Methods and compositions for restoring sensitivity to the antitumorigenic effects of antiestrogen therapy and/or cytotoxic therapy and inducing cell apoptosis are provided. Contacting tumor cells to GP88 antagonists (e.g., anti-GP88 antibodies, anti-GP88 antisense nucleic acids, GP88 siRNA, and small molecules) induces apoptosis and restores sensitivity to the antitumorigenic effects of antiestrogen therapy and cytotoxic therapy.