RESUMEN
In this study, quantitative isobologram studies showed that treatment with gemcitabine and doxorubicin, known inducers of ceramide generation, in combination, supra-additively inhibited the growth of human UM-SCC-22A cells in situ. Then, possible involvement of the human homologue of yeast longevity assurance gene 1 (LASS1)/C(18)-ceramide in chemotherapy-induced cell death in these cells was examined. Gemcitabine/doxorubicin combination treatment resulted in the elevation of mRNA and protein levels of LASS1 and not LASS2-6, which was consistent with a 3.5-fold increase in the endogenous (dihydro)ceramide synthase activity of LASS1 for the generation of C(18)-ceramide. Importantly, the overexpression of LASS1 (both human and mouse homologues) enhanced the growth-inhibitory effects of gemcitabine/doxorubicin with a concomitant induction of caspase-3 activation. In reciprocal experiments, partial inhibition of human LASS1 expression using small interfering RNA (siRNA) prevented cell death by about 50% in response to gemcitabine/doxorubicin. In addition, LASS1, and not LASS5, siRNA modulated the activation of caspase-3 and caspase-9, but not caspase-8, in response to this combination. Treatment with gemcitabine/doxorubicin in combination also resulted in a significant suppression of the head and neck squamous cell carcinoma (HNSCC) tumor growth in severe combined immunodeficiency mice bearing the UM-SCC-22A xenografts. More interestingly, analysis of endogenous ceramide levels in these tumors by liquid chromatography/mass spectroscopy showed that only the levels of C(18)-ceramide, the main product of LASS1, were elevated significantly (about 7-fold) in response to gemcitabine/doxorubicin when compared with controls. In conclusion, these data suggest an important role for LASS1/C(18)-ceramide in gemcitabine/doxorubicin-induced cell death via the activation of caspase-9/3 in HNSCC.
