Gene expression studies in human astrocytoma, with emphasis on oncostatin M induced effects

Abstract

Brain tumors comprise about one percent of all cancers and astrocytoma is the most common primary brain malignancy. According to the World Health Organization system, the astrocytomas are divided into four grades according to degree of malignancy. Grade I, pilocytic astrocytoma (PA), is the least malignant form and most often affects children. Grade IV, glioblastoma multiforme (GBM), is the most malignant astrocytoma and has very poor prognosis, with an average survival of 9 - 11 months despite current therapies. This project started as a screening project, in which RT-PCR was used to search for over-expressed cytokines in human brain tumors. In our first paper, we discovered that members of the IL-6 cytokine family were highly expressed in different types of brain tumors. One of them was oncostatin M (OSM), a multifunctional cytokine, produced in the central nervous system after injury, inflammation and various diseases. Immunohistochemistry analysis showed that the tumor cells contained OSM in their cytoplasm, suggesting they produce this factor. The expression of OSM in non-tumor brain tissue was found to be very low or below detection limit. The function of OSM are unclear with conflicting reports of growth stimulatory or inhibitory effects in various cell types. OSM is known to signal through gp130 and the JAK/STAT pathway. In our second paper, we investigated the effects of OSM in vitro by stimulating glioblastoma cell cultures with OSM. STAT1 and STAT3 were immediately phosphorylated, indicating presence of a functional JAK/STAT pathway. We however, could not detect any changes in proliferation or in apoptosis after addition of OSM to cell cultures.In our third paper gene expression were examined by microarray in a glioblastoma cell line before and after addition of OSM. Twelve up-regulated and six down-regulated genes were identified in the U1242MG after stimulation with OSM. Among the up-regulated genes were: CH3L1 and PLAU which both affect cell migration, degradation and remodeling of extracellular matrix. MT2A was also induced and it has a function in metal ion binding and is known to be implicated in the regulation of cell proliferation and as an inhibitor of apoptosis. Another up-regulated gene was EPAS1, which is important for tumor adaptation to hypoxia and induces the expression of VEGF. These are all cancer promoting properties and therefore, we conclude that OSM may function as a master switch for several genes that contribute to the invasive nature of GBM.In the fourth paper, microarray were used to search for differentially expressed genes in PA and GBM. We hoped to find genes that could help explain the malignant behavior of GBM. Fourteen differentially expressed genes were identified and further tested in an extended tumor panel. SLC1A2, HNT, PLEKHB1 and STOM were low expressed in the GBM group. SLC1A2 expression has been shown to decrease glioma cell proliferation; HNT may promote cell adhesion; PLEKHB1 encodes a signal transduction protein with unknown function and STOM is a mediator of ion transport. Loss of these genes and their functions may be important for the highly malignant growth of this tumor group.