Self-renewal gene tracking to identify tumour-initiating cells associated with metastatic potential

Tumour-initiating cells (TICs) are rare cancer cells isolated from tumours of different origins including high-grade tumours that sustain neoplasic progression and development of metastatic disease. They harbour deregulated stem cells pathways and exhibit an unchecked ability to self-renew, a property essential for tumour progression. Among the essential factors maintaining embryonic stem (ES) cells properties, OCT-4 (also known as POU5F1) has been detected in tumours of different origins. Although ectopic expression results in dysplasic growth restricted to epithelial tissues, overexpression expands the proportion of immature cells in teratomas. However, OCT-4-expressing cells have not been purified from spontaneously occurring tumours, thus information concerning their properties is rather scant. Here, using p53-/- mice expressing green fluorescent protein and the puromycin resistance gene under the control of the Oct-4 promoter, we show that OCT-4 is expressed in 5% onwards of the undifferentiated tumour cell populations derived from different organs. OCT-4 expression was low as compared with ES cells, but was associated with a 'stemness' signature and expression of the chemokine receptor CXCR4. These cells displayed cancer stem cell features, including increased self-renewal and differentiation ability in vitro and in vivo. They not only formed allografts containing immature bone regions but also disseminated into different organs, including lung, liver and bone. Experiments based on RNA interference revealed that Oct-4 expression drives both their engraftment and metastasis formation. This work points out the crucial contribution of Oct-4-expressing TICs in the hierarchical organization of the malignant potential, leading to metastasis formation. Consequently, it provides an appropriate model to develop novel therapies aiming to strike down TICs by targeting self-renewal genes, therefore efficient to reduce tumour growth and metastatic disease.

Cloning of hOST-PTP: the only example of a protein-tyrosine-phosphatase the function of which has been lost between rodent and human

Protein-tyrosine-phosphatases (PTP-ases), in concert with protein tyrosine kinases, control various biological activities such as cell growth and differentiation. In rodents, around 40 PTP-ases have been described. Functional orthologue for each of these PTP-ases have been identified in human, except for OST-PTP. OST-PTP is a transmembrane PTP-ase with a restricted tissue distribution. In silico analysis on public sequence databases reveals a human OST-PTP gene orthologue that encompasses 21 kb on chromosome 1q32.1. Using RT-PCR we isolated a 4 kb hOST-PTP transcript. hOST-PTP cDNA sequence exhibits numerous disablements indicating that it does not code for a PTP-ase but is rather a pseudogene with unique features. Indeed, (i) it has no "functional" parent in the human genome, (ii) it has retained an "intron-exon" structure, and (iii) it is transcribed in a regulated manner. Interestingly, we found two ESTs, from domesticated pig and from cow that exhibit ORF that would predict a functional OST-PTP orthologue in Artiodactyls. Taken together, these results indicate that OST-PTP is the only PTP-ase the function of which has been lost during the evolution process between rodents and human.

Coordinated Up-regulation by hypoxia of adrenomedullin and one of its putative receptors (RDC-1) in cells of the rat blood-brain barrier

Adrenomedullin (ADM) is a potent hypotensive peptide, which is produced during sepsis and ischemia. We demonstrate here that hypoxia induced a time-dependent increase of both ADM mRNA and protein expressions in cultured astrocytes and endothelial cells from rat brain microvessels. Gene reporter analyses showed a 2-fold increase in ADM gene transcription which was suppressed when the ADM promoter was deleted of its hypoxia responsive element. Hypoxia increased 7-fold the stability of pre-formed ADM mRNAs. Rat brain microvessels expressed mRNAs coding for the different putative ADM receptors but they did not respond to exogenous ADM and calcitonin gene-related peptide by the formation of cAMP. In contrast, ADM and calcitonin gene-related peptide increased the formation of cAMP in astrocytes and their actions were potentiated about 2-fold after hypoxia. Messenger RNA species coding for three putative ADM receptors (the L1 orphan receptor, RDC-1, and calcitonin receptor-like receptor) and accessory proteins (receptor-activity modifying proteins) were present in astrocytes. Hypoxia selectively up-regulated expression of RDC-1 receptor mRNAs. The results indicate that ADM and RDC-1 are hypoxia-sensitive genes and that RDC-1 receptors may mediate some actions of ADM in hypoxic astrocytes.

Expression of vascular endothelial growth factor-b in human astrocytoma

Growth of human malignant gliomas is stringently dependent on an angiogenic process that probably involves vascular endothelial growth factor (VEGF). Expressions of mRNA coding for the different forms of VEGF were analyzed in surgical specimens from human astrocytomas. Low levels of placental growth factor (PGF) and VEGFC mRNA were observed in polymerase chain reaction, but not in Northern blot experiments. VEGF mRNA was found in some but not all grade and grade IV astrocytomas. VEGFB mRNA was observed in all tissue samples analyzed irrespective of the tumor grade. A new splice variant of VEGFB (VEGFB155) that lacks exons 5 and 6 is described. Expressions of VEGF mRNA in cultured glioblastomas cells were upregulated by hypoxia, but the sensitivity of the cells to hypoxia was reduced as compared with normal rat astrocytes. VEGF expression was depressed by dexamethasone. Expressions of VEGFB mRNA were affected neither by hypoxia nor by dexamethasone. The results indicate a coexpression of VEGF mRNA and VEGFB mRNA in human astrocytomas. Expression of VEGFB is markedly different from that of VEGF. Possible roles of VEGFB as a cofactor for hypoxia-induced angiogenesis in human astrocytomas are discussed.

Vascular endothelial growth factors and angiogenesis

VEGF was discovered in 1989. Research -conducted over the past 10 years has -demonstrated that VEGF is a major regulator of angiogenesis and vasculogenesis. This paper reviews the molecular data on the multiple forms of VEGF, their signalling and accessory receptors. Five genes encoding VEGF-like proteins have been identified; the different isoforms of each VEGF molecule are generated by alternative splicing mechanisms. The different VEGF's recognize signalling tyrosine kinase receptors (Flt-1, Flk-1 and Flt-4) and accessory receptors. VEGF expression is stimulated by hypoxia-dependent and -independent mechanisms. Hypoxic responses are mediated by specific transcription factors that are expressed in a tissue dependent fashion and that are developmentally regulated. VEGF is thought to play a role in tumor angiogenesis and may contribute to cardioprotection in ischemic heart -diseases. Its role in pulmonary hypertension induced by chronic hypoxia is discussed. This review also stresses the difficulty of applying results from in vitro -studies to in vivo situations.

Regulation of vascular endothelial growth factor expression in human keratinocytes by retinoids

Vascular endothelial growth factor (VEGF) is overexpressed in hyperproliferative diseases, such as psoriasis and cancers, which are characterized by increased angiogenesis. Experimentally, VEGF overexpression can be induced by the treatment of cell cultures and biological tissues with phorbol esters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA). Using normal human keratinocytes in conventional cultures and skin grafted onto nude mice in vivo, we show that retinoids can inhibit TPA-mediated VEGF gene induction at the transcriptional level. Because retinoids are biologically active either by interacting with the nuclear retinoic acid receptors or by interfering with the activator protein 1 (AP1) transcription factor, we studied the effect of the retinoic acid derivative CD 2409, which exhibits strong anti-AP1 activity but does not bind to the known retinoic acid receptors in vitro. The results demonstrate that the inhibition of VEGF expression by retinoids only depends on their anti-AP1 activity and does not require gene transactivation via retinoic acid response elements. Because the VEGF promoter contains four potential AP1 binding sites, we used different promoter constructs to identify the functional site responsible for TPA induction and retinoid inhibition. This site turned out to be localized at position -621 of the 5' flanking region of the VEGF gene.

Hypoxia-induced transcriptional activation of vascular endothelial growth factor is inhibited by serum

Expression of vascular endothelial growth factor (VEGF) by cultured vascular smooth muscle cells was analyzed. Serum and hypoxia had nearly additive actions on VEGF mRNA expression. The function of the VEGF promoter in smooth muscle cells was analyzed using transient luciferase reporter assays. Serum and hypoxia stimulated expression of luciferase. The presence of hypoxia response element (HRE) was necessary for the hypoxic induction. AP-1 sequences located upstream of HRE and AP-2/Sp-1 sequences located downstream of HRE are not necessary. Hypoxic responses were best observed in serum-deprived cells. They were largely absent in serum-stimulated cells. Serum did not suppress the hypoxic response by interfering with the hypoxia sensor mechanism or with the signaling cascade that leads to the activation of HIF-1. It is concluded that growth-promoting cytokines regulate hypoxic gene induction in smooth muscle cells.

P2X7 receptor activation-induced contraction and lysis in human saphenous vein smooth muscle

In cutaneous veins where purinergic neurotransmission is more prominent compared with in deep vessels, physiological and pathological roles of nerve-released ATP have been described. Neuronally released ATP has been reported to act through activation of unidentified ionotropic P2X receptor(s). This study analyzed P2X receptor subtypes expressed in human saphenous vein smooth muscle and their physiological functions. Transcripts for both hP2X1 receptors, already identified in other smooth muscles, and, surprisingly, hP2X7 receptors known to be responsible for the cytotoxic effect of ATP in macrophages were detected by Northern blot analysis in total RNA from saphenous vein smooth muscle. ATP and other P2X receptor agonists [alphabeta-methylene-ATP, 2-methylthio-ATP, and 2',3'-(4-benzoyl)benzoyl-ATP] dose-dependently contracted venous rings, but the contraction induced by 2-methylthio-ATP was more transient than that evoked by the other P2X agonists. The effect of hP2X1 agonists involved the activation of a rapidly desensitizing cation current recorded in freshly isolated myocytes. The action of hP2X7 receptor agonists was related to a maintained nondesensitizing cation current. In addition, hP2X7 receptor activation formed membrane pores that were permeable to large molecules. hP2X1 and hP2X7 receptors coexpressed in COS cells did not associate to form heteromultimers. Our data indicate that both hP2X1 and hP2X7 receptors are expressed as 2 separated populations of channels in human saphenous vein myocytes and are involved in ATP-induced tension. We suggest that cell lysis consequent to hP2X7 receptor-induced pore formation contributes to the disorganization and decrease in the amount of contractile myocytes in the media of varicose veins.

Cardiac expressions of HIF-1 alpha and HLF/EPAS, two basic loop helix/PAS domain transcription factors involved in adaptative responses to hypoxic stresses

Expression of many mammalian genes is regulated by oxygen tension. HIF-1 alpha and HLF/EPAS are two basic helix-loop-helix/PAS domain transcription factors that bind to hypoxia sensitive elements in the promoters /enhancers of hypoxia sensitive genes such as vascular endothelial growth factor (VEGF). This paper describes the structure of rat HIF-1 alpha and analyses expressions HIF-1 alpha and of HLF/EPAS mRNAs in lung and cardiac tissues from the rat. HLF/EPAS mRNAs appear at birth in the two tissues and are maintained at high levels throughout adult life. HIF-1 alpha mRNAs are expressed at a constant level during lung development. Their abundance increase transiently at birth in cardiac tissues. Cultured cardiomyocytes from new born rats only express HIF-1 alpha mRNAs. HIF-1 alpha mRNA expression is increased by phorbol myristate acetate but not by anoxia or cobalt. The results indicate (i) that HIF-1 alpha and HLF/EPAS are expressed in a cell specific manner and (ii) that the hypoxic induction of VEGF mRNA expression by isolated cardiomyocytes is independent of HLF/EPAS. Finally, they suggest that protein kinase C may prime hypoxia induced gene regulation by inducing expression of HIF-1 alpha mRNAs.

Differential regulation of cardiac heme oxygenase-1 and vascular endothelial growth factor mRNA expressions by hemin, heavy metals, heat shock and anoxia

Increasing attention has been paid to the effects of hypoxia, heavy metals and heat shocks on gene expression and to the similarities in their actions. This paper compares mRNA levels of two putative hypoxia, heavy metal and heat shock sensitive genes: heme oxygenase-1 (HO-1) and vascular endothelial growth factor (VEGF) in myocyte-enriched cultures of neonatal rat heart cells. HO-1 mRNA expression is stimulated by hemin, Cd2+, Co2+ and heat shocks but not by Ni2+ or Mn2+. It is stimulated by long (13h) but not by short (4h) periods of anoxia. Conversely, VEGF mRNA expression is stimulated by short as well as long periods of anoxia, by Cd2+, Co2+, Ni2+ and Mn2+ but not by hemin or heat shocks. The results suggest that heavy metals, anoxia and heat shocks exert their effects on VEGF and HO-1 mRNA expression through separate though potentially overlapping mechanisms. Increased expressions of HO-1 and VEGF may be both cardioprotective under hypoxic/ischemic conditions.

Contributions of NO synthase and heme oxygenase to cGMP formation by cytokine and hemin treated brain capillary endothelial cells

Two mechanisms contribute to cGMP formation by soluble guanylyl cyclase (i) NO production by NO synthase and (ii) CO production by heme oxygenase. We analyze here the contributions of these two pathways to IL1, TNF, lipopolysaccharide and hemin treated brain capillary endothelial cells. Cytokines and LPS induced cGMP formation in manners that were completely prevented by LY 83,583, methylene blue and by cyclosporin A. They were partially inhibited by inhibitor of NO synthase. Cyclosporin A acts by a posttranscriptional mechanism. Cells constitutively expressed mRNAs for heme oxygenase-1. Expression was enhanced by hemin but not by IL1 or lipopolysaccharide. Induction of heme oxygenase-1 and its inhibition by Sn protoporphyrin IX had no effect on cGMP levels.

Cobalt stimulates the expression of vascular endothelial growth factor mRNA in rat cardiac cells

We have previously demonstrated that an exposure of rat cardiomyocytes to anoxia induces the expression of mRNAs coding for vascular endothelial growth factor (VEGF). The action of anoxia was mimicked by Co, Ni and Mn. The actions of Co and of anoxia were not additive and did not involve AP-1 binding sites. Experiments using actinomycin D and cycloheximide indicated that VEGF mRNA levels in cardiac cells are regulated both at transcriptional and post transcriptional levels. It is concluded that an oxygen sensing mechanism is present in cardiac cells and controls the expression of VEGF mRNAs. It may be important for the neovascularization of ischemic myocardium.

Hypoxia is a strong inducer of vascular endothelial growth factor mRNA expression in the heart

Vascular endothelial growth factor (VEGF) is a potent and specific mitogen for vascular endothelial cells. A 3.9 kb VEGF transcript is expressed by all cardiac tissues from rat, mouse and guinea pig examined. VEGF expression was not developmentally regulated. The major form of VEGF mRNAs expressed by cardiac tissues coded for VEGF188. Myocyte enriched and fibroblast enriched cultures of new born rat heart cells also expressed VEGF transcripts but the major mRNA found coded for VEGF164. The expression of VEGF mRNA in myocyte enriched cultures of new born rat ventricles was increased 2 fold by serum, 5 fold by phorbol myristate acetate and 7 fold by hypoxic conditions. We conclude that hypoxic conditions may promote cardiac capillary cell growth by inducing VEGF expression.

Expression of vascular endothelial growth factor by cultured endothelial cells from brain microvessels

Vascular endothelial growth factor (VEGF) is a potent and specific mitogen for vascular endothelial cells that may be involved in tumor angiogenesis. Cultured capillary endothelial cells from rat brain (BCEC) express transcripts for VEGF as assayed by Northern blots and polymerase chain reaction analysis. The three forms of VEGF (VEGF120, VEGF164 and VEGF188) are produced, VEGF188 being detected in lower amounts. The sequence of rat VEGF188 was determined. Rat and human exons 6 differ at only one position (human Tyr134-->rat Phe133). Transcripts for VEGF were observed in different clones of rat BCEC, in bovine BCEC but not in bovine aortic endothelial cells.

Function of vasoactive factors in the cerebral microcirculation

The properties of brain capillary endothelial cells (BCECs) have been analyzed. BCECs express two types of receptor sites for endothelins (ETs), and ETA-like receptor, and an ETB-like receptor that is not coupled to phospholipase C but whose occupancy activates Na+/H+ exchange activity. The ETA receptor is positively coupled to phospholipase C and negatively coupled to adenylate cyclase. BCECs, unlike aortic endothelial cells, express high-affinity receptor sites for C-type natriuretic peptide. They respond to exogenous nitric oxide (NO) and to NO donor molecules by large activations of soluble guanylate cyclase. They produce little cGMP in response to A23187 or to agonists of phospholipase C but do so after an exposure to interleukin-1. The physiological consequence of the high reactivity of BCECs to vasoactive factors is discussed.

Endothelins inhibit adenylate cyclase in brain capillary endothelial cells

The action of endothelins (Et) on cAMP formation was studied in endothelial cells from rat brain microvessels. Et-1 and Et-3 had no action by themselves. They both inhibited cholera toxin stimulated adenylate cyclase by about 50%. K0.5 values were observed at 2 nM and 40 nM for Et-1 and Et-3 respectively, indicating an involvement of a low affinity Et-3 receptor. Coupling to adenylate cyclase was achieved by a pertussis toxin sensitive mechanism. Another action of endothelins in brain capillary endothelial cells was to stimulate phospholipase C. This action involved a low affinity Et-3 receptor and a pertussis toxin insensitive mechanism. It is concluded that in brain capillary endothelial cells, ETA like receptors are coupled to phospholipase C and to adenylate cyclase via two different mechanisms.

Endothelins activate Na+/H+ exchange in brain capillary endothelial cells via a high affinity endothelin-3 receptor that is not coupled to phospholipase C

Endothelial cells from brain microvessels (BCEC) express high affinity receptor sites for endothelin-1 that recognize endothelin-3 with a low affinity (Vigne, P., Marsault, R., Breittmayer, J.P. & Frelin, C. (1990) Biochem. J. 266, 415-420). Binding experiments using 125I-endothelin-3 showed the presence in BCEC of a new class of receptor sites that had a high affinity for endothelin-3 (Kd = 0.8 nM), endothelin-1 (Kd = 0.8 nM), and sarafotoxin S6b (Kd = 0.3 nM). Endothelins activated phospholipase C in BCEC and produced transient increases in intracellular Ca2+ with properties of a low affinity endothelin-3 receptor. Endothelins also increased 22Na+ uptake via the Na+/H+ antiporter in BCEC. Concentrations for half-maximum activation (endothelin-1, 0.5 nM; sarafotoxin S6b, 1 nM; endothelin-3, 2 nM) were close to the Kd values determined in 125I-endothelin-3-binding experiments. The action of endothelins on Na+/H+ exchange was not mimicked by phorbol myristate acetate, it was not reversed by staurosporine, and it did not correlate with the phosphorylation of the 80-kDa protein. These results indicated that the action of endothelins on Na+/H+ exchange did not involve protein kinase C. It is concluded that BCEC coexpress two types of functional receptor sites for endothelins: (i) a high affinity endothelin-1, low affinity endothelin-3 receptor that is coupled to phospholipase C and to intracellular Ca2+ mobilization, and (ii) a high affinity endothelin-1, high affinity endothelin-3 receptor that controls Na+/H+ exchange activity via a protein kinase C-independent mechanism.

Functional properties of high- and low-affinity receptor subtypes for endothelin-3

Endothelial cells from brain microvessels express two types of endothelin (ET) receptor. The first receptor subtype (defined as E alpha) shows a high affinity for ET-1, a low affinity for ET-3, and it is coupled to phospholipase C. The second subtype (E beta) shows a high affinity for both ET-1 and ET-3. It is not coupled to phospholipase C, but its activation leads to an increased activity of the Na+/H+ exchanger via a protein kinase C-independent mechanism. Brain astrocytes also express a high-affinity ET-3 receptor. However, unlike that of brain capillary endothelial cells, this receptor is coupled to phospholipase C and it may be a third type of endothelin receptor (E gamma). Thus, it seems that by using both binding and functional criteria, at least three subtypes of endothelin receptor can be distinguished: a low-affinity ET-3 receptor and two high-affinity ET-3 receptors that are coupled to different intracellular signaling pathways.

Histamine as a growth factor and chemoattractant for human carcinoma and melanoma cells: action through Ca2(+)-mobilizing H1 receptors

Histamine receptors are present on the surface of various normal and tumor-derived cell types, where their biological function is incompletely understood. Here we report that histamine not only stimulates cell proliferation under serum-free conditions, but also is chemotactic for human carcinoma (Hela and A431) and melanoma (A875) cells expressing H1 type receptors. Histamine was found to be a potent activator of phospholipase C, leading to polyphosphoinositide hydrolysis and subsequent intracellular Ca2+ mobilization. In addition, histamine also causes the protein kinase C-mediated activation of Na+/H+ exchange, as evidenced by an amiloride-sensitive rise in cytoplasmic pH. All histamine-induced responses, including chemotaxis and DNA synthesis, are completely inhibited by the H1 receptor antagonist pyrilamine, but not by cimetidine, an inhibitor of histamine H2 type receptors. Our results suggest that histamine may have a previously unrecognized role in the migration and proliferation of cells expressing H1 receptors.

Phorbol esters and chemotactic factor induce distinct changes in cytoplasmic Ca2+ and pH in granulocytic like HL60 cells

Differentiation of HL60 cells into neutrophil-like cells after exposure to dimethylsulfoxide is accompanied by an increase in intracellular pH (pHi) which results from an increased activity of the Na+/H+ antiporter at physiological pHi value, but not at acidic pHi values. The functional responses of differentiated HL60 cells to the chemotactic peptide, N-formylmethionylleucylphenylalanine (fMLP), and to phorbol myristate acetate (PMA) were studied. In differentiated cells fMLP produced a large increase in cytosolic Ca2+ levels and a small biphasic change in pHi, whereas PMA produced cellular acidification, which was potentiated by ethylisopropylamiloride and no change in [Ca2+]i. In undifferentiated HL60 cells, PMA produced the opposite effect on pHi, i.e., a cellular alkalinization. The extent of the acidification produced by PMA in differentiated HL60 cells correlated with the production of reactive oxygen species.

Single-cell analysis of the intracellular pH and its regulation during the monocytic differentiation of U937 human leukemic cells

Monocytic differentiation of U937 cells induced by retinoic acid is accompanied by a 0.2-pH-unit cell alkalinisation. The effect of retinoic acid on intracellular pH (pHi) develops slowly and it precedes the differentiation of the cells by 24 h. Heterogeneity in cellular pHi values was assessed using flow cytometry. It was higher at the differentiated stage than at the undifferentiated stage. It was reduced under conditions of clamped pHi values. Two membrane mechanisms allow U937 cells to recover from an intracellular acidosis. These are the Na+/H+ exchange system and a Na+-dependent HCO3-/Cl- exchange system. The increase in the pHi observed after monocytic differentiation resulted from a twofold increase in the maximum activity of the Na+/H+ exchange system with no change in the activity of the bicarbonate-dependent system. The properties of interaction of the Na+/H+ exchanger of U937 cells with Na+, Li+, amiloride and its derivatives were defined and appeared to be unique to human leukemic cells.

An increase in intracellular pH is a general response of promonocytic cells to differentiating agents

Intracellular pH (pHi) was measured in HL60 and U937 cells before and after differentiation into monocyte-macrophage like cells. 12-O-Tetradecanoyl phorbol-13-acetate (PMA), butyrate, interferon, retinoic acid and 1,25-dihydroxyvitamin D3 all increased pHi. The increases elicited were rapid with PMA, much slower with retinoic acid and interferon and still slower with 1,25-dihydroxyvitamin D3. Increases in pHi are due to an activation of the Na+/H+ exchange system. High pHi values are unlikely to serve as an early intracellular signal for initiating monocytic differentiation.

Fatty acid composition of HL-60 cells is modified upon proliferation arrest and differentiation

The human leukemic cell line HL-60 undergoes differentiation to granulocytic-like cells in response to dimethyl sulfoxide (DMSO) or retinoic acid (RA). This differentiation is accompanied by an arrest in cell proliferation. Studies have implicated alterations in the phospholipid fatty acid (FA) composition as a result of HL-60 differentiation. However, changes in FA's are also known to occur during the arrest of cellular proliferation. Using a highly efficient capillary gas-liquid chromatography technique, the phospholipid FA composition of HL-60 and of DMSO-resistant and RA-resistant HL-60 subclones was determined in proliferating cells, in density-arrested cells, and in terminally differentiated cells. The same specific modifications in some of the FAs of the three cell lines were observed when proliferation was inhibited by cell density; 16:0 and 18:2n-6 were decreased and 22:6n-3 increased. Moreover, 16 and 18 dimetylacetals were both increased when proliferation was decreased, indicating modifications in plasmalogen contents. Granulocytic differentiation of HL-60 cells and of its subclones with DMSO and/or RA provoked modifications in phospholipid FAs different from that found in density-arrested, undifferentiated cells such as decreases in monoenoic FAs of 16 and 18 carbons as well as an increase in arachidonic acid, the major polyunsaturated FA. The biological significance of these changes upon arrest of proliferation and differentiation are discussed. These results indicate that, when arrest of proliferation accompanies differentiation, these two phenomena can be responsible for different changes and, whenever possible, they have to be considered separately in order to know which modifications are effectively due to differentiation itself.

The regulation of the intracellular pH in cells from vertebrates

Eukaryotic cells control their intracellular pH using ion-transporting systems that are situated in the plasma membrane. This paper describes the different mechanisms that are involved and how their activity is regulated.

Properties of the Na+-dependent Cl-/HCO3- exchange system in U937 human leukemic cells

U937 cell possess two mechanisms that allow them to recover from an intracellular acidification. The first mechanism is the amiloride-sensitive Na+/H+ exchange system. The second system involves bicarbonate ions. Its properties have been defined from intracellular pH (pHi) recovery experiments, 22Na+ uptake experiments, 36Cl- influx and efflux experiments. Bicarbonate induced pHi recovery of the cells after a cellular acidification to pHi = 6.3 provided that Na+ ions were present in the assay medium. Li+ or K+ could not substitute for Na+. The system seemed to be electroneutral. 22Na+ uptake experiments showed the presence of a bicarbonate-stimulated uptake pathway for Na+ which was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonate. The bicarbonate-dependent 22Na+ uptake component was reduced by depleting cells of their internal Cl- and increased by removal of external Cl-. 36Cl- efflux experiments showed that the presence of both external Na+ and bicarbonate stimulated the efflux of 36Cl- at a cell pHi of 6.3. Finally a 36Cl- uptake pathway was documented. It was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonate (K0.5 = 10 microM) and bicarbonate (K0.5 = 2 mM). These results are consistent with the presence in U937 cells of a coupled exchange of Na+ and bicarbonate against chloride. It operates to raise the intracellular pH. Its pHi and external Na+ dependences were defined. No evidence for a Na+-independent Cl-/HCO3- exchange system could be found. The Na+-dependent Cl-/HCO3- exchange system was relatively insensitive to (aryloxy)alkanoic acids which are potent inhibitors of bicarbonate-induced swelling of astroglia and of the Li(Na)CO3-/Cl- exchange system of human erythrocytes. It is concluded that different anionic exchangers exist in different cell types that can be distinguished both by their biochemical properties and by their pharmacological properties.

PSL, a nuclear cell-cycle associated antigen is increased during retinoic acid-induced differentiation of HL-60 cells

PSL(p55) is a nuclear 55kD antigen present in various mammalian cell systems, which has been first identified by use of human autoimmune antibodies (Barque et al. 1983, EMBO J. 2, 743). It has been shown to be associated with interphase chromatine and to be synthesized in during the S phase of the cell cycle. In this work, we have analysed the status of PSL in promyelocytic HL-60 human cells in exponential or stationary growth, or undergoing granulocytic differentiation in presence of Retinoic acid. By use of 2-dimensional electrophoresis, PSL was found to be composed of two acidic proteins designated p55A and p55B. Unexpectedly, estimated 10-20 fold higher amounts of each species were found in cells treated for 5 days with 10(-6)M Retinoic acid, than in asynchronously growing cells or resting cells. Moreover, the p55A protein was phosphorylated during the process. On the basis of these results, PSL appears to be involved in some steps of the granulocytic differentiation process.

Differentiation of human promyelocytic HL 60 cells by retinoic acid is accompanied by an increase in the intracellular pH. The role of the Na+/H+ exchange system

Retinoic acid, which induces the differentiation of HL 60 cells to granulocytes, produces a cell alkalinization from pHi = 7.03 to pHi = 7.37. The half-maximum effect of retinoic acid is observed at 10 nM. The effect of retinoic acid on the pHi develops slowly, and it precedes the differentiation of the cells. A cell alkalinization is also observed after differentiation of the cells by dimethyl sulfoxide. It is not observed using etretinate, a synthetic retinoid that does not promote the differentiation of HL 60 cells. Two pHi regulating mechanisms coexist in HL 60 cells. The Na+/H+ exchange system is the major mechanism that allows HL 60 cells to recover from an intracellular acidosis. A second mechanism is a Na-HCO3 cotransport system. During differentiation of the cells by retinoic acid, a 2-fold increase in the activity of the Na+/H+ exchange system is observed, while the activity of the NaHCO3 cotransport remains constant. The properties of interaction of the Na+/H+ exchanger with internal H+, external Na+, and Li+ as well as with amiloride and its derivatives are defined. The Na+/H+ exchanger of HL 60 cells is characterized by unusually low affinities for alkali cations and a high affinity for amiloride and its derivatives. The pHi dependence of the exchanger is not modified after differentiation by retinoic acid. It is concluded that the mechanism of activation of the Na+/H+ exchanger by retinoic acid is distinct from the short-term effect produced by mitogens and phorbol esters which change the pHi dependence of the system.

Retinoic-acid-induced differentiation of HL-60 cells is associated with biphasic activation of the Na+-K+ pump

The human promyelocytic leukemia cell line, HL-60, can be induced to differentiate into granulocyte-like cells when cultured in the presence of 10(-6) M retinoic acid (RA) for several days. Following the addition of RA two kinds of changes occur. First, there are early changes that comprise an increase in the intracellular concentration of sodium ions [Na]i, which reaches its maximum after 6 h, and an increase in the activity of the Na+-pump, which is reflected by an ouabain-sensitive K+ influx that peaks at 8 h (170% of the control value) and that occurs without any change in the number of pump molecules, as measured by the binding of 3H-ouabain. Second, beginning after 12 h of culture with RA, a decrease in the number of ouabain-binding sites occurs, this being accompanied by an increase in the number of K+ ions actively transported by each site. The effect of modulation of Na+-pump activity on the RA-induced differentiation of HL-60 cells was studied using low, noncytotoxic concentrations of ouabain which, although alone having no differentiating effect, accelerated and potentiated the effect of RA on differentiation. When added in combination, these drugs induced rapid stimulation of the Na+-pump, which reached its peak after 2 h. These results indicate that a concomitant increase in the level of [Na+]i and in the activity of the Na+-pump constitute primary events in the interaction between RA and HL-60 cells, and that cation fluxes may play a role in the initiation of the process of differentiation.

[3H]Ouabain binding and 86 rubidium uptake during the dimethyl sulfoxide-induced differentiation of human promyelocytic leukemia HL-60 cells

When HL-60 cells were induced to differentiate into granulocyte-like cells by culture in the presence of 1.3% dimethyl sulfoxide, an increase in the rate of ouabain-sensitive 86Rb transport was observed during the first 6 h followed by a constant decrease which became stable on day 4 at about 40% of control level. By contrast, the number of ouabain binding sites remained constant during the first 24 h then decreased with the same kinetics as that of the 86Rb transport rate. These results suggest that alterations in ionic fluxes, through activation of the sodium pump, are part of the early events resulting in HL-60 cell differentiation.