BackgroundFGF21 is a promising intervention therapy for metabolic diseases as fatty liver, obesity and diabetes. Recent results suggest that FGF21 is highly expressed in hepatocytes under metabolic stress caused by starvation, hepatosteatosis, obesity and diabetes. Hepatic FGF21 elicits metabolic benefits by targeting adipocytes of the peripheral adipose tissue through the transmembrane FGFR1-KLB complex. Ablation of adipose FGFR1 resulted in increased hepatosteatosis under starvation conditions and abrogation of the anti-obesogenic action of FGF21. These results indicate that FGF21 may be a stress responsive hepatokine that targets adipocytes and adipose tissue for alleviating the damaging effects of stress on the liver. However, it is unclear whether hepatic induction of FGF21 is limited to only metabolic stress, or to a more general hepatic stress resulting from liver pathogenesis and injury.MethodsIn this survey-based study, we examine the nature of hepatic FGF21 activation in liver tissues and tissue sections from several mouse liver disease models and human patients, by quantitative PCR, immunohistochemistry, protein chemistry, and reporter and CHIP assays. The liver diseases include genetic and chemical-induced HCC, liver injury and regeneration, cirrhosis, and other types of liver diseases.ResultsWe found that mouse FGF21 is induced in response to chemical (DEN treatment) and genetic-induced hepatocarcinogenesis (disruptions in LKB1, p53, MST1/2, SAV1 and PTEN). It is also induced in response to loss of liver mass due to partial hepatectomy followed by regeneration. The induction of FGF21 expression is potentially under the control of stress responsive transcription factors p53 and STAT3. Serum FGF21 levels correlate with FGF21 expression in hepatocytes. In patients with hepatitis, fatty degeneration, cirrhosis and liver tumors, FGF21 levels in hepatocytes or phenotypically normal hepatocytes are invariably elevated compared to normal health subjects.ConclusionFGF21 is an inducible hepatokine and could be a biomarker for normal hepatocyte function. Activation of its expression is a response of functional hepatocytes to a broad spectrum of pathological changes that impose both cellular and metabolic stress on the liver. Taken together with our recent data, we suggest that hepatic FGF21 is a general stress responsive factor that targets adipose tissue for normalizing local and systemic metabolic parameters while alleviating the overload and damaging effects imposed by the pathogenic stress on the liver. This study therefore provides a rationale for clinical biomarker studies in humans.

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Stem and progenitor cells maintain a robust DNA replication program during the tissue expansion phase of embryogenesis. The unique mechanism that protects them from the increased risk of replication-induced DNA damage, and hence permits self-renewal, remains unclear. To determine whether the genome integrity of stem/progenitor cells is safeguarded by mechanisms involving molecules beyond the core DNA repair machinery, we created a nucleostemin (a stem and cancer cell-enriched protein) conditional-null allele and showed that neural-specific knockout of nucleostemin predisposes embryos to spontaneous DNA damage that leads to severe brain defects in vivo. In cultured neural stem cells, depletion of nucleostemin triggers replication-dependent DNA damage and perturbs self-renewal, whereas overexpression of nucleostemin shows a protective effect against hydroxyureainduced DNA damage. Mechanistic studies performed in mouse embryonic fibroblast cells showed that loss of nucleostemin triggers DNA damage and growth arrest independently of the p53 status or rRNA synthesis. Instead, nucleostemin is directly recruited to DNA damage sites and regulates the recruitment of the core repair protein, RAD51, to hydroxyurea-induced foci. This work establishes the primary function of nucleostemin in maintaining the genomic stability of actively dividing stem/progenitor cells by promoting the recruitment of RAD51 to stalled replication-induced DNA damage foci.DNA damage repair | homologous recombination | conditional knockout | replication fork stalling | neural development S tem and progenitor cells play critical roles in embryonic organogenesis, adult tissue regeneration, and tumor development. To maintain self-renewing proliferation, they must be protected from replication-induced DNA damage that limits the proliferative lifespan of most dividing cells. Replication-induced DNA damage may occur spontaneously as a result of stalled and collapsed replication forks, caused by the slowing of the DNA replication machinery over replication "trouble" zones or previously unrepaired damage sites (1-3). Alternatively, replication stalling can be triggered by drugs that deplete the endogenous nucleotide pool (e.g., hydroxyurea) or inhibit the activity of DNA replication (e.g., camptothecin). Prolonged replication stalling will lead to the collapse of replication machinery and doublestrand DNA breaks (DSBs), causing cell cycle arrest and genomic instability (4, 5). To date, it remains unclear how stem and progenitor cells survive the increased risk of replication-induced DNA damage during this hyperactive mitotic window of embryogenesis.Nucleostemin (NS) is a stem cell-enriched nucleolar protein (6). Its biological significance has been illustrated by the early embryonic lethal phenotype of NS germ-line knockout mice (7), which ineluctably hinders further analyses of its in vivo function beyond the blastula stage. To date, the mechanism of NS action in vivo remains unclear. Although some suggested a connection to the p53 pathway (6,(8)(9)(10), ...

Summary Nucleolar disassembly occurs during mitosis and nucleolar stress, releasing several MDM2-interactive proteins residing in the nucleolus that share the common activity of p53 stabilization. Here, we demonstrated that mobilization of nucleostemin (NS), a cancer and stem cell-enriched nucleolar protein, plays the opposite role by stabilizing MDM2 and suppressing p53 functions. Our results showed that NS increases the protein stability and nucleoplasmic retention of MDM2, and competes with L23 for MDM2 binding. These activities are significantly elevated when NS is released into the nucleoplasm by mutations that abolish its nucleolar localization or by chemotherapeutic agents that disassemble the nucleoli. NS depletion decreases MDM2 protein, increases the transcriptional activities without changing the protein level of p53, and triggers G2/M arrest and cell death in U2OS but not in H1299 cells. This work reveals that nucleoplasmic relocation of NS during nucleolar disassembly safeguards the G2/M transit and survival of continuously dividing cells by MDM2 stabilization and p53 inhibition.

Telomeric repeat binding factor 1 (TRF1) is a component of the multiprotein complex “shelterin,” which organizes the telomere into a high-order structure. TRF1 knockout embryos suffer from severe growth defects without apparent telomere dysfunction, suggesting an obligatory role for TRF1 in cell cycle control. To date, the mechanism regulating the mitotic increase in TRF1 protein expression and its function in mitosis remains unclear. Here, we identify guanine nucleotide-binding protein-like 3 (GNL3L), a GTP-binding protein most similar to nucleostemin, as a novel TRF1-interacting protein in vivo. GNL3L binds TRF1 in the nucleoplasm and is capable of promoting the homodimerization and telomeric association of TRF1, preventing promyelocytic leukemia body recruitment of telomere-bound TRF1, and stabilizing TRF1 protein by inhibiting its ubiquitylation and binding to FBX4, an E3 ubiquitin ligase for TRF1. Most importantly, the TRF1 protein-stabilizing activity of GNL3L mediates the mitotic increase of TRF1 protein and promotes the metaphase-to-anaphase transition. This work reveals novel aspects of TRF1 modulation by GNL3L.