Transforming growth factor-beta-like activity in tumors of the central nervous system

Recent developments on immunotherapy for brain cancer

INTRODUCTION

Brain tumors are a unique class of cancers since they are anatomically shielded from normal immunosurveillance by the blood-brain barrier, lack a normal lymphatic drainage system and reside in a potently immunosuppressive environment. Of the primary brain cancers, glioblastoma multiforme (GBM) is the most common and aggressive in adults. Although treatment options include surgery, radiation and chemotherapy, the average lifespan of GBM patients remains at only 14.6 months post-diagnosis.

AREAS COVERED

A review of key cellular and molecular immune system mediators in the context of brain tumors including TGF-β, cytotoxic T cells, Tregs, CTLA-4, PD-1 and IDO is discussed. In addition, prognostic factors, currently utilized immunotherapeutic strategies, ongoing clinical trials and a discussion of new or potential immunotherapies for brain tumor patients are considered.

EXPERT OPINION

Current drugs that improve the quality of life and overall survival in patients with brain tumors, especially for GBM, are poorly effective. This disease requires a reanalysis of currently accepted treatment strategies, as well as newly designed approaches. Here, we review the fundamental aspects of immunosuppression in brain tumors, new and promising immunotherapeutic drugs as well as combinatorial strategies that focus on the simultaneous inhibition of immunosuppressive hubs, both in immune and brain tumor cells, which is critical to consider for achieving future success for the treatment of this devastating disease.

Different release of cytokines into the cerebrospinal fluid following surgery for intra- and extra-axial brain tumours

To elucidate the role of cytokines in brain repair processes and in local inflammation after neurosurgical procedures, cerebrospinal fluid (CSF) samples from 8 patients with intra-axial tumours and 8 patients with extra-axial tumours were analysed for interleukin (IL)-1 beta, IL-1 receptor antagonist (IL-1 ra), IL-6, IL-8, IL-10, and tumour necrosis factor (TNF)-alpha at the beginning and after surgery. Levels of IL-6 and IL-8 increased dramatically in all patients just hours after surgery and fell during subsequent days. IL-1 beta was found only in low amounts in the CSF of both patient groups. Other cytokines demonstrated different courses. In patients with intra-axial tumours IL-1 ra peaked two to four hours after surgery with a subsequent decrease. In patients with extra-axial tumours there was a continuous low-level IL-1 ra release into the CSF without a peak. TNF-alpha was not present in detectable levels in the CSF after surgery for extra-axial tumours but was found to peak two to four hours after surgery for intra-axial tumours. IL-10 was detected in the CSF of both patient groups, but a higher peak was seen after surgery for extra-axial tumours. These results suggest different requirements for the cytokine response and an involvement of different cell types in cytokine release. However, the analysis of the CSF from both patient groups showed no differences in cell counts and populations, with a mild pleocytosis being present in both patient groups after surgery. Therefore, we conclude that after surgery for extra-axial tumours cytokines were predominately produced by non-immune cells stimulated through hypoxia or mechanical irritation. After surgery for intra-axial tumours with a significant brain injury immune cells-activated by necrotic material-seen to be involved in the process of cytokine synthesis. In these cases an additional IL-1ra and TNF-alpha peak was found and these cytokines may be markers for cerebral injury.

In vitro properties of a newly established medulloblastoma cell line, MCD-1

Medulloblastomas are poorly differentiated brain tumors believed to arise from primitive pleuripotential stem cells, and tend to express mixed neuronal and glial properties. In the present study, we examined immunohistochemical and neurotransmitter phenotypic properties in a newly established medulloblastoma cell line, MCD-1. MCD-1 cells were immortal, not contact-inhibited, but did not grow in soft agar. Immunohistochemical studies showed positive staining for neurofilament protein (NF), neuron-specific enolase (NSE), synaptophysin, MAP 2, tau, NCAM 180, vimentin, and S-100 protein. The cells expressed specific uptake of glutamate, serotonin, and choline, but not GABA or dopamine. A significant increase in process extension was seen in response to agents that enhance intracellular cyclic AMP, especially 3-isobutyl-1-methylxanthine (IBMX). Process formation induced by IBMX was associated with a decrease in cell proliferation as evidenced by a reduction in numbers of cells incorporating 5-bromo-2-deoxyuridine (BrdU). No increase in process extension was observed following exposure to NGF or retinoic acid. MCD-1 cells were shown to produce transforming growth factor beta (TGF beta), and were immunopositive for mutant p53. Transfection assays with the PG13-Luc reporter plasmid, which contains a p53-responsive enhancer element and a luciferase reporter gene, suggested MCD-1 cells are deficient in wild-type p53 and do not activate p53 on treatment with the anticancer agent adriamycin. The MCD-1 cell line is suggested to represent an abnormally differentiated cell type, which has some properties consistent with a multipotent neuronal phenotype while retaining some properties of immature cells of a glial lineage. The MCD-1 cell line can be used to provide a model of a medulloblastoma cell line that is resistant to growth-controlling and anticancer agents.

Inhibition of proliferation and induction of differentiation in medulloblastoma- and astrocytoma-derived cell lines with phenylacetate

The authors investigated the effects of a nontoxic differentiation inducer, phenylacetate (PA), on neuroectodermal tumor-derived cell lines. Treatment of medulloblastoma (Daoy and D283 MED) and glioma (U-251MG, C6, and RG2) cell lines resulted in a dose-dependent decline in DNA synthesis and cell proliferation, associated with accumulation in the G0/G1 phase of the cell cycle. Phenylacetate decreased transforming growth factor (TGF)-beta 2 production by medulloblastoma Daoy cells. Neutralizing antibodies against either TGF beta 2 or TGF beta 1 failed to block the growth arrest observed. This suggests that, unlike other differentiation agents, such as retinoic acid, the effect of PA on medulloblastoma proliferation is not mediated by a TGF beta pathway. In addition to cytostasis, PA induced marked morphological changes in U-251MG and C6 glioma cells associated with increased abundance of glial fibrillary acidic protein-positive processes. Although the morphology of PA-treated medulloblastoma cells was not significantly altered, the D283 MED cells exhibited increased expression of neurofilament proteins and Hu antigen, indicative of differentiation along a neuronal pathway. The effects of PA on the medulloblastoma cell lines were compared to its effects on the human neuroblastoma cell line BE(2)C, which is capable of a bidirectional differentiation toward a neuronal or a glial/schwann cell pathway. In BE(2)C cells, PA induced differentiation toward a schwann/glial cell-like phenotype, suggesting that the choice of differentiation pathway is cell type and agent specific. These in vitro antiproliferative and differentiation inducing effects of PA suggest that this agent warrants further evaluation as a potential therapeutic modality for the treatment of medulloblastoma and malignant glioma in humans.

The failure of current immunotherapy for malignant glioma. Tumor-derived TGF-beta, T-cell apoptosis, and the immune privilege of the brain

Human malignant gliomas are rather resistant to all current therapeutic approaches including surgery, radiotherapy and chemotherapy as well as antibody-guided or cellular immunotherapy. The immunotherapy of malignant glioma has attracted interest because of the immunosuppressed state of malignant glioma patients which resides mainly in the T-cell compartment. This T-cell suppression has been attributed to the release by the glioma cells of immunosuppressive factors like transforming growth factor-beta (TGF-beta) and prostaglandins. TGF-beta has multiple effects in the immune system, most of which are inhibitory. TGF-beta appears to control downstream elements of various cellular activation cascades and regulates the expression of genes that are essential for cell cycle progression and mitosis. Since TGF-beta-mediated growth arrest of T-cell lines results in their apoptosis in vitro, glioma-derived TGF-beta may prevent immune-mediated glioma cell elimination by inducing apoptosis of tumor-infiltrating lymphocytes in vivo. T-cell apoptosis in the brain may be augmented by the absence of professional antigen-presenting cells and of appropriate costimulating signals. Numerous in vitro studies predict that tumor-derived TGF-beta will incapacitate in vitro-expanded and locally administered lymphokine-activated killer cells (LAK-cells) or tumor-infiltrating lymphocytes. Thus, TGF-beta may be partly responsible for the failure of current adoptive cellular immunotherapy of malignant glioma. Recent experimental in vivo studies on non-glial tumors have corroborated that neutralization of tumor-derived TGF-beta activity may facilitate immune-mediated tumor rejection. Current efforts to improve the efficacy of immunotherapy for malignant glioma include various strategies to enhance the immunogenicity of glioma cells and the cytotoxic activity of immune effector cells, e.g., by cytokine gene transfer. Future strategies of cellular immunotherapy for malignant glioma will have to focus on rendering glioma cell-targeting immune cells resistent to local inactivation and apoptosis which may be induced by TGF-beta and other immunosuppressive molecules at the site of neoplastic growth. Cytotoxic effectors targeting Fas/APO-1, the receptor protein for perforin-independent cytotoxic T-cell killing, might be promising, since Fas/APO-1 is expressed by glioma cells but not by untransformed brain cells, and since Fas/APO-1-mediated killing in vitro is not inhibited by TGF-beta.

TM-1 cells from an established human malignant glioma cell line produce PDGF, TGF-alpha, and TGF-beta which cooperatively play a stimulatory role for an autocrine growth promotion

We have previously established a human malignant glioma cell line, TM-1. TM-1 cells could proliferate in the serum-free medium. In the present study, immunochemical analysis demonstrated that platelet-derived growth factor (PDGF), transforming growth factor (TGF)-alpha, and TGF-beta are present in the serum-free medium conditioned by growing TM-1 cells. While the cells appeared to possess a single type of binding sites for epidermal growth factor (EGF) with properties comparable to those determined for other tumor cells, the conditioned medium did not contain EGF.PDGF, TGF-alpha, and EGF added exogenously to serum-free media stimulated thymidine incorporation into DNA of TM-1 cells. In addition, antibodies specific for PDGF and TGF-alpha suppressed this activity. These results indicate autocrine and stimulatory roles of PDGF and TGF-alpha for the proliferation of TM-1 cells. As observed for other tumor cells, TGF-beta by itself weakly suppressed thymidine incorporation by TM-1 cells. However, TGF-beta employed in combination with TGF-alpha or EGF appeared to stimulate thymidine incorporation, suggesting that a cooperative action of TGF-beta with different growth factors may be involved in the stimulatory growth regulation at least for TM-1 cells.

A sequential cell kinetic study of meningioma cells in primary explant culture using bromodeoxyuridine

The present study was undertaken to evaluate the sequential BrdU-LI at weekly intervals upto four weeks in 18 primary explant cultures of meningiomas. This revealed three distinct patterns of growth which could be arbitrarily defined as 'degenerating' (group I), 'proliferating' (group II) and 'adaptive' (group III) types. Interestingly two cases of malignant and two of recurrent meningiomas fell into the 'degenerating' group I pattern. The possible explanations for the observed relatively higher in vitro LI values compared to lower in vivo values as reported in the literature and the theoretical implications of the three distinct patterns of sequential LI values are discussed.

Human brain tumor cyst fluid is mitogenic for primary astrocytes in culture

Tumor cyst fluid from five patients with malignant glial neoplasms was assayed for mitogenic activity by measuring [3H]thymidine uptake by rat astrocyte cultures in serum-free defined media. Cyst fluid from all patients stimulated [3H]thymidine uptake three- to fourfold in astrocyte cultures in comparison with untreated controls. Mitogenic activity was found to be soluble and resistant to freezing, but inactivated by heat and trypsin. The activity was retained by ultrafiltration through a 100-kd molecular weight filter. It appears from these results that mitogenic factor(s) for nonneoplastic astrocytes are present in human brain tumor cyst fluid and that such factors appear to be protein in nature and associated with a complex of molecular weight greater than 100,000.

Inflammatory leukocytes associated with increased immunosuppression by glioblastoma

In order to determine the in vivo immune response in glioblastoma, monoclonal and polyclonal antibodies specific for inflammatory leukocytes and immunoregulatory products were utilized to stain tissue from four surgical specimens. The more activated the inflammatory cells, the more activated the tumors appeared to be. In the tumor with the largest infiltration (Case 3), inflammatory cells were stained for interferon-gamma, interleukin-2, interleukin-1 beta, lymphotoxin, tumor necrosis factor-alpha, and transforming growth factor-beta. The tumor cells also expressed interleukin-1 beta, interleukin-6, transforming growth factor-beta, tumor necrosis factor-alpha, and prostaglandin E. In contrast, in the tumor with the least inflammatory response (Case 1), the tumor cells did not express any cytokines. Expression of cytokines by glioma cells was modest in the two cases with modest inflammatory responses. Cellular inflammation, primarily consisting of T cells and macrophages with few or no B cells or natural killer cells, was two- to 15-fold greater outside the tumor than within. In contrast to leukocytes outside the tumor, which were activated and expressing class II major histocompatibility antigens, leukocytes within the tumor parenchyma or at the tumor's edge were negative for these antigens. In the four specimens studied here, the tumor cells themselves were also negative for class II major histocompatibility antigens. These findings, although preliminary, suggest that inflammatory cells within gliomas are inactivated and that glioma cells may increase the expression of immunosuppressive cytokines in response to an increased lymphocyte infiltrate. This observation, if corroborated by more extensive studies, may help to explain the failure of immune treatments in glioblastoma multiforme.

Inflammatory cell infiltrates vary in experimental primary and metastatic brain tumors

We have studied the cellular immune response that accompanies primary and metastatic brain cancers induced experimentally in rats by inoculation of RG-2 glioma and Walker 256 (W256) carcinoma cells, respectively. The inflammatory cell infiltrates were characterized with lectin histochemistry to visualize microglial cells and macrophages and with immunohistochemistry, using a panel of monoclonal antibodies, to detect major histocompatibility complex (MHC), lymphocytic, and macrophage antigens. The metastatic tumor was composed of a loose stroma with multiple, often large, necrotic areas, whereas the RG-2 glioma was composed of a dense collection of tumor cells showing only rare necrotic foci. Both tumor types were heavily infiltrated with microglia and/or macrophages, and these were positive for MHC Class II (Ia) antigens. Expression of MHC Class I antigens was absent from RG-2 glioma cells, but it was present in W256 metastatic carcinoma cells. The metastatic tumor was also characterized by a much heavier infiltrate of lymphocytes, as shown by the presence of cells positive for CD4, CD8, and leukocyte common antigens. These lymphocytic markers were absent from reactive microglia in the W256 carcinoma, whereas they were present in the RG-2 glioma. Polymorphonuclear leukocytes were seen only in the metastatic tumor. Our study delineates differences between the inflammatory cell infiltrates found in metastatic brain tumors and those found in primary brain tumors. The differences in cell composition and immunophenotype may indicate a more effective antitumor response in the metastatic tumor that could account for the observed tissue destruction.

Immunophenotypic analysis of infiltrating leukocytes and microglia in an experimental rat glioma

The appearance and cellular distribution of major histocompatibility complex (MHC), as well as lymphocytic and macrophage antigens has been studied in a fully developed experimental rat forebrain glioma. Activated microglial cells and microglia-derived macrophages expressing CR3 complement receptor molecules and MHC class II (Ia) antigen were found throughout the tumor, and with increased density along the tumor's periphery. MHC class I antigen expression was entirely absent from tumor cells, and found only occasionally on microglia. The expression of leukocyte common antigen, and CD4 and CD8 antigens was conspicuous throughout the tumor, and associated with lymphocytes, perivascular cells, and microglia. Cells expressing the ED2 macrophage epitope were almost exclusively of the perivascular type and revealed a distribution dissimilar to that of cells positive for Ia antigen. The ED2 epitope was found sporadically on ramified microglial cells. The results show that despite heavy infiltration with blood mononuclear and CNS microglial cells, the tumor showed no evidence of destruction caused by inflammatory cells. Possible mechanisms of tumor immunosuppressive activity preventing the full immunological activation of microglia and blood mononuclear cells are discussed.

Expression of platelet-derived growth factor and transforming growth factor and their correlation with cellular morphology in glial tumors

This study was undertaken to evaluate the role of two sets of growth factors, platelet-derived growth factor (PDGF) and transforming growth factor-beta (TGF-beta), in the induction and maintenance of glial tumors and their phenotypic expression. Explants from eight malignant tumors, five benign tumors, and two nontumor glial cells were analyzed for levels of messenger ribonucleic acid (mRNA) expression of PDGFA, PDGFB, TGF-beta 1, and TGF-beta 2. Results were normalized to the mRNA expression of tubulin, a "housekeeping" gene present in glial cells. Of the 15 explants tested, PDGFB was seen in six, all of which were malignant tumors; PDGFA was seen in all 15 with much higher levels expressed in malignant tumors; and TGF-beta 1 and TGF-beta 2 were seen in all 15 without a clear difference between cell types, although expression tended to be higher in malignant tumors. This project supports the theory that the induction and maintenance of glial tumors is likely to be a multifactorial phenomenon.

TGF beta 1 and TGF beta 2 are potential growth regulators for low-grade and malignant gliomas in vitro: evidence in support of an autocrine hypothesis

Low-grade astrocytomas, anaplastic astrocytomas and glioblastomas in vitro were found to ubiquitously produce the mRNA of transforming growth factor-beta (TGF beta). TGF beta 1 and TGF beta 2 mRNA were expressed to a lesser degree among the hyperdiploid malignant gliomas. By radioreceptor assay of conditioned medium, TGF beta was secreted predominantly in latent form, in both latent and active form, or only in active form within a panel of low-grade and malignant gliomas. The TGF beta receptor (types I, II, and III) was evident among the glioma lines. Many near-diploid gliomas were growth-inhibited by TGF beta 1 and TGF beta 2 in vitro. Most hyperdiploid glioblastomas showed a positive mitogenic response to exogenous TGF beta 1 and TGF beta 2. A synergistic or additive mitogenic interaction with epidermal growth factor and insulin was observed among some. Under serum-free conditions, anti-TGF beta antibody neutralized the expected growth-regulatory effect of endogenous TGF beta, thus establishing the specificity of the response in vitro. TGF beta 1 also enhanced the clonogenicity of certain gliomas which had been growth-stimulated in monolayer. Thus, basic elements in support of an autocrine hypothesis have been demonstrated: TGF beta mRNA was expressed among low-grade and malignant gliomas, TGF beta was secreted in latent and/or active form into conditioned media and appeared to serve as an endogenous regulator of glioma proliferation in vitro. The mitogenic response, either positive or negative, correlated with the degree of anaplasia and karyotypic divergence.

A cell line of human malignant astrocytoma producing autocrine growth factor

A cell line was established from an anaplastic astrocytoma from a 69-yr-old female. The cells have been serially subcultured over 300 times for 6 yr without showing any sign of cell senescence. Their doubling time is about 36 h. The cells are fusiform and often hexagonal in sparse culture, but become spindle-shaped and formed mosaic structure in confluent culture. Under electron microscopy, intermediate filaments were randomly distributed in the cytoplasma, especially in the perinuclear space. The chromosome number was near tetraploid and varied from 86 to 94 chromosomes with a modal number of 91. The alpha and beta subunits of S-100 protein, vimentin, and glial fibrillary acidic protein (GFAP), which are reliable markers of astrocytic cells, were demonstrated in a large number of cells by immunoperoxidase staining. The results of immunoblotting showed that the expression of vimentin was much higher than that of GFAP. The tumorigenicity of the cells was revealed by xenografting into nude mice, which were X-irradiated before inoculation. Culture medium conditioned by the cells promoted growth of these cells in serum-free conditions and of normal rat glial cells in serum-depleted culture. The growth-promoting effect of conditioned medium was lost by trypsinization and reduced by boiling. These findings suggest that these cells are derived from neoplastic astrocytic cells and secrete a self-acting polypeptide growth-promoting factor into the culture medium.

Immunoregulators in the nervous system

The nervous system, through the production of neuroregulators (neurotransmitters, neuromodulators and neuropeptides) can regulate specific immune system functions, while the immune system, through the production of immunoregulators (immunomodulators and immunopeptides) can regulate specific nervous system functions. This indicates a reciprocal communication between the nervous and immune systems. The presence of immunoregulators in the brain and cerebrospinal fluid is the result of local synthesis--by intrinsic and blood-derived macrophages, activated T-lymphocytes that cross the blood-brain barrier, endothelial cells of the cerebrovasculature, microglia, astrocytes, and neuronal components--and/or uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. Acute and chronic pathological processes (infection, inflammation, immunological reactions, malignancy, necrosis) stimulate the synthesis and release of immunoregulators in various cell systems. These immunoregulators have pivotal roles in the coordination of the host defense mechanisms and repair, and induce a series of immunological, endocrinological, metabolical and neurological responses. This review summarizes studies concerning immunoregulators--such as interleukins, tumor necrosis factor, interferons, transforming growth factors, thymic peptides, tuftsin, platelet activating factor, neuro-immunoregulators--in the nervous system. It also describes the monitoring of immunoregulators by the central nervous system (CNS) as part of the regulatory factors that induce neurological manifestations (e.g., fever, somnolence, appetite suppression, neuroendocrine alterations) frequently accompanying acute and chronic pathological processes.

Transforming growth factor (TGF)-beta like activity of intracranial meningioma and its effect on cell growth

Polypeptides, characterized by their ability to confer a transformed phenotype on an untransformed indicator cell have been isolated directly from surgical specimens of intracranial meningioma by using an acid/ethanol extraction procedure. Transforming activity in meningeal cells was based on the ability to induce NRK 49F rat kidney fibroblasts to form colonies in soft agar. This polypeptide was separated by gel filtration into two fragments of 15 and 40 kilodalton (kDa) molecular weight. Among other cases of brain neoplasms, one case of glioblastoma multiforme had moderate TGF-beta activity, but medulloblastoma and neurinoma had no activity. Purified TGF-beta also stimulated DNA synthesis in primary cultured meningioma cells, but no effect was seen in U 251MG human glioma cells. While the physiological function of TGF-beta is still ill-defined and the molecular character of its receptor has not been analyzed, intracranial meningiomas are noted to have TGF-beta-like activity. TGF-beta also induces the DNA synthesis of cultured meningioma cells. From these results, TGF-beta would be considered one of the growth promoting factors in meningioma.

Isolation and partial purification of growth factors with TGF-like activity from human malignant gliomas

The effect of concentrated conditioned medium from each of eight human malignant glioma cell lines on the growth of indicator cells (normal rat kidney fibroblasts (NRK), clone 14) was determined in monolayer and in soft agar assay systems. The conditioned medium from all cell lines was mitogenic in the monolayer assay, but only SF-210, U-343 MG-A, and U-251 MG produced soluble factors that caused NRK cells to grow in soft agar. The soluble growth-promoting factors from these three cell lines were acid- and heat-stable (60 degrees C for 30 minutes) but were inactivated by trypsin (100 microns/ml) and dithiothreitol (50 microM). The growth factors from SF-210 and U-343 MG-A were further purified by molecular-sieve chromatography. The partially purified growth factor from U-343 MG-A retained transforming growth factor (TGF)-like activity, had a molecular weight of 9 kD, was potentiated by TGF-beta in the soft agar assay, competed effectively with 125I-epidermal growth factor (EGF) radiolabeled for the EGF receptor on A 431 epidermoid carcinoma cells, and was completely inhibited by monoclonal antibodies to TGF-alpha. The partially purified growth factor from SF-210 had a molecular weight of 17 kD, was not inhibited by monoclonal antibodies to platelet-derived growth factor (PDGF) or TGF-alpha, and did not bind to a heparin-Sepharose column. These results imply that U-343 MG-A secretes a growth factor with TGF-alpha-like activity, and SF-210 secretes a TGF with neither TGF-alpha nor TGF-beta activity.

Immunomodulators and feeding regulation: a humoral link between the immune and nervous systems

Cells of the nervous and immune systems have specific receptors for humoral substances that originate in both systems. These elements establish a bidirectional information exchange network between the nervous and immune systems. In particular, neuroregulators (neurotransmitters and neuromodulators) can modulate specific immune system function(s) and immunoregulators (immunomodulators) can modulate specific nervous system function(s). Modulation of immune functions by neuroregulators has been receiving considerable attention; however, modulation of nervous system functions by immunomodulators has been little studied. The presence of immunomodulators in the brain and cerebrospinal fluid may represent local synthesis by astrocytes, microglia, endothelial cells, intrinsic macrophages and blood-derived lymphocytes which cross the blood-brain barrier, or the concentration of substances derived from the peripheral blood. Acute and chronic inflammatory processes, malignancy, and immunological reactions stimulate the synthesis and release of immunomodulators in various cell systems. These immunomodulators have pivotal roles in the coordination of the host defense mechanisms and repair and induce a series of endocrine, metabolic, and neurologic responses. This paper focuses on the effects of immunomodulators (interleukins, tumor necrosis factor, tuftsin, platelet activating factor, and others) on the central nervous system (CNS), in particular, on feeding regulation. It is proposed that an immunomodulatory system regulates food intake by a direct action in the CNS through a specific neuro-immuno interaction. This regulatory system may be operative during acute and chronic disease.

Transforming growth factor-beta activity is potentiated by heparin via dissociation of the transforming growth factor-beta/alpha 2-macroglobulin inactive complex

The control of smooth muscle cell (SMC) proliferation is determined by the combined actions of mitogens, such as platelet-derived growth factor, and the opposing action of growth inhibitory agents, such as heparin and transforming growth factor-beta (TGF-beta). The present studies identify an interaction between heparin and TGF-beta in which heparin potentiates the biological action of TGF-beta. Using a neutralizing antibody to TGF-beta, we observed that the short term antiproliferative effect of heparin depended upon the presence of biologically active TGF-beta. This effect was observed in rat and bovine aortic SMC and in CCL64 cells, but not in human saphenous vein SMC. Binding studies demonstrated that the addition of heparin (100 micrograms/ml) to medium containing 10% plasma-derived serum resulted in a 45% increase in the specific binding of 125I-TGF-beta to cells. Likewise, heparin induced a twofold increase in the growth inhibitory action of TGF-beta at concentrations of TGF-beta near its apparent dissociation constant. Using 125I-labeled TGF-beta, we demonstrated that TGF-beta complexes with the plasma component alpha 2-macroglobulin, but not with fibronectin. Heparin increases the electrophoretic mobility of TGF-beta apparently by freeing TGF-beta from its complex with alpha 2-macroglobulin. Dextran sulfate, another highly charged antiproliferative molecule, but not chondroitin sulfate or dermatan sulfate, similarly modified TGF-beta's mobility. Relatively high, antiproliferative concentrations of heparin (1-100 micrograms/ml) were required to dissociate the TGF-beta/alpha 2-macroglobulin complex. Thus, it appears that the antiproliferative effect of heparin may be partially attributed to its ability to potentiate the biological activity of TGF-beta by dissociating it from alpha 2-macroglobulin, which normally renders it inactive. We suggest that heparin-like agents may be important regulators of TGF-beta's biological activity.

Growth factors, feeding regulation and the nervous system

A variety of growth factors and their receptors are present in the nervous system. Growth factors can modulate specific nervous system functions others than those related to growth, development, and tissue repair. The presence of growth factors in the brain and cerebrospinal fluid is the result of local synthesis (by neuronal, glial, vascular, and mononuclear phagocyte components), and uptake from the peripheral blood through the blood-brain barrier (in specific cases) and circumventricular organs. This paper focuses on the effects of a heterogeneous group of growth factors (acidic and basic fibroblast growth factors, insulin-like growth factors, epidermal growth factor, platelet-derived growth factor, interleukin-1 and others) on the central nervous system (CNS), in particular, on feeding regulation. Recent evidence supporting participation of growth factors in the regulation of feeding by a direct action at the level of the CNS is reviewed. Various growth factors have the ability to suppress short- and long-term food intake (FI), whereas others affect only short-term FI, or do not affect FI. Acute and chronic pathological processes stimulate the synthesis and release of growth factors in various cellular systems, and monitoring of growth factors by the CNS could be part of the regulatory signals that induce FI suppression frequently accompanying acute and chronic disease. Thus, it is proposed that a system regulating FI through growth factor-dependent mechanisms may be operative during specific physiological or pathological conditions.