Ceramide response post-photodamage is absent after treatment with HA14-1

Ceramide generation during curcumin-induced apoptosis is controlled by crosstalk among Bcl-2, Bcl-xL, caspases and glutathione

Curcumin exhibits anti-cancer properties manifested by activation of pro-apoptotic signaling. We have demonstrated earlier that apoptosis of HL-60 human leukemia cells induced by curcumin is controlled by ceramide generated by neutral sphingomyelinase (nSMase) which contributes to sphingomyelin synthase (SMS) inhibition favoring accumulation of ceramide in cells. Here we report that the activity of nSMase, ceramide accumulation and death of HL-60 cells are inhibited by overexpression of Bcl-xL or Bcl-2 proteins, while down-regulation of nSMase interferes with degradation of Bcl-2 but not Bcl-xL. Activation of nSMase in curcumin-treated cells requires the activity of apoptosis initiator caspase-8 and executioner caspase-3, whereas nSMase depletion prevents activation of caspase-3, but not caspase-8. These data place nSMase activation downstream of caspase-8 and Bcl-xL and indicate a mutual regulation between nSMase and caspase-3 activity on one hand, and Bcl-2 level on the other hand in curcumin-treated cells. The activation of nSMase and ceramide accumulation also depended on the depletion of glutathione. The depletion of glutathione required the activity of caspase-8 and caspase-3 as well as the down-regulation of Bcl-2 and Bcl-xL. Together, the data indicate a crosstalk among Bcl-2, Bc-xL, caspases and glutathione during curcumin-induced apoptosis and point to the superior role of caspase-8 activity, Bcl-xL down-regulation and glutathione depletion in the pro-apoptotic cascade leading to nSMase activation and generation of ceramide.

Suppression of sphingomyelin synthase 1 by small interference RNA is associated with enhanced ceramide production and apoptosis after photodamage

We have shown that overexpression of SMS1, an enzyme that converts de novo ceramide into sphingomyelin, is accompanied by attenuated ceramide response and apoptotic resistance after photodamage with the photosensitizer Pc 4 (photodynamic therapy; PDT). To test whether SMS1 overexpression-related effects after PDT can be reversed, in this study SMS1 was downregulated in Jurkat T lymphoma/leukemia cells using small inhibitory RNA (siRNA) for SMS1. Compared to scrambled (control) siRNA-transfectants, in SMS1 siRNA-transfected cells the activity of SMS at rest was downregulated with concomitant decrease in sphingomyelin mass. In SMS1 siRNA-transfected cells increases in ceramides were higher than in control siRNA-transfectants after PDT. Similar findings were obtained for dihydroceramides suggesting the involvement of de novo ceramide pathway. PDT-induced DEVDase (caspase-3-like) activation was enhanced in SMS1 siRNA-transfected cells compared to their control counterparts. The data show that RNA interference-dependent downregulation of SMS1 is associated with increased accumulation of ceramide and dihydroceramide with concomitant sensitization of cells to apoptosis after photodamage. Similarly, in SMS2 siRNA-transfected cells, downregulation of SMS activity was accompanied by potentiated DEVDase activation post-photodamage. These findings suggest that SMS is a potential novel molecular target that can augment therapeutic efficacy of PDT.