Cell cycle dependence of inositol phosphate levels in neuroblastoma cells

Complex changes in cellular inositol phosphate complement accompany transit through the cell cycle

Inositol polyphosphates other than Ins(1,4,5)P3 are involved in several aspects of cell regulation. For example, recent evidence has implicated InsP6, Ins(1,3,4,5,6)P5 and their close metabolic relatives, which are amongst the more abundant intracellular inositol polyphosphates, in chromatin organization, DNA maintenance, gene transcription, nuclear mRNA transport, membrane trafficking and control of cell proliferation. However, little is known of how the intracellular concentrations of inositol polyphosphates change through the cell cycle. Here we show that the concentrations of several inositol polyphosphates fluctuate in synchrony with the cell cycle in proliferating WRK-1 cells. InsP6, Ins(1,3,4,5,6)P5 and their metabolic relatives behave similarly: concentrations are high during G1-phase, fall to much lower levels during S-phase and rise again late in the cycle. The Ins(1,2,3)P3 concentration shows especially large fluctuations, and PP-InsP5 fluctuations are also very marked. Remarkably, Ins(1,2,3)P3 turns over fastest during S-phase, when its concentration is lowest. These results establish that several fairly abundant intracellular inositol polyphosphates, for which important biological roles are emerging, display dynamic behaviour that is synchronized with cell-cycle progression.

Human meningiomas possess muscarinic acetylcholine receptors: stimulation of phosphatidylinositol turnover by carbachol

The effect of carbachol, an acetylcholine receptor agonist, on rate of phosphatidylinositol (PI) turnover in cultured human meningioma cells was investigated. Exposure of meningioma cells for 2 h to carbachol (3.12-200 mumol/L) resulted in a dose-dependent stimulation of PI turnover to a maximum of 5.5-fold over basal controls. A time course study showed stimulation of IP3 formation after 30 s followed by increases in IP1 and IP2. The stimulatory effect of carbachol on PI turnover was completely abolished by the muscarinic receptor antagonist, atropine, but was unaltered by the nicotinic antagonist, hexamethonium. Reverse-transcription of meningioma-derived RNA into cDNA followed by amplification by the polymerase chain reaction using specific primers revealed presence of ml type muscarinic receptor mRNA. These results provide evidence that human meningioma cells possess muscarinic acetylcholine receptors the activation of which leads to PI hydrolysis.

The role of ions and second messengers in circadian clock function

The fact that single cells can exhibit circadian rhythmicity simultaneously in quite different processes, such as those of photosynthesis, bioluminescence, and cell division, suggests that membrane-bound compartmentalization is important for temporal organization. Since these rhythms, as well as others, are known to be affected by changes in the ionic environment and are probably membrane-bound systems, it is not surprising that transmembrane ion transport or flux has been proposed to be a key feature of the underlying circadian oscillator(s). Likewise, signal transduction along the entrainment pathway leading to the clock, among the elements, or "gears," of the timing loop itself, and within the output pathway between the oscillator and its "hands" likely is mediated by ions and second messengers. In this overview, we examine the theoretical and experimental evidence supporting the possible roles of intracellular free calcium and cyclic AMP in these capacities, particularly in view of the fact that oscillations in the concentrations of both species have been proposed to form the basis of pacemaker activity and other biological rhythms.

Perturbation of host-cell membrane is a primary mechanism of HIV cytopathology

Cytopathic viruses injure cells by a number of different mechanisms. The mechanism by which HIV-1 injures T cells was studied by temporally examining host-cell macromolecular syntheses, stages of the cell cycle, and membrane permeability following acute infection. T cells cytopathically infected at an m.o.i. of 1-5 grew normally for 24-72 hr, depending on the cell line, followed by the first manifestation of cell injury, slowing of cell division. At that time significant amounts of unintegrated HIV DNA and p24 core protein became detectable, and acridine orange flow cytometric cell cycle studies demonstrated the presence of fewer cells in the G2/M stage of the cell cycle. There was no change in the frequency of cells in the S-stage, and metabolic pulsing with radioactive precursors demonstrated that host-cell DNA, RNA, and protein syntheses were normal at that time and normal up to the time cells started to die (approximately 24 hr later), when all three decreased. Cellular lipid synthesis, however, was perturbed when cell multiplication slowed, with phospholipid synthesis reduced and neutral lipid synthesis enhanced. Permeability of the host-cell membrane to small molecules, such as Ca2+ and sucrose, was slightly enhanced early postinfection, and by the time of slowing of cell division, host membrane permeability was greatly increased to both Ca2+ and sucrose (Stokes radius 5.2 A) but not to inulin (Stokes radium 20 A). These changes in host-cell membrane permeability and phospholipid synthesis were not observed in acutely infected H9 cells, which are not susceptible to HIV cytopathology. Thus, HIV-1 appeared to predominantly injure T cells by perturbing host-cell membrane permeability and lipid synthesis, which is similar to the cytopathic mechanisms of paramyxoviruses.