New growth factors for imaginal discs

Several families of peptide growth factors are implicated in regulating cell growth and proliferation of vertebrate cells in culture. Genetic studies in Drosophila implicate some of these factors in growth control in vivo. A recent report identifies a new family of growth factors, related to chitinase enzymes, required by Drosophila imaginal disc cells in culture. It will be of interest to determine how such factors relate to size regulation during development.

Lineage-tracing cells born in different domains along the PD axis of the developing Drosophila leg

Patterning of the developing limbs by the secreted signaling proteins Wingless, Hedgehog and Dpp takes place while the imaginal discs are growing rapidly. Cells born in regions of high ligand concentration may be displaced through growth to regions of lower ligand concentration. We have used a novel lineage-tagging method to address the reversibility of cell fate specification by morphogen gradients. We find that responses to Hedgehog and Dpp in the wing disc are readily reversible. In the leg, we find that cells readily adopt more distal fates, but do not normally shift from distal to proximal fate. However, they can do so if given a growth advantage. These results indicate that cell fate specification by morphogen gradients remains largely reversible while the imaginal discs grow. In other systems, where growth and patterning are uncoupled, nonreversible specification events or ‘ratchet’ effects may be of functional significance.

Regulation of LIM homeodomain activity in vivo: a tetramer of dLDB and apterous confers activity and capacity for regulation by dLMO

Dorsal-ventral axis formation in the Drosophila wing depends on the activity of the LIM homeodomain transcription factor Apterous and its cofactor, dLDB/Chip. We present evidence that Apterous activity depends on the formation of a LIM homeodomain dimer bridged by a dimer of cofactor. We show that Apterous activity levels are regulated in vivo by dLMO, an antagonist of homodimer formation. Making use of a constitutively active form of Apterous and dominant-negative forms of Apterous and dLDB/Chip, we show that the normal function of dLMO is to downregulate Apterous activity and that the dLMO mutant phenotype is due to excess Apterous activity. These findings may point to a general mechanism for regulation of LIM homeodomain protein activity.

The nomenclature of cell death: recommendations of an ad hoc Committee of the Society of Toxicologic Pathologists

The last several years have seen considerable confusion regarding the terms "apoptosis" and "necrosis" in pathology. This situation prompted the Society of Toxicologic Pathologists to charter the Committee on the Nomenclature of Cell Death, which was charged with making recommendations about the use of the terms "apoptosis" and "necrosis" in toxicity studies. The Committee recommends use of the term "necrosis" to describe findings comprising dead cells in histological sections, regardless of the pathway by which the cells died. The modifiers "apoptotic" and "oncotic" or "mixed apoptotic and oncotic" are recommended to specify the predominant morphological cell death pathway or pathways, when appropriate. Other standard modifiers, indicating the lesion distribution and severity, may also be used in conjunction with these. "Individual cell necrosis" (also known as "single cell necrosis") may be either of the apoptotic, oncotic, or mixed types. In many cases, more traditional terms such as "coagulation necrosis" may be used to convey a meaning similar to oncotic necrosis. It is important that pathologists use terms that accurately and concisely convey the level of information appropriate to the study's needs. Furthermore, toxicologic pathologists should actively help to disseminate these recommendations to other biologists and to regulatory authorities.

Formation of morphogen gradients in the Drosophila wing

Patterning of multicellular fields requires mechanisms to coordinate developmental decisions made by populations of cells. Evidence is accumulating that the necessary information is provided by localized sources of secreted signalling proteins which act as morphogens. We review evidence that Wingless, Dpp and Hedgehog proteins act as morphogens in the developing wing of Drosophila and discuss recent work illustrating that signalling helps to shape their activity gradients by regulating ligand distribution and by modulating the responsiveness of target cells. These studies suggest that there is more to being a morphogen than formation of a ligand gradient by passive diffusion.

Localization of vascular endothelial growth factor in nontumorous human pituitaries

Vascular endothelial growth factor (VEGF) is a key mediator of endothelial cell proliferation, angiogenesis, and vascular permeability. Our aim was to investigate whether VEGF is expressed in various cell types of the human pituitary. Eight nontumorous pituitaries were investigated by histology, immunocytochemistry, double immunostaining, and immunoelectron microscopy.Immunocytochemistry, including double immunostaining, showed VEGF immunoreactive cells to be distributed throughout the adenohypophysis. Immunopositivity was evident in all adenohypophysial cell types, but was colocalized mainly with somatotroph and stellate cell antigen (i.e., growth hormone and S-100 protein). Weak to moderate VEGF immunoreactivity was also noted in posterior lobe pituicytes as well as in most endothelial and perivascular smooth muscle cells.In situ hybridization confirmed these findings in demonstrating a strong signal in corticotrophs, somatotrophs, and stellate cells. Immunoelectron microscopy showed VEGF to be present mainly within secretory granules where it colocalized with the full spectrum of adenohypophysial hormones. The subcellular distribution of VEGF suggests that hypothalamic factors play a role in its release from adenohypophysial cells. Further studies are required to examine the possible role of VEGF in affecting both the pituitary's vasculature and endocrine activity.

Proximodistal axis formation in the Drosophila leg: subdivision into proximal and distal domains by Homothorax and Distal-less

The developing legs of Drosophila are subdivided into proximal and distal domains by the activity of the homeodomain proteins Homothorax (Hth) and Distal-less (Dll). The expression domains of Dll and Hth are initially reciprocal. Wingless and Dpp define both domains by activating Dll and by repressing Hth in the distal region of the disc. Wg and Dpp do not act through Dll to repress Hth. Hth functions to reduce the sensitivity of proximal cells to Wg and Dpp. This serves to limit the effective range of these signals in regulating later-acting genes such as Dac. We present evidence that proximal and distal cells tend to sort-out from one another. Cells forced to express Hth are unable to mix with distal cells. Likewise, cells forced to express Dll are unable to mix with proximal cells. Clones of cells unable to express Dll in the distal region sort-out from the disc. Clones of cells unable to express Hth lose the specialized population of cells at the interface between proximal and distal territories and cause fusion between body wall and leg segments. These observations suggest that sorting-out behavior of Hth- and Dll-expressing cells contributes to subdivision of the leg into proximal and distal domains.

Notchless encodes a novel WD40-repeat-containing protein that modulates Notch signaling activity

Signaling by Notch family receptors is involved in many cell-fate decisions during development. Several modifiers of Notch activity have been identified, suggesting that regulation of Notch signaling is complex. In a genetic screen for modifiers of Notch activity, we identified a gene encoding a novel WD40-repeat protein. The gene is called Notchless, because loss-of-function mutant alleles dominantly suppress the wing notching caused by certain Notch alleles. Reducing Notchless activity increases Notch activity. Overexpression of Notchless in Xenopus or Drosophila appears to have a dominant-negative effect in that it also increases Notch activity. Biochemical studies show that Notchless binds to the cytoplasmic domain of Notch, suggesting that it serves as a direct regulator of Notch signaling activity.

Dpp receptor levels contribute to shaping the Dpp morphogen gradient in the Drosophila wing imaginal disc

Axis formation in the Drosophila wing depends on the localized expression of the secreted signaling molecule Decapentaplegic (Dpp). Dpp acts directly at a distance to specify discrete spatial domains, suggesting that it functions as a morphogen. Expression levels of the Dpp receptor thick veins (tkv) are not uniform along the anterior-posterior axis of the wing imaginal disc. Receptor levels are low where Dpp induces its targets Spalt and Omb in the wing pouch. Receptor levels increase in cells farther from the source of Dpp in the lateral regions of the disc. We present evidence that Dpp signaling negatively regulates tkv expression and that the level of receptor influences the effective range of the Dpp gradient. High levels of tkv sensitize cells to low levels of Dpp and also appear to limit the movement of Dpp outside the wing pouch. Thus receptor levels help to shape the Dpp gradient.

Identification of chromosomal bands replicating early in the S phase of normal human fibroblasts

Normal human fibroblasts (NHF1) were released from confluence arrest (G0) and replated in medium containing bromodeoxyuridine (BrdU) and aphidicolin. Despite severe reduction in the rate of DNA synthesis by aphidicolin, cells reentering the cell cycle incorporated BrdU at regions of the human genome that replicated very early in S phase. After removal of aphidicolin and BrdU from the tissue culture medium, cells were collected in mitosis. Q-banding with 4', 6-diamidino-2-phenylindole/actinomycin D was used to identify metaphase chromosomes. A monoclonal anti-BrdU antibody and a fluorescein isothiocyanate (FITC)-conjugated goat anti-mouse antibody were used to identify the BrdU-labeled sites. The criterion for scoring DNA replication sites was the detection of FITC fluorescence at homologous regions of both sister chromatids. Early replicating regions mapped within R-bands, but not all R-bands incorporated BrdU. Chromosomal bands 1p36.1, 8q24.1, 12q13, 15q15, 15q22, and 22q13 were labeled in 53% or more of the copies of these chromosomes in the data set, suggesting that these sites replicated very early in S phase. Chromosomal band 15q22 was the most frequently labeled site (64%), which indicates that it contains some of the earliest replicating sequences in normal human fibroblasts.

Beadex encodes an LMO protein that regulates Apterous LIM-homeodomain activity in Drosophila wing development: a model for LMO oncogene function

Formation of the dorsal-ventral axis of the Drosophila wing depends on activity of the LIM-homeodomain protein Apterous (Ap). Here we report that Ap activity levels are modulated by dLMO, the protein encoded by the Beadex (Bx) gene. Overexpression of dLMO in Bx mutants interferes with Apterous function. Conversely, Bx loss-of-function mutants fail to down-regulate Apterous activity at late stages of wing development. Biochemical analysis shows that dLMO protein competes for binding of Apterous to its cofactor Chip. These data suggest that Apterous activity depends on formation of a functional complex with Chip and that the relative levels of dLMO, Apterous, and Chip determine the level of Apterous activity. The dominant interference mechanism of dLMO action may serve as a model for the mechanism by which LMO oncogenes cause cancer when misexpressed in T cells.

Boundary formation in Drosophila wing: Notch activity attenuated by the POU protein Nubbin

Cell interactions mediated by Notch-family receptors have been implicated in the specification of tissue boundaries in vertebrate and insect development. Although Notch ligands are often widely expressed, tightly localized activation of Notch is critical for the formation of sharp boundaries. Evidence is presented here that the POU domain protein Nubbin contributes to the formation of a sharp dorsoventral boundary in the Drosophila wing. Nubbin represses Notch-dependent target genes and sets a threshold for Notch activity that defines the spatial domain of boundary-specific gene expression.

13C NMR study of the effects of leptin treatment on kinetics of hepatic intermediary metabolism

The recent discovery of leptin receptors in peripheral tissue raises questions about which of leptin's biological actions arise from direct effects of the hormone on extraneural tissues and what intracellular mechanisms are responsible for leptin's effects on carbohydrate and lipid metabolism. The present study is focused on the action of leptin on hepatic metabolism. Nondestructive 13C NMR methodology was used to follow the kinetics of intermediary metabolism by monitoring flux of 13C-labeled substrate through several multistep pathways. In perfused liver from either ob/ob or lean mice, we found that acute treatment with leptin in vitro modulates pathways controlling carbohydrate flux into 13C-labeled glycogen, thereby rapidly enhancing synthesis by an insulin-independent mechanism. Acute treatment of ob/ob liver also caused a rapid stimulation of long-chain fatty acid synthesis from 13C-labeled acetyl-CoA by the de novo synthesis route. Chronic leptin treatment in vivo induced homeostatic changes that resulted in a tripling of the rate of glycogen synthesis via the gluconeogenic pathway from [2-13C]pyruvate in ob/ob mouse liver perfused in the absence of the hormone. Consistent with the 13C NMR results, leptin treatment of the ob/ob mouse in vivo resulted in significantly increased hepatic glycogen synthase activity. Chronic treatment with leptin in vivo exerted the opposite effect of acute treatment in vitro and markedly decreased hepatic de novo synthesis of fatty acids in ob/ob mouse liver. In agreement with the 13C NMR findings, activities of hepatic acetyl-CoA carboxylase and fatty acid synthase were significantly reduced by chronic treatment of the ob/ob mouse with leptin. Our data represent a demonstration of direct effects of leptin in the regulation of metabolism in the intact functioning liver.

Tumorigenicity of sodium ascorbate in male rats

Sodium ascorbate, like other sodium salts such as saccharin, glutamate, and bicarbonate, produces urinary alterations when fed at high doses to rats, which results in mild superficial urothelial cytotoxicity and regeneration but not tumors in a standard 2-year bioassay. Sodium saccharin was shown to produce a low incidence of bladder tumors in rats if administered in a two-generation bioassay. In the present study, we evaluated sodium ascorbate in a two-generation bioassay that involved feeding to the male and female parental F344 rats for 4 weeks before mating, feeding the dams during gestation and lactation, and then feeding the weaned (at 28 days of age) male F1 generation rats for the remainder of their lifetime (up to 128 weeks of the experiment). Dietary levels of 1.0, 5.0, and 7.0% sodium ascorbate were tested. At 5.0 and 7.0% sodium ascorbate, there was an increase in urinary bladder urothelial papillary and nodular hyperplasia and the induction of a few papillomas and carcinomas. There was a dose-responsive increase in renal pelvic calcification and hyperplasia and inhibition of the aging nephropathy of rats even at the level of 1% sodium ascorbate. Because the short-term urothelial effects of sodium ascorbate in rats are inhibited by treatments producing urinary acidification to pH < 6.0, we coadministered high doses of long-term NH4Cl to groups of rats with 5.0 or 7.0% sodium ascorbate to evaluate the long-term effects. The combination of 7.0% sodium ascorbate plus 2.78% NH4Cl in the diet was toxic, and the group was terminated early during the course of the experiment. The group fed 5.0% sodium ascorbate plus 2.04% NH4Cl showed complete inhibition of the urothelial effects of sodium ascorbate and significant inhibition of its renal effects. We also demonstrated the presence of a calcium phosphate-containing urinary precipitate in rats fed sodium ascorbate at all doses, in a dose-responsive manner. The formation of the precipitate was inhibited by coadministration with NH4Cl. The proliferative effects of sodium ascorbate on the male rat urinary tract in this study are similar to those seen with sodium saccharin when administered in a two-generation bioassay. Mechanistic information suggests that this is a high-dose, rat-specific phenomenon.

Urinary physiologic and chemical metabolic effects on the urothelial cytotoxicity and potential DNA adducts of o-phenylphenol in male rats

ortho-Phenylphenol (OPP), a fungicide and antibacterial agent with food residues, is carcinogenic to rat bladder. The present studies provide information on changes in urinary composition and urinary metabolites, urothelial cytotoxicity and regenerative hyperplasia, and DNA adducts in male F344 rats fed OPP. An initial experiment evaluated dietary doses of 0, 1,000, 4,000, and 12,500 ppm OPP fed for 13 weeks. There was no evidence of urinary calculi, microcrystalluria, or calcium phosphate-containing precipitate, but urothelial cytotoxicity and hyperplasia occurred at the highest dose only. In a second experiment, rats were fed dietary OPP levels of 0, 800, 4,000, 8,000, and 12,500 ppm. Urinary pH was > 7 in all groups. Urinary volume was increased at the 2 highest doses with consequent decreases in osmolality, creatinine, and other solutes. Total urinary OPP metabolite excretions were increased, mostly excreted as conjugates of OPP and of phenylhydroquinone. Free OPP or free metabolites accounted for less than 2% excreted in the urine without a dose response. Urothelial toxicity and hyperplasia occurred only at doses of 8,000 and 12,500 ppm. OPP-DNA adducts were not detected in the urothelium at any dose. In summary, OPP produces cytotoxicity and proliferation of the urothelium at dietary doses > or = 8,000 ppm without formation of urinary solids. The paucity of unconjugated metabolites and the lack of OPP-DNA adducts suggests that OPP is acting as a bladder carcinogen in male rats by inducing cytotoxicity and hyperplasia without it or its metabolites directly binding to DNA.

Enhanced tumorigenicity of rat bladder squamous cell carcinoma cells after abrogation of gap junctional intercellular communication

We previously demonstrated a clear tendency for actively communicating rat bladder carcinoma cell lines with elevated expression of connexin 43 mRNA to possess strong tumorigenicity. In the present study, immunohistochemical analysis established that normal bladder epithelium did not express connexin 43 protein, but bladder carcinomas often expressed the protein, particularly on the membranes of cells within areas of squamous cell differentiation. To investigate the role of connexin 43 overexpression in rat bladder carcinoma cells, an anti-sense connexin 43 expression vector was transfected into BC31 cells having a high communication capacity. In the resultant transfectants, there was little or no communication capacity and connexin 43 expression. The growth rate in vitro was not changed compared to that of cells treated with the vector alone (without the anti-sense sequence), but tumorigenicity in nude mice was dramatically enhanced. The results indicate that connexin 43 overexpression in rat bladder carcinogenesis is related to squamous cell differentiation, and the protein can have tumor suppressor characteristics, as in other organs.

Systematic gain-of-function genetics in Drosophila

A modular misexpression system was used to carry out systematic gain-of-function genetic screens in Drosophila. The system is based on inducible expression of genes tagged by insertion of a P-element vector carrying a GAL4-regulated promoter oriented to transcribe flanking genomic sequences. To identify genes involved in eye and wing development, the 2300 independent lines were screened for dominant phenotypes. Among many novel genes, the screen identified known genes, including hedgehog and decapentaplegic, implicated in these processes. A genetic interaction screen for suppressors of a cell migration defect in a hypomorphic slow border cells mutant identified known genes with likely roles in tyrosine kinase signaling and control of actin cytoskeleton, among many novel genes. These studies demonstrate the ability of the modular misexpression system to identify developmentally important genes and suggest that it will be generally useful for genetic interaction screens.

Long-term feeding of sodium saccharin to nonhuman primates: implications for urinary tract cancer

BACKGROUND

It was observed in the early 1970s that saccharin produced bladder cancer in rats. However, it has been unclear whether sodium saccharin when consumed by humans poses a substantial carcinogenic hazard. Numerous epidemiologic studies have not shown any evidence of increased urothelial proliferation associated with ingestion of sodium saccharin.

PURPOSE

Our purpose was to determine the effects of long-term feeding of sodium saccharin to three species of nonhuman primates.

METHODS

Twenty monkeys of three species (six African green, seven rhesus, six cynomolgus, and one hybrid [of rhesus male and cynomolgus female parentage]) were treated with sodium saccharin (25 mg in the diet/kg body weight daily for 5 days a week) beginning within 24 hours after birth and continuing for up to 24 years. Sixteen monkeys (seven rhesus and nine cynomolgus) served as controls. During their last 2 years of life, urine was collected from selected treated and control animals and evaluated for various urinary chemistries and for the presence of calculi, microcrystalluria, and precipitate. Urinary bladders were examined by light microscopy and by scanning electron microscopy.

RESULTS

Sodium saccharin treatment had no effect on the urine or urothelium in any of these monkeys. There was no evidence of increased urothelial cell proliferation, and there was no evidence of formation of solid material in the urine.

CONCLUSION

Although the dose of sodium saccharin administered to these monkeys was only five to 10 times the allowable daily intake for humans, the results provide additional evidence that sodium saccharin is without a carcinogenic effect on the primate urinary tract.

Frequent p53 mutations and occasional loss of chromosome 4 in invasive bladder carcinoma induced by N-butyl-N-(4-hydroxybutyl)nitrosamine in B6D2F1 mice

B6D2F1 mice (45/group) were treated with N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) or uracil as follows: Group 1 received 0.05% BBN in drinking water for the entire experiment, Group 2 received 5 mg of BBN by gastric gavage in 0.1 mL of 20% ethanol twice per week for 10 wk, Group 3 received a 2.5% uracil-containing diet for the entire experiment, and Group 4 was controls (received 0.1 ml of 20% ethanol by gavage twice per week for 10 wk). The surviving mice in Group 1 were killed after week 26 and those in the other groups after week 30. By week 15, three of 11 Group 1 and one of 15 Group 2 mice had bladder carcinoma. By 26 and 30 wk, respectively, invasive carcinomas were observed in 33 of 34 and six of 21 mice in Groups 1 and 2 and renal pelvic carcinomas in 11 of 34 and three of 21 mice in Groups 1 and 2. Four of 19 uracil-treated mice had bladder nodular hyperplasia. By polymerase chain reaction-single-strand conformation polymorphism and sequence analyses, 16 of 20 and two of five bladder carcinomas from Groups 1 and 2, respectively, showed mutations in the p53 gene. Ha-ras mutation was present in one case. Loss of heterozygosity analysis with simple-sequence length polymorphism markers for chromosome 4 showed that 10 of 21, two of 15, and nine of 13 mice in Groups 1-3, respectively, had heterozygous or homozygous deletions. B6D2F1 mice are therefore susceptible to the urothelial carcinogenic effects of BBN and develop frequent p53 mutations and chromosome 4 deletions. Chromosome 4 deletions were also seen with uracil.

Urinary bladder carcinogenesis

Urinary bladder carcinogenesis in rodents bears numerous similarities to the diseases in humans. In rats, the process progresses through the morphologic stages of simple hyperplasia, papillary and nodular hyperplasia, papilloma, noninvasive, and invasive carcinoma. In mice, the pathogenesis can be similar or can follow a sequence of marked dysplasia with or without hyperplasia, leading to carcinoma in situ and ultimately to high-grade invasive carcinoma. Although the papillary and nonpapillary diseases appear to be related in rodents and in humans, they are distinct morphologically, biologically, and molecularly. Numerous classes of genotoxic chemicals have been identified as bladder carcinogens in rodents, and some of these have also been identified as carcinogenic in humans, most notably, aromatic amines, nitrosamines, and cyclophosphamide. In contrast, nongenotoxic chemicals appear to be highly specific with respect to species, strain, diet, agent, dose, and mechanism. For some, it is unclear whether the results at high doses in rodents can be extrapolated to low doses or to humans, e.g., chemicals that cause bladder cancer only at high doses related to the formation of calculi. Numerous observations in rodents can assist in identifying possible mechanisms involved for these nongenotoxic chemicals and therefore can be important for a rational evaluation of human risk.