In the adult human brain, normal astrocytes constitute nearly 40% of the total central nervous system (CNS) cell population and may assume a star-shaped configuration resembling epithelial cells insofar as the astrocytes remain intimately associated, through their cytoplasmic extensions, with the basement membrane of the capillary endothelial cells and the basal lamina of the glial limitans externa. Although their exact function remains unknown, in the past, astrocytes were thought to subserve an important supportive role for neurons, providing a favorable ionic environment, modulating extracellular levels of neurotransmitters, and serving as spacers that organize neurons. In immunohistochemical preparations, normal, reactive, and neoplastic astrocytes may be positively identified and distinguished from other CNS cell types by the expression of the astrocyte-specific intermediate filament glial fibrillary acidic protein (GFAP). Glial fibrillary acidic protein is a 50-kD intracytoplasmic filamentous protein that constitutes a portion of, and is specific for, the cytoskeleton of the astrocyte. This protein has proved to be the most specific marker for cells of astrocytic origin under normal and pathological conditions. Interestingly, with increasing astrocytic malignancy, there is progressive loss of GFAP production. As the human gene for GFAP has now been cloned and sequenced, this review begins with a summary of the molecular biology of GFAP including the proven utility of the GFAP promoter in targeting genes of interest to the CNS in transgenic animals. Based on the data provided the authors argue cogently for an expanded role of GFAP in complex cellular events such as cytoskeletal reorganization, maintenance of myelination, cell adhesion, and signaling pathways. As such, GFAP may not represent a mere mechanical integrator of cellular space, as has been previously thought. Rather, GFAP may provide docking sites for important kinases that recognize key cellular substrates that enable GFAP to form a dynamic continuum with microfilaments, integrin receptors, and the extracellular matrix.

No abstract

Astrocytic tumors frequently exhibit defects in the expression or activity of proteins that control cellcycle progression. Inhibition of kinase activity associated with cyclin/cyclin-dependent kinase co-complexes by cyclin-dependent kinase inhibitors is an important mechanism by which the effects of growth signals are down-regulated. We undertook the present study to determine the role of p57 KIP2 (p57) in human astrocytomas. We demonstrate here that whereas p57 is expressed in fetal brain tissue, specimens of astrocytomas of varying grade and permanent astrocytoma cell lines do not express p57, and do not contain mutations of the p57 gene by multiplex-heteroduplex analysis. However, the inducible expression of p57 in three well-characterized human astrocytoma cell lines (U343 MG-A, U87 MG, and U373 MG) using the tetracycline repressor system leads to a potent proliferative block in G 1 as determined by growth curve and flow cytometric analyses. After the induction of p57, retinoblastoma protein, p107, and E2F-1 levels diminish, and retinoblastoma protein is shifted to a hypophosphorylated form. Morphologically, p57-induced astrocytoma cells became large and flat with an expanded cytoplasm. The inducible expression of p57 leads to the accumulation of senescence-associated ␤-galactosidase marker within all astrocytoma cell lines such that ϳ75% of cells were positive at 1 week after induction. Induction of p57 in U373 astrocytoma cells generated a small population of cells (ϳ15%) that were nonviable, contained discrete nuclear fragments on The most common brain tumor is the astrocytoma accounting for ϳ65% of all primary brain tumors. The malignant astrocytoma has a very poor prognosis primarily because of its highly proliferative and invasive nature. As with other neoplasms with increased proliferative potential, malignant astrocytomas demonstrate dysregulation of various components of the cell cycle machinery. Altered expression of positive growth regulators such as growth factors, cyclins, and cyclin-dependent kinases (CDKs), or the loss of negative regulators, including cyclin-dependent kinase inhibitors (CKIs) and the retinoblastoma protein (pRB) have all been demonstrated in malignant astrocytomas. 1,2 The CDKs phosphorylate pRB to release cells from cell-cycle arrest. In contrast with CDKs, the CKIs inhibit cyclin-CDK complexes and transduce internal or external growth suppressive signals. Accordingly, all CKIs may be construed as candidate tumor suppressor genes.The CKIs are divided into two families, the INK4 and the CIP/KIP, which are defined on the basis of their structural homology and mechanism of action. The CIP/KIP family includes three structurally related members, p21 CIP1/WAF1 , 3,4 p27 KIP1 , 5,6 and a recently isolated and cloned third member, p57 KIP2 (p57). 7-10 These three CKIs share a common N-terminal domain for binding to and inhibiting the kinase activity of CDK-cyclin complexes. Mouse p57 consists of four structurally distinct domains, a CDK inhibitory domain, a proline-rich domain, an a...

Ribozymes targeted to aberrant EGFR mRNA can inhibit the growth of tumors formed by cells that express this mRNA.

Elastin has been identified within the meninges and the microvasculature of the normal human brain. However, the role that elastin plays in either facilitating astrocytoma cell attachment to these structures or modulating astrocytoma invasion has not been previously characterized. We have recently shown that astrocytoma cell lines and specimens produce tropoelastin, and express the 67 kDa elastin binding protein (EBP*). In the present report, we have established that astrocytoma cells attach to elastin as a substrate in vitro. The U87 MG astrocytoma cell line demonstrated the greatest degree of adhesion. In addition, all astrocytoma cell lines examined were capable of penetrating and migrating through an intact elastin membrane, and of degrading tritiated‐elastin, a process that could be prevented by the pre‐incubation of astrocytoma cells with EDTA, but not with α1‐antitrypsin. Astrocytoma cells were also capable of penetrating 1 mm sections of human brain tissue maintained as organotypic cultures. Interestingly, the invasive potential of cultured astrocytoma cells plated on organotypic cultures of human brain was significantly increased after exposure to elastin degradation products (κ‐elastin), which interact with astrocytoma cell surface EBP. Our data show that astrocytoma cells express a functional 67 kDa EBP, enabling them to potentially recognize and attach to elastin as a substrate. These data also suggest that this elastin receptor may be involved in processes which regulate regional astrocytoma invasion. GLIA 25:179–189, 1999. © 1999 Wiley‐Liss, Inc.