SBDS-deficiency results in specific hypersensitivity to Fas stimulation and accumulation of Fas at the plasma membrane.
Shwachman-Diamond syndrome (SDS) is an inherited disorder characterized by reduced cellularity in the bone marrow and exocrine pancreas. Most patients have mutations in the SBDS gene, whose functions are unknown. We previously showed that cells deficient in the SBDS protein are characterized by accelerated apoptosis and Fas hypersensitivity, suggesting that the protein might play an important role in Fas-mediated apoptosis. To study the mechanism of Fas hypersensitivity, we compared shRNA-mediated SBDS-knockdown HeLa cells and SDS marrow CD34+ cells for their sensitivity to several groups of apoptosis inducers. Marked hypersensitivity was noticed in response to Fas stimulation, but not to tumor necrosis factor-alpha, DNA-damaging agents, transcription inhibition or protein synthesis inhibition. To identify the Fas signaling factors that cause hypersensitivity, we analyzed the expression of the pathway's proteins. We found that Fas accumulated at the plasma membrane in SBDS-knockdown cells with corresponding expression of Fas transcript 1, the main Fas transcript which contains both the transmembrane domain and the death domain. However, the total levels of Fas protein and mRNA were comparable to controls, and Fas internalization occurred normally. Expression of FADD, caspase-8 and -3 were not elevated and the pathway inhibitors: ERK, c-FLIP and XIAP were not decreased. These results suggest that SBDS loss results in abnormal accumulation of Fas at the plasma membrane, where it sensitizes the cells to stimulation by Fas ligand.
Watanabe
Ken-Ichiro
K
Cell Biology Program, Research Institute, The Hospital for Sick Children, University of Toronto, Toronto, ON, Canada.
Ambekar
Chhaya
C
Wang
Hanming
H
Ciccolini
Amanda
A
Schimmer
Aaron D
AD
Dror
Yigal
Y
D018952:Antigens, CD34 / Q000096:biosynthesis; D017209:Apoptosis; D001854:Bone Marrow Cells / Q000378:metabolism; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein / Q000378:metabolism; D053148:Caspase 3 / Q000378:metabolism; D053181:Caspase 8 / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism*; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D053222:Fas Ligand Protein / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D009154:Mutation; D011506:Proteins / Q000235:genetics / Q000502:physiology*; D051636:X-Linked Inhibitor of Apoptosis Protein / Q000378:metabolism; D019014:fas Receptor / Q000378:metabolism*
D018952:Antigens, CD34; D053446:CASP8 and FADD-Like Apoptosis Regulating Protein; C506386:CFLAR protein, human; D053222:Fas Ligand Protein; D011506:Proteins; C470521:SBDS protein, human; D051636:X-Linked Inhibitor of Apoptosis Protein; D019014:fas Receptor; D048049:Extracellular Signal-Regulated MAP Kinases; D053148:Caspase 3; D053181:Caspase 8