Bax mediates the apoptosis-sensitizing effect of maspin.
Maspin, a serine protease inhibitor (serpin), can suppress tumor growth and metastasis in vivo and tumor cell motility and invasion in vitro. This may occur through maspin-mediated inhibition of pericellular proteolysis. In a recent report, we provided evidence that maspin may also suppress tumor progression by enhancing cellular sensitivity to apoptotic stimuli. To our knowledge, maspin is the only proapoptotic serpin among all of the serpins implicated thus far in apoptosis regulation. The goal of the present study is to identify the specific target molecule(s), the modification of which by maspin renders tumor cells sensitive to chemotherapeutic agents. Our cellular, molecular, and biochemical studies demonstrate an essential role of Bax in the proapoptotic effect of maspin. First, Bax was up-regulated in maspin-transfected prostate and breast tumor cells, whereas the levels of other Bcl-2 family members including Bcl-2, Bcl-xl, and Bak remained unchanged. Second, on apoptosis induction, a greater amount of Bax was translocated from cytosol to mitochondria in maspin-transfected cells. After treatment with a Bax-silencing small interfering RNA, maspin-transfected cells became significantly more resistant to drug-induced apoptosis. Consistently, the release of cytochrome c and Smac/DIABLO from mitochondria was more responsive to apoptosis stimuli in maspin-transfected cells than in the mock-transfected cells. Third, the apoptosis induction of maspin-transfected cells was associated with increased activation of both caspase-8 and caspase-9. However, a caspase-9-specific inhibitor blocked the sensitization effect of maspin in a dose-dependent and time-dependent manner, demonstrating a rate-limiting role for caspase-9. In line with the central role of the Bax-mediated mitochondrial apoptotic pathway, maspin sensitized the apoptotic response of breast and prostate carcinoma cells to various drugs, ranging from death ligands to endoplasmic reticulum stress. The link between maspin and Bax up-regulation explains the loss of maspin-expressing tumor cells in invasive breast and prostate carcinomas. Our data reveal a novel mechanism for tumor suppressive maspin and suggest that maspin may be used as a modifier for apoptosis-based cancer therapy.
Liu
Jiayou
J
Department of Pathology, Wayne State University School of Medicine, Detroit, Michigan 48201, USA.
Yin
Shuping
S
Reddy
Neelima
N
Spencer
Craig
C
Sheng
Shijie
S
D017209:Apoptosis; D051017:Apoptosis Regulatory Proteins; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases / Q000378:metabolism; D004789:Enzyme Activation; D016147:Genes, Tumor Suppressor; D006801:Humans; D008297:Male; D008562:Membrane Glycoproteins / Q000494:pharmacology; D008928:Mitochondria / Q000502:physiology; D011471:Prostatic Neoplasms / Q000473:pathology; D011506:Proteins / Q000502:physiology*; D011518:Proto-Oncogene Proteins / Q000502:physiology*; D019253:Proto-Oncogene Proteins c-bcl-2; D015843:Serpins / Q000502:physiology*; D053221:TNF-Related Apoptosis-Inducing Ligand; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology; D051636:X-Linked Inhibitor of Apoptosis Protein; D051028:bcl-2-Associated X Protein
D051017:Apoptosis Regulatory Proteins; C494826:BAX protein, human; D008562:Membrane Glycoproteins; D011506:Proteins; D011518:Proto-Oncogene Proteins; D019253:Proto-Oncogene Proteins c-bcl-2; C085344:SERPIN-B5; D015843:Serpins; D053221:TNF-Related Apoptosis-Inducing Ligand; C505653:TNFSF10 protein, human; D014409:Tumor Necrosis Factor-alpha; D051636:X-Linked Inhibitor of Apoptosis Protein; C496866:XIAP protein, human; D051028:bcl-2-Associated X Protein; C505407:CASP3 protein, human; C505530:CASP8 protein, human; C506433:CASP9 protein, human; D053148:Caspase 3; D053181:Caspase 8; D053453:Caspase 9; D020169:Caspases