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Apoptosis is a programmed cell death that efficiently removes damaged cells to maintain tissue homeostasis. Defect in apoptotic machinery can lead to tumor development, progression, and resistance to chemotherapy. PUMA (p53 upregulated modulator of apoptosis) and BAX (BCL2-associated X protein) are among the most well-known inducers of apoptosis. It has been reported that expression levels of BAX and PUMA are controlled at the posttranslational level by phosphorylation. However, the posttranslational regulation of these proapoptotic proteins remains largely unexplored. In this study, using biochemical, molecular biology, flow cytometric, and immunohistochemistry techniques, we show that PUMA and BAX are the direct target of the F-box protein FBXL20, which restricts their cellular levels. FBXL20 directs the proteasomal degradation of PUMA and BAX in a protein kinase AKT1-dependent manner to promote cancer cell proliferation and tumor growth. Interestingly, inactivation of AKT1 results in activation of another protein kinase GSK3α/β, which facilitates the proteasomal degradation of FBXL20 by another F-box protein, FBXO31. Thus, a switch between two signaling kinases AKT1 and GSK3α/β modulates the functional activity of these proapoptotic regulators, thereby determining cell survival or death. RNAi-mediated ablation of FBXL20 results in increased levels of PUMA as well as BAX, which further enhances the sensitivity of cancer cells to chemotherapeutic drugs. We showed that high level expression of FBXL20 in cancer cells reduces therapeutic drug-induced apoptosis and promotes chemoresistance. Overall, this study highlights the importance of targeting FBXL20 in cancers in conjunction with chemotherapy and may represent a promising anticancer strategy to overcome chemoresistance.

Aberrant activation of β‐catenin has been implicated in a variety of human diseases, including cancer. In spite of significant progress, the regulation of active Wnt/β‐catenin‐signaling pathways is still poorly understood. In this study, we show that F‐box protein 16 (FBXO16) is a putative tumor suppressor. It is a component of the SCF (SKP1‐Cullin1‐F‐box protein) complex, which targets the nuclear β‐catenin protein to facilitate proteasomal degradation through the 26S proteasome. FBXO16 interacts physically with the C‐terminal domain of β‐catenin and promotes its lysine 48‐linked polyubiquitination. In addition, it inhibits epithelial‐to‐mesenchymal transition (EMT) by attenuating the level of β‐catenin. Therefore, depletion of FBXO16 leads to increased levels of β‐catenin, which then promotes cell invasion, tumor growth, and EMT of cancer cells. Furthermore, FBXO16 and β‐catenin share an inverse correlation of cellular expression in clinical breast cancer patient samples. In summary, we propose that FBXO16 functions as a putative tumor suppressor by forming an SCFFBXO16 complex that targets nuclear β‐catenin in a unique manner for ubiquitination and subsequent proteasomal degradation to prevent malignancy. This work suggests a novel therapeutic strategy against human cancers related to aberrant β‐catenin activation. © 2019 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The tumor suppressor p53 plays a critical role in maintaining genomic stability. In response to genotoxic stress, p53 levels increase and induce cell-cycle arrest, senescence, or apoptosis, thereby preventing replication of damaged DNA. In unstressed cells, p53 is maintained at a low level. The major negative regulator of p53 is MDM2, an E3 ubiquitin ligase that directly interacts with p53 and promotes its polyubiquitination, leading to the subsequent destruction of p53 by the 26S proteasome. Following DNA damage, MDM2 is degraded rapidly, resulting in increased p53 stability. Because of the important role of MDM2 in modulating p53 function, it is critical to understand how MDM2 levels are regulated. Here we show that the F-box protein FBXO31, a candidate tumor suppressor encoded in 16q24.3 for which there is loss of heterozygosity in various solid tumors, is responsible for promoting MDM2 degradation. Following genotoxic stress, FBXO31 is phosphorylated by the DNA damage serine/threonine kinase ATM, resulting in increased levels of FBXO31. FBXO31 then interacts with and directs the degradation of MDM2, which is dependent on phosphorylation of MDM2 by ATM. FBXO31-mediated loss of MDM2 leads to elevated levels of p53, resulting in growth arrest. In cells depleted of FBXO31, MDM2 is not degraded and p53 levels do not increase following genotoxic stress. Thus, FBXO31 is essential for the classic robust increase in p53 levels following DNA damage.

The F-box protein FBXO31 is a tumor suppressor that is encoded in 16q24.3, for which there is loss of heterozygosity in various solid tumors. FBXO31 serves as the substrate-recognition component of the SKP/Cullin/F-box protein class of E3 ubiquitin ligases and has been shown to direct degradation of pivotal cell-cycle regulatory proteins including cyclin D1 and the p53 antagonist MDM2. FBXO31 levels are normally low but increase substantially following genotoxic stress through a mechanism that remains to be determined. Here we show that the low levels of FBXO31 are maintained through proteasomal degradation by anaphase-promoting complex/cyclosome (APC/C). We find that the APC/C coactivators CDH1 and CDC20 bind to a destruction-box (D-box) motif present in FBXO31 to promote its polyubiquitination and degradation in a cell-cycle-regulated manner, which requires phosphorylation of FBXO31 on serine-33 by the prosurvival kinase AKT. Following genotoxic stress, phosphorylation of FBXO31 on serine-278 by another kinase, the DNA damage kinase ATM, results in disruption of its interaction with CDH1 and CDC20, thereby preventing FBXO31 degradation. Collectively, our results reveal how alterations in FBXO31 phosphorylation, mediated by AKT and ATM, underlie physiological regulation of FBXO31 levels in unstressed and genotoxically stressed cells.

Genetic lesions that activate KRAS account for ∼30% of the 1.6 million annual cases of lung cancer. Despite clinical need, KRAS is still undruggable using traditional small-molecule drugs/inhibitors. When oncogenic Kras is suppressed by RNA interference, tumors initially regress but eventually recur and proliferate despite suppression of Kras Here, we show that tumor cells can survive knockout of oncogenic Kras, indicating the existence of Kras-independent survival pathways. Thus, even if clinical KRAS inhibitors were available, resistance would remain an obstacle to treatment. Kras-independent cancer cells exhibit decreased colony formation in vitro but retain the ability to form tumors in mice. Comparing the transcriptomes of oncogenic Kras cells and Kras knockout cells, we identified 603 genes that were specifically up-regulated in Kras knockout cells, including the Fas gene, which encodes a cell surface death receptor involved in physiological regulation of apoptosis. Antibodies recognizing Fas receptor efficiently induced apoptosis of Kras knockout cells but not oncogenic Kras-expressing cells. Increased Fas expression in Kras knockout cells was attributed to decreased association of repressive epigenetic marks at the Fas promoter. Concordant with this observation, treating oncogenic Kras cells with histone deacetylase inhibitor and Fas-activating antibody efficiently induced apoptosis, thus bypassing the need to inhibit Kras. Our results suggest that activation of Fas could be exploited as an Achilles' heel in tumors initiated by oncogenic Kras.

Acetylation of p53 is indispensable for its transcriptional activities and induction of apoptosis upon DNA damage. Here, we show that chromatin remodelling protein SMAR1 inhibits p53 acetylation and p53 dependent apoptosis by repressing p300 expression in response to DNA damage. The repression of p300 expression by SMAR1 is relieved upon treatment with proteosomal inhibitors MG132 and Lactacystin. We demonstrate that SMAR1 interacts with p53–p300 transcriptional complex and SMAR1 overexpression antagonizes p300 interaction with p53 and suppresses activation of p53 apoptotic targets and p53 regulated miRNA miR-34a. Conversely, knockdown of SMAR1 promotes p300 accumulation and p53 acetylation while ectopic expression of p300 rescues SMAR1 inhibition on p53. Collectively, these results indicate that SMAR1 is an important player in p300–p53 regulated DNA damage signalling pathway and can exert its effect on apoptosis in a transcription independent manner.

Aberrant NF-kappaB activity promotes tumorigenesis. However, NF-kappaB also inhibits tumor growth where tumor suppressor pathways remain unaltered. Thus, its role in tumorigenesis depends upon the function of other cellular factors. Tumor suppressor SMAR1 down-modulated in high grade breast cancers is regulated by p53 and is reported to interact and stabilize p53. Because both SMAR1 and NF-kappaB are involved in tumorigenesis, we investigated the effect of SMAR1 upon NF-kappaB activity. We show that SMAR1 induction by doxorubicin or overexpression produces functional NF-kappaB complexes that are competent for binding to NF-kappaB consensus sequence. However, SMAR1 induced p65-p50 complex is phosphorylation- and transactivation-deficient. Induction of functional NF-kappaB complexes stems from down-regulation of IkappaBalpha transcription through direct binding of SMAR1 to the matrix attachment region site present in IkappaBalpha promoter and recruitment of corepressor complex. Real time PCR array for NF-kappaB target genes revealed that SMAR1 down-regulates a subset of NF-kappaB target genes that are involved in tumorigenesis. We also show that SMAR1 inhibits tumor necrosis factor alpha-induced induction of NF-kappaB suggesting that activation of NF-kappaB by SMAR1 is independent and different from classical pathway. Thus, for the first time we report that a tumor suppressor protein SMAR1 can modulate NF-kappaB transactivation and inhibit tumorigenesis by regulating NF-kappaB target genes.

International audience; Interleukin-8 (IL-8) is a pleiotropic chemokine involved in metastasis and angiogenesis of breast tumors. The expression of IL-8 is deregulated in metastatic breast carcinomas owing to aberrant NF-κB activity, which is known to positively regulate IL-8 transcription. Earlier, we have shown that tumor suppressor SMAR1 suppresses NF-κB transcriptional activity by modulating IκBα function. Here, we show that NF-κB target gene IL-8, is a direct transcriptional target of SMAR1. Using chromatin immunoprecipitation and reporter assays, we demonstrate that SMAR1 binds to IL-8 promoter MAR (matrix attachment region) and recruits HDAC1 dependent co-repressor complex. Further, we also show that SMAR1 antagonizes p300-mediated acetylation of RelA/p65, a post-translational modification indispensable for IL-8 transactivation. Thus, we decipher a new role of SMAR1 in NF-κB dependent transcriptional regulation of pro-angiogenic chemokine IL-8.

Changes in the composition of nuclear matrix associated proteins contribute to alterations in nuclear structure, one of the major phenotypes of malignant cancer cells. The malignancy-induced changes in this structure lead to alterations in chromatin folding, the fidelity of genome replication and gene expression programs. The nuclear matrix forms a scaffold upon which the chromatin is organized into periodic loop domains called matrix attachment regions (MAR) by binding to various MAR binding proteins (MARBPs). Aberrant expression of MARBPs modulates the chromatin organization and disrupt transcriptional network that leads to oncogenesis. Dysregulation of nuclear matrix associated MARBPs has been reported in different types of cancers. Some of these proteins have tumor specific expression and are therefore considered as promising diagnostic or prognostic markers in few cancers. SMAR1 (scaffold/matrix attachment region binding protein 1), is one such nuclear matrix associated protein whose expression is drastically reduced in higher grades of breast cancer. SMAR1 gene is located on human chromosome 16q24.3 locus, the loss of heterozygosity (LOH) of which has been reported in several types of cancers. This review elaborates on the multiple roles of nuclear matrix associated protein SMAR1 in regulating various cellular target genes involved in cell growth, apoptosis and tumorigenesis.

Pyrazinamide (PZA) is an important first-line antituberculosis drug because of its sterilizing activity against semidormant tubercle bacilli. In spite of its very high in vivo activity, its in vitro activity is not apparent unless an acidic environment is available, which makes PZA susceptibility testing difficult by conventional methods. The present study was, therefore, planned to assess the performance of the colorimetric BacT/ALERT 3D system and compare the results with those from conventional tests, i.e., the Löwenstein-Jensen (LJ) proportion method (pH 4.85) and Wayne's pyrazinamidase (PZase) assay, using 107 clinical isolates. The concordance among all of these tests was 89.71% after the first round of testing and reached 92.52% after resolution of the discordant results by retesting. Prolonged incubation of the PZase tube for up to 10 days was found to increase the specificity of the PZase test. The concordances between LJ proportion and BacT/ALERT 3D, LJ proportion and the PZase assay, and BacT/ALERT 3D and the PZase assay were found to be 99.06%, 93.46%, and 92.52%, respectively. Using the LJ results as the gold standard, the sensitivities of BacT/ALERT 3D and the PZase assay were 100 and 82.85%, respectively, while the specificity was 98.61% for both of the tests. The difference between the sensitivities of BacT/ALERT 3D and the PZase assay was significant ( P = 0.025). The mean turnaround times for the detection of resistant and susceptible results by BacT/ALERT 3D were 8.04 and 11.32 days, respectively. While the major limitations associated with the PZase assay and the LJ proportion method are lower sensitivity in previously treated patients and a longer time requirement, respectively, the BacT/ALERT 3D system was found to be rapid, highly sensitive, and specific.