Roles of p16INk a and PDGF-B in glioma

Sammanfattning: Gliomas are diffuse astrocytic tumors of the central nervous system (CNS) which usually affect adults. This thesis work has focused on molecular mechanisms of altered growth control in glioma, and the significance of two gene products belonging to pathways commonly affected in glioma, p16INK4a and PDGF-B, has been investigated in vitro and in vivo.The INK4a locus is one of the most frequently lossed tumor suppressor gene of human gliomas. It encodes a G1 specific cell cycle inhibitor, p16INK4a, which was ectopically expressed in the human malignant glioma cell line U-1242 MG. Forced expression caused a G1 arrest and senescence in the cells, defined by an inability to induce DNA replication at subconfluency in the presence of serum, an enlarged cell morphology and expression of a senescence marker, senescence-associated β-galactosidase (SA-β-gal).Many studies have implicated a role for platelet-derived growth factor (PDGF) in human glioma. We established a mouse brain tumor model by injecting a murine retrovirus containing the PDGP-B cDNA together with wild-type virus, intracranially in neonatal mice. The induced malignant brain tumors displayed features of human gliomas. Tumor cells were nestin positive and expressed PDGP-B and PDGF-Rα mRNA, suggesting an autocrine stimulation of a neuroepithelial progenitor cell as the mechanism of tumor induction. A cell line was established from one PDGF-B induced tumor. Cultured tumor cells expressed PDGF-B causing autophosphorylation of endogenous PDGF receptors. A specific PDGF tyrosine kinase inhibitor caused a striking inhibition of PDGF receptor tyrosine phosphorylation and cell proliferation, showing a dependence of PDGF stimulation in initiation as well as progression of these tumors. Furthermore, the contribution of insertional mutagenesis in tumor development was analysed. Retroviral insertions in genomic DNA from 15 tumors were investigated with inverse polymerase chain reaction (IPCR) and sequencing. Preliminary data indicate the presence of a common integration site in a sequence mapped to a defined region of human chromosome 22, corresponding to mouse chromosome 15.