6-substituted purines: a novel class of inhibitors of endogenous protein degradation in isolated rat hepatocytes

Functional role of autophagy in testicular and ovarian steroidogenesis

Autophagy is an evolutionarily conserved cellular recycling process that maintains cellular homeostasis. Despite extensive research in endocrine contexts, the role of autophagy in ovarian and testicular steroidogenesis remains elusive. The significant role of autophagy in testosterone production suggests potential treatments for conditions like oligospermia and azoospermia. Further, influence of autophagy in folliculogenesis, ovulation, and luteal development emphasizes its importance for improved fertility and reproductive health. Thus, investigating autophagy in gonadal cells is clinically significant. Understanding these processes could transform treatments for endocrine disorders, enhancing reproductive health and longevity. Herein, we provide the functional role of autophagy in testicular and ovarian steroidogenesis to date, highlighting its modulation in testicular steroidogenesis and its impact on hormone synthesis, follicle development, and fertility therapies.

Induction of α-Phellandrene on Autophagy in Human Liver Tumor Cells

α-Phellandrene (α-PA) is a cyclic monoterpene. To investigate the induction of autophagy by α-PA and its mechanism, human liver tumor cells (J5) were incubated with α-PA and analyzed for cell viability and the molecular regulation of pre-autophagosome origination and autophagosome formation. According to the results, PI3K-I, mTOR, and Akt protein levels were decreased after α-PA treatment compared to those of the control group (p < 0.05). The phosphorylation of Bcl-2, and PI3K-III, LC3-II and Beclin-1 protein levels in J5 cells were increased after α-PA treatment (p < 0.05). In addition, α-PA up-regulated nuclear p53 and down-regulated cytoplasmic p53 expression in J5 cells. The NF-κB pathway was activated, as indicated by increase in cytosolic phosphorylated IκB, nuclear NF-κB levels, and the DNA-binding activity of NF-κB after α-PA treatment in J5 cells (p < 0.05). These results suggest that α-PA can induce J5 cell autophagy by regulating mTOR and LC-3II expression, p53 signaling, and NF-κB activation in J5 cells.

Autophagy in tobacco BY-2 cells cultured under sucrose starvation conditions: isolation of the autolysosome and its characterization

Tobacco culture cells carry out a large-scale degradation of intracellular proteins in order to survive under sucrose starvation conditions. We have previously suggested that this bulk degradation of cellular proteins is performed by autophagy, where autolysosomes formed de novo act as the major lytic compartments. The digestion process in autolysosomes can be retarded by addition of the cysteine protease inhibitor E-64c to the culture medium, resulting in the accumulation of autolysosomes. In the present study, we have investigated several properties of autolysosomes in tobacco cells. Electron microscopy showed that the autolysosomes contain osmiophilic particles, some of which resemble partially degraded mitochondria. It also revealed the presence of two kinds of autolysosome precursor structures; one resembled the isolation membrane and the other the autophagosome of mammalian cells. Immunofluorescence microscopy showed that autolysosomes contain acid phosphatase, in accordance with cytochemical enzyme analyses by light and electron microscopy in a previous study. Autolysosomes isolated by cell fractionation on Percoll gradients showed the localization of acid phosphatase, vacuolar H(+)-ATPase and cysteine protease. These results show that starvation-induced autophagy in tobacco cells follows a macroautophagic-type response similar to that described for other eukaryotes. However, our results indicate that, although the plant vacuole is often described as being equivalent to the lysosome of the animal cell, a new low pH lytic compartment-the autolysosome-also contributes to proteolytic degradation when tobacco cells are subjected to sucrose deprivation.

Modulatory effects of 6-carboxymethylthiopurine on activated murine macrophages

The immunological activity of macrophages against pathogens in hosts includes the phagocytosis and the production of nitric oxide. We report herein the investigation of the effect of 6-carboxymethylthiopurine on nitric oxide production by murine macrophages as well as its effect on the cell viability and proliferation after stimulus with Mycobacterium bovis bacille Calmette-Guérin, interferon-gamma or a combination of both. J774A.1 macrophages stimulated or not by bacille Calmette-Guérin (20 microg/mL), interferon-gamma or both, were cultured in the presence of 6-carboxymethylthiopurine (125, 250 and 500 microm). Nitric oxide production was measured by the Griess method and cell viability/proliferation by the diphenyltetrazolium assay [3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide]. We observed an increase of J774A.1 cell proliferation after stimulus with bacille Calmette-Guérin at 125, 250 and 500 microm (69.1, 124.0 and 89.7%, respectively) and with interferon-gamma at 125 and 250 microm (64.8% and 61.7%, respectively) (p < 0.05). In all cultures treated with 6-carboxymethylthiopurine, interferon-gamma-activated nitric oxide production by J774A.1 cells decreased as well as when subjected to interferon-gamma plus bacille Calmette-Guérin stimuli at 500 microm (p < 0.05). Altogether these data point to an anti-inflammatory effect of 6-carboxymethylthiopurine on stimulated macrophages.

Autophagy-physiology and pathophysiology

"Autophagy" is a highly conserved pathway for degradation, by which wasted intracellular macromolecules are delivered to lysosomes, where they are degraded into biologically active monomers such as amino acids that are subsequently re-used to maintain cellular metabolic turnover and homeostasis. Recent genetic studies have shown that mice lacking an autophagy-related gene (Atg5 or Atg7) cannot survive longer than 12 h after birth because of nutrient shortage. Moreover, tissue-specific impairment of autophagy in central nervous system tissue causes massive loss of neurons, resulting in neurodegeneration, while impaired autophagy in liver tissue causes accumulation of wasted organelles, leading to hepatomegaly. Although autophagy generally prevents cell death, our recent study using conditional Atg7-deficient mice in CNS tissue has demonstrated the presence of autophagic neuron death in the hippocampus after neonatal hypoxic/ischemic brain injury. Thus, recent genetic studies have shown that autophagy is involved in various cellular functions. In this review, we introduce physiological and pathophysiological roles of autophagy.

Use of protease inhibitors for detecting autophagy in plants

In cultured tobacco (BY-2) cells, autophagy seems to be induced under nutrient-starvation conditions, whereas in root cells from Arabidopsis and barley, it occurs constitutively though is activated under nutrient starvation conditions. In both cases, protease inhibitors such as E-64, E-64c, antipain, and leupeptin block autophagy at the step of degradation of the cytoplasm enclosed in lysosomes/vacuoles, and cause the accumulation of autolysosomes (lysosomes containing parts of the cytoplasm) and/or of many cytoplasmic inclusions in the central vacuoles. Both types of autophagy are inhibited by 3-methyladenine, which is known as a potent inhibitor of autophagy in mammalian cells. Thus, using protease inhibitors and 3-methyladenine provides us with a method useful for analyzing autophagy in plant cells. This chapter describes protocols for detecting autophagic compartments in BY-2 cells and in the root-tip cells of Arabidopsis and barley by microscopy.

Synthesis and biological evaluation of some 6-substituted purines

We report herein the synthesis and the in vitro antileishmanial evaluation of a series of 6-substituted purines. The most active compounds against Leishmania amazonensis promastigotes were 6-(3'-chloropropylthio)purine 2 [11,12] [corrected] 6-(3'-(thioethylamine)propylthio)purine 5, 6-(alpha-aceticacidthio)purine 7 and 6-(6'-deoxy-1'-O-methyl-beta-D-ribofuranose)purine 14 with an IC(50)=50, 50, 39 and 29 microM, respectively.

AtATG Genes, Homologs of Yeast Autophagy Genes, are Involved in Constitutive Autophagy in Arabidopsis Root Tip Cells

In Arabidopsis root tips cultured in medium containing sufficient nutrients and the membrane-permeable protease inhibitor E-64d, parts of the cytoplasm accumulated in the vacuoles of the cells from the meristematic zone to the elongation zone. Also in barley root tips treated with E-64, parts of the cytoplasm accumulated in autolysosomes and pre-existing central vacuoles. These results suggest that vacuolar and/or lysosomal autophagy occurs constitutively in these regions of cells. 3-Methyladenine, an inhibitor of autophagy, inhibited the accumulation of such inclusions in Arabidopsis root tip cells. Such inclusions were also not observed in root tips prepared from Arabidopsis T-DNA mutants in which AtATG2 or AtATG5, an Arabidopsis homolog of yeast ATG genes essential for autophagy, is disrupted. In contrast, an atatg9 mutant, in which another homolog of ATG is disrupted, accumulated a significant number of vacuolar inclusions in the presence of E-64d. These results suggest that both AtAtg2 and AtAtg5 proteins are essential for autophagy whereas AtAtg9 protein contributes to, but is not essential for, autophagy in Arabidopsis root tip cells. Autophagy that is sensitive to 3-methyladenine and dependent on Atg proteins constitutively occurs in the root tip cells of Arabidopsis.

3-methyladenine inhibits autophagy in tobacco culture cells under sucrose starvation conditions

Tobacco (Nicotiana tabacum) culture cells perform autophagy and degrade cellular proteins in response to sucrose starvation. When protein degradation is blocked by the cysteine protease inhibitor E-64c, lysosomes containing particles of cytoplasm (autolysosomes) accumulate in the cells. Therefore, using light microscopy, we can determine whether cells have performed autophagy. In this study, we investigated whether or not 3-methyladenine (3-MA), which is a known inhibitor of autophagy in mammalian cells, blocks autophagy in tobacco culture cells. The accumulation of autolysosomes was blocked by the addition to the culture media of 5 mM 3-MA together with E-64c. We did not detect autolysosomes or structures thought to be involved with autophagy, such as autophagosomes, accumulating in these cells, as observed by electron microscopy. 3-MA blocked cellular protein degradation without any effect on cellular protease activity. In mammalian cells, phosphatidylinositol 3-kinase (PtdIns 3-kinase) is a putative target of 3-MA. The PtdIns 3-kinase inhibitors wortmannin and LY294002 also inhibited the accumulation of autolysosomes in tobacco culture cells. These results suggest that (1) 3-MA inhibits autophagy by blocking the formation of autophagosomes in tobacco culture cells, and (2) PtdIns 3-kinase is essential for autophagy in tobacco cells.

Inhibition of hepatocytic autophagy by adenosine, adenosine analogs and AMP

Autophagy, measured in isolated rat hepatocytes as the sequestration of electroinjected [3H]raffinose, was moderately (17%) inhibited by adenosine (0.4 mM) alone, but more strongly (85%) in the presence of the adenosine deaminase inhibitor, 2'-deoxycoformycin (50 microM), suggesting that metabolic deamination of adenosine limited its inhibitory effectiveness. The adenosine analogs, 6-methylmercaptopurine riboside and N6,N6-dimethyladenosine, inhibited autophagy by 89% and 99%, respectively, at 0.5 mM, probably reflecting the adenosine deaminase-resistance of their 6-substitutions. 5-Iodotubercidin (10 microM), an adenosine kinase inhibitor, blocked the conversion of adenosine to AMP and largely abolished the inhibitory effects of both adenosine and its analogs, indicating that AMP/nucleotide formation was required for inhibition of autophagy. Inhibition by adenosine of autophagic protein degradation, measured as the release of [14C]valine from prelabelled protein, was similarly potentiated by deoxycoformycin and prevented by iodotubercidin. Inhibition of autophagy by added AMP, ADP or ATP (0.3-1 mM) was, likewise, potentiated by deoxycoformycin and prevented by iodotubercidin, suggesting dephosphorylation to adenosine and intracellular re-phosphorylation to AMP. Suppression of autophagy by AMP may be regarded as a feedback inhibition of autophagic RNA degradation, or as an aspect of the general down-regulation of energy-requiring processes that occurs under conditions of ATP depletion, when AMP levels are high.

Inhibition of hepatocytic autophagy by adenosine, aminoimidazole-4-carboxamide riboside, and N6-mercaptopurine riboside. Evidence for involvement of amp-activated protein kinase

To examine the role of AMP-activated protein kinase (AMPK; EC 2.7.1. 109) in the regulation of autophagy, rat hepatocytes were incubated with the AMPK proactivators, adenosine, 5-amino-4-imidazole carboxamide riboside (AICAR), or N6-mercaptopurine riboside. Autophagic activity was inhibited by all three nucleosides, AICAR and N6-mercaptopurine riboside being more potent (IC50 = 0.3 mM) than adenosine (IC50 = 1 mM). 2'-Deoxycoformycin, an adenosine deaminase (EC 3.5.4.4) inhibitor, increased the potency of adenosine 5-fold, suggesting that the effectiveness of adenosine as an autophagy inhibitor was curtailed by its intracellular deamination. 5-Iodotubercidin, an adenosine kinase (EC 2.7.1.20) inhibitor, abolished the effects of all three nucleosides, indicating that they needed to be phosphorylated to inhibit autophagy. A 5-iodotubercidin-suppressible phosphorylation of AICAR to 5-aminoimidazole-4-carboxamide riboside monophosphate was confirmed by chromatographic analysis. AICAR, up to 0.4 mM, had no significant effect on intracellular ATP concentrations. Because activated AMPK phosphorylates and inactivates 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase (EC 1.1.1.88), the rate-limiting enzyme in cholesterol synthesis, the strong inhibition of hepatocytic cholesterol synthesis by all three nucleosides confirmed their ability to activate AMPK under the conditions used. Lovastatin and simvastatin, inhibitors of HMG-CoA reductase, strongly suppressed cholesterol synthesis while having no effect on autophagic activity, suggesting that AMPK inhibits autophagy independently of its effects on HMG-CoA reductase and cholesterol metabolism.

Role of cAMP in the regulation of hepatocytic autophagy

To assess the role of cAMP in the regulation of autophagy, we examined the effects of cAMP analogues and cAMP-elevating agents on freshly isolated rat hepatocytes, using electroinjected [3H]raffinose as an autophagy probe. Glucagon was found to stimulate, inhibit or have no effect on autophagy, depending on the inclusion of metabolites like pyruvate (which caused ATP depletion and autophagy suppression) and amino acids (a complete mixture that antagonized pyruvate) in the incubation medium. Inhibition was also observed with theophylline, a cAMP-elevating inhibitor of cyclic nucleotide phosphodiesterases, and with the adenylyl cyclase activator deacetylforskolin. At low concentrations of deacetylforskolin, the inhibition could be abolished by amino acids. N6,2'-O-Dibutyryladenosine 3',5'-monophosphate (Bt2-cAMP) strongly inhibited both autophagic sequestration of [3H]raffinose and overall autophagic protein degradation; again, amino acids abolished the autophagy-inhibitory effect of low Bt2-cAMP concentrations. Several other cAMP analogues (8-thiomethyl-cAMP, N6-benzoyl-cAMP, (S)-5,6-dichloro-1-D-ribofuranosylbenzimidazole 3',5'-[thio]monophosphate, (S)-8-bromoadenosine 3',5'-[thio]monophosphate) inhibited autophagy as well. The effect of Bt2-cAMP was rapid, dose-dependent, reversible and did not require concomitant protein synthesis. Neither Bt2-cAMP nor deacetylforskolin reduced intracellular ATP levels or cell viability, ruling out inhibition of autophagy by non-specific cytotoxicity. The autophagy-inhibitory effect of Bt2-cAMP could be substantially antagonized (40-50%) by KT-5720, a specific inhibitor of the cAMP-dependent protein kinase A, and by the nonspecific protein kinase inhibitor K-252a. Somewhat surprisingly, KN-62 and KT-5926, allegedly specific inhibitors of Ca2+/calmodulin-dependent protein kinase II and myosin light chain kinase, respectively, were also Bt2-cAMP-antagonistic. These results suggest that cAMP regulates the early, sequestrational step of hepatocytic autophagy by a highly conditional, dual mechanism, inhibition being predominant under most conditions in freshly isolated hepatocytes, whereas stimulation reportedly predominates in vivo. The effect of cAMP is probably mediated by protein kinase A, but other protein kinases would appear to participate in the regulation of autophagic sequestration as well.

Cellular vacuoles induced by Helicobacter pylori originate from late endosomal compartments

Pathogenic strains of Helicobacter pylori cause progressive vacuolation and death of epithelial cells. To identify the nature of vacuoles, the distribution of markers of various membrane traffic compartments was studied. Vacuoles derive from the endocytic pathway since they include the fluid-phase marker Lucifer yellow. Early endosome markers such as rab5, transferrin, and transferrin receptor, as well as the lysosomal hydrolase cathepsin D, are excluded from these structures. In contrast, the vacuolar membrane is specifically stained by affinity-purified antibodies against rab7, a small GTPase, localized to late endosomal compartments. The labeling of rab7 on vacuolar membranes increases as vacuolation progresses, without a concomitant increase of cellular rab7. Cell vacuolation is inhibited by the microtubule-depolymerizing agents nocodazole and colchicine. Taken together, these findings indicate that the vacuoles specifically originate from late endosomal compartments.

Autophagy and other vacuolar protein degradation mechanisms

Autophagic degradation of cytoplasm (including protein, RNA etc.) is a non-selective bulk process, as indicated by ultrastructural evidence and by the similarity in autophagic sequestration rates of various cytosolic enzymes with different half-lives. The initial autophagic sequestration step, performed by a poorly-characterized organelle called a phagophore, is subject to feedback inhibition by purines and amino acids, the effect of the latter being potentiated by insulin and antagonized by glucagon. Epinephrine and other adrenergic agonists inhibit autophagic sequestration through a prazosin-sensitive alpha 1-adrenergic mechanism. The sequestration is also inhibited by cAMP and by protein phosphorylation as indicated by the effects of cyclic nucleotide analogues, phosphodiesterase inhibitors and okadaic acid. Asparagine specifically inhibits autophagic-lysosomal fusion without having any significant effects on autophagic sequestration, on intralysosomal degradation or on the endocytic pathway. Autophaged material that accumulates in prelysosomal vacuoles in the presence of asparagine is accessible to endocytosed enzymes, revealing the existence of an amphifunctional organelle, the amphisome. Evidence from several cell types suggests that endocytosis may be coupled to autophagy to a variable extent, and that the amphisome may play a central role as a collecting station for material destined for lysosomal degradation. Protein degradation can also take place in a 'salvage compartment' closely associated with the endoplasmic reticulum (ER). In this compartment unassembled protein chains are degraded by uncharacterized proteinases, while resident proteins return to the ER and assembled secretory and membrane proteins proceed through the Golgi apparatus. In the trans-Golgi network some proteins are proteolytically processed by Ca(2+)-dependent proteinases; furthermore, this compartment sorts proteins to lysosomes, various membrane domains, endosomes or secretory vesicles/granules. Processing of both endogenous and exogenous proteins can occur in endosomes, which may play a particularly important role in antigen processing and presentation. Proteins in endosomes or secretory compartments can either be exocytosed, or channeled to lysosomes for degradation. The switch mechanisms which decide between these options are subject to bioregulation by external agents (hormones and growth factors), and may play an important role in the control of protein uptake and secretion.

Carboxy terminally truncated forms of ribophorin I are degraded in pre-Golgi compartments by a calcium-dependent process

Two COOH terminally truncated variants of ribophorin I (RI), a type I transmembrane glycoprotein of 583 amino acids that is segregated to the rough portions of the ER and is associated with the protein-translocating apparatus of this organelle, were expressed in permanent HeLa cell transformants. Both variants, one membrane anchored but lacking part of the cytoplasmic domain (RL467) and the other consisting of the luminal 332 NH2-terminal amino acids (RI332), were retained intracellularly but, in contrast to the endogenous long lived, full length ribophorin I (t 1/2 = 25 h), were rapidly degraded (t 1/2 less than 50 min) by a nonlysosomal mechanism. The absence of a measurable lag phase in the degradation of both truncated ribophorins indicates that their turnover begins in the ER itself. The degradation of RI467 was monophasic (t 1/2 = 50 min) but the rate of degradation of RI332 molecules increased about threefold approximately 50 min after their synthesis. Several pieces of evidence suggest that the increase in degradative rate is the consequence of the transport of RI332 molecules that are not degraded during the first phase to a second degradative compartment. Thus, when added immediately after labeling, ionophores that inhibit vesicular flow out of the ER, such as carbonyl cyanide m-chlorophenylhydrazone (CCCP) and monensin, suppressed the second phase of degradation of RI332. On the other hand, when CCCP was added after the second phase of degradation of RI332 was initiated, the degradation was unaffected. Moreover, in cells treated with brefeldin A the degradation of RI332 became monophasic, and took place with a half-life intermediate between those of the two normal phases. These results point to the existence of two subcellular compartments where abnormal ER proteins can be degraded. One is the ER itself and the second is a non-lysosomal pre-Golgi compartment to which ER proteins are transported by vesicular flow. A survey of the effects of a variety of other ionophores and protease inhibitors on the turnover of RI332 revealed that metalloproteases are involved in both phases of the turnover and that the maintenance of a high Ca2+ concentration is necessary for the degradation of the luminally truncated ribophorin.

Both endocytic and endogenous protein degradation in fibroblasts is stimulated by serum/amino acid deprivation and inhibited by 3-methyladenine

Incubation of BHK-21 hamster fibroblasts in a serum- and amino acid-deficient medium caused a 3-fold increase in the degradation of endogenous protein, a doubling of the degradation of endocytosed epidermal growth factor, and an eightfold increase in the degradation of endocytosed alpha 2-macroglobulin. 3-Methyladenine (3MA) inhibited the deprivation-induced lysosomal degradation of both endogenous and endocytosed protein, but had no effect on basal (non-induced) degradation. 3MA also inhibited deprivation-induced protein degradation in human IMR-90 fibroblasts. Some inhibition of protein synthesis and of endocytic uptake of alpha 2-macroglobulin was observed in 3MA-treated BHK-21 cells, whereas cellular ATP levels were unaffected. These results are different from those obtained with isolated hepatocytes, and suggest that in some cells both endogenous and endocytic protein degradation may be accelerated as part of a general deprivation response.

Azathioprine, a genotoxic agent to be considered non-genotoxic in man

Azathioprine, an immunosuppressive drug, has been used for 25 years. Azathioprine is rapidly converted into a number of metabolites after absorption. Maximum blood levels in experimental animals (mice) were 11.3 micrograms/ml after a dosage of 33.3 mg/kg. Generally, levels of less than 1 microgram/ml are found. As azathioprine is ineffective in hypoxanthine guanine phosphoribosyltransferase (HPRT)-deficient patients, it will be clear that for immunosuppressive activity azathioprine must be metabolised. Regarding mutagenic activity, its mutagenicity for bacteria seems irrelevant for man because the nitroimidazole moiety can be reduced by bacteria but not or hardly at all by mammalian tissues. So 6-mercaptopurine (a metabolite of azathioprine) and its metabolites should be regarded as the active compounds. In vitro azathioprine can induce chromosome aberrations and other cytogenetic events at high, non-physiological doses. However, in view of the low blood levels it is unlikely that azathioprine can induce chromosome aberrations in kidney transplant patients. It is more probable that azathioprine inhibits the elimination of such aberrant cells through its immunosuppressive activity. It should be pointed out that in microbial mutagenicity systems also, azathioprine concentrations that are not reached in patients are needed to obtain an increased mutation rate.

3-Methyladenine, an inhibitor of autophagy, has multiple effects on metabolism

3-Methyladenine is generally used as an inhibitor of autophagy [P. O. Seglen & P. B. Gordon (1982) Proc. Natl Acad. Sci. USA 79, 1889-1892]. Using isolated hepatocytes, we observed that 3-methyladenine has other effects as well. 1. 3-Methyladenine promoted glycogen breakdown and inhibited flux through phosphofructokinase and pyruvate kinase. These effects proved to be unrelated to inhibition of autophagic proteolysis and were caused by cAMP, which slightly increased in the presence of 3-methyladenine. 2. Addition of 3-methyladenine to intact hepatocytes increased the intralysosomal pH and caused a lower density of the lysosomal population upon centrifugation in a Percoll density gradient. No increase in the intralysosomal pH was effected by 3-methyladenine in isolated lysosomes.

Energy dependence of autophagic protein degradation in isolated rat hepatocytes

The effect of small changes in intracellular ATP on autophagic flux was studied in isolated rat hepatocytes by using inhibitors of ATP production or by varying the metabolic conditions. The following observations were made. There was a linear relationship between endogenous protein degradation and intracellular ATP, the rate of proteolysis declining with decreasing ATP concentrations. 15% of the maximal proteolysis is either independent of ATP or has a very high affinity for this metabolite. There was a linear relationship between the autophagic sequestration of cytosolic [14C]sucrose and intracellular ATP, the sequestration rate decreasing with decreasing ATP concentrations. ATP depletion did not cause release of [14C]sucrose previously sequestered in autophagosomes and lysosomes at high ATP levels. Intracellular accumulation of chloroquine, used as an indicator of the pH inside lysosomes and other acidic cell compartments, diminished with decreasing cellular ATP content. Amino acids inhibited proteolysis without affecting ATP levels or chloroquine accumulation. We conclude from the high sensitivity of autophagy towards relatively small changes in the concentration of intracellular ATP that, besides amino acids, ATP is a very important factor in controlling the rate of autophagy in rat hepatocytes.

Accounting for the ATP-consuming processes in rabbit reticulocytes

The report deals with a detailed balance of ATP production and consumption of the rabbit reticulocyte. The sum-total of ATP produced amounts to 135 mmol . 1-1 . h-1. About 70% of the ATP consumption has been accounted for by specific processes. The main contributing processes are the globin synthesis with about 28%, the Na+, K+-ATPase with 23% and the proteolysis with more than 15%. 30% of ATP consumption has not been accounted for. Cycloheximide (20 microM) leads to a dissociation between synthesis and degradation of proteins, which argues against any obligatory connection between these processes. More than 90% of the lysine liberated from mitochondria by proteolysis were reutilized for the globin synthesis demonstrating the high nitrogen economy of reticulocytes. Each of the ATP-consuming processes studied appears to control ATP production in an independent manner without competition with each other.

Effects of exogenous amines on mammalian cells, with particular reference to membrane flow

We have reviewed the evidence that amines accumulate in intracellular vesicles of low pH, such as lysosomes and endosomes. There is consequent elevation of intravesicular pH, and inhibition of receptor-ligand dissociation often results from this pH change. We have argued that the capacity for fusion of such vesicles is also reduced by the high pH. We suggest that the variety of effects of amines on membrane flow and macromolecular transport we describe are at least partly due to such reduced fusion (Figs. 1 and 2). We propose that an internal low pH may facilitate heterologous vesicle-vesicle and vesicle-plasma membrane fusion. There is some evidence that clathrin can accelerate phospholipid vesicle fusion in vitro at low pH (Blumenthal et al., 1983) but no direct evidence on the role of intravesicular pH. This idea is consistent not only with the preceding discussion, but also with the fact that the intracellular membrane-bound compartments least involved in fusion events (e.g. mitochondria) are of neutral or alkaline internal pH. Membrane fusion is certainly required for the formation of vesicles at the periphery of the Golgi apparatus, and possibly earlier in the transport and processing of biosynthetic products in the Golgi (Bergeron et al., 1982). Thus the accumulation of amines in the Golgi may be responsible for several effects on the flow of macromolecules along their translocation pathways. The status of the plasma membrane in this view is complex. It might be argued that the pH dictating the fusion step in endocytosis is that of the extracellular fluid, in which case the inhibitory effects of amines on this process are not explained. However, the rapidity of acidification of the newly formed endocytic vesicles allows the possibility that plasma membrane invaginations might temporarily sequester areas which are of lower pH than that of the bulk extracellular fluid even before fusion, since the proton pumping enzyme(s) are probably present on the plasma membrane. Were this the case, then an acid pH could again be a factor determining membrane fusion at the plasma membrane. The inhibition of endocytosis by weak bases thus may again reflect elevation of pH in a sequestered compartment. From the data on the dependence of response on the concentration of amines, we anticipate that most responses involving membrane flow will be biphasic, with inhibitory effects at low amine concentration, giving way to stimulatory ones at higher concentrations. We suggest that the reported dichotomy between different amines in intracellular membrane fusion systems (D'Arcy Hart, 1982) may result from this concentration dependence.(ABSTRACT TRUNCATED AT 400 WORDS)