CD38/CD31 interactions activate genetic pathways leading to proliferation and migration in chronic lymphocytic leukemia cells

Human CD38 is a pleiotropic glycoprotein belonging to a family of enzymes/receptors involved in the catabolism of extracellular nucleotides. CD38-receptor activities are regulated through binding to the nonsubstrate ligand CD31. CD38 expression above a critical threshold is a negative prognostic marker for chronic lymphocytic leukemia (CLL) patients. Activation of CD38 by means of agonistic monoclonal antibodies or the CD31 ligand induces proliferation and immunoblast differentiation of CLL cells. Here we define the genetic signature that follows long-term in vitro interactions between CD38(+) CLL lymphocytes and CD31(+) cells. The emerging profile confirms that the CD31/CD38 axis activates genetic programs relevant for proliferative responses. It also indicates a contribution of this pathway to the processes mediating migration and homing. These results further support the notion that the CD31/CD38 axis is part of a network of accessory signals that modify the microenvironment, favoring localization of leukemic cells to growth-permissive sites.

The epithelial-mesenchymal transition induced by keratinocyte growth conditions is overcome by E6 and E7 from HPV16, but not HPV8 and HPV38: characterization of global transcription profiles

The aim of this study was to evaluate the growth properties of primary human keratinocytes expressing E6 and E7 proteins, which are from either the beta- or alpha-genotypes, under different culture conditions. We demonstrated that keratinocytes expressing E6 and E7, from both HPV8 and 38, irreversibly underwent the epithelial-mesenchymal transition (EMT) when grown on plastic with FAD medium (F12/DMEM/5%FBS). Expression of E6/E7 from HPV16 was capable of fully overcoming the FAD-induced EMT. Immortalization was only observed in HPV16-transduced cell lines, while the more proliferating phenotype of both KerHPV8 and 38 was mainly related to FAD-induced EMT. Microarray analysis of exponentially growing cells identified 146 cellular genes that were differentially regulated in HPV16 compared to HPV8- and 38-transduced cells. A large accumulation of transcripts associated with epidermal development and differentiation was observed in HPV16-transduced cells, whereas transcripts of genes involved in the extracellular matrix, multicellular organismal processes, and inflammatory response were affected in HPV8 and 38-transduced cells.

CD38 modulates CXCL12-mediated chemotaxis: a novel mechanism in homing of chronic lymphocytic leukemia cells (98.7)

Human CD38, an ectoenzyme involved in the catabolism of NAD, can also bind the non-substrate ligand CD31, resulting in activation and proliferation of distinct lymphocyte populations.

CD38 is clinically used as a negative prognostic marker for chronic lymphcytic leukemia (CLL) patients. We adopted CLL as a model to study the relevance of CD38 in vivo. CD38 signals trigger proliferation and increase survival of CLL cells. Further, the simultaneous expression of CD38 and ZAP-70 defines a genetically distinct subset of CLL cells with heightened responses to CXCL12, a critical chemokine in CLL homing from blood to lymphoid organs.

Here we show that CD38 is directly involved in CXCL12-mediated homing. CD38 signals facilitate short- (ERK1/2 phosphorylation) and long-term (chemotaxis) effects induced by CXCL12. Assays using i) a specific inhibitor of CD38 enzymatic activities, ii) agonistic mAbs, and iii) microarray technology indicate that increased homing depends on the receptor functions of CD38. Conversely, blocking anti-CD38 mAbs significantly weaken CXCL12 responses. This is due to physical association on the membrane between CD38 and CXCR4 (the CXCL12 receptor), as shown by i) co-immunoprecipitation, ii) confocal microscopy and iii) measurement of CXCL12 binding to its receptor. These results indicate that CD38 modulates CXCL12 signals, representing a novel mechanism in CLL chemotaxis and homing.

Analysis of gene expression in early-stage ovarian cancer

PURPOSE

Gene expression profile was analyzed in 68 stage I and 15 borderline ovarian cancers to determine if different clinical features of stage I ovarian cancer such as histotype, grade, and survival are related to differential gene expression.

EXPERIMENTAL DESIGN

Tumors were obtained directly at surgery and immediately frozen in liquid nitrogen until analysis. Glass arrays containing 16,000 genes were used in a dual-color assay labeling protocol.

RESULTS

Unsupervised analysis identified eight major patient partitions, one of which was statistically associated to overall survival, grading, and histotype and another with grading and histotype. Supervised analysis allowed detection of gene profiles clearly associated to histotype or to degree of differentiation. No difference was found between borderline and grade 1 tumors. As to recurrence, a subset of genes able to differentiate relapsers from nonrelapsers was identified. Among these, cyclin E and minichromosome maintenance protein 5 were found particularly relevant, as their expression was inversely correlated to progression-free survival (P = 0.00033 and 0.017, respectively).

CONCLUSIONS

Specific molecular signatures define different histotypes and prognosis of stage I ovarian cancer. Mucinous and clear cells histotypes can be distinguished from the others regardless of tumor grade. Cyclin E and minichromosome maintenance protein 5, whose expression was found previously to be related to a bad prognosis of advanced ovarian cancer, appear to be potential prognostic markers in stage I ovarian cancer too, independent of other pathologic and clinical variables.

Post-apoptotic tumors are more palatable to dendritic cells and enhance their antigen cross-presentation activity

Critical issues for cytotoxic lymphocyte (CTL) cross-priming are (a) the maturation state of dendritic cells (DC), (b) the source of the tumor-associated antigens (TAA) and (c) the context in which they are delivered to DCs. Drug-induced apoptosis has recently been implicated in CTL cross-priming. However, since drug-treatment produces in vivo more tumor cells than the DC default apoptotic clearance program can cope with, they are expected to proceed to secondary necrosis and change their molecular pattern. Here we have addressed this issue on renal carcinoma cells (RCC) by using different apoptotic stimuli. UVC, but not gamma-irradiation or anthracyclins, induced after 4h treatment of the RCC cell line K1 a combination of apoptotic (phosphatydilserine and calreticulin plasma membrane mobilization) and necrotic (membrane incompetence) features. Heat shock protein (Hsp)-70 and chromatin-bound high mobility box 1 HMGB1 protein, typical of necrosis, were released during the further 20h and thus made accessible to co-cultured monocyte-derived immature (i) DC. UVC-treated, secondary necrotic RCC cell lines were cross-presented with higher efficiency by cytokine-matured (m) DC than their early apoptotic (i.e. gamma-irradiated) counterpart. Upstream events such as increased tumor uptake, activation of genes involved in the antigen-processing machinery, and increased expression of costimulatory and maturation molecules were also observed after loading iDC with secondary necrotic, but not apoptotic, tumor cells. These data offer a description of the molecular and immunogenic characteristics of post-apoptotic tumors which can be exploited to increase the efficiency of in vivo and ex vivo TAA delivery to the DC cross-presentation pathway.

In melanocytic lesions the fraction of BRAF V600E alleles is associated with sun exposure but unrelated to ERK phosphorylation

BRAF(V600E) mutation has been frequently reported in different types of melanocytic lesions, but its role in melanomagenesis is poorly understood, having been associated with either the proliferative-induced MAPK pathway activation or the acquisition of oncogene-driven senescence. The presence of BRAF alterations has been related to sun exposure, although the molecular mechanisms underlying this event are only partly known. To elucidate the relationships among BRAF/NRAS alterations, MAPK pathway activation, and sun exposure, we examined 22 acquired nevi and 18 cutaneus melanomas from 38 patients. Microdissected tissues from each lesion were subjected to BRAF/NRAS mutation analysis by sequencing, allele-specific PCR and pyrosequencing assay. The same lesions were also examined for the expression of phosphorylated ERK1/2. Phototype and an accurate history of sun exposure were evaluated for each patient. BRAF(V600E) mutation was detected in 50% of the acquired nevi and in 70% of the cutaneus melanomas in the absence of NRAS alterations. The fraction of alleles carrying BRAF(V600E) substitution was variable but strongly associated with sun exposure. In contrast, no relationship was evidenced between the presence of this mutation and patients' phototype, phosphorylated ERK1/2 expression, or Clark's level. Our findings indicate that in melanocytic lesions, BRAF(V600E) mutation can affect a subset of the cells and is associated with the type and quantity of sun exposure. This mutation is independent of the nevo-melanoma progression and unrelated to ERK phosphorylation, suggesting that alternative mechanisms to the MAPK activation are also involved in this type of transformation.

Identification of novel vascular markers through gene expression profiling of tumor-derived endothelium

BACKGROUND

Targeting tumor angiogenesis and vasculature is a promising strategy for the inhibition of tumor growth and dissemination. Evidence suggests that tumor vasculature expresses unique markers that distinguish it from normal vasculature. Our efforts focused on the molecular characterization of endothelial cells (EC) in the search for selective markers of tumor vasculature that might be helpful for the development of effective therapeutic approaches.

RESULTS

We investigated by microarray analysis the gene expression profiles of EC purified and cultured from tumor (ovarian carcinoma [HOC-EC]) and normal (human adrenal gland [HA-EC]) tissue specimens. We found distinct transcriptional features characterizing the EC of different origin, and identified 158 transcripts highly expressed by HOC-EC. We analyzed four of these genes, ADAM23, FAP, GPNMB and PRSS3, which were not previously known to be expressed by endothelium. In vitro experiments confirmed the higher expression of the selected genes in tumor-derived endothelium with no expression in tumor cells. In vivo investigation by in situ hybridization established that ADAM23, GPNMB and PRSS3 expression is localized on blood vessels of human cancer specimens.

CONCLUSION

These findings elucidate some of the molecular features of the tumor endothelium. Comparative transcriptomic analysis allowed us to determine molecular differences of tumor and normal tissue-derived endothelium and to identify novel markers that might be exploited to selectively target tumor vasculature.

Telomere damage induced by the G-quadruplex ligand RHPS4 has an antitumor effect

Functional telomeres are required for the replicability of cancer cells. The G-rich strand of telomeric DNA can fold into a 4-stranded structure known as the G-quadruplex (G4), whose stabilization alters telomere function limiting cancer cell growth. Therefore, the G4 ligand RHPS4 may possess antitumor activity. Here, we show that RHPS4 triggers a rapid and potent DNA damage response at telomeres in human transformed fibroblasts and melanoma cells, characterized by the formation of several telomeric foci containing phosphorylated DNA damage response factors gamma-H2AX, RAD17, and 53BP1. This was dependent on DNA repair enzyme ATR, correlated with delocalization of the protective telomeric DNA-binding protein POT1, and was antagonized by overexpression of POT1 or TRF2. In mice, RHPS4 exerted its antitumor effect on xenografts of human tumor cells of different histotype by telomere injury and tumor cell apoptosis. Tumor inhibition was accompanied by a strong DNA damage response, and tumors overexpressing POT1 or TRF2 were resistant to RHPS4 treatment. These data provide evidence that RHPS4 is a telomere damage inducer and that telomere disruption selectively triggered in malignant cells results in a high therapeutic index in mice. They also define a functional link between telomere damage and antitumor activity and reveal the key role of telomere-protective factors TRF2 and POT1 in response to this anti-telomere strategy.

From single gene to integrative molecular concept MAPS: pitfalls and potentials of microarray technology

Microarray experiments have a large variety of applications and several important achievements have been obtained by means of this technology, especially within the field of whole genome expression profiling, which undoubtedly is the most diffused world-wide. Nevertheless, care must be taken in unconditionally applying such high-throughput techniques and in extracting/interpreting their results. Both the validity and the reproducibility of microarray-based clinical research have recently been challenged. Pitfalls and potentials of the microarray technology for gene expression profiling are critically reviewed in this paper.

CD38 and ZAP-70 are functionally linked and mark CLL cells with high migratory potential

Our interest in chronic lymphocytic leukemia (CLL) derives primarily from the exploitation of human diseases as strategic models for defining the in vivo biological roles of CD38. Using this model, we showed that CD38 triggers robust proliferation/survival signals modulated through the interactions with the CD31 ligand expressed by nurse-like cells and by the stromal/endothelial components. By analyzing a cohort of 56 patients with clinically and molecularly characterized CLL, we show that (1) patients with CD38(+)/ZAP-70(+) are characterized by enhanced migration toward Stromal derived factor-1alpha (SDF-1alpha)/CXCL12; (2) CD38 ligation leads to tyrosine phosphorylation of ZAP-70, showing that these markers are functionally linked; (3) ZAP-70 represents a limiting factor for the CD38 pathway in the CLL context, as shown by studying CD38-mediated signal transduction in 26 molecularly characterized patients; and (4) the CLL subgroup of patients defined on the basis of migratory potential is marked by a specific genetic signature, with a significant number of differentially expressed genes being involved in cell-cell interactions and movement. Altogether, the results of this work provide biological evidence for why the combined analysis of CD38 and ZAP-70 expression as determined in several clinical trials results in more dependable identification of patients with CLL who have aggressive disease.

Notch1 is a p53 target gene involved in human keratinocyte tumor suppression through negative regulation of ROCK1/2 and MRCKalpha kinases

Little is known about the regulation and function of the Notch1 gene in negative control of human tumors. Here we show that Notch1 gene expression and activity are substantially down-modulated in keratinocyte cancer cell lines and tumors, with expression of this gene being under p53 control in these cells. Genetic suppression of Notch signaling in primary human keratinocytes is sufficient, together with activated ras, to cause aggressive squamous cell carcinoma formation. Similar tumor-promoting effects are also caused by in vivo treatment of mice, grafted with keratinocytes expressing oncogenic ras alone, with a pharmacological inhibitor of endogenous Notch signaling. These effects are linked with a lesser commitment of keratinocytes to differentiation, an expansion of stem cell populations, and a mechanism involving up-regulation of ROCK1/2 and MRCKalpha kinases, two key effectors of small Rho GTPases previously implicated in neoplastic progression. Thus, the Notch1 gene is a p53 target with a role in human tumor suppression through negative regulation of Rho effectors.

Response to SDF-1 defines a subset of CD38+/ZAP-70+ CLL patients with a common genetic signature and aggressive behavior (96.23)

Chronic lymphocytic leukemia (CLL), traditionally considered a disease of failed apoptosis, is now seen as characterized by a different propensity to grow according to microenvironmental signals. CD38 (surface receptor) and ZAP-70 (cytoplasmic molecule) independently predict unfavorable outcome in CLL patients. We reported that CD38 is a receptor mediating interactions with nurse-like cells (NLC) and transducing proliferation signals. The production by NLC of the attractant SDF-1α and the presence of ZAP-70 in CXCR4 pathway suggested that CD38 and ZAP-70 control CLL cell migration. We show that CD38+/ZAP-70+ patients feature enhanced ability to respond to SDF-1α. The functional link between the two molecules was confirmed by showing that i) CD38 engagement leads to ZAP-70 phosphorylation in CLL cells and in ad hoc modified B line expressing a ZAP-70-myc construct. Further, ii) CD38 signals are effective only in presence of ZAP-70, as seen in 22 molecularly characterized patients.

Lastly, a separation of the patient cohort based on migration, brought to the identification of 23 differentially expressed genes: 12 are involved in cell motility, migration and neoplastic transformation.

This work provides biological evidence explaining why the combination of CD38 and ZAP-70 in routine clinics results in a more efficient identification of CLL patients with aggressive disease.

Sponsors: CLL-GRF, AIRC, FIRMS.

Cross-regulation between Notch and p63 in keratinocyte commitment to differentiation

Notch signaling promotes commitment of keratinocytes to differentiation and suppresses tumorigenesis. p63, a p53 family member, has been implicated in establishment of the keratinocyte cell fate and/or maintenance of epithelial self-renewal. Here we show that p63 expression is suppressed by Notch1 activation in both mouse and human keratinocytes through a mechanism independent of cell cycle withdrawal and requiring down-modulation of selected interferon-responsive genes, including IRF7 and/or IRF3. In turn, elevated p63 expression counteracts the ability of Notch1 to restrict growth and promote differentiation. p63 functions as a selective modulator of Notch1-dependent transcription and function, with the Hes-1 gene as one of its direct negative targets. Thus, a complex cross-talk between Notch and p63 is involved in the balance between keratinocyte self-renewal and differentiation.

c-Myc phosphorylation is required for cellular response to oxidative stress

Aside from the well-established roles of c-Myc in the regulation of cell cycle, differentiation, and apoptosis, a recent picture is beginning to emerge linking c-Myc to the regulation of metabolic pathways. Here, we define a further function for c-Myc in determining cellular redox balance, identifying glutathione (GSH) as the leading molecule mediating this process. The link between c-Myc and GSH is gamma-glutamyl-cysteine synthetase (gamma-GCS), the rate-limiting enzyme catalyzing GSH biosynthesis. Indeed, c-Myc transcriptionally regulates gamma-GCS by binding and activating the promoters of both gamma-GCS heavy and light subunits. Exposure to H2O2 enhances c-Myc recruitment to gamma-GCS regulatory regions through ERK-dependent phosphorylation. Phosphorylation at Ser-62 is required for c-Myc recruitment to gamma-GCS promoters and determines the cellular response to oxidative stress induced by different stimuli. Thus, the c-Myc phosphorylation-dependent activation of the GSH-directed survival pathway can contribute to oxidative stress resistance in tumor cells, which generally exhibit deregulated c-Myc expression.

Negative control of keratinocyte differentiation by Rho/CRIK signaling coupled with up-regulation of KyoT1/2 (FHL1) expression

Rho GTPases integrate control of cell structure and adhesion with downstream signaling events. In keratinocytes, RhoA is activated at early times of differentiation and plays an essential function in establishment of cell-cell adhesion. We report here that, surprisingly, Rho signaling suppresses downstream gene expression events associated with differentiation. Similar inhibitory effects are exerted by a specific Rho effector, CRIK (Citron kinase), which is selectively down-modulated with differentiation, thereby allowing the normal process to occur. The suppressing function of Rho/CRIK on differentiation is associated with induction of KyoT1/2, a LIM domain protein gene implicated in integrin-mediated processes and/or Notch signaling. Like activated Rho and CRIK, elevated KyoT1/2 expression suppresses differentiation. Thus, Rho signaling exerts an unexpectedly complex role in keratinocyte differentiation, which is coupled with induction of KyoT1/2, a LIM domain protein gene with a potentially important role in control of cell self renewal.

Molecular characterisation of two human cancer cell lines selected in vitro for their chemotherapeutic drug resistance to ET-743

ET-743 (Yondelis(TM), Trabectedin) isolated from the tunicate Ecteinascidia turbinata, is being tested in phase II clinical trials in Europe and the United States of America (USA). Studies with different solid tumours have shown antitumour activity in advanced, pre-treated sarcomas as well as in drug-resistant breast and ovarian cancer. The primary mechanism of action for ET-743 has not been fully elucidated and different models have been suggested to explain its molecular mechanism of action. ET-743 binds tightly to the minor groove of DNA and previous data have suggested that ET-743 acts by interfering with RNA transcription. To further investigate the mechanism of in vitro drug resistance, we evaluated the gene expression profile in ovarian and chondrosarcoma cell lines selected for resistance to ET-743. We found 70 genes whose expression was modulated in both drug-resistant cell lines when compared with their respective parental drug-sensitive cell lines. This pattern of gene expression seems to be selective for ET-743-resistant cells, since ovarian cancer cells resistant to paclitaxel did not share the same gene expression changes. Data presented in this study reveal different molecular pathways that could be involved in the cellular mechanism of ET-743 resistance.

Interpretation of expression-profiling results obtained from different platforms and tissue sources: examples using prostate cancer data

The analysis of expression signatures is a powerful tool for the classification of cancer and other tissue samples. Several protocols and platforms are available on the market, and these lead to both confirmatory and complementary results. We review the main processing techniques for cross-platform comparisons and the different tissue sources for cancer profiling. Some examples and the cross-interpretation of bibliographic data related to prostate cancer are also presented.

High commitment of embryonic keratinocytes to terminal differentiation through a Notch1-caspase 3 regulatory mechanism

Embryonic cells are expected to possess high growth/differentiation potential, required for organ morphogenesis and expansion during development. However, little is known about the intrinsic properties of embryonic epithelial cells due to difficulties in their isolation and cultivation. We report here that pure keratinocyte populations from E15.5 mouse embryos commit irreversibly to differentiation much earlier than newborn cells. Notch signaling, which promotes keratinocyte differentiation, is upregulated in embryonic keratinocyte and epidermis, and elevated caspase 3 expression, which we identify as a transcriptional Notch1 target, accounts in part for the high commitment of embryonic keratinocytes to terminal differentiation. In vivo, lack of caspase 3 results in increased proliferation and decreased differentiation of interfollicular embryonic keratinocytes, together with decreased activation of PKC-delta, a caspase 3 substrate which functions as a positive regulator of keratinocyte differentiation. Thus, a Notch1-caspase 3 regulatory mechanism underlies the intrinsically high commitment of embryonic keratinocytes to terminal differentiation.

Glutathione influences c-Myc-induced apoptosis in M14 human melanoma cells

The objective of this article is to dissect the mechanisms by which the down-regulation of c-Myc induces programmed cell death in melanoma cells. In stable and doxycycline-inducible M14 melanoma cells, down-regulation of c-Myc induced apoptosis subsequent to a decrease in the intracellular reduced glutathione content and a concomitant accumulation of its oxidized form. This redox alteration was associated with a decrease of the enzyme activities of gamma-glutamyl-cysteine synthetase and NADPH-dependent GSSG reductase, as well as a consequent glutathione release in the extracellular medium. Cytochrome c was released into the cytosol at very early stages of apoptosis induction, long before detectable production of reactive oxygen species and activation of caspase-9 and -3. Macroarray analysis revealed that down-regulation of c-Myc produced striking changes in gene expression in the section related to metabolism, where the expression of gamma-glutamyl-cysteine synthetase and GSSG reductase was found to be significantly reduced. The addition of N-acetyl-l-cysteine or glutathione ethyl ester inhibited the apoptotic process, thus confirming the key role of glutathione in programmed cell death induced by c-Myc.

Changes in gene expression profile induced by the anticancer agent Aplidine in Molt-4 leukemic cell lines

Microarray technique was employed to study differences in gene expression profile induced by Aplidine treatment in the Molt-4 human leukemic T cell line. Aplidine is a novel marine compound purified from caribbean tunicate (sea squirt) Aplidium Albicans. Despite promising anti-tumor activity, few data are available on its mechanism of action. Exponentially growing cells were treated with Aplidine concentrations close to its 5IC50 for 1 hour and RNA samples collected after 0.5, 1, 6 and 24 hours of recovery in drug free medium. 32P labelled cDNAs were hybridized against Atlas Human Cancer arrays onto which 588 cDNAs were spotted. Genes involved in different cellular pathways, (such as growth factors, signal transduction or transcription factors) were found modulated by the drug. Even if the data obtained in the present study cannot be conclusive, several hypothesis on Aplidine's mechanism of action are indicated that will be the subject of future studies.

Cell cycle phase perturbations and apoptosis in tumour cells induced by aplidine

Aplidine, dehydrodidemnin B, is a marine depsipeptide isolated from the Mediterranean tunicate Aplidium albicans currently in phase II clinical trial. In human Molt-4 leukaemia cells Aplidine was found to be cytotoxic at nanomolar concentrations and to induce both a G(1) arrest and a G(2) blockade. The drug-induced cell cycle perturbations and subsequent cell death do not appear to be related to macromolecular synthesis (protein, RNA, DNA) since the effects occur at concentrations (e.g. 10 nM) in which macromolecule synthesis was not markedly affected. Ten nM Aplidine for 1 h inhibited ornithine decarboxylase activity, with a subsequently strong decrease in putrescine levels. This finding has questionable relevance since addition of putrescine did not significantly reduce the cell cycle perturbations or the cytotoxicity of Aplidine. The cell cycle perturbations caused by Aplidine were also not due to an effect on the cyclin-dependent kinases. Although the mechanism of action of Aplidine is still unclear, the cell cycle phase perturbations and the rapid induction of apoptosis in Molt-4 cells appear to be due to a mechanism different from that of known anticancer drugs.

Variability in the timing of G(1)/S transition

Cell cycle duration and phase transition times are not fixed, even within homogeneous cell populations growing under optimal environmental conditions. We investigate G(1) phase variability from the molecular point of view and propose a mathematical approach to model the protein interactions regulating the transition from the G(1) phase to the phase of DNA synthesis. The mathematical model has some connections with flow cytometry experimental data.

Desynchronization rate in cell populations: mathematical modeling and experimental data

We characterize the kinetics of two cancer cell lines: IGROV1 (ovarian carcinoma) and MOLT4 (leukemia). By means of flow cytometry, we selected two populations from exponentially growing in vitro cell lines, depending on the cells' DNA synthesis activity during a preceding labeling period. For these populations we determined the time course of the percentages of cells in different phases of the cycles, sampling every 3 hr for 60 hr. Initially, semi-synchronous populations quickly converged to a stable age distribution, which is typical of the cell line (at equilibrium); this desynchronization reflects the intercell variability in cell cycle duration. By matching these experimental observations to mathematical modelling, we related the convergence rate toward the asymptotic distribution (R) and the period of the phase-percentage oscillations (T), to the mean cell cycle duration and its coefficient of variation. We give two formulas involving the above-mentioned parameters. Since T and R can be drawn by fitting our data to an asymptotic formula obtained from the model, we can estimate the other two kinetic parameters. IGROV1 cells have a shorter mean cell cycle time, but higher intercell variability than the leukemia line, which takes longer to lose synchrony.

Behavioral choices based on patch selection: a model using aggregation methods

The aim of this work is to study the influence of patch selection on the dynamics of a system describing the interactions between two populations, generically called 'population N' and 'population P'. Our model may be applied to prey-predator systems as well as to certain host-parasite or parasitoid systems. A situation in which population P affects the spatial distribution of population N is considered. We deal with a heterogeneous environment composed of two spatial patches: population P lives only in patch 1, while individuals belonging to population N migrate between patch 1 and patch 2, which may be a refuge. Therefore they are divided into two patch sub-populations and can migrate according to different migration laws. We make the assumption that the patch change is fast, whereas the growth and interaction processes are slower. We take advantage of the two time scales to perform aggregation methods in order to obtain a global model describing the time evolution of the total populations, at a slow time scale. At first, a migration law which is independent on population P density is considered. In this case the global model is equivalent to the local one, and under certain conditions, population P always gets extinct. Then, the same model, but in which individuals belonging to population N leave patch 1 proportionally to population P density, is studied. This particular behavioral choice leads to a dynamically richer global system, which favors stability and population coexistence. Finally, we study a third example corresponding to the addition of an aggregative behavior of population N on patch 1. This leads to a more complicated situation in which, according to initial conditions, the global system is described by two different aggregated models. Under certain conditions on parameters a stable limit cycle occurs, leading to periodic variations of the total population densities, as well as of the local densities on the spatial patches.