Cell-specific toxicity of a chimeric protein composed of interleukin-6 and Pseudomonas exotoxin (IL6-PE40) on tumor cells

Recombinant toxins in haematologic malignancies and solid tumours

Recombinant toxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. They are fusion proteins, which contain toxin and ligand regions, and are produced in Escherichia coli. The ligand may be a growth factor or a fragment of an antibody, and the toxin is usually one of the two bacterial toxins: Pseudomonas exotoxin and diphtheria toxin. Compared to the earlier generation chemical conjugates of ligands and toxins, recombinant toxins have many advantages, including homogeneity with respect to the connection between the ligand and toxin, ease and yield of production and small size. A variety of chemotherapy-resistant haematologic and solid tumours have been targeted with recombinant toxins, and clinical trials with many of them have recently demonstrated their effectiveness. Moreover, their unwanted toxic effects are different from those of most chemotherapeutic agents, supporting the expectation that they can be combined with existing modalities to improve the clinical resources available to treat cancer in humans.

IL-6 signaling by STAT3 participates in the change from hyperplasia to neoplasia in NRP-152 and NRP-154 rat prostatic epithelial cells

BACKGROUND

STAT3 phosphorylation is associated with the neoplastic state in many types of cancer, including prostate cancer. We investigated the role of IL-6 signaling and phosphorylation of STAT3 in 2 rat prostatic epithelial lines. NRP-152 and NRP-154 cells were derived from the same rat prostate, yet the NRP-152 cells are not tumorigenic while the NRP-154 cells are tumorigenic. These lines are believed to represent 2 of the stages in the development of prostate cancer, hyperplasia and neoplasia. Differences in signaling pathways should play a role in the 2 phenotypes, hyperplastic and neoplastic.

METHODS

We looked at the phosphorylation state of STAT3 by intracellular flow cytometry, using phospho-specific antibodies to STAT3. We used the same method to examine IL-6 production by the cell lines. We also measured apoptosis by binding of fluorescent annexin V to the cells.

RESULTS

Although both cells lines made IL-6 constitutively, phosphorylated-STAT3 was present in untreated NRP-154 cells, but not in NRP-152 cells. Treatment with dexamethasone inhibited the IL-6 production of NRP-152 cells, but enhanced that of NRP-154 cells. Treatment with the JAK2 inhibitor AG490 induced apoptosis in NRP-152, but not NRP-154 cells.

CONCLUSIONS

We conclude from these experiments that STAT3 activity plays a role in the phenotype of NRP-154 cell, but not NRP-152 cells. The significance of alternative IL-6 signaling pathways in the different phenotypes of the 2 cell lines is discussed.

Interleukin-6 activates phosphatidylinositol-3 kinase, which inhibits apoptosis in human prostate cancer cell lines

BACKGROUND

A number of recent studies have identified interleukin (IL)-6 as an important regulator of prostate cancer growth. Here, we investigate the potential interaction of IL-6 with phosphatidylinositol (PI)-3 kinase, a key growth regulatory enzyme, in prostate cancer cell lines.

METHODS

Tyrosine phosphorylation of p85, the regulatory subunit of PI-3 kinase, in the human prostate cancer cell lines LNCaP and PC-3 was assessed by sequential immunoprecipitation with anti-p85 antibody and immunoblotting with anti-phosphotyrosine. The effects of wortmannin, an inhibitor of PI-3 kinase, and/or IL-6 on cell growth were assessed by MTT assays. DNA laddering experiments were performed to assay for programmed cell death.

RESULTS

Tyrosine phosphorylation of p85 is upregulated by IL-6 in both LNCaP and PC-3. IL-6 promotes coprecipitation of p85 with gp130, the signal-transducing component of the IL-6 receptor. Inhibition of PI-3 kinase with wortmannin induces programmed cell death in PC-3 cells. In contrast, wortmannin has no effect on LNCaP growth when used alone; however, combined with IL-6, wortmannin promotes apoptosis in these cells.

CONCLUSIONS

PI-3 kinase is involved in IL-6 signal transduction and delivers an antiapoptotic signal in human prostate cancer cell lines.

Characterization of the role of IL-6 in the progression of prostate cancer

BACKGROUND

We recently identified IL-6, a pleiotropic cytokine implicated in the neoplastic process of a variety of neoplasms, as a mediator of prostate cancer morbidity. In the present study, we investigated the expression of members of the IL-6 supergene family and related cytokines and the potential role of IL-6 in prostate cancer growth regulation.

METHODS

Five established human prostate cancer cell lines were screened by ELISA for production of granulocyte colony-stimulating factor (G-CSF), leukemia inhibitory factor (LIF), ciliary neurotrophic factor (CNTF), oncostatin M (OSM), tumor necrosis factor (TNF), interleukin-1 (IL-1), and granulocyte macrophage colony-stimulating factor (GM-CSF). Expression of the ligand-binding component of the IL-6 receptor, IL-6Rp80, was evaluated by ELISA and RT-PCR. Sequential immunoprecipitation and immunoblotting were performed to assay for expression of the signal-transducing component of the IL-6 receptor, gp130. The effects of IL-6 on cell growth were assessed by MTT assays.

RESULTS

The three hormone-refractory cell lines, DU-145, TSU, and PC-3, secreted distinct combinations of cytokines (DU-145: IL-6, GM-CSF; TSU: IL-6, LIF; PC-3: IL-6, G-CSF, LIF, IL-1, GM-CSF), each uniformly expressing IL-6. In contrast, neither of the two hormone-dependent cell lines, LNCaP-ATCC and LNCaP-GW, secreted significant quantities of any of the cytokines analyzed. None of the cell lines secreted detectable quantities of OSM, CNTF, or TNF. All cell lines, irrespective of hormone status, expressed both Il-6Rp80 and gp130. Addition of IL-6 in vitro inhibited growth of hormone-dependent cells, but had no effect on hormone-refractory lines. Anti-IL-6 neutralizing antibody inhibited growth of hormone-refractory cells.

CONCLUSIONS

IL-6 appears to undergo a functional transition from paracrine growth inhibitor to autocrine growth stimulator during progression of prostate cancer to the hormone-refractory phenotype.

Immunotoxins for targeted cancer therapy

Immunotoxins constitute a new modality for the treatment of cancer, since they target cells displaying specific surface-receptors or antigens. Immunotoxins contain a ligand such as a growth factor, monoclonal antibody, or fragment of an antibody which is connected to a protein toxin. After the ligand subunit binds to the surface of the target cell, the molecule internalizes and the toxin kills the cell. Bacterial toxins which have been targeted to cancer cells include Pseudomonas exotoxin and diphtheria toxin, which are well suited to forming recombinant single-chain or double-chain fusion toxins. Plant toxins include ricin, abrin, pokeweed antiviral protein, saporin and gelonin, and have generally been connected to ligands by disulfide-bond chemistry. Immunotoxins have been produced to target hematologic malignancies and solid tumors via a wide variety of growth factor receptors and antigens. Challenges facing the clinical application of immunotoxins are discussed.

Mechanisms of inhibition of IL-6-mediated immunoglobulin secretion by dexamethasone and suramin in human lymphoid and myeloma cell lines

The cytokine IL-6 has been proposed as an autocrine growth factor in multiple myeloma, and is also required for stimulation of immunoglobulin production and secretion in normal plasma cells and myeloma cells. In this study, we showed that secreted IL-6 is detectable by Western blot analysis in a panel of lymphoid and myeloma cell lines. Previous studies in our laboratory have shown that dexamethasone and suramin inhibit cell proliferation and IL-6-mediated immunoglobulin secretion in various lymphoblastoid and myeloma cell lines. In the present study, we present study, we present data to examine mechanisms by which dexamethasone and suramin inhibit IL-6-mediated immunoglobulin secretion in the lymphoid cell line SKW 6.4. Cells treated with rIL-6 or the IC10 concentration of dexamethasone respectively undergo a doubling of intracellular IgM. Moreover, rIL-6 and dexamethasone additively stimulate cells to accumulate intracellular IgM. In contrast, cells treated with the IC10 concentration of suramin undergo no significant alteration of total cellular IgM, and do not respond to IL-6 with an increase in intracellular IgM. Northern blot analysis demonstrates that cells treated with exogenous rIL-6 and/or dexamethasone respectively undergo a coordinate one to three fold increase of kappa and mu chain mRNA expression, while there is a 30-40% decrease of kappa and mu chain mRNA when cells are treated with suramin and suramin plus rIL-6. Western blot analysis shows that levels of intracellular IL-6 modestly increase when cells are treated with exogenous rIL-6, whereas treatment with dexamethasone plus rIL-6 causes a 70% decrease of immunoreactive IL-6 protein in comparison with untreated cells. An rtPCR analysis of IL-6 mRNA expression shows an abolished signal in response to dexamethasone or rIL-6 and/or dexamethasone. Using a flow cytometric assay, it is demonstrated that suramin inhibits IL-6 binding to its receptor. Taken together, these results indicate that SKW 6.4 cells treated with rIL-6 and/or dexamethasone undergo increased expression of IgM mRNA leading to increased intracellular IgM levels. Treatment with suramin or suramin plus rIL-6 does not alter the IL-6 protein level or the mRNA levels for IL-6 and IL-6 receptor. Suramin treatment causes a moderate decrease in IgM mRNA, and this is associated with a decreased intracellular level of IgM in SKW 6.4 cells. Overall these findings support the concept that IL-6 is an autocrine factor for immunoglobulin production and secretion in myeloma cells. Suramin interferes with IL-6 binding to its receptor and/or decreases IL-6 receptor expression. Dexamethasone has neither of these effects on IL-6 receptor expression or IL-6 binding to its receptor, and we postulate that it acts through a block in secretion or in degradation of intracellular immunoglobulin by decreasing IL-6 mRNA expression and IL-6 protein content. These studies suggest that the combination of suramin and dexamethasone not only synergistically growth inhibit myeloma cells but also act in concert to inhibit immunoglobulin secretion and represent a therapeutic approach worthy of further investigation.

Interleukin 6 receptor mRNA in prostate carcinomas and benign prostate hyperplasia

We investigated the presence of interleukin 6 (IL6) receptors in human prostate carcinomas and benign prostatic hyperplasias. Interleukin 6 receptor expression was measured at the mRNA level by slot blot analysis using a probe that recognizes mRNA encoding the 80-kDa subunit of the IL6 receptor. Significant expression was found in 29 of 37 (78%) samples of benign prostate hyperplasia (BPH) and in 17 of 17 prostate carcinomas. Quantitative analysis of the expression level revealed that 11 of the 29 positive hyperplasia tissues (38%) and 4 carcinoma samples (23.5%) expressed equal or higher levels of IL6 receptor mRNA than the human hepatoma cell line PLC/PRF5, which contains about 2300 IL6 receptors per cell. We also measured IL6 receptor mRNA levels in three human prostate carcinoma cell lines LNCaP, DU145 and PC3, which are known to contain IL6 receptors because they are sensitive to the cytotoxic action of an IL6-toxin fusion protein. We were not able to detect IL6 receptor expression with the slot blot procedure, but we were able to detect IL6 receptor mRNA using a very sensitive PCR assay. Our data provide evidence that IL6 may play a role in the growth of benign and malignant prostate tumors and suggest that the IL6 receptor could be a target for the delivery of therapeutic agents in prostate cancer.

Differential sensitivity of human myeloma cell lines and normal bone marrow colony forming cells to a recombinant diphtheria toxin-interleukin 6 fusion protein

The cytotoxicity of a recombinant interleukin 6 (IL-6)-diphtheria toxin (DT) fusion protein towards human myeloma cell lines was investigated. DAB389-IL-6 inhibited protein synthesis and methylcellulose colony formation by U266 myeloma cells. In the clonogenic assay, the fusion protein approached the level of cytotoxicity achieved by native DT. The specificity of killing by DAB389-IL-6 was demonstrated by inhibition of cytotoxicity by a molar excess of free rhIL-6. The effect of DAB389-IL-6 on colony formation by six OCI-My cell lines was assessed. Similar to U266 cells, colony growth by the OCI-My 5 and -My 2 cell lines was inhibited in a simple dose dependent manner. However, a biphasic effect was observed for the IL-6 dependent OCI-My 4 cells; DAB389-IL-6 stimulated colony formation at low (< or = 10(-11) M) concentrations, yet was inhibitory at higher doses. Three other cell lines whose growth was not altered by IL-6 were relatively unaffected by DAB389-IL-6, despite their sensitivity to native DT. Flow cytometric analysis for IL-6 receptor expression using phycoerythrin-conjugated IL-6 demonstrated specific binding sites on both DAB389-IL-6 sensitive and certain insensitive cell lines, suggesting that other factors in addition to the expression of IL-6 receptors are involved in killing by the fusion toxin. Despite evidence for a role of IL-6 in myeloid cell development, normal bone marrow was insensitive to the IL-6 fusion toxin. In cultures containing both normal bone marrow and U266 cells DAB389-IL-6 effectively inhibited the growth of U266 myeloma colonies but had little effect on normal bone marrow erythroid, granulocyte and mixed erythroid/granulocyte colony growth. From these experiments we conclude that DAB389-IL-6 is specifically cytotoxic towards a subset of IL-6-responsive human myeloma cell lines and may be useful, in some cases, in the selective elimination of tumour cells from mixed populations of normal and malignant cells.

Targeting growth factor receptors with fusion toxins

Recombinant toxins which bind to growth factor receptors have been prepared and used to kill cells responsible for malignant or autoimmune disease. Our strategy has been to genetically fuse ligands to different forms of Pseudomonas exotoxin which due to mutations or deletions do not bind to normal cells. The resulting recombinant chimeric toxins, in concentrations often less than 1 ng/ml, selectively kill cells expressing the appropriate growth factor receptor. The ligand may be a growth factor, such as transforming growth factor alpha (TGF alpha), interleukin 6 (IL6) or interleukin 2 (IL2), or single chain antigen binding proteins, such as the variable heavy and light regions of the monoclonal antibody anti-Tac. These chimeric toxins kill not only established cell lines but also fresh tumor cells from patients and display anti-tumor activity toward human malignant tumors in nude mice. While clinical trials are beginning with some of these agents, work continues to improve the effectiveness of recombinant chimeric toxins, and to widen the scope of disorders which might be treated by this approach.

Inhibition of human antigen-specific memory B cell response in vitro by a diphtheria toxin-related interleukin 2 fusion protein

Recombinant diphtheria toxin-related interleukin-2 fusion protein (DAB486IL-2) is specifically cytotoxic for cells bearing the high-affinity IL-2 receptor (p55/75). We evaluated the effects of DAB486IL-2 on the generation of tetanus toxoid (TT)-specific IgG antibody-forming cells in 6-day cocultures of human splenocytes and TT-coupled Sepharose beads. The results indicate that a significant portion (30-75%) of the anti-tetanus toxoid IgG response in vitro was susceptible to inhibition by 10(-10) M DAB486IL-2. The inhibition required both the IL-2 portion of the fusion protein and an active toxin moiety and was greater when the IL-2 toxin was added on Day 3 as compared with Day 0 of culture. The induction of the p55 (Tac) subunit of the IL-2R was demonstrable by two-color flow cytometry on a small percentage (5%) of B cells and on a higher percentage (10%) of non-B cells 3 days after exposure to TT-coupled Sepharose. Short-term (2 hr) treatment of T and B cell subpopulations separated on Day 3 of culture followed by remixing indicated that while activated T helper cells were most strongly inhibited by DAB486IL-2, up to 50% of the TT-specific IgG response was inhibited by treatment of B cells alone with DAB486IL-2. Our results suggest that a strategy of eliminating human memory B cells by a combination of antigen activation and properly timed administration of a recombinant lymphokine-toxin fusion protein is feasible.

Mitotoxins: growth factor-targeted cytotoxic molecules

When the selective specificity and exquisite affinity of growth factors for their receptors is conferred to protein toxins, the chimeric molecules so generated become potent cytotoxins. Chimera are produced by the chemical conjugation of the two proteins or by expression of fusion proteins in bacterial expression systems. The toxic moiety, usually a ribosome-inactivating protein or a fragment of a bacterial toxin, is internalized into target cells by receptor-mediated endocytosis. Release of the receptor's ligand in the endosome allows the toxic moiety to exert its action on protein synthesis. Accordingly, the potent catalytic activity of the toxin results in a sensitivity of some cells to picomolar quantities of the mitotoxin. This review discusses the numerous growth factor-toxins that have been created and describes some of their applications.