Constitutive secretion of serum albumin requires reversible protein tyrosine phosphorylation events intrans-Golgi

Inflammatory liver tissue formation using oxygen permeable membrane based culture platform

During chronic liver injury, inflammation leads to liver fibrosis, particularly due to the activation of hepatic stellate cells (HSCs). The involvement of inflammatory cytokines in HSC activation and the interplay among different liver cells are elaborated. To examine their interactions in vitro, many cultured liver tissue models are performed in organoid or spheroid culture with random 3D structure. Herein, we demonstrated the hierarchical coculture of primary rat hepatocytes with non-parenchymal cells such as the human-derived HSC line (LX-2) and liver sinusoidal endothelial cell line (TMNK-1). The cocultured tissue had high usability with simple operation of separating solid and liquid phases with improved liver functions such as albumin production and hepatic cytochrome P450 3A4 activity. We also studied the effects of stimulation by both oxygen tension and the key pro-fibrogenic cytokine, transforming growth factor beta (TGF-β), on HSC activation. Gene expression of collagen type I and alpha-smooth muscle actin were enhanced in the hierarchical coculture under lower oxygen tension and TGF-β1 stimulation. Therefore, this hierarchical in vitro cocultured liver tissue could provide a useful platform as a disease model for elucidating the interactions of various liver cell types and biochemical signals in future liver fibrogenesis studies.

Hepatic Protein and Phosphoprotein Signatures of Alcohol-Associated Cirrhosis and Hepatitis

Alcohol-associated liver disease is a global health care burden, with alcohol-associated cirrhosis (AC) and alcohol-associated hepatitis (AH) being two clinical manifestations with poor prognosis. The limited efficacy of standard of care for AC and AH highlights a need for therapeutic targets and strategies. The current study aimed to address this need through the identification of hepatic proteome and phosphoproteome signatures of AC and AH. Proteomic and phosphoproteomic analyses were conducted on explant liver tissue (test cohort) and liver biopsies (validation cohort) from patients with AH. Changes in protein expression across AH severity and similarities and differences in AH and AC hepatic proteome were analyzed. Significant alterations in multiple proteins involved in various biological processes were observed in both AC and AH, including elevated expression of transcription factors involved in fibrogenesis (eg, Yes1-associated transcriptional regulator). Another finding was elevated levels of hepatic albumin (ALBU) concomitant with diminished ALBU phosphorylation, which may prevent ALBU release, leading to hypoalbuminemia. Furthermore, altered expression of proteins related to neutrophil function and chemotaxis, including elevated myeloperoxidase, cathelicidin antimicrobial peptide, complement C3, and complement C5 were observed in early AH, which declined at later stages. Finally, a loss in expression of mitochondria proteins, including enzymes responsible for the synthesis of cardiolipin was observed. The current study identified hepatic protein signatures of AC and AH as well as AH severity, which may facilitate the development of therapeutic strategies.

Construction and evaluation of BSA-CaP nanomaterials with enhanced transgene performance via biocorona-inspired caveolae-mediated endocytosis

Non-viral nanovectors have attracted much attention owing to their ability to condense genetic materials and their ease of modification. However, their poor stability, low biocompatibility and gene degradation in endosomes or lysosomes has significantly hampered their application in vivo and in the clinic. In an attempt to overcome these difficulties a series of bovine serum albumin (BSA)-calcium phosphate (CaP) nanoparticles were constructed. The CaP condenses with DNA to form nanocomplexes coated with a biomimetic corona of BSA. Such complexes may retain the inherent endocytosis profile of BSA, with improved biocompatibility. In particular the transgene performance may be enhanced by stimulating the cellular uptake pathway via caveolae-mediated endocytosis. Two methods were employed to construct and optimize the formulation of BSA-CaP nanomaterials. The optimized BSA-CaP-50-M2 nanoparticles prepared by our second method exhibited good stability, negligible cytotoxicity and enhanced transgene performance with long-term expression for 72 h in vivo even with a single dose. Determination of the cellular uptake pathway and Western blot revealed that cellular uptake of the designed BSA-CaP-50-M2 nanoparticles was mainly via caveolae-mediated endocytosis in a non-degradative pathway in which the biomimetic uptake profile of BSA was retained.

mRNA destabilization improves glycemic responsiveness of transcriptionally regulated hepatic insulin gene therapy in vitro and in vivo

BACKGROUND

Hepatic insulin gene therapy (HIGT) employing a glucose and insulin sensitive promoter to direct insulin transcription can lower blood sugars within 2 h of an intraperitoneal glucose challenge. However, post-challenge blood sugars frequently decline to below baseline. We hypothesize that this 'over-shoot' hypoglycemia results from sustained translation of long-lived transgene message, and that reducing pro-insulin message half-life will ameliorate post-challenge hypoglycemia.

METHODS

We compared pro-insulin message content and insulin secretion from primary rat hepatocytes expressing insulin from either a standard construct (2xfur), or a construct producing a destabilized pro-insulin message (InsTail), following exposure to stimulating or inhibitory conditions.

RESULTS

Hepatocytes transduced with a 2xfur construct accumulated pro-insulin message, and exhibited increased insulin secretion, under conditions that both inhibit or stimulate transcription. By contrast, pro-insulin message content remained stable in InsTail expressing cells, and insulin secretion increased less than 2xfur during prolonged stimulation. During transitions from stimulatory to inhibitory conditions, or vice versa, amounts of pro-insulin message changed more rapidly in InsTail expressing cells than 2xfur expressing cells. Importantly, insulin secretion increased during the transition from stimulation to inhibition in 2xfur expressing cells, although it remained unchanged in InsTail expressing cells. Use of the InsTail destabilized insulin message tended to more rapidly reduce glucose induced glycemic excursions, and limit post-load hypoglycemia in STZ-diabetic mice in vivo.

CONCLUSIONS

The data obtained in the present study suggest that combining transcriptional and post-transcriptional regulatory strategies may reduce undesirable glycemic excursion in models of HIGT.

Genistein induces cytokinesis failure through RhoA delocalization and anaphase chromosome bridging

Genistein, an isoflavone abundantly present in soybeans, possesses anticancer properties and induces growth inhibition including cell cycle arrest and apoptosis. Although abnormal cell division, such as defects in chromosome segregation and spindle formation, and polyploidization have been described, the mechanisms underlying the induction of abnormal cell division are unknown. In this study, we examined the effect of genistein on cell division in cells that are synchronized in M phase, since genistein treatment delays mitotic entry in asynchronous cells. HeLa S3 cells were arrested at the G2 phase and subsequently released into the M phase in presence of genistein. Immunofluorescence staining showed that genistein treatment delays M phase progression. Time-lapse analysis revealed that the delay occurs until anaphase onset. In addition, genistein treatment induces cleavage furrow regression, resulting in the generation of binucleated cells. Central spindle formation, which is essential for cytokinesis, is partially disrupted in genistein-treated cells. Moreover, aberrant chromosome segregation, such as a chromosome bridge and lagging chromosome, occurs through progression of cytokinesis. RhoA, which plays a role in the assembly and constriction of an actomyosin contractile ring, is delocalized from the cortex of the ingressing cleavage furrow. These results suggest that genistein treatment induces binucleated cell formation through cleavage furrow regression, which is accompanied by chromosome bridge formation and RhoA delocalization. Our results provide the mechanism that underlies genistein-induced polyploidization, which may be involved in genistein-induced growth inhibition.

Cre activity in fetal albCre mouse hepatocytes: Utility for developmental studies

The albCre transgene, having Cre recombinase driven by the serum albumin (alb) gene promoter, is commonly used to generate adult mice having reliable hepatocyte-specific recombination of loxP-flanked ("floxed") alleles. Based on previous studies, it has been unclear whether albCre transgenes are also reliable in fetal and juvenile mice. Perinatal liver undergoes a dynamic transition from being predominantly hematopoietic to predominantly hepatic. We evaluated Cre activity during this transition in albCre mice using a sensitive two-color fluorescent reporter system. From fetal through adult stages, in situ patterns of Cre-dependent recombination of the reporter closely matched expression of endogenous Alb mRNA or protein, indicating most or all hepatocytes, including those in fetal and juvenile livers, had expressed Cre and recombined the reporter. Our results indicate the albCre transgene is effective in converting simple floxed alleles in fetal and neonatal mice and is an appropriate tool for studies on hepatocyte development.

A traffic-activated Golgi-based signalling circuit coordinates the secretory pathway

As with other complex cellular functions, intracellular membrane transport involves the coordinated engagement of a series of organelles and machineries; however, the molecular basis of this coordination is unknown. Here we describe a Golgi-based signalling system that is activated by traffic and is involved in monitoring and balancing trafficking rates into and out of the Golgi complex. We provide evidence that the traffic signal is due to protein chaperones that leave the endoplasmic reticulum and reach the Golgi complex where they bind to the KDEL receptor. This initiates a signalling reaction that includes the activation of a Golgi pool of Src kinases and a phosphorylation cascade that in turn activates intra-Golgi trafficking, thereby maintaining the dynamic equilibrium of the Golgi complex. The concepts emerging from this study should help to understand the control circuits that coordinate high-order cellular functions.

Regulation of endogenous apolipoprotein E secretion by macrophages

Apolipoprotein E has critical roles in the protection against atherosclerosis and is understood to follow the classical constitutive secretion pathway. Recent studies have indicated that the secretion of apoE from macrophages is a regulated process of unexpected complexity. Cholesterol acceptors such as apolipoprotein A-I, high density lipoprotein, and phospholipid vesicles can stimulate apoE secretion. The ATP binding cassette transporter ABCA1 is involved in basal apoE secretion and in lipidating apoE-containing particles secreted by macrophages. However, the stimulation of apoE secretion by apoA-I is ABCA1-independent, indicating the existence of both ABCA1-dependent and -independent pathways of apoE secretion. The release of apoE under basal conditions is also regulated, requiring intact protein kinase A activity, intracellular calcium, and an intact microtubular network. Mathematical modeling of apoE turnover indicates that whereas some pools of apoE are committed to either secretion or degradation, other pools can be diverted from degradation toward secretion. Targeted inhibition or stimulation of specific apoE trafficking pathways will provide unique opportunities to regulate the biology of this important molecule.

Kalirin/Trio Rho guanine nucleotide exchange factors regulate a novel step in secretory granule maturation

The molecular mechanisms involved in the maturation of secretory granules, organelles that store hormones and neuropeptides, are poorly understood. As granule content proteins are processed, the composition of granule membranes changes, yielding constitutive-like secretion of immature content proteins and producing secretagogue-responsive mature granules. Constitutive-like secretion was not previously recognized as a process subject to regulation. We show that Kalirin and Trio, homologous Rho guanine nucleotide exchange factors (GEFs), which interact with a secretory granule resident protein, modulate cargo secretion from immature granules. Some of the Kalirin and Trio isoforms expressed in neuroendocrine cells colocalize with immature granules. Overexpression of their N-terminal GEF domain (GEF1) enhances secretion from immature granules, depleting cells of secretory cargo in the absence of secretagogue. This response requires GEF1 activity and is mimicked by Kalirin/Trio substrates Rac1 and RhoG. Accordingly, selective pharmacological inhibition of endogenous GEF1 activity decreases secretagogue-independent release of hormone precursors, accumulating product peptide in mature secretory granules. Kalirin/Trio modulation of cargo secretion from immature granules provides secretory cells with an extra layer of control over the sets of peptides released. Control of this step enhances the range of physiological responses that can be elicited, whereas lack of control could have pathological consequences.

Secretion of apolipoprotein E from macrophages occurs via a protein kinase A and calcium-dependent pathway along the microtubule network

Macrophage-specific expression of apolipoprotein (apo)E protects against atherosclerosis; however, the signaling and trafficking pathways regulating secretion of apoE are unknown. We investigated the roles of the actin skeleton, microtubules, protein kinase A (PKA) and calcium (Ca2+) in regulating apoE secretion from macrophages. Disrupting microtubules with vinblastine or colchicine inhibited basal secretion of apoE substantially, whereas disruption of the actin skeleton had no effect. Structurally distinct inhibitors of PKA (H89, KT5720, inhibitory peptide PKI(14-22)) all decreased basal secretion of apoE by between 50% to 80% (P<0.01). Pulse-chase experiments demonstrated that inhibition of PKA reduced the rate of apoE secretion without affecting its degradation. Confocal microscopy and live cell imaging of apoE-green fluorescent protein-transfected RAW macrophages identified apoE-green fluorescent protein in vesicles colocalized with the microtubular network, and inhibition of PKA markedly inhibited vesicular movement. Chelation of intracellular calcium ([Ca2+]i) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate-acetoxymethyl ester (BAPTA-AM) inhibited apoE secretion by 77.2% (P<0.01). Injection of c57Bl6 apoE+/+ bone marrow-derived macrophages into the peritoneum of apoE-/- C57Bl6 mice resulted in time-dependent secretion of apoE into plasma, which was significantly inhibited by transient exposure of macrophages to BAPTA-AM and colchicine and less effectively inhibited by H89. We conclude that macrophage secretion of apoE occurs via a PKA- and calcium-dependent pathway along the microtubule network.

Leptin biosynthetic pathway in white adipocytes

The aim of this study was to determine through morphological and biochemical means the biosynthetic and secretory pathway followed by leptin in adipocytes. Immunocytochemistry revealed the presence of leptin in the rough endoplasmic reticulum, the Golgi apparatus, and in numerous small vesicles along the plasma membrane of white adipo cytes. In vitro, isolated adipocytes under nonstimulated conditions (basal) continuously secreted leptin while their intra cellular content remained unchanged. When adipocytes were stimulated with insulin, leptin cellular content and secretion increased in parallel and were significantly different from basal secretion only after 45 min. L-leucine and L-glutamate also strongly stimulated leptin synthesis and secretion. These stimulating effects were abolished by cycloheximide and brefeldin A. The transcriptional inhibitor actinomycin D did not have any effects in either basal or stimulated conditions. Leptin mRNA levels were not affected by any stimulating or inhibiting agents. Finally, norepinephrine, isoproterenol, CL316243, and palmitate inhibited the effects of insulin, L-leucine, and L-glutamate on leptin synthesis. We thus conclude that (i) adipocytes continuously synthesize and secrete leptin along a rough endoplasmic reticulum-Golgi secretory vesicles pathway, (ii) an increase in leptin secretion requires increased de novo synthesis, and (iii) short-term leptin secretion does not involve changes in mRNA levels.

Two distinct chloride ion requirements in the constitutive protein secretory pathway

The role of chloride ions in regulated secretion is well described but remains poorly characterised in the constitutive system. In the liver, newly synthesised proalbumin is transported to the trans Golgi network where it is converted to albumin by a furin protease and then immediately secreted. We used this acid-dependent hydrolysis and the measurement of specific protein secretion rates to examine the H+ and Cl- ion dependence of albumin synthesis and secretion, a major constitutive protein secretory event in all mammals. Using permeabilised primary rat hepatocytes we show that ordinarily chloride ions are essential for the processing of proalbumin to albumin. However Cl- is not required for transport which continues but releases solely proalbumin. Prior treatment of the cells with Tris (used as a membrane-permeable weak base to neutralise Golgi luminal pH) both eliminated the formation of albumin and very greatly reduced secretion. After washing out Tris, both authentic secretion and processing could be restarted if Cl-, ATP, GTP, cAMP, Ca2+ and cytosolic proteins were added. Hence a requirement for chloride in transport, in addition to processing, can be uncovered by first neutralising pH gradients. Furthermore, the chloride channel blocker DIDS (4,4-diisothiocyanostilbene 2,2-disulphonic acid) reversibly inhibited the constitutive secretory pathway. However, the total mass of proalbumin detectable in DIDS-treated cells fell to 36% of control while the fraction processed to albumin remained almost constant. This clearly dissociates a large part of the Cl- requirement of the constitutive protein secretory pathway from the function of known liver Golgi Cl- channels.