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 induction of apoptosis in malignant cells by a large number of agents including a wide variety of chemotherapeutic drugs. CD437 (6-[3-(1-adamantyl)-4 hydroxyphenyl]-2-naphthalene carboxylic acid) is a novel retinoid that induces apoptosis in a number of malignant cells through a unique mechanism of action. The addition of 1 μM CD437 to HL-60/NEO cells resulted in capase 3 (CPP32) activation and poly(ADP-ribose) polymcrase (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/bcl-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 terms of their growth inhibition, suggesting that CD437-mediated inhibition of growth and induction of apoptosis represent two distinct and separable processes. In addition, CD437 induces G₁ arrest as well as p21^WAFI/CIPI mRNA expression in these cells despite the overexpression of bcl-2 or bcl-X_L. CD437 induced mitochondrial instability as indicated by cytochrome c leakage into the cytoplasm in all three cell lines. CD437 also induced growth inhibition and apoptosis of an apoptosis-resistant variant of the HL-60 cell line (HCW-2), which switched expression from bcl-2 to bcl-X_L. CD437-mediated apoptosis is not accompanied by downregulation of bcl-2 or bcl-X_L or upregulation of bax. The reason for the inability of bcl-2 or bcl-X_L overexpression to inhibit CD437-mediated apoptosis is unclear. The ability of CD437 to initiate apoptosis in a spectrum of malignant cells without interference from bcl-2 or bcl-X_L overexpression suggests that CD437 may possess significant therapeutic potential in the treatment of malignancy.