Tumor suppressor genes may represent an important new therapeutic modality in the treatment of human
glioblastoma (GBM).
p16(INK4A) is a tumor suppressor gene with mutation and/or deletion found in many human
tumors, including
glioblastomas,
melanoma, and
leukemias. RT-2 rat GBM cell line was used to investigate if the p16 gene induces dominant suppression of
glioblastoma growth. Close to 100% of
tumor cells were infected by high titer pCL retrovirus encoding the full-length human p16
cDNA at 5 m.o.i. Infected cells showed a 98% reduction in colony forming assay and a 60% reduction in growth curves in vitro compared to vector control. Exogenous overexpression of p16 induced hypophosphorylation of
Rb protein by Western blot analysis. Intracranial injection of p16-infected
tumor cells into syngeneic rats resulted in a 95% reduction in
tumor volume compared to the controls. Intratumoral injection of p16 retrovirus resulted in
tumor necrosis and prominent human p16 transgene expressions. Proliferation marker
PCNA was not detected in these human p16-expressed RT-2
tumor cells, suggesting the cells were unable to enter into S phase after p16 expression. In addition, direct repeat intracranial
injections of p16 retrovirus prolonged animal survival 3.2-fold compared to the controls (48.4 +/- 13.4 vs 15.0 +/- 2.1 days, p < 0.001). Two out of ten rats were found with dormant
tumors at day 60 after p16 retrovirus injection. These results showed that p16 is effective in inhibiting GBM growth in situ. The mechanisms of
tumor growth reduction and
necrosis in vivo might be due to G1 arrest triggered by p16 expression.