Down‐regulation of mitochondrial gene expression by the anti‐tumor arotinoid mofarotene (Ro 40‐8757)

To understand the mechanism of action of the antitumor arotinoid mofarotene (Ro 40‐8757), differential screening of cDNA libraries with cDNA probes prepared from treated or untreated breast‐cancer cells was performed. Several genes were identified that appeared to be regulated by mofarotene, including a mitochondrial gene encoding a subunit of NADH dehydrogenase (NDI). This gene was down‐regulated in the breast‐cancer cell line MDA‐MB‐231 after treatment with the arotinoid for 3 to 6 hr. Down‐regulation of NDI was detected in 2 other breast‐carcinoma cell lines (ZR‐75‐1 and MCF‐7) and a pancreatic cancer cell line (BxPC3), but not in the normal fibroblast cell line Wi‐38 or several other tumor cell lines. This effect was blocked by addition of cycloheximide to the medium. The retinoids, all‐trans and 9‐cis retinoic acids, did not affect the expression of NDI in MDA‐MB‐231 cells, demonstrating that mofarotene was not acting through the nuclear retinoic‐acid receptors. In the estrogen‐receptor‐expressing breast‐cancer line ZR‐75‐1, tamoxifen had no effect on NDI expression. The cytotoxic drugs doxorubicin, 5‐FU and vincristine also had no effect on regulation of this gene. Two mitochondrial proteins encoded in the nucleus, ATPase β subunit and mitochondrial transcription factor I, were not down‐regulated by mofarotene. Addition of mofarotene to cells incubated in glucose‐free medium led to their death. These results indicate that down‐regulation of mitochondrial gene transcription is specific to mofarotene and may explain, in part, the anti‐proliferative effects of this compound.