Changes induced in astrocyte cathepsin D by cytokines and leupeptin

Cathepsin D is widely, but unevenly, distributed among cells and is capable of degrading a number of neural peptides and proteins. The present study was undertaken to examine the level of cathepsin D in astrocytes that might be relevant to its induction in inflammatory demyelination. Primary astrocytes were cultured from neonatal rat cerebrums according to the method of McCarthy and de Vellis. Based on staining for cell markers, cultures were greater than 95% astrocytes and less than 3% microglia. Under serum-free conditions, leupeptin induced a 1.4- to 2.0-fold increase, maximal by 48 hours, in cathepsin D protein quantified by a radioimmunoassay. Cathepsin D enzymatic activity, inhibitable by pepstatin, also increased. Northern blot analysis demonstrated that leupeptin also increased cathepsin D mRNA expression. Kinetic analysis indicated that maximal cathepsin D mRNA levels are detected 24 h after stimulation with leupeptin. Exposure of astrocytes under the same conditions to rat recombinant interferon-gamma, human recombinant tumor necrosis factor-alpha, human recombinant interleukin-1 beta, lipopolysaccharide, calcium ionophore, or a combination of these reagents did not increase the level of cathepsin D above controls. These results indicate that astrocytic cathepsin D mRNA and protein can be induced by selected materials. Furthermore, the effects attributed to leupeptin as a proteinase inhibitor may be modified by its ability to increase cathepsin D activity.

Differential tumor necrosis factor alpha expression by astrocytes from experimental allergic encephalomyelitis-susceptible and -resistant rat strains

There is evidence that the cytokine tumor necrosis factor alpha (TNF-alpha) contributes to the pathogenesis of neurological autoimmune diseases such as multiple sclerosis (MS) and experimental allergic encephalomyelitis (EAE). TNF-alpha exerts damaging effects on oligodendrocytes, the myelin-producing cell of the central nervous system (CNS), and myelin itself. We have recently demonstrated TNF-alpha expression from astrocytes induced by lipopolysaccharide (LPS), interferon gamma (IFN-gamma), and interleukin 1 beta (IL-1 beta). Astrocytes secrete TNF-alpha in response to LPS alone, and can be primed by IFN-gamma to enhance LPS-induced TNF-alpha production. IFN-gamma and IL-1 beta, cytokines known to be present in the CNS during neurological disease states, do not induce TNF-alpha production alone, but act synergistically to stimulate astrocyte TNF-alpha expression. Inbred Lewis and Brown-Norway (BN) rats differ in genetic susceptibility to EAE, which is controlled in part by major histocompatibility complex (MHC) genes. We examined TNF-alpha gene expression by astrocytes derived from BN rats (resistant to EAE) and Lewis rats (highly susceptible). Astrocytes from BN rats express TNF-alpha mRNA and protein in response to LPS alone, yet IFN-gamma does not significantly enhance LPS-induced TNF-alpha expression, nor do they express appreciable TNF-alpha in response to the combined stimuli of IFN-gamma/IL-1 beta. In contrast, astrocytes from Lewis rats express low levels of TNF-alpha mRNA and protein in response to LPS, and are extremely responsive to the priming effect of IFN-gamma for subsequent TNF-alpha gene expression. Also, Lewis astrocytes produce TNF-alpha in response to IFN-gamma/IL-1 beta. The differential TNF-alpha production by astrocytes from BN and Lewis strains is not due to the suppressive effect of prostaglandins, because the addition of indomethacin does not alter the differential pattern of TNF-alpha expression. Furthermore, Lewis and BN astrocytes produce another cytokine, IL-6, in response to LPS, IFN-gamma, and IL-1 beta in a comparable fashion. Peritoneal macrophages and neonatal microglia from Lewis and BN rats are responsive to both LPS and IFN-gamma priming signals for subsequent TNF-alpha production, suggesting that differential TNF-alpha expression by the astrocyte is cell type specific. Taken together, these results suggest that differential TNF-alpha gene expression in response to LPS and IFN-gamma is strain and cell specific, and reflects both transcriptional and post-transcriptional control mechanisms.(ABSTRACT TRUNCATED AT 400 WORDS)

Induction and regulation of class II major histocompatibility complex mRNA expression in astrocytes by interferon-gamma and tumor necrosis factor-alpha

Astrocytes can function as antigen-presenting cells (APC) upon expression of class II major histocompatibility complex (MHC) antigens, which are induced by interferon-gamma (IFN-gamma). Previous data from this laboratory had shown that the cytokine tumor necrosis factor-alpha (TNF-alpha) enhances IFN-gamma-mediated class II antigen expression on astrocytes. We have now investigated the effect of IFN-gamma and TNF-alpha on class II MHC mRNA expression in astrocytes using Northern blot analysis. Astrocytes do not constitutively express mRNA for class II MHC. Kinetic analysis of class II MHC mRNA expression in IFN-gamma-treated cells demonstrated an 8 h time lag, which was followed by an increase over the next 16 h. Optimal expression of class II mRNA was detected after a 24 h incubation with IFN-gamma. This level of expression was further enhanced by the simultaneous addition of IFN-gamma and TNF-alpha to the astrocytes, while TNF-alpha alone had no effect on class II mRNA expression. TNF-alpha does not act by increasing the stability of IFN-gamma-induced class II mRNA, indicating its action is not at that specific level of post-transcriptional control. Furthermore, astrocyte class II mRNA expression was inhibited when cycloheximide (CHX) was added together with IFN-gamma or IFN-gamma/TNF-alpha, and when CHX was added up to 4 h after treatment with IFN-gamma or IFN-gamma/TNF-alpha. These results indicate that astrocyte class II mRNA expression is mediated by newly synthesized proteins induced by IFN-gamma and/or IFN-gamma/TNF-alpha. The expression of class II antigens on astrocytes, and cytokine modulation of their expression, may be important in the initiation and perpetuation of intracerebral immune responses.

Induction and regulation of interleukin-6 gene expression in rat astrocytes

Cells that produce interleukin-6 (IL-6) require the presence of signaling molecules since this cytokine is not normally constitutively expressed. It is now established that astrocytes produce IL-6; however, the precise inducing molecules and the kinetics of their action have not yet been clearly identified. In the current study, we show that either interleukin-1 beta (IL-1 beta) or tumor necrosis factor-alpha (TNF-alpha) exert a strong inducing signal for IL-6 in primary rat astrocytes. When the two cytokines are added together the response is synergistic, suggesting that each cytokine may induce IL-6 gene expression by different pathways. Interferon-gamma (IFN-gamma) does not affect IL-6 expression although if it is added in conjunction with IL-1 beta, an augmented induction of IL-6 occurs. In addition to the cytokines, bacterial lipopolysaccharide (LPS) and the calcium ionophore, A23187, induce IL-6 expression. IL-6 expression can be blocked by the glucocorticoid analogue, dexamethasone. IL-6 induction by LPS/Ca2+ ionophore is more sensitive to the suppressive effects of dexamethasone than is IL-6 induction by TNF-alpha/IL-1 beta. Cycloheximide (CHX), an inhibitor of protein synthesis, markedly increased levels of IL-6 mRNA in both unstimulated and stimulated astrocytes, indicating that ongoing protein synthesis is not required for astrocyte IL-6 gene expression. We propose that astrocyte-produced IL-6 may have a role in augmenting intracerebral immune responses in neurological diseases such as multiple sclerosis (MS), AIDS dementia complex (ADC), and viral infections. These diseases are characterized by infiltration of lymphoid and mononuclear cells into the central nervous system (CNS), and intrathecal production of immunoglobulins. IL-6 may act to promote terminal differentiation of B cells in the CNS, leading to immunoglobulin synthesis.

Tumor necrosis factor production and receptor expression by a human malignant glioma cell line, D54-MG

Human malignant gliomas possess some of the same immune-related functions as astrocytes do. For instance, they are capable of secreting various immunoregulatory molecules and expressing HLA-DR antigens on their surface. The human malignant glioma cell line, D54-MG, was used to investigate the proliferative effects of tumor necrosis factor-alpha (TNF-alpha) and the expression of specific surface receptors for TNF-alpha. Additionally, we were interested in examining whether D54-MG cells are capable of synthesizing and secreting biologically active TNF-alpha. D54-MG cells responded in a mitogenic fashion upon incubation with TNF-alpha for 48 h under serum-free conditions. 125I-labeled TNF-alpha was used in this study to investigate the expression of receptors specific for TNF-alpha on D54-MG cells. Scatchard analysis of our receptor binding data produced curvilinear plots indicating there are two distinct receptor sites for TNF-alpha. From these data, we calculated that there are approximately 3500 high affinity and 24,666 low affinity binding sites per cell. Pretreating these cells with interferon-gamma (IFN-gamma) resulted in a 2-fold increase in the number of high affinity binding sites and a moderate increase in the number of low affinity binding sites, with no appreciable change in binding affinity (Kd) of either site. D54-MG cells were unable to constitutively secrete TNF-alpha; however, upon stimulation, these cells synthesize and secrete biologically active TNF-alpha. Polyclonal antisera reactive with human macrophage-derived TNF-alpha neutralized the cytotoxicity of D54-MG-derived TNF-alpha, demonstrating that the cytotoxic activity was in fact due to TNF-alpha. Our observations indicate that TNF-alpha could act in an autocrine fashion to induce the proliferation of this malignant glioma cell line and that TNF-alpha exerts its effect by binding to specific TNF-alpha receptors whose expression was enhanced by IFN-gamma.

Tumor necrosis factor-alpha production by astrocytes. Induction by lipopolysaccharide, IFN-gamma, and IL-1 beta

Astrocytes have the capacity to secrete or respond to a variety of cytokines including IL-1, IL-6, IL-3, and TNF-alpha. In this study, we have examined the capacity of astrocytes to secrete TNF-alpha in response to a variety of biologic stimuli, particularly cytokines such as IL-1 and IFN-gamma, which are known to be present in the central nervous system during neurologic diseases associated with inflammation. Rat astrocytes do not constitutively produce TNF-alpha, but have the ability to secrete TNF-alpha in response to LPS, and can be primed by IFN-gamma to respond to a suboptimal dose of LPS. IFN-gamma and IL-1 beta alone do not induce TNF-alpha production, however, the combined treatment of IFN-gamma and IL-1 beta results in a striking synergistic effect on astrocyte TNF-alpha production. Astrocyte TNF-alpha protein production induced by a combined treatment of either IFN-gamma/LPS or IFN-gamma/IL-1 beta occurs in a dose- and time-dependent manner, and appears to require a "priming signal" initiated by IFN-gamma, which then renders the astrocyte responsive to either a suboptimal dose of LPS or IL-1 beta. Astrocyte TNF-alpha production by IFN-gamma/LPS stimulation can be inhibited by the addition of anti-rat IFN-gamma antibody, whereas IFN-gamma/IL-1-induced TNF-alpha production is inhibited by antibody to either IFN-gamma or IL-1 beta. Polyclonal antisera reactive with mouse macrophage-derived TNF-alpha neutralized the cytotoxicity of IFN-gamma/LPS and IFN-gamma/IL-1 beta-induced astrocyte TNF-alpha, demonstrating similarities between these two sources of TNF-alpha. We propose that astrocyte-produced TNF-alpha may have a pivotal role in augmenting intracerebral immune responses and inflammatory demyelination due to its diverse functional effects on glial cells such as oligodendrocytes and astrocytes themselves.

Tumor necrosis factor-alpha production by astrocytes. Induction by lipopolysaccharide, IFN-gamma, and IL-1 beta

Astrocytes have the capacity to secrete or respond to a variety of cytokines including IL-1, IL-6, IL-3, and TNF-alpha. In this study, we have examined the capacity of astrocytes to secrete TNF-alpha in response to a variety of biologic stimuli, particularly cytokines such as IL-1 and IFN-gamma, which are known to be present in the central nervous system during neurologic diseases associated with inflammation. Rat astrocytes do not constitutively produce TNF-alpha, but have the ability to secrete TNF-alpha in response to LPS, and can be primed by IFN-gamma to respond to a suboptimal dose of LPS. IFN-gamma and IL-1 beta alone do not induce TNF-alpha production, however, the combined treatment of IFN-gamma and IL-1 beta results in a striking synergistic effect on astrocyte TNF-alpha production. Astrocyte TNF-alpha protein production induced by a combined treatment of either IFN-gamma/LPS or IFN-gamma/IL-1 beta occurs in a dose- and time-dependent manner, and appears to require a "priming signal" initiated by IFN-gamma, which then renders the astrocyte responsive to either a suboptimal dose of LPS or IL-1 beta. Astrocyte TNF-alpha production by IFN-gamma/LPS stimulation can be inhibited by the addition of anti-rat IFN-gamma antibody, whereas IFN-gamma/IL-1-induced TNF-alpha production is inhibited by antibody to either IFN-gamma or IL-1 beta. Polyclonal antisera reactive with mouse macrophage-derived TNF-alpha neutralized the cytotoxicity of IFN-gamma/LPS and IFN-gamma/IL-1 beta-induced astrocyte TNF-alpha, demonstrating similarities between these two sources of TNF-alpha. We propose that astrocyte-produced TNF-alpha may have a pivotal role in augmenting intracerebral immune responses and inflammatory demyelination due to its diverse functional effects on glial cells such as oligodendrocytes and astrocytes themselves.

Tumor necrosis factor-alpha enhances interferon-gamma-mediated class II antigen expression on astrocytes

Astrocytes can function as antigen-presenting cells (APC) upon expression of class II antigens, which are induced by interferon-gamma (IFN-gamma). Tumor necrosis factor-alpha (TNF-alpha) can act synergistically with IFN-gamma with respect to class II expression on a variety of cells. As brain cells themselves can secrete TNF-like factors upon stimulation, we examined the effect of TNF-alpha on IFN-gamma-mediated class II induction on astrocytes. TNF-alpha alone had no effect on class II expression, but did synergize with IFN-gamma for enhanced expression of class II antigens. The specificity of TNF-alpha activity was demonstrated by blocking the amplifying effect of TNF-alpha with a polyclonal anti-TNF-alpha antibody. Kinetic analysis of the synergistic effect indicated that optimal TNF-alpha enhancement of class II expression was observed when astrocytes were pretreated with IFN-gamma 12-24 h prior to TNF-alpha addition. A possible mechanism for the synergistic action between IFN-gamma and TNF-alpha may be increased TNF-alpha receptor expression by IFN-gamma. Astrocytes treated with IFN-gamma for 24 h express more TNF-alpha receptors (3900/cell) than do untreated astrocytes (2483/cell), with no significant change in the binding affinity (Kd). These results suggest that the synergistic activity of TNF-alpha requires an inductive signal from IFN-gamma, which in part may be increased TNF-alpha receptor expression. Altogether, our observations indicate that TNF-alpha enhances ongoing class II major histocompatibility complex gene expression in rat astrocytes, which in this system is initially induced by IFN-gamma. TNF-alpha exerts its effect by binding to high affinity TNF-alpha receptors on astrocytes, whose expression is also enhanced by IFN-gamma. These two cytokines work in concert to elevate class II expression on astrocytes, an event which can contribute to initiation and/or perpetuation of intracerebral immune responses.

Monoclonal idiotypic and anti-idiotypic antibodies produced by immunization with peptides specified by a region of human myelin basic protein mRNA and its complement

Murine monoclonal antibodies (MAbs) selective for an idiotope on a monoclonal antibody (IgG1/kappa) to human myelin basic protein (MBP) peptide 80-89 were prepared by immunization with a synthetic decapeptide specified by RNA that is complementary to the mRNA for human MBP peptide 80-89. The monoclonal anti-idiotypic antibody (anti-ID) reacted with the MAb to human MBP peptide 80-89 but not with a MAb to bovine MBP peptide 79-88 or to murine myeloma IgG1. The reaction between the monoclonal anti-ID and the MAb to the human MBP peptide 80-89 could be inhibited by human MBP peptide 80-89 and to a more limited degree with human MBP peptide 76-85 and bovine MBP peptide 79-88, but not by human MBP peptides 69-81 and 85-96. Practically, the use of a complementary peptide for stimulating an anti-ID response permits a more selective and feasible method for preparing anti-ID reagents. Theoretically, these results provide further support for the molecular basis of the network hypothesis.

Human B cell growth factor enhances proliferation and glial fibrillary acidic protein gene expression in rat astrocytes

The proliferation and differentiation of astrocytes are fundamental events in the normal development and function of the central nervous system (CNS), and may also contribute to the pathogenesis of a number of neurological diseases. Products of T lymphocytes can stimulate proliferation of astrocytes, but the nature of the T lymphocyte-derived molecule(s) responsible for this response is unknown. The present study was undertaken to examine several well-characterized T lymphocyte-derived factors for their ability to stimulate cultured primary rat astrocytes. While recombinant human interleukin-2 (IL-2), interleukin-3 (IL-3), interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-6 (IL-6), and rat or human recombinant interferon-gamma (IFN-gamma) have no proliferative effect on astrocytes, a human T cell-derived B cell growth factor (BCGF) does. This BCGF, termed 2B11, had previously been characterized by its ability to enhance the proliferation of anti-mu-stimulated human B cells, while not influencing B cell immunoglobulin synthesis. High performance liquid chromatography (HPLC)-purified 2B11-BCGF (MW approximately 20,000 daltons) stimulates the proliferation of astrocytes in a dose-dependent fashion. Purified 2B11-BCGF also induced morphological differentiation and increased mRNA transcripts for glial fibrillary acidic protein (GFAP) in rat astrocytes. In addition to demonstrating the absence of effect of other known lymphokines, the effect on astrocytes attributed to 2B11-BCGF was confirmed by blocking its activity with a monoclonal antibody specific for 2B11-BCGF. Absorption experiments demonstrated that when BCGF activity was absorbed out by large, activated human B cells, astrocyte-stimulatory activity was also depleted. Rat astrocytes were able to partially absorb out both BCGF and astrocyte-stimulatory activity. These results suggest that 2B11-BCGF is responsible for stimulating astrocyte proliferation, and that human B cells and rat astrocytes may share a similar receptor for BCGF. These findings indicate that the growth and differentiation of astrocytes can be influenced by a T cell-derived lymphokine, 2B11-BCGF, whose activity thus far appears to be distinct from other reported cytokines.

Response of human glioblastoma cells to recombinant interleukin-2

We investigated the ability of glioma cells to respond to T cell-derived lymphokines. The growth of astrocytoma and mixed glioblastoma cell lines, as assessed by DNA synthesis, was inhibited in the presence of supernatants derived from mitogen-stimulated human T cells, an HTLV-II-transformed human T cell line, Mo, and human interleukin-2 (IL-2). The mixed glioblastoma cell line, 138-MG-C, was subjected to limiting dilution analysis, and two cell lines (5D7, 5C5) were derived which were homogeneous with respect to staining for galactocerebroside (GalC) (100%). These two GalC+ glioblastoma cell lines proliferated in the presence of high concentrations of recombinant human interleukin-2 (RIL-2). Additionally, these cell lines bear receptors for the IL-2 molecule as determined by immunofluorescent staining with various anti-IL-2 receptor antibodies.

Myelin basic protein-specific RNA levels in interleukin-2-stimulated oligodendrocytes

In the immune system, T- and B-cell proliferation, as well as B-cell immunoglobulin secretion, is induced by interleukin-2 (IL-2), a T-cell-derived lymphokine. IL-2 also influences the growth of glial cells, specifically, the proliferation and maturation of oligodendrocytes. Studies were conducted to investigate further IL-2-induced maturation of oligodendrocytes through its effect on the regulation of the myelin basic protein (MBP) gene. A cDNA probe specific for rat MBP and a double-antibody radioimmunoassay for MBP were used to quantitate MBP mRNA and protein levels in oligodendrocytes under different experimental conditions. We demonstrate that both MBP mRNA and protein levels are increased in IL-2-stimulated oligodendrocytes. MBP mRNA levels increase within 8 h after IL-2 stimulation, peak between 24 and 48 h, and then decline slightly. MBP protein levels increase 24 h after stimulation and peak at 72 h. MBP mRNA transcripts in the range of 2.0-2.4 kilobases are present in cultured rat oligodendrocytes, which are similar to the MBP mRNA transcripts detected in whole rat and mouse brain. These mRNA transcripts are specifically increased in quantity after oligodendrocyte stimulation with IL-2. These results suggest that one component of oligodendrocyte differentiation/maturation--MBP mRNA and protein expression--can be regulated in part by IL-2.

Remyelination by oligodendrocytes stimulated by antiserum to spinal cord

The new synthesis of myelin and the proliferation of oligodendrocytes was stimulated by serum from syngeneic mice immunized with homogenized spinal cord (SCH). Treatment with this antiserum produced a 10-fold increase in the area of remyelination in spinal cords that had become demyelinated previously as a result of infection by Theiler's murine encephalomyelitis virus. Inflammation was decreased in regions of white matter that showed remyelination. Oligodendrocytes exposed to anti-SCH in vitro incorporated three to five times more [3H]thymidine than resting cells did and expressed more myelin basic protein in their cytoplasm, suggesting stimulation of myelinogenesis. Thus, there is a factor present in anti-SCH antiserum that stimulates central nervous system-type remyelination. This finding may provide clues for the therapy of patients with demyelinating disorders such as multiple sclerosis.

Stimulation of oligodendroglial proliferation and maturation by interleukin-2

There exists considerable evidence that the growth of glial cells can be influenced by T-cell-derived lymphokines and monokines. Astrocytes proliferate in the presence of mitogen- or antigen-stimulated T-cell supernatants, supernatants from human T-lymphotropic virus (HTLV)-transformed T cells, and purified human interleukin-1 (IL-1; ref. 4). Oligodendrocytes proliferate and differentiate when incubated with supernatants from mitogen-activated or HTLV-transformed T cells. In addition, we have recently purified a T-cell-derived lymphokine of relative molecular mass 30,000, termed glial growth promoting factor (GGPF), which specifically stimulates the proliferation of oligodendrocytes. The traditional role of interleukins 1 and 2 is in the initiation, propagation and regulation of the immune response. IL-1, released by a variety of cells including monocytes, stimulates T cells to produce IL-2; IL-2 in turn induces the expansion of T cells that is critical for immune responsiveness. Recently, IL-2 has been shown to induce B-cell proliferation and immunoglobulin secretion, indicating that its action is not restricted to T cells. We now report that recombinant human IL-2 influences the growth of glial cells--specifically, the proliferation and differentiation of oligodendrocytes. IL-2 may have a role in the inflammatory neural lesions of multiple sclerosis patients and in the growth of brain glia during injury or disease.