Identification and characterization of mouse PSF1-binding protein, SLD5

Ferroptosis: A New Promising Target for Lung Cancer Therapy

Ferroptosis is a new type of regulatory cell death that differs from autophagy, apoptosis, necrosis, and pyroptosis; it is caused primarily by the accumulation of iron and lipid peroxides in the cell. Studies have shown that many classical signaling pathways and biological processes are involved in the process of ferroptosis. In recent years, investigations have revealed that ferroptosis plays a crucial role in the progression of tumors, especially lung cancer. In particular, inducing ferroptosis in cells can inhibit the growth of tumor cells, thereby reversing tumorigenesis. In this review, we summarize the characteristics of ferroptosis from its underlying basis and role in lung cancer and provide possible applications for it in lung cancer therapies.

LSH interacts with and stabilizes GINS4 transcript that promotes tumourigenesis in non-small cell lung cancer

Background

Elucidating mechanisms in oncogenes and epigenetic modifiers are needed to gain insights into the etiology and treatment of cancer, regulation of oncogene by chromatin modifiers at post-transcriptional level is critical and remains unclear. We have investigated the role of GINS4 in NSCLC.

Methods

The expression of chromatin modifier lymphoid-specific helicase (LSH) and GINS4 was assessed in tumor and normal tissue from 79 patients with NSCLC with clinical characteristics. HBE, A549, H358, and H522, PC9, 95C and 95D were cultured after overexpression or silencing of GIAT4RA. Cell proliferation assay, cell migration and invasion assays, plate colony formation assay, immunofluorescence assay, Operetta® high-content screening and analysis, Western blot analysis and Co-Immunoprecipitation (Co-IP) assay, RNA immunoprecipitation assay and tumor growth assay was used to address the potential interplay of between GINS4 and LSH, and the functional of GINS4.

Results

GINS4 is highly expressed in lung cancer cells and tissues, and GINS4 expression is not association with clinical risk factors, but linked with clinical stage and lymphatic metastasis status. Higher expression of GINS4 poorly linked with overall survival in lung adenocarcinomas. Furthermore, GINS4 promoted many characteristics of tumorigenesis including cell growth, clonal formation, migration and invasion, epithelial–mesenchymal transition, tumor sphere and tumor growth in vivo. Interestingly, our results demonstrated that LSH increases GINS4 expression through binding to 3’UTR region of GINS4 and stabilizing its mRNA levels. Finally, LSH overexpression rescues GINS4 knockdown-induced features.

Conclusions

GINS4 facilitates lung cancer progression by promoting key characteristics of tumor potential, and LSH epigenetically interacts with and stabilizes GINS4 transcripts.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1276-y) contains supplementary material, which is available to authorized users.

Influenza virus matrix protein M1 interacts with SLD5 to block host cell cycle

Influenza virus matrix 1 protein (M1) is highly conserved and plays essential roles at many stages of virus life cycle. Here, we used a yeast two-hybrid system to identify the host protein SLD5, a component of the GINS complex, which is essential for the initiation of DNA replication in eukaryotic cells, as a new M1 interacting protein. M1 from several different influenza virus strains all interacted with SLD5. Overexpression of SLD5 suppressed influenza virus replication. Transient, stable, or inducible expression of M1 induced host cell cycle blockade at G0/G1 phase. Moreover, SLD5 partially rescued M1 expression- or influenza virus infection-induced G0/G1 phase accumulation in cell lines and primary mouse embryonic fibroblasts. Importantly, SLD5 transgenic mice exhibited higher resistance and improved lung epithelial regeneration after virus infection compared with wild-type mice. Therefore, influenza virus M1 blocks host cell cycle process by interacting with SLD5. Our finding reveals the multifunctional nature of M1 and provides new insight for understanding influenza virus-host interaction.

Regulation of SLD5 gene expression by miR-370 during acute growth of cancer cells

SLD5 is a member of the GINS complex, essential for DNA replication in eukaryotes. It has been reported that SLD5 is involved in early embryogenesis in the mouse, and cell cycle progression and genome integrity in Drosophila. SLD5 may be involved in malignant tumor progression, but its relevance in human cancer has not been determined. Here, we found strong SLD5 expression in both human bladder cancer tissues from patients and cell lines. Knockdown of SLD5 using small interfering RNA resulted in reduction of cell growth both in vitro and an in vivo xenograft model. Moreover, we found that high levels of SLD5 in bladder cancer cells result from downregulation of microRNA (miR)-370 that otherwise suppresses its expression. High level expression of DNA-methyltransferase (DNMT) 1 and IL-6 were also observed in bladder cancer cells. Knockdown of IL-6 led to downregulation of DNMT1 and SLD5 expression, suggesting that IL-6-induced overexpression of DNMT1 suppresses miR-370, resulting in high SLD5 expression. Our findings could contribute to understanding tumorigenic processes and progression of human bladder cancer, whereby inhibition of SLD5 could represent a novel strategy to prevent tumor growth.

Tumor microenvironment for cancer stem cells

Tumor tissues consist of heterogeneous cancer cells including cancer stem cells (CSCs) that can terminally differentiate into cancer cells. Tissue-specific stem cells in normal organs maintain their stemness in a specific microenvironment, the stem cell niche; several studies have suggested that there are specific microenvironments that maintain CSCs in an immature phenotype. Cell types in a CSC niche vary from fibroblasts, to endothelial cells, immune cells, and so on; these non-cancer cells have been suggested to change their original features in the normal tissue/organ and to acquire a phenotype that protects CSCs from anticancer therapies. Therefore, to kill CSCs, we need to understand the cellular and molecular mechanisms involved in the maintenance of the immature phenotype of CSCs and in drug resistance.

DNA damage enhanced by the attenuation of SLD5 delays cell cycle restoration in normal cells but not in cancer cells

SLD5 is a member of the GINS complex composed of PSF1, PSF2, PSF3 and SLD5, playing a critical role in the formation of the DNA replication fork with CDC45 in yeast. Previously, we had isolated a PSF1 orthologue from a murine hematopoietic stem cell DNA library and were then able to identify orthologues of all the other GINS members by the yeast two hybrid approach using PSF1 as the bait. These GINS orthologues may also function in DNA replication in mammalian cells because they form tetrameric complexes as observed in yeast, and gene deletion mutants of both PSF1 and SLD5 result in a lack of epiblast proliferation and early embryonic lethality. However, we found that PSF1 is also involved in chromosomal segregation in M phase, consistent with recent suggestions that homologues of genes associated with DNA replication in lower organisms also regulate cellular events other than DNA replication in mammalian cells. Here we analyzed the function of SLD5 other than DNA replication and found that it is active in DNA damage and repair. Attenuation of SLD5 expression results in marked DNA damage in both normal cells and cancer cells, suggesting that it protects against DNA damage. Attenuation of SLD5 delays the DNA repair response and cell cycle restoration in normal cells but not in cancer cells. These findings suggest that SLD5 might represent a therapeutic target molecule acting at the level of tumor stromal cells rather than the cancerous cells themselves, because development of the tumor microenvironment could be delayed or disrupted by the suppression of its expression in the normal cell types within the tumor.

Requirement of SLD5 for early embryogenesis

SLD5 forms a GINS complex with PSF1, PSF2 and PSF3, which is essential for the initiation of DNA replication in lower eukaryotes. Although these components are conserved in mammals, their biological function is unclear. We show here that targeted disruption of SLD5 in mice causes a defect in cell proliferation in the inner cell mass, resulting in embryonic lethality at the peri-implantation stage, indicating that SLD5 is essential for embryogenesis. Moreover, this phenotype of SLD5 mutant mice is quite similar compared with that of PSF1 mutant mice. We have previously reported that haploinsufficiency of PSF1 resulted in failure of acute proliferation of bone marrow hematopoietic stem cells (HSCs) during reconstitution of bone marrow ablated by 5-FU treatment. Since SLD5 was highly expressed in bone marrow, we investigated its involvement in bone marrow reconstitution after bone marrow ablation as observed in PSF1 heterozygous mutant mice. However, heterozygous deletion of the SLD5 gene was found not to significantly affect bone marrow reconstitution. On the other hand, abundant SLD5 expression was observed in human cancer cell lines and heterozygous deletion of the gene attenuated tumor progression in a murine model of spontaneous gastric cancer. These indicated that requirement and dependency of SLD5 for cell proliferation is different in different cell types.

The initiation step of eukaryotic DNA replication

Eukaryotic initiation of DNA replication is a tightly regulated process. In the yeasts, S-phase-specific cyclin Cdk1 complex as well as Dfb4-Cdc7 kinase phosphorylate the initiation factors Sld2 and Sld3. These factors form a ternary complex with another initiation factor Dbp11 in their phosphorylated state, and associate with the origin of replication. This complex mediates the loading of Cdc45. A second complex called GINS and consisting of Sld5 and Psf1, 2 and 3 is also loaded onto the origin during the initiation process, in an interdependent manner with the Sld2/Sld3/Dpb11 complex. Both complexes cooperate in the recruitment of the replicative DNA polymerases, thus executing the initiation and subsequent establishment of the replication fork. Cdc45 and GINS are essential, well-conserved factors that are retained at the elongating replication fork. They form a stable helicase complex with MCM2-7 and mediate its contact to the replicative DNA polymerases. In contrast, the Sld2/Sld3/Dpb11 complex critical for the initiation is not retained by the elongating replication fork. Sld2 displays limited homology to the amino-terminal region of RecQL4 helicase, which may represent its metazoan orthologue, whereas Sld3 homologues have been identified only in fungi. Dbp11 and its fission yeast homologue Cut5 are members of a large family of BRCT-containing proteins including human TopBP1 and fruit fly Mus101. Similar principles of regulation apply also to human initiation of DNA replication, despite obvious differences in the detailed mechanisms. The regulatory initiation cascade is intimately intertwined with the cell cycle apparatus as well as the checkpoint control.

Both alleles of PSF1 are required for maintenance of pool size of immature hematopoietic cells and acute bone marrow regeneration

Hematopoietic stem cells (HSCs) have a very low rate of cell division in the steady state; however, under conditions of hematopoietic stress, these cells can begin to proliferate at high rates, differentiate into mature hematopoietic cells, and rapidly reconstitute ablated bone marrow (BM). Previously, we isolated a novel evolutionarily conserved DNA replication factor, PSF1 (partner of SLD5-1), from an HSC-specific cDNA library. In the steady state, PSF1 is expressed predominantly in CD34+KSL (c-kit+/Sca-1+/Lineage−) cells and progenitors, whereas high levels of PSF1 expression are induced in KSL cells after BM ablation. In 1-year-old PSF1+/− mice, the pool size of stem cells and progenitors is decreased. Whereas young PSF1+/− mutant mice develop normally, are fertile, and have no obvious differences in hematopoiesis in the steady state compared with wild-type mice, intravenous injection of 5-fluorouracil (5-FU) is lethal in PSF1+/− mice, resulting from a delay in induction of HSC proliferation during ablated BM reconstitution. Overexpression studies revealed that PSF1 regulates molecular stability of other GINS components, including SLD5, PSF2, and PSF3. Our data indicate that PSF1 is required for acute proliferation of HSCs in the BM of mice.

Inhibition of axonal outgrowth in the tumor environment: involvement of class 3 semaphorins

That tumors lack innervation is dogma in the field of pathology, but the molecular determinants of this phenomenon remain elusive. We studied the effects of conditioned media from Colon 26 and B16 mouse tumor cell lines on the axonal outgrowth and cellular differentiation of embryonic Institute of Cancer Research (ICR) mouse dorsal root ganglion cells. Tumor-conditioned media suppressed dorsal root ganglion axonal extension but had no effect on neuronal or glial differentiation. We found that the tumor cells expressed most of the class 3 semaphorins - axon guidance molecules. Blocking the activity of class 3 semaphorins with the soluble receptor neuropilin-1 significantly counteracted the tumor-induced inhibition of axonal extension. Together, these results suggest a role for tumor-secreted class 3 semaphorins in selectively inhibiting axonal outgrowth of dorsal root ganglion neurons.

Comprehensive expression profiling of tumor cell lines identifies molecular signatures of melanoma progression

BACKGROUND

Gene expression profiling has revolutionized our ability to molecularly classify primary human tumors and significantly enhanced the development of novel tumor markers and therapies; however, progress in the diagnosis and treatment of melanoma over the past 3 decades has been limited, and there is currently no approved therapy that significantly extends lifespan in patients with advanced disease. Profiling studies of melanoma to date have been inconsistent due to the heterogeneous nature of this malignancy and the limited availability of informative tissue specimens from early stages of disease.

METHODOLOGY/PRINCIPLE FINDINGS

In order to gain an improved understanding of the molecular basis of melanoma progression, we have compared gene expression profiles from a series of melanoma cell lines representing discrete stages of malignant progression that recapitulate critical characteristics of the primary lesions from which they were derived. Here we describe the unsupervised hierarchical clustering of profiling data from melanoma cell lines and melanocytes. This clustering identifies two distinctive molecular subclasses of melanoma segregating aggressive metastatic tumor cell lines from less-aggressive primary tumor cell lines. Further analysis of expression signatures associated with melanoma progression using functional annotations categorized these transcripts into three classes of genes: 1) Upregulation of activators of cell cycle progression, DNA replication and repair (CDCA2, NCAPH, NCAPG, NCAPG2, PBK, NUSAP1, BIRC5, ESCO2, HELLS, MELK, GINS1, GINS4, RAD54L, TYMS, and DHFR), 2) Loss of genes associated with cellular adhesion and melanocyte differentiation (CDH3, CDH1, c-KIT, PAX3, CITED1/MSG-1, TYR, MELANA, MC1R, and OCA2), 3) Upregulation of genes associated with resistance to apoptosis (BIRC5/survivin). While these broad classes of transcripts have previously been implicated in the progression of melanoma and other malignancies, the specific genes identified within each class of transcripts are novel. In addition, the transcription factor NF-KB was specifically identified as being a potential "master regulator" of melanoma invasion since NF-KB binding sites were identified as consistent consensus sequences within promoters of progression-associated genes.

CONCLUSIONS/SIGNIFICANCE

We conclude that tumor cell lines are a valuable resource for the early identification of gene signatures associated with malignant progression in tumors with significant heterogeneity like melanoma. We further conclude that the development of novel data reduction algorithms for analysis of microarray studies is critical to allow for optimized mining of important, clinically-relevant datasets. It is expected that subsequent validation studies in primary human tissues using such an approach will lead to more rapid translation of such studies to the identification of novel tumor biomarkers and therapeutic targets.