as cells progressed through G1, reaching a plateau in early S phase. PKR protein level increased following serum stimulation reaching a peak at the G2+M boundary and declining thereafter. In contrast, PKR kinase activity exhibited two peaks, in early G1 and at the G1/S boundary, declining sharply in early S phase. Thus, the activity profile did not follow the protein profile indicating a tight regulation of PKR at the level of activity. In T98G cells expressing the catalytically inactive PKRK296R the dsRNA-induced activation of NF-kappaB and IRF-1 was suppressed and the mutant cells exhibited resistance to stress induced apoptosis. Cell cycle distribution analysis showed that the mutant expressing cells exhibited longer G1 phase and fewer cells engaged in S phase. Furthermore, early passage mouse embryo fibroblasts derived from PKR knockout mice grew more slowly compared with the control cells. Taken together these results suggest that PKR may play a role in cell cycle progression.

Interferon induction in turkeys by oral administration of the imidazoquinolinamine S-28828 and modulation of the pathogenesis of Escherichia coli

A synthetic imidazoquinolinamine, S-28828, has been shown to be an effective antiviral and antitumor agent in mammals. This immune modifier induces a number of cytokines such as interferons, tumor necrosis factor-alpha, interleukins and granulocyte-macrophage colony-stimulating factors in mammals. We showed that when turkeys were given S-28828 orally, high serum titers of IFN were induced in a dose-dependent manner. Turkeys, once stimulated by S-28828, became refractory to IFN production by repeated stimulation. S-28828 induced spleen, bone marrow and peripheral leukocytes to produce IFN in vitro. Splenic adherent cells were the main producers of IFN after in vitro stimulation. S-28828-induced IFN was identified as type I IFN that was pH-resistant but heat-labile. We examined the effect of a high dose (100 mg kg(-1) body weight) of S-28828 on the pathogenesis of E. coli in turkeys. Treatment with S-28828 increased mortality in infected birds and impaired E. coli clearance from the liver. The enhancement of the pathogenicity of E. coli by S-28828 may have been due to the massive release of cytokines inducing a shock-like syndrome in infected turkeys.

Thioredoxin reductase mediates cell death effects of the combination of beta interferon and retinoic acid

Interferons (IFNs) and retinoids are potent biological response modifiers. By using JAK-STAT pathways, IFNs regulate the expression of genes involved in antiviral, antitumor, and immunomodulatory actions. Retinoids exert their cell growth-regulatory effects via nuclear receptors, which also function as transcription factors. Although these ligands act through distinct mechanisms, several studies have shown that the combination of IFNs and retinoids synergistically inhibits cell growth. We have previously reported that IFN-beta-all-trans-retinoic acid (RA) combination is a more potent growth suppressor of human tumor xenografts in vivo than either agent alone. Furthermore, the IFN-RA combination causes cell death in several tumor cell lines in vitro. However, the molecular basis for these growth-suppressive actions is unknown. It has been suggested that certain gene products, which mediate the antiviral actions of IFNs, are also responsible for the antitumor actions of the IFN-RA combination. However, we did not find a correlation between their activities and cell death. Therefore, we have used an antisense knockout approach to directly identify the gene products that mediate cell death and have isolated several genes associated with retinoid-IFN-induced mortality (GRIM). In this investigation, we characterized one of the GRIM cDNAs, GRIM-12. Sequence analysis suggests that the GRIM-12 product is identical to human thioredoxin reductase (TR). TR is posttranscriptionally induced by the IFN-RA combination in human breast carcinoma cells. Overexpression of GRIM-12 causes a small amount of cell death and further enhances the susceptibility of cells to IFN-RA-induced death. Dominant negative inhibitors directed against TR inhibit its cell death-inducing functions. Interference with TR enzymatic activity led to growth promotion in the presence of the IFN-RA combination. Thus, these studies identify a novel function for TR in cell growth regulation.

How cells respond to interferons

Interferons play key roles in mediating antiviral and antigrowth responses and in modulating immune response. The main signaling pathways are rapid and direct. They involve tyrosine phosphorylation and activation of signal transducers and activators of transcription factors by Janus tyrosine kinases at the cell membrane, followed by release of signal transducers and activators of transcription and their migration to the nucleus, where they induce the expression of the many gene products that determine the responses. Ancillary pathways are also activated by the interferons, but their effects on cell physiology are less clear. The Janus kinases and signal transducers and activators of transcription, and many of the interferon-induced proteins, play important alternative roles in cells, raising interesting questions as to how the responses to the interferons intersect with more general aspects of cellular physiology and how the specificity of cytokine responses is maintained.

p130, p107, and pRb are differentially regulated in proliferating cells and during cell cycle arrest by alpha-interferon

We have determined how the phosphorylation of the retinoblastoma family (pRb, p107, and p130) is governed in individual cell cycle phases of Daudi B-cells during cell cycle exit triggered by alpha-interferon (alpha-IFN). alpha-IFN causes dephosphorylation of pRb and loss of p130 phosphorylated Form 3. However, the change in p130 phosphorylation in response to alpha-IFN occurs before dephosphorylation of pRb is complete because loss of p130 Form 3 occurs throughout the cell cycle prior to complete arrest in G1, whereas pRb is dephosphorylated only in G1. In contrast, p107 is dephosphorylated and is then depleted from cells as they exit the cell cycle. p130, predominantly in Form 1, and hypophosphorylated pRb bind an E2F DNA binding site; p130 complexes E2F-4, whereas pRb binds both E2F-4 and E2F-1. The phosphorylated forms of E2F-4 that bind to the E2F DNA site are different from hyperphosphorylated E2F-4, which predominates in primary hemopoietic cells in G0. We conclude that although cell cycle arrest induced by alpha-IFN may be mediated in part by formation of a complex containing p130 and E2F-4, alpha-IFN does not induce hyperphosphorylation of E2F-4, which characterizes primary hemopoietic cells in G0.

An in vitro model for cytogenetic conversion in CML. Interferon-alpha preferentially inhibits the outgrowth of malignant stem cells preserved in long-term culture

IFN-alpha has been shown to prolong survival in chronic myeloid leukemia patients, but its mechanism of action is still not understood. The human cobblestone area-forming cell (CAFC) assay allows for the measurement of the concentration of normal as well as malignant stem cells, while their progeny can be measured in parallel long-term culture (LTC) in flasks. Using CAFC and LTC assays, we have examined direct effects of IFN-alpha (500; 5,000 IU/ml) on the maintenance and outgrowth of CD34-enriched normal and malignant stem cells, obtained from six patients with an established major cytogenetic response to IFN-alpha and from four nonresponding patients. CAFC concentrations were not affected by IFN-alpha. In contrast, IFN-alpha strongly inhibited the clonogenic output in flask LTC. Nucleated cells (NC) produced in LTC were evaluated by fluorescent in situ hybridization (FISH) for the presence of the Philadelphia (Ph) translocation. After 8 wk of LTC, the percentage of Ph+ NCs produced was significantly more inhibited by IFN-alpha in responding patients than in nonresponders. Control LTC without IFN-alpha showed no significant differences of Ph+ NC production between responders and nonresponders. These findings provide the first in vitro model for cytogenetic conversion and suggest that direct antiproliferative effects of IFN-alpha account for the cytogenetic response observed clinically.

Abrogation of betaglycan attenuates TGF-beta-mediated inhibition of embryonic murine lung branching morphogenesis in culture

Although betaglycan (TGF-beta type III receptor) is known to enhance TGF-beta ligand binding to its type II receptor in murine lung epithelial cell lines, the biological significance of this phenomenon in the process of lung organogenesis is not understood. Betaglycan gene expression was detected in embryonic murine lungs undergoing branching morphogenesis in ex vivo culture. Antisense betaglycan oligodeoxynucleotides (ODN) resulted in up to 56% stimulation of lung branching morphogenesis in culture, while betaglycan mRNA and protein expression levels were suppressed by 90 and 82%, respectively. Following abrogation of betaglycan expression with antisense oligodeoxynucleotide, embryonic lungs were relatively insensitive to TGF-beta: TGF-beta2 (0.5 ng/ml) and TGF-beta1 (20 ng/ml), respectively, down-regulated lung morphogenesis by 38 and 34% in control cultures, whereas TGF-beta-induced inhibition was attenuated to 13 and 26% respectively, in the presence of betaglycan antisense oligodeoxynucleotides. TGF-beta neutralizing antibodies also prevented TGF-beta-mediated inhibition of lung branching in culture, supporting the speculation that autocrine/paracrine TGF-beta signaling is minimal in the absence of betaglycan. Betaglycan was immunolocalized mainly to the epithelial cells in developing airways, a spatial distribution which overlaps with that of TGF-beta type II receptor. Furthermore, abrogation of endogenous betaglycan gene expression prevented the characteristic down-regulation of cyclin A and surfactant protein C (SP-C) mRNAs by exogenous TGF-beta ligands. These results show that betaglycan expression is essential for optimal TGF-beta signaling during embryonic lung development. We therefore conclude that the abrogation of endogenous betaglycan attenuates endogenous autocrine and/or paracrine TGF-beta-mediated negative regulation of lung organogenesis.

Differential myeloma cell responsiveness to interferon-alpha correlates with differential induction of p19(INK4d) and cyclin D2 expression

Interferon-alpha (IFN-alpha) has been used as therapy for the treatment of a variety of viral diseases and malignancies including multiple myeloma. The effectiveness of interferon-alpha in treating multiple myeloma, however, has been somewhat variable, and the mechanism(s) accounting for this is not well understood. As a means to examine the basis for the differential effectiveness of this cytokine, we have analyzed IFN-alpha-mediated modulation of the cell cycle in two human myeloma cell lines. These two cell lines, ANBL-6 and KAS-6/1, display dramatically different outcomes in response to this cytokine. Although IFN-alpha inhibited the growth of ANBL-6 cells by blocking cell cycle progression from G0/G1 to S phase, IFN-alpha stimulated cell cycle progression in KAS-6/1 cells. Moreover, the effects of IFN-alpha on cell cycle progression correlated with the phosphorylation status of the retinoblastoma protein. Of interest, IFN-alpha increased cyclin D2 expression and cyclin-dependent kinase activity in the KAS-6/1 cells but not in the ANBL-6 cells. To determine whether the differential effects of IFN-alpha on myeloma cell cycle progression could also result from differences in the expression of cyclin-dependent kinase inhibitors, we examined the effects of IFN-alpha on the induction of cyclin-dependent kinase inhibitors with broad regulatory function (p21 and p27) and those with specificity for G1-associated cyclin-cyclin-dependent kinase complexes (p15, p16, p18, and p19). Although we failed to detect an effect of IFN-alpha on expression levels of p21, p15, p16, or p18, IFN-alpha treatment of the ANBL-6 cell line resulted in induction of p19 expression, whereas it was without effect on the KAS-6/1 cell line. These results suggest that heterogeneity in IFN-alpha-mediated growth effects in myeloma cells correlates with differential induction of cyclin D2 and p19(INK4d) expression.

Growth control mechanisms in multiple myeloma

Interleukin-6 (IL-6) is the major growth factor for the malignant plasma cell clone in patients with multiple myeloma (MM). Although interferon-alpha (IFN-alpha) has been widely used as maintenance therapy in MM, controversy exists as to its clinical utility. This review summarizes data showing that cell growth arrest brought about by type I (IFNs-alpha/beta) and type II (IFN-gamma) IFNs occurs in part through utilization/modification of various components of the otherwise stimulatory Jak-STAT and Ras signaling pathways triggered by IL-6. Recent experimental results indicating that IFN-alpha acts as a survival factor for certain myeloma cell lines and frequently induces endogenous IL-6 expression may help to explain the conflicting clinical findings obtained in this heterogeneous disease with this usually potent growth inhibitor. By comparison, consistent antiproliferative activity exhibited by IFN-gamma on IL-6-dependent myeloma cell lines and primary myeloma cells from patients suggests that further investigation of the possible value of this cytokine in the treatment of MM may be warranted.

Differential expression of the p27Kip1 mRNA in IFN-sensitive and resistant cell lines

IFNs arrest the growth of a small cell lung cancer (SCLC) cell line NCI-H82 in the G1 phase but not the growth of the derived cell line NCI-H82R. Progression through the G1 phase is controlled by positive and negative regulatory genes. Oncoviral genes can override this control. In this study, we compared the effects of human IFN alpha 2b on the mRNA levels of the Cdk inhibitor p27Kip1 in NCI-H82, NCI-H82R and HPV 16 E7-transfected NCI-H82 cell lines. Induction of the 2-5 Oligoadenylate synthetase (2-5 OAS) gene was used as a marker of IFN-dependent signal transduction The expression of p27Kip1 mRNA increased at 48 and 72 hr after IFN alpha 2b addition in sensitive cells. In contrast, p27Kip1 mRNA had only slight variations in both the resistant and E7-transfected cells. Interestingly, the E7-transfected NCI-H82 cells became resistant to the IFN alpha 2b anti-proliferative effect. Our results suggest that p27Kip1 could be a key mediator of the IFN alpha 2b-induced growth arrest and that HPV 16 E7 might affect p27Kip1 inducibility, originating IFN alpha 2b-resistant cells.

The Cytoplasmic Domain of Stem Cell Antigen CD34 Is Essential for Cytoadhesion Signaling But Not Sufficient for Proliferation Signaling

CD34 is widely used as a marker in the identification and purification of human hematopoietic stem and progenitor cells; however, its function within hematopoiesis is largely unknown. We have investigated the contribution of cytoplasmic domain of CD34 in cytoadhesion signaling and proliferation signaling in hematopoietic cells. Engagement of particular determinants of CD34 by monoclonal antibodies leads to homotypic adhesiveness of the full-length CD34-transfected BaF3 cells. However, this homotypic adhesiveness is abrogated in BaF3 cells transfected with the truncated CD34 lacking the cytoplasmic domain. Cytoadhesion signaling through the cytoplasmic domain of CD34 cannot be restored through that of erythropoietin receptor (EPOR) or granulocyte colony-stimulating factor receptor (G-CSFR), suggesting that the cytoplasmic domain of CD34 is required for its signal transduction of cellular adhesion. In constrast, we show that replacing the cytoplasmic domain of EPOR or G-CSFR with that of CD34 abolished growth signal transduction in response to EPO or G-CSF in the chimeric receptor-transfected BaF3, 32D, and FDCP1 cells, whereas the wild-type EPOR- or G-CSFR-transfected cells responded to EPO or G-CSF growth signaling well. These results suggest that the cytoplasmic portion of CD34 may not contain the elements necessary to transduce a proliferative signal in hematopoietic cells. Thus, the function of CD34 in hematopoiesis is primarily on hematopoietic cell adhesion.

The polyoma virus T antigen interferes with interferon-inducible gene expression

Murine polyoma virus (MPyV) is a small DNA virus that induces tumors in multiple tissues of infected host. In this investigation, we show that cell lines derived from wild type virus-induced breast tumors are resistant to the growth inhibitory action of interferon beta (IFN-beta). Furthermore, replication of heterologous viruses such as vesicular stomatitis virus and encephalomyocarditis virus was not inhibited by IFN-beta in these cells. This effect was due to inhibition of IFN-stimulated gene expression by viral T antigen. Activation of IFN-stimulated gene factor 3 was inhibited in cells derived from a tumor induced by wild-type MPyV but not those from a mutant that lacks the pRB binding site of the large T antigen. Similarly IFN-gamma-inducible gene expression was also inhibited in cells transformed by wild-type virus. The levels of components of IFN-stimulated gene factor 3 and signal transducing Janus tyrosine kinases were comparable between the cells transformed by the wild-type and mutant viruses. The viral large T antigen bound to Janus tyrosine kinase 1 and inactivated signaling through IFN receptors. Thus, these studies identify a mechanism of viral resistance to IFN action.

Association of Stat3-dependent transcriptional activation of p19INK4D with IL-6-induced growth arrest

Signal transducer and activator of transcription 3 (Stat3) is the major mediator of the IL-6-induced signals regulating growth and differentiation. In the M1 myeloleukemic cell line, Stat3 is a critical transcription factor causing repression of c-myc and c-myb genes, expression of junB and IRF1, growth arrest at G1, and subsequent macrophage differentiation. To understand the mechanisms by which Stat3 causes such effects, we searched for other Stat3-regulated genes possibly involved in growth arrest. We identified this inducible molecule as p19INK4D using a specific antibody. Both p19INK4D mRNA and protein were rapidly induced by IL-6 treatment without requiring de novo protein synthesis and the induction was fully suppressed by dominant-negative forms of Stat3. Thus both Stat3-regulated events, repressions of c-myc and c-myb and induction of p19INK4D, are likely to be involved in IL-6-induced growth arrest in M1 cells.

Formation of the early-region-2 transcription-factor-1-retinoblastoma-protein (E2F-1-RB) transrepressor and release of the retinoblastoma protein from nuclear complexes containing cyclin A is induced by interferon alpha in U937V cells but not in interferon-alpha-resistant U937VR cells

We have analysed the different regulation of cell-cycle-relevant proteins by interferon alpha (IFN alpha) in IFN alpha-sensitive and resistant U937 leukemic cell lines. In contrast to the INF alpha-sensitive U937 variant cell line U937V, the IFN alpha-resistant derivative (U937VR) is insensitive to the antiproliferative activity of IFN alpha. As we found no differences between these cell lines concerning the induction by IFN alpha of the pathway involving tyrosine-protein kinases and the signal transducer and activator of transcription (Jak-Stat), we examined whether cell-cycle-regulating proteins are differently affected by IFN alpha in U937VR and U937VR cells. In U937V cells IFN alpha induced the formation of the complex between early-region-2 transcription factor 1 (E2F-1) and retinoblastoma protein (RB) which is known to repress transcription of E2F-1-inducible genes, necessary for cell cycle progression. Formation of this complex was not inducible by IFN alpha in U937VR cells, although the suitable binding partners (E2F-1 and under-phosphorylated RB) were present. Interestingly, treatment of nuclear extracts from logarithmically growing U937V and U937VR cells with an antiserum against cyclin A that disrupts cyclin-A-containing complexes, led to the formation of the E2F-1-RB complex, suggesting the presence of under-phosphorylated (active) RB, trapped in nuclear complexes that contain cyclin A. This suggestion was supported by combined immunoprecipitation/western blot experiments that revealed a physical interaction between phosphorylated as well as under-phosphorylated forms of RB and cyclin A complex(es) in U937V and U937VR cells. RB, especially the under-phosphorylated form, was released by treatment with IFN alpha from this complex(es) in the case of U937V cells but not U937VR cells. We conclude that the missing induction of the E2F-1-RB transrepressor by IFN alpha and the failure to release RB from cyclin-A-containing complexes might contribute to the resistance of U937VR cells to the antiproliferative effects of IFN alpha.

The retinoblastoma protein: a master regulator of cell cycle, differentiation and apoptosis

The retinoblastoma susceptibility gene is a tumour suppressor and its product retinoblastoma protein (pRb) has been known for 10 years as a repressor of progression towards S phase. Its major activity was supposed to be sequestration or inactivation of the transcription factor E2F which is required for activation of S phase genes. However, within recent years growing evidence has been accumulating for a more general function of pRb at both the transcriptional level and the cellular level. pRb not only regulates the activity of certain protein-encoding genes but also the activity of RNA polymerase pol I and pol III transcription. This protein appears to be the major player in a regulatory circuit in the late G1 phase, the so-called restriction point. Moreover, it is involved in regulating an elusive switch point between cell cycle, differentiation and apoptosis. Here, it seems to cooperate with another major tumour suppressor, p53. Thus, pRb sits at the interface of the most important cell-regulatory processes and therefore deserves close attention by specialists from different fields of research. This review provides an introduction to the complex functions of pRb.