Preconditioning-induced cytoprotection in hepatocytes requires Ca(2+)-dependent exocytosis of lysosomes

Targeting lysosomal quality control as a therapeutic strategy against aging and diseases

Previously, lysosomes were primarily referred to as the digestive organelles and recycling centers within cells. Recent discoveries have expanded the lysosomal functional scope and revealed their critical roles in nutrient sensing, epigenetic regulation, plasma membrane repair, lipid transport, ion homeostasis, and cellular stress response. Lysosomal dysfunction is also found to be associated with aging and several diseases. Therefore, function of macroautophagy, a lysosome-dependent intracellular degradation system, has been identified as one of the updated twelve hallmarks of aging. In this review, we begin by introducing the concept of lysosomal quality control (LQC), which is a cellular machinery that maintains the number, morphology, and function of lysosomes through different processes such as lysosomal biogenesis, reformation, fission, fusion, turnover, lysophagy, exocytosis, and membrane permeabilization and repair. Next, we summarize the results from studies reporting the association between LQC dysregulation and aging/various disorders. Subsequently, we explore the emerging therapeutic strategies that target distinct aspects of LQC for treating diseases and combatting aging. Lastly, we underscore the existing knowledge gap and propose potential avenues for future research.

Adenosine A2A Receptor Activation Regulates Niemann-Pick C1 Expression and Localization in Macrophages

Adenosine plays an important role in modulating immune cell function, particularly T cells and myeloid cells, such as macrophages and dendritic cells. Cell surface adenosine A_2A receptors (A_2AR) regulate the production of pro-inflammatory cytokines and chemokines, as well as the proliferation, differentiation, and migration of immune cells. In the present study, we expanded the A_2AR interactome and provided evidence for the interaction between the receptor and the Niemann-Pick type C intracellular cholesterol transporter 1 (NPC1) protein. The NPC1 protein was identified to interact with the C-terminal tail of A_2AR in RAW 264.7 and IPMФ cells by two independent and parallel proteomic approaches. The interaction between the NPC1 protein and the full-length A_2AR was further validated in HEK-293 cells that permanently express the receptor and RAW264.7 cells that endogenously express A_2AR. A_2AR activation reduces the expression of NPC1 mRNA and protein density in LPS-activated mouse IPMФ cells. Additionally, stimulation of A_2AR negatively regulates the cell surface expression of NPC1 in LPS-stimulated macrophages. Furthermore, stimulation of A_2AR also altered the density of lysosome-associated membrane protein 2 (LAMP2) and early endosome antigen 1 (EEA1), two endosomal markers associated with the NPC1 protein. Collectively, these results suggested a putative A_2AR-mediated regulation of NPC1 protein function in macrophages, potentially relevant for the Niemann-Pick type C disease when mutations in NPC1 protein result in the accumulation of cholesterol and other lipids in lysosomes.

DUSP12 acts as a novel endogenous protective signal against hepatic ischemia-reperfusion damage by inhibiting ASK1 pathway

Ischemia–reperfusion injury (IRI) consequent to major liver surgery is a still unmet clinical problem. The activation of endogenous systems of hepatoprotection can prevent the damaging effects of ischemia–reperfusion (IR) as shown by the phenomenon known as ‘ischemic preconditioning’. The identification of endogenous signal mediators of hepatoprotection is of main interest since they could be targeted in future therapeutic interventions. Qiu et al. recently reported in Clin. Sci. (Lond.) (2020) 134(17), 2279–2294, the discovery of a novel protective molecule against hepatic IR damage: dual-specificity phosphatase 12 (DUSP12). IR significantly decreased DUSP12 expression in liver whereas DUSP12 overexpression in hepatocytes protected IRI and DUSP12 deletion in DUSP12 KO mice exacerbated IRI. The protective effects of DUSP12 depended on apoptosis signal-regulating kinase 1 (ASK1) and acted through the inhibition of the ASK1-dependent kinases c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). These results enlighten DUSP12 as a novel intermediate negative regulator of the pro-inflammatory and pro-apoptotic ASK1/JNK-p38 MAPK pathway activated during hepatic IR and identify DUSP12 as potential therapeutic target for IRI.

The adenosine A2A receptor agonist T1-11 ameliorates neurovisceral symptoms and extends the lifespan of a mouse model of Niemann-Pick type C disease

Niemann-Pick C is a fatal neurovisceral disorder caused, in 95% of cases, by mutation of NPC1 gene. Therapeutic options are extremely limited and new "druggable" targets are highly warranted. We previously demonstrated that the stimulation of the adenosine A_2A receptor (A_2AR) normalized the pathological phenotype of cellular models of NPC1. Since the validation of A_2ARs as a therapeutic target for NPC1 can be obtained only conducting studies in in vivo models of the disease, in the present paper, the effects of two agonists of A_2ARs were evaluated in the mouse model Balb/c Npc1^nih, hereafter indicated as NPC1-/-. The agonists CGS21680 (2.5 and 5mg/kg/day by intraperitoneal injection) and T1-11 (50mg/kg/day in drinking water) were administered at a presymptomatic stage of the disease of NPC1-/- mice (PN28 and PN30, respectively); the experimental groups were the following: vehicle-treated WT mice (N=16 for both CGS and T1-11 treatments); vehicle-treated NPC1-/- mice (N=14 for CGS and 12 for T1-11 treatment); CGS-treated NPC1-/- mice (N=7) and T1-11-treated NPC1-/- mice (N=11). The efficacy of the treatments was evaluated by comparing vehicle-treated and CGS or T1-11-treated NPC1-/- mice for their motor deficits (analyzed by both rotarod and footprint tests), hippocampal cognitive impairment (by Novel Object Recognition (NOR) test), cerebellar neurodegeneration (Purkinje neurons counting), and cholesterol and sphingomyelin accumulation in spleen and liver. Finally, the effect of both agonists on survival was evaluated by applying a humane late endpoint (weight loss >30% of peak weight, punched posture and reduced activity in the cage). The results demonstrated that, while CGS21680 only slightly attenuated cognitive deficits, T1-11 ameliorated motor coordination, significantly improved cognitive impairments, increased the survival of Purkinje neurons and reduced sphingomyelin accumulation in the liver. More importantly, it significantly prolonged the lifespan of NPC1-/- mice. In vitro experiments conducted in a neuronal model of NPC1 demonstrated that the ability of T1-11 to normalize cell phenotype was mediated by the selective activation of A_2ARs and modulation of intracellular calcium levels. In conclusion, our results fully confirm the validity of A_2ARs as a new target for NPC1 treatment. As soon as new ligands with improved pharmacokinetic characteristics (i.e. orally active, with brain bioavailability and metabolic stability) will be obtained, A_2AR agonists could represent a breakthrough in the treatment of NPC.

Adenosine A2a receptor stimulation blocks development of nonalcoholic steatohepatitis in mice by multilevel inhibition of signals that cause immunolipotoxicity

Lipotoxicity and immunoinflammation are associated with the evolution of steatosis toward nonalcoholic steatohepatitis (NASH). This study reports the ability of adenosine A2a receptor (A2aR) activation to inhibit NASH development by modulating the responses of CD4⁺ T-helper (Th) cells to avoid an immuno-mediated potentiation of lipotoxicity. The effect of the A2aR agonist CGS21680 on immunoinflammatory signals, CD4⁺Th cell infiltration and immunolipotoxicity was analyzed in steatotic C57BL/6 mice fed with a methionine-choline-deficient (MCD) diet and in mouse hepatocytes exposed to palmitic acid (PA). CGS21680 inhibited NASH development in steatotic mice and decreased cytokines and chemokines involved in Th cell recruitment or polarization (namely CXCL10, CCL2, tumor necrosis factor alfa [TNFα], tumor growth factor [TGFβ], and IL-12). CGS21680 also reduced the expansion of Th17, Th22, and Th1 cells and increased the immunosuppressive activity of T regulatory cells. In PA-treated mice hepatocytes, CGS21680 inhibited the production of CXCL10, TNFα, TGFβ, IL-12, and CCL2; CGS21680 also prevented JNK-dependent lipotoxicity and its intensification by IL-17 or IL-17 plus IL-22 through Akt/PI3-kinase stimulation and inhibition of the negative regulator of PI3-kinase, (phosphatase and tensin homologue deleted from chromosome 10 (PTEN), which is upregulated by IL-17. In MCD livers, CGS21680 reduced JNK activation and PTEN expression and increased Akt phosphorylation. In conclusion, A2aR stimulation inhibited NASH development by reducing Th17 cell expansion and inhibiting the exacerbation of the IL-17-induced JNK-dependent lipotoxicity. These data promote the implementation of further studies to evaluate the potential clinical application of A2aR agonists that, by being able to function as both cytoprotective and immunomodulatory agents, could efficiently antagonize the multi-faced pathogenesis of NASH.

Role of OATP2A1 in PGE(2) secretion from human colorectal cancer cells via exocytosis in response to oxidative stress

Chronic inflammation induced by reactive oxygen species is associated with increased risk of developing colorectal cancer (CRC), and prostaglandin E2 (PGE2), which serves as a key mediator of inflammatory responses, plays an important role in CRC initiation and progression. Therefore, in the present study, we aimed to investigate the role of prostaglandin transporter OATP2A1/SLCO2A1 in the changes of PGE2 disposition in CRC cells in response to oxidative stress. H2O2 induced translocation of cytoplasmic OATP2A1 to plasma membranes in LoVo and COLO 320DM cells, but not in Caco-2 cells. The shift of subcellular OATP2A1 was abolished in the presence of anti-oxidant N-acetyl-L-cysteine or an inhibitor of protein kinase C, which evokes exocytosis. Exposure of LoVo cells to H2O2 caused an increase in the amount of extracellular PGE2 without changing the sum of intra- and extracellular PGE2. OATP2A1 knockdown decreased extracellular PGE2 in LoVo cells. In addition, extracellular PGE2 was significantly reduced by exocytosis inhibitor cytochalasin D, suggesting that H2O2-induced PGE2 release occurs in an exocytotic manner. Furthermore, mRNA expression of vascular endothelial growth factor (VEGF) was significantly reduced in LoVo cells by knockdown of OATP2A1. These results suggest that cytoplasmic OATP2A1 likely facilitates PGE2 loading into suitable intracellular compartment(s) for efficient exocytotic PGE2 release from CRC cells exposed to oxidative stress.

Pharmacological Preconditioning by Adenosine A2a Receptor Stimulation: Features of the Protected Liver Cell Phenotype

Ischemic preconditioning (IP) of the liver by a brief interruption of the blood flow protects the damage induced by a subsequent ischemia/reperfusion (I/R) preventing parenchymal and nonparenchymal liver cell damage. The discovery of IP has shown the existence of intrinsic systems of cytoprotection whose activation can stave off the progression of irreversible tissue damage. Deciphering the molecular mediators that underlie the cytoprotective effects of preconditioning can pave the way to important therapeutic possibilities. Pharmacological activation of critical mediators of IP would be expected to emulate or even to intensify its salubrious effects. In vitro and in vivo studies have demonstrated the role of the adenosine A2a receptor (A2aR) as a trigger of liver IP. This review will provide insight into the phenotypic changes that underline the resistance to death of liver cells preconditioned by pharmacological activation of A2aR and their implications to develop innovative strategies against liver IR damage.

The stimulation of adenosine A2A receptors ameliorates the pathological phenotype of fibroblasts from Niemann-Pick type C patients

Niemann-Pick type C1 (NPC1) disease is a rare neurovisceral disorder characterized by intracellular accumulation of unesterified cholesterol, sphingolipids, and other lipids in the lysosomal compartment. A deregulation of lysosomal calcium has been identified as one of the earliest steps of the degenerative process. Since adenosine A2A receptors (A2ARs) control lysosome trafficking and pH, which closely regulates lysosomal calcium, we hypothesized a role for these receptors in NPC1. The aim of this study was to evaluate the effects of the A2AR agonist CGS21680 on human control and NPC1 fibroblasts. We show that CGS21680 raises lysosomal calcium levels and rescues mitochondrial functionality (mitochondrial inner membrane potential and expression of the complex IV of the mitochondrial respiratory chain), which is compromised in NPC1 cells. These effects are prevented by the selective blockade of A2ARs by the antagonist ZM241385. The effects of A2AR activation on lysosomal calcium are not mediated by the cAMP/PKA pathway but they appear to involve the phosphorylation of ERK1/2. Finally, CGS21680 reduces cholesterol accumulation (Filipin III staining), which is the main criterion currently used for identification of a compound or pathway that would be beneficial for NPC disease, and such an effect is prevented by the Ca(2+) chelator BAPTA-AM. Our findings strongly support the hypothesis that A2AR agonists may represent a therapeutic option for NPC1 and provide insights on their mechanisms of action.

Cathepsin L inhibition by the small molecule KGP94 suppresses tumor microenvironment enhanced metastasis associated cell functions of prostate and breast cancer cells

Metastasis remains the major cause of therapeutic failure, poor prognosis and high mortality in breast and prostate cancer patients. Aberrant microenvironments including hypoxia and acidic pH are common features of most solid tumors that have been long associated with enhanced metastasis and poor patient outcomes. Novel approaches to reduce metastatic incidences and improve overall survival of cancer patients clearly are needed. The crucial role of Cathepsin L (CTSL) in the dissemination of tumor cells has led to the development of novel cathepsin L inhibition strategies. The present study evaluated the ability of KGP94, a small molecule inhibitor of CTSL, to impair the metastatic phenotype of prostate (PC-3ML) and breast (MDA-MB-231) cancer cells both under normal and aberrant microenvironmental conditions. To assess the role of CTSL in hypoxia and acidosis triggered metastasis associated cell functions, secreted CTSL levels were determined under conditions pertinent to the tumor microenvironment. Acute exposures to hypoxic or acidic conditions significantly elevated secreted CTSL levels either through an increase in intracellular CTSL levels or through activation of lysosomal exocytosis or both, depending on the tumor type. Increases in CTSL secretion closely paralleled enhanced tumor cell migration and invasion suggesting that CTSL could be an essential factor in tumor microenvironment triggered metastasis. Importantly, KGP94 treatment led to marked attenuation of tumor cell invasion and migration under both normal and aberrant microenvironmental conditions suggesting that it may have significant utility as an anti-metastatic agent.

Involvement of lysosomal exocytosis in the excretion of mesoporous silica nanoparticles and enhancement of the drug delivery effect by exocytosis inhibition

The exocytosis of phosphonate modified mesoporous silica nanoparticles (P-MSNs) is demonstrated and lysosomal exocytosis is identified as the mechanism responsible for this event. Regulation of P-MSN exocytosis can be achieved by inhibiting or accelerating lysosomal exocytosis. Slowing down P-MSN exocytosis enhances the drug delivery effect of CPT-loaded P-MSNs by improving cell killing.

Expression and function of TRP channels in liver cells

The liver plays a central role in whole body homeostasis by mediating the metabolism of carbohydrates, fats, proteins, drugs and xenobiotic compounds, and bile acid and protein secretion. Hepatocytes together with endothelial cells, Kupffer cells, smooth muscle cells, stellate and oval cells comprise the functioning liver. Many members of the TRP family of proteins are expressed in hepatocytes. However, knowledge of their cellular functions is limited. There is some evidence which suggests the involvement of TRPC1 in volume control, TRPV1 and V4 in cell migration, TRPC6 and TRPM7 in cell proliferation, and TRPPM in lysosomal Ca(2+) release. Altered expression of some TRP proteins, including TRPC6, TRPM2 and TRPV1, in tumorigenic cell lines may play roles in the development and progression of hepatocellular carcinoma and metastatic liver cancers. It is likely that future experiments will define important roles for other TRP proteins in the cellular functions of hepatocytes and other cell types of which the liver is composed.

Intracellular calcium signaling pathways during liver ischemia and reperfusion

Calcium plays a major role in intracellular signaling mechanisms during ischemia reperfusion (I/R) injury of a liver cell. Under ischemic conditions, the absence of oxygen arrests oxidative phosphorylation, thereby eliminating the energy source by which hepatocellular mechanisms maintain homeostasis of calcium. This, in turn, leaves nonselective plasma membrane influx pores unopposed and results in a net increase in intracellular calcium concentrations. Subsequent reperfusion marks the onset and progression of apoptosis and necrosis, as it involves inflammatory responses as well as free-radical formation due to re-oxygenation of cells. These processes destroy the structural integrity of organelles, leading to disruptive redistribution of calcium between cellular and subcellular compartments. This initial elevation and later imbalance of intracellular calcium concentrations associated with I/R induce various molecular responses within each organelle. In the cytoplasm, a series of pro-apoptotic pathways involving various calcium sensitive enzymes are activated. The injury is further exacerbated in the endoplasmic reticulum (ER) due to the malfunction of mechanisms responsible for intracellular calcium sequestration. Both the mitochondria and the nucleus are also adversely affected, as their structural integrity and physiologic functions are disrupted. To date, however, the precise pathophysiology of these calcium-mediated signaling pathways is not fully understood due to its complex nature. This review aims to systematically examine the current literature about individual molecular signaling pathways in the cytoplasm, ER, mitochondria, and the nucleus prior to causing time-sensitive progression of permanent tissue injury.

Caffeine protects against disruptions of the blood-brain barrier in animal models of Alzheimer's and Parkinson's diseases

Sporadic Alzheimer's disease (AD) and Parkinson's disease (PD) are two of the most common neurodegenerative diseases and as such they represent major public health problems. Finding effective treatments for AD and PD represents an unmet and elusive goal largely because these diseases are chronic and progressive, and have a complicated and ill-understood pathogenesis. Although the underlying mechanisms are not fully understood, caffeine, the most commonly ingested psychoactive drug in the world, has been shown in human and animal studies to be protective against AD and PD. One mechanism implicated in the pathogenesis of AD and PD is blood-brain barrier (BBB) dysfunction and we reported recently that caffeine exerts protective effects against AD and PD at least in part by keeping the BBB intact. The present review focuses on the role of BBB dysfunction in the pathogenesis of AD and PD, caffeine's protective effects against AD and PD, and potential mechanisms whereby caffeine protects against BBB leakage.

Prolactin promotes the secretion of active cathepsin D at the basal side of rat mammary acini

Cathepsin D (CD), a lysosomal aspartic protease present in mammary tissue and milk in various molecular forms, is also found in the incubation medium of mammary acini in molecular forms that are proteolytically active on prolactin at a physiological pH. Because prolactin controls the vesicular traffic in mammary cells, we studied, in vivo and in vitro, its effects on the polarized transport and secretion of various forms of CD in the rat mammary gland. CD accumulated in vesicles not involved in endocytosis in the basal region of cells. Prolactin increased this accumulation and the release of endosomal active single-chain CD at the basal side of acini. The CD-mediated proteolysis of prolactin, leading to the antiangiogenic 16-kDa form, at a physiological pH, was observed only in conditioned medium but not milk. These data support the novel concept that an active molecular form of CD, secreted at the basal side of the mammary epithelium, participates in processing blood-borne prolactin outside the cell, this polarized secretion being controlled by prolactin itself.

Adenosine-dependent activation of hypoxia-inducible factor-1 induces late preconditioning in liver cells

The cellular mechanisms by which ischemic preconditioning increases liver tolerance to ischemia/reperfusion injury are still poorly understood. This study investigated the role of the hypoxia-inducible factor-1 (HIF-1) in the protection associated with the late phase of liver preconditioning. Late preconditioning was induced in primary cultured rat hepatocytes by a transient (10 minute) hypoxic stress or by 15 minutes incubation with the adenosine A(2A) receptors agonist CGS21680 24 hours before exposure to 90 minutes of hypoxia in a serum-free medium. Late preconditioning induced the nuclear translocation of HIF-1 and the expression of carbonic anhydrase IX (CAIX), a HIF-1-regulated transmembrane enzyme that catalyzes bicarbonate production. Such effects were associated with prevention of hepatocyte killing by hypoxia and the amelioration of intracellular acidosis and Na+ accumulation. The inhibition of PKC-mediated and PI3-kinase-mediated signals with, respectively, chelerythrine and wortmannin abolished HIF-1 activation and blocked both CAIX expression and the protective action of late preconditioning. CAIX expression was also prevented by interfering with the transcriptional activity of HIF-1 using a dominant negative HIF-1beta subunit. The inhibition of CAIX with acetazolamide or the block of bicarbonate influx with disodium-4-acetamido-4'-isothiocyanato-stilben-2,2'-disulfonate also reverted the protective effects of late preconditioning on intracellular acidosis and Na+ accumulation.

CONCLUSION

The stimulation of adenosine A(2A) receptors induced late preconditioning in liver cells through the activation of HIF-1. HIF-1-induced expression of CAIX increases hepatocyte tolerance to ischemia by maintaining intracellular Na+ homeostasis. These observations along with the importance of HIF-1 in regulating cell survival indicates HIF-1 activation as a possible key event in liver protection by late preconditioning.

Ca(2+) -permeable channels in the hepatocyte plasma membrane and their roles in hepatocyte physiology

Hepatocytes are highly differentiated and spatially polarised cells which conduct a wide range of functions, including intermediary metabolism, protein synthesis and secretion, and the synthesis, transport and secretion of bile acids. Changes in the concentrations of Ca(2+) in the cytoplasmic space, endoplasmic reticulum (ER), mitochondria, and other intracellular organelles make an essential contribution to the regulation of these hepatocyte functions. While not yet fully understood, the spatial and temporal parameters of the cytoplasmic Ca(2+) signals and the entry of Ca(2+) through Ca(2+)-permeable channels in the plasma membrane are critical to the regulation by Ca(2+) of hepatocyte function. Ca(2+) entry across the hepatocyte plasma membrane has been studied in hepatocytes in situ, in isolated hepatocytes and in liver cell lines. The types of Ca(2+)-permeable channels identified are store-operated, ligand-gated, receptor-activated and stretch-activated channels, and these may vary depending on the animal species studied. Rat liver cell store-operated Ca(2+) channels (SOCs) have a high selectivity for Ca(2+) and characteristics similar to those of the Ca(2+) release activated Ca(2+) channels in lymphocytes and mast cells. Liver cell SOCs are activated by a decrease in Ca(2+) in a sub-region of the ER enriched in type1 IP(3) receptors. Activation requires stromal interaction molecule type 1 (STIM1), and G(i2alpha,) F-actin and PLCgamma1 as facilitatory proteins. P(2x) purinergic channels are the only ligand-gated Ca(2+)-permeable channels in the liver cell membrane identified so far. Several types of receptor-activated Ca(2+) channels have been identified, and some partially characterised. It is likely that TRP (transient receptor potential) polypeptides, which can form Ca(2+)- and Na(+)-permeable channels, comprise many hepatocyte receptor-activated Ca(2+)-permeable channels. A number of TRP proteins have been detected in hepatocytes and in liver cell lines. Further experiments are required to characterise the receptor-activated Ca(2+) permeable channels more fully, and to determine the molecular nature, mechanisms of activation, and precise physiological functions of each of the different hepatocyte plasma membrane Ca(2+) permeable channels.

The expression of prolactin and its cathepsin D-mediated cleavage in the bovine corpus luteum vary with the estrous cycle

In the corpus luteum (CL), blood vessels develop, stabilize, and regress. This process depends on the ratio of pro- and antiangiogenic factors, which change during the ovarian cycle. The present study focuses on the possible roles of 23,000 (23K) prolactin (PRL) in the bovine CL and its antiangiogenic NH(2)-terminal fragments after extracellular cleavage by cathepsin D (Cath D). PRL RNA and protein were demonstrated in the CL tissue, in luteal endothelial cells, and in steroidogenic cells. Cath D was detected in CL tissue, cell extracts, and corresponding cell supernatants. In the intact CL, 23K PRL levels decreased gradually, whereas Cath D levels concomitantly increased between early and late luteal stages. In vitro, PRL cleavage occurred in the presence of acidified homogenates of CL tissue, cells, and corresponding cell supernatants. Similar fragments were obtained with purified Cath D, and their appearance was inhibited by pepstatin A. The aspartic protease specific substrate MOCAc-GKPILF~FRLK(Dnp)-D-R-NH(2) was cleaved by CL cell supernatants, providing further evidence for Cath D activity. The 16,000 PRL inhibited proliferation of luteal endothelial cells accompanied by an increase in cleaved caspase-3. In conclusion, 1) the bovine CL is able to produce PRL and to process it into antiangiogenic fragments by Cath D activity and 2) PRL cleavage might mediate angioregression during luteolysis.

Purinergic P2Y2 receptors promote hepatocyte resistance to hypoxia

BACKGROUND/AIMS

ATP stimulation of purinergic P2 receptors (P2YR and P2XR) regulates several hepatic functions. Here we report the involvement of ATP-mediated signals in enhancing hepatocyte tolerance to lethal stress.

METHODS

The protection given by purinergic agonists was investigated in rat hepatocytes exposed to hypoxia.

RESULTS

ATP released after hypotonic stress (200 mOsm/L) as well as P2YR agonists prevented hepatocyte killing by hypoxia with efficiency ranking UTP > ATPgammaS > ADPbetaS, whereas the P2XR agonist, methylene-adenosine-5'-triphosphate, was ineffective. Adenosine-5'-O-3-thiotriphosphate (ATPgammaS; 100 micromol/L) also prevented Na+ -overload in hypoxic cells by inhibiting the Na+/H+ exchanger, without interfering with hypoxic acidosis. ATPgammaS activated Src and promoted a Src-dependent stimulation of both ERK1/2 and p38MAPK. Blocking p38MAPK with SB203580 reverted the protection given by ATPgammaS on both cell viability and Na+ accumulation, whereas ERK1/2 inhibition with PD98058 was ineffective. An increased phosphorylation of ERK1/2 was also evident in untreated hypoxic hepatocytes. PD98058 ameliorated Na+ accumulation and cell death caused by hypoxia. Hepatocyte pre-treatment with ATPgammaS reverted ERK1/2 activation in hypoxic cells. SB203580 blocked the effects of ATPgammaS on both ERK1/2 and Na+/H+ exchanger.

CONCLUSIONS

The activation of p38MAPK by P2Y2R increases hepatocyte resistance to hypoxia by down-modulating ERK1/2-mediated signals that promote Na+ influx through the Na+/H+ exchanger.

Hepatic ischemia-reperfusion injury: roles of Ca2+ and other intracellular mediators of impaired bile flow and hepatocyte damage

Liver resection and liver transplantation have been successful in the treatment of liver tumors and end-stage liver disease. This success has led to an expansion in the pool of patients potentially treatable by liver surgery and, in the case of transplantation, to a shortage of liver donors. At present, there are significant numbers of potential candidates for liver resection and liver donation who have fatty livers, are aged, or have livers damaged by chemotherapy. All of these are at high risk for ischemic reperfusion (IR) injury. The aims of this review are to assess current knowledge of the clinical effectiveness of ischemic preconditioning and intermittent ischemia in reducing IR damage in liver surgery; to evaluate the use of bile flow as a sensitive indicator of IR liver damage; and to analyze the molecular mechanisms, especially intracellular Ca2+, involved in IR injury and ischemic preconditioning. It is concluded that bile flow is a sensitive indicator of IR injury. Together with reactive oxygen species (ROS) and other extracellular and intracellular signaling molecules, intracellular Ca2+ in hepatocytes plays a key role in the normal regulation of bile flow and in IR-induced injury and cell death. Ischemic preconditioning is an effective strategy to reduce IR injury but there is considerable scope for improvement, especially in patients with fatty and aged livers. The development of effective new strategies to reduce IR injury will depend on improved understanding of the molecular mechanisms involved, especially by gaining a better perspective of the relative importance of the various intrahepatocyte signaling pathways involved.

PI3K-dependent lysosome exocytosis in nitric oxide-preconditioned hepatocytes

We investigated the signal mediators and the cellular events involved in the nitric oxide (NO)-induced hepatocyte resistance to oxygen deprivation in isolated hepatocytes treated with the NO donor (Z)-1-(N-methyl-N-[6-(N-methylammoniohexyl)amino])diazen-1-ium-1,2-diolate (NOC-9). NOC-9 greatly induced PI3K activation, as tested by phosphorylation of PKB/Akt. This effect was prevented by either 1H-(1,2,4)-oxadiazolo-(4,3)-quinoxalin-1-one, an inhibitor of the soluble guanylate cyclase (sGC), or KT5823, an inhibitor of cGMP-dependent kinase (cGK), as well as by farnesyl protein transferase inhibitor, which blocks the function of Ras GTPase. Bafilomycin A, an inhibitor of the lysosome-type vacuolar H+-ATPase, cytochalasin D, which disrupts the cytoskeleton-dependent organelle traffic, and wortmannin, which inhibits the PI3K-dependent traffic of lysosomes, all abolished the NOC-9-induced hepatocyte protection. The treatment with NOC-9 was associated with the PI3K-dependent peripheral translocation and fusion with the plasma membrane of lysosomes and the appearance at the cell surface of the vacuolar H+-ATPase. Inhibition of sGC, cGK, and Ras, as well as the inhibition of PI3K by wortmannin, prevented the exocytosis of lysosomes and concomitantly abolished the protective effect of NOC-9 on hypoxia-induced pHi and [Na+]i alterations and cell death. These data indicate that NO increases hepatocyte resistance to hypoxic injury by activating a pathway involving Ras, sGC, and cGK that determines PI3K-dependent exocytosis of lysosomes.

Cyclophilin A is secreted by a vesicular pathway in vascular smooth muscle cells

Reactive oxygen species (ROS) contribute to the pathogenesis of atherosclerosis in part by promoting vascular smooth muscle cell (VSMC) growth. Previously we demonstrated that cyclophilin A (CyPA) is a secreted oxidative stress-induced factor (SOXF) that promotes inflammation, VSMC growth, and endothelial cell apoptosis. However, the mechanisms that regulate CyPA secretion are unknown. In this study, we hypothesized that ROS-induced CyPA secretion from VSMC requires a highly regulated process of vesicle transport, docking, and fusion at the plasma membrane. Conditioned medium and plasma membrane sheets were prepared by exposing VSMC to 1 micromol/L LY83583, which generates intracellular superoxide. A vesicular transport mechanism was confirmed by colocalization at the plasma membrane with vesicle-associated membrane protein (VAMP). CyPA transport to the plasma membrane and secretion were significantly increased by LY83583. Reduction of VAMP-2 expression by small interfering RNA inhibited LY83583-induced CyPA secretion. Pretreatment with 3 micromol/L cytochalasin D, an actin depolymerizing agent, abrogated CyPA secretion. Infection with dominant-negative RhoA and Cdc42 adenovirus inhibited CyPA secretion by 72% and 63%, respectively, whereas dominant-negative Rac1 had a small effect (11%). Pretreatment with the Rho kinase inhibitor Y27632 (3 to 30 micromol/L) and myosin II inhibitor blebbistatin (1 to 10 micromol/L) inhibited CyPA secretion in a dose-dependent manner. Simvastatin (3 to 30 micromol/L) also dose-dependently inhibited LY83583-induced CyPA secretion likely via decreased isoprenylation of small GTPases. Our findings define a novel VSMC vesicular secretory pathway for CyPA that involves actin remodeling and myosin II activation via RhoA-, Cdc42-, and Rho kinase-dependent signaling events.

Intrinsic Resistance of Hepatocytes to Complement-Mediated Injury

When activated on or in the vicinity of cells, complement usually causes loss of function and sometimes cell death. Yet the liver, which produces large amounts of complement proteins, clears activators of complement and activated complexes from portal blood without obvious injury or impaired function. We asked whether and to what extent hepatocytes resist injury and loss of function mediated by exposure to complement. Using cells isolated from porcine livers as a model system, we found that, in contrast to endothelial cells, hepatocytes profoundly resist complement-mediated lysis and exhibit normal synthetic and conjugative functions when complement is activated on their surface. The resistance of hepatocytes to complement-mediated injury was not a function of cell surface control of the complement cascade but rather an intrinsic resistance of the cells dependent on the PI3K/Akt pathway. The resistance of hepatocytes to complement might be exploited in developing approaches to the treatment of hepatic failure or more broadly to the treatment of complement-mediated disease.

Cathepsin D released by lactating rat mammary epithelial cells is involved in prolactin cleavage under physiological conditions

The 16 kDa prolactin fragment arises from partial proteolysis of the native 23 kDa prolactin pituitary hormone. The mammary gland has been involved in this processing, although it has not been clarified whether it occurs in stroma or epithelial cells or extracellularly. Also, the processing enzyme has not been defined yet. Here we show that the incubation medium of stroma-deprived mammary acini from lactating rat contains an enzymatic activity able to cleave, in a temperature- and time-dependent fashion, the 23 kDa prolactin to generate a 16 kDa prolactin detectable under reducing conditions. This cleavage was not impaired in the presence of hirudin, a thrombin inhibitor, but strongly weakened in the presence of pepstatin A, a cathepsin D inhibitor. Cathepsin D immuno-depletion abolished the capability of acini-conditioned medium to cleave the 23 kDa prolactin. Brefeldin A treatment of acini, a condition that largely abolished the apical secretion of milk proteins, did not impair the secretion of the enzymatically active single chain of cathepsin D. These results show that mature cathepsin D from endosomes or lysosomes is released, likely at the baso-lateral site of mammary epithelial cells, and that a cathepsin D-dependent activity is required to effect, under physiological conditions, the cleavage of 23 kDa prolactin in the extracellular medium. This is the first report demonstrating that cathepsin D can perform a limited proteolysis of a substrate at physiological pH outside the cell.

Biosynthesis and alternate targeting of the lysosomal cysteine protease cathepsin L

Upregulation of cathepsin L expression, whether during development or cell transformation, or mediated by ectopic expression from a plasmid, alters the targeting of the protease and thus its physiological function. Upregulated procathepsin L is targeted to small dense core vesicles and to the dense cores of multivesicular bodies, as well as to lysosomes and to the plasma membrane for selective secretion. The multivesicular vesicles resemble secretory lysosomes characterized in specialized cell types in that they are endosomes that stably store an upregulated protein and they possess the tetraspanin CD63. Morphologically the multivesicular endosomes also resemble late endosomes, but they store procathepsin L, not the active protease, and they are not the major site for LAMP-1 accumulation. Distinction between the lysosomal proenzyme and active protease thus identifies two populations of multivesicular endosomes in fibroblasts, one a storage compartment and one an enzymatically active compartment. A distinctive targeting pathway using aggregation is utilized to enrich the storage endosomes with a particular lysosomal protease that can potentially activate and be secreted.