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-I 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-I,
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 beta 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.