INDUCTION OF SENESCENCE IN HUMAN-MALIGNANT GLIOMA-CELLS BY P16(INK4A)

p16(INK4A) is a G1-specific cell cycle inhibitor which maps to human c hromosome 9p21, a region frequently mutated or deleted in cancer cell lines and primary tumors. In glioblastomas the frequency of homozygous deletions is 40-70% making it one of the most common mutations in thi s tumor type. We have analysed the significance of the loss of this ge ne in gliomas by introducing the cDNA for p16(INK4A) into the human gl ioma cell line U-1242 MG which has a deleted CDKN2 locus. We used the tetracycline repressable vector system and obtained two stably transfe cted clones that expressed p16(INK4A) upon induction. p16(INK4A) expre ssion caused a G1 arrest and enlargement of the cells similar to that of senescent cells. When staining for Senescence-Associated beta-galac tosidase activity, described to be specific for senescent cells, we co uld show that the enlarged cells specifically gave a positive staining reaction. This senescence phenotype was dependent on the continuous e xpression of p16(INK4A) since it was reversed upon reintroduction of t etracycline suppression. Thus, the induced expression of p16(INK4A) in these glioma cells reverted their immortal phenotype and caused an im mediate cellular senescence.