Effect of reactive oxygen species on iron regulatory protein activity

Iron may be important in catalyzing excessive production of reactive oxygen species (ROS). Cellular iron homeostasis is regulated by iron regulatory proteins (IRPs), which bind to iron-responsive elements (IRE) of mRNAs for ferritin and transferrin receptor (TfR) modulating iron uptake and sequestration, respectively. Although iron is the main regulator of IRP activity, IRP is also influenced by other factors, including the redox state. Therefore, IRP might be sensitive to pathophysiological alterations of redox state caused by ROS. However, previous studies have produced diverging evidence on the effect of oxidative injury on IRP. Results obtained in an animal model close to a pathophysiological condition, such as ischemia reperfusion of the liver as well as in a cell-free system involving an enzymatic source of O2 and H2O2, indicate that IRP is downregulated by oxidative stress. In fact, IRP activity is inhibited at early times of post-ischemic reperfusion. Moreover, the concerted action of O2 and H2O2 produced by xanthine oxidase in a cell-free system caused a remarkable inhibition of IRP activity. IRP seems a direct target of ROS; in fact, in vivo inhibition can be prevented by the antioxidant N-acetylcysteine and by interleukin-1 receptor antagonist. In addition, modulation of iron levels of the cell-free assay did not affect the downregulation imposed by xanthine oxidase. Conceivably, downregulation of IRP activity by O2 and H2O2 may facilitate iron sequestration into ferritin, thus limiting the pro-oxidant challenge of iron.

The secondary alcohol metabolite of doxorubicin irreversibly inactivates aconitase/iron regulatory protein-1 in cytosolic fractions from human myocardium

Anticancer therapy with doxorubicin (DOX) is limited by severe cardiotoxicity, presumably reflecting the intramyocardial formation of drug metabolites that alter cell constituents and functions. In a previous study, we showed that NADPH-supplemented cytosolic fractions from human myocardial samples can enzymatically reduce a carbonyl group in the side chain of DOX, yielding a secondary alcohol metabolite called doxorubicinol (DOXol). Here we demonstrate that DOXol delocalizes low molecular weight Fe(II) from the [4Fe-4S] cluster of cytoplasmic aconitase. Iron delocalization proceeds through the reoxidation of DOXol to DOX and liberates DOX-Fe(II) complexes as ultimate by-products. Under physiologic conditions, cluster disassembly abolishes aconitase activity and forms an apoprotein that binds to mRNAs, coordinately increasing the synthesis of transferrin receptor but decreasing that of ferritin. Aconitase is thus converted into an iron regulatory protein-1 (IRP-1) that causes iron uptake to prevail over sequestration, forming a pool of free iron that is used for metabolic functions. Conversely, cluster reassembly converts IRP-1 back to aconitase, providing a regulatory mechanism to decrease free iron when it exceeds metabolic requirements. In contrast to these physiologic mechanisms, DOXol-dependent iron release and cluster disassembly not only abolish aconitase activity, but also affect irreversibly the ability of the apoprotein to function as IRP-1 or to reincorporate iron within new Fe-S motifs. This damage is mediated by DOX-Fe(II) complexes and reflects oxidative modifications of -SH residues having the dual role to coordinate cluster assembly and facilitate interactions of IRP-1 with mRNAs. Collectively, these findings describe a novel mechanism of cardiotoxicity, suggesting that intramyocardial formation of DOXol may perturb the homeostatic processes associated with cluster assembly or disassembly and the reversible switch between aconitase and IRP-1. These results may also provide a guideline to design new drugs that mitigate the cardiotoxicity of DOX.

Response of monocyte iron regulatory protein activity to inflammation: abnormal behavior in genetic hemochromatosis

In genetic hemochromatosis (GH), iron overload affects mainly parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells. We previously found that RE cells from GH patients had an inappropriately high activity of iron regulatory protein (IRP), the key regulator of intracellular iron homeostasis. Elevated IRP should reflect a reduction of the iron pool, possibly because of a failure to retain iron. A defect in iron handling by RE cells that results in a lack of feedback regulation of intestinal absorption might be the basic abnormality in GH. To further investigate the capacity of iron retention in RE cells of GH patients, we used inflammation as a model system as it is characterized by a block of iron release from macrophages. We analyzed the iron status of RE cells by assaying IRP activity and ferritin content after 4, 8, and 24 hours of incubation with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma). RNA-bandshift assays showed that in monocytes and macrophages from 16 control subjects, IRP activity was transiently elevated 4 hours after treatment with LPS and IFN-gamma but remarkably downregulated thereafter. Treatment with NO donors produced the same effects whereas an inducible Nitric Oxide Synthase (iNOS) inhibitor prevented them, which suggests that the NO pathway was involved. Decreased IRP activity was also found in monocytes from eight patients with inflammation. Interestingly, no late decrease of IRP activity was detected in cytokine-treated RE cells from 12 GH patients. Ferritin content was increased 24 hours after treatment in monocytes from normal subjects but not in monocytes from GH patients. The lack of downregulation of IRP activity under inflammatory conditions seems to confirm that the control of iron release from RE cells is defective in GH.

Nitric oxide-mediated induction of ferritin synthesis in J774 macrophages by inflammatory cytokines: role of selective iron regulatory protein-2 downregulation

Cytokine-treated macrophages represent a useful model to unravel the molecular basis of reticuloendothelial (RE) iron retention in inflammatory conditions. In the present study, we showed that stimulation of murine macrophage J774 cells with interferon (IFN)-gamma/lipopolysaccharide (LPS) resulted in a nitric oxide-dependent modulation of the activity of iron regulatory proteins (IRP)-1 and 2, cytoplasmic proteins which, binding to RNA motifs called iron responsive elements (IRE), control ferritin translation. Stimulation with cytokines caused a small increase of IRP-1 activity and a strong reduction of IRP-2 activity accompanied by increased ferritin synthesis and accumulation. Cytokines induced only a minor increase of H chain ferritin mRNA, thus indicating that IRP-2-mediated posttranscriptional regulation plays a major role in the control of ferritin expression. This was confirmed by direct demonstration that the translational repression function of IRP was impaired in stimulated cells. In fact, translation in cell-free extracts of a reporter transcript under the control of an IRE sequence was repressed less efficiently by IRP-containing lysates from cytokine-treated cells than by lysates from control cells. Our findings throw light on the role of IRP-2 showing that: (1) this protein responds to a stimulus in opposite fashion to IRP-1; (2) when abundantly expressed, as in J774 cells, IRP-2 is sufficient to regulate intracellular iron metabolism in living cells; and (3) by allowing increased ferritin synthesis, IRP-2 may play a role in the regulation of iron homeostasis in RE cells during inflammation.

Mitochondrial respiratory chain deficiency leads to overexpression of antioxidant enzymes

U937 cell growth in the presence of either chloramphenicol or ethidium bromide rapidly leads to respiratory deficiency. The novel finding of this report is that this response is paralleled by a specific increase in Se-dependent and independent glutathione peroxidase activities as well as of glutathione peroxidase and heme oxygenase mRNAs. Under the same experimental conditions, catalase activity and catalase mRNA do not show appreciable changes. These results can be explained by an increased formation of H2O2 at the early times of development of respiratory deficiency followed by induction of antioxidant enzymes.

Haemodynamic alteration in patients undergoing chronic haemodialysis

Fifteen elderly patients, 13 of them undergoing chronic haemodialysis, 1 acute and 1 coming from Continuous Ambulatory Peritoneal Dialysis (CAPD) either with no significant cardiovascular alteration or presenting various cardiovascular pathologies were studied to investigate the possibility of onset of hypotensive episodes during dialytic treatment depending on cardiac or vascular alteration in the patients. Monitoring of the arterial pressure on the contralateral arm and on the lower limbs by using the Takeda System, made it possible to compute the Windsor Index (WI). The figures obtained were correlated to the Ejection Fraction Index (EFI) to investigate the relation between WI alteration and haemodynamic variations in the patient. The results show that cardiothoracic recirculation is much more present in those patients with pathologies that affect EFI which worsens during dialysis due to the loss of fluid. Moreover the results obtained from the two patients with temporary access and no evident cardiovascular pathology show the constancy of the haemodynamic parameters throughout the dialytic treatment.

Haemodynamic Alteration in Patients Undergoing Chronic Haemodialysis

Fifteen elderly patients, 13 of them undergoing chronic haemodialysis, 1 acute and 1 coming from Continuous Ambulatory Peritoneal Dialysis (CAPD) either with no significant cardiovascular alteration or presenting various cardiovascular pathologies were studied to investigate the possibility of onset of hypotensive episodes during dialytic treatment depending on cardiac or vascular alteration in the patients. Monitoring of the arterial pressure on the contralateral arm and on the lower limbs by using the Takeda System, made it possible to compute the Windsor Index (WI). The figures obtained were correlated to the Ejection Fraction Index (EFI) to investigate the relation between WI alteration and haemodynamic variations in the patient. The results show that cardiothoracic recirculation is much more present in those patients with pathologies that affect EFI which worsens during dialysis due to the loss of fluid. Moreover the results obtained from the two patients with temporary access and no evident cardiovascular pathology show the constancy of the haemodynamic parameters throughout the dialytic treatment.

Induction of ferritin synthesis in ischemic-reperfused rat liver: analysis of the molecular mechanisms

BACKGROUND & AIMS

Iron may catalyze the production of reactive oxygen species (ROS) during postischemic reoxygenation. Ferritin, a cellular iron storage protein, can either represent a source of iron or perform a cytoprotective action against ROS. The aim of this study was to address the role of ferritin in postischemic reperfusion.

METHODS

Transcriptional and posttranscriptional mechanisms controlling ferritin gene expression were studied in reperfused rat livers.

RESULTS

Proteolysis reduced ferritin levels 2 hours after reperfusion, but a concomitant increase of synthesis, accompanied by enhanced transcription and accumulation of H and L ferritin subunit messenger RNAs (mRNAs), almost re-established normal ferritin content at 4 hours. Pretreatment with interleukin 1 receptor antagonist (IL-1RA) did not prevent the rise of ferritin mRNAs. RNA bandshift assays showed that the activity of the iron regulatory proteins (IRPs), which control ferritin mRNA translation, declined early after reperfusion and recovered progressively thereafter. Pretreatment with either the antioxidant N-acetyl cysteine or IL-1RA was sufficient to prevent almost completely down-regulation of IRP activity.

CONCLUSIONS

Postischemic reperfusion causes degradation of ferritin, possibly increasing iron levels. However, induction of ferritin gene transcription, possibly mediated by ferritin-derived iron and ROS-mediated inactivation of IRP, which allows translation of ferritin mRNAs, counteracts this effect and concurs to reestablish the amount of ferritin, which may thus act to limit reperfusion damage.

Hereditary hyperferritinemia-cataract syndrome: relationship between phenotypes and specific mutations in the iron-responsive element of ferritin light-chain mRNA

Recent reports have described families in whom a combination of elevated serum ferritin not related to iron overload and congenital nuclear cataract is transmitted as an autosomal dominant trait. We have studied the molecular pathogenesis of hyperferritinemia in two families showing different phenotypic expression of this new genetic disorder. Serum ferritin levels ranged from 950 to 1,890 microg/L in affected individuals from family 1, and from 366 to 635 microg/L in those from family 2. Cataract was clinically manifested in family 1 and asymptomatic in family 2. By using monoclonal antibodies specific for the H and L ferritin subunits, serum ferritin was found to be essentially L type in both normal and affected individuals. The latter also showed normal amounts of H-type ferritin in circulating mononuclear cells; on the contrary, L-type ferritin contents were 13 times normal in family 1 and five times normal in family 2 on average. Serum ferritin was glycosylated in both normal and affected individuals. There was a close relationship between mononuclear cell L-type ferritin content and serum ferritin concentration (r = 0.95, P < .00001), suggesting that the excess production of ferritin in cells was directly responsible for the hyperferritinemia. The dysregulated L-subunit synthesis was found to result from different point mutations in a noncoding sequence of genomic L-subunit DNA, which behaves as an mRNA cis-acting element known as iron regulatory element (IRE). Affected individuals from family 1 were heterozygous for a point mutation (a single G to A change) in the highly conserved, three-nucleotide motif forming the IRE bulge. Affected members from family 2 were heterozygous for a double point mutation in the IRE lower stem. Using a gel retardation assay, the observed molecular lesions were shown to variably reduce the IRE affinity for an iron regulatory protein (IRP), which inhibits ferritin mRNA translation. The direct relationship between the degree of hyperferritinemia and severity of cataract suggests that this latter is the consequence of excessive ferritin production within the lens fibers. These findings provide strong evidence that serum ferritin is a byproduct of intracellular ferritin synthesis and that the L-subunit gene on chromosome 19 is the source of glycosylated serum ferritin. From a practical standpoint, this new genetic disorder should be taken into account by clinicians when facing a high serum ferritin in an apparently healthy person.

Inappropriately high iron regulatory protein activity in monocytes of patients with genetic hemochromatosis

In genetic hemochromatosis (GH), excess iron is deposited in parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells until the later stages of the disease. As iron absorption is inversely related to RE cells stores, a failure of RE to retain iron has been proposed as the basic defect in GH. In RE cells of GH subjects, we examined the activity of iron regulatory protein (IRP), a reliable indicator of the elusive regulatory labile iron pool, which modulates cellular iron homeostasis through control of ferritin (Ft) and transferrin receptor gene expression. RNA-bandshift assays showed a significant increase in IRP activity in monocytes from 16 patients with untreated GH compared with 28 control subjects (1.5-fold) and five patients with secondary hemochromatosis (SH) with similar iron burden (fourfold). In 17 phlebotomy-treated GH patients, IRP activity did not differ from that of control subjects. In both GH and SH monocyte-macrophages, Ft content increased by twofold and the L subunit-rich isoferritin profile was unchanged as compared with controls. IRP activity was still upregulated in vitro in monocyte-derived macrophages of GH subjects but, following manipulations of iron levels, was modulated normally. Therefore, the sustained activity of monocyte IRP found in vivo in monocytes of GH patients is not due to an inherent defect of its control, but is rather the expression of a critical abnormality of iron metabolism, eg, a paradoxical contraction of the regulatory iron pool. By preventing Ft mRNA translation, high IRP activity in monocytes may represent a molecular mechanism contributing to the inadequate Ft accumulation and insufficient RE iron storage in GH.

Superoxide and hydrogen peroxide-dependent inhibition of iron regulatory protein activity: a protective stratagem against oxidative injury

Cellular iron homeostasis is regulated by the cytoplasmic iron regulatory protein (IRP), which binds to iron-responsive elements (IRE) of mRNAs, modulating iron uptake and sequestration, respectively. When iron is scarce, IRP binds to IRE and coordinately increases the synthesis of transferrin receptor and decreases that of ferritin, thus providing the cell with readily available free iron. When iron is in excess, IRP does not bind and iron sequestration prevails over iron uptake. We have found that incubation of rat liver lysates with xanthine oxidase (XO), which generates superoxide (O2-.) and hydrogen peroxide (H2O2), caused a remarkable but reversible inhibition of IRP activity, as the formation of IRE-IRP decreased by 70-80% but returned to baseline values upon exposure to a reducing agent like 2-mercaptoethanol. IRP inhibition was prevented by separate or simultaneous addition of superoxide dismutase and catalase, showing that both O2-. and H2O2 were involved. By contrast, iron chelators and hydroxyl radical scavengers did not impede the inhibition of IRP, suggesting that O2-. and H2O2 acted independently of free iron sources. Ferritin enhanced IRP inhibition, but this process involved tightly bound iron centers that shunted reducing equivalents from XO and returned them to oxygen, thus increasing the formation of O2-. In agreement with the exclusive role of O2-. and H2O2, XO also inhibited recombinant human IRP in the absence of iron. These results demonstrate that O2-. and H2O2 can directly but reversibly down-regulate the RNA-binding activity of IRP, causing transient decrease of free iron that otherwise would convert them into more potent oxidants such as hydroxyl radicals or equally aggressive iron-peroxo complexes. This establishes a novel protective stratagem against oxidative injury under pathophysiologic conditions characterized by the excessive generation of O2-. and H2O2.

Nitric-oxide-mediated activation of iron-regulatory protein controls hepatic iron metabolism during acute inflammation

The molecular regulation of intracellular iron metabolism has been studied in the livers of rats undergoing an acute inflammatory reaction following turpentine injection. Treatment induced an increase in the steady-state level of the transferrin receptor (TfR) mRNA, peaking 18 h after treatment and returning to control levels 24 h after treatment, with no change in TfR gene transcription. RNA band-shift assays documented an activation of the cytoplasmic RNA-binding protein called the iron-regulatory protein (IRP), in parallel with a rise in the amount of TfR transcripts. A 2-3-fold increase in the amount of H and L ferritin subunit mRNAs was found 12-18 h after turpentine treatment. Surprisingly, higher accumulation of ferritin mRNAs did not result in appreciable differences in the liver ferritin content. This might be due to the concomitant rise of IRP activity, which is known to prevent ferritin mRNA translation. The absence of significant changes in the total iron and ferritin contents prompted us to investigate the role of nitric oxide (NO), an inflammatory mediator which is also known to modulate the activity of IRP. Northern-blot analysis showed a marked enhancement in the expression of the inducible form of nitric oxide synthase mRNA in turpentine-treated rats. Furthermore, the activation of IRP and the increase of the TfR mRNA content that occur in turpentine-treated rats were abolished by treatment with N5-nitro-L-arginine, a specific nitric oxide synthase inhibitor. The present data suggest that NO-mediated activation of IRP regulates alterations of hepatic iron homeostasis that occur in acute inflammation.

Induction of ferritin synthesis by oxidative stress. Transcriptional and post-transcriptional regulation by expansion of the "free" iron pool

Ferritin, by regulating the "free" intracellular iron pool, controls iron-catalyzed generation of reactive oxygen species, but its role in oxidative damage is still unclear. We show that ferritin synthesis is significantly stimulated in the liver of rats subjected to oxidative stress by treatment with phorone, a glutathione-depleting drug. RNA-bandshift assays document reduced activity of iron regulatory factor, in particular of IRFB, the cytoplasmic protein that post-transcriptionally controls ferritin mRNA translation. Furthermore, Northern blot analysis shows increased accumulation of H and L subunit mRNAs, and nuclear run-on experiments provide evidence of transcriptional activation. Direct measurements of intracellular free iron levels by EPR indicate that the increased ferritin synthesis can be mediated by an expansion of the free iron pool. An early drop of ferritin content after phorone treatment indicates that part of the iron that fuels the free pool might derive from ferritin degradation. Present data seem to suggest that, under conditions of oxidative stress, liver ferritin can represent either a pro- or an anti-oxidant in a time-dependent manner. In fact, its early degradation contributes to expand the intracellular free iron pool that, later on, activates multiple molecular mechanisms to reconstitute ferritin content, thus limiting the pro-oxidant challenge of iron.

Duodenal ferritin synthesis in genetic hemochromatosis

BACKGROUND/AIMS

The molecular defect of genetic hemochromatosis (GH) is unknown. It is believed that low expression of duodenal ferritin in GH is caused by tissue or cell specific defect of ferritin synthesis. Our study was designed to ascertain whether the control of duodenal ferritin synthesis in GH was defective.

METHODS

Expression at the single cell level of H and L ferritin messenger RNAs and protein and activity of the iron regulatory factor, which controls the translation of ferritin messenger RNA, were assessed in 43 duodenal biopsy specimens from individuals with GH, secondary hemochromatosis (SH), anemia, or normal iron balance.

RESULTS

Signal for ferritin H and L subunit messenger RNAs was detected in both absorptive and nonabsorptive cells by in situ hybridization, but in 10 of 14 patients with untreated GH, the signal was lower than in patients with SH or normal subjects. However, immunostaining for ferritin protein documented a diffuse/cytoplasmic pattern, whereas a supranuclear/granular staining was found in normal subjects or patients with SH. The spontaneous activity of duodenal iron regulatory factor was consistently higher in patients with GH than in normal subjects or subjects with anemia or SH.

CONCLUSIONS

In patients with GH, ferritin gene transcription is preserved in both absorptive and nonabsorptive intestinal cells. Low accumulation of ferritin is not caused by a defective control of ferritin synthesis but by low expression of ferritin messenger RNA and sustained activity of iron regulatory factor.

Transferrin receptor gene expression during rat liver regeneration. Evidence for post-transcriptional regulation by iron regulatory factorB, a second iron-responsive element-binding protein

Transferrin receptor (TfR) expression is regulated by iron at the level of mRNA stability through a factor (IRF/IRE-BP) which binds to specific iron-responsive elements (IRE). On the other hand, growth-dependent regulation of TfR expression is generally believed to be transcriptionally controlled. We analyzed the molecular mechanisms that control TfR gene expression at the onset of cell proliferation in vivo during liver regeneration after partial hepatectomy. The amount of TfR mRNA increased considerably after partial hepatectomy while run-on assays did not show significant changes in TfR gene transcription. RNA band-shift assays documented a significant activation of IRF/IRE-BP specific for the faster migrating IRE-protein complex (IRFB). These changes occurred in the absence of modifications of total liver iron concentration but together with a significant decrease of ferritin content. Moreover, when extreme variations of liver iron content were achieved by either chronic iron overload or severe iron deficiency, liver regeneration was unable to influence IRE-binding activity. We conclude that IRF/IRE-BP-mediated post-transcriptional control can fully account for TfR mRNA induction during liver cell proliferation in vivo. IRF/IRE-BP activation in the absence of changes in total tissue iron content might depend either on a drop of iron levels into the regulatory pool or on a relatively iron-independent mechanism specific for the faster migrating complex.

Expression of a growth arrest specific gene (gas-6) during liver regeneration: molecular mechanisms and signalling pathways

A set of growth arrest-specific (gas) genes negatively regulated by serum has been identified. To define the role of gas genes in a model of cell proliferation in vivo we analyzed the expression of one of these genes (gas-6) during liver regeneration after partial hepatectomy (PH). We found that gas-6 mRNA was down-regulated 4 hours after PH, within the G0 to G1 transition. Later on, gas-6 mRNA increased over the level found in normal liver with a peak at 16 hours, before the onset of DNA synthesis. This surge was probably triggered by an inflammatory response caused by the surgical trauma, because an increase of similar extent occurring with the same time course was present in livers of sham-operated and turpentine-treated rats. Comparison of mRNA steady state levels with nuclear transcription rates indicated that gas-6 expression is post-transcriptionally regulated. As we found that down-regulation of gas-6 expression was prevented by treatment with Actinomycin D, a labile protein might be involved in the determination of gas-6 mRNA stability. To investigate the mitogenic signals controlling gas-6 expression during liver regeneration we treated hepatectomized rats with a specific alpha-1-adrenoceptor blocker (prazosin) as well as with drugs which modify intracellular calcium levels. The decrease of gas-6 mRNA 4 hours after PH was prevented by prazosin and by neomycin, an inhibitor of calcium release from endogenous stores. These findings suggest that down-regulation of gas-6 expression during hepatic regeneration is triggered by catecholamines interaction with alpha-1-adrenergic receptors and by subsequent calcium release. In addition we found that the rise of gas-6 gene expression occurring at 16 hours after PH was not affected by prazosin but was inhibited by trifluoperazine. Therefore, we suggest that up-regulation of gas-6 gene expression is mediated by the interaction of calcium with calmodulin, independently of catecholamines.

Course and prognosis of 132 patients with diabetic non ketotic hyperosmolar state

One hundred and thirty two episodes of diabetic non ketotic hyperosmolar states were studied after a prospective schedule of treatment was designed. The admission data, the prognostic factors and their outcome were analyzed. Initial high osmolarity, urea and sodium plasma levels and low plasma pH were related to the admission level of consciousness (p < 0.01). High glucose, osmolarity, urea and sodium plasma levels at entry were related to the admission level of dehydration (p < 0.01). In multivariate regression analysis, osmolarity was the most influential variable in both the level of consciousness and the admission level of dehydration (p < 0.0001). Twenty two patients died (16.9%). Septic shock was the most frequent cause of death (31%) and mortality was higher in patients with cardiovascular disease (acute myocardial infarction or stroke) as the precipitating factor for diabetic hyperosmolar state (p < 0.002). Older age, low blood pressure, low sodium, pH and bicarbonate plasma levels, and high urea plasma levels were related to mortality (p < 0.01). In multiple regression analysis, urea was the most influential mortality risk factor (p < 0.0118). Non survivors received higher doses of insulin than survivors (p < 0.01). All these data suggest that it is not the hyperosmolarity itself, but the hemodynamical state of the patients, which is the most influential factor on the prognosis of a diabetic hyperosmolar state.

Estrogen-regulated expression of a growth arrest specific gene (gas-1) in rat uterus

gas-1 belongs to a family of growth arrest specific genes negatively regulated after growth induction of arrested cells. We report the expression of gas-1 in an in vivo system of cell proliferation. gas-1 mRNA accumulates progressively in the uterus of ovariectomized rats with a peak at three weeks after surgery. After estrogen treatment gas-1 mRNA levels decrease within two hours, at a time when c-myc expression is greatly increased, and return to pretreatment levels at 48 hours. Treatment with cycloheximide does not prevent estrogen-induced down-regulation of gas-1 mRNA levels. The present results show that: i) estrogen affects the uterine growth state by regulating the expression of both positively and negatively acting genes, ii) gas-1 expression is controlled by cellular growth state also in vivo.

Molecular basis of reduced albumin gene expression in hepatoma cell lines with different growth rates

To study the relationship between expression of a differentiated function and cellular proliferation we analyzed the molecular mechanisms controlling albumin gene expression in rat liver and in rat hepatoma cell lines of different growth rates: low (MH1C1), intermediate (FAO), and high (3924A), as evaluated by both cell doubling time and histone H3 gene expression. In comparison with the liver, albumin accumulation was reduced in FAO cells and absent in MH1C1 and 3924A cells. Thus, these hepatoma cell lines do not show the inverse relationship between cellular growth rate and expression of a differentiation marker that has been often described in several hepatic and nonhepatic cellular systems. This conclusion is further reinforced by the finding that all cell lines, irrespective of their growth rate, failed to express alpha-fetoprotein mRNA, a dedifferentiation marker, and that two other liver-abundant genes, transferrin and apolipoprotein A-1, are regulated in a way similar to that of albumin. The defect in albumin accumulation was due to decreased or lacking synthesis which, in turn, was accompanied by reduction or absence of albumin mRNA. Run-on transcription assays provided evidence for diminished or absent albumin gene transcription. Southern blot analysis revealed that the structure of the albumin gene is preserved in all the tumor cell lines; however, we found a different methylation state of the albumin gene that correlated well with albumin expression.

Protein synthesis and gene expression in transplanted and postischemic livers

The expression of some genes has been comparatively studied in transplanted rat liver and in liver reperfused after ischemia in situ. Experiments on protein synthesis by tissue slices from cold-stored or transplanted livers show that rat livers that retain a good capacity for protein synthesis during storage undergo a profound impairment in the capacity for protein synthesis during the first hours after transplantation. This recovers in the following hours. There is never any indication of synthesis of stress proteins, and of hsp 70 in particular. The steady-state level of mRNAs for albumin, transferrin, and beta-actin, which are well expressed in reperfused postischemic livers in vivo, are reduced early after transplantation and recover only many hours later. Run-on analysis shows that an early defect in transcription and a partial recovery of this process later on are responsible for these changes. The steady-state levels of the same mRNAs are well maintained in donor livers preserved in University of Wisconsin solution for at least 12 hr, and less satisfactorily in Euro-Collins solution. Results of run-on analysis parallel the data on mRNA levels. The behavior of these mRNAs is, therefore, clearly different in reperfused and transplanted liver. The early stages of liver transplantation seem to be characterized by a depressed capacity of gene expression, without the reactive phenomenon of activation of stress protein genes that occurs in reperfused postischemic livers.

Stress proteins and reperfusion stress in the liver

Blood reperfusion after temporary liver ischemia induces the expression of heat shock genes and the synthesis of heat shock proteins (hsps), in particular hsp 70. Induction requires a certain duration of ischemia, suggesting that cell damage before reperfusion is essential for activation of heat shock genes. The expression of the hsp 70 gene is preceded by activation of the cellular protooncogenes c-fos and c-jun. However, the product of these genes, which is transcription factor AP-1, seems unnecessary for activation of the hsp 70 gene, which does not require the integrity of protein synthesis. Hsp genes seem to behave as "early response genes," enabling the cell to respond to emergency situations.

Ferritin synthesis on polyribosomes attached to the endoplasmic reticulum

The evidence that ferritin is synthesized both on free polyribosomes and on polyribosomes attached to the endoplasmic reticulum is reviewed. Evidence that some ferritin is secreted from cells after synthesis on bound polyribosomes was found to be inconclusive.

Expression of a growth arrest specific gene (gas-1) in transformed cells

A set of growth arrest-specific (gas) genes negatively regulated by serum has been identified. We report the analysis of the expression of one of them (gas-1) in transformed cells. We found a down regulation of gas-1 expression in NIH 3T3 cells transfected in vitro with an activated Ha-ras oncogene. In five chemically-induced mouse tumours grown in vivo the amounts of gas-1 mRNA were largely different but not related to the proliferating activity (evaluated by both H3 histone expression and 3H-thymidine incorporation into DNA). The amount of gas-1 mRNA in the tumours was in general higher than in normal tissues. Expression of c-myc was also evaluated and found to be high in tumours which exhibited low gas-1 expression. Two fibrosarcomas, CA-2 and CB-20, with similar phenotype, similar growth rate, different expression of c-myc and 100-fold difference in gas-1 expression were further investigated and gas-1 expression was found to be correlated with the expression of a differentiated function (as judged from collagen expression). Cell lines derived from CA-2 and CB-20 and maintained under different culture conditions showed that the cell cycle regulation and serum response of gas-1 expression were lost in CA-2. The higher steady state level of gas-1 mRNA in spite of a shorter mRNA half life suggests that in CB-20 cells the gas-1 gene is transcribed faster than in CA-2 cells indicating that transcriptional regulation is the major determinant of gas-1 gene expression in tumour cells. The finding of gas-1 expression in tumour cells suggests that its expression is not sufficient to maintain cells into quiescence, however, as a marker specific for the G0 phase, it could be useful, in conjunction with other growth related genes, to define the cell cycle distribution of a cell population.

Ferritin mRNAs on rat liver membrane-bound polysomes synthesize ferritin that does not translocate across membranes

Ferritin is a typical intracellular protein but small amounts are also present in serum and other biological fluids. The source and physiological significance of serum ferritin are still obscure. The presence of ferritin mRNAs on polysomes bound to endoplasmic reticulum (ER) could be relevant for the secretion of ferritin. By Northern blot analysis we found significant amounts of both L and H subunit mRNAs on rat liver membrane-bound polysomes. Immunoprecipitation of translational products of membrane-bound polysomes with anti-rat liver ferritin antibody showed that ferritin is actually synthesized on ER membranes. Analysis of RNA extracted from salt-washed rat liver microsomes demonstrated that ferritin mRNAs are translated by polysomes tightly bound to ER membranes. Following iron treatment, both the amount of H and L subunit mRNAs and ferritin synthesis increased sharply in both free and bound polysomal fractions. Translation of membrane-bound polysomes in the presence of microsomal membranes indicated that ferritin is not processed by signal sequence cleavage or glycosylation and is not translocated into ER membranes. Ferritin mRNAs found on membrane-bound polysomes are associated with ER in a specific way, however, their products do not seem to follow the classic secretory pathway and therefore the significance of the large amount of ferritin mRNAs in the bound ribosome fraction remains unclear.