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

The novel retinoid 6-[3-(1-adamantyl)-4-hydroxyphenyl]-2-naphtalene carboxylic acid (AHPN/CD437), a retinoic acid receptor (RAR)gamma activator, has been found to inhibit the growth and to induce apoptosis of a wide variety of malignant cell types including solid tumors and various leukemias. Interestingly, CD437 is able to induce apoptosis in some all-trans-retinoic acid (ATRA)-resistant models. In a number of experimental systems, the early apoptotic stage that precedes nuclear chromatinolysis consists in mitochondrial alterations, including a disruption of the inner mitochondrial transmembrane potential (delta(psi)m) mediated by the mitochondrial permeability transition (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 fragmentation. The same sequence of events is observed during the CD437-induced apoptosis in L363, a RARgamma-negative human myeloma cell line, as well as RPMI 8226 cytoplasts (anucleate cells). Indeed, RPMI 8226 cells and cytoplasts manifest a similar degree in delta(psi)m loss, phosphatidylserine exposure, and caspase activation in response to CD437, which indicates that nuclear effects cannot account for the apoptogenic potential of CD437. The mitochondrial release of cytochrome c, the activation of caspases as well as nuclear signs of CD437-induced apoptosis are fully prevented by the MPT inhibitory compound cyclosporin A. Purified mitochondria can be directly induced to undergo MPT with CD437 but not with ATRA. In a cell-free in vitro system consisting of exposing mitochondrial supernatants to isolated nuclei, only supernatants from CD437-treated mitochondria provoke chromatin condensation, whereas supernatants from mitochondria treated with ATRA, or with the combination of CD437 and cyclosporin A, remain inactive. In conclusion, these results suggest that the rapid execution of CD437-induced apoptosis is a nucleus-independent (and probably RARgamma-independent) phenomenon involving mitochondria and MPT.