The novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphtalene carboxylic acid can trigger apoptosis through a mitochondrial pathway independentof the nucleus

The novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphtalene carboxy lic acid (AHPN/CD437), a retinoic acid receptor (RAR)gamma activator, has b een found to inhibit the growth and to induce apoptosis of a wide variety o f malignant cell types including solid tumors and various leukemias. Intere stingly, CD437 is able to induce apoptosis in some all-trans-retinoic acid (ATRA)-resistant models. In a number of experimental systems, the early apo ptotic stage that precedes nuclear chromatinolysis consists in mitochondria l alterations, including a disruption of the inner mitochondrial transmembr ane potential (Delta psi(m)) mediated by the mitochondrial permeability tra nsition (MPT). Similarly CD437 causes RPMI 8226, a human myeloma cell line, to undergo a rapid Delta psi(m), disruption that precedes other apoptotic alterations such as the generation of reactive oxygen species and DNA fragm entation. The same sequence of events is observed during the CD437-induced apoptosis in L363, a RAR gamma-negative human myeloma cell Line, as well as RPMI 8226 cytoplasts (anucleate cells). Indeed, RPMI 8226 cells and cytopl asts manifest a similar degree in Delta psi(m) loss, phosphatidylserine exp osure, and caspase activation in response to CD437, which indicates that nu clear effects cannot account for the apoptogenic potential of CD437. The mi tochondrial release of cytochrome c, the activation of caspases as well as nuclear signs of CD437-induced apoptosis are fully prevented by the MPT inh ibitory compound cyclosporin A. Purified mitochondria can be directly induc ed to undergo MPT with CD437 but not with ATRA. In a cell-free in vitro sys tem consisting of exposing mitochondrial supernatants to isolated nuclei, o nly supernatants from CD437-treated mitochondria provoke chromatin condensa tion, whereas supernatants from mitochondria treated with ATRA, or with the combination of CD437 and cyclosporin A, remain inactive. In conclusion, th ese results suggest that the rapid execution of CD437-induced apoptosis is a nucleus-independent (and probably RAR gamma-independent) phenomenon invol ving mitochondria and MPT.