High SEPT9_v1 expression in human breast cancer cells is associated with oncogenic phenotypes

The mechanistic role of NAT10 in cancer: Unraveling the enigmatic web of oncogenic signaling

N-acetyltransferase 10 (NAT10), a versatile enzyme, has gained considerable attention as a significant player in the complex realm of cancer biology. Its enigmatic role in tumorigenesis extends across a wide array of cellular processes, impacting cell growth, differentiation, survival, and genomic stability. Within the intricate network of oncogenic signaling, NAT10 emerges as a crucial agent in multiple cancer types, such as breast, lung, colorectal, and leukemia. This compelling research addresses the intricate complexity of the mechanistic role of NAT10 in cancer development. By elucidating its active participation in essential physiological processes, we investigate the regulatory role of NAT10 in cell cycle checkpoints, coordination of chromatin remodeling, and detailed modulation of the delicate balance between apoptosis and cell survival. Perturbations in NAT10 expression and function have been linked to oncogenesis, metastasis, and drug resistance in a variety of cancer types. Furthermore, the bewildering interactions between NAT10 and key oncogenic factors, such as p53 and c-Myc, are deciphered, providing profound insights into the molecular underpinnings of cancer pathogenesis. Equally intriguing, the paradoxical role of NAT10 as a potential tumor suppressor or oncogene is influenced by context-dependent factors and the cellular microenvironment. This study explores the fascinating interplay of genetic changes, epigenetic changes, and post-translational modifications that shape the dual character of NAT10, revealing the delicate balance between cancer initiation and suppression. Taken together, this overview delves deeply into the enigmatic role of NAT10 in cancer, elucidating its multifaceted roles and its complex interplay with oncogenic networks.

Septin9 DNA methylation is associated with breast cancer recurrence or metastasis

OBJECTIVE

We aimed to explore the prognostic value of Septin9 DNA methylation in breast cancer.

METHODS

Breast cancer patients with and without recurrence or metastasis and matched non-breast cancer patients were screened retrospectively from 2014 to 2016. Bisulfite conversion and fluorescence quantitative methylation-specific polymerase chain reaction were used to detect the Septin9 methylation status and distribution levels in patient breast tissues.

RESULTS

Septin9 DNA methylation was more frequent in breast cancer tissues than in non-breast cancer tissues, but was not significantly correlated with any relevant breast cancer patient clinicopathological characteristic. Septin9 methylation rates were higher in patients with recurrence or metastasis. Septin9 methylation, tumor size, lymph node status, and progesterone receptor (PR) expression could influence prognosis. Septin9 methylation was significantly associated with worse disease-free survival in breast cancer patients, with receiver operating characteristic curve analysis indicating that it had good prognostic ability, with an area under the curve (AUC) value of 0.719. The AUC values increased when Septin9 methylation was combined with tumor size, lymph node status, and PR to predict prognosis.

CONCLUSIONS

Septin9 DNA methylation was an independent predictors of breast cancer prognostic risk. This could possibly help improve comprehensive prognosis prediction methods when combined with other risk factors.

N4-Acetylcytidine Drives Glycolysis Addiction in Gastric Cancer via NAT10/SEPT9/HIF-1α Positive Feedback Loop

Anti-angiogenic therapy has long been considered a promising strategy for solid cancers. Intrinsic resistance to hypoxia is a major cause for the failure of anti-angiogenic therapy, but the underlying mechanism remains unclear. Here, it is revealed that N4-acetylcytidine (ac4C), a newly identified mRNA modification, enhances hypoxia tolerance in gastric cancer (GC) cells by promoting glycolysis addiction. Specifically, acetyltransferase NAT10 transcription is regulated by HIF-1α, a key transcription factor of the cellular response to hypoxia. Further, acRIP-sequencing, Ribosome profiling sequencing, RNA-sequencing, and functional studies confirm that NAT10 in turn activates the HIF-1 pathway and subsequent glucose metabolism reprogramming by mediating SEPT9 mRNA ac4C modification. The formation of the NAT10/SEPT9/HIF-1α positive feedback loop leads to excessive activation of the HIF-1 pathway and induces glycolysis addiction. Combined anti-angiogenesis and ac4C inhibition attenuate hypoxia tolerance and inhibit tumor progression in vivo. This study highlights the critical roles of ac4C in the regulation of glycolysis addiction and proposes a promising strategy to overcome resistance to anti-angiogenic therapy by combining apatinib with ac4C inhibition.

BORG family proteins in physiology and human disease

The binder of rho GTPases (BORG)/Cdc42 effector proteins (Cdc42EP) family is composed of five Rho GTPase binding proteins whose functions and mechanism of actions are of emerging interest. Here, we review recent findings pertaining to the family as a whole and consider how these change our understanding of cellular organization. Recent studies have implicated BORGs in both fundamental physiology and in human diseases, mainly cancers. An emerging pattern suggests that BORG family members cancer-promoting properties are related to their ability to regulate the cytoskeleton, with many impacting the organization of acto-myosin stress fibers. This is consistent with the broader literature indicating that BORG family members are regulators of both the septin and actin cytoskeleton networks. The exact mechanism through which BORGs modify the cytoskeleton is not clear, but we consider here a few data-supported and speculative possibilities. Finally, we delve into how the Rho GTPase Cdc42 modifies BORG function in cells. This remains open-ended as Cdc42's effects on BORGs appear cell type- and cell state-dependent. Collectively, these data point to the importance of the BORG family and suggest broader themes in their function and regulation.

SEPT9 Expression in Hepatic Nodules: An Immunohistochemical Study of Hepatocellular Neoplasm and Metastasis

The methylated SEPT9 DNA ( mSEPT9 ) in plasma is a US Food and Drug Administration (FDA)-approved screening biomarker in colorectal cancer and is emerging as a promising diagnostic and prognostic biomarker in hepatocellular carcinoma (HCC). We evaluated the SEPT9 protein expression by immunohistochemistry (IHC) in various hepatic tumors from 164 hepatectomies and explants. Cases diagnosed as HCC (n=68), hepatocellular adenoma (n=31), dysplastic nodule (n=24), and metastasis (n=41) were retrieved. SEPT9 stain was performed on representative tissue blocks showing tumor/liver interface. For HCC, archived IHC (SATB2, CK19, CDX2, CK20, and CDH17) slides were also reviewed. The findings were correlated with demographics, risk factors, tumor size, alpha fetoprotein levels at diagnosis, T stage and oncologic outcomes, with significance defined as P <0.05. Percentage of SEPT9 positivity differed significantly among hepatocellular adenoma (3%), dysplastic nodule (0%), HCC (32%), and metastasis (83%, P <0.001). Compared with patients with SEPT9- HCC, those with SEPT9+ HCC were older (70 vs. 63 y, P =0.01). The extent of SEPT9 staining correlated with age ( rs =0.31, P =0.01), tumor grade ( rs =0.30, P =0.01), and extent of SATB2 staining ( rs =0.28, P =0.02). No associations were found between SEPT9 staining and tumor size, T stage, risk factors, CK19, CDX2, CK20, or CDH17 expression, alpha fetoprotein levels at diagnosis, METAVIR fibrosis stage, and oncologic outcome in the HCC cohort. SEPT9 is likely implicated in liver carcinogenesis in a HCC subset. Similar to mSEPT9 DNA measurement in liquid biopsies, SEPT9 staining by IHC may prove helpful as an adjunct diagnostic biomarker with potential prognostic ramifications.

Down-regulation of lncRNA LUADT1 suppresses cervical cancer cell growth by sequestering microRNA-1207-5p

Emerging evidence has proved the essential roles of long non-coding RNAs (lncRNAs) in cervical carcinoma (CC). LncRNA lung adenocarcinoma-associated transcript 1 (LUADT1) is overexpressed and plays an oncogenic role in various cancers; however, the function and clinical values of LUADT1 in CC remain unclear. In this study we found that LUADT1 is highly expressed in CC tissues and cells. Up-regulated LUADT1 is significantly correlated with the more aggressive status and poorer survival of CC patients. studies show that LUADT1 depletion suppresses CC proliferation, and leads to cell apoptosis and cell cycle arrest. Furthermore, the xenograft mouse assay demonstrates that LUADT1 knockdown remarkably suppresses tumor growth. Mechanistically, LUADT1 binds to miR-1207-5p and inhibits miR-1207-5p expression in CC cells. Septin 9 (SEPT9) is identified as a miR-1207-5p target which is negatively regulated by LUADT1. Overexpression of SEPT9 abrogates the suppressed proliferation of CC cells induced by LUADT1 knockdown. These results demonstrate that LUADT1 sponges miR-1207-5p and consequently modulates SEPT9 expression in CC. Our study suggests the possible application of LUADT1 as a prognostic and therapeutic target to inhibit CC.

Revised subunit order of mammalian septin complexes explains their in vitro polymerization properties

Septins are conserved GTP-binding cytoskeletal proteins that polymerize into filaments by end-to-end joining of hetero-oligomeric complexes. In human cells, both hexamers and octamers exist, and crystallography studies predicted the order of the hexamers to be SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7, while octamers are thought to have the same core, but with SEPT9 at the ends. However, based on this septin organization, octamers and hexamers would not be expected to copolymerize due to incompatible ends. Here we isolated hexamers and octamers of specific composition from human cells and show that hexamers and octamers polymerize individually and, surprisingly, with each other. Binding of the Borg homology domain 3 (BD3) domain of Borg3 results in distinctive clustering of each filament type. Moreover, we show that the organization of hexameric and octameric complexes is inverted compared with its original prediction. This revised septin organization is congruent with the organization and behavior of yeast septins suggesting that their properties are more conserved than was previously thought.

CDC42EP5/BORG3 modulates SEPT9 to promote actomyosin function, migration, and invasion

Fast amoeboid migration is critical for developmental processes and can be hijacked by cancer cells to enhance metastatic dissemination. This migratory behavior is tightly controlled by high levels of actomyosin contractility, but how it is coupled to other cytoskeletal components is poorly understood. Septins are increasingly recognized as novel cytoskeletal components, but details on their regulation and contribution to migration are lacking. Here, we show that the septin regulator Cdc42EP5 is consistently required for amoeboid melanoma cells to invade and migrate into collagen-rich matrices and locally invade and disseminate in vivo. Cdc42EP5 associates with actin structures, leading to increased actomyosin contractility and amoeboid migration. Cdc42EP5 affects these functions through SEPT9-dependent F-actin cross-linking, which enables the generation of F-actin bundles required for the sustained stabilization of highly contractile actomyosin structures. This study provides evidence that Cdc42EP5 is a regulator of cancer cell motility that coordinates actin and septin networks and describes a unique role for SEPT9 in melanoma invasion and metastasis.

Structure and function of Septin 9 and its role in human malignant tumors

The treatment and prognosis of malignant tumors are closely related to the time when the tumors are diagnosed; the earlier the diagnosis of the tumor, the better the prognosis. However, most tumors are not detected in the early stages of screening and diagnosis. It is of great clinical significance to study the correlation between multiple pathogeneses of tumors and explore simple, safe, specific, and sensitive molecular indicators for early screening, diagnosis, and prognosis. The Septin 9 (SEPT9) gene has been found to be associated with a variety of human diseases, and it plays a role in the development of tumors. SEPT9 is a member of the conserved family of cytoskeletal GTPase, which consists of a P-loop-based GTP-binding domain flanked by a variable N-terminal region and a C-terminal region. SEPT9 is involved in many biological processes such as cytokinesis, polarization, vesicle trafficking, membrane reconstruction, deoxyribonucleic acid repair, cell migration, and apoptosis. Several studies have shown that SEPT9 may serve as a marker for early screening, diagnosis, and prognosis of some malignant tumors, and have the potential to become a new target for anti-cancer therapy. This article reviews the progress in research on the SEPT9 gene in early screening, diagnosis, and prognosis of tumors.

SEPT9_i1 regulates human breast cancer cell motility through cytoskeletal and RhoA/FAK signaling pathway regulation

Increasing cell mobility is the basis of tumor invasion and metastasis, and is therefore a therapeutic target for preventing the spread of many types of cancer. Septins are a family of cytoskeletal proteins with GTPase activity, and play a role in many important cellular functions, including cell migration. SEPT9 isoform 1 protein (SEPT9_i1) has been associated with breast tumor development and the enhancement of cell migration; however, the exact mechanism of how SEPT9_i1 might affect breast cancer progression remains to be elucidated. Here, we report that the expression of SEPT9_i1 positively correlated with paxillin, and both were significantly upregulated in invasive breast cancer tissues of patients with lymph node metastases. Lentivirus-mediated shRNA knockdown of SEPT9 in MCF-7 cells diminished tumor cell migration, focal adhesion (FA) maturation and the expression of β-actin, β-tubulin, Cdc42, RhoA, and Rac, whereas overexpression of SEPT9_i1 in SEPT9-knockdown MCF-7 cells promoted cell migration, FA maturation and relevant protein expression. Furthermore, overexpression of SEPT9_i1 in MCF-7 cells markedly increased FAK/Src/paxillin signaling, at least in part through RhoA/ROCK1 upstream activation. Transcriptome profiling suggested that SEPT9_i1 may directly affect “Focal adhesion” and “Regulation of actin cytoskeleton” signaling mechanisms. Finally, overexpression of SEPT9_i1 markedly enhanced lung metastases in vivo 6 weeks after tumor inoculation. These findings suggest that a mechanism of Septin-9-induced aberrant cancer cell migration is through cytoskeletal regulation and FA modulation, and encourages the use of SEPT9 as novel therapeutic target in the prevention of tumor metastasis.

Septin 9 isoforms promote tumorigenesis in mammary epithelial cells by increasing migration and ECM degradation through metalloproteinase secretion at focal adhesions

The cytoskeletal interacting protein Septin 9 (SEPT9), a member of the septin gene family, has been proposed to have oncogenic functions. It is a known hot spot of retroviral tagging insertion and a fusion partner of both de novo and therapy-induced mixed lineage leukemia (MLL). Of all septins, SEPT9 holds the strongest link to cancer, especially breast cancer. Murine models of breast cancer frequently exhibit Sept9 amplification in the form of double minute chromosomes, and about 20% of human breast cancer display genomic amplification and protein over expression at the SEPT9 locus. Yet, a clear mechanism by which SEPT9 elicits tumor-promoting functions is lacking.

To obtain unbiased insights on molecular signatures of SEPT9 upregulation in breast tumors, we overexpressed several of its isoforms in breast cancer cell lines. Global transcriptomic profiling supports a role of SEPT9 in invasion. Functional studies reveal that SEPT9 upregulation is sufficient to increase degradation of the extracellular matrix, while SEPT9 downregulation inhibits this process. The degradation pattern is peripheral and associated with focal adhesions (FA), where it is coupled with increased expression of matrix metalloproteinases. SEPT9 overexpression induces MMP upregulation in human tumors and in culture models and promotes MMP3 secretion to the media at FAs. Downregulation of SEPT9 or chemical inhibition of septin filament assembly impairs recruitment of MMP3 to FAs. Our results indicate that SEPT9 promotes upregulation and both trafficking and secretion of MMPs near FAs, thus enhancing migration and invasion of breast cancer cells.

SREBP1 drives Keratin-80-dependent cytoskeletal changes and invasive behavior in endocrine-resistant ERα breast cancer

Approximately 30% of ERα breast cancer patients relapse with metastatic disease following adjuvant endocrine therapies. The connection between acquisition of drug resistance and invasive potential is poorly understood. In this study, we demonstrate that the type II keratin topological associating domain undergoes epigenetic reprogramming in aromatase inhibitors (AI)-resistant cells, leading to Keratin-80 (KRT80) upregulation. KRT80 expression is driven by de novo enhancer activation by sterol regulatory element-binding protein 1 (SREBP1). KRT80 upregulation directly promotes cytoskeletal rearrangements at the leading edge, increased focal adhesion and cellular stiffening, collectively promoting cancer cell invasion. Shearwave elasticity imaging performed on prospectively recruited patients confirms KRT80 levels correlate with stiffer tumors. Immunohistochemistry showed increased KRT80-positive cells at relapse and, using several clinical endpoints, KRT80 expression associates with poor survival. Collectively, our data uncover an unpredicted and potentially targetable direct link between epigenetic and cytoskeletal reprogramming promoting cell invasion in response to chronic AI treatment.

Septin filament coalignment with microtubules depends on SEPT9_i1 and tubulin polyglutamylation, and is an early feature of acquired cell resistance to paclitaxel

Cancer cell resistance to taxanes is a complex, multifactorial process, which results from the combination of several molecular and cellular changes. In breast cancer cells adapted to long-term paclitaxel treatment, we previously identified a new adaptive mechanism that contributes to resistance and involves high levels of tubulin tyrosination and long-chain polyglutamylation coupled with high levels of septin expression, especially that of SEPT9_i1. This in turn led to higher CLIP-170 and MCAK recruitment to microtubules to enhance microtubule dynamics and therefore counteract the stabilizing effects of taxanes. Here, we explored to which extent this new mechanism alone could trigger taxane resistance. We show that coupling septins (including SEPT9_i1) overexpression together with long-chain tubulin polyglutamylation induce significant paclitaxel resistance in several naive (taxane-sensitive) cell lines and accordingly stimulate the binding of CLIP-170 and MCAK to microtubules. Strikingly, such resistance was paralleled by a systematic relocalization of septin filaments from actin fibers to microtubules. We further show that this relocalization resulted from the overexpression of septins in a context of enhanced tubulin polyglutamylation and reveal that it could also be promoted by an acute treatment with paclitaxel of sensitve cell displaying a high basal level of SEPT9_i1. These findings point out the functional importance and the complex cellular dynamics of septins in the onset of cell resistance to death caused by microtubule-targeting antimitotic drugs of the taxane family.

Septin 9 isoform 1 (SEPT9_i1) specifically interacts with importin-α7 to drive hypoxia-inducible factor (HIF)-1α nuclear translocation

We have shown previously that septin 9 isoform 1 (SEPT9_i1) protein associates with hypoxia-inducible factor (HIF)-1α to augment HIF-1 transcriptional activity by driving its importin-α-mediated nuclear translocation. Using in vitro and in vivo binding assays we identified that HIF-1α interacts with importin-α5 and importin-α7 in prostate cancer cells but only importin-α7 interacts with SEPT9_i1. The interaction with importin-α7 was dependent on the first 25 amino acids of SEPT9_i1 that are unique compared to other members of the mammalian septin family. Depletion of endogenous importin-α7 reduced HIF-1α levels in the nucleus. Our results provide evidence that there are importin-α specificities in the cytosolic/nuclear translocation process of HIF-1α protein, which may act differently under certain pathophysiological circumstances where SEPT9_i1 is overexpressed.

Imaging of hypoxia-inducible factor 1α and septin 9 interaction by bimolecular fluorescence complementation in live cancer cells

Hypoxia-inducible factor 1 (HIF-1) is a major mediator of the hypoxic response involved in tumor progression. We had earlier described the interaction between septin 9 isoform 1 (SEPT9_i1) protein and the oxygen-regulated subunit, HIF-1α. SEPT9_i1 is a member of the conserved family of GTP-binding cytoskeleton septins. SEPT9_i1 stabilizes HIF-1α and facilitates its cytoplasmic-nuclear translocation. We utilized split yellow fluorescent protein (YFP) bimolecular fluorescence complementation (BiFC) methodology to monitor the interaction between HIF-1α and SEPT9_i1 in live cells. N-terminal (YN) and C-terminal (YC) split YFP chimeras with HIF-1α and SEPT9_i1 on both their amino and carboxyl termini were generated. HIF-1α and SEPT9_i1 chimeras were expressed in cancer cells and screened for functional complementation. SEPT9_i1-YN and YC-HIF-1α formed a long-lived highly stable complex upon interaction. The BiFC signal was increased in the presence of hypoxia-mimicking agents. In contrast, YC-ΔHLH-HIF-1α chimera, which lacked the helix-loop-helix domain that is essential for the interaction with SEPT9_i1 as well as the expression of SEPT9_i1 252-379 amino acids fragment required for the interaction with HIF-1α, significantly reduced the BiFC signal. The signal was also reduced when cells were treated with 17-N-allylamino-17-demethoxygeldanamycin, an HSP90 inhibitor that inhibits HIF-1α. It was increased with fourchlorfenuron, a small molecule that increases the interaction between HIF-1α and SEPT9_i1. These results reconfirmed the interaction between HIF-1α and SEPT9_i1 that was imaged in live cells. This BiFC system represents a novel approach for studying the real-time interaction between these two proteins and will allow high-throughput drug screening to identity compounds that disrupt this interaction.

Hepatitis B virus X induces inflammation and cancer in mice liver through dysregulation of cytoskeletal remodeling and lipid metabolism

Hepatitis B virus X protein (HBx) participates in the occurrence and development processes of hepatocellular carcinoma (HCC) as a multifunctional regulation factor. However, the underlying molecular mechanism remains obscure. Here, we describe the use of p21^HBx/+ mouse and SILAM (Stable Isotope Labeling in Mammals) strategy to define the pathological mechanisms for the occurrence and development of HBx induced liver cancer. We systematically compared a series of proteome samples from regular mice, 12- and 24-month old p21^HBx/+ mice representing the inflammation and HCC stages of liver disease respectively and their nontransgenic wild-type (WT) littermates. Totally we identified 22 and 97 differentially expressed proteins out of a total of 2473 quantified proteins. Bioinformatics analysis suggested that the lipid metabolism and CDC42-induced cytoskeleton remodeling pathways were strongly activated by the HBx transgene. Interestingly, the protein-protein interaction MS study revealed that HBx directly interacted with multiple proteins in these two pathways. The same effect of up-regulation of cytoskeleton and lipid metabolism related proteins, including CDC42, CFL1, PPARγ and ADFP, was also observed in the Huh-7 cells transfected with HBx. More importantly, CFL1 and ADFP were specifically accumulated in HBV-associated HCC (HBV-HCC) patient samples, and their expression levels were positively correlated with the severity of HBV-related liver disease. These results provide evidence that HBx induces the dysregulation of cytoskeleton remodeling and lipid metabolism and leads to the occurrence and development of liver cancer. The CFL1 and ADFP might be served as potential biomarkers for prognosis and diagnosis of HBV-HCC.

The requirement of SEPT2 and SEPT7 for migration and invasion in human breast cancer via MEK/ERK activation

Septins are a novel class of GTP-binding cytoskeletal proteins evolutionarily conserved from yeast to mammals and have now been found to play a contributing role in a broad range of tumor types. However, their functional importance in breast cancer remains largely unclear. Here, we demonstrated that pharmaceutical inhibition of global septin dynamics would greatly suppress proliferation, migration and invasiveness in breast cancer cell lines. We then examined the expression and subcellular distribution of the selected septins SEPT2 and SEPT7 in breast cancer cells, revealing a rather variable localization of the two proteins with cell cycle progression. To determine the role of both septins in mediating malignant behavior of cancer cells, we used RNA silencing to specifically deplete endogenous SEPT2 or SEPT7 in highly invasive breast cancer cell line MDA-MB-231. Our findings showed that SEPT2/7 depletion had virtually identical inhibitory effects on cellular proliferation, apoptosis, migration and invasion. Moreover, the opposite performance in migration and invasion was observed after enforced expression of SEPT2/7 in the same cell line. We further demonstrated MEK/ERK activation, but not other MAPKs and AKT, was positively correlated with the protein levels of SEPT2 and SEPT7. Additionally, in SEPT2/7-overexpressing cells, the MEK specific inhibitor U0126 was able to correct the high active status of MEK/ERK while normalizing the increased invasive behaviors of these cells. Taken together, these results strongly suggest that SEPT2 and SEPT7 are involved in breast carcinogenesis and may serve as valuable therapeutic targets for breast cancer.

DNA methylation profiles in cancer diagnosis and therapeutics

Cancer initiation and proliferation is regulated by both epigenetic and genetic events with epigenetic modifications being increasingly identified as important targets for cancer research. DNA methylation catalyzed by DNA methyltransferases (DNMTs) is one of the essential epigenetic mechanisms that control cell proliferation, apoptosis, differentiation, cell cycle, and transformation in eukaryotes. Recent progress in epigenetics revealed a deeper understanding of the mechanisms of tumorigenesis and provided biomarkers for early detection, diagnosis, and prognosis in cancer patients. Although DNA methylation biomarker possesses potential contributing to precision medicine, there are still limitations to be overcome before it reaches clinical setting. Hence, the current status of DNA methylation biomarkers was reviewed and the future use in clinic was also predicted.

Methylation of Septin9 mediated by DNMT3a enhances hepatic stellate cells activation and liver fibrogenesis

Liver fibrosis, resulting from chronic and persistent injury to the liver, is a worldwide health problem. Advanced liver fibrosis results in cirrhosis, liver failure and even hepatocellular cancer (HCC), often eventually requiring liver transplantation, poses a huge health burden on the global community. However, the specific pathogenesis of liver fibrosis remains not fully understood. Numerous basic and clinical studies have provided evidence that epigenetic modifications, especially DNA methylation, might contribute to the activation of hepatic stellate cells (HSCs), the pivotal cell type responsible for the fibrous scar in liver. Here, reduced representation bisulfite sequencing (RRBS) and bisulfite pyrosequencing PCR (BSP) analysis identified hypermethylation status of Septin9 (Sept9) gene in liver fibrogenesis. Sept9 protein was dramatically decreased in livers of CCl4-treated mice and immortalized HSC-T6 cells exposed to TGF-β1. Nevertheless, the suppression of Sept9 could be blocked by DNMT3a-siRNA and DNA methyltransferase inhibitor, 5-aza-2'-deoxycytidine (5-azadC). Overexpressed Sept9 attenuated TGF-β1-induced expression of myofibroblast markers α-SMA and Col1a1, accompanied by up-regulation of cell apoptosis-related proteins. Conversely, RNAi-mediated silencing of Sept9 enhanced accumulation of extracellular matrix. These observations suggested that Sept9 contributed to alleviate liver fibrosis might partially through promoting activated HSCs apoptosis and this anti-fibrogenesis effect might be blocked by DNMT-3a mediated methylation of Sept9. Therefore, pharmacological agents that inhibit Sept9 methylation and increase its expression could be considered as valuable treatments for liver fibrosis.

High SEPT9_i1 protein expression is associated with high-grade prostate cancers

Septins are a family of GTP-binding cytoskeleton proteins expressed in many solid tumors. Septin 9 (SEPT9) in particular was found overexpressed in diverse carcinomas. Herein, we studied the expression of SEPT9 isoform 1 protein (SEPT9_i1) in human prostate cancer specimens. We utilized immunohistochemical staining to study the expression of SEPT9_i1 protein. Staining level was analyzed in association with clinical characteristics and the pathological Gleason grade and score. Fifty human prostate cancer specimens (42 primary tumors and 8 metastatic lesions) were stained by SEPT9_i1 antibody and analyzed. SEPT9_i1 protein was expressed in prostate cancer cells but absent in normal epithelial cells. The intensity of staining was correlated proportionally to pretreatment prostate-specific antigen (PSA) blood levels and Gleason score (P < 0.05). SEPT9_i1 was highly expressed in all metastatic lesions. A significant assocation between SEPT9_i1 expression and high Gleason score on multivariate linear regression analysis was found. We conclude that SEPT9_i1 is expressed in high-grade prostate tumors suggesting it has a significant role in prostate tumorigenesis and that it could serve as a molecular marker for prostate tumor progression.

Cytosolic chaperones mediate quality control of higher-order septin assembly in budding yeast

Septin hetero-oligomers polymerize into cytoskeletal filaments with essential functions in many eukaryotic cell types. Mutations within the oligomerization interface that encompasses the GTP-binding pocket of a septin (its "G interface") cause thermoinstability of yeast septin hetero-oligomer assembly, and human disease. When coexpressed with its wild-type counterpart, a G interface mutant is excluded from septin filaments, even at moderate temperatures. We show that this quality control mechanism is specific to G interface mutants, operates during de novo septin hetero-oligomer assembly, and requires specific cytosolic chaperones. Chaperone overexpression lowers the temperature permissive for proliferation of cells expressing a G interface mutant as the sole source of a given septin. Mutations that perturb the septin G interface retard release from these chaperones, imposing a kinetic delay on the availability of nascent septin molecules for higher-order assembly. Un-expectedly, the disaggregase Hsp104 contributes to this delay in a manner that does not require its "unfoldase" activity, indicating a latent "holdase" activity toward mutant septins. These findings provide new roles for chaperone-mediated kinetic partitioning of non-native proteins and may help explain the etiology of septin-linked human diseases.

Cell and molecular biology of septins

Septins are a family of GTP-binding proteins that assemble into cytoskeletal filaments. Unlike other cytoskeletal components, septins form ordered arrays of defined stoichiometry that can polymerize into long filaments and bundle laterally. Septins associate directly with membranes and have been implicated in providing membrane stability and serving as diffusion barriers for membrane proteins. In addition, septins bind other proteins and have been shown to function as multimolecular scaffolds by recruiting components of signaling pathways. Remarkably, septins participate in a spectrum of cellular processes including cytokinesis, ciliogenesis, cell migration, polarity, and cell-pathogen interactions. Given their breadth of functions, it is not surprising that septin abnormalities have also been linked to human diseases. In this review, we discuss the current knowledge of septin structure, assembly and function, and discuss these in the context of human disease.

Septin 9 amplification and isoform-specific expression in peritumoral and tumor breast tissue

Septins are a large family of GTP-binding proteins abnormally expressed in many solid tumors. Septin 9 (SEPT9) in particular has been found overexpressed in diverse human tumors including breast, head and neck, ovarian, endometrial, kidney, and pancreatic cancer. Although we previously reported SEPT9 amplification in breast cancer, we now show specifically that high-grade breast carcinomas, the subtype with worst clinical outcome, exhibit a significant increase in SEPT9 copy number when compared with other tumor grades. We also present, for the first time, a sensitive and quantitative measure of seven (SEPT9_v1 through SEPT9_v7) isoform variant mRNA levels in mammary epithelial cells. SEPT9_v1, SEPT9_v3, SEPT9_v6, and SEPT9_v7 isoforms were expressed at the highest levels followed by SEPT9_v2 and SEPT9_v5, whereas SEPT9_v4 was almost undetectable. Although most of the isoforms were upregulated in primary tumor tissues relative to the patient-matched peritumoral tissues, SEPT9_v4 remained the lowest expressing isoform. This comprehensive analysis of SEPT9 provides substantial evidence for increased SEPT9 expression as a consequence of genomic amplification and is the first study to profile SEPT9_v1 through SEPT9_v7 isoform-specific mRNA expression in tumor and nontumor tissues from patients with breast cancer.

Novel septin 9 repeat motifs altered in neuralgic amyotrophy bind and bundle microtubules

Novel septin 9 repeat motifs interact with the acidic C-terminal tails of β-tubulin to promote microtubule bundling and asymmetric neurite growth.

Septin 9 (SEPT9) interacts with microtubules (MTs) and is mutated in hereditary neuralgic amyotrophy (HNA), an autosomal-dominant neuropathy. The mechanism of SEPT9 interaction with MTs and the molecular basis of HNA are unknown. Here, we show that the N-terminal domain of SEPT9 contains the novel repeat motifs K/R-x-x-E/D and R/K-R-x-E, which bind and bundle MTs by interacting with the acidic C-terminal tails of β-tubulin. Alanine scanning mutagenesis revealed that the K/R-R/x-x-E/D motifs pair electrostatically with one another and the tails of β-tubulin, enabling septin–septin interactions that link MTs together. SEPT9 isoforms lacking repeat motifs or containing the HNA-linked mutation R88W, which maps to the R/K-R-x-E motif, diminished intracellular MT bundling and impaired asymmetric neurite growth in PC-12 cells. Thus, the SEPT9 repeat motifs bind and bundle MTs, and thereby promote asymmetric neurite growth. These results provide the first insight into the mechanism of septin interaction with MTs and the molecular and cellular basis of HNA.

SEPT9_i1 is required for the association between HIF-1α and importin-α to promote efficient nuclear translocation

Septin 9 isoform 1 (SEPT9_i1) protein associates with hypoxia-inducible factor (HIF)-1α to augment HIF-1 transcriptional activity. The first 25 amino acids of SEPT9_i1 (N 25) are unique compared with other members of the mammalian septin family. This N 25 domain is critical for HIF-1 activation by SEPT9_i1 but not essential for the protein-protein interaction. Here, we show that expression of N 25 induces a significant dose-dependent inhibition of HIF-1 transcriptional activity under normoxia and hypoxia without influencing cellular HIF-1α protein levels. In vivo, N 25 expression inhibits proliferation, tumor growth and angiogenesis concomitant with decreased expression levels of intratumoral HIF-1 downstream genes. Depletion of endogenous SEPT9_i1 or the exogenous expression of N 25 fragment reduces nuclear HIF-1α levels accompanied by reciprocal accumulation of HIF-1α in the cytoplasm. Mechanistically, SEPT9_i1 binds to importin-α through N 25 depending on its bipartite nuclear localization signal, to scaffold the association between HIF-1α and importin-α, which leads to facilitating HIF-1α nuclear translocation. Our data explore a new and a previously unrecognized role of a septin protein in the cytoplasmic-nuclear translocation process. This new level in the regulation of HIF-1α translocation is critical for efficient HIF-1 transcriptional activation that could be targeted for cancer therapeutics.

DNA methylation as an early diagnostic marker of cancer (Review)

DNA methylation is one of the essential epigenetic mechanisms that are closely correlated with the mechanisms underlying cell growth, differentiation and transformation in eukaryotes. Global changes in the epigenetic landscape are considered to be a hallmark of cancer. The initiation and progression of cancer are mediated through epigenetic modifications along with genetic alterations. Aberrant methylation of promoter regions is an epigenetic abnormality of the human genome that is highly characteristic of cancer. In this review, we aimed to summarize our current understanding of the alterations in the epigenetic landscape and investigate the potential use of DNA and RNA methylation in effective molecular treatment strategies.

Aberrant septin 9 DNA methylation in colorectal cancer is restricted to a single CpG island

Background

The septin 9 gene (SEPT9) codes for a GTP-binding protein associated with filamentous structures and cytoskeleton formation. SEPT9 plays a role in multiple cancers as either an oncogene or a tumor suppressor gene. Regulation of SEPT9 expression is complex and not well understood; however, hypermethylation of the gene was recently introduced as a biomarker for early detection of colorectal cancer (CRC) and has been linked to cancer of the breast and of the head and neck. Because the DNA methylation landscape of different regions of SEPT9 is poorly understood in cancer, we analyzed the methylation patterns of this gene in distinct cell populations from normal and diseased colon mucosa.

Methods

Laser capture microdissection was performed to obtain homogeneous populations of epithelial and stromal cells from normal, adenomatous, and tumorous colon mucosa. Microdissected samples were analyzed using direct bisulfite sequencing to determine the DNA methylation status of eight regions within and near the SEPT9 gene. Septin-9 protein expression was assessed using immunohistochemistry (IHC).

Results

Regions analyzed in SEPT9 were unmethylated in normal tissue except for a methylation boundary detected downstream of the largest CpG island. In adenoma and tumor tissues, epithelial cells displayed markedly increased DNA methylation levels (>80%, p <0.0001) in only one of the CpG islands investigated. SEPT9 methylation in stromal cells increased in adenomatous and tumor tissues (≤50%, p <0.0001); however, methylation did not increase in stromal cells of normal tissue close to the tumor. IHC data indicated a significant decrease (p <0.01) in Septin-9 protein levels in epithelial cells derived from adenoma and tumor tissues; Septin-9 protein levels in stromal cells were low in all tissues.

Conclusions

Hypermethylation of SEPT9 in adenoma and CRC specimens is confined to one of several CpG islands of this gene. Tumor-associated aberrant methylation originates in epithelial cells; stromal cells appear to acquire hypermethylation subsequent to epithelial cells, possibly through field effects. The region in SEPT9 with disease-related hypermethylation also contains the CpGs targeted by a novel blood-based screening test (Epi proColon®), providing further support for the clinical relevance of this biomarker.

Mitotic regulation of SEPT9 protein by cyclin-dependent kinase 1 (Cdk1) and Pin1 protein is important for the completion of cytokinesis

Precise cell division is essential for multicellular development, and defects in this process have been linked to cancer. Septins are a family of proteins that are required for mammalian cell division, but their function and mode of regulation during this process are poorly understood. Here, we demonstrate that cyclin-dependent kinase 1 (Cdk1) phosphorylates septin 9 (SEPT9) upon mitotic entry, and this phosphorylation controls association with the proline isomerase, Pin1. Both SEPT9 and Pin1 are critical for mediating the final separation of daughter cells. Expression of mutant SEPT9 that is defective in Pin1 binding was unable to rescue cytokinesis defects caused by SEPT9 depletion but rather induced dominant-negative defects in cytokinesis. However, unlike SEPT9 depletion, Pin1 was not required for the accumulation of the exocyst complex at the midbody. These results suggest that SEPT9 plays multiple roles in abscission, one of which is regulated by the action of Cdk1 and Pin1.

Forchlorfenuron disrupts SEPT9_i1 filaments and inhibits HIF-1

Forchlorfenuron (FCF) is a synthetic plant cytokinin that has been shown to alter yeast and mammalian septin organization. Septins are a highly conserved family of GTP-binding cytoskeletal proteins. Mammalian septins are involved in diverse cellular processes including tumorigenesis. We have been studying the interaction between septin 9 isoform 1 (SEPT9_i1) and hypoxia inducible factor-1α (HIF-1α), the oxygen regulated subunit of HIF-1. HIF-1 is a key transcription factor in the hypoxic responses pathway, and its activation has been observed in carcinogenesis and numerous cancers. SEPT9_i1/HIF-1α interaction plays an important role in upregulation of HIF-1 transcriptional activity by preventing HIF-1α's ubiquitination and degradation leading to increased tumor growth and angiogenesis. We tested the hypothesis whether FCF affects SEPT9_i1 filamentous structures and consequently HIF-1 pathway in cancer cells. We showed that FCF suppresses tumorigenic properties, including proliferation, migration and transformation, in prostate cancer cells. FCF did not alter SEPT9_i1 steady state protein expression levels but it affected its filamentous structures and subcellular localization. FCF induced degradation of HIF-1α protein in a dose- and time-dependent manner. This inhibition was also shown in other common cancer types tested. Rapid degradation of HIF-1α protein levels was accompanied by respective inhibition in HIF-1α transcriptional activity. Moreover, HIF-1α protein half-life was markedly decreased in the presence of FCF compared with that in the absence of FCF. The FCF-induced degradation of HIF-1α was mediated in a significant part via the proteasome. To the best of our knowledge, this is the first demonstration of specific manipulation of septin filaments by pharmacological means having downstream inhibitory effects on the HIF-1 pathway.

Septin filament organization in Saccharomyces cerevisiae

Septins are a family of GTP-binding, membrane-interacting cytoskeletal proteins, highly conserved and essential in all eukaryotes (with the exception of plants). Septins play important roles in a number of cellular events that involve membrane remodeling and compartmentalization. One such event is cytokinesis, the last stage of cell division. While cytokinesis is ultimately achieved via the mechanical contraction of an actomyosin ring at the septum, determination of the location where cytokinesis will take place, and recruitment of factors involved in signaling events leading to septation requires the activity of septins. We are working towards dissecting the properties of septins from the budding yeast Saccharomyces cerevisiae, where they were first discovered as cell cycle mutants. In our studies we have employed several complementary electron microscopy techniques to describe the organization and structure of septins both in vitro and in situ.

The structure and properties of septin 3: a possible missing link in septin filament formation

The human genome codes for 13 members of a family of filament-forming GTP-binding proteins known as septins. These have been divided into four different subgroups on the basis of sequence similarity. The differences between the subgroups are believed to control their correct assembly into heterofilaments which have specific roles in membrane remodelling events. Many different combinations of the 13 proteins are theoretically possible and it is therefore important to understand the structural basis of specific filament assembly. However, three-dimensional structures are currently available for only three of the four subgroups. In the present study we describe the crystal structure of a construct of human SEPT3 which belongs to the outstanding subgroup. This construct (SEPT3-GC), which includes the GTP-binding and C-terminal domains, purifies as a nucleotide-free monomer, allowing for its characterization in terms of GTP-binding and hydrolysis. In the crystal structure, SEPT3-GC forms foreshortened filaments which employ the same NC and G interfaces observed in the heterotrimeric complex of human septins 2, 6 and 7, reinforcing the notion of 'promiscuous' interactions described previously. In the present study we describe these two interfaces and relate the structure to its tendency to form monomers and its efficiency in the hydrolysis of GTP. The relevance of these results is emphasized by the fact that septins from the SEPT3 subgroup may be important determinants of polymerization by occupying the terminal position in octameric units which themselves form the building blocks of at least some heterofilaments.

SEPTIN12 genetic variants confer susceptibility to teratozoospermia

It is estimated that 10-15% of couples are infertile and male factors account for about half of these cases. With the advent of intracytoplasmic sperm injection (ICSI), many infertile men have been able to father offspring. However, teratozoospermia still remains a big challenge to tackle. Septins belong to a family of cytoskeletal proteins with GTPase activity and are involved in various biological processes e.g. morphogenesis, compartmentalization, apoptosis and cytokinesis. SEPTIN12, identified by c-DNA microarray analysis of infertile men, is exclusively expressed in the post meiotic male germ cells. Septin12(+/+)/Septin12(+/-) chimeric mice have multiple reproductive defects including the presence of immature sperm in the semen, and sperm with bent neck (defect of the annulus) and nuclear DNA damage. These facts make SEPTIN12 a potential sterile gene in humans. In this study, we sequenced the entire coding region of SEPTIN12 in infertile men (n = 160) and fertile controls (n = 200) and identified ten variants. Among them is the c.474 G>A variant within exon 5 that encodes part of the GTP binding domain. The variant creates a novel splice donor site that causes skipping of a portion of exon 5, resulting in a truncated protein lacking the C-terminal half of SEPTIN12. Most individuals homozygous for the c.474 A allele had teratozoospermia (abnormal sperm <14%) and their sperm showed bent tail and de-condensed nucleus with significant DNA damage. Ex vivo experiment showed truncated SEPT12 inhibits filament formation in a dose-dependent manner. This study provides the first causal link between SEPTIN12 genetic variant and male infertility with distinctive sperm pathology. Our finding also suggests vital roles of SEPT12 in sperm nuclear integrity and tail development.

SEPT9 occupies the terminal positions in septin octamers and mediates polymerization-dependent functions in abscission

Septins are filamentous guanosine triphosphatase-binding proteins that are required for cytokinesis in a wide range of organisms from yeast to man. Several septins, including SEPT9, have been found to be altered in cancers, but their roles in malignancy and cytokinesis remain unclear. It is known that they assemble into rod-shaped oligomeric complexes that join end-on-end to form filaments, but whether SEPT9 incorporates into these complexes and how it does so are unanswered questions. We used tandem affinity purification of mammalian septin complexes to show that SEPT9 occupies a terminal position in an octameric septin complex. A mutant SEPT9, which cannot self-associate, disrupted septin filament formation and resulted in late abscission defects during cytokinesis but did not affect septin-dependent steps earlier in mitosis. These data suggest that mammalian SEPT9 holds a terminal position in the septin octamers, mediating abscission-specific polymerization during cytokinesis.

Steps solidifying a role for SEPT9 in breast cancer suggest that greater strides are needed

Septins comprise a conserved family of GTPase proteins. Of these, human SEPT9 has been widely implicated in cancers of epithelial origin, including breast cancer, as well as leukemia. In a previous issue of Breast Cancer Research, Connolly and colleagues present compelling data further supporting a role for SEPT9 isoforms in early breast cancer development as well as evidence suggesting that cellular localization patterns of SEPT9 isoforms may contribute to oncogenesis.

Mammalian septins: dynamic heteromers with roles in cellular morphogenesis and compartmentalization

The septins are a family of GTP-binding proteins, evolutionarily conserved from yeast through to mammals, with roles in multiple core cellular functions. Here we provide an overview of our current knowledge of septin structure and function and focus mainly on mammalian septins, but gain much insight by drawing on knowledge of septins in other organisms. We describe their genomic and transcriptional complexity: a complexity manifest also in the diversity of scaffold structures that septins can form. Septin complexes can act to localize interacting proteins at specific intracellular locales and can also define membrane compartments by defining diffusion barriers. By such activities, septins can contribute to the definition of spatial asymmetry and cell polarity and we suggest a potential role in stem cell biology. Finally, we review the evidence that septins contribute to various disease states and argue that it is a breakdown in the tight regulation of their expression (particularly of individual isoforms), and also their inherent ability to oligomerize, which is pathogenic. Study of the perturbation of septin complex formation in disease will provide valuable insights into septin biology and will be a fertile ground for study.

SEPT9_i1 and genomic instability: mechanistic insights and relevance to tumorigenesis

Septins are highly conserved cytoskeletal GTP-binding proteins implicated in numerous cellular processes from apoptosis to vesicle trafficking. Septins have been associated with leukemia and solid tumor malignancies, including breast, ovarian, and prostate. We previously reported that high SEPT9_i1 expression in human mammary epithelial cell lines (HMECs) led to malignant cellular phenotypes such as increased cell proliferation, invasiveness, motility, and genomic instability. Our goal here was to better understand how SEPT9_i1 expression might contribute to genomic instability and malignant progression. First, we confirmed that even transient expression of SEPT9_i1 was sufficient to increase aneuploidy in HMECs. We then analyzed SEPT9_i1 by immunoprecipitation and immunofluorescence studies and found that SEPT9_i1 interacts with both α and γ tubulin. SEPT9_i1 expressing cells demonstrated dramatic chromosome segregation defects, centrosome amplification and cytokinesis defects, suggesting two possible molecular mechanisms contributing to the development of genomic instability. This suggests that SEPT9_i1 may promote genomic instability through both cytokinesis and mitotic spindle defects which lead to chromosome missegregation.

The emerging functions of septins in metazoans

Septins form a subfamily of highly related GTP-binding proteins conserved from eukaryotic protists to mammals. In most cases, septins function in close association with cell membranes and the actin and microtubule cytoskeleton to regulate a wide variety of key cellular processes. Further underscoring their importance, septin abnormalities are associated with several human diseases. Remarkably, septins have the ability to polymerize into assemblies of different sizes in vitro and in vivo. In cells, these structures act in the formation of diffusion barriers and scaffolds that maintain subcellular polarity. Here, we focus on the emerging roles of vertebrate septins in ciliogenesis, neurogenesis, tumorigenesis and host-pathogen interactions, and discuss whether unifying themes underlie the molecular function of septins in health and disease.

Data-independent proteomic screen identifies novel tamoxifen agonist that mediates drug resistance

A label-free quantitative variation of the recently developed data-independent shotgun proteomic method precursor acquisition independent from ion count (PAcIFIC) was used to identify novel proteins implicated in cancer progression and resistance. Specifically, this screen identified the pro-metastatic protein anterior gradient 2 (AGR2) as significantly up-regulated in tamoxifen-treated cells. Highlighting the need for direct proteome profiling methods like PAcIFIC, neither data-dependent gas-phase fractionation nor a transcriptomic screen detected AGR2 protein/transcript at significantly up-regulated levels. Further cell-based experiments using human cancer cell lines and in vivo xenografts confirmed the PAcIFIC hypothesis that AGR2 is up-regulated in MCF-7 cells post tamoxifen treatment and that it is implicated in drug resistance mediation.

The influence of methylated septin 9 gene on RNA and protein level in colorectal cancer

Colorectal cancer is one of the leading death causes in the world. Specificity and sensitivity of the present screening methods are unsuitable and their compliance is too low. Nowadays the most effective method is the colonoscopy, because it gives not only macroscopic diagnosis but therapeutic possibility as well, however the compliance of the patients is very low. Hence development of new diagnostic methods is needed. Altered expression of septin 9 was found in several tumor types including colorectal cancer. The aim of this study was to detect the methylation related mRNA and protein expression changes of septin 9 in colorectal adenoma-dysplasia-carcinoma sequence and to analyze its reversibility by demethylation treatment. Septin 9 protein expression showed significant difference between normal and colorectal cancer (CRC) samples (p < 0,001). According to biopsy microarray results, septin 9 mRNA expression decreased in the progression of colon neoplastic disease (p < 0,001). In laser microdissected epithelial cells, septin 9 significantly underexpressed in CRC compared to healthy controls (p < 0,001). The expression of septin9_v1 region was higher in the healthy samples, while septin9_v2, v4, v4*, v5 overexpression were detected in cancer epithelial cells compared to normal. The septin 9 mRNA and protein levels of HT29 cells increased after demethylation treatment. The increasing methylation of septin 9 gene during colorectal adenoma-dysplasia-carcinoma sequence progression is reflected in the decreasing mRNA and protein expression, especially in the epithelium. These changes can be reversed by demethylation agents converting this screening marker gene into therapeutic target.

Modulation of septin and molecular motor recruitment in the microtubule environment of the Taxol-resistant human breast cancer cell line MDA-MB-231

Cell resistance to low doses of paclitaxel (Taxol) involves a modulation of microtubule (MT) dynamics. We applied a proteomic approach based on 2-DE coupled with MS to identify changes in the MT environment of Taxol-resistant breast cancer cells. Having established a proteomic pattern of the microtubular proteins extracted from MDA-MB-231 cells, we verified by Western blotting that in resistant cells, α- and β-tubulins (more specifically the βIII and βIV isotypes) increased. Interestingly, four septins (SEPT2, 8, 9 and 11), which are GTPases involved in cytokinesis and in MT/actin cytoskeleton organization, were overexpressed and enriched in the MT environment of Taxol-resistant cells compared to their sensitive counterpart. Changes in the MT proteome of resistant cells also comprised increased kinesin-1 heavy chain expression and recruitment on MTs while dynein light chain-1 was downregulated. Modulation of motor protein recruitment around MTs might reflect their important role in controlling MT dynamics via the organization of signaling pathways. The identification of proteins previously unknown to be linked to taxane-resistance could also be valuable to identify new biological markers of resistance.

Septin 9 isoform expression, localization and epigenetic changes during human and mouse breast cancer progression

Introduction

Altered expression of Septin 9 (SEPT9), a septin coding for multiple isoform variants, has been observed in several carcinomas, including colorectal, head and neck, ovarian and breast, compared to normal tissues. The mechanisms regulating its expression during tumor initiation and progression in vivo and the oncogenic function of its different isoforms remain elusive.

Methods

Using an integrative approach, we investigated SEPT9 at the genetic, epigenetic, mRNA and protein levels in breast cancer. We analyzed a panel of breast cancer cell lines, human primary tumors and corresponding tumor-free areas, normal breast tissues from reduction mammoplasty patients, as well as primary mammary gland adenocarcinomas derived from the polyoma virus middle T antigen, or PyMT, mouse model. MCF7 clones expressing individual GFP-tagged SEPT9 isoforms were used to determine their respective intracellular distributions and effects on cell migration.

Results

An overall increase in gene amplification and altered expression of SEPT9 were observed during breast tumorigenesis. We identified an intragenic alternative promoter at which methylation regulates SEPT9_v3 expression. Transfection of specific GFP-SEPT9 isoforms in MCF7 cells indicates that these isoforms exhibit differential localization and affect migration rates. Additionally, the loss of an uncharacterized SEPT9 nucleolar localization is observed during tumorigenesis.

Conclusions

In this study, we found conserved in vivo changes of SEPT9 gene amplification and overexpression during human and mouse breast tumorigenesis. We show that DNA methylation is a prominent mechanism responsible for regulating differential SEPT9 isoform expression and that breast tumor samples exhibit distinctive SEPT9 intracellular localization. Together, these findings support the significance of SEPT9 as a promising tool in breast cancer detection and further emphasize the importance of analyzing and targeting SEPT9 isoform-specific expression and function.

Septin roles in tumorigenesis

Septins are a family of cytoskeleton related proteins consisting of 14 members that associate and interact with actin and tubulin. From yeast to humans, septins maintain a conserved role in cytokinesis and they are also involved in a variety of other cellular functions including chromosome segregation, DNA repair, migration and apoptosis. Tumorigenesis entails major alterations in these processes. A substantial body of literature reveals that septins are overexpressed, downregulated or generate chimeric proteins with MLL in a plethora of solid tumors and in hematological malignancies. Thus, members of this gene family are emerging as key players in tumorigenesis. The analysis of septins during cancer initiation and progression is challenged by the presence of many family members and by their potential to produce numerous isoforms. However, the development and application of advanced technologies is allowing for a more detailed analysis of septins during tumorigenesis. Specifically, such applications have led to the establishment and validation of SEPT9 as a biomarker for the early detection of colorectal cancer. This review summarizes the current knowledge on the role of septins in tumorigenesis, emphasizing their significance and supporting their use as potential biomarkers in various cancer types.

Distinct roles of septins in cytokinesis: SEPT9 mediates midbody abscission

Septins are a family of GTP-binding proteins implicated in mammalian cell division. Most studies examining the role of septins in this process have treated the family as a whole, thus neglecting the possibility that individual members may have diverse functions. To address this, we individually depleted each septin family member expressed in HeLa cells by siRNA and assayed for defects in cell division by immunofluorescence and time-lapse microscopy. Depletion of SEPT2, SEPT7, and SEPT11 causes defects in the early stages of cytokinesis, ultimately resulting in binucleation. In sharp contrast, SEPT9 is dispensable for the early stages of cell division, but is critical for the final separation of daughter cells. Rescue experiments indicate that SEPT9 isoforms containing the N-terminal region are sufficient to drive cytokinesis. We demonstrate that SEPT9 mediates the localization of the vesicle-tethering exocyst complex to the midbody, providing mechanistic insight into the role of SEPT9 during abscission.

A draft of the human septin interactome

Background

Septins belong to the GTPase superclass of proteins and have been functionally implicated in cytokinesis and the maintenance of cellular morphology. They are found in all eukaryotes, except in plants. In mammals, 14 septins have been described that can be divided into four groups. It has been shown that mammalian septins can engage in homo- and heterooligomeric assemblies, in the form of filaments, which have as a basic unit a hetero-trimeric core. In addition, it has been speculated that the septin filaments may serve as scaffolds for the recruitment of additional proteins.

Methodology/Principal Findings

Here, we performed yeast two-hybrid screens with human septins 1–10, which include representatives of all four septin groups. Among the interactors detected, we found predominantly other septins, confirming the tendency of septins to engage in the formation of homo- and heteropolymeric filaments.

Conclusions/Significance

If we take as reference the reported arrangement of the septins 2, 6 and 7 within the heterofilament, (7-6-2-2-6-7), we note that the majority of the observed interactions respect the “group rule”, i.e. members of the same group (e.g. 6, 8, 10 and 11) can replace each other in the specific position along the heterofilament. Septins of the SEPT6 group preferentially interacted with septins of the SEPT2 group (p<0.001), SEPT3 group (p<0.001) and SEPT7 group (p<0.001). SEPT2 type septins preferentially interacted with septins of the SEPT6 group (p<0.001) aside from being the only septin group which interacted with members of its own group. Finally, septins of the SEPT3 group interacted preferentially with septins of the SEPT7 group (p<0.001). Furthermore, we found non-septin interactors which can be functionally attributed to a variety of different cellular activities, including: ubiquitin/sumoylation cycles, microtubular transport and motor activities, cell division and the cell cycle, cell motility, protein phosphorylation/signaling, endocytosis, and apoptosis.

High SEPT9_v1 Expression Is Associated with Poor Clinical Outcomes in Head and Neck Squamous Cell Carcinoma

The purpose of this work was to determine SEPT9_v1 expression levels in head and neck squamous cell carcinoma (HNSCC) and to analyze whether SEPT9_v1 expression is relevant to clinical outcomes. Recently, the SEPT9 isoform SEPT9_v1 has been implicated in oncogenesis, and methylation of the SEPT9 promoter region was reported in HNSCC. These findings led us to hypothesize that SEPT9_v1 could be differently expressed in HNSCC. To determine whether SEPT9_v1 is expressed in HNSCC, tissue microarray immunohistochemical analysis was performed using a SEPT9_v1-specific antibody. Tissue microarrays stained with a polyclonal SEPT9_v1-specific antibody was used to determine protein expression levels in HNSCC tissue samples, some with known clinical outcomes. This analysis showed that SEPT9_v1 is in fact highly expressed in HNSCC compared with normal epithelium, and high expression levels directly correlated with poor clinical outcomes. Specifically, a high SEPT9_v1 expression was associated with decreased disease-specific survival (P = .012), time to indication of surgery at primary site (P = .008), response to induction chemotherapy (P = .0002), and response to chemotherapy (P = .02), as well as advanced tumor stage (P = .012) and N stage (P = .0014). The expression of SEPT9_v1 was also strongly correlated with smoking status (P = .00094). SEPT9_v1 is highly expressed in HNSCC, and a high expression of SEPT9_v1 is associated with poor clinical outcomes. These data indicate that SEPT9_v1 warrants additional investigation as a potential biomarker for HNSCC.

A translocation in acute lymphoblastic leukemia that cytogenetically mimics the recurrent MLL-AFF1 translocation and fuses SEPT11 to MLL

A 55-year-old man sought care for aggressive acute lymphoblastic leukemia (ALL), which developed 8 years after he had received chemotherapeutic treatment for nephrotic syndrome. The sole cytogenetic abnormality observed in bone marrow-derived metaphases was a t(4;11)(q21;q23), which is a frequently occurring translocation in ALL. However, subsequent reverse transcriptase-polymerase chain reaction for the expected mixed lineage leukemia [trithorax homolog, Drosophila] (MLL)-AFF1 fusion transcript was negative. Further fluorescence in situ hybridization (FISH) analysis narrowed the 4q21 breakpoint down to a 250-kb region proximal of AFF1. This comprised four genes, of which septin11 (SEPT11) was further analyzed. Reverse transcriptase-polymerase chain reaction revealed expression of a chimeric MLL-SEPT11 transcript, thus identifying what is to our knowledge a hitherto undescribed translocation in ALL. Sequence analysis of cDNA showed in-frame fusion of MLL exon 11 to SEPT11 exon 2. This MLL-SEPT11 fusion is cytogenetically indistinguishable from the recurrent t(4;11)(q21;q23). Thus, it is crucial to characterize cytogenetic aberrations in leukemia by molecular methods, even in cases where a known recurrent translocation is presumed. This report expands the spectrum of ALL-related translocations and hypothesizes on the mechanism leading to the MLL-SEPT11 fusion. Five septins have been identified thus far as MLL fusion partners in leukemia. Their putative oncogenic role may be related to forced MLL dimerization by the septin coiled coil and GTP-binding domains, which could convert MLL to an oncogene.

Alternative splicing of sept9a and sept9b in zebrafish produces multiple mRNA transcripts expressed throughout development

BACKGROUND

Septins are involved in a number of cellular processes including cytokinesis and organization of the cytoskeleton. Alterations in human septin-9 (SEPT9) levels have been linked to multiple cancers, whereas mutations in SEPT9 cause the episodic neuropathy, hereditary neuralgic amyotrophy (HNA). Despite its important function in human health, the in vivo role of SEPT9 is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Here we utilize zebrafish to study the role of SEPT9 in early development. We show that zebrafish possess two genes, sept9a and sept9b that, like humans, express multiple transcripts. Knockdown or overexpression of sept9a transcripts results in specific developmental alterations including circulation defects and aberrant epidermal development.

CONCLUSIONS/SIGNIFICANCE

Our work demonstrates that sept9 plays an important role in zebrafish development, and establishes zebrafish as a valuable model organism for the study of SEPT9.

Targeted knockdown of SEPT9_v1 inhibits tumor growth and angiogenesis of human prostate cancer cells concomitant with disruption of hypoxia-inducible factor-1 pathway

Hypoxia-inducible factor-1 (HIF-1) is a key transcription factor in the hypoxic response pathway. We recently identified a novel interaction between HIF-1alpha and the mammalian septin family member, septin 9 protein, isoform 1 (SEPT9_i1), a protein product of septin 9 transcript variant 1 (SEPT9_v1). Septins are a highly conserved family of GTP-binding cytoskeletal proteins that are implicated in multiple cellular functions, including oncogenesis. SEPT9_i1 binds and stabilizes HIF-1alpha protein and stimulates HIF-1 transcriptional activity by preventing its RACK1-mediated ubiquitination and degradation. SEPT9_i1-HIF-1 activation promotes tumor growth and angiogenesis. The effect of SEPT9_v1 silencing in prostate cancer cells was studied. SEPT9_v1 stable knockdown was generated in PC-3 cells using a specific shRNA. SEPT9_v1 silencing reduced HIF-1alpha protein expression and inhibited HIF-1 transcriptional activity. SEPT9_v1 knockdown affected cell morphology, deregulated cell cycle, and decreased migration. The antiproliferative effect of shSEPT9_v1 was abolished in HIF-1alpha knockout colon cancer cells. In vivo, SEPT9_i1 depletion reduced HIF-1alpha protein expression, cellular proliferation, tumor growth, and angiogenesis. These results provide new insights and validation for applying SEPT9_v1 as a potential target for antitumor therapy by interrupting the HIF-1 pathway.