Ionizing radiation induces apoptosis and inhibits neuronal differentiation in rat neural stem cells via the c-Jun NH2-terminal kinase (JNK) pathway

A substantial number of neural stem cells (NSCs) continue to proliferate and generate neurons in the central nervous system throughout life. Ionizing radiation, an important adjuvant therapy for glioma patients, may damage NSCs and cause neuronal deficits, such as cognitive dysfunction and memory impairment. However, the precise mechanism of radiation effects on death and differentiation of NSCs remains largely unknown. Here, we found that radiation induced apoptosis in NSCs via the mitochondrial pathway, upregulating the ratio of Bax to Bcl-2 and releasing cytochrome c into the cytoplasm. Radiation also inhibited neuronal differentiation of NSCs by 50%. Of the three stress-associated mitogen-activated protein kinases (MAPKs), only c-Jun NH(2)-terminal kinase (JNK) was activated in NSCs after radiation. Interestingly, JNK inhibition by the specific inhibitor SP600125 rescued NSCs from apoptosis and improved neuronal differentiation. Furthermore, we examined whether radiation directly inhibits neuronal differentiation or not. Radiation did not affect the promoter activity of NeuroD, a basic helix-loop-helix transcription factor that regulates the expression of neuronal differentiation markers. Radiation induced more apoptosis in NeuroD-positive cells than NeuroD-negative cells. We concluded that radiation activates JNK and induces apoptosis, especially in neural progenitor cells, resulting in the inhibition of neurogenesis. Our findings raise the possibility that JNK inhibition has therapeutic potential in protecting NSCs from the adverse effects of radiation.

Histone deacetylase inhibitors, N-butyric acid and trichostatin A, induce caspase-8- but not caspase-9-dependent apoptosis in human malignant glioma cells

Histone deacetylase (HDAC) inhibitors have both apoptotic and differentiating effects on various tumor cells. However, the mechanisms underlying the effect of HDAC inhibitors remain unclear. In this study, we investigated the function of anti-proliferative effects of HDAC inhibitors, N-butyric acid and trichostatin A, on human malignant glioma cell lines, U251-MG and D54. MTT assay showed a dose-dependent inhibition of cellular proliferation in both cell lines. Cell cycle analysis revealed increased sub-G1 population in both lines, and G1 arrest only in U251-MG cells. Induction of apoptosis was also supported by the occurrence of DNA fragmentation in tumor cells treated with HDAC inhibitors. Furthermore, caspase inhibition assay indicated that HDAC inhibitor-induced apoptosis was caspase-dependent. Neither mitochondrial membrane potential nor the expression of caspase-9 was changed by treatment with HDAC inhibitors, suggesting the possibility that HDAC inhibitor-induced apoptosis was not mediated by the mitochondrial cell death pathway. On the other hand, immunoblot assay confirmed increased expression of caspase-8 in both lines, and elevation of p21 but not p27 protein in U251-MG cells following HDAC inhibitor treatment. Taken together, the HDAC inhibitors, N-butyric acid and trichostatin A, induce caspase-8- but not caspase-9-dependent apoptosis with or without p21-mediated G1 arrest in human malignant glioma cells.

Role of autophagy in temozolomide-induced cytotoxicity for malignant glioma cells

Autophagy is originally named as a process of protein recycling. It begins with sequestering cytoplasmic organelles in a membrane vacuole called autophagosome. Autophagosomes then fuse with lysosomes, where the materials inside are degraded and recycled. To date, however, little is known about the role of autophagy in cancer therapy. In this study, we present that temozolomide (TMZ), a new alkylating agent, inhibited the viability of malignant glioma cells in a dose-dependent manner and induced G2/M arrest. At a clinically achievable dose (100 microM), TMZ induced autophagy, but not apoptosis in malignant glioma cells. After the treatment with TMZ, microtubule-associated protein light-chain 3 (LC3), a mammalian homologue of Apg8p/Aut7p essential for amino-acid starvation-induced autophagy in yeast, was recruited on autophagosome membranes. When autophagy was prevented at an early stage by 3-methyladenine, a phosphatidylinositol 3-phosphate kinase inhibitor, not only the characteristic pattern of LC3 localization, but also the antitumor effect of TMZ was suppressed. On the other hand, bafilomycin A1, a specific inhibitor of vacuolar type H(+)-ATPase, that prevents autophagy at a late stage by inhibiting fusion between autophagosomes and lysosomes, sensitized tumor cells to TMZ by inducing apoptosis through activation of caspase-3 with mitochondrial and lysosomal membrane permeabilization, while LC3 localization pattern stayed the same. These results indicate that TMZ induces autophagy in malignant glioma cells. Application of an autophagy inhibitor that works after the association of LC3 with autophagosome membrane, such as bafilomycin A1, is expected to enhance the cytotoxicity of TMZ for malignant gliomas.

Inhibition of platelet-derived growth factor signalling induces autophagy in malignant glioma cells

Malignant gliomas highly coexpress platelet-derived growth factor (PDGF) and its receptor, suggesting the presence of an autocrine loop. Therefore, disruption of PDGF ligand/receptor complex represents a promising strategy for the treatment of malignant gliomas. However, the mechanisms of the antitumour effect exerted by the inhibition of PDGF-mediated cell growth remain unclear. In the present study, using anti-PDGF neutralising antibody, we investigated the effect of the inhibition of PDGF signalling on malignant glioma U87-MG, D54, and T98G cells with high levels of PDGF-A and -B. As a control, normal fibroblast MRC5 cells expressing low levels of PDGF-A and -B were used. Treatment with anti-PDGF neutralising antibody did not affect the expressions of PDGF-A, PDGF-B, and Akt, but suppressed the level of phosphorylated Akt in tumour cells, indicating the inhibition of PDGF signalling. The cell viability of all malignant glioma cells tested in this study was significantly inhibited in a time-dependent manner following the treatment compared to that of fibroblast cells (P<0.02 to <0.05). The antitumour effect of anti-PDGF antibody was suppressed by the activation of Akt and enhanced by the downregulation of Akt. Interestingly, the inhibition of PDGF signalling induced the development of acidic vesicular organelles and the autophagosome membrane association of the microtubule-associated protein light chain 3, which are characteristic of autophagy, in malignant glioma cells, while apoptotic cell death was not observed. Together these findings imply a new concept of autophagy for PDGF autocrine inhibition in malignant gliomas.

Inhibition of telomerase activity in malignant glioma cells correlates with their sensitivity to temozolomide

Temozolomide (TMZ, 3,4-dihydro-3-methyl-4-oxoimidazo [5,1-d]-as-tetrazine-8-carboxamide) is a new alkylating agent with promising antitumour efficacy for malignant gliomas. The resistance of tumour cells to TMZ is primarily associated with levels of the alkylguanine alkyltransferase (AGT). O(6)-benzylguanine (O(6)-BG), an inhibitor for AGT, reduced resistance to TMZ. Recently, it has been demonstrated that chemosensitivity of tumour cells is related to a decline in telomerase activity. However, it is unknown if TMZ sensitivity of malignant glioma cells correlates with telomerase. In this study, using malignant glioma cells with low levels of AGT (U373-MG and U87-MG) and high levels of AGT (T98G), we investigated the association among AGT, telomerase, and TMZ sensitivity. U373-MG and U87-MG cells were sensitive to TMZ (IC(50) for a 2-day treatment=100 microM), while T98G cells were resistant to TMZ (IC(50) for a 2-day treatment >500 microM). Treatment with TMZ (100 microM) suppressed telomerase activity in U373-MG and U87-MG cells in a time-dependent manner, but not in T98G cells. The downregulation of telomerase activity in U373-MG and U87-MG cells was due to inhibition of the human telomerase reverse-transcriptase (hTERT) gene expression at the transcriptional level. This inhibitory effect was induced by interfering with transcription factor Sp1 binding sites of the hTERT core promoter. Interestingly, O(6)-BG not only sensitised T98G cells to TMZ, but also suppressed telomerase activity. These findings suggest that response of malignant glioma cells to TMZ can be monitored by reduction in telomerase activity. Therefore, quantification of telomerase activity during or after treatment with TMZ may be a useful marker to detect treatment efficacy.

Antitumour effect of cyclin-dependent kinase inhibitors (p16(INK4A), p18(INK4C), p19(INK4D), p21(WAF1/CIP1) and p27(KIP1)) on malignant glioma cells

Cyclin-dependent kinase inhibitors (CDKIs) are considered as novel anticancer agents because of their ability to induce growth arrest or apoptosis in tumour cells. It has not yet been fully determined, however, which CDKI is the best candidate for the treatment of malignant gliomas and whether normal brain tissues are affected by CDKI expression. Using recombinant adenoviral vectors that express CDKIs (p16(INK4A), p18(INK4C), p19(INK4D), p21(WAF1/CIP1) and p27(KIP1)), we compared the antitumour effect of CDKIs on malignant glioma cell lines (A172, GB-1, T98G, U87-MG, U251-MG and U373-MG). p27(KIP1) showed higher ability to suppress the growth of all tumour cells tested than other CDKIs. Interestingly, overexpression of p27(KIP1) induced autophagic cell death, but not apoptosis in tumour cells. On the other hand, p27(KIP1) overexpression did not inhibit the viability of cultured astrocytes (RNB) nor induced autophagy. Overall, our findings suggest that gene transfer of p27(KIP1) may be a promising approach for the therapy of malignant gliomas.

Treatment of malignant glioma cells with the transfer of constitutively active caspase-6 using the human telomerase catalytic subunit (human telomerase reverse transcriptase) gene promoter

Because the apoptotic pathway is often disrupted in tumor cells, its genetic restoration is a very attractive approach for the treatment of tumors. To treat malignant gliomas with this approach, it would be preferred to restrict induction of apoptosis to tumor cells by establishing a tumor-specific expression system. Telomerase is an attractive target because the vast majority of malignant gliomas have telomerase activity whereas normal brain cells do not. Activation of telomerase is tightly regulated at the transcriptional level of the telomerase catalytic subunit [human telomerase reverse transcriptase, (hTERT)]. Therefore, we hypothesized that using a hTERT promoter-driven vector system, an apoptosis-inducible gene may be preferentially restricted to telomerase- or hTERT-positive tumor cells. In this study, we constructed an expression vector consisting of the constitutively active caspase-6 (rev-caspase-6) under the hTERT promoter (hTERT/rev-caspase-6) and then investigated its antitumor effect on malignant glioma cells. The rationale for using the rev-caspase-6 gene is because it induces apoptosis independent of the initiator caspases. We demonstrated that the hTERT/rev-caspase-6 construct induced apoptosis in hTERT-positive malignant glioma cells, but not in hTERT-negative astrocytes, fibroblasts, and alternative lengthening of telomeres cells. In addition, the growth of s.c. tumors in nude mice was significantly suppressed by the treatment with hTERT/rev-caspase-6 construct. The present results strongly suggest that the telomerase-specific transfer of the rev-caspase-6 gene under the hTERT promoter is a novel targeting approach for the treatment of malignant gliomas.

Differentiated regulation of allo-antigen presentation by different types of murine microglial cell lines

We established granulocyte-macrophage colony-stimulating factor (GM-CSF)-dependent murine microglial clones and investigated the immune properties of four individual clones. All four clones expressed MHC class I and CD54 (ICAM-1) at similar levels. The 5-2, Ra2, and 6-3 clones expressed CD80 (B7-1), CD86 (B7-2), and MHC class II at low, medium, and high levels, respectively. Only the 6-3 clone expressed CD40. Generally, the levels of co-stimulation and CD 40 signals had a profound effect on the response to antigens. The 5-2, Ra2, and 6-3 clones, however, stimulated allogenic T-cell proliferation to the same extent or less compared to spleen cells. Although the 6-1 clone expressed co-stimulatory and MHC molecules at levels similar to Ra2, it suppressed allogenic T-cell proliferation, unlike Ra2. Thus, allo-antigen presentation by microglial clones was not correlated with the expression of CD40 and co-stimulatory molecules. When microglial clones were fixed with paraformaldehyde, they enhanced IL-2-dependent T-cell proliferation according to the level of their expression of co-stimulatory molecules. Furthermore, conditioned medium from the 6-1 clone inhibited the T-cell response to allo-antigen. This indicates that some factor(s) derived from a microglial subtype may play an important role in the regulation of T-cell proliferation in addition to the molecules involved in antigen presentation. Moreover, these results also suggest that there may be specialized subtypes of microglia that regulate the immune response in the CNS.

Melanotic cerebral astrocytoma: case report and literature review

We describe the case of 47-year-old man with a cystic, melanotic temporal lobe astrocytoma who had a history of complex partial seizures. The tumor mass was made up of two histologically different regions: one consisted of spindle-shaped and pleomorphic cells often with foamy or vacuolated cytoplasm, while the other consisted of fairly uniform spindle-shaped cells, many of which contained dark-brown intracytoplasmic pigment. Desmoplasia was also noted in the latter region of the tumor. No features suggestive of malignancy, such as mitotic figures, necrotic foci or endothelial vascular proliferation, were observed throughout the tumor. Immunohistochemically, the tumor cells in both regions were positive for glial fibrillary acidic protein. Ultrastructural examination of the pigmented region showed the presence of melanosomal melanin in the tumor cells. Apart from the partial pigmentation, the entire histological picture resembled a pleomorphic xanthoastrocytoma. To our knowledge, only two cases of similar melanotic astrocytic tumors have been described previously. Interestingly, the astrocytic tumors in both of these patients were also clinically associated with epilepsy, were located in the temporal lobe, and were histologically benign.