A novel lysine-rich domain and GTP binding motifs regulate the nucleolar retention of human guanine nucleotide binding protein, GNL3L

Knockdown of GNL3L Alleviates the Progression of COPD Through Inhibiting the ATM/p53 Pathway

Background

Chronic obstructive pulmonary disease (COPD) is a persistent chronic bronchitis disease, and its potential biomarkers have not been fully expounded. This study aims to explore the role of Guanine nucleotide binding protein like-3-like (GNL3L) in COPD induced by cigarette smoking (CS) in vivo.

Methods

Two microarray datasets of COPD were selected to screen differentially expressed genes (DEGs). A protein-protein interaction network was constructed to find hub genes. The COPD model was conducted using CS/LPS-induced mouse and cigarette smoke extract induced human bronchial epithelial cells. The pathological changes of lung tissue in mice were observed by hematoxylin-eosin staining and mean linear intercept. Cell viability was measured by CCK8 assay. Oxidative stress-related indicators, inflammatory factors, and ATM/p53 related-proteins were assessed using ELISA and Western blot.

Results

In this study, there were 110 common DEGs identified from the two datasets (GSE5058 and GSE38974). The key gene GNL3L was the optimal indicator to distinguish between samples with COPD and healthy controls. Through the in vivo and in vitro experiments, GNL3L knockdown significantly improved the pathological features of CS/LPS-induced COPD mice, promoted cell viability, inhibited inflammation (IL-1β, IL-8, and TNF-α), oxidative stress (MDA, SOD, and CAT), and ATM/p53 related-proteins (ATM, p53, and p21).

Conclusion

GNL3L is a novel biomarker of COPD, and knockdown of GNL3L participates in the progression of COPD by inhibiting ATM/p53 pathway.

Multi-Omics Analysis of GNL3L Expression, Prognosis, and Immune Value in Pan-Cancer

Simple Summary

Guanine nucleotide-binding protein-like 3-like (GNL3L) is a novel GTP-binding nucleolar protein. In this study, we analyzed the expression, prognosis, and immune roles of GNL3L in pan-cancer from multiple omics analyses. The final results showed that GNL3L is differentially expressed in a variety of cancers, plays a prognostic role, and has good immune value. Moreover, GNL3L may affect the occurrence of cancer through processes such as ribonucleoprotein, ribosomal RNA processing, and cell proliferation. At the same time, we established an esophageal cancer (ESCA) prediction model with strong predictive ability and proved that GNL3L can significantly affect the proliferation ability of esophageal cancer cells through clone formation assays. In conclusion, GNL3L is an important biomarker.

Abstract

Guanine nucleotide-binding protein-like 3-like protein (GNL3L) is a novel, evolutionarily conserved, GTP-binding nucleolar protein. This study aimed to investigate the expression, prognosis, and immune value of GNL3L in pan-cancer from multiple omics analyses. Firstly, the expression and prognostic value of GNL3L in pan-cancer were discussed using the TIMER2 database, the GEPIA database, the cBioportal database, COX regression analysis, and enrichment analysis. The association of GNL3L with tumor mutational burden (TMB), tumor microsatellite instability (MSI), mismatch repair (MMR) genes, and immune cells was then analyzed. Finally, an esophageal cancer (ESCA) prediction model was established, and GNL3L clone formation assays were performed. The final results showed that GNL3L is differentially expressed in the vast majority of cancers, is associated with the prognosis of various cancers, and may affect cancer occurrence through processes such as ribonucleoprotein, ribosomal RNA processing, and cell proliferation. At the same time, it was found that the correlation between GNL3L and TMB, MSI, MMR, and various immune cells is significant. The established ESCA prediction model had a strong predictive ability, and GNL3L could significantly affect the proliferation of esophageal cancer cells. In conclusion, GNL3L may serve as an important prognostic biomarker and play an immunomodulatory role in tumors.

High expression of guanine nucleotide-binding protein-like-3-like is associated with poor prognosis in esophageal cancer

Guanine nucleotide-binding protein-like-3-like (GNL3L) is required for processing ribosomal pre-rRNA and cell proliferation and is upregulated in many types of cancer. This study is aimed to investigate the clinical significance of GNL3L in esophageal cancer. The mRNA and protein expression levels of GNL3L were determined by using quantitative real-time polymerase chain reaction and immunohistochemistry, respectively. GNL3L was localized in both cytoplasm and nucleus. The expression levels of GNL3L in esophageal cancer tissues were significantly higher than those in adjacent nonmalignant tissues. High GNL3L expression was associated with pathologic type and poor differentiation. Patients with high GNL3L expression had shorter overall survival (OS) than those with low GNL3L expression. Multivariate Cox regression analysis revealed that GNL3L expression was an independently predictive factor for the OS of patient with esophageal cancer. The Gene Expression Profiling Interactive Analysis (GEPIA) databases also showed that GNL3L was upregulated in esophageal cancer, which was closely associated with an unfavorable prognosis of patients with esophageal cancer. Taken together, our findings suggest that GNL3L is upregulated in esophageal cancer, which is linked to the progression of the disease. As a result, GNL3L could be used as a biomarker for esophageal cancer.

Chemoresistance-Associated Silencing of miR-4454 Promotes Colorectal Cancer Aggression through the GNL3L and NF-κB Pathway

Guanine nucleotide-binding protein-like-3-like (GNL3L) is a crucial regulator of NF-κB signaling that is aberrantly activated during diverse chemoresistance-associated cellular processes. However, the molecular mechanisms of GNL3L tumor initiation and resistant state are largely unknown. Moreover, the identification of predictive biomarkers is necessary to effectively generate therapeutic strategies for metastatic human colorectal cancer (CRC). This study aims to identify how cells acquire resistance to anticancer drugs and whether the downregulation of miR-4454 is associated with the progression of CRC. Here, we have shown that the overexpression of miR-4454 in resistant tumors is a crucial precursor for the posttranscriptional repression of GNL3L in human chemoresistant CRC progression, and we used doxycycline induced miR-4454 overexpression that significantly reduced tumor volume in a subcutaneous injection nude mice model. Together, these observations highlight that the downregulation of miR-4454 in resistant clones is prominently responsible for maintaining their resistance against anticancer drug therapy. Our study indicates that the development of miR-4454 as a microRNA-based therapeutic approach to silence GNL3L may remarkably reduce oncogenic cell survival that depends on GNL3L/NF-κB signaling, making miR-4454 a candidate for treating metastatic human CRC.

Interplay between human nucleolar GNL1 and RPS20 is critical to modulate cell proliferation

Human Guanine nucleotide binding protein like 1 (GNL1) belongs to HSR1_MMR1 subfamily of nucleolar GTPases. Here, we report for the first time that GNL1 promotes cell cycle and proliferation by inducing hyperphosphorylation of retinoblastoma protein. Using yeast two-hybrid screening, Ribosomal protein S20 (RPS20) was identified as a functional interacting partner of GNL1. Results from GST pull-down and co-immunoprecipitation assays confirmed that interaction between GNL1 and RPS20 was specific. Further, GNL1 induced cell proliferation was altered upon knockdown of RPS20 suggesting its critical role in GNL1 function. Interestingly, cell proliferation was significantly impaired upon expression of RPS20 interaction deficient GNL1 mutant suggest that GNL1 interaction with RPS20 is critical for cell growth. Finally, the inverse correlation of GNL1 and RPS20 expression in primary colon and gastric cancers with patient survival strengthen their critical importance during tumorigenesis. Collectively, our data provided evidence that cross-talk between GNL1 and RPS20 is critical to promote cell proliferation.

Reading the Evolution of Compartmentalization in the Ribosome Assembly Toolbox: The YRG Protein Family

Reconstructing the transition from a single compartment bacterium to a highly compartmentalized eukaryotic cell is one of the most studied problems of evolutionary cell biology. However, timing and details of the establishment of compartmentalization are unclear and difficult to assess. Here, we propose the use of molecular markers specific to cellular compartments to set up a framework to advance the understanding of this complex intracellular process. Specifically, we use a protein family related to ribosome biogenesis, YRG (YlqF related GTPases), whose evolution is linked to the establishment of cellular compartments, leveraging the current genomic data. We analyzed orthologous proteins of the YRG family in a set of 171 proteomes for a total of 370 proteins. We identified ten YRG protein subfamilies that can be associated to six subcellular compartments (nuclear bodies, nucleolus, nucleus, cytosol, mitochondria, and chloroplast), and which were found in archaeal, bacterial and eukaryotic proteomes. Our analysis reveals organism streamlining related events in specific taxonomic groups such as Fungi. We conclude that the YRG family could be used as a compartmentalization marker, which could help to trace the evolutionary path relating cellular compartments with ribosome biogenesis.

Leucine Zipper Down-regulated in Cancer-1 (LDOC1) interacts with Guanine nucleotide binding protein-like 3-like (GNL3L) to modulate Nuclear Factor-kappa B (NF-κB) signaling during cell proliferation

Guanine nucleotide binding protein-like 3-like (GNL3L) is an evolutionarily conserved putative nucleolar GTPase belonging to the HSR1-MMR1 family. In the present study, using protein-protein interaction assays, we show that Leucine Zipper Down-regulated in Cancer-1 (LDOC1) is a novel interacting partner of GNL3L. Furthermore, our results reveal that ectopic expression of LDOC1 destabilizes endogenous GNL3L levels and down modulates GNL3L-induced cell proliferation, in contrast, the knockdown of LDOC1 potentiates cell proliferation upon GNL3L expression. Interestingly, GNL3L upregulates NF-κB dependent transcriptional activity by modulating the expression of NF-κB subunit p65, which is reversed upon co-expression of LDOC1 with GNL3L. GNL3L also potentiates TNF-α mediated NF-κB activity. In addition, anti-apoptotic function of GNL3L is impaired upon p65 knockdown, suggesting its critical role in GNL3L mediated cell proliferation/survival. An inverse correlation of GNL3L and LDOC1 expression profiles in various tumor tissues from BioXpress database indicate their critical role in cancer. Collectively, our data provides evidence that GNL3L-LDOC1 interplay regulates cell proliferation through the modulation of NF-κB pathway during tumorigenesis.

GNL3L Is a Nucleo-Cytoplasmic Shuttling Protein: Role in Cell Cycle Regulation

GNL3L is an evolutionarily conserved high molecular weight GTP binding nucleolar protein belonging to HSR1-MMR1 subfamily of GTPases. The present investigation reveals that GNL3L is a nucleo-cytoplasmic shuttling protein and its export from the nucleus is sensitive to Leptomycin B. Deletion mutagenesis reveals that the C-terminal domain (amino acids 501–582) is necessary and sufficient for the export of GNL3L from the nucleus and the exchange of hydrophobic residues (M567, L570 and 572) within the C-terminal domain impairs this process. Results from the protein-protein interaction analysis indicate that GNL3L interaction with CRM1 is critical for its export from the nucleus. Ectopic expression of GNL3L leads to lesser accumulation of cells in the ‘G2/M’ phase of cell cycle whereas depletion of endogenous GNL3L results in ‘G2/M’ arrest. Interestingly, cell cycle analysis followed by BrdU labeling assay indicates that significantly increased DNA synthesis occurs in cells expressing nuclear export defective mutant (GNL3L^∆NES) compared to the wild type or nuclear import defective GNL3L. Furthermore, increased hyperphosphorylation of Rb at Serine 780 and the upregulation of E2F1, cyclins A2 and E1 upon ectopic expression of GNL3L^∆NES results in faster ‘S’ phase progression. Collectively, the present study provides evidence that GNL3L is exported from the nucleus in CRM1 dependent manner and the nuclear localization of GNL3L is important to promote ‘S’ phase progression during cell proliferation.

Dissecting the expression landscape of RNA-binding proteins in human cancers

BACKGROUND

RNA-binding proteins (RBPs) play important roles in cellular homeostasis by controlling gene expression at the post-transcriptional level.

RESULTS

We explore the expression of more than 800 RBPs in sixteen healthy human tissues and their patterns of dysregulation in cancer genomes from The Cancer Genome Atlas project. We show that genes encoding RBPs are consistently and significantly highly expressed compared with other classes of genes, including those encoding regulatory components such as transcription factors, miRNAs and long non-coding RNAs. We also demonstrate that a set of RBPs, numbering approximately 30, are strongly upregulated (SUR) across at least two-thirds of the nine cancers profiled in this study. Analysis of the protein-protein interaction network properties for the SUR and non-SUR groups of RBPs suggests that path length distributions between SUR RBPs is significantly lower than those observed for non-SUR RBPs. We further find that the mean path lengths between SUR RBPs increases in proportion to their contribution to prognostic impact. We also note that RBPs exhibiting higher variability in the extent of dysregulation across breast cancer patients have a higher number of protein-protein interactions. We propose that fluctuating RBP levels might result in an increase in non-specific protein interactions, potentially leading to changes in the functional consequences of RBP binding. Finally, we show that the expression variation of a gene within a patient group is inversely correlated with prognostic impact.

CONCLUSIONS

Overall, our results provide a roadmap for understanding the impact of RBPs on cancer pathogenesis.

A comparative study of nucleostemin family members in zebrafish reveals specific roles in ribosome biogenesis

Nucleostemin (NS) is an essential protein for the growth and viability of developmental stem cells. Its functions are multi-faceted, including important roles in ribosome biogenesis and in the p53-induced apoptosis pathway. While NS has been well studied, the functions of its family members GNL2 and GNL3-like (GNL3L) remain relatively obscure despite a high degree of sequence and domain homology. Here, we use zebrafish lines carrying mutations in the ns family to compare and contrast their functions in vertebrates. We find the loss of zebrafish ns or gnl2 has a major impact on 60S large ribosomal subunit formation and/or function due to cleavage impairments at distinct sites of pre-rRNA transcript. In both cases this leads to a reduction of total protein synthesis. In contrast, gnl3l loss shows relatively minor rRNA processing delays that ultimately have no appreciable effects on ribosome biogenesis or protein synthesis. However, the loss of gnl3l still results in p53 stabilization, apoptosis, and lethality similarly to ns and gnl2 loss. The depletion of p53 in all three of the mutants led to partial rescues of the morphological phenotypes and surprisingly, a rescue of the 60S subunit collapse in the ns mutants. We show that this rescue is due to an unexpected effect of p53 loss that even in wild type embryos results in an increase of 60S subunits. Our study presents an in-depth description of the mechanisms through which ns and gnl2 function in vertebrate ribosome biogenesis and shows that despite the high degree of sequence and domain homology, gnl3l has critical functions in development that are unrelated to the ribosome.

Subcellular distribution of the human putative nucleolar GTPase GNL1 is regulated by a novel arginine/lysine-rich domain and a GTP binding domain in a cell cycle-dependent manner

GNL1, a putative nucleolar GTPase, belongs to the MMR1-HSR1 family of large GTPases that are emerging as crucial coordinators of signaling cascades in different cellular compartments. Members of this family share very closely related G-domains, but the signals and pathways regulating their subcellular localization with respect to cell growth remain unknown. To understand the nuclear transport mechanism of GNL1, we have identified a novel arginine/lysine-rich nucleolar localization signal in the NH(2)-terminus that is shown to translocate GNL1 and a heterologous protein to the nucleus/nucleolus in a pathway that is independent of importin-α and importin-β. In addition, the present investigation provided evidence that GNL1 localized to the nucleus and the nucleolus only in G2 stage, in contrast to its cytoplasmic localization in the G1 and S phases of the cell cycle. Using heterokaryon assay, we have demonstrated that GNL1 shuttles between the nucleus and the cytoplasm and that the motif between amino acids 201 and 225 is essential for its export from the nucleus by a signal-mediated CRM1-independent pathway. Alanine-scanning mutagenesis of conserved residues within G-domains suggests that the G2 motif is critical for guanine nucleotide triphosphate (GTP) binding of GNL1 and further showed that nucleolar retention of GNL1 is regulated by a GTP-gating-mediated mechanism. Expression of wild-type GNL1 promotes G2/M transition, in contrast to the G-domain mutant (G2m), which fails to localize to the nucleolus. These data suggest that nucleolar translocation during G2 phase may be critical for faster M-phase transition during cell proliferation. Replacement of conserved residues within the G5 motif alters the stability of GNL1 without changing GTP binding activity. Finally, our data suggest that ongoing transcription is essential for the efficient localization of GNL1 to the nucleolus. Overall, the results reported here demonstrate that multiple mechanisms are involved in the translocation of GNL1 to the nucleolus in a cell cycle-dependent manner to regulate cell growth and proliferation.

Signals and pathways regulating nucleolar retention of novel putative nucleolar GTPase NGP-1(GNL-2)

NGP-1(GNL-2) is a putative GTPase, overexpressed in breast carcinoma and localized in the nucleolus. NGP-1 belongs to the MMR1-HSR1 family of large GTPases that are emerging as crucial coordinators of signaling cascades in different cellular compartments. The members of this family share very closely related G-domains, but the signals and pathways regulating their subcellular localization and their functional relevance remain unknown. To improve our understanding of the nuclear transport mechanism of NGP-1, we have identified two nucleolar localization signals (NoLS) that are independently shown to translocate NGP-1 as well the heterologous protein to the nucleolus. Site-specific mutagenesis and immunofluorescence studies suggest that the tandem repeats of positively charged amino acids are critical for NGP-1 NoLS function. Interestingly, amino-terminal (NGP-1(1-100)) and carboxyl-terminal (NGP-1(661-731)) signals independently interact with receptors importin-β and importin-α, respectively. This investigation, for the first time, provides evidence that the interaction of importin-α with C-terminal NoLS (NGP-1(661-731)) was able to target the heterologous protein to the nucleolar compartment. Structural modeling analysis and alanine scanning mutagenesis of conserved G-domains suggest that G4 and G5 motifs are critical for GTP binding of NGP-1 and further show that the nucleolar localization of NGP-1 is regulated by a GTP gating-mediated mechanism. In addition, our data suggest that an ongoing transcription is essential for efficient localization of NGP-1 to the nucleolus. We have observed a high level of NGP-1 expression in the mitogen-activated primary human peripheral blood mononuclear cells (hPBMC) as well as in human fetal brain-derived neural precursor cells (hNPCs) in comparison to cells undergoing differentiation. Overall, the results suggest that multiple mechanisms are involved in the localization of NGP-1 to the nucleolus for the regulation of nucleolar function in cell growth and proliferation.

Conservation of complex nuclear localization signals utilizing classical and non-classical nuclear import pathways in LANA homologs of KSHV and RFHV

ORF73 latency-associated nuclear antigen (LANA) of the Kaposi's sarcoma-associated herpesvirus (KSHV) is targeted to the nucleus of infected cells where it binds to chromatin and mediates viral episome persistence, interacts with cellular proteins and plays a role in latency and tumorigenesis. A structurally related LANA homolog has been identified in the retroperitoneal fibromatosis herpesvirus (RFHV), the macaque homolog of KSHV. Here, we report the evolutionary and functional conservation of a novel bi-functional nuclear localization signal (NLS) in KSHV and RFHV LANA. N-terminal peptides from both proteins were fused to EGFP or double EGFP fusions to examine their ability to induce nuclear transport of a heterologous protein. In addition, GST-pull down experiments were used to analyze the ability of LANA peptides to interact with members of the karyopherin family of nuclear transport receptors. Our studies revealed that both LANA proteins contain an N-terminal arginine/glycine (RG)-rich domain spanning a conserved chromatin-binding motif, which binds directly to importin β1 in a RanGTP-sensitive manner and serves as an NLS in the importin β1-mediated non-classical nuclear import pathway. Embedded within this domain is a conserved lysine/arginine-(KR)-rich bipartite motif that binds directly to multiple members of the importin α family of nuclear import adaptors in a RanGTP-insensitive manner and serves as an NLS in the classical importin α/β-mediated nuclear import pathway. The positioning of a classical bipartite kr-NLS embedded within a non-classical rg-NLS is a unique arrangement in these viral proteins, whose nuclear localization is critical to their functionality and to the virus life cycle. The ability to interact with multiple import receptors provides alternate pathways for nuclear localization of LANA. Since different import receptors can import cargo to distinct subnuclear compartments, a multifunctional NLS may provide LANA with an increased ability to interact with different nuclear components in its multifunctional role to maintain viral latency.

Role of the nucleoplasmin 2 C-terminal domain in the formation of nucleolus-like bodies in mouse oocytes

Nucleolus-like bodies (NLBs) are characteristic structures found in the germinal vesicles of mammalian oocytes. Although these structures are essential for embryonic development, their composition, precise function, and mechanism of formation have not been elucidated. Here, we used immunoblotting and EGFP fusion protein fluorescence to demonstrate that murine nucleoplasmin 2 (NPM2) is a component of mouse NLBs and that the targeting of NPM2 to NLBs is regulated by a lysine-rich, 16-aa C-terminal motif (K-rich motif). When the K-rich motif was fused to another nuclear protein, MafG, the resultant fusion protein accumulated in NLBs but not in the nucleoli of somatic cells, suggesting that the K-rich motif functions to target NPM2 specifically to NLBs. To investigate the role of the K-rich motif in NLB formation, we replaced the endogenous NPM2 in growing oocytes with a mutant NPM2 protein lacking the K-rich motif (NPM2(C16del)). Growing oocytes surrounded by granulosa layers were coinjected with NPM2(C16del) mRNA and with small-interfering RNA targeting NPM2 (siNpm2), which was used to degrade the endogenous NPM2 mRNA. After culture in vitro, the NLBs in the resulting full-grown oocytes were significantly smaller than those in control oocytes that had been coinjected with siNpm2 and NPM2 mRNA, indicating that the K-rich motif is necessary for NLB development. Together, these results suggest that NPM2 targeting of NLBs is regulated by the K-rich motif and is essential for the formation of NLBs.

Nucleolar trafficking of nucleostemin family proteins: common versus protein-specific mechanisms

The nucleolus has begun to emerge as a subnuclear organelle capable of modulating the activities of nuclear proteins in a dynamic and cell type-dependent manner. It remains unclear whether one can extrapolate a rule that predicts the nucleolar localization of multiple proteins based on protein sequence. Here, we address this issue by determining the shared and unique mechanisms that regulate the static and dynamic distributions of a family of nucleolar GTP-binding proteins, consisting of nucleostemin (NS), guanine nucleotide binding protein-like 3 (GNL3L), and Ngp1. The nucleolar residence of GNL3L is short and primarily controlled by its basic-coiled-coil domain, whereas the nucleolar residence of NS and Ngp1 is long and requires the basic and the GTP-binding domains, the latter of which functions as a retention signal. All three proteins contain a nucleoplasmic localization signal (NpLS) that prevents their nucleolar accumulation. Unlike that of the basic domain, the activity of NpLS is dynamically controlled by the GTP-binding domain. The nucleolar retention and the NpLS-regulating functions of the G domain involve specific residues that cannot be predicted by overall protein homology. This work reveals common and protein-specific mechanisms underlying the nucleolar movement of NS family proteins.

GNL3L inhibits activity of estrogen-related receptor gamma by competing for coactivator binding

Guanine nucleotide binding protein-like 3 (GNL3L) is the closest homologue of a stem cell-enriched factor nucleostemin in vertebrates. They share the same yeast orthologue, Grn1p, but only GNL3L can rescue the growth-deficient phenotype in Grn1-null yeasts. To determine the unique function of GNL3L, we identified estrogen-related receptor gamma (ERRgamma) as a GNL3L-specific binding protein. GNL3L and ERRgamma are coexpressed in the eye, kidney and muscle, and co-reside in the nucleoplasm. The interaction between GNL3L and ERRgamma requires the intermediate domain of GNL3L and the AF2-domain of ERRgamma. Gain-of- and loss-of-function experiments show that GNL3L can inhibit the transcriptional activities of ERR genes in a cell-based reporter system, which does not require the nucleolar localization of GNL3L. We further demonstrate that GNL3L is able to reduce the steroid receptor coactivator (SRC) binding and the SRC-mediated transcriptional coactivation of ERRgamma. This work reveals a novel mechanism that negatively regulates the transcriptional function of ERRgamma by GNL3L through coactivator competition.

Rat homolog of PinX1 is a nucleolar protein involved in the regulation of telomere length

Human PinX1 involves in regulation of telomere length. Here, we describe the function of a rat homolog of PinX1. Rat PinX1 (rPinX1) was cloned from WB-F344, a rat hepatic stem-like epithelial cell. It encodes a protein of 331 amino acids with 70% homology to human PinX1 and 91% homology to mouse. Northern analysis revealed that rPinX1 is expressed in both somatic and germ tissues, most abundantly in heart, liver and testis. Co-localization with a nucleolar protein, fibrillarin, showed that rPinX1 resides in the nucleolus. Analysis of truncated mutants revealed that an internal K,E/D region seems to be important for nucleolar localization. A stable cell line expressing rPinX1 was established in NIH3T3, a mouse-transformed embryonic fibroblast cell line, and stable cells were subcultured for more than 150 population doublings. The growth of stable rPinX1 cells slowed down at late passages, and a fraction of these cells exhibited increased size and stained positively for senescence-associated beta-galactosidase. Overexpression of rPinX1 in NIH3T3 cells resulted in gradual telomere shortening over successive passages. However, the telomeric 3' overhang was not altered by PinX1 expression. This study demonstrates that a rat homolog of human PinX1 is a nucleolar protein, and that overexpression of rPinX1 induces cellular senescence and telomere shortening, but has no effect on 3' overhang length. The function of PinX1 in regulating telomere length is conserved in rodents, and this study may provide insight into the mechanism by which a nucleolar protein can regulate telomere length.