The p53-transcriptional target, BTG2 TIS21/PC3 , was previously identified as an antiproliferative gene. However, the precise biological functions of the protein product remain to be elucidated. BTG2 TIS21/PC3 expression is induced in vivo during neurogenesis, and the gene is transiently expressed in vitro in rat pheochromocytoma PC12 cells after induction of neuronal differentiation by addition of nerve growth factor (NGF). These observations suggest that BTG2 TIS21/PC3 is functionally significant during the neuronal differentiation process. To test this hypothesis, a vector that expressed BTG2 TIS21/PC3 under the control of an inducible promoter was introduced into PC12 cells. Growth arrest and differentiation in response to NGF were greatly enhanced by BTG2 TIS21/PC3 overexpression. Furthermore, an antisense oligonucleotide complementary to BTG2 TIS21/PC3 mRNA, which was able to inhibit endogenous BTG2 TIS21/PC3 expression, triggered programmed cell death in differentiated PC12 cells. These observations confirm that BTG2 TIS21/PC3 expression promotes neuronal differentiation and that it is required for survival of terminally differentiated cells.

The phosphoaminothiol WR1065, the active metabolite of the pro-drug amifostine (WR2721), protects cultured cells and tissues against cytotoxic exposure to radiation or chemotherapeutic agents. We show here that WR1065 and the pro-drug WR2721 activate the p53 tumor suppressor protein and induce the expression of the cyclin-dependent kinase inhibitor p21waf-1 in the breast cancer cell line MCF-7, and in the mouse ®broblast cell line balb/c 3T3. Using two MCF-7 derived cell lines, MN1 and MDD2, we show that induction of p21waf-1 is detectable in MN1 (expressing a functional p53) but not in MDD2 (p53 disabled). These eects are observed at concentrations of WR1065 (0.5 to 1 mM) identical to those required to protect against cytotoxicity by hydrogen peroxide. Induction of p53 is not prevented by addition of aminoguanidine, an inhibitor of Cudependent amine-oxidases which blocks the extra-cellular degradation of WR1065 into toxic metabolites. Moreover, spermidine, a natural polyamine structurally related to amifostine, does not activate p53. Induction of p53 by WR1065 results in a delay in the G1/S transition in MCF-7 and MN-1 cells, but not in the p53 disabled cells MDD2. These data indicate that WR1065, a polyamine analog with thiol anti-oxidant properties, activates a cell cycle check-point involving p53.