Endocrine Abstracts

Combination chemotherapy for breast cancer offers the potential for improving drug efficacy through enhancement of cancer cell killing. Theoretically, the mechanistic basis underlying this approach may depend on multiple targets and/or amplification/interaction of putative proapoptotic signals. The synthetic retinoid N-(4-hydroxyphenyl) retinamide (4HPR) has proved effective as a chemopreventative and therapeutic agent in breast cancer, however, the mechanisms mediating this action are unknown. Recent studies have demonstrated an enhancement of both 4HPR-induced apoptosis and nitric oxide (NO) production when the retinoid is combined with the immunosuppressant cyclosporin A (CyA), Lim et al, Int J Cancer, 101, 243-7 (2002). The present study investigates a potential role for the putative stress-response mediators ceramide and NO during the action of this novel drug combination modality on MDA MB 231 breast cancer cells.

CyA potentiated the dose-dependent decrease in cell survival caused by 4HPR during a 72h growth period. This was preceded by an induction of apoptosis after 24h as detected by ELISA of cytoplasmic nucleosomes. 4HPR also caused a dose-dependent increase in ceramide production after 24h as measured by incorporation of 3 H serine into C₁₈ and C₂₄ ceramide species. This ceramide production was greatly enhanced in the presence of CyA and blocked by the serine palmitoyl transferase inhibitor, myriocin. This inhibition of ceramide production by myriocin did not prevent the decrease in cell survival due to the drug combination but did partially reduce the apoptotic response. Neither the NO synthase inhibitors, L-NMMA or L-NAME, nor the NO donors sodium nitropusside or nitro-glutathione, had any effect on ceramide production or cell survival.

These data confirm the efficacy of 4HPR plus CyA-induced cytotoxicity in MDA MB 231 cells. Notwithstanding the enhancement of NO and ceramide signalling in response to this treatment, neither signal appears to be the major mediator of this anti-proliferative action.