Human SLFN5 is a transcriptional co-repressor of STAT1-mediated interferon responses and promotes the malignant phenotype in glioblastoma.

We provide evidence that the IFN-regulated member of the Schlafen (SLFN) family of proteins, SLFN5, promotes the malignant phenotype in glioblastoma multiforme (GBM). Our studies indicate that SLFN5 expression promotes motility and invasiveness of GBM cells, and that high levels of SLFN5 expression correlate with high-grade gliomas and shorter overall survival in patients suffering from GBM. In efforts to uncover the mechanism by which SLFN5 promotes GBM tumorigenesis, we found that this protein is a transcriptional co-repressor of STAT1. Type-I IFN treatment triggers the interaction of STAT1 with SLFN5, and the resulting complex negatively controls STAT1-mediated gene transcription via interferon stimulated response elements. Thus, SLFN5 is both an IFN-stimulated response gene and a repressor of IFN-gene transcription, suggesting the existence of a negative-feedback regulatory loop that may account for suppression of antitumor immune responses in glioblastoma.

Involvement of polypyrimidine tract-binding protein (PTBP1) in maintaining breast cancer cell growth and malignant properties.

We have investigated some roles of splicing factor polypyrimidine tract-binding protein (PTBP1) in human breast cancer. We found that PTBP1 was upregulated in progressively transformed human mammary epithelial cells (HMECs), as well as in breast tumor cell lines compared with HMECs with finite growth potential and found that the level of PTBP1 correlated with the transformation state of HMECs. Knockdown of PTBP1 expression substantially inhibited tumor cell growth, colony formation in soft agar and in vitro invasiveness of breast cancer cell lines, a result similar to what we have reported in ovarian cancer. However, ectopic expression of PTBP1 (as a PTBP1-EGFP fusion protein) did not enhance the proliferation of immortalized HMEC. Rather, PTBP1 expression promoted anchorage-independent growth of an immortalized HMEC as assessed by increased colony formation in soft agar. In addition, we found that knockdown of PTBP1 expression led to upregulation of the expression of the M1 isoform of pyruvate kinase (PKM1) and increase of the ratio of PKM1 vs PKM2. PKM1 has been reported to promote oxidative phosphorylation and reduce tumorigenesis. Correspondingly, we observed increased oxygen consumption in PTBP1-knockdown breast cancer cells. Together, these results suggest that PTBP1 is associated with breast tumorigenesis and appears to be required for tumor cell growth and maintenance of transformed properties. PTBP1 exerts these effects, in part, by regulating the splicing of pyruvate kinase, and consequently alters glucose metabolism and contributes to the Warburg effect.

Knockdown of splicing factor SRp20 causes apoptosis in ovarian cancer cells and its expression is associated with malignancy of epithelial ovarian cancer.

Our previous study revealed that two splicing factors, polypyrimidine tract-binding protein (PTB) and SRp20, were upregulated in epithelial ovarian cancer (EOC) and knockdown of PTB expression inhibited ovarian tumor cell growth and transformation properties. In this report, we show that knockdown of SRp20 expression in ovarian cancer cells also causes substantial inhibition of tumor cell growth and colony formation in soft agar and the extent of such inhibition appeared to correlate with the extent of suppression of SRp20. Massive knockdown of SRp20 expression triggered remarkable apoptosis in these cells. These results suggest that overexpression of SRp20 is required for ovarian tumor cell growth and survival. Immunohistochemical staining for PTB and SRp20 of two specialized tissue microarrays, one containing benign ovarian tumors, borderline/low malignant potential (LMP) ovarian tumors as well as invasive EOC and the other containing invasive EOC ranging from stage I to stage IV disease, reveals that PTB and SRp20 are both expressed differentially between benign tumors and invasive EOC, and between borderline/LMP tumors and invasive EOC. There were more all-negative or mixed staining cases (at least two evaluable section cores per case) in benign tumors than in invasive EOC, whereas there were more all-positive staining cases in invasive EOC than in the other two disease classifications. Among invasive EOC, the majority of cases were stained all positive for both PTB and SRp20, and there were no significant differences in average staining or frequency of positive cancer cells between any of the tumor stages. Therefore, the expression of PTB and SRp20 is associated with malignancy of ovarian tumors but not with stage of invasive EOC.