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

Hypoxia and the hypoxia-inducible-factor pathway in glioma growth and angiogenesis

Glioblastomas, like other solid tumors, have extensive areas of hypoxia and necrosis. The importance of hypoxia in driving tumor growth is receiving increased attention. Hypoxia-inducible factor 1 (HIF-1) is one of the master regulators that orchestrate the cellular responses to hypoxia. It is a heterodimeric transcription factor composed of alpha and beta subunits. The alpha subunit is stable in hypoxic conditions but is rapidly degraded in normoxia. The function of HIF-1 is also modulated by several molecular mechanisms that regulate its synthesis, degradation, and transcriptional activity. Upon stabilization or activation, HIF-1 translocates to the nucleus and induces transcription of its downstream target genes. Most important to gliomagenesis, HIF-1 is a potent activator of angiogenesis and invasion through its upregulation of target genes critical for these functions. Activation of the HIF-1 pathway is a common feature of gliomas and may explain the intense vascular hyperplasia often seen in glioblastoma multiforme. Activation of HIF results in the activation of vascular endothelial growth factors, vascular endothelial growth factor receptors, matrix metalloproteinases, plasminogen activator inhibitor, transforming growth factors alpha and beta, angiopoietin and Tie receptors, endothelin-1, inducible nitric oxide synthase, adrenomedullin, and erythropoietin, which all affect glioma angiogenesis. In conclusion, HIF is a critical regulatory factor in the tumor microenvironment because of its central role in promoting proangiogenic and invasive properties. While HIF activation strongly promotes angiogenesis, the emerging vasculature is often abnormal, leading to a vicious cycle that causes further hypoxia and HIF upregulation.

Interaction of transforming growth factor-beta (TGF-beta) and epidermal growth factor (EGF) in human glioma cells

Gliomas are characterized by a deregulation of growth factor production and growth factor receptors expression, e.g. overproduction of the cytokine transforming growth factor-beta (TGF-beta) and overexpression/constitutive activation of receptors for the epidermal growth factor (EGF). Potential interactions of such growth factors and their signaling cascades could enhance the malignancy of these tumors. Therefore, we investigated the effects of TGF-beta and EGF alone and in combination on the proliferation of glioma cells cultivated from eight solid human WHO grade IV gliomas and one glioma cell line, analyzed the expression and intactness of the TGF-beta-signaling molecules Samd-4 and -2, and the phosphorylation of the EGF-signaling kinases ERK 1/2. The effects were divergent and complex: Whereas EGF mostly stimulated glioma cell proliferation, TGF-beta either enhanced, inhibited or had no significant effect on proliferation. In combination, co-stimulation and inhibition of the EGF-induced mitogenic activity could be observed. Smad-4/-2 were expressed in all glioma cells, one point mutation at base 1595 in Smad-4 did not affect its protein sequence. In part of the glioma cells, reduced phosphorylation of ERK 1/2 and expression of cyclin-dependent kinase inhibitor 1 or p21 was observed in co-stimulation experiments. These experiments show that TGF-beta can inhibit EGF-mediated effects only in some gliomas, whereas it enhances it in others. The interaction of both factors is very complex and varies between different gliomas.

Modulation of tissue-type plasminogen activator expression by platelet activating factor in human glioma cells

PURPOSE

For tumor growth, proteolytic remodeling of the extracellular matrix (ECM) is a key factor. To determine proteolytic activity in human glioma cells, fibrinolytic activity, mRNA expression of fibrinolytic factors, and fibrinolytic inhibitors were studied in human glioma cell lines. The effect of platelet activating factor (PAF), a potent mediator of inflammatory and immune responses, on this fibrinolytic activity was also examined.

METHODS

The fibrinolytic activities of conditioned medium and cell lysates from human glioma cell lines, A172, T98G, U87 and TM1 were studied by fibrin plate zymography. mRNA expression of tissue plasminogen activator (tPA), urokinase-type plasminogen activator (uPA) and plasminogen activator inhibitors (PAI-1, PAI-2) was measured by Northern blot analysis. PAF was added to the medium, and its effects on cell proliferation, fibrinolytic activity, mRNA expression of plasminogens and inhibitors were studied.

RESULTS

mRNA expression of plasminogens and inhibitors differed between individual cell lines. Only the medium and cell lysates from A172 cells revealed fibrinolytic activity. A172 cells showed mRNA expression of tPA. PAF at low concentrations, such as 1 nM, stimulated A172 cell proliferation, and high concentrations of PAF inhibited proliferation. PAF stimulated tPA release into the conditioned medium. mRNA expression of tPA was stimulated by low concentrations of PAF and inhibited by high concentrations.

CONCLUSION

The variability of mRNA expression of plasminogen activators (PAs) between different glioma cell lines may indicate that plasminogens and their inhibitors do not directly correlate with brain tumor growth. PAF may be an important factor in the local control of fibrinolytic activity in glioma and its proliferation.

Tissue factor and cancer procoagulant expressed by glioma cells participate in their thrombin-mediated proliferation

The relationship between coagulation cascade activation and glioma cell proliferation was examined. The human glioma cell lines T98G, TM-1 and normal human astrocyte cell strain (NHA) were examined. Using anti-tissue factor (TF) antibody, immunocytochemical detection of TF antigen was obtained in both cell lines and cell strain. TF antigen in cell lysates was also measured by enzyme linked immunosorbent assay (ELISA). In a one-stage clotting assay, T98G, TM-1 and NHA revealed procoagulant activity (PCA) in normal human plasma and factor VII deficient plasma. PCA in normal human plasma was significantly inhibited by both inhibitory anti-TF antibody and cysteine protease inhibitor HgCl2. This result indicates that T98G, TM-1 and NHA cells express not only TF but also cancer procoagulant (CP) at the same time. In a cell proliferation assay, thrombin induced proliferation in T98G and TM-1 cells in a dose-dependent fashion and in NHA cell in a bell-shaped fashion. This mitogenic stimulant was inhibited by the specific thrombin inhibitor hirudin. The combinations of coagulation factors II, V, and X with or without factor VII induced proliferation in T98G, TM-1, and NHA cells. The maximal mitogenic stimulatory effects were larger in glioma cells than in NHA. These mitogenic stimulatory effects were also inhibited by hirudin. Each coagulation factor on its own or in any other combination of coagulation factors had no proliferative effect. Thus, these mitogenic stimulatory effects were considered to be the effect of thrombin. In conclusion, T98G and TM-1 human glioma cells express two different types of procoagulants TF and CP. In the presence of coagulation factors, these glioma cells can generate thrombin and this thrombin generation is capable of inducing glioma cell proliferation in vitro.

What role(s) for TGFalpha in the central nervous system?

Transforming growth factor alpha (TGFalpha) is a member of the epidermal growth factor (EGF) family with which it shares the same receptor, the EGF receptor (EGFR or erbB1). Identified since 1985 in the central nervous system (CNS), its functions in this organ have started to be determined during the past decade although numerous questions remain unanswered. TGFalpha is widely distributed in the nervous system, both glial and neuronal cells contributing to its synthesis. Although astrocytes appear as its main targets, mediating in part TGFalpha effects on different neuronal populations, results from different studies have raised the possibility for a direct action of this growth factor on neurons. A large array of experimental data have thus pointed to TGFalpha as a multifunctional factor in the CNS. This review is an attempt to present, in a comprehensive manner, the very diverse works performed in vitro and in vivo which have provided evidences for (i) an intervention of TGFalpha in the control of developmental events such as neural progenitors proliferation/cell fate choice, neuronal survival/differentiation, and neuronal control of female puberty onset, (ii) its role as a potent regulator of astroglial metabolism including astrocytic reactivity, (iii) its neuroprotective potential, and (iv) its participation to neuropathological processes as exemplified by astroglial neoplasia. In addition, informations regarding the complex modes of TGFalpha action at the molecular level are provided, and its place within the large EGF family is precised with regard to the potential interactions and substitutions which may take place between TGFalpha and its kindred.

Plasma platelet-derived growth factor-B chain is elevated in patients with extensively large brain tumour

The plasma concentration of the platelet-derived growth factor (PDGF)-B chain and the plasma platelet factor 4 (PF4) levels were measured in 17 healthy controls and 55 brain tumour patients. In the 17 normal controls, the plasma PDGF-B and PF4 levels were 523 +/- 157 pg/ml (mean +/- SD) and 84 +/- 37 ng/ml, respectively. In the brain tumour patients, these values were 881 +/- 854 pg/ml and 93 +/- 64 ng/ml, respectively. The plasma PDGF-B concentration was elevated above the upper limit of normal individuals in 12 (22%) of the 55 patients. However, since the corresponding PF4 levels suggested the platelet activation, the increased plasma PDGF-B may have originated from platelets. To address this, platelet releasing experiments were performed on citrated blood samples from 5 normal individuals. The plasma PDGF-B and PF4 levels from the 17 normal controls and those observed in the platelet releasing experiments correlated with a regression line of Y = 240 + 4.86X (Y:PDGF, X:PF4). There were only 6 (11%) patients whose plasma PDGF-B level was elevated above the 95% confidence limit estimated from the corresponding PF4 value. In these patients, the tumour volumes were extensively large, and those elevated PDGF-B values decreased after treatment and became elevated again in three patients with recurrent glioblastoma. Although the plasma tumour-derived PDGF-B was detected only in an extensively large brain tumour, it might be a useful plasma marker evaluating the effects of therapy and prognosis in such patients.