Abstracts 2007

Abstract : To optimize the temporal patterning of drug delivery used in cancer chronotherapy, we resort to an automaton model describing the transitions through the successive phases of the cell cycle. The model accounts for the progressive desynchronization of cells due to the variability of the durations of the cell cycle phases, and for the entrainment of the cell cycle by the circadian clock. Focusing on the cytotoxic effect of the anticancer drug 5-fluorouracil (5-FU), which kills cells in the S phase, we compare the effect of continuous infusion of 5-FU with various circadian patterns of 5-FU administration that peak either at 4 a.m., 10 a.m., 4 p.m., or 10 p.m. The model indicates that the cytotoxic effect of 5-FU is minimum for the circadian delivery peaking at 4 a.m., and maximum for the continuous infusion or the circadian pattern peaking at 4 p.m. These results fit well with experimental observations and illustrate how the modeling approach based on the cell cycle automaton may help to predict the cytotoxic effect of anticancer drugs affecting various phases of the cell cycle.

Abstract : To optimize the temporal patterning of drug delivery used in cancer chronotherapy, we resort to an automaton model describing the transitions through the successive phases of the cell cycle. The model accounts for the progressive desynchronization of cells due to the variability of the durations of the cell cycle phases, and for the entrainment of the cell cycle by the circadian clock. Focusing on the cytotoxic effect of 5-fluorouracil (5-FU), which kills cells exposed to this anticancer drug during the S phase, we compare the effect of continuous infusion of 5-FU with various circadian patterns of 5-FU administration that peak either at 4 a.m., 10 a.m., 4 p.m., or 10 p.m. The model indicates that the cytotoxic effect of 5-FU is minimum for the circadian delivery peaking at 4 a.m., and maximum for the continuous infusion or the circadian pattern peaking at 4 p.m. These results fit well with experimental observations and illustrate how the modeling approach based on the cell cycle automaton may help to predict the differential cytotoxic effect of chronomodulated chemotherapy on host and tumor cells.

Abstract : Many essential physiological processes are controlled by calcium. To ensure reliability and specificity, calcium signals are highly organized in time and space in the form of oscillations and waves. Interesting findings have been obtained at various scales, ranging from the stochastic opening of a single calcium channel to the intercellular calcium wave spreading through an entire organ. A detailed understanding of calcium dynamics thus requires a link between observations at different scales. It appears that some regulations such as calcium-induced calcium release or PLC activation by calcium, as well as the weak diffusibility of calcium ions play a role at all levels of organization in most cell types. To comprehend how calcium waves spread from one cell to another, specific gap-junctional coupling and paracrine signaling must also be taken into account. On the basis of a pluridisciplinar approach ranging from physics to physiology, a unified description of calcium dynamics is emerging, which could help understanding how such a small ion can mediate so many vital functions in living systems.

Abstract : At intermediate intensities, stress induces oscillations in the nucleocytoplasmic shuttling of the transcription factor Msn2 in budding yeast. Activation by stress results in a reversible translocation of Msn2 from the cytoplasm to the nucleus. This translocation is negatively controlled by the cAMP-PKA pathway through Msn2 phosphorylation. Here we show that the nuclear localization signal (NLS) of Msn2 is necessary and sufficient to promote the nucleocytoplasmic oscillations of the transcription factor. Because the NLS is controlled by protein kinase A (PKA) phosphorylation, we use a computational model to investigate the possibility that the cAMP-PKA pathway could function as an oscillator driving the periodic shuttling of Msn2. The model indicates that sustained oscillations of cAMP can indeed occur in a range bounded by two critical values of stress intensity, owing to the negative feedback exerted by PKA on cAMP accumulation. We verify the predictions of the model in mutants by showing that suppressing this negative-feedback loop prevents the oscillatory shuttling but still promotes the stress-induced nuclear localization of Msn2. The physiological significance of Msn2 oscillations is discussed in the light of the frequency encoding of cellular rhythms

Abstract : The establishment of thresholds along morphogen gradients in the embryo is poorly understood. Using mathematical modeling, we show that mutually inhibitory gradients can generate and position sharp morphogen thresholds in the embryonic space. Taking vertebrate segmentation as a paradigm, we demonstrate that the antagonistic gradients of retinoic acid (RA) and Fibroblast Growth Factor (FGF) along the presomitic mesoderm (PSM) may lead to the coexistence of two stable steady states. Here, we propose that this bistability is associated with abrupt switches in the levels of FGF and RA signaling, which permit the synchronized activation of segmentation genes, such as mesp2, in successive cohorts of PSM cells in response to the segmentation clock, thereby defining the future segments. Bistability resulting from mutual inhibition of RA and FGF provides a molecular mechanism for the all-or-none transitions assumed in the "clock and wavefront" somitogenesis model. Given that mutually antagonistic signaling gradients are common in development, such bistable switches could represent an important principle underlying embryonic patterning.

Abstract : Benzene polyphosphates containing phosphate groups on one ring are Ins(1,4,5)P₃ 5-phosphatase inhibitors when evaluated against type-I Ins(1,4,5)P₃ 5-phosphatase. A novel biphenyl derivative, biphenyl 2,3',4,5',6-pentakisphosphate, with five phosphate groups on two rings was synthesized: It inhibited the activity of two inositol 5-phosphatases: type I and SHIP2 with Ins(1,3,4,5)P₄ as substrate. The inhibition was competitive with respect to the substrate. IC50 value measured in rat hepatocytes, which contains the native Ins(1,4,5)P₃ 5-phosphatase, was in the micromolar range at 1.0 M Ins(1,4,5)P₃ as substrate. Biphenyl 2,3',4,5',6-pentakisphosphate did not affect the activity of Ins(1,4,5)P₃ 3-kinase A in the 5­100 M range. Surprisingly, experimental evidence supports an effect of biphenyl 2,3',4,5',6-pentakisphosphate at the level of the Ins(1,4,5)P₃ receptor. Finally, when injected into rat hepatocytes, the analog affected the frequency of Ca2 oscillations in a positive or negative way depending on its concentration. At very high concentrations of the analog, Ca²⁺ oscillations were even suppressed. These data were interpreted as a dual effect of the biphenyl 2,3',4,5',6-pentakisphosphate on cytosolic [Ca2 ] increases: an activation effect through an increase in Ins(1,4,5)P₃ level via Ins(1,4,5)P₃ 5-phosphatase inhibition and an inhibitory effect, which was exerted directly on the Ins(1,4,5)P₃ receptor. Thus, our data show for the first time that the frequency of Ca2 oscillations in response to a Ca²⁺ -mobilizing agonist can be controlled by inhibitors of type-I Ins(1,4,5)P₃ 5-phosphatase.‹ Vandeput, F., Combettes, L., Mills, S. J., Backers, K., Wohlkönig, A., Parys, J. B., De Smedt, H., Missiaen, L., Dupont, G., Potter, B. V. L., Erneux, C. Biphenyl 2,3',4,5',6-pentakisphosphate, a novel inositol polyphosphate surrogate, modulates Ca²⁺ responses in rat hepatocytes. FASEB J. 21, 000-000 (2007)