Overexpression of the Bcl-2 proto-oncogene in
tumor cells confers resistance against chemotherapeutic drugs. In this study, we describe how the novel pyrrolo-1,5-benzoxazepine compound 7-[[dimethylcarbamoyl]oxy]-6-(2-naphthyl)pyrrolo-[2,1-d] (1,5)-benzoxazepine (PBOX-6) selectively induces apoptosis in Bcl-2-overexpressing
cancer cells, whereas it shows no cytotoxic effect on normal peripheral blood mononuclear cells.
PBOX-6 overcomes Bcl-2-mediated resistance to apoptosis in
chronic myelogenous leukemia (CML) K562 cells by the time- and dose-dependent phosphorylation and inactivation of antiapoptotic Bcl-2 family members Bcl-2 and Bcl-XL.
PBOX-6 also induces Bcl-2 phosphorylation and apoptosis in wild-type T
leukemia CEM cells and cells overexpressing Bcl-2. This is in contrast to chemotherapeutic agents such as
etoposide,
actinomycin D, and ultraviolet irradiation, whereby overexpression of Bcl-2 confers resistance against apoptosis. In addition,
PBOX-6 induces Bcl-2 phosphorylation and apoptosis in wild-type Jurkat
acute lymphoblastic leukemia cells and cells overexpressing Bcl-2. However, Jurkat cells containing a Bcl-2 triple mutant, whereby the principal Bcl-2 phosphorylation sites are mutated to
alanine, demonstrate resistance against Bcl-2 phosphorylation and apoptosis.
PBOX-6 also induces the early and transient activation of c-Jun NH2-terminal
kinase (JNK) in CEM cells. Inhibition of JNK activity prevents Bcl-2 phosphorylation and apoptosis, implicating JNK in the upstream signaling pathway leading to Bcl-2 phosphorylation. Collectively, these findings identify Bcl-2 phosphorylation and inactivation as a critical step in the apoptotic pathway induced by
PBOX-6 and highlight its potential as an effective antileukemic agent.