Quiescent cells

The differences between palytoxin and PDBu with respect to kinetics, temperature dependence, and effect on low affinity binding suggest that these two different types of tumor promoters may be acting through different mechanisms. Further, in contrast to PDBu, the effect of palytoxin is not readily reversible (33). To determine where the two pathways differ, we compared the relative ability of palytoxin and PDBu to inhibit EGF binding in protein kinase C depleted cells. Swiss 3T3 cells were depleted of protein kinase C to different extents by exposing confluent quiescent cells to 0, 20, 200, or 2000 nM PDBu for 72 hr. Previous results indicate that this treatment depletes cells of protein kinase C activity in a dose-dependent manner (31). 

The provocative initial biological activities reported for PatA, primarily the 20,000-fold difference in toxicity toward cancer cells (P388) versus a quiescent cell line (BSC) (Northcote et al, 1991), led us to undertake a total synthesis of this natural product that would ultimately enable detailed mode of action studies. We envisioned the synthesis and subsequent union of three principal fragments, namely, enyne acid 4, /1-lactam 10, and dienylstannane 14 (Fig. 14.1). A crucial aspect of this plan was a late-stage Stille coupling to append the expected labile trienyl... 

Sutteeluty, H., et ah, p45SKP2 promotes p27Kipl degradation and induces S phase in quiescent cells. 

Such damage is more likely to occur when DNA is stripped of the proteins that stabilise it (e.g. histones). Consequently, the concentration of p53 is high in rapidly dividing cells but low in quiescent cells. 

Sutterluty H., Chatelain, E., Marti, A., Wirbelauer, C., Seufler, M., Muller, U. and Krek, W. (1999). p45Skp2 promotes p27Kipl degradation and induces S phase in quiescent cells. Nature Cell Biol. 1, 207-214. 

Willimott S, Baou M, Huf S et al (2007) Separate ceU culture conditions to promote proliferation or quiescent cell survival in chronic lymphocytic leukemia. Leuk Lymphoma 48 1647-1650... 

Interestingly, the selectivity of the hydoxamic acid inhibitor of DPP IV was crucial in experiments to establish the role and existence of quiescent cell proline dipeptidase, QPP, a serine protease whose inhibition leads to the death of quiescent peripheral blood monocytes [104]. All other DPP IV inhibitors did not exhibit sufficient selectivity to discriminate between DPP IV and QPP. 

M. Chiravuri, T. Schmitz, K. Yardley, R. Underwood, Y. Dayal, B.T. Huber, A novel apoptotic pathway in quiescent lymphocytes identified by inhibition of a post-proline cleaving aminodipeptidase A candidate target protease, quiescent cell proline dipeptidase, J. Immunol. 163 (1999) 3092-3099. 

Many of the enzymes participating in de novo synthesis of deoxyribonucleotide triphosphates, as well as those responsible for interconversion of deoxyribonucleotides, increase in activity when cells prepare for DNA synthesis. The need for increased DNA synthesis occurs under three circumstances (1) when the cell proceeds from the G0, or resting, stage of the cell cycle to the S, or synthetic or replication, stage (fig. 23.26) (2) when it performs repair after extensive DNA damage and (3) after infection of quiescent cells with virus. When cells leave G0, for example, enzymes such as thymidylate synthase and ribonucleotide reductase, increase as well as the corresponding mRNAs. These increases in enzyme amount supplement allosteric controls that increase the activity of each enzyme molecule. Corresponding decreases in amounts of these enzymes and their mRNAs occur when DNA synthesis is completed. 

Didanosine is a synthetic purine nucleoside analog that inhibits the activity of reverse transcriptase in HIV-1, HIV-2, other retroviruses and zidovudine-resistant strains. A nucleobase carrier helps transport it into the cell where it needs to be phosphorylated by 5 -nucleoiidase and inosine 5 -monophosphate phosphotransferase to didanosine S -monophosphate. Adenylosuccinate synthetase and adenylosuccinate lyase then convert didanosine 5 -monophosphate to dideoxyadenosine S -monophosphate, followed by its conversion to diphosphate by adenylate kinase and phosphoribosyl pyrophosphate synthetase, which is then phosphorylated by creatine kinase and phosphoribosyl pyrophosphate synthetase to dideoxyadenosine S -triphosphate, the active reverse transcriptase inhibitor. Dideoxyadenosine triphosphate inhibits the activity of HIV reverse transcriptase by competing with the natural substrate, deoxyadenosine triphosphate, and its incorporation into viral DNA causes termination of viral DNA chain elongation. It is 10-100-fold less potent than zidovudine in its antiviral activity, but is more active than zidovudine in nondividing and quiescent cells. At clinically relevant doses, it is not toxic to hematopoietic precursor cells or lymphocytes, and the resistance to the drug results from site-directed mutagenesis at codons 65 and 74 of viral reverse transcriptase. 

Lentiviruses can be very effective vectors for gene therapy since they can change the expression of genes in target cells for up to 6 months. They are useful for nondividing and terminally differentiated cells including muscle cells, hepato-cytes, neurons, macrophages, retinal photoreceptors and hematopoietic stem cells. However, lentiviruses cannot enter quiescent cells in which reverse transcription is blocked. 

In eukaryotes, the cell cycle consists of Glr S, G2 and M phases. Most differences in the cycle times of different cells are due to differences in the length of the G, phase. Quiescent cells are said to be in the G0 phase. 

On stimulation of quiescent cells with growth factors or serum there is a rapid increase in the transmembrane flux of Na+, K+ and H+ and a mobilisation of Ca2+ from intracellular stores. The increase in Ca2+ concentration can be mimicked by treating cells with the Ca2+ ionophore, A23187. There quickly follows a series of events leading to changes in gene expression and cell structure and eventually to DNA replication and cell division. 

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