Reactive Oxygen Species Regulate Spontaneous and Fas-Mediated Apoptosis in SBDS-Deficient Cells.

Background and hypotheses: Shwachman-Diamond syndrome (SDS) is an autosomal recessive disorder characterized by bone marrow failure, pancreatic insufficiency, and a marked propensity for myelodysplastic syndrome and leukemia. Approximately 90% of the patients have mutations in the SBDS gene, but the function of the gene is unknown. We previously showed that marrow cells from SDS patients and SBDS-deficient HeLa Cells are characterized by accelerated apoptosis, overexpression of Fas and hypersensitive to Fas stimulation. Involvement of reactive oxygen species (ROS; oxidative stress) have been shown to be related to Fas hypersensitivity and overexpression in a variety of cell types. Therefore, we hypothesized that functional deficiency in SBDS in cells that express Fas could lead to impaired ROS generation and a subsequent increase in spontaneous and Fas-mediated apoptosis and decrease in cell growth.

Methods: We used shRNA-mediated SBDS-knockdown HeLa cells as a model. We investigated whether SBDS-deficiency increases ROS levels and if antioxidants can rescue the cell growth and apoptosis phenotype. To measure ROS formation cells were incubated with DCFH-DA and fluorescence measured in Gemini Spectra MAX microplate reader. Staining with annexin V and propidium iodide was done to determine apoptosis and necrotic cell death. MTT assay was used to measure cell viability.

Results: ROS levels in SBDS knockdown cells were significantly increased compared to control. Apoptosis analysis by annexin V and propidium iodide showed a marked decrease in cell viability in the SBDS-knockdown cells. NAC treatment decreased ROS levels, enhanced ERK phosphorylation (pERK), improved cell viability, and decreased apoptotic and necrotic cell death. Stimulation of the Fas signaling pathway by CH-11 (activating anti-Fas antibody) and Fas ligand showed increased ROS production in SBDS Knockdown cells. CH-11 treatment showed a marked increase in apoptotic and necrotic cell death after 24 and 48hrs incubation. Cell viability decreased by 40% and 80% after 24 and 48hrs incubation with CH-11. Treatment with NAC lowered ROS levels, enhanced pERK expression, protected the cells from Fas-mediated early apoptosis and improved cell survival.

Conclusion: We have demonstrated that stable loss of SBDS results in increased ROS levels, leading to apoptotic and necrotic cell death. Thus, increased baseline and Fas-stimulated ROS could result in increased sensitivity to apoptosis and necrotic cell death. NAC appeared to reverse the ROS-mediated decrease in cell survival and apoptotic cell death. Our data support the novel concept that SBDS may be a homeostatic regulator of oxidative stress