Downregulation of POLD4 in Calu6 cells results in G1-S blockage through suppression of the Akt-Skp2-p27 pathway

UCHL3 induces radiation resistance and acquisition of mesenchymal phenotypes by deubiquitinating POLD4 in glioma stem cells

BACKGROUND

The high degree of intratumoral genomic heterogeneity is a major obstacle for glioblastoma (GBM) tumors, one of the most lethal human malignancies, and is thought to influence conventional therapeutic outcomes negatively. The proneural-to-mesenchymal transition (PMT) of glioma stem cells (GSCs) confers resistance to radiation therapy in glioblastoma patients. POLD4 is associated with cancer progression, while the mechanisms underlying PMT and tumor radiation resistance have remained elusive.

METHOD

Expression and prognosis of the POLD family were analyzed in TCGA, the Chinese Glioma Genome Atlas (CGGA) and GEO datasets. Tumorsphere formation and in vitro limiting dilution assay were performed to investigate the effect of UCHL3-POLD4 on GSC self-renewal. Apoptosis, TUNEL, cell cycle phase distribution, modification of the Single Cell Gel Electrophoresis (Comet), γ-H2AX immunofluorescence, and colony formation assays were conducted to evaluate the influence of UCHL3-POLD4 on GSC in ionizing radiation. Coimmunoprecipitation and GST pull-down assays were performed to identify POLD4 protein interactors. In vivo, intracranial xenograft mouse models were used to investigate the molecular effect of UCHL3, POLD4 or TCID on GCS.

RESULT

We determined that POLD4 was considerably upregulated in MES-GSCs and was associated with a meagre prognosis. Ubiquitin carboxyl terminal hydrolase L3 (UCHL3), a DUB enzyme in the UCH protease family, is a bona fide deubiquitinase of POLD4 in GSCs. UCHL3 interacted with, depolyubiquitinated, and stabilized POLD4. Both in vitro and in vivo assays indicated that targeted depletion of the UCHL3-POLD4 axis reduced GSC self-renewal and tumorigenic capacity and resistance to IR treatment by impairing homologous recombination (HR) and nonhomologous end joining (NHEJ). Additionally, we proved that the UCHL3 inhibitor TCID induced POLD4 degradation and can significantly enhance the therapeutic effect of IR in a gsc-derived in situ xenograft model.

CONCLUSION

These findings reveal a new signaling axis for GSC PMT regulation and highlight UCHL3-POLD4 as a potential therapeutic target in GBM. TCID, targeted for reducing the deubiquitinase activity of UCHL3, exhibited significant synergy against MES GSCs in combination with radiation.

Expression Profile, Molecular Association, and Clinical Significance of POLD4 in Glioblastoma

The POLD4 gene encodes a subunit (δ4) of DNA polymerase delta, which is a key enzyme involved in DNA replication and repair. Recent studies have suggested that POLD4 plays a crucial role in developing certain cancers. However, there is a lack of knowledge regarding the role of POLD4 in the context of glioblastoma (GBM). Therefore, in this study we have used various cancer bioinformatics tools to explore the role of POLD4 in glioblastoma. Data from various sources were accessed to analyze POLD4 gene expression and estimate tumor-infiltrating immune cells in glioblastoma. Methylation data were retrieved using the MEXPRESS web browser and analyzed. UALCAN webserver was used to analyze the protein expression of POLD4. Gene correlation and pathway enrichment analysis were performed using cBioPortal and GSEA software, respectively. Afterward, survival analysis was performed. POLD4 was significantly upregulated in glioblastoma at both gene and protein levels in GBM, and ROC curve analysis revealed it as a potential biomarker in glioblastoma. GSEA analysis of TCGA-GBM pan-cancer study exhibited that POLD4 expression was associated with critical pathways, such as interferon-gamma response, G2M checkpoint, inflammatory response, E2F targets, EMT transition, and KRAS signaling pathways. Furthermore, POLD4 expression was positively correlated with DNA methylation at 3 CpG sites, including Cg16509978, with a Pearson correlation coefficient value of 0.398 (p-value ≤ 0.01), while the promoter region had a positive correlation but was not significant. In addition, POLD4 is significantly linked with poor OS, PFS, and DFS. We also found association of POLD4 expression with altered immune cell infiltration. In conclusion, POLD4 is significantly upregulated in glioblastoma and may be used as a potential diagnostic or prognostic biomarker for GBM patients. However, to establish the same a large cohort study is needed. Using TCGA data and various cancer bioinformatics tools mentioned above we observed very high level of gene and protein expression of POLD4 in glioblastoma patients. The expression of POLD4 was significantly correlated with inflammatory and oncogenic pathways and it also has a significant correlation with adverse outcome in patients with glioblastoma.

POLD4 Promotes Glioma Cell Proliferation and Suppressive Immune Microenvironment: A Pan-Cancer Analysis Integrated with Experimental Validation

POLD4 plays a crucial part in the complex machinery of DNA replication and repair as a vital component of the DNA polymerase delta complex. In this research, we obtained original information from various publicly available databases. Using a blend of R programming and internet resources, we initiated an extensive examination into the correlation between POLD4 expression and the various elements of cancers. In addition, we performed knockdown experiments in glioma cell lines to authenticate its significant impact. We discovered that POLD4 is upregulated in various malignant tumors, demonstrating a significant correlation with poor patient survival prognosis. Using function analysis, it was uncovered that POLD4 exhibited intricate associations with signaling pathways spanning multiple tumor types. Subsequent investigations unveiled the close association of POLD4 with the immune microenvironment and the effectiveness of immunotherapy. Drugs like trametinib, saracatinib, and dasatinib may be used in patients with high POLD4. Using experimental analysis, we further confirmed the overexpression of POLD4 in gliomas, as well as its correlation with glioma recurrence, proliferation, and the suppressive immune microenvironment. Our research findings indicate that the expression pattern of POLD4 not only serves as a robust indicator of prognosis in cancer patients but also holds promising potential as a new focus for treatment.

The Mechanism and Prognostic Value of DNA Polymerase δ Subunits in Hepatocellular Carcinoma: Implications for Precision Therapy

PURPOSE

Polymerase δ (POLD) proteins is a pivotal B-family DNA polymerase in the process of genome replication and repair and are comprised of POLD1-4. The predictive value of POLDs in hepatocellular carcinoma (HCC) has not been evaluated until now.

PATIENTS AND METHODS

A total of 369 hepatocellular carcinoma samples and 50 adjacent normal samples were enrolled from the TCGA-LIHC database, and the GSE10186 database was also used. Transcription, methylation and genetic alteration status of HCC patients were evaluated by GEPIA, Kaplan-Meier plotter, cBioPortal, MethHC, MethSurv. SurvExpress was employed to generate the overall prognosis prediction signature of POLDs. POLDs coexpressed genes were explored and enriched in potential pathways. K-M curves were generated to compare the different survival results in different groups, while ROC curves were used to validate the efficiency of the POLD signature.

RESULTS

All four POLD subunits were highly expressed in HCC tumor tissues. POLD1-3 and increased mRNA levels were also positively associated with advanced tumor stage and OS prognosis. Methylation in the promoter of POLDs affects mRNA expression and OS, especially for some specific CpG sites. Meanwhile, POLDs could preferably predict the prognosis for patients who suffered from a high gene mutation burden. We evaluated the combined prognostic predictive value of four POLD subunits in both the TCGA-LIHC and GSE10186 databases and recognized the statistically significant HR of the high-risk group, along with the reliable predictive value. The coexpressed gene sets and annotation results showed that the POLD coexpressed genes were mostly associated with DNA repair and cell cycle regulation pathways.

CONCLUSION

POLD is an essential predictive factor for the prognosis of HCC. The united signature could precisely identify unfavorable clinical outcome of HCC.

Regulation and Modulation of Human DNA Polymerase δ Activity and Function

This review focuses on the regulation and modulation of human DNA polymerase δ (Pol δ). The emphasis is on the mechanisms that regulate the activity and properties of Pol δ in DNA repair and replication. The areas covered are the degradation of the p12 subunit of Pol δ, which converts it from a heterotetramer (Pol δ4) to a heterotrimer (Pol δ3), in response to DNA damage and also during the cell cycle. The biochemical mechanisms that lead to degradation of p12 are reviewed, as well as the properties of Pol δ4 and Pol δ3 that provide insights into their functions in DNA replication and repair. The second focus of the review involves the functions of two Pol δ binding proteins, polymerase delta interaction protein 46 (PDIP46) and polymerase delta interaction protein 38 (PDIP38), both of which are multi-functional proteins. PDIP46 is a novel activator of Pol δ4, and the impact of this function is discussed in relation to its potential roles in DNA replication. Several new models for the roles of Pol δ3 and Pol δ4 in leading and lagging strand DNA synthesis that integrate a role for PDIP46 are presented. PDIP38 has multiple cellular localizations including the mitochondria, the spliceosomes and the nucleus. It has been implicated in a number of cellular functions, including the regulation of specialized DNA polymerases, mitosis, the DNA damage response, mouse double minute 2 homolog (Mdm2) alternative splicing and the regulation of the NADPH oxidase 4 (Nox4).

4-Nitroquinoline-1-oxide effects human lung adenocarcinoma A549 cells by regulating the expression of POLD4

The aim of the present study was to explore the expression of POLD4 in human lung adenocarcinoma A549 cells under 4-nitroquinoline-1-oxide (4NQO) stimulation to investigate the role of POLD4 in smoking-induced lung cancer. The lung cancer A549 cell line was treated with 4NQO, with or without MG132 (an inhibitor of proteasome activity), and subsequently the POLD4 level was determined by western blot analysis. Secondly, the cell sensitivity to 4NQO and Taxol was determined when the POLD4 expression level was downregulated by siRNA. The POLD4 protein levels in the A549 cells decreased following treatment with 4NQO; however, MG132 could reverse this phenotype. Downregulation of the POLD4 expression by siRNA enhanced A549 cell sensitivity to 4NQO, but not to Taxol. In conclusion, 4NQO affects human lung adenocarcinoma A549 cells by regulating the expression of POLD4.

Expression of the p12 subunit of human DNA polymerase δ (Pol δ), CDK inhibitor p21(WAF1), Cdt1, cyclin A, PCNA and Ki-67 in relation to DNA replication in individual cells

We recently reported that the p12 subunit of human DNA polymerase δ (Pol δ4) is degraded by CRL4(Cdt2) which regulates the licensing factor Cdt1 and p21(WAF1) during the G1 to S transition. Presently, we performed multiparameter laser scanning cytometric analyses of changes in levels of p12, Cdt1 and p21(WAF1), detected immunocytochemically in individual cells, vis-à-vis the initiation and completion of DNA replication. The latter was assessed by pulse-labeling A549 cells with the DNA precursor ethynyl-2'-deoxyribose (EdU). The loss of p12 preceded the initiation of DNA replication and essentially all cells incorporating EdU were p12 negative. Completion of DNA replication and transition to G2 phase coincided with the re-appearance and rapid rise of p12 levels. Similar to p12 a decline of p21(WAF1) and Cdt1 was seen at the end of G1 phase and all DNA replicating cells were p21(WAF1) and Cdt1 negative. The loss of p21(WAF1) preceded that of Cdt1 and p12 and the disappearance of the latter coincided with the onset of DNA replication. Loss of p12 leads to conversion of Pol δ4 to its trimeric form, Pol δ3, so that the results provide strong support to the notion that Pol δ3 is engaged in DNA replication during unperturbed progression through the S phase of cell cycle. Also assessed was a correlation between EdU incorporation, likely reflecting the rate of DNA replication in individual cells, and the level of expression of positive biomarkers of replication cyclin A, PCNA and Ki-67 in these cells. Of interest was the observation of stronger correlation between EdU incorporation and expression of PCNA (r = 0.73) than expression of cyclin A (r = 0.47) or Ki-67 (r = 0.47).