Chenzhong Xu

The histone acetyltransferase (HAT) KAT7/HBO1/MYST2 plays a crucial role in the pre-replication complex (pre-RC) formation, DNA replication and cell proliferation via acetylation of histone H4 and H3. In a search for protein kinase D1 (PKD1)-interacting proteins, we have identified KAT7 as a potential PKD1 substrate. We show that PKD1 directly interacts and phosphorylates KAT7 at Thr97 and Thr331 in vitro and in vivo. PKD1-mediated phosphorylation of KAT7 enhances its expression levels and stability by reducing its ubiquitination-mediated degradation. Significantly, the phospho-defective mutant KAT7-Thr97/331A attenuates histone H4 acetylation levels, MCM2/6 loading on the chromatin, DNA replication and cell proliferation. Similarly, PKD1 knockdown decreases, whereas the constitutive active mutant PKD1-CA increases histone H4 acetylation levels and MCM2/6 loading on the chromatin. Overall, these results suggest that PKD1-mediated phosphorylation of KAT7 may be required for pre-RC formation and DNA replication.

Human telomerase RNA component hTERC comprises multiple motifs that contribute to hTERC biogenesis, holoenzyme activity, and enzyme recruitment to telomeres. hTERC contains several guanine tracts (G-tracts) at its 5′-end, but its associated proteins and potential roles in telomerase function are still poorly understood. The heterogeneous nuclear ribonucleoproteins F, H1, and H2 (hnRNP F/H) are splicing factors that preferentially bind to poly(G)-rich sequences RNA. Here, we demonstrate that hnRNP F/H associate with both hTERC and telomerase holoenzyme to regulate telomerase activity. We reveal hnRNP F/H bind to the 5′-end region of hTERC in vitro and in vivo, and identify the first three G-tracts of hTERC and qRRM1 domain of hnRNP F/H are required for their interaction. Furthermore, hnRNP F/H also directly interact with telomerase holoenzyme. Functionally, we show that hnRNP F/H plays important roles in modulating telomerase activity and telomere length. Moreover, hnRNP F/H deletion greatly impair cancer and stem cell proliferation, and induce stem cell senescence, while hnRNP F/H overexpression delay stem cell senescence. Collectively, our findings unveil a novel role of hnRNP F/H as the binding partners of hTERC and telomerase holoenzyme to regulate telomerase function.

The activation of transcription factor NF-kappa B is currently identified as one of the driving forces to the aging process. Genetic impairment of NF-kappa B signaling pathway or pharmacological inhibition of NF-kappa B activity has been shown to extend healthspan and lifespan in animal models, and delay or reduce many age-related symptoms. However, the aging intervention strategies based on NF-kappa B inhibition by the suitable small molecular compound is currently still lacking. The water-soluble dimethylaminomicheliolide (DMAMCL), can inhibit NF-kappa B activity and is currently undergoing clinical trials. In this study, we showed that 15 months of DMAMCL administration started in 1-year old male mice was well-tolerated and safe, and improved or had little effect on some age-associated symptoms, such as neurobehavioral phenotypes, physical performance, cardiac function, hematological parameters, immune aging phenotypes, clinical chemistry parameters, and glucose homeostasis. At the molecular level, DMAMCL administration mitigated serum levels of several age-associated inflammatory cytokines, including IL-6, IL-1 alpha, IL-1 beta, TNF-alpha, IFN-gamma, and CXCL2, and inhibited NF-kappa B activity in several aged tissues. Collectively, our results indicate that current strategy of DMAMCL administration may has little effect on aging process in mice, and provide basic clues to further exploit the possibility of DMAMCL-based aging intervention to promote healthy aging.

Senescent cells display the senescence-associated secretory phenotype (SASP) which plays important roles in cancer, aging, etc. Cell surface-bound IL-1α is a crucial SASP factor and acts as an upstream regulator to induce NF-κB activity and subsequent SASP genes transcription. IL-1α exports to cell surface via S100A13 protein-dependent non-classical secretory pathway. However, the status of this secretory pathway during cellular senescence and its role in cellular senescence remain unknown. Here, we show that S100A13 is up-regulated in various types of cellular senescence. S100A13 overexpression increases cell surface-associated IL-1α level, NF-κB activity and subsequent multiple SASP genes induction, whereas S100A13 knockdown has an opposite role. We also exhibit that Cu2+ level is elevated during cellular senescence. Lowering Cu2+ level decreases cell surface-bound IL-1α level, NF-κB activity and SASP production. Moreover, S100A13 overexpression promotes oncogene Ras-induced cell senescence (Ras OIS), Doxorubicin-induced cancer cell senescence (TIS) and replicative senescence, while impairment of non-classical secretory pathway of IL-1α delays cellular senescence. In addition, intervention of S100A13 affects multiple SASP and cellular senescence mediators including p38, γ-H2AX, and mTORC1. Taken together, our findings unveil a critical role of the non-classical secretory pathway of IL-1α in cellular senescence and SASP regulation.

The nucleolus is an important organelle that is responsible for the biogenesis of ribosome RNA (rRNA) and ribosomal subunits assembly. It is also deemed to be the center of metabolic control, considering the critical role of ribosomes in protein translation. Perturbations of rRNA synthesis are closely related to cell proliferation and tumor progression. Telomeric repeat-binding factor 2 (TRF2) is a member of shelterin complex that is responsible for telomere DNA protection. Interestingly, it was recently reported to localize in the nucleolus of human cells in a cell-cycle-dependent manner, while the underlying mechanism and its role on the nucleolus remained unclear. In this study, we found that nucleolar and coiled-body phosphoprotein 1 (NOLC1), a nucleolar protein that is responsible for the nucleolus construction and rRNA synthesis, interacted with TRF2 and mediated the shuttle of TRF2 between the nucleolus and nucleus. Abating the expression of NOLC1 decreased the nucleolar-resident TRF2. Besides, the nucleolar TRF2 could bind rDNA and promoted rRNA transcription. Furthermore, in hepatocellular carcinoma (HCC) cell lines HepG2 and SMMC7721, TRF2 overexpression participated in the nucleolus stress-induced rRNA inhibition and cell-cycle arrest.

Senescent cells develop a senescence-associated secretory phenotype (SASP). The factors secreted by cells with a SASP have multiple biological functions that are mediated in an autocrine or paracrine manner. However, the status of the protein kinase D1 (PKD1; also known as PRKD1)-mediated classical protein secretory pathway, from the trans-Golgi network (TGN) to the cell surface, during cellular senescence and its role in the cellular senescence response remain unknown. Here, we show that the activities or quantities of critical components of this pathway, including PKD1, ADP-ribosylation factor 1 (ARF1) and phosphatidylinositol 4-kinase IIIβ (PI4KIIIβ), at the TGN are increased in senescent cells. Blocking of this pathway decreases IL-6 and IL-8 (hereafter IL-6/IL-8) secretion and results in IL-6/IL-8 accumulation in SASP-competent senescent cells. Inhibition of this pathway reduces IL-6/IL-8 secretion during Ras oncogene-induced senescence (OIS), retards Ras OIS and alleviates its associated ER stress and autophagy. Finally, targeting of this pathway triggers cell death in SASP factor-producing senescent cells due to the intracellular accumulation of massive amounts of IL-6/IL-8. Taken together, our results unveil the hyperactive state of the protein secretory pathway in SASP-competent senescent cells and its critical functions in mediating SASP factor secretion and the Ras OIS process, as well as in determining the fate of senescent cells.

Sirtuin6(SIRT6) has been implicated as a key factor in aging and aging-related diseases. However, the role of SIRT6 in cellular senescence has not been fully understood. Here, we show that SIRT6 repressed the expression of p27Kip1 (p27) in cellular senescence. The expression of SIRT6 was reduced during cellular senescence, whereas enforced SIRT6 expression promoted cell proliferation and antagonized cellular senescence. In addition, we demonstrated that SIRT6 promoted p27 degradation by proteasome and SIRT6 decreased the acetylation level and protein half-life of p27. p27 acetylation increased its protein stability. Furthermore, SIRT6 directly interacted with p27. Importantly, p27 was strongly acetylated and had a prolonged protein half-life with the reduction of SIRT6 when cells were senescent, compared with those young cells. Finally, SIRT6 markedly rescued senescence induced by p27. Our findings indicate that SIRT6 decreases p27 acetylation, leading to its degradation via ubiquitin-proteasome pathway and then delays cellular senescence.

Senescent cells display a senescence-associated secretory phenotype (SASP) which contributes to tumor suppression, aging, and cancer. However, the underlying mechanisms for SASP regulation are not fully elucidated. SIRT1, a nicotinamide adenosine dinucleotide-dependent deacetylase, plays multiple roles in metabolism, inflammatory response, and longevity, etc. However, its posttranscriptional regulation and its roles in cellular senescence and SASP regulation are still elusive. Here, we identify the RNA-binding protein hnRNP A1 as a posttranscriptional regulator of SIRT1, as well as cell senescence and SASP regulator. hnRNP A1 directly interacts with the 3′ untranslated region of SIRT1 mRNA, promotes its stability, and increases SIRT1 expression. hnRNP A1 delays replicative cellular senescence and prevents from Ras OIS via upregulation of SIRT1 expression to deacetylate NF-κB, thus blunting its transcriptional activity and subsequent IL-6/IL-8 induction. hnRNP A1 overexpression promotes cell transformation and tumorigenesis in a SIRT1-dependent manner. Together, our findings unveil a novel posttranscriptional regulation of SIRT1 by hnRNP A1 and uncover a critical role of hnRNP A1-SIRT1–NF-κB pathway in regulating cellular senescence and SASP expression.