Cytokine triggered molecular pathways that control cell cycle arrest

An anti-cancer surveillance by the interplay between interferon-beta and retinoblastoma protein RB1

Interferon-beta (IFN-β), an extracellular cytokine that initiates signaling pathways for gene regulation, has been demonstrated to function as a tumor suppressor protein through lentiviral gene transduction. In this article, I review the relevant previous works and propose a cell cycle-based, tumor suppressor protein-mediated mechanism of anti-cancer surveillance. IFN-β induces a tumor cell cycle alteration that leads to S phase accumulation, senescence entry, and a loss of tumorigenicity in solid tumor cells. IFN-β does not show a significant cell cycle effect in their normal counterparts. Retinoblastoma protein RB1, another tumor suppressor protein, tightly controls the cell cycle and differentiation of normal cells, preventing them from being significantly impacted by the IFN-β effect. The interplay between IFN-β and RB1 acts as a mechanism of cell cycle-based, tumor suppressor protein-mediated anti-cancer surveillance that can selectively suppress solid tumor or proliferating transformed cells from the loss of control leading to cancer. This mechanism has important implications for the treatment of solid tumors.

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).

Inhibition of human cancer-cell proliferation by long double-stranded RNAs

Three different enzymatically synthesized long double-stranded RNAs (dsRNAs) [448 bp homologous to the third exon of c-myc messenger RNA (mRNA) (dsMyc); 473 bp homologous to enhanced green fluorescent protein (EGFP) mRNA (dsEGFP) and control interferon inducer poly(I:C)] were studied for antiproliferative and gene-silencing activities in KB-3-1, SK-N-MC, and IMR-32 human cancer cell lines. Simple incubation with these dsRNAs did not affect the expression of c-myc gene and the proliferation of KB-3-1 and IMR-32 cells, but inhibited the proliferation of SK-N-MC cells. Transfection of KB-3-1 and SK-N-MC cells using Oligofectamine-dsRNAs complexes resulted in dose-dependent inhibition of c-myc and beta-actin genes expression and proliferation. The data show that dsMyc, acting both as interferon inducer and as gene-specific interfering RNA, is more effective as c-myc inhibitor than other tested dsRNAs. The most efficient inhibition of proliferation was displayed by dsEGFP RNA, dsMyc and poly(I:C) were effective only when used in higher concentrations. Our data indicate that transfection of studied dsRNAs causes an increase in apoptotic and dead cells number in the cell population. This proapoptotic activity correlates with dsRNAs-induced antiproliferative activity. However the difference in cell growth between dsRNA-treated and Oligofectamine-only treated cells can not be attributed only to the loss of cells due to the apoptosis; it also indicates some retardation of cell cycle progression caused by dsRNA.

Identification and characterization of GRIM-1, a cell-death-associated gene product

Using a genome-wide technical knockout, we isolated a newly identified set of GRIM (genes associated with retinoid-interferon-induced mortality) genes; GRIM genes mediate IFN- and retinoic-acid (RA)-induced cell death. Here, we describe the isolation and characterization of one such gene, GRIM-1. Three proteins, with identical C-termini, were produced from the GRIM-1 open reading frame when this gene was transcribed and translated in vitro. These protein isoforms, designated GRIM-1alpha, GRIM-1beta and GRIM-1gamma, differentially suppressed growth via apoptosis in various cell lines. We also show that a caspase-dependent mechanism generates the proapoptotic GRIM-1 isoforms. Lastly, GRIM-1 isoforms differentially blocked maturation of 18S ribosomal RNA, consistent with their respective growth-suppressive ability. Together, these studies identified a novel protein involved in growth suppression and cell death.

Dynamic regulation of epithelial cell fate and barrier function by intercellular junctions

In the intestine, a single layer of epithelial cells effectively separates potentially harmful luminal content from the underlying tissue. The importance of an intact mucosal layer is highlighted by pathological disorders of the gut such as inflammatory bowel disease, in which disruption of the epithelial barrier leads to severe inflammation of the submucosal tissue compartments. Epithelial barrier function is provided by tightly regulated intercellular junctions, which consist of a plethora of membrane-associated and transmembrane proteins organized in discreet, spatially restricted complexes. Classically, these complexes are known to be dynamic seals for fluids and small molecules, as well as to provide mechanical strength by anchoring cell-cell contacts to the cytoskeleton. Rather than just acting as simple gates and adapters, however, junctional complexes themselves can relay extracellular stimuli to the epithelium and initiate cellular responses such as differentiation and apoptosis. In this review, we will highlight recent studies by our group and others which discuss how junctional proteins can promote outside-to-inside signaling and modulate epithelial cell fate. Unraveling the complex crosstalk between epithelial cells and their intercellular junctions is essential to understanding how epithelial barrier function is maintained in vivo and might provide new strategies for the treatment of inflammatory disorders of the intestine.

Evaluation of newly developed combination therapy of intra-arterial 5-fluorouracil and systemic pegylated interferon alpha-2b for advanced hepatocellular carcinoma with portal venous invasion: preliminary results

AIM

Prognosis is extremely poor for advanced hepatocellular carcinoma (HCC) in patients with portal invasion. The present study evaluated the efficacy of combined intra-arterial 5-fluorouracil (5-FU) and systemic pegylated interferon (PEG-IFN)alpha-2b in patients with advanced HCC.

METHODS

The subjects comprised nine HCC patients with portal vein thrombosis treated using subcutaneous administration of PEG-IFNalpha-2b (50-100 microg on day 1 of every week, for 4 weeks) and intra-arterial infusion of 5-FU (250 mg/day for 5 h on days 1-5 of every week, for 4 weeks). For four patients with hepatitis C virus (HCV) infection, oral administration of ribavirin (400-800 mg/day) was added. At the end of every cycle, response to therapy was assessed using Response Evaluation Criteria in Solid Tumors (RECIST) guidelines.

RESULTS

Partial response (PR) was observed in seven of nine patients, with stable or progressive disease in the remaining two patients. Tumors were resectable in three patients displaying PR after treatment. Tumor markers decreased significantly after therapy. Serum HCV-RNA titers were markedly decreased and became undetectable in all patients with HCV infection. National Cancer Institute-Common Toxicity Criteria: version 3.0 (NCI-CTC) grade 3 thrombocytopenia was seen in one case at the end of treatment, but was resolved with cessation of treatment. Other adverse effects were manageable.

CONCLUSION

Combination therapy with intra-arterial 5-FU and systemic PEG-IFNalpha-2b may be useful as a palliative treatment for patients with advanced HCC. A prospective controlled trial using a larger population of patients with advanced HCC is needed to evaluate this new combination therapy.

Interferon-alpha and 5-fluorouracil combination therapy after palliative hepatic resection in patients with advanced hepatocellular carcinoma, portal venous tumor thrombus in the major trunk, and multiple nodules

BACKGROUND

The authors reported previously the beneficial effects of interferon (IFN)-alpha/5-fluorouracil (5-FU) combination therapy for patients with advanced hepatocellular carcinoma (HCC) who have tumor thrombi in the major portal branches. In this report, the authors describe the results from IFN/5-FU chemotherapy for patients who underwent palliative hepatic resection for advanced HCC with tumor thrombus in the main trunk of the portal vein and multiple nodules in the whole liver. In addition, they evaluated the correlation between the response to such therapy and expression of IFN-alpha type 2 receptor (IFNAR2).

METHODS

From October 1999 to December 2004, 30 patients with advanced HCC, tumor thrombi in the main trunk of the portal vein, and multiple nodules in the whole liver (Vp4 and grade 3 intrahepatic metastases) were recruited for this study. They underwent palliative hepatic resection followed by at least 2 courses of IFN/5-FU. IFNAR2 expression levels were determined by immunohistochemistry.

RESULTS

No major treatment-related complications were noted. An objective response was noted in 10 patients (33.3%) and included a complete response in 6 patients (20%), a partial response in 4 patients (13.3%), no response in 1 patient (3.3%), and progressive disease in 19 patients (63.4%). IFNAR2 expression was detected in 20 of 30 patients (66.7%). There was a significant difference in overall survival between patients with positive and negative IFNAR2 expression cases (P<.0025), and a significant correlation was observed between IFNAR2 expression and response to IFN/5-FU combination therapy (P=.0199).

CONCLUSIONS

Adjunct IFN/5-FU therapy is a promising modality for patients with advanced HCC, tumor thrombi in the major trunk, and multiple nodules after palliative hepatic resection. The results from this study indicated that the response to such therapy seemed to be correlated with IFNAR2 expression.

Similar effects of recombinant interferon-alpha-2b and natural interferon-alpha when combined with intra-arterial 5-fluorouracil for the treatment of advanced hepatocellular carcinoma

AIM

Intra-arterial 5-fluorouracil (5-FU) plus interferon (IFN) combination therapy is effective against advanced hepatocellular carcinoma (HCC) with portal vein tumour thrombosis. In this study, we compared the efficiency and safety of recombinant IFN-alpha-2b with natural IFN-alpha as components of the combination therapy.

METHODS

Consecutive HCC patients (n=31) with portal vein tumour thrombosis were enrolled in this prospective study. They received combination therapy of 5-FU and either recombinant IFN-alpha-2b (R group, n=15) or natural IFN-alpha (N group, n=16). We compared the two groups for the early response rate, adverse reactions, time to progression (TTP) and survival rates. In addition, we assessed the cost-effectiveness of each protocol.

RESULTS

The early response rate (R: 26.7%, N: 31.2%), median TTP (R: 5.8 months, N: 5.6 months) and median survival time (R: 7.5 months, N: 6.5 months) were not significantly different between the R and N groups. There were no differences in adverse reactions between the two groups. The estimated cost-effectiveness ratio of recombinant IFN-alpha-2b was better than natural IFN-alpha.

CONCLUSIONS

In our protocol of combination therapy, there were no significant differences between recombinant IFN-alpha-2b and natural IFN-alpha with regard to early response to therapy, adverse effects, TTP and survival rates. 5-FU could be combined with either recombinant IFN-alpha-2b or natural IFN-alpha, although the cost-effectiveness of the former warrants its use clinically.

Interferon-induced sensitization to apoptosis is associated with repressed transcriptional activity of the hTERT promoter in multiple myeloma

The aim of the present study was to explore hTERT as a target for IFN-induced sensitization to apoptosis in multiple myeloma (MM). IFN-alpha and IFN-gamma downregulated telomerase activity in the IL-6-dependent MM cell line U-266-1970. In MM cells undergoing IFN-induced sensitization to Fas-mediated apoptosis, the repression of telomerase was increased as compared to IFN-alpha treatment alone. Similar to the sensitization effect of IFN, the use of a dominant negative IkappaBalpha vector inhibiting hTERT activity via transcriptional targeting resulted in augmentation of Fas-mediated apoptosis. The mechanism underlying the reduction of telomerase activity by IFN was shown to be transcriptional repression of the hTERT gene. The present study does not support a direct effect of IFN on NF-kappaB binding to the hTERT promoter as underlying the transcriptional repression. We conclude that one potential mechanism whereby IFNs induce apoptosis sensitization is by repressing hTERT transcription and telomerase activity, thereby constituting attractive targets for MM therapy.

Treatment of hepatocellular carcinoma with major portal vein thrombosis by combined therapy with subcutaneous interferon-alpha and intra-arterial 5-fluorouracil; role of type 1 interferon receptor expression

We previously reported the beneficial effects of combination therapy of interferon (IFN)-α/5-fluorouracil (FU) for advanced hepatocellular carcinoma (HCC) with tumour thrombi in the major portal branches. This report describes the results of longer follow-up and includes more than double the number of patients relative to the original report, and evaluates the role of IFN-α/type 2 interferon receptor (IFNAR2) expression on the response to the combination therapy. The study subjects were 55 patients with advanced HCC and tumour thrombi in the major branches of the portal vein (Vp3 or 4). They were treated with at least two courses of IFN-α/5-FU without major complication. In the 55 patients, 24 (43.6%) showed objective response (eight (14.5%) showed complete response, 16 (29.1%) partial response), four (7.3%) showed no response, and 27 (49.1%) showed progressive disease. Immunohistochemically, IFNAR2 expression was detected in nine out of 13 (69.2%) patients. There was significant difference in the time-to-progression survival (P=0.0002) and the overall survival (P<0.0001) between IFNAR2-positive and -negative cases. There was a significant correlation between IFNAR2 expression and response to IFN-α/5-FU combination therapy in univariate analysis (P=0.0070). IFN-α/5-FU combination therapy is a promising modality for advanced HCC with tumour thrombi in the major portal branches and could significantly depend on IFNAR2 expression.

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.

Gene expression profiles in human mesothelioma cell lines in response to interferon-γ treatment

Interferon-gamma (IFN-gamma) has been found to be antiproliferative and antitumoral in malignant mesothelioma (MM), but the MM cell response to IFN-gamma has not been fully characterized so far. We investigated gene expression profiles in human MM cell lines (HMCLs) exposed to IFN-gamma. Four HMCLs showing different sensitivities to the antiproliferative effect of IFN-gamma, two of them presenting a defect in the JAK/STAT signaling pathway, were treated with 500 IU/mL of IFN-gamma. Gene expression patterns were studied at 6 and 72 hours after exposure to the IFN-gamma, using a cDNA array technique. Six genes were studied with real-time reverse transcription-polymerase chain reaction. The gene expression profiles in response to IFN-gamma were shown to differ in different HMCLs. Numerous genes involved in cell proliferation and cell adhesion were regulated by IFN-gamma in JAK/STAT-deficient HMCLs. Genes possibly involved in regulation of cell proliferation included CDC2, PLK1, and IGFBP4, as well as several genes involved in cell-cell interactions and cell adhesion. The cDNA array technique revealed differences in expression pathways, especially those involved in cell growth, cell adhesion, and cell proliferation, between IFN-gamma-resistant and -sensitive MM cell lines. We found that the changes in gene expression profiles of HMCLs exposed to IFN-gamma were also related to features other than the antiproliferative response.

Alternate interferon signaling pathways

More than a half a century ago, interferons (IFN) were identified as antiviral cytokines. Since that discovery, IFN have been in the forefront of basic and clinical cytokine research. The pleiotropic nature of these cytokines continues to engage a large number of investigators to define their actions further. IFN paved the way for discovery of Janus tyrosine kinase (JAK)-signal transducing activators of transcription (STAT) pathways. A number of important tumor suppressive pathways are controlled by IFN. Several infectious pathogens counteract IFN-induced signaling pathways. Recent studies indicate that IFN activate several new protein kinases, including the MAP kinase family, and downstream transcription factors. This review not only details the established IFN signaling paradigms but also provides insights into emerging alternate signaling pathways and mechanisms of pathogen-induced signaling interference.

Alcohol, injury, and cellular immunity

It is widely accepted that alcohol exposure is a causative factor in the occurrence of burn or other traumatic injury. It is less well known that individuals who have consumed alcohol before sustaining an injury suffer from increased morbidity and mortality compared with the morbidity and mortality of non-alcohol-consuming subjects with similar injuries. Complications due to bacterial infection are the most common burn sequelae in injured patients and are frequently associated with depressed immunity. Independently, alcohol exposure and injury have been shown to influence cellular immunity negatively. These changes in immunity are closely linked to injury- or alcohol-induced alterations in the cytokine milieu in both clinical studies and animal models. Not surprisingly, the combination of insult of alcohol exposure and burn injury results in immune suppression that is greater in magnitude and duration compared with either insult alone. The combined effects of alcohol and injury on immunity have been examined in a limited number of studies. However, results of these studies support the suggestion that altered cytokine production is an integral part of the immune dysregulation and increased mortality that is observed. In particular, the increased presence of macrophage-derived mediators observed after burn or alcohol exposure alone seems to be synergistically increased in a combined injury model. Although more research is needed, it is likely that therapeutic modalities that include manipulation of cytokine networks to boost cellular immunity may improve outcome for patients who sustain injuries subsequent to consuming alcohol.

Direct evidence that IFN-beta functions as a tumor-suppressor protein

Interferon-beta (IFN-beta) induces aberrant cell cycle progression as well as cytotoxicity and apoptosis. However, the relationship between the cell cycle alteration and the induction of cytotoxicity/apoptosis is unknown. Here, we report the first demonstration that the IFN-beta-induced direct cytotoxic/apoptotic effect can be separated functionally from its cell cycle effect. By using lentiviral transduction, we generated human tumor cells that stably expressed IFN-beta and were resistant to its direct cytotoxic/apoptotic effect. Despite this resistance to apoptosis, these cells showed significant S phase accumulation as measured by both FACS analyses and bromodeoxyuridine (BrdU) incorporation. Although the cells proliferated in the presence of high levels of IFN-beta, they had lost their tumorigenicity in mice. A portion of these cells was observed to undergo a tumor cell-specific senescence. Therefore, our study revealed a direct tumor-suppressor function of IFN-beta. This tumor-suppressor function was independent of IFN-beta-induced direct cytotoxic effect. It was also distinct from the IFN-beta-induced immunologic antitumor response, an indirect effect of IFN-beta. We conclude that the antiproliferative effect on human tumor cells is the collective activities of the direct cytotoxic/apoptotic effect, the cell cycle alteration that occurs as predominantly S phase accumulation, and less frequently other cell cycle effects, such as G(1) arrest, and the promotion of tumor cells into a senescent-like state.

Review: IFN-alpha/beta receptor interactions to biologic outcomes: understanding the circuitry

Type I interferons (IFNs), which include the IFN-alphas, IFN-beta, IFN-omega, IFN-kappa, and IFN-tau, are an evolutionarily conserved group of secreted cytokines that serve as potent extracellular mediators of host defense and homeostasis. Binding of IFNs to specific cell surface receptors results in the activation of multiple intracellular signaling cascades, leadingto the synthesis of proteins that mediate antiviral, growth inhibitory and immunomodulatory responses. In the past decade, considerable information has accumulated pertaining to the different signalingpathways that are activated by the type I IFNs. Although many of the literature findings are specific to defined cell systems or are tissue restricted, the intent of this review is to place these signaling cascades and their effectors in the context of distinct biologic outcomes.

Cytotoxicity of combinations of IFN-beta and chemotherapeutic drugs

Interferon-beta (IFN-beta) induces various antiproliferative activities. In solid tumor cells, IFN-beta inhibits cell cycle progression, which mainly occurs as S phase accumulation. The IFN-beta-induced cell cycle effect has been implicated in the antitumor effect of combinations of IFN-beta and chemotherapeutic drugs. In this report, we characterized the viability of various human tumor cells in vitro after combination treatment with IFN-beta protein and the chemotherapeutic drugs, cis-platinum (II) diamine dichloride (cisplatin), 5-fluorouracil (5-FU), paclitaxel (Taxol) and gemcitabine. IFN-beta could significantly potentiate the cytotoxicity of these chemotherapeutic drugs. The potentiating effect was observed after pretreatment of tumor cells with IFN-beta but did not require the constant presence of IFN-beta. The potentiating effect correlated with the sensitivity of the tumor cells to the IFN-beta-induced cytotoxicity. Furthermore, chemotherapeutic drugs also potentiated the cytotoxicity of IFN-beta. We conclude that the cell cycle effect per se did not determine the ability of IFN-beta to potentiate the cytotoxicity of chemotherapeutic drugs. We suggest that the combination of local IFN-beta gene therapy with chemotherapy could be an effective cancer treatment.

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.

ERK1 and ERK2 activate CCAAAT/enhancer-binding protein-beta-dependent gene transcription in response to interferon-gamma

Interferons (IFNs) regulate the expression of a number of cellular genes by activating the JAK-STAT pathway. We have recently discovered that CCAAAT/enhancer-binding protein-beta (C/EBP-beta) induces gene transcription through a novel IFN response element called the gamma-IFN-activated transcriptional element (Roy, S. K., Wachira, S. J., Weihua, X., Hu, J., and Kalvakolanu, D. V. (2000) J. Biol. Chem. 275, 12626-12632. Here, we describe a new IFN-gamma-stimulated pathway that operates C/EBP-beta-regulated gene expression independent of JAK1. We show that ERKs are activated by IFN-gamma to stimulate C/EBP-beta-dependent expression. Sustained ERK activation directly correlated with C/EBP-beta-dependent gene expression in response to IFN-gamma. Mutant MKK1, its inhibitors, and mutant ERK suppressed IFN-gamma-stimulated gene induction through the gamma-IFN-activated transcriptional element. Ras and Raf activation was not required for this process. Furthermore, Raf-1 phosphorylation negatively correlated with its activity. Interestingly, C/EBP-beta-induced gene expression required STAT1, but not JAK1. A C/EBP-beta mutant lacking the ERK phosphorylation site failed to promote IFN-stimulated gene expression. Thus, our data link C/EBP-beta to IFN-gamma signaling through ERKs.

Regulation of c-myc expression by IFN-gamma through Stat1-dependent and -independent pathways

Interferons (IFNs) inhibit cell growth in a Stat1-dependent fashion that involves regulation of c-myc expression. IFN-gamma suppresses c-myc in wild-type mouse embryo fibroblasts, but not in Stat1-null cells, where IFNs induce c-myc mRNA rapidly and transiently, thus revealing a novel signaling pathway. Both tyrosine and serine phosphorylation of Stat1 are required for suppression. Induced expression of c-myc is likely to contribute to the proliferation of Stat1-null cells in response to IFNs. IFNs also suppress platelet-derived growth factor (PDGF)-induced c-myc expression in wild-type but not in Stat1-null cells. A gamma-activated sequence element in the promoter is necessary but not sufficient to suppress c-myc expression in wild-type cells. In PKR-null cells, the phosphorylation of Stat1 on Ser727 and transactivation are both defective, and c-myc mRNA is induced, not suppressed, in response to IFN-gamma. A role for Raf-1 in the Stat1-independent pathway is revealed by studies with geldanamycin, an HSP90-specific inhibitor, and by expression of a mutant of p50(cdc37) that is unable to recruit HSP90 to the Raf-1 complex. Both agents abrogated the IFN-gamma-dependent induction of c-myc expression in Stat1-null cells.

Basic fibroblast growth factor stimulates cytosolic phospholipase A2, phospholipase C-gamma1 and phospholipase D through distinguishable signaling mechanisms

Fibroblast growth factors (FGFs) stimulate proliferation, differentiation and motility of different cell types. The cellular effects of FGF are transduced by its interaction with any one of four members of a family of high affinity, cell surface FGF receptors (FGFRs) that have autophosphorylating tyrosine kinase activity. Activation of FGFR causes release of various low molecular weight signaling molecules which are required for the pleotropic effects of FGFs. We report here that basic FGF plays critical role in membrane phospholipid hydrolysis in NIH 3T3 cells that are stably transfected with FGFR1. Upon binding to FGFR1, basic FGF stimulates cytosolic form of phospholipase A2 (cPLA2), phospholipase C-gamma1 (PLC-gamma1) and phospholipase D (PLD), the key enzymes for the production of various lipid second messengers, in a tyrosine kinase-dependent manner. In addition to tyrosine phosphorylation, cPLA2 catalytic activation requires serine phosphorylation by p42 mitogen-activated protein (MAP) kinase and possibly pertussis toxin-sensitive G-protein coupling. On the other hand, phosphatidyl inositol 4,5 bisphosphate (PIP2) hydrolysis requires direct phosphorylation at tyrosine residue of the PLC-gamma1 isozyme. The activation of PLD needs direct or indirect receptor tyrosine kinase and protein kinase C (PKC) activities. Additionally, it also requires botulinum toxin C-sensitive Rho-like G-protein activation. All these results suggest that the pleotropic effects of FGF are exerted through its tyrosine kinase receptors and individual effectors are activated via distinguishable signaling mechanisms according to the cell's need.

Molecular mechanisms underlying interferon-alpha-induced G0/G1 arrest: CKI-mediated regulation of G1 Cdk-complexes and activation of pocket proteins

One prominent effect of IFNs is their cell growth-inhibitory activity. The mechanism behind this inhibition of proliferation is still not fully understood. In this study, the effect of IFN-alpha treatment on cell cycle progression has been analysed in three lymphoid cell lines, Daudi, U-266 and H9. Examination of the growth-arrested cell populations shows that Daudi cells accumulate in a G0-like state, whereas U-266 cells arrest later in G1. H9 cells are completely resistant to IFN-alpha's cell growth-inhibitory effects. The G0/G1-phase arrest is preceded by a rapid induction of the cyclin-dependent kinase inhibitors (CKIs), p21 and p15. In parallel, the activities of the G1 Cdks are significantly reduced. In addition to p21/p15 induction, IFN-alpha regulates the expression of another CKI, p27, presumably by a post-transcriptional mechanism. In the G1 Cdk-complexes, there is first an increased binding of p21 and p15 to their respective kinases. At longer exposure times, when Cdk-bound p15 and p21 decline, p27 starts to accumulate. Furthermore, we found that IFN-alpha not only suppresses the phosphorylation of pRb, but also alters the phosphorylation and expression of the other pocket proteins p130 and p107. These data suggest that induction of p21/p15 is involved in the primary IFN-alpha response inhibiting G1 Cdk activity, whereas increased p27 expression is part of a second set of events which keep these Cdks in their inactive form. Moreover, elevated levels of p27 correlated with a dissociation of cyclin E/Cdk2-p130 or p107 complexes to yield cyclin E/Cdk2-p27 complexes. In resistant H9 cells, which possess a homozygous deletion of the p15/p16 genes and lack p21 protein expression, IFN-alpha causes no detectable changes in p27 expression and, furthermore, no effects are observed on either pocket proteins in this cell line. Taken together, these data suggest that the early decline in G1 Cdk activity, subsequent changes in phosphorylation of pocket proteins, and G1/G0 arrest following IFN-alpha treatment, is not primarily due to loss of the G1 kinase components, but result from the inhibitory action of CKIs on these complexes.

Novel biologic approaches to hematologic malignancies

Biologic agents have made a major impact on the treatment of hematologic malignancies and will continue to play a major role as we better understand their function in normal and malignant cell regulation. The examples provided in this chapter are a brief introduction to the potential of these agents. Many have yet to be used in conjunction with current cytotoxic therapy in these diseases, and perhaps combinations will prove even more successful. This possibility provides substantial ground for further investigation and therapy.

Human cytomegalovirus UL69 protein induces cells to accumulate in G1 phase of the cell cycle

Earlier studies have revealed that human cytomegalovirus rapidly inhibits the growth of fibroblasts, blocking cell cycle progression at multiple points, including the G1-to-S-phase transition. The present study demonstrates that the UL69 protein, a virus-encoded constituent of the virion, is able to arrest cell cycle progression when introduced into uninfected cells. Expression of the UL69 protein causes U2 OS cells and primary human fibroblasts to accumulate within the G1 compartment of the cell cycle, and serum fails to induce the progression of quiescent human fibroblasts into the S phase when the protein is present. Therefore, the UL69 protein is at least partially responsible for the cell cycle block that is instituted after infection of permissive cells with human cytomegalovirus.

Interferon and malignant disease--how does it work and why doesn't it always?

Since their first use at the clinic almost 30 years ago. interferons (IFNS) have become an accepted therapy in a range of malignancies. Although IFN All induce remissions in some patients, they are of no benefit, or at best, lead only to minor improvements in the great majority of patients. This review considers possible mechanisms underlying the antitumour effects of IFN, and discusses possible reasons for resistance to IFN therapy in patients with malignant disease.

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.

The promyelocytic leukemia zinc finger protein affects myeloid cell growth, differentiation, and apoptosis

The promyelocytic leukemia zinc finger (PLZF) gene, which is disrupted in therapy-resistant, t(11;17)(q23;q21)-associated acute promyelocytic leukemia (APL), is expressed in immature hematopoietic cells and is down-regulated during differentiation. To determine the role of PLZF in myeloid development, we engineered expression of PLZF in murine 32Dcl3 cells. Expression of PLZF had a dramatic growth-suppressive effect accompanied by accumulation of cells in the G0/G1 compartment of the cell cycle and an increased incidence of apoptosis. PLZF-expressing pools also secreted a growth-inhibitory factor, which could explain the severe growth suppression of PLZF-expressing pools that occurred despite the fact that only half of the cells expressed high levels of PLZF. PLZF overexpression inhibited myeloid differentiation of 32Dcl3 cells in response to granulocyte and granulocyte-macrophage colony-stimulating factors. Furthermore, cells that expressed PLZF appeared immature as demonstrated by morphology, increased expression of Sca-1, and decreased expression of Gr-1. These findings suggest that PLZF is an important regulator of cell growth, death, and differentiation. Disruption of PLZF function associated with t(11;17) may be a critical event leading to APL.

G1/S regulatory mechanisms from yeast to man

Cyclin-dependent kinases play a key role in promoting and regulating the transition from G1 to S phase in all eukaryotic organisms. The kinase activities involved are distinguished from those participating in other cell cycle phase transitions in that they are driven by a class of specialised G1-specific cyclins. Although the G1 regulatory components have diverged structurally in the course of evolution, the regulatory mechanisms and principles remain highly conserved from yeast to vertebrates. An important issue that remains is that of identifying the principal targets phosphorylated by G1 cyclin-dependent kinases.

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.

How does interferon exert its cell growth inhibitory effect?

The interferons (IFNs) have become accepted therapy in a range of haematological and non-haematological malignancies. The mechanism behind IFN's antitumour action is, however, unclear. Interferons (IFNs) are capable of modulating a variety of cellular responses. One prominent effect of IFNs is their cell growth inhibitory activity, which has also been suggested to be of major importance in their antitumour action. In the present review we will discuss the cellular events leading to a decreased number of cells following IFN treatment, the molecular mechanisms underlying these phenomena, and the importance of these effects in a clinical perspective.

Activation of the STAT signaling pathway can cause expression of caspase 1 and apoptosis

Protein tyrosine kinases activate the STAT (signal transducer and activator of transcription) signaling pathway, which can play essential roles in cell differentiation, cell cycle control, and development. However, the potential role of the STAT signaling pathway in the induction of apoptosis remains unexplored. Here we show that gamma interferon (IFN-gamma) activated STAT1 and induced apoptosis in both A431 and HeLa cells, whereas epidermal growth factor (EGF) activated STAT proteins and induced apoptosis in A431 but not in HeLa cells. EGF receptor autophosphorylation and mitogen-activated protein kinase activation in response to EGF were similar in both cell lines. The breast cancer cell line MDA-MB-468 exhibited a similar response to A431 cells, i.e., STAT activation and apoptosis correlatively resulted from EGF or IFN-gamma treatment. In addition, in a mutant A431 cell line in which STAT activation was abolished, no apoptosis was induced by either EGF or IFN-gamma. We further demonstrated that both EGF and IFN-gamma induced caspase 1 (interleukin-1beta converting enzyme [ICE]) gene expression in a STAT-dependent manner. IFN-gamma was unable to induce ICE gene expression and apoptosis in either JAK1-deficient HeLa cells (E2A4) or STAT1-deficient cells (U3A). However, ICE gene expression and apoptosis were induced by IFN-gamma in U3A cells into which STAT1 had been reintroduced. Moreover, both EGF-induced apoptosis and IFN-gamma-induced apoptosis were effectively blocked by Z-Val-Ala-Asp-fluoromethylketone (ZVAD) in all the cells tested, and studies from ICE-deficient cells indicated that ICE gene expression was necessary for IFN-gamma-induced apoptosis. We conclude that activation of the STAT signaling pathway can induce apoptosis through the induction of ICE gene expression.

DAP-5, a novel homolog of eukaryotic translation initiation factor 4G isolated as a putative modulator of gamma interferon-induced programmed cell death

A functional approach to gene cloning was applied to HeLa cells in an attempt to isolate cDNA fragments which convey resistance to gamma interferon (IFN-gamma)-induced programmed cell death. One of the rescued cDNAs, described in this work, was a fragment of a novel gene, named DAP-5. Analysis of a DAP-5 full-length cDNA clone revealed that it codes for a 97-kDa protein that is highly homologous to eukaryotic translation initiation factor 4G (eIF4G, also known as p220). According to its deduced amino acid sequence, this novel protein lacks the N-terminal region of eIF4G responsible for association with the cap binding protein eIF4E. The N-terminal part of DAP-5 has 39% identity and 63% similarity to the central region of mammalian p220. Its C-terminal part is less homologous to the corresponding region of p220, suggesting that it may possess unique functional properties. The rescued DAP-5 cDNA fragment which conveyed resistance to IFN-gamma-induced cell death was expressed from the vector in the sense orientation. Intriguingly, it comprised part of the coding region which corresponds to the less conserved C-terminal part of DAP-5 and directed the synthesis of a 28-kDa miniprotein. The miniprotein exerted a dual effect on HeLa cells. Low levels of expression protected the cells from IFN-gamma-induced programmed cell death, while high levels of expression were not compatible with continuous cell growth. The relevance of DAP-5 protein to possible changes in a cell's translational machinery during programmed cell death and growth arrest is discussed.

Double-stranded RNA-dependent protein kinase mediates c-Myc suppression induced by type I interferons

The antiproliferative functions of interferons result from specific effects that these cytokines exert on several cell cycle-controlling genes. The possible coupling between the interferon-responsive genes that are directly transactivated by the interferon signaling and the genes that constitute the basic machinery of the cell cycle is not clear yet. We report in this work that interferon-induced double-stranded RNA-activated kinase (PKR) is one of the specific mediators of the antiproliferative effects of the cytokine. Transfections of M1 myeloid leukemia cells with two catalytically inactive mutant forms of PKR abrogated the ability of interferon to suppress c-Myc without interfering with the pRB/cyclin D responses. As a consequence, these genetically manipulated cells displayed a small but significant reduction in their growth sensitivity to interferons, a phenotype that characterizes a single pathway disruption. Transfection of the parental M1 cells with the functional wild-type human PKR restricted their proliferation in the absence of interferons. This PKR-mediated growth inhibition could be efficiently rescued by the ectopic expression of deregulated c-myc. Taken together these results prove the existence of direct or indirect links between PKR and c-Myc suppression, thereby placing this gene along one of the complementary growth suppressive pathways that are triggered by interferons.

Enhancement of antiproliferative activity of gamma interferon by the specific inhibition of tyrosine dephosphorylation of Stat1

Gamma interferon (IFN-gamma) signals to the nucleus through the activation, by tyrosine phosphorylation, of the latent cytoplasmic transcription factor Stat1 (signal transducer and activator of transcription). It has been demonstrated that the activity of Stat1 is dependent on tyrosine phosphorylation which is regulated by Jak tyrosine kinases as well as by the as-yet-unidentified protein tyrosine phosphatase. We report that the N-terminal domain of Stat1, which is highly conserved among all STAT family members, is required for its tyrosine dephosphorylation. A single amino acid substitution (Arg-31 to Ala) in the Stat1 N-terminal domain inhibited Stat1 tyrosine dephosphorylation. The deletion of the Stat1 N-terminal domain resulted in a mutant Stat1 protein which was constitutively phosphorylated on Tyr-701. Upon IFN-gamma stimulation, the tyrosine phosphorylation of this mutant protein was further enhanced but was not down-regulated by protein tyrosine phosphatase in vivo. When expressed in NIH 3T3 cells, this mutant protein greatly enhanced the antiproliferative activity of IFN-gamma. We suggest that the N-terminal domains of STATs are crucial for modulating STAT activities through regulating the tyrosine dephosphorylation of STATs.

Interferons and the tumor cell

Optimal use of interferons (IFNs) for the treatment of tumor disease requires experimental work in order to precisely define IFN actions. We have pointed out three modes of such actions relevant for the antitumor efficacy exerted by IFNs: effects on apoptosis, effects on genes involved in malignant transformation and effects on angiogenesis. These are but three selected areas forming a basis for the development of optimal IFN therapy. Further experimental work, undertaken in these and additional IFN areas, is mandatory for the most effective clinical use of IFNs for the treatment of tumor disease.

Transcriptionally active Stat1 is required for the antiproliferative effects of both interferon alpha and interferon gamma

Type I (alpha, beta) and type II (gamma) interferons (IFNs) can restrict the growth of many cell types. INF-stimulated gene transcription, a key early event in IFN response, acts through the Janus kinase-signal transducers and activators of transcription pathway, in which both IFN-alpha and IFN-gamma activate the transcription factor Stat1. A cell line lacking Stat1 (U3A) was not growth-arrested by IFN-alpha or IFN-gamma, and experiments were carried out with U3A cells permanently expressing normal or various mutant forms of Stat1 protein. Only cells in which complete Stat1 activity was available (Stat1alpha) were growth-inhibited by IFN-gamma. A mutant that supports 20-30% normal transcription did not cause growth restraint. In contrast, IFN-alpha growth restraint was imposed by cells producing Stat1beta, which lacks transcriptional activation potential. This parallels earlier results showing the truncated Stat1 can function in IFN-alpha gene activation. In addition to experiments on long-term cultured cells, we also found that wild-type primary mouse embryonic fibroblasts were inhibited by IFNs, but fibroblasts from Stat1-deficient mouse embryos were not inhibited by IFNs.

Alpha interferon suppresses the cyclin D3 and cdc25A genes, leading to a reversible G0-like arrest

Alpha interferon is a potent growth inhibitor of Daudi Burkitt's lymphoma cells. We show here that alpha-interferon signaling interacted simultaneously with several components of the basic cell cycle machinery, causing cells to enter into a state that had many features characteristic of the G0 state. Within a few hours after alpha-interferon treatment, cyclin D3 mRNA and protein levels dropped to undetectable levels and, in parallel, the activities of cyclin A- and cyclin E-associated kinases were significantly reduced. The latter resulted from the rapid alpha-interferon-mediated elimination of cdc25A, a phosphatase that is required for antagonism of negative tyrosine phosphorylation of cdk2 in cyclin-cdk complexes. This regulation represents a novel mechanism through which an external inhibitory cytokine interacts with the cell cycle machinery. At later time points after alpha-interferon treatment, the levels of the 55-kDa slowly migrating hyperphosphorylated form of cyclin E and of cyclin A were also reduced. The antiproliferative effects were reversible, and cultures from which alpha interferon was removed reentered S phase after a lag that typically corresponded to approximately two doubling times. During this lag period, the expression of cyclin D3 and cyclin A, as well as of the cdc25A phosphatase, continued to be switched off, in spite of the removal of alpha interferon from the cell surface. In contrast, c-myc, which represents another downstream target gene that is subjected to negative regulation by alpha interferon, was relieved from suppression much earlier, concomitant with the decay in early signaling of the cytokine. The delayed pattern of cyclin reexpression provides evidence that alpha-interferon signaling imposes a G0-like state on this system.

Antiproliferative and c-myc mRNA suppressive effect of tranilast on newborn human vascular smooth muscle cells in culture

1. Newborn human vascular smooth muscle cells (VSMCs) proliferated faster and were more sensitive to platelet-derived growth factor-BB (PDGF-BB) than those from adults. In this study, we investigated mechanism of the inhibitory effect of tranilast on PDGF-BB-induced proliferation of VSMCs from newborns. 2. Tranilast (30-300 microM) concentration-dependently inhibited the VSMC proliferation in randomly growing cultures stimulated with PDGF-BB. 3. Tranilast (30-300 microM) concentration-dependently inhibited the [3H]-thymidine incorporation into DNA in VSMCs that had been synchronized by 48 h serum depletion and then stimulated by addition of PDGF-BB. However, tranilast had little influence on unscheduled DNA synthesis in quiescent cells or on RNA and protein synthesis, unlike aphidicolin, actimomycin D, and cycloheximide. 4. In synchronized VSMC cultures, tranilast still inhibited the PDGF-BB-induced DNA synthesis even when added 18 h after stimulation of the quiescent cells. The mode of the antiproliferative action of tranilast was different from that of NiCl2, genistein, or staurosporin. In addition, flow cytometry of synchronized VSMCs treated with tranilast revealed a blockade of PDGF-inducible cell-cycle progression at the G1/S checkpoint. 5. Northern blotting showed that tranilast (30-300 microM) concentration-dependently suppressed constitutive c-myc mRNA expression even when added 18 h after PDGF-BB-stimulation of quiescent VSMCs. Tranilast still had an inhibitory effect on the induction of c-myc mRNA when de novo protein synthesis was inhibited by cycloheximide and did not shorten the degradation of c-myc mRNA at the post-transcriptional level, demonstrating that tranilast directly inhibited c-myc mRNA expression at the transcriptional level. 6. These results suggest that the inhibitory effect of tranilast on PDGF-BB-induced proliferation is due to S-phase blockade and may be, at least in part, involved in the direct suppression of c-myc gene expression. Tranilast did not cause cell toxicity and may therefore hold promising potential for the prevention of vascular proliferative diseases.

Why do so many cancer patients fail to respond to interferon therapy?

Since their first use in the clinic some 25 years ago, interferons (IFNs) have become accepted therapy in a range of cancer forms. However, although in some patients they induce remission, in the great majority they are of no benefit or, at best, lead only to minor improvements. This review considers possible reasons for these failures.

Isolation of DAP3, a novel mediator of interferon-gamma-induced cell death

Interaction of certain cytokines with their corresponding cell-surface receptors induces programmed cell death. Interferon-gamma induces in HeLa cells a type of cell death with features characteristic of programmed cell death. Here, we report the isolation of a novel gene, DAP3 (death-associated protein-3), involved in mediating interferon-gamma-induced cell death. The rescue of this gene was performed by a functional selection approach of gene cloning that is based on transfection with an antisense cDNA expression library. The antisense RNA-mediated inactivation of the DAP3 gene protected the cells from interferon-gamma-induced cell death. This property endowed the cells expressing it with a growth advantage in an environment restrictive due to the continuous presence of interferon-gamma and thus provided the basis of its selection. The gene is transcribed into a single 1.7-kilobase mRNA, which is ubiquitously expressed in different tissues and codes for a 46-kDa protein carrying a potential P-loop motif. Ectopic expression of DAP3 in HeLa cells was not compatible with cell growth, resulting in a 16-fold reduction in the number of drug-resistant stable clones. The data presented suggest that DAP3 is a positive mediator of cell death induced by interferon-gamma.

Involvement of nitric oxide in IFN-gamma-mediated reduction of microvessel smooth muscle cell proliferation

Previous studies in our laboratory demonstrated that murine cerebral microvessel smooth muscle cells (SMC) activate syngeneic CD4+ T-cells in vitro. These T-cells, or their culture supernatants, in turn, strongly inhibit proliferation of the SMC. The present study focuses on IFN-gamma as a mediator of inhibition of SMC proliferation, and addresses the molecular mechanism of this inhibition. IFN-gamma profoundly reduced the proliferation of murine brain microvessel smooth muscle cells in vitro. Three lines of evidence indicate that nitric oxide contributed to this effect: (1) IFN-gamma-mediated inhibition of proliferation correlated with the quantity of nitrite, a stable breakdown product of nitric oxide, in culture supernatants; (2) the addition of N(g)- monomethyl-l-arginine, and inhibitor of nitric oxide synthesis, restored proliferation to control or near control levels; and (3) the addition of hemoglobin, which has a high affinity for, and thus sequesters nitric oxide, also resulted in significant restoration of the proliferative response. However, the nitric oxide donating chemical sodium nitro-prusside, at concentrations up to 100 microM, had no direct cytostatic effect. These results suggest that nitric oxide is a necessary but insufficient component in IFN-gamma-mediated inhibition of microvessel smooth muscle cell proliferation. TNF-alpha also stimulated nitric oxide production by the smooth muscle cells, but was not as potent as IFN-gamma at inhibiting proliferation. Knowledge of the physiological effects of lymphokines on cells of the brain microvasculature will contribute towards a better understanding of inflammatory processes in diseases such as multiple sclerosis and infectious encephalitis.

Expression of interleukin 2 receptors on human carcinoma cell lines and tumor growth inhibition by interleukin 2

We have previously shown that human squamous cell carcinomas (SCC) express the interleukin 2 receptor (IL2R)-alpha and -beta chains, and that the ligand, IL2, directly inhibits growth of the tumor in vitro and in vivo in the tumor xenograft-nude mice model. We now show that the alpha and beta chains of IL2R are expressed on a variety of human carcinoma cell lines and on normal human keratinocytes in early-stage cultures. While all carcinoma cells in a population expressed IL2R-alpha and -beta proteins, in keratinocytes obtained from different normal donors, variable proportions of cells were positive, as measured by flow cytometry. The carcinoma lines and 2/5 keratinocyte lines studied were also found to contain transcripts for the IL2R-gamma chain detectable by combined reverse transcription-PCR (RT-PCR) and hybridization with the specific cDNA probe. Incubation of the gastric (HR) or renal cell carcinoma (RCC) cell lines, but not of other IL2R+ carcinoma cell lines or normal keratinocytes, in the presence of IL2 resulted in dose-dependent inhibition of tumor cell growth. Monoclonal antibodies (MAbs) specific for IL2R-beta chain completely reversed this growth inhibitory effect of IL2. The ligand, IL2, also down-regulated surface expression of its own receptor and of intercellular adhesion molecule-I (ICAM-I) or class I major histocompatibility complex (MHC) antigens on IL2R+ tumor cells. All carcinoma cells studied incubated in the presence of IL2 exhibited significantly increased sensitivity to growth-inhibitory effects of other cytokines such as interferon (IFN)-gamma, tumor necrosis factor (TNF)-alpha or transforming growth factor (TGF)-beta. IL2 inhibited growth of the HR cells by arresting a significant proportion of tumor cells in the G0/G1 phase of the cell cycle. Thus, IL2 can have direct effects on IL2R+ carcinoma cells, leading to changes in growth or to increases in sensitivity of tumor cells to cytostatic activities of other cytokines.