Coupling of proadipocyte growth arrest and differentiation. II. A cell cycle model for the physiological control of cell proliferation

Experimental evidence is presented that supports a cell cycle model showing that there are five distinct biological processes involved in proadipocyte differentiation. These include: (a) growth arrest at a distinct state in the G1 phase of the cell cycle; (b) nonterminal differentiation; (c) terminal differentiation; (d) loss of the differentiated phenotype; and (e) reinitiation of cell proliferation. Each of these events is shown to be regulated by specific human plasma components or other physiological factors. At two states designated GD and GD', coupling of growth arrest and differentiation is shown to occur. We propose that these mechanisms for the coupling of growth arrest and differentiation are physiologically significant and mimic the regulatory processes that control stem cell proliferation in vivo.     

Cell-cycle-regulatory elements and the control of cell differentiation in the budding yeast

The stable differentiation of cells into other cell types typically involves dramatic reorganization of cellular structures and functions. This often includes remodeling of the cell cycle and the apparatus that controls it. Here we review our understanding of the role and regulation of cell cycle control elements during cell differentiation in the yeast, Saccharomyces cerevisiae. Although the process of differentiation may be more overtly obvious in metazoan organisms, those systems are by nature more difficult to study at a mechanistic level. We consider the relatively well-understood mechanisms by which mating-type switching and the pheromone-induced differentiation of gametes are coupled to the cell cycle as well as the more obscure mechanisms that govern the remodeling of the cell cycle during meiosis and filamentous growth. In some cases, the cell cycle is a primary stimulus for differentiation whereas, in other cases, the signals that promote differentiation alter the cell cycle. Thus, despite relative simplicity of these processes in yeast, the nature of the interplay between the cell cycle and differentiation is diverse.     

Pathocycles: Ustilago maydis as a model to study the relationships between cell cycle and virulence in pathogenic fungi

Activation of virulence in pathogenic fungi often involves differentiation processes that need the reset of the cell cycle and induction of a new morphogenetic program. Therefore, the fungal capability to modify its cell cycle constitutes an important determinant in carrying out a successful infection. The dimorphic fungus Ustilago maydis is the causative agent of corn smut disease and has lately become a highly attractive model in addressing fundamental questions about development in pathogenic fungi. The different morphological and genetic changes of U. maydis cells during the pathogenic process advocate an accurate control of the cell cycle in these transitions. This is why this model pathogen deserves attention as a powerful tool in analyzing the relationships between cell cycle, morphogenesis, and pathogenicity. The aim of this review is to summarize recent advances in the unveiling of cell cycle regulation in U. maydis. We also discuss the connection between cell cycle and virulence and how cell cycle control is an important downstream target in the fungus-plant interaction.     

Terminal Myeloid Differentiation is Uncoupled from Cell Cycle Arrest

It has been assumed that terminal myeloid differentiation and cell cycle arrest are coupled processes, and that prohibiting cell cycle arrest blocks differentiation. Previously we have shown that, using the murine M1 myeloid leukemic cell line, deregulated expression of the proto-oncogene c-myc results in cells that cannot be induced to undergo terminal differentiation and continued to proliferate. It has also been shown that ectopic expression of Egr-1 abrogated the c-Myc block in terminal myeloid differentiation, yet there was no accumulation of cells in the G0/G1 phase of the cell cycle. In this study we conclusively demonstrate that M1Myc/Egr-1 cells terminally differentiate while still actively cycling and synthesizing DNA, concluding that the terminal myeloid differentiation program is uncoupled from growth arrest. How deregulated expression/activation of proto-oncogenes that promote cell cycle progression interferes with differentiation and how differentiation is regulated independently of cell cycle control are discussed, as well as the implications with regard to differentiation therapy.     

Mathematical Model of Acute Myeloblastic Leukaemia: an Investigation of the Relevant Kinetic Parameters

A simple model of acute myeloblastic leukaemia (AML) development is introduced, explicitly including cell growth, cell differentiation and cell-cell interaction. Each of these processes is described by a single model parameter. It is hypothesized that the leukaemic cell is characterized by an alteration of only one of these processes. the kinetic behaviour of the AML system is examined separately for possible alterations of each of the three parameters describing the three processes involved. It is shown that, on the basis of the existing data on AML kinetics, the alteration of the growth and cell-cell interaction parameters can be eliminated as a possible source of AML. Thus kinetic data support the modification of the differentiation process as the origin of the AML state. Further, the growth characteristics of normal and leukaemic cells in the presence of each other are analysed. It is shown how the initial growth of leukaemic cells depends on the difference in the differentiation of normal and leukaemic cells and how the same difference determines the decay of normal cells in the presence of the predominantly leukaemic population. Correlations between the kinetic parameters of the normal and leukaemic populations are suggested to characterize the leukaemic state.     

Mathematical model of acute myeloblastic leukaemia: an investigation of the relevant kinetic parameters

A simple model of acute myeloblastic leukaemia (AML) development is introduced, explicitly including cell growth, cell differentiation and cell-cell interaction. Each of these processes is described by a single model parameter. It is hypothesized that the leukaemic cell is characterized by an alteration of only one of these processes. The kinetic behaviour of the AML system is examined separately for possible alterations of each of the three parameters describing the three processes involved. It is shown that, on the basis of the existing data on AML kinetics, the alteration of the growth and cell-cell interaction parameters can be eliminated as a possible source of AML. Thus kinetic data support the modification of the differentiation process as the origin of the AML state. Further, the growth characteristics of normal and leukaemic cells in the presence of each other are analysed. It is shown how the initial growth of leukaemic cells depends on the difference in the differentiation of normal and leukaemic cells and how the same difference determines the decay of normal cells in the presence of the predominantly leukaemic population. Correlations between the kinetic parameters of the normal and leukaemic populations are suggested to characterize the leukaemic state.     

Heterocyst differentiation and pattern formation in cyanobacteria: a chorus of signals

Heterocyst differentiation in filamentous cyanobacteria provides an excellent prokaryotic model for studying multicellular behaviour and pattern formation. In Anabaena sp. strain PCC 7120, for example, 5-10% of the cells along each filament are induced, when deprived of combined nitrogen, to differentiate into heterocysts. Heterocysts are specialized in the fixation of N(2) under oxic conditions and are semi-regularly spaced among vegetative cells. This developmental programme leads to spatial separation of oxygen-sensitive nitrogen fixation (by heterocysts) and oxygen-producing photosynthesis (by vegetative cells). The interdependence between these two cell types ensures filament growth under conditions of combined-nitrogen limitation. Multiple signals have recently been identified as necessary for the initiation of heterocyst differentiation, the formation of the heterocyst pattern and pattern maintenance. The Krebs cycle metabolite 2-oxoglutarate (2-OG) serves as a signal of nitrogen deprivation. Accumulation of a non-metabolizable analogue of 2-OG triggers the complex developmental process of heterocyst differentiation. Once heterocyst development has been initiated, interactions among the various components involved in heterocyst differentiation determine the developmental fate of each cell. The free calcium concentration is crucial to heterocyst differentiation. Lateral diffusion of the PatS peptide or a derivative of it from a developing cell may inhibit the differentiation of neighbouring cells. HetR, a protease showing DNA-binding activity, is crucial to heterocyst differentiation and appears to be the central processor of various early signals involved in the developmental process. How the various signalling pathways are integrated and used to control heterocyst differentiation processes is a challenging question that still remains to be elucidated.     

Patterns of Cell Division,Cell Differentiation and Cell Elongation in Epidermis and Cortex of Arabidopsis pedicels in the Wild Type and in erecta

Plant organ shape and size are established during growth by a predictable, controlled sequence of cell proliferation, differentiation, and elongation. To understand the regulation and coordination of these processes, we studied the temporal behavior of epidermal and cortex cells in Arabidopsis pedicels and used computational modeling to analyze cell behavior in tissues. Pedicels offer multiple advantages for such a study, as their growth is determinate, mostly one dimensional, and epidermis differentiation is uniform along the proximodistal axis. Three developmental stages were distinguished during pedicel growth: a proliferative stage, a stomata differentiation stage, and a cell elongation stage. Throughout the first two stages pedicel growth is exponential, while during the final stage growth becomes linear and depends on flower fertilization. During the first stage, the average cell cycle duration in the cortex and during symmetric divisions of epidermal cells was constant and cells divided at a fairly specific size. We also examined the mutant of ERECTA, a gene with strong influence on pedicel growth. We demonstrate that during the first two stages of pedicel development ERECTA is important for the rate of cell growth along the proximodistal axis and for cell cycle duration in epidermis and cortex. The second function of ERECTA is to prolong the proliferative phase and inhibit premature cell differentiation in the epidermis. Comparison of epidermis development in the wild type and erecta suggests that differentiation is a synchronized event in which the stomata differentiation and the transition of pavement cells from proliferation to expansion are intimately connected.     

Wnt and FGF signals interact to coordinate growth with cell fate specification during limb development

A fundamental question in developmental biology is how does an undifferentiated field of cells acquire spatial pattern and undergo coordinated differentiation? The development of the vertebrate limb is an important paradigm for understanding these processes. The skeletal and connective tissues of the developing limb all derive from a population of multipotent progenitor cells located in its distal tip. During limb outgrowth, these progenitors segregate into a chondrogenic lineage, located in the center of the limb bud, and soft connective tissue lineages located in its periphery. We report that the interplay of two families of signaling proteins, fibroblast growth factors (FGFs) and Wnts, coordinate the growth of the multipotent progenitor cells with their simultaneous segregation into these lineages. FGF and Wnt signals act together to synergistically promote proliferation while maintaining the cells in an undifferentiated, multipotent state, but act separately to determine cell lineage specification. Withdrawal of both signals results in cell cycle withdrawal and chondrogenic differentiation. Continued exposure to Wnt, however, maintains proliferation and re-specifies the cells towards the soft connective tissue lineages. We have identified target genes that are synergistically regulated by Wnts and FGFs, and show how these factors actively suppress differentiation and promote growth. Finally, we show how the spatial restriction of Wnt and FGF signals to the limb ectoderm, and to a specialized region of it, the apical ectodermal ridge, controls the distribution of cell behaviors within the growing limb, and guides the proper spatial organization of the differentiating tissues.     

A mechanism of intracellular timing and its cooperation with extracellular signals in controlling cell proliferation and differentiation, an amended hypothesis.

Various observations suggest that an intracellular timer is involved in the control of cell proliferation and differentiation that supplements control by extracellular signaling and depends on quantitative relations between cytoplasm and nucleus. To further elucidate the mechanism of this timer, we examined the results of experiments with mice in which cell cycle regulating genes were inactivated: the inactivation of negative cell cycle regulators extends cell proliferation, whereas inactivation of positive regulators decreases cell proliferation. We conclude that this is caused in the former case by shortening of G1 which decreases the cytoplasmic growth rate per cell cycle, whereas in the latter case this rate is increased due to G1 prolongation. This is consistent with our hypothesis according to which the cytoplasmic/nuclear ratio must increase to a certain level to induce end stage differentiation and cell cycle arrest. A new basis of this hypothesis is the fact that end stage differentiation requires large quantities of membranous cytoplasmic structures that the cells are unable to produce de novo. Embryonic cells, however, possess only few of these structures. The only feasible way to multiply these structures is by growing more cytoplasm per cell cycle than needed for a doubling so that successively, the level of the cytoplasmic/nuclear ratio is reached that is required for differentiation. A consequence is that the cytoplasmic growth rate per cell cycle determines the number of amplification divisions. We suggest that the differentiation signal may be triggered when a differentiation-preventing protein (for example Bcl-2) is diluted out by the expansion of cytoplasmic membrane structures, thus simultaneously determining the cell size. The intracellular timer and extracellular signals cooperate in adjusting cell production to the organism's need and in determining when and how the cells respond to extracellular signals or transmit extracellular signals.     

Plant development meets cell proliferation in Madrid.

Cell division is intimately intertwined with plant development, and the mechanisms that link the control of cell proliferation and differentiation with the processes of organogenesis, morphogenesis, and growth are starting to be understood. A recent Juan March meeting explored this interface, and revealed a rich seam of exciting work that is leading toward an integrated view of the role of cell proliferation in the unfolding of developmental programs.     

Decreased neoblast progeny and increased cell death during starvation-induced planarian degrowth

The development of a complex multicellular organism requires a careful coordination of growth, cell division, cell differentiation and cell death. All these processes must be under intricate and coordinated control, as they have to be integrated across all tissues. Freshwater planarians are especially plastic, in that they constantly replace somatic tissues from a pool of adult somatic stem cells and continuously undergo growth and degrowth as adult animals in response to nutrient availability. During these processes they appear to maintain perfect scale of tissues and organs. These life history traits make them an ideal model system to study growth and degrowth. We have studied the unique planarian process of degrowth. When food is not available, planarians are able to degrow to a minimum size, without any signs of adverse physiological outcomes. For example they maintain full regenerative capacity. Our current knowledge of how this is regulated at the molecular and cellular level is very limited. Planarian degrowth has been reported to result from a decrease in cell number rather than a decrease in cell size. Thus one obvious explanation for degrowth would be a decrease in stem cell proliferation. However evidence in the literature suggests this is not the case. We show that planarians maintain normal basal mitotic rates during degrowth but that the number of stem cell progeny decreases during starvation and degrowth. These observations are reversed upon feeding, indicating that they are dependent on nutritional status. An increase in cell death is also observed during degrowth, which is not rapidly reversed upon feeding. We conclude that degrowth is a result of cell death decreasing cell numbers and that the dynamics of neoblast self-renewal and differentiation adapt to nutrient conditions to allow maintenance of the neoblast population during the period of starvation.     

Derangement of growth and differentiation control in oncogenesis.

Human neoplasms develop following the progressive accumulation of genetic and epigenetic alterations to oncogenes and tumor suppressor genes. These alterations confer a growth advantage to the cancer cell, leading to its clonal proliferation, invasion into surrounding tissues, and spread to distant organs. Genes that are altered in neoplasia affect three major biologic pathways that normally regulate cell growth and tissue homeostasis: the cell cycle, apoptosis, and differentiation. While each of these pathways can be defined by a unique set of molecular events, they are not biologically separate. Rather, they function more as an integrated molecular network, and perturbations in one pathway can have profound consequences on another. Insights into what distinguishes the regulation of growth and differentiation in a normal cell versus a cancer cell have led to the development of novel anticancer therapies.     

The EGF-like homeotic protein dlk affects cell growth and interacts with growth-modulating molecules in the yeast two-hybrid system

Levels of dlk, an EGF-like homeotic protein, are critical for several differentiation processes. Because growth and differentiation are, in general, exclusive of each other, and increasing evidence indicates that Dlk1 expression changes in tumorigenic processes, we studied whether dlk could also affect cell growth. We found that, in response to glucocorticoids, Balb/c 3T3 cells with diminished levels of dlk expression develop foci-like cells that have lost contact inhibition, display altered morphology, and grow faster than control cell lines. Balb/c 3T3 cells spontaneously growing more rapidly are also dlk-negative cells. Moreover, screening by the yeast two-hybrid system, using Dlk1 constructs as baits, resulted in the isolation of GAS1 and acrogranin cDNAs. Interestingly, these proteins are cysteine-rich molecules involved in the control of cell growth. Taken together, these observations suggest that dlk may participate in a network of interactions controlling how the cells respond to growth or differentiation signals.     

Insulin receptor-mediated signaling via phospholipase C-γ regulates growth and differentiation in Drosophila

Coordination between growth and patterning/differentiation is critical if appropriate final organ structure and size is to be achieved. Understanding how these two processes are regulated is therefore a fundamental and as yet incompletely answered question. Here we show through genetic analysis that the phospholipase C-γ (PLC-γ) encoded by small wing (sl) acts as such a link between growth and patterning/differentiation by modulating some MAPK outputs once activated by the insulin pathway; particularly, sl promotes growth and suppresses ectopic differentiation in the developing eye and wing, allowing cells to attain a normal size and differentiate properly. sl mutants have previously been shown to have a combination of both growth and patterning/differentiation phenotypes: small wings, ectopic wing veins, and extra R7 photoreceptor cells. We show here that PLC-γ activated by the insulin pathway participates broadly and positively during cell growth modulating EGF pathway activity, whereas in cell differentiation PLC-γ activated by the insulin receptor negatively regulates the EGF pathway. These roles require different SH2 domains of PLC-γ, and act via classic PLC-γ signaling and EGF ligand processing. By means of PLC-γ, the insulin receptor therefore modulates differentiation as well as growth. Overall, our results provide evidence that PLC-γ acts during development at a time when growth ends and differentiation begins, and is important for proper coordination of these two processes.