Biomedical subjects

Geng Liu

Publications and source records attributed to Geng Liu.

9 recordsLinked to original sources

PubMed2021

Dissecting spatial heterogeneity and the immune-evasion mechanism of CTCs by single-cell RNA-seq in hepatocellular carcinoma.

Little is known about the transcriptomic plasticity and adaptive mechanisms of circulating tumor cells (CTCs) during hematogeneous dissemination. Here we interrogate the transcriptome of 113 single CTCs from 4 different vascular sites, including hepatic vein (HV), peripheral artery (PA), peripheral vein (PV) and portal vein (PoV) using single-cell full-length RNA sequencing in hepatocellular carcinoma (HCC) patients. We reveal that the transcriptional dynamics of CTCs were associated with stress response, cell cycle and immune-evasion signaling during hematogeneous transportation. Besides, we identify chemokine CCL5 as an important mediator for CTC immune evasion. Mechanistically, overexpression of CCL5 in CTCs is transcriptionally regulated by p38-MAX signaling, which recruites regulatory T cells (Tregs) to facilitate immune escape and metastatic seeding of CTCs. Collectively, our results reveal a previously unappreciated spatial heterogeneity and an immune-escape mechanism of CTC, which may aid in designing new anti-metastasis therapeutic strategies in HCC.

Aged↗

PubMed2006

p21 delays tumor onset by preservation of chromosomal stability.

The p53 protein suppresses tumorigenesis by initiating cellular functions such as cell cycle arrest and apoptosis in response to DNA damage. A p53 mutant, p53R172P, which is deficient for apoptosis but retains a partial cell cycle arrest function, delays tumor onset in mice. Remarkably, lymphomas arising in Trp53(515C/515C) mice (encoding p53R172P) retain stable genomes. Given the dominant role of p21 in p53 cell cycle control, we crossed Trp53(515C/515C) mice onto a p21-null background to determine whether p21 was required for maintaining chromosomal stability and delaying tumor onset. Loss of p21 completely abolished the cell cycle arrest function of p53R172P and accelerated tumor onset in Trp53(515C/515C) mice. Cytogenetic examination of Trp53(515C/515C) p21(-/-) sarcomas and lymphomas revealed aneuploidy and chromosomal aberrations that were absent in Trp53(515C/515C) malignancies. Thus, p21 coupled p53-dependent checkpoint control and preservation of chromosomal stability, and cooperated with apoptosis in suppressing tumor onset in mice.

Animals↗

PubMed2006

The p66 and p12 subunits of DNA polymerase delta are modified by ubiquitin and ubiquitin-like proteins.

Modification by ubiquitin-like proteins is now known to be important for the functions of many proteins involved in DNA replication and repair. We have investigated the modification of human DNA polymerase delta by ubiquitin and SUMO proteins. We find that while the p125 and p50 subunits were not modified, the p12 subunit is ubiquitinated and the p66 subunit can be modified by ubiquitin and SUMO3. We show that levels of p12 are regulated by the proteasome, either directly or indirectly, through a mechanism that is not dependent upon p12 ubiquitination. We have mapped two sites of SUMO3-specific modification on the p66 subunit. SUMOylation by SUMO3 but not SUMO2 is unusual: their level of homology is so high that they are normally classified as variants of the same protein. However, our findings show that these two proteins can be distinguished in vivo and may have specific functions.

Amino Acid Sequence↗

PubMed2006

Synergistic roles of Mdm2 and Mdm4 for p53 inhibition in central nervous system development.

Loss of Mdm2 or Mdm4 leads to embryo lethal phenotypes that are p53-dependent. To determine whether Mdm2 and Mdm4 inhibit p53 function redundantly in a more restricted cell type, conditional alleles were crossed to a neuronal specific Cre transgene to delete Mdm2 and Mdm4 in the CNS. Mice lacking Mdm2 in the CNS developed hydranencephaly at embryonic day 12.5 due to apoptosis, whereas Mdm4 deletion showed a proencephaly phenotype at embryonic day 17.5 because of cell cycle arrest and apoptosis. The deletion of both genes, strikingly, contributed to an even earlier and more severe CNS phenotype. Additionally, Mdm2 and Mdm4 had a gene dosage effect, because loss of three of the four Mdm alleles also showed a more accelerated CNS phenotype than deletion of either gene alone. All phenotypes were rescued by deletion of p53. Thus, these in vivo data demonstrate the importance of Mdm4 independent of Mdm2 in inhibition of p53.

Animals↗

PubMed2006

Induced endoplasmic reticulum (ER) stress and binding of over-expressed ER specific chaperone GRP78/BiP with dimerized epidermal growth factor receptor in mammalian cells exposed to low concentration of N-methyl-N'-nitro-N-nitrosoguanidine.

Previously we have shown that alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) can induce the clustering of epidermal growth factor receptor (EGFR) in human amnion FL cells. However, the biological consequence of MNNG-induced clustering is different from that of epidermal growth factor (EGF)-induced clustering. In addition, MNNG strongly blocks the autophosphorylation of EGFR in response to its ligand, we speculate it might be due to the altered conformation of EGFR by MNNG alkylation, or the binding of some unknown suppressive molecules to EGFR, which could lead to the down-regulation of EGFR pathway. In this study, we further demonstrated that EGFR could not be phosphorylated by EGF in lysates prepared from MNNG-pretreated cell. In addition, it was found that the clustering of EGFR induced by low concentration (<or=1 microM) of MNNG on cell surface was indeed the dimerization of EGFR; however, unlike EGF treatment, the dimerization initiated by MNNG was irreversible upon mild-acid washing. Besides, in accordance with our previous results, the recruitment of adaptor proteins Grb-2/Sos1, which play key roles in activating ensuing RAS-MAPK pathway, was also suppressed. Interestingly, we found that endoplasmic reticulum (ER) stress participates in MNNG-induced down-regulation of EGFR signaling. It was demonstrated that the ER specific chaperone, glucose-regulated protein 78 (GRP78/BiP) formed a stable complex with EGFR in MNNG-treated cell. However, in the presence of 1mM ATP, EGF induced phosphorylation of tyrosine residues of EGFR can be revitalized in lysates prepared from MNNG pretreated cells. We also found that MNNG can induce ER stress or unfolded protein response (UPR) which is characterized by induced expression of ER-stress response proteins, such as GRP78/BiP, GADD153/CHOP, and activation of ER-localized caspase-12. Therefore, it is concluded MNNG is also an ER stress inducer. In MNNG-exposed cells, ER stress plays an important role in the blockage of EGFR-signaling pathway by forming a stable complex of EGFR/BiP.

Amnion↗

PubMed2005

p53 isoforms can regulate p53 transcriptional activity.

The recently discovered p53-related genes, p73 and p63, express multiple splice variants and N-terminally truncated forms initiated from an alternative promoter in intron 3. To date, no alternative promoter and multiple splice variants have been described for the p53 gene. In this study, we show that p53 has a gene structure similar to the p73 and p63 genes. The human p53 gene contains an alternative promoter and transcribes multiple splice variants. We show that p53 variants are expressed in normal human tissue in a tissue-dependent manner. We determine that the alternative promoter is conserved through evolution from Drosophila to man, suggesting that the p53 family gene structure plays an essential role in the multiple activities of the p53 family members. Consistent with this hypothesis, p53 variants are differentially expressed in human breast tumors compared with normal breast tissue. We establish that p53beta can bind differentially to promoters and can enhance p53 target gene expression in a promoter-dependent manner, while Delta133p53 is dominant-negative toward full-length p53, inhibiting p53-mediated apoptosis. The differential expression of the p53 isoforms in human tumors may explain the difficulties in linking p53 status to the biological properties and drug sensitivity of human cancer.

Alternative Splicing↗

PubMed2005

p21 stability: linking chaperones to a cell cycle checkpoint.

Progression through the cell cycle is regulated by numerous proteins, one of which is the cyclin-dependent kinase inhibitor, p21. A new study identifies a novel protein complex that stabilizes p21. The stability of this complex is critical in effecting the p53-mediated cell cycle checkpoint.

Animals↗

PubMed2004

Gain of function of a p53 hot spot mutation in a mouse model of Li-Fraumeni syndrome.

Individuals with Li-Fraumeni syndrome carry inherited mutations in the p53 tumor suppressor gene and are predisposed to tumor development. To examine the mechanistic nature of these p53 missense mutations, we generated mice harboring a G-to-A substitution at nucleotide 515 of p53 (p53+/515A) corresponding to the p53R175H hot spot mutation in human cancers. Although p53+/515A mice display a similar tumor spectrum and survival curve as p53+/- mice, tumors from p53+/515A mice metastasized with high frequency. Correspondingly, the embryonic fibroblasts from the p53515A/515A mutant mice displayed enhanced cell proliferation, DNA synthesis, and transformation potential. The disruption of p63 and p73 in p53-/- cells increased transformation capacity and reinitiated DNA synthesis to levels observed in p53515A/515A cells. Additionally, p63 and p73 were functionally inactivated in p53515A cells. These results provide in vivo validation for the gain-of-function properties of certain p53 missense mutations and suggest a mechanistic basis for these phenotypes.

Animals↗

PubMed2003

Chromosome stability, in the absence of apoptosis, is critical for suppression of tumorigenesis in Trp53 mutant mice.

The p53 protein integrates multiple upstream signals and functions as a tumor suppressor by activating distinct downstream genes. At the cellular level, p53 induces apoptosis, cell cycle arrest and senescence. A rare mutant form of p53 with the amino acid substitution R175P, found in human tumors, is completely defective in initiating apoptosis but still induces cell cycle arrest. To decipher the functional importance of these pathways in spontaneous tumorigenesis, we used homologous recombination to generate mice with mutant p53-R172P (the mouse equivalent of R175P in humans). Mice inheriting two copies of this mutation (Trp53(515C/515C)) escape the early onset of thymic lymphomas that characterize Trp53-null mice. At 7 months of age, 90% of Trp53-null mice had died, but 85% of Trp53(515C/515C) mice were alive and tumor-free, indicating that p53-dependent apoptosis was not required for suppression of early onset of spontaneous tumors. The lymphomas and sarcomas that eventually developed in Trp53(515C/515C) mice retained a diploid chromosome number, in sharp contrast to aneuploidy observed in tumors and cells from Trp53-null mice. The ability of mutant p53-R172P to induce a partial cell cycle arrest and retain chromosome stability are crucial for suppression of early onset tumorigenesis.

Animals↗