It has been shown that SIRT7 regulates rDNA transcription and that reduced SIRT7 levels inhibit tumor growth. This anti-tumor effect could be due to reduced Pol I activity and perturbed ribosome biogenesis. In this study, using pulse labeling with RNA and amino acid analogs, we found that SIRT7 knockdown efficiently suppressed both RNA and protein synthesis. Surprisingly, SIRT7 knockdown preferentially inhibited protein synthesis over rDNA transcription, whereas the levels of both were reduced to similar extents following Pol I knockdown. Using an affinity purification mass spectrometry approach and functional analyses of the resulting SIRT7 interactome, we identified and validated SIRT7 interactions with proteins involved in ribosomal biogenesis. Indeed, SIRT7 co-fractionated with monoribosomes within a sucrose gradient. Using reciprocal isolations, we determined that SIRT7 interacts specifically with mTOR and GTF3C1, a component of the Pol III transcription factor TFIIIC2 complex. Further studies found that SIRT7 knockdown triggered an increase in the levels of LC3B-II, an autophagosome marker, suggesting a link between SIRT7 and the mTOR pathway. Additionally, we provide several lines of evidence that SIRT7 plays a role in modulating Pol III function. Immunoaffinity purification of SIRT7-GFP from a nuclear fraction demonstrated specific SIRT7 interaction with five out of six components of the TFIIIC2 complex, but not with the TFIIIA or TFIIIB complex, the former of which is required for Pol III-dependent transcription of tRNA genes. ChIP assays showed SIRT7 localization to the Pol III targeting genes, and SIRT7 knockdown triggered a reduction in tRNA levels. Taken together, these data suggest that SIRT7 may regulate Pol III transcription through mTOR and the TFIIIC2 complex. We propose that SIRT7 is involved in multiple pathways involved in ribosome biogenesis, and we hypothesize that its down-regulation may contribute to an antitumor effect, partly through the inhibition of protein synthesis. Molecular &
Studies in animal models have indicated that dietary isothiocyanates (ITCs) exhibit cancer preventive activities through carcinogen detoxification-dependent and -independent mechanisms. The carcinogen detoxification-independent mechanism of cancer prevention by ITCs has been attributed at least in part to their ability to induce apoptosis of transformed (initiated) cells (e.g. through suppression of IB kinase and nuclear factor B as well as other proposed mechanisms). In the current studies we show that ITC-induced apoptosis of oncogene-transformed cells involves thiol modification of DNA topoisomerase II (Top2) based on the following observations. 1) siRNAmediated knockdown of Top2␣ in both SV40-transformed MEFs and Ras-transformed human mammary epithelial MCF-10A cells resulted in reduced ITC sensitivity. 2) ITCs, like some anticancer drugs and cancer-preventive dietary components, were shown to induce reversible Top2␣ cleavage complexes in vitro. 3) ITC-induced Top2␣ cleavage complexes were abolished by co-incubation with excess glutathione. In addition, proteomic analysis revealed that several cysteine residues on human Top2␣ were covalently modified by benzyl-ITC, suggesting that ITC-induced Top2␣ cleavage complexes may involve cysteine modification. Interestingly, consistent with the thiol modification mechanism for Top2␣ cleavage complex induction, the thiol-reactive selenocysteine, but not the nonthiol-reactive selenomethionine, was shown to induce Top2␣ cleavage complexes. In the aggregate, our results suggest that thiol modification of Top2␣ may contribute to apoptosis induction in transformed cells by ITCs.
Several G-rich oligodeoxynucleotides (ODNs), which are capable of forming G-quadruplexes, have been shown to exhibit antiproliferative activity against tumor cell lines and antitumor activity in nude mice carrying prostate and breast tumor xenografts. However, the molecular basis for their antitumor activity remains unclear. In the current study, we showed that a variety of telomeric G-tail oligodeoxynucleotides (TG-ODNs) exhibited antiproliferative activity against many tumor cells in culture. Systematic mutational analysis of the TG-ODNs suggests that the antiproliferative activity depends on the G-quadruplex conformation of these TG-ODNs. TG-ODNs were also shown to induce poly(ADPribose) polymerase-1 cleavage, phosphatidylserine flipping, and caspase activation, indicative of induction of apoptosis. TG-ODN-induced apoptosis was largely ataxia telangiectasia mutated (ATM) dependent. Furthermore, TG-ODN-induced apoptosis was inhibited by the c-Jun NH 2 -terminal kinase (JNK) inhibitor SP600125. Indeed, TG-ODNs were shown to activate the JNK pathway in an ATM-dependent manner as evidenced by elevated phosphorylation of JNK and c-Jun. Interestingly, a number of G-quadruplex ODNs (GQ-ODN) derived from nontelomeric sequences also induced ATM/JNKdependent apoptosis, suggesting a possible common mechanism of tumor cell killing by GQ-ODNs. (Cancer Res 2006; 66(24): 11808-16)
The chemokine CC motif ligand 2 (CCL2) is important in recruiting tumor-associated macrophages and is involved in the development of castration-resistance prostate cancer (CRPC) after androgen-deprivation therapy (ADT); however, the underlying mechanism remains unclear. We found that inactivation of the androgen receptor (AR) reduces a transcriptional repressor (SAM pointed domain-containing ETS transcription factor, SPDEF) of CCL2, which mediates epithelial-to-mesenchymal transition (EMT) of prostate tumor cells. Cell lines derived from a prostate-specific Pten/Trp53-null mouse and capable of a spontaneous EMT were utilized for identification of CCL2, and showed that reduced SPDEF expression was associated with an elevated CCL2-activated EMT. AR signaling inhibits CCL2 through a SPDEF-mediated mechanism in that the SPDEF recognizes the CCL2 promoter and transcriptionally represses its activity. Ectopically expressed SPDEF reduced the EMT and rescued expression of CCL2 in SPDEF-expressing cells, which induced the EMT and promotes malignant functions of prostate cancer cells. In tissues from prostate cancer patients with ADT, low SPDEF levels were correlated with high CCL2 expression compared to patients without ADT. We present a novel mechanism that contributes to the EMT and metastatic phenotype observed in a subset of ADT-resistant prostate cancer, where the CCL2 is stimulated through the inactivated of AR-mediated SPDEF.
Androgen deprivation therapy (ADT) targeting the androgen receptor (AR) is a standard therapeutic regimen for treating prostate cancer. However, most tumors progress to metastatic castration-resistant prostate cancer after ADT. We identified the type 1, 2, and 4 collagen-binding protein transforming growth factor-β (TGFβ)-induced protein (TGFBI) as an important factor in the epithelial-to-mesenchymal transition (EMT) and malignant progression of prostate cancer. In prostate cancer cell lines, AR signaling stimulated the activity of the transcription factor SPDEF, which repressed the expression of ADT, AR antagonism, or overexpression of TGFBI inhibited the activity of SPDEF and enhanced the proliferation rates of prostate cancer cells. Knockdown of TGFBI suppressed migration and proliferation in cultured cells and reduced prostate tumor growth and brain and bone metastasis in xenograft models, extending the survival of tumor-bearing mice. Analysis of prostate tissue samples collected before and after ADT from the same patients showed that ADT reduced the nuclear abundance of SPDEF and increased the production of TGFBI. Our findings suggest that induction of TGFBI promotes prostate cancer growth and metastasis and can be caused by dysregulation or therapeutic inhibition of AR signaling.
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