Effects of 2,4-dinitrophenol and dinactin on heat-sensitive and ecdysone-specific puffs of Drosophila salivary gland chromosomes in vitro

Sodium salicylate decreases intracellular ATP, induces both heat shock factor binding and chromosomal puffing, but does not induce hsp 70 gene transcription in Drosophila

Sodium salicylate has long been known to be an inducer of the heat shock puffs and presumably heat shock gene transcription in the polytene chromosomes of Drosophila salivary gland cells. Stress-induced transcription of the heat shock genes is mediated by the transcription factor known as Heat Shock Factor (HSF). In yeast, sodium salicylate has been reported to induce the DNA binding of HSF but not heat shock gene transcription itself, and similar findings have been reported in human cells. This apparent discrepancy in the induction of certain aspects of the heat shock response between these organisms prompted us to carefully reexamine the induction of the heat shock response in Drosophila salivary gland cells of third instar larvae and Drosophila tissue culture (SL2) cells. Sodium salicylate (3-30 mM) decreases intracellular ATP levels in SL2 cells and induces HSF binding activity in SL2 and salivary gland cells in a dose-dependent manner. Despite the induction of HSF binding and heat shock puffs in polytene chromosomes, we found no evidence for increased hsp 70 gene transcription suggesting that chromosomal puffing and gene transcription may be separable events. Salicylate did not induce the HSF hyperphosphorylation that is normally associated with HSF activation. Furthermore, salicylate (30 mM) prevented heat-induced hyperphosphorylation of HSF and hsp 70 gene transcription indicating that salicylate's inhibitory effect on hsp 70 transcription may be independent of its effect on HSF binding activity. We propose that the reduction in intracellular ATP caused by the addition of salicylate likely plays a role in the activation of HSF binding and the inhibition of both HSF hyperphosphorylation and hsp 70 gene transcription.

Heat shock protein synthesis is affected by intracellular pH: inhibition by monensin-induced alkalosis in C6 rat glioma cells

The effect of intracellular pH (pHi) on heat shock protein (HSP) synthesis was investigated in C6 rat glioma cells. pHi changes were analysed by means of fluorescence spectroscopy in a perfused monitoring system allowing continuous measurements before, during and after treatments. HSP induction was determined by means of Western blots and autoradiographs. A 20 min heat shock (HS) of 44 degrees C decreased the pHi from 7.36 to 7.05 during exposure [17] and elicited the synthesis of heat shock proteins 2-8 h later. A pHi decrease, brought about by low extracellular pH (pHe) of 4.5 and 5.0 or 5.5, induced HSP synthesis after 1 h or 3 h, respectively. During these treatments, pHi decreased to values significantly lower than that caused by HS. Three h exposure to pHe 6.2, however, was not inductive. These results indicate that the heat shock-induced pHi decrease alone is not sufficient to stimulate HSP synthesis. In order to investigate the effect of alkaline pHi on the induction of HSP by heat, pHi was increased prior to HS treatments. Preincubation of cells at pHe ranging from 6.8 to 8.0 had little effect on pHi and on HSP synthesis. A shift of pHi to more alkaline values was achieved by adding the H+/Na+ exchanger monensin at alkaline pHe. Twenty microM monensin raised the pHi and inhibited the HSP induction depending on the pHe values: as pHe was increased from pH 7.2 to 8.0 HSP synthesis was increasingly inhibited. Monensin also diminished the HS-induced drop of pHi particularly at higher pHe. The result showed that neither a lower pHi nor a drop of pHi during HS is a necessary prerequisite for the induction, whereas alkalosis inhibits the synthesis of HSP.

Oxidative stress as a causal factor in differentiation and aging: a unifying hypothesis

In this article, the authors have pointed out flaws in the current version of the free radical hypothesis of aging and have advanced a new hypothesis that reconciles and encapsulates existing information. The main premise of this hypothesis is that aging is a continuation of development and is thus influenced by genetically programmed phenomena. Completion of various genetic programs and the duration of life are linked to a metabolic potential which is itself a genetically determined sum of energy expenditure. Nevertheless, the rate at which metabolic potential is reached is linked to the rate of metabolism and the level of oxidative stress both of which are influenced by epigenetic stimuli. The current version of the free radical hypothesis postulates that partially reduced oxygen species are produced in aerobic cells in an uncontrolled fashion and do not play any useful physiological function. The principle tenet of the free radical hypothesis is that molecular damage is the underlying cause of aging and that O2- radicals and derivatives induce most of the damage sustained by cells during aging. The authors regard this hypothesis as flawed because it fails to explain either low randomly occurring damage can lead to age-associated changes that are species-specific, or the sequential nature of the changes that occur in aging organisms. In contrast to the free radical hypothesis, our hypothesis can explain the specific and sequential nature of aging-related changes because they are postulated to be neither dependent upon uncontrolled damage nor the cellular capacity to prevent it. Instead, the authors suggest that the damage accumulated during aging is a secondary effect rather than a direct cause of senescence. The authors have shown that cells exert control not only on their level of antioxidant defense but also on their rate of oxidant production. The authors postulate that aging is the terminal stage of development, and as such is influenced genetically. The authors also postulate that a definite sum of energy is required to complete the genetic programs associated with aging. Thus, the rate of aging is linked to the level of oxidative stress; the rate of energy utilization is postulated to determine the level of oxidative stress. Oxidative stress is one of the factors which appears to govern changes in gene expression during differentiation and we suggest that it causes alterations in gene expression during aging. In the authors revised hypothesis, free radicals promote aging by affecting specific genetic programs and the incidental damage they inflict in cells is only a by-product of this process.(ABSTRACT TRUNCATED AT 400 WORDS)

Oxidative influence on development and differentiation: an overview of a free radical theory of development

Metabolic gradients exist in developing organisms and are believed to influence development. It has been postulated that the effects of these gradients on development result from differential oxygen supplies to tissues. Oxygen has been found to influence the course of development. Cells and tissues in various stages of differentiation exhibit discrete changes in their antioxidant defenses and in parameters of oxidation. Metabolically generated oxidants have been implicated as one factor that directs the initiation of certain developmental events. Also implicated as factors that modulate developmental processes are the cellular distribution of ions and the cytoskeleton both of which can be influenced by oxidants. The interaction of oxidants with ion balance and cytoskeleton is discussed.

A role for reduced oxygen species in heat induced cell killing and the induction of thermotolerance

A model suggesting a role for superoxide (O2-) and hydrogen peroxide (H2O2) in heat induced cytotoxicity and development of thermotolerance is proposed: (1) Heat shock increases cellular generation of O2- and H2O2 in proportion to the severity of the heat shock. (2) Heat induced generation of O2- or H2O2 in excess of the ability of the antioxidant enzymes to remove these toxic species causes heat induced cell injury and cytotoxicity. This damage is caused by lipid peroxidation, leading to disruption of the cytoskeleton and calcium metabolism. (3) The flux of O2- and H2O2 generated by heat shock induces the synthesis of additional antioxidant enzymes. Other treatments which induce thermotolerance also cause oxidative stress and induce the antioxidant enzymes. The ability of various agents to modify heat induced cytotoxicity and development of thermotolerance is reviewed in light of this model.

Modulators of the eukaryotic heat shock response

A wide variety of agents other than heat have been reported to induce or suppress heat shock protein (hsp) synthesis in eukaryotic cells. Such agents, termed 'modulators', include inhibitors of respiration, low molecular weight nutrients, oxygen, hormones, sulfhydryl reagents, ionophores and amino acid analogues. The evidence for modulation is critically reviewed and common mechanisms of action that may relate modulation to induction by heat are discussed.

Abnormal proteins serve as eukaryotic stress signals and trigger the activation of heat shock genes

Heat shock protein (hsp) genes, a group of ubiquitous genes, are activated by various metabolic stresses. The suggestion that denaturation of intracellular proteins may be produced by the metabolic stresses and then signal the activation of the hsp genes was examined by co-injection of purified proteins and hsp genes into frog oocytes. Activation of hsp genes was observed if the proteins were denatured prior to injection but not if they were introduced in their native form. Furthermore, the activation of hsp genes by abnormal proteins and by heat shock appears to occur by a common mechanism. A model for the transcriptional regulation of the genes is based on competition for degradation between abnormal intracellular proteins and a labile regulatory factor.

Oxygen free radicals play a role in cellular differentiation: an hypothesis

Evidence from a variety of sources supports the view that oxygen free radicals play a role in cellular differentiation. It is postulated that cellular differentiation is accompanied by changes in the redox state of cells. Differentiated cells have a relatively more prooxidizing or less reducing intracellular environment than the undifferentiated or dedifferentiated cells. Changes in the redox balance during differentiation appear to be due to an increase in the rate of O2- generation. Differentiated cells, in general, exhibit higher rates of cyanide-resistant respiration, cyanide-insensitive SOD activity, and peroxide concentration and lower levels of GSH as compared to undifferentiated cells. The effects of free radicals on cellular differentiation may be mediated by the consequent changes in ionic composition.

Kinetics and models of the Drosophila heat-shock system

The puffing of Drosophila heat-shock genes after (1) a step-wise temperature increase ("heat-shock"); (2) recovery from anaerobiosis; and (3) incubation with uncoupling reagents was expressed as percent of the maximal size and normalized to the time scale. Data were taken from the literature and new measurements. In addition, puffing was measured after a 30-min temperature pulse and after two 30-min pulses. The latter experiment revealed a second, smaller increase in puff-size. Data on RNA and protein synthesis in Drosophila cells were collected from the literature and also normalized. From the available data, a feed-back control system is derived that consists of a controlled variable x, possibly a metabolic function of the mitochondria, interacting with an activator molecule which exists in an active (A+) and an inactive (A-) configuration. A+ activates the heat-shock genes which in turn produce their mRNA (y) and proteins (z) which then change the controlled variable x into a new steady state. A modified version of this model assumes a feed-back control of the heat-shock proteins on the activator molecule. A mathematical model of this system (Goodwin, 1965) was simulated by computer and compared with the experimental results.

Heat shock phenomena in Chironomus tentans I. In vivo effects of heat, overheat, and quenching on salivary chromosome puffing

Incubation of 4th instar larvae of Chironomus tentans at elevated temperatures leads in salivary and Malpighian chromosomes to the appearance of 4-5 new puffs. Previously present puffs, particularly Balbiani rings in salivary chromosomes, become drastically reduced. The reactions of region IV-5C and Balbiani ring 1 and 2 in salivary glands are quantitatively analyzed. Statistically significant heat shock effects are observed already after 5 min and reach a maximum between 30 and 60 min. The effective temperature range is small (between 33 to 40 degrees C) with an optimum at 37 degrees C. Above 40 degrees C, i.e., at overheat shock temperatures, heat shock reactions are suppressed. Larvae heat or overheat shocked for 1-7 h or 15-30 min, respectively, survive when returned to normal culturing temperatures. The recovery from heat shock of the puffing pattern occurs in two phases: a fast one (10-20 min) and a slow one (up to 5 h) sometimes separated by a period of backlash. Quenching of overheat shocked larvae does not result in a delayed heat shock reaction.

Changes of enzyme activity in larval salivary glands following the induction of respiration dependent puffs in giant chromosomes of Drosophila melanogaster

Changes in the activity of 3 enzymes--lactate dehydrogenase (LDH), soluble malate dehydrogenase (sMDH), and pyruvate kinase (PK)--in homogenates of larval salivary glands of Drosophila melanogaster were studied before and after the induction of heat sensitive puffs with trinactin. The activities of 2 enzymes (LDH and sMDH) were enhanced after puff induction, an effect that was abolished in the presence of cycloheximide. Pyruvate kinase activity did not change after puff induction. The results indicate a relation between the increase of LDH and sMDH activity due to de novo synthesis and the induction of heat sensitive puffs.

The effects of sodium, potassium and ATP on a developmental puff sequence in Drosophila salivary glands in vitro

Salivary glands of late third instar larvae of Drosophila melanogaster were isolated at a developmental stage when the release of ecdysone had already taken place. They were then incubated in a chemically defined medium. An ecdysone-dependent developmental puff sequence was measured in vitro and influenced by adding various substances or by changing iso-osmotically the sodium or potassium content of the medium. Trinactin, valinomycin N-ethylmaleimide and KCN blocked the puff sequence, i.e. the regression of the early and the induction of the late ecdysone-dependent puffs probably by increasing the Na+ influx and depleting the ATP content of the cell. A medium that contained Na+ as the only monovalent cation decreased the size of the late ecdysone-dependent puffs and increased the size of other puffs, such as 50 CD. Addition of tetrodotoxin to the normal medium had the opposite effect, i.e., it increased 63 E and inhibited 50 CD. Na+ free medium, inhibition of K+ flux by tetraethylammonium chloride, and application of ouabain did not considerably influence the size of the puffs measured. It is concluded from these results that Na+ in particular has an inhibitory effect on the induction of late ecdysone-dependent puffs. Na+ (and perhaps also K+) may act by modulating the effect of proteins that are involved in gene control mechanisms.