Rho regulation: DLC proteins in space and time

Rho GTPases function as molecular switches that connect changes of the external environment to intracellular signaling pathways. They are active at various subcellular sites and require fast and tight regulation to fulfill their role as transducers of extracellular stimuli. New imaging technologies visualizing the active states of Rho proteins in living cells elucidated the necessity of precise spatiotemporal activation of the GTPases. The local regulation of Rho proteins is coordinated by the interaction with different guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that turn on and off GTPase signaling to downstream effectors. GEFs and GAPs thus serve as critical signaling nodes that specify the amplitude and duration of a particular Rho signaling pathway. Despite their importance in Rho regulation, the molecular aspects underlying the spatiotemporal control of the regulators themselves are still largely elusive. In this review we will focus on the Deleted in Liver Cancer (DLC) family of RhoGAP proteins and summarize the evidence gathered over the past years revealing their different subcellular localizations that might account for isoform-specific functions. We will also highlight the importance of their tightly controlled expression in the context of neoplastic transformation.

The Rho-specific GAP protein DLC3 coordinates endocytic membrane trafficking

Membrane trafficking is known to be coordinated by small GTPases, but the identity of their regulators, the guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) that ensure balanced GTPase activation at different subcellular sites is largely elusive. Here, we show in living cells that deleted in liver cancer 3 (DLC3, also known as STARD8) is a functional Rho-specific GAP protein, the loss of which enhances perinuclear RhoA activity. DLC3 is recruited to Rab8-positive membrane tubules and is required for the integrity of the Rab8 and Golgi compartments. Depletion of DLC3 impairs the transport of internalized transferrin to the endocytic recycling compartment (ERC), which is restored by the simultaneous downregulation of RhoA and RhoB. We further demonstrate that DLC3 loss interferes with epidermal growth factor receptor (EGFR) degradation associated with prolonged receptor signaling. Taken together, these findings identify DLC3 as a novel component of the endocytic trafficking machinery, wherein it maintains organelle integrity and regulates membrane transport through the control of Rho activity.

[Oscar Vulpius and tendon transfer in the hand]

Oscar Vulpius dealt extensively with the problem of tendon transfer for motoric deficits. An analysis of his works shows that the knowledge of tendon healing, aftercare and operative techniques was already very widely advanced at the beginning of the last century.

The RhoGAP protein Deleted in Liver Cancer 3 (DLC3) is essential for adherens junctions integrity

Epithelial cell-cell contacts are mediated by E-cadherin interactions, which are regulated by the balanced local activity of Rho GTPases. Despite the known function of Rho at adherens junctions (AJs), little is known about the spatial control of Rho activity at these sites. Here we provide evidence that in breast epithelial cells the Deleted in Liver Cancer 3 (DLC3) protein localizes to AJs and is essential for E-cadherin function. DLC3 is a still poorly characterized RhoA-specific GTPase-activating protein that is frequently downregulated in various types of cancer. We demonstrate that DLC3 depletion leads to mislocalization of E-cadherin and catenins, which was associated with impaired cell aggregation and increased migration. This is explained by aberrant local Rho signaling because ROCK inhibition restored cell-cell contacts in DLC3 knockdown cells. We thus identify DLC3 as a novel negative regulator of junctional Rho and propose that DLC3 loss contributes to carcinogenesis by compromising epithelial integrity.

Studies on the recovery of crown gall tumor cells

Our previous studies have shown that, while a persistent but potentially reversible suppression of the tumorous state appears to be a characteristic feature of the vegetative phase of teratoma shoot growth in the crown gall disease of plants, a recovery from that state occurs during the reproductive phase. An analysis has now been made of the reproductive process in an attempt to define the precise stage at which recovery occurs. The results of this analysis have shown that diploid somatic cells of teratoma-derived flower parts such as those found in petals and filaments are inherently neoplastic. On the other hand, haploid cells of plants grown from anthers of the same flowers and diploid cells of F(1) generation plants grown from teratoma-derived seed have, by generally accepted criteria, recovered from the tumorous state. These findings have been interpreted to mean that the loss of neoplastic properties occurs in crown gall teratoma cells during meiosis rather than during fertilization or later stages of the reproductive process.An analysis of more than 2000 teratoma-derived tumor shoots has shown, moreover, that a recovery from the tumorous state may also occur, although apparently as a very rare event, during the vegetative phase of teratoma shoot growth.

8-bromoadenosine 3':5'-cyclic monophosphate as a promoter of cell division in excised tobacco pith parenchyma tissue

8-Bromoadenosine 3':5'-cyclic monophosphate, when used in association with an auxin, can completely replace the cell-division-promoting activity of either a cytokinesin or a 6-substituted adenylyl cytokinin in excised tobacco pith parenchyma tissue. The 8-bromo derivative of adenosine 3':5'-cyclic monophosphate was found to be far more resistant to degradation by plant adenosine 3':5'-cyclic monophosphate phosphodiesterases than was adenosine 3':5'-cyclic monophosphate. These findings appear to provide further support for the suggestion made earlier that the cytokinesins, which are potent inhibitors of both plant and animal adenosine 3':5'-cyclic monophosphate phosphodiesterases, exert their cell-division-promoting effects as regulators of adenosine 3':5'-cyclic monophosphate.

Regulation of human MC2-R gene expression by CREB, CREM, and ICER in the adrenocortical cell line Y1

The MC2-Receptor (melanocortin 2 receptor, MC2-R) is a Gs-protein coupled receptor that is upregulated by its own ligand ACTH and by forskolin. The mechanisms regulating MC2-R expression are still unclear. We therefore investigated the role of the stimulatory transcription factors CREB and CREM and the inhibitory factor ICER for regulation of human MC2-R expression. We cotransfected mouse adrenocortical Y1 cells with luciferase reporter gene vectors containing full length and deleted human MC2-R promoter constructs with expression plasmids for CREB, CREBS133A, CREMtau, CREMtauS117A, or ICER. Direct protein-DNA interaction was investigated by EMSA. Wild type CREB did not significantly affect promoter activity due to high endogenous CREB activity. However, CREBS133A decreased forskolin stimulated MC2-R promoter activity by 48+/-5% (mean+/-SEM) while unstimulated values remained unchanged. CREMtau moderately increased basal and forskolin stimulated luciferase activity in a dose-dependent manner (maximum effect 252+/-24% and 186+/-13% VS. control vector, respectively). While this effect required the full length promoter, cAMP stimulation was retained in shorter constructs. ICER reduced basal luciferase activity in Y1 cells by 17+/-28%, but completely abolished forskolin stimulation. Although 5'-deletion constructs mapped the minimum promoter region required for ICER effect to the shortest -64/+40 construct, direct protein DNA interaction in this promoter region could not be identified by EMSA. Moreover, mutation of the SF-1 binding sites, which retained ICER dependent inhibition, excluded SF-1 to be required for this effect. We conclude from these data that transcription factors of the CREB/CREM/ATF family have a moderate effect on human MC2-R promoter activity, but seem to play a minor role in transmitting stimulation of the cAMP pathway to increased MC2-R expression.

[Ender nailing of per and subtrochanteric fractures. Results and complications]

An evaluation of 84 trochanteric and subtrochanteric fractures, treated with Ender pins in the Department of Orthopaedic Surgery (University of Heidelberg) proved to be of instant success in 66 (78%) cases. A reoperation had to be performed in 10 (12%) cases, in 3 of these reoperation the pins had to be redriven. The use of Ender pins failed in 7 patients, a total hip arhtroplasty became necessary. The mortality was 14%. A third of the cases showed an external malrotation which, however, is of no relevance in older people. The intermedullary fixation with Ender pins is a useful operative procedure. Stable internal fixation can be achieved even in sever trochanteric ans subtrochanteric fractures. In case the form of the fracture, the age and the general condition of the patient are satisfactory, a total hip arthroplasty should be considered.

An epigenetic model for the origin of cancer

A unifying concept that appears to provide an understanding of cancer as a fundamental scientific problem is presented. This concept, which was initially developed on the basis of experiments using the relatively uncomplicated plant tumor systems, now appears applicable to animal and human tumors as well. Evidence is provided to show that the tumor problem is fundamentally a problem of anomalous cellular differentiation and that the heritable cellular change that underlies the tumorous state is similar to that which underlies cell specialization occurring during the normal course of development in all higher organisms. Both cellular differentiation and tumorigenesis depend for their expression on the persistent activation of select but, in part, different genes (whether normal, foreign, or both) present in the genome of a cell. Since heritable cellular changes of this kind may be induced by physical, chemical, and biological agents of the most diverse type, and since cells may remain totipotent during the time that they exhibit the tumor phenotype, the results reported here suggest that whether the normal or tumor phenotype is expressed is determined by how the genetic information present in a cell is regulated in the cell. Regulation leading to the establishment and maintenance of the tumorous state may be accomplished in different ways by the different types of oncogenic agents. Thus cancer and related neoplastic diseases appear to have a common underlying heritable cellular change in which the diverse manifestations of the tumorous state commonly observed would simply reflect different expressions of this heritable cellular change.

Genetic and biochemical studies on the suppression of and a recovery from the tumorous state in higher plants

Four neoplastic diseases of plants: crown gall, which is caused by Ti plasmid DNA; Black's wound tumor disease by an RNA virus; the Kostoff genetic tumors by chromosomal imbalance; and habituation, which results from a spontaneous activation of select biosynthetic systems, have been analyzed and compared. It has been found that both the development of a capacity for autonomous growth and the nature of the heritable cellular change that underlies tumorigenesis are similar in the four instances. All develop a capacity for autonomous growth as a result of the persistent activation of select biosynthetic systems, the products of which are concerned with cell growth and division. That the persistent activation of these biosynthetic systems does not involve heritable changes of an irreversible type is indicated by the finding that a reversal of the neoplastic state occurred in three of the test systems. Since the tumor cells in these instances were found to remain totipotent the results suggest that whether the normal or tumor phenotype is expressed is determined by how the genetic information is regulated in a cell. Regulation appears to be accomplished in part through positive feedback control mechanisms. Foreign genetic information could act either in a regulatory manner to persistently activate normal biosynthetic systems or it could code for one or more essential but normally limiting substance(s) and thus replace a substance(s) that in the case of the Kostoff tumors or habituation is specified by host cell genes, or it could do both. In either case, the foreign genetic information can be regulated in much the same manner as are the host cell genes to give rise to either the normal or tumor phenotype.

Suppression of the neoplastic state with the acquisition of specialized functions in cells, tissues, and organs of crown gall teratomas of tobacco

The neoplastic state in cells of tissues and organs that develop from cloned lines of crown gall teratomas of tobacco may be completely but reversibly suppressed. Stems and leaves found on teratoma shoots may appear morphologically normal and such organs contain all of the specialized cell types and are histologically and functionally indistinguishable from those found in normal tobacco shoots of comparable age. When however, specialized cells of several different kinds that are present in stems and leaves of the teratomas are excised from the plant and grown on a basic culture medium they again assume their neoplastic properties. The results of this study indicate that the morphogenetic factors and mechanisms that govern so precisely growth, cellular differentiation, and organogenesis during the normal course of development can completely suppress the tumorous state, leading to the formation of cells, tissues, and organs that appear normal in every respect but are, in fact, inherently neoplastic. Whether the normal or tumor phenotype is expressed appears to depend on the activation or repression of select biosynthetic systems, one of which, the auxin sytems, has been identified here.

A comparative study of cytokinesins I and II and zeatin riboside: a reply to Carlos Miller

Two cell-division-promoting factors, which have partition coefficients of 1.9 and 2.75 determined by 500-tube countercurrent distribution in a butanolwater system, have been repeatedly isolated in this laboratory from crown gall tumor tissues of Vinca rosea L. These substances have been given the trivial names cytokinesin I and cytokinesin II, respectively. Chemical and mass spectrometric analyses suggest that both cytokinesins are substituted hypoxanthines and are thus very different compounds from the 6-substituted adenyl cytokinins. Carlos Miller, using a very different and far more drastic isolation procedure, obtained one main cell-division-promoting factor from these same tumor tissues, which he identified as ribosyl-trans-zeatin. On the basis of this finding, and without an attempt to repeat our studies, questions have been raised by Miller concerning the existence of the cytokinesins as biologically active substances. We have, therefore, compared some pertinent physical, chemical, and biological properties of the cytokinesins with those of zeatin riboside, have demonstrated that these three substances can be cleanly separated from one another by a number of different methods and that each behaves as a pure substance in the several systems, and, finally, we have shown that the cytokinesins are not contaminated with ribosyl-trans-zeatin and thus do not owe their biological activity to such a contaminant.

Programming of cells for death under experimental conditions: relevance to the tumor problem

An attempt was made in this study to determine more precisely the nature of the factors that are involved in the programming of cells for a form of terminal cellular differentiation that results in death. These studies demonstrated that both the cytokinesins, which are potent inhibitors of plant and animal adenosine 3':5'-cyclic monophosphate phosphodiesterases, and 8-bromoadenosine 3':5'-cyclic monophosphate, which is a stable, biologically active form of adenosine 3':5'-cyclic monophosphate, are highly effective in encouraging differentiation of parenchyma cells into tracheary elements with accompanying death. Since adenosine 3':5'-cyclic monophosphate and theophylline when used together were also effective, the results reported here suggest that adenosine 3':5'-cyclic monophosphate is somehow importantly involved in the conversion of parenchyma cells into tracheary elements in this system. The possible significance to the tumor problem generally of the programming of cells for terminal differentiation, with or without resulting death, is discussed.

Roles of calcium, serum, plasma, and folic acid in the control of proliferation of normal and Rous sarcoma virus-infected chicken fibroblasts

In a culture medium of pH 7.4 and a folic acid concentration of 100 mug/liter that contains 5% heat-inactivated chicken plasma rather than serum, the rate of proliferation of normal chicken fibroblasts is determined by the concentration of calcium. Proliferation, rapid when the calcium concentration is physiological, decreases when the calcium concentration is reduced. At a very low calcium concentration, in this culture medium, normal fibroblasts are maintained without proliferation, whereas those infected with Rous sarcoma virus proliferate rapidly. This proliferative inactivity of normal fibroblasts does not involve contact-inhibition, since the effect is observed at low, as well as higher, culture densities. When a physiological amount of calcium is added to cultures of normal fibroblasts that have been maintained in very low calcium-plasma medium for 3 days, labeled thymidine uptake and protein synthesis are strongly stimulated, and cell division follows. The use of heat-inactivated chicken serum, instead of plasma, in this medium appears to strongly sensitize normal fibroblasts to the mitogenic action of calcium. In a plasma-containing culture medium of physiological calcium concentration and a folate concentration of 5 mug/liter, neither normal nor Rous sarcoma virus-infected fibroblasts proliferate to an appreciable extent. The use of serum, however, instead of plasma results in rapid proliferation of both normal and infected cells, as does increase in the folate concentration of the plasma-containing medium to 100 mug/liter.The fact that while normal fibroblasts are maintained without proliferation in low calcium-plasma medium, Rous sarcoma virus-infected fibroblasts proliferate rapidly, indicates that the effect of calcium is regulatory rather than permissive. These results suggest that the proliferation of normal fibroblasts is initiated by a cellular function involving calcium, and that the autonomous proliferation of the neoplastic fibroblasts results either from increased calcium uptake or from an alteration or a hypass of that function. The results also suggest that serum contains a mitogenic factor(s) not present in plasma, possibly a "wound hormone" for fibroblasts.

The inhibition of plant and animal adenosine 3':5'-cyclic monophosphate phosphodiesterases by a cell-division-promoting substance from tissues of higher plant species

One member of a new class of cell-division-promoting factors, that has been given the trivial name of cytokinesin I, is a potent inhibitor of adenosine 3':5'-cyclic monophosphate phosphodiesterases of both plant and animal origin. Since an adenylate cyclase has been demonstrated in this study to be present in plant cells, the results suggest that cytokinesin I may be exerting its biological effects in promoting division in cells of higher plant species as a regulator of adenosine 3':5'-cyclic monophosphate.

Studies on the distribution and properties of a new class of cell division--promoting substances from higher plant species

One member of a new class of cell division-promoting substances, which are nicotinamide derivatives, has been found to be present in dividing cells of tobacco and cactus. These plants are taxonomically far removed from one another and from Vinca rosea L., the plant species from which the new substances were first isolated. Because of their apparent wide distribution among dicotyledonous plant species, the question is raised as to whether the nicotinamide derivatives rather than the purine cytokinins may not, in fact, be the naturally occurring cell division factors that are directly involved in promoting cytokinesis in higher plant species. Unequivocal evidence is presented to show that the nicotinamide derivatives do not owe their biological activity to contamination by 6-substituted purine cytokinins.

On the inhibition of tumor inception in the crown-gall disease with the use of ribonuclease A

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