NSrp70 is a lymphocyte-essential splicing factor that controls thymocyte development.

Alternative pre-mRNA splicing is a critical step to generate multiple transcripts, thereby dramatically enlarging the proteomic diversity. Thus, a common feature of most alternative splicing factor knockout models is lethality. However, little is known about lineage-specific alternative splicing regulators in a physiological setting. Here, we report that NSrp70 is selectively expressed in developing thymocytes, highest at the double-positive (DP) stage. Global splicing and transcriptional profiling revealed that NSrp70 regulates the cell cycle and survival of thymocytes by controlling the alternative processing of various RNA splicing factors, including the oncogenic splicing factor SRSF1. A conditional-knockout of Nsrp1 (NSrp70-cKO) using CD4Cre developed severe defects in T cell maturation to single-positive thymocytes, due to insufficient T cell receptor (TCR) signaling and uncontrolled cell growth and death. Mice displayed severe peripheral lymphopenia and could not optimally control tumor growth. This study establishes a model to address the function of lymphoid-lineage-specific alternative splicing factor NSrp70 in a thymic T cell developmental pathway.

USP41 Enhances Epithelial-Mesenchymal Transition of Breast Cancer Cells through Snail Stabilization.

Ubiquitination, one of many post-translational modifications, causes proteasome-mediated protein degradation by attaching ubiquitin to target proteins. Multiple deubiquitinases inhibit the ubiquitination pathway by removing the ubiquitin chain from protein, thus contributing to the stabilization of substrates. USP41 contributes to invasion, apoptosis and drug resistance in breast and lung cancer cells. However, the detailed mechanism and role of USP41 in breast cancer have not been elucidated. USP41 was overexpressed and showed poor prognosis according to the aggressive phenotype of breast cancer cells. Knockdown of USP41 inhibited migration and growth of breast cancer cells, whereas overexpression of USP41 increased cell growth and migration. In addition, depletion of USP41 downregulated Snail protein expression, an epithelial-mesenchymal transition marker, but not mRNA expression. Furthermore, USP41 interacted with and inhibited ubiquitination of Snail, resulting in the increase in Snail stabilization. Therefore, these data demonstrated that USP41 increases migration of breast cancer cells through Snail stabilization.

Tubeimoside-1 Enhances TRAIL-Induced Apoptotic Cell Death through STAMBPL1-Mediated c-FLIP Downregulation.

Tubeimoside-1 (TBMS-1), a traditional Chinese medicinal herb, is commonly used as an anti-cancer agent. In this study, we aimed to investigate its effect on the sensitization of cancer cells to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL). Our results revealed that even though monotherapy using TBMS-1 or TRAIL at sublethal concentrations did not affect cancer cell death, combination therapy using TBMS-1 and TRAIL increased apoptotic cell death. Mechanistically, TBMS-1 destabilized c-FLIP expression by downregulating STAMBPL1, a deubiquitinase (DUB). Specifically, when STAMBPL1 and c-FLIP bound together, STAMBPL1 deubiquitylated c-FLIP. Moreover, STAMBPL1 knockdown markedly increased sensitivity to TRAIL by destabilizing c-FLIP. These findings were further confirmed in vivo using a xenograft model based on the observation that combined treatment with TBMS-1 and TRAIL decreased tumor volume and downregulated STAMBPL1 and c-FLIP expression levels. Overall, our study revealed that STAMBPL1 is essential for c-FLIP stabilization, and that STAMBPL1 depletion enhances TRAIL-mediated apoptosis via c-FLIP downregulation.

Inhibition of USP2 Enhances TRAIL-Mediated Cancer Cell Death through Downregulation of Survivin.

Ubiquitin-specific protease 2 (USP2) is a deubiquitinase belonging to the USPs subfamily. USP2 has been known to display various biological effects including tumorigenesis and inflammation. Therefore, we aimed to examine the sensitization effect of USP2 in TRAIL-mediated apoptosis. The pharmacological inhibitor (ML364) and siRNA targeting USP2 enhanced TNF-related apoptosis-inducing ligand (TRAIL)-induced cancer cell death, but not normal cells. Mechanistically, USP2 interacted with survivin, and ML364 degraded survivin protein expression by increasing the ubiquitination of survivin. Overexpression of survivin or USP2 significantly prevented apoptosis through cotreatment with ML364 and TRAIL, whereas a knockdown of USP2 increased sensitivity to TRAIL. Taken together, our data suggested that ML364 ubiquitylates and degrades survivin, thereby increasing the reactivity to TRAIL-mediated apoptosis in cancer cells.

Melatonin induces apoptotic cell death through Bim stabilization by Sp1-mediated OTUD1 upregulation.

Melatonin, secreted by the pineal gland, regulates the circadian rhythms and also plays an oncostatic role in cancer cells. Previously, we showed that melatonin induces the expression of Bim, a pro-apoptotic Bcl-2 protein, at both the transcriptional and post-translational levels. In the present study, we investigated the molecular mechanisms underlying the melatonin-mediated Bim upregulation through post-translational regulation. We found that ovarian tumor domain-containing protein 1 (OTUD1), a deubiquitinase belonging to the OTU protein family, is upregulated by melatonin at the mRNA and protein levels. OTUD1 knockdown inhibited melatonin-induced Bim upregulation and apoptosis in cancer cells. OTUD1 directly interacted with Bim and inhibited its ubiquitination. Melatonin-induced OTUD1 upregulation caused deubiquitination at the lysine 3 residue of Bim, resulting in its stabilization. In addition, melatonin-induced activation of Sp1 was found to be involved in OTUD1 upregulation at the transcriptional level, and pharmacological inhibition and genetic ablation of Sp1 (siRNA) interrupted melatonin-induced OTUD1-mediated Bim upregulation. Furthermore, melatonin reduced tumor growth and induced upregulation of OTUD1 and Bim in a mouse xenograft model. Notably, Bim expression levels correlated with OTUD1 levels in patients with renal clear cell carcinoma. Thus, our results demonstrated that melatonin induces apoptosis by stabilizing Bim via Sp1-mediated OTUD1 upregulation.

Inhibition of BMI-1 Induces Apoptosis through Downregulation of DUB3-Mediated Mcl-1 Stabilization.

BMI-1, a polycomb ring finger oncogene, is highly expressed in multiple cancer cells and is involved in cancer cell proliferation, invasion, and apoptosis. BMI-1 represents a cancer stemness marker that is associated with the regulation of stem cell self-renewal. In this study, pharmacological inhibition (PTC596) or knockdown (siRNA) of BMI-1 reduced cancer stem-like cells and enhanced cancer cell death. Mechanistically, the inhibition of BMI-1 induced the downregulation of Mcl-1 protein, but not Mcl-1 mRNA. PTC596 downregulated Mcl-1 protein expression at the post-translational level through the proteasome-ubiquitin system. PTC596 and BMI-1 siRNA induced downregulation of DUB3 deubiquitinase, which was strongly linked to Mcl-1 destabilization. Furthermore, overexpression of Mcl-1 or DUB3 inhibited apoptosis by PTC596. Taken together, our findings reveal that the inhibition of BMI-1 induces Mcl-1 destabilization through downregulation of DUB3, resulting in the induction of cancer cell death.

Lucanthone, Autophagy Inhibitor, Enhances the Apoptotic Effects of TRAIL through miR-216a-5p-Mediated DR5 Upregulation and DUB3-Mediated Mcl-1 Downregulation.

A lucanthone, one of the family of thioxanthenones, has been reported for its inhibitory effects of apurinic endonuclease-1 and autophagy. In this study, we investigated whether lucanthone could enhance tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in various cancer cells. Combined treatment with lucanthone and TRAIL significantly induced apoptosis in human renal carcinoma (Caki and ACHN), prostate carcinoma (PC3), and lung carcinoma (A549) cells. However, combined treatment did not induce apoptosis in normal mouse kidney cells (TCMK-1) and normal human skin fibroblast (HSF). Lucanthone downregulated protein expression of deubiquitinase DUB3, and a decreased expression level of DUB3 markedly led to enhance TRAIL-induced apoptosis. Ectopic expression of DUB3 inhibited combined treatment with lucanthone and TRAIL-induced apoptosis. Moreover, lucanthone increased expression level of DR5 mRNA via downregulation of miR-216a-5p. Transfection of miR-216a-5p mimics suppressed the lucanthone-induced DR5 upregulation. Taken together, these results provide the first evidence that lucanthone enhances TRAIL-induced apoptosis through DR5 upregulation by downregulation of miR-216a-5p and DUB3-dependent Mcl-1 downregulation in human renal carcinoma cells.

Magnolol Enhances the Therapeutic Effects of TRAIL through DR5 Upregulation and Downregulation of c-FLIP and Mcl-1 Proteins in Cancer Cells.

Magnolol is a biologically active compound, isolated from the Chinese herb Magnolia, that regulates antiproliferative, anticancer, antiangiogenic and antimetastatic activities. We found that magnolol sensitizes TRAIL-induced apoptotic cell death via upregulation of DR5 and downregulation of cellular FLICE-inhibitory protein (c-FLIP) and Mcl-1 in cancer cells, but not in normal cells. Mechanistically, magnolol increased ATF4-dependent DR5 expression at the transcription level, and knockdown of ATF4 markedly inhibited magnolol-induced DR5 upregulation. Silencing DR5 with siRNA prevented combined treatment with magnolol and TRAIL-induced apoptosis and PARP cleavage. Magnolol induced proteasome-mediated Mcl-1 downregulation, while magnolol-induced c-FLIP downregulation was regulated, at least in part, by lysosomal degradation. Our results revealed that magnolol enhanced TRAIL-induced apoptosis via ATF4-dependent DR5 upregulation and downregulation of c-FLIP and Mcl-1 proteins.

Inhibition of Drp1 Sensitizes Cancer Cells to Cisplatin-Induced Apoptosis through Transcriptional Inhibition of c-FLIP Expression.

Mitochondrial fragmentation occurs during the apoptosis. Dynamin-related protein 1 (Drp1) acts as an important component in mitochondrial fission machinery and can regulate various biological processes including apoptosis, cell cycle, and proliferation. The present study demonstrates that dysfunction of mitochondrial dynamics plays a pivotal role in cisplatin-induced apoptosis. Inhibiting the mitochondrial fission with the specific inhibitor (Mdivi-1) did not affect apoptotic cell death in low concentrations (<10 µM). Interestingly, mdivi-1 enhanced cisplatin-induced apoptosis in cancer cells, but not in normal cells. Particularly in the presence of mdivi-1, several human cancer cell lines, including renal carcinoma cell line Caki-1, became vulnerable to cisplatin by demonstrating the traits of caspase 3-dependent apoptosis. Combined treatment induced downregulation of c-FLIP expression transcriptionally, and ectopic expression of c-FLIP attenuated combined treatment-induced apoptotic cell death with mdivi-1 plus cisplatin. Collectively, our data provide evidence that mdivi-1 might be a cisplatin sensitizer.

Axl Inhibitor R428 Enhances TRAIL-Mediated Apoptosis Through Downregulation of c-FLIP and Survivin Expression in Renal Carcinoma.

R428, a selective small molecule Axl inhibitor, is known to have anti-cancer effects, such as inhibition of invasion and proliferation and induction of cell death in cancer cells. The Axl receptor tyrosine kinase is highly expressed in cancer cells and the level of Axl expression is associated with survival, metastasis, and drug resistance of many cancer cells. However, the effect of Axl inhibition on overcoming anti-cancer drugs resistance is unclear. Therefore, we investigated the capability of Axl inhibition as a therapeutic agent for the induction of TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) sensitivity. In this study, R428 markedly sensitized cancer cells to TRAIL-induced apoptotic cell death, but not in normal human skin fibroblast (HSF) and human umbilical vein cells (EA.hy926). Moreover, knockdown of Axl by siRNA also increased TRAIL-induced apoptosis. R428 decreased c-FLIP proteins levels via induction of miR-708 expression and survivin protein levels at the post-translational level, and we found that knockdown of Axl also decreased both c-FLIP and survivin protein expression. Overexpression of c-FLIP and survivin markedly inhibited R428 plus TRAIL-induced apoptosis. Furthermore, R428 sensitized cancer cells to multiple anti-cancer drugs-mediated cell death. Our results provide that inhibition of Axl could improve sensitivity to TRAIL through downregulation of c-FLIP and survivin expression in renal carcinoma cells. Taken together, Axl may be a tempting target to overcome TRAIL resistance.