Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: UNIPROT:P04637 (p53)
 77,613 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The rat cell line REF52 is not permissive for gene amplification. Simian virus 40 tumor (T) antigen converts these cells to a permissive state, as do dominant negative mutants of p53, suggesting that the effect of T antigen is due mainly to its ability to bind to p53. To manipulate permissivity, we introduced a temperature-sensitive mutant of T antigen (tsA58) into REF52 cells and selected for resistance to N-(phosphonacetyl)-L-aspartate (PALA). Most freshly isolated PALA-resistant colonies, each of approximately 200 cells, selected at a permissive temperature, arrested when shifted to a nonpermissive temperature. Growth arrest was stable, with no evidence of apoptosis, as long as T antigen was absent but was reversed when T antigen was restored. In contrast, PALA-resistant clones grown to approximately 10(7) cells at a permissive temperature did not arrest when shifted to a nonpermissive temperature. All PALA-resistant clones examined had amplified carbamoyl-phosphate synthetase-aspartate transcarbamoylase-dihydroorotase (CAD) genes, present in structures consistent with a mechanism involving bridge-breakage-fusion (BBF) cycles. We propose that p53-mediated growth arrest operates only early during the complex process of gene amplification, when newly formed PALA-resistant cells contain broken DNA, generated in BBF cycles. During propagation under permissive conditions, the broken DNA ends are healed, and, even though the p53-mediated pathway is still intact at a nonpermissive temperature and the cells contain amplified DNA, they are not arrested in the absence of broken DNA. The data support the hypothesis that BBF cycles are an important mechanism of amplification and that the broken DNA generated in each cycle is a key signal that regulates permissivity for gene amplification.
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PMID:p53-dependent growth arrest of REF52 cells containing newly amplified DNA. 772 43

Amplification in rodent cells usually involves bridge-breakage-fusion (BBF) cycles initiated either by end-to-end fusion of sister chromatids, or by chromosome breakage. In contrast, in human cells, resistance to the antimetabolite N-(phosphonacetyl)-L-aspartate (PALA) can be mediated by several different mechanisms that lead to overexpression of the target enzyme carbamyl-P synthetase, aspartate transcarbamylase, dihydro-orotase (CAD). Mechanisms involving BBF cycles account for only a minority of CAD amplification events in the human fibrosarcoma cell line HT 1080. Here, formation of a 2p isochromosome and overexpression of CAD by other types of amplification events (and even without amplification) are much more prevalent. Broken DNA is recognized by mammalian cells with intact damage-recognition pathways, as a signal to arrest or to die. Loss of these pathways by, for example, loss of p53 or pRb tumour suppressor function, or by increased expression of ras and myc oncogenes, causes non-permissive rat and human cells to become permissive both for amplification and for other manifestations of DNA damage. In cells that are already permissive, amplification can be stimulated by overexpressing oncogenes such as c-myc or ras, or by damaging DNA in a variety of ways. To supplement genetic analysis of amplification in mammalian cells, an amplification selection has been established in Schizosaccharomyces pombe. Selection with LiCl yields cells with amplified sod2 genes in structures related to those observed in mammalian cells. The effect on amplification in S. pombe can now be tested for any mutation in a gene involved in repair of damaged DNA or in normal cellular responses to DNA damage.
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PMID:Regulation and mechanisms of gene amplification. 774 53

Genomic stability was investigated in Chinese hamster ovary (CHO) and human hepatocellular carcinoma HepG2 cells selected for growth in the presence of cytotoxic concentrations of N-(phosphonacetyl-L-asparate) (PALA). In CHO cells selected with 9 x LD50 PALA the carbamyl p-synthetase, aspartate transcarbamylase and dihydroorotase (CAD) gene complex was amplified two-fold while in HepG2 cells selected at comparable PALA concentrations a 7- to 10-fold increase in the CAD gene was observed. Concomitant with amplification of the CAD gene were increases in CAD mRNA and protein expression in both CHO and HepG2 cells. In long-term cultures of HepG2 cells the CAD gene underwent spontaneous amplification (5-fold) in the absence of PALA treatment with increasing passage number. In an attempt to define proteins and/or family of proteins that may either directly or indirectly influence DNA amplification potential through a mechanism of enhanced genomic instability, immobilized pH gradient-two-dimensional polyacrylamide gel electrophoresis (IPG 2-D PAGE) analysis of silver-stained nuclear cytoplasmic polypeptides concomitant with PALA resistance and CAD amplification was performed. Analysis of silver-stained polypeptides from 3 x LD50 PALA-selected CHO and HepG2 cells revealed no significant alterations in polypeptide expression. In CHO cells selected at 5 x and 7 x PALA LD50, and HepG2 cells selected at 5 x and 9 x PALA LD50, one subset of 4-8 polypeptides (pl: pI 7.2-7.6/36-38 kDa) were increased 2- to 3-fold in both 5 x and 7 x- and 5 x and 9 x LD50 PALA-selected CHO and HepG2, respectively, while five relatively neutral-to-basic, low M(r) polypeptides (p2: 18/7.30; p3: 16/7.00; p4: 14/7.00; p5: 14/7.40; and p6: 13.5/7.00) were markedly increased in CHO cells selected at 7 x LD50 PALA. In addition to these PALA-associated increases, four polypeptides (p7a: pI 6.50/40 kDa; p7b: 6.55/40; p7c: 6.60/40; and p7d: 6.65/40) were significantly increased in high-passage (p159) HepG2 cells undergoing spontaneous CAD gene amplification in the absence of PALA exposure. In CHO cells, polypeptides p7 a, b, d were increased while the expression of p7c (pI 6.60/40 kDa) was unaltered in 7 x LD50-treated CHO cells. Although neither the identity nor biological function of polypeptides 1-7 is known, a proposed mechanism involving interaction with certain growth regulatory proteins such as p53 for mediating genomic instability is given.
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PMID:Protein alterations associated with gene amplification in cultured human and rodent cells. 885 14

p53 is essential for the cellular responses to DNA damage that help to maintain genomic stability. Protective p53-dependent cell-cycle checkpoints are activated in response to a wide variety of stresses, including not only DNA damage but also arrest of DNA synthesis and of mitosis. In addition to its role in activating the G(1) and G(2) checkpoints, p53 also helps to protect cells in S phase when they are starved for DNA precursors by treatment with the specific aspartate transcarbamylase inhibitor N-phosphonacetyl-l-aspartate (PALA), which blocks the synthesis of pyrimidine nucleotides. Even though p53 is activated, PALA-treated cells expressing low levels of p53 or lacking expression of p21 do not arrest in G(1) or G(2) but are blocked in S phase instead. In the complete absence of p53, PALA-treated cells continue to synthesize DNA slowly and eventually progress through S phase, suffering severe DNA damage that in turn triggers apoptosis. Expression of the secreted protein macrophage inhibitory cytokine 1 (MIC-1), a member of the TGF-beta superfamily, increases substantially after PALA treatment, and application of exogenous MIC-1 or its constitutive expression from a cDNA provides remarkable protection of p53-null cells from PALA-mediated apoptosis, arguing that the p53-dependent secretion of MIC-1 provides a major part of such protection. Stimulation of MIC-1-dependent S phase arrest in normal gut epithelial cells might help to revitalize the clinical use of PALA, which has been limited by gut toxicity.
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PMID:Macrophage inhibitory cytokine 1 mediates a p53-dependent protective arrest in S phase in response to starvation for DNA precursors. 1705 Jun 87