Ceramide-induced apoptosis of human thyroid cancer cells resistant to apoptosis by irradiation

Ionizing radiation (IR) induces apoptosis through, in part, cell membrane b reakdown signals. Ceramide and diacylglycerol (DAG) are released after IR e xposure, which act as second messengers to induce proapoptotic and antiapop totic signals, respectively. We have previously shown, however, that thyroi d cells are relatively resistant to IR-induced apoptosis. To investigate th e mechanism of thyroid cell resistance to IR-related apoptosis, we determin ed the effects of ceramide and its release following exposure of human thyr oid cancer cell lines to IR. Exogenous C-2-ceramide (10-100 mu M) activated the apoptosis process in all cell lines used. Exogenous C-2-ceramide also activated a stress kinase, c-Jun N-terminal kinase (JNK). The apoptotic act ion of ceramide was attenuated by serum or simultaneous activation of prote in kinases C and A by phorbol esters and forskolin. Furthermore, 2-5 Gy IR had a differential effect on ceramide and DAG release in human thyroid cell s; a weak and transient release of ceramide but a strong and sustained rele ase of DAG. Our results indicated that the radioresistance properties of th yroid cancer cells probably reflect the dominance of anti-apoptotic signals , evoked by growth factor(s) and DAG, which override the apoptotic effect o f ceramide released by human thyroid cells on exposure to IR, in spite of a ctivation of proapoptotic pathway downstream of ceramide.