OVEREXPRESSION OF BCL-2 OR BCL-X-L FAILS TO INHIBIT APOPTOSIS MEDIATED BY A NOVEL RETINOID

Overexpression of bcl-2 or bcl-X-L has been found to inhibit the induc tion of apoptosis in malignant cells by a large number of agents inclu ding a wide variety of chemotherapeutic drugs. CD437 {6-[3-(1-adamanty l)-4 hydroxyphenyl]-2-naphthalene carboxylic acid} is a novel retinoid that induces apoptosis in a number of malignant cells through it uniq ue mechanism of action. The addition of 1 mu M CD437 to HL-60/NEO cell s resulted in capase 3 (CPP32) activation and poly(ADP-ribose) polymer ase (PARP) cleavage in 3 h whereas in bcl-2- or bcl-X-L-overexpressing HL-60 cells CD437 induced CPP32 activation and PARP cleavage in 6 h. Although 50 and 300 nM CD437 were required to induce PARP cleavage in HL-60/NEO and HL-60/bcl-2, HL-60/bcI-X-L cells, respectively, maximal apoptosis in both cell lines was achieved utilizing 300 nM CD437. All three cell lines, however, share identical dose-response curves in ter ms of their growth inhibition, suggesting that CD437-mediated inhibiti on of growth and induction of apoptosis represent two distinct and sep arable processes. In addition, CD437 induces G(1) arrest as well as p2 1(WAFL/CIPI) mRNA expression in these cells despite the overexpression of bcl-2 or bcl-X-L. CD437 induced mitochondrial instability as indic ated by cytochrome c leakage into the cytoplasm in all three cell line s. CD437 also induced growth inhibition and apoptosis of an apoptosis- resistant variant of the HL-60 cell line (HCW-2), which switched expre ssion from bcl-2 to bcl-XL. CD437-mediated apoptosis is not accompanie d by downregulation of bcl-2 or bci-X-L or upregulation of bar. The re ason for the inability of bcl-2 or bcl-X-L overexpression to inhibit C D437-mediated apoptosis is unclear. The ability of CD437 to initiate a poptosis in a spectrum of malignant cells without interference from bc l-2 or bcl-X-L overexpression suggests that CD437 may possess signific ant therapeutic potential in the treatment of malignancy.