I. Garcin

Immediate neuroendocrine signaling after partial hepatectomy through acute portal hyperpressure and cholestasis

BACKGROUND & AIMS Early neuroendocrine pathways contribute to liver regeneration after partial hepatectomy (PH). We investigated one of these pathways involving acute cholestasis, immediate portal hyperpressure, and arginine vasopressin (AVP) secretion. METHODS Surgical procedure (PH, Portal vein stenosis (PVS), bile duct ligation (BDL), spinal cord lesion (SCL)) and treatments (capsaicin, bile acids (BA), oleanolic acid (OA)) were performed on rats and/or wild type or TGR5 (GPBAR1) knock-out mice. In these models, the activation of AVP-secreting supraoptic nuclei (SON) was analyzed, as well as plasma BA, AVP, and portal vein pressure (PVP). Plasma BA, AVP, and PVP were also determined in human living donors for liver transplantation. RESULTS Acute cholestasis (mimicked by BDL or BA injection) as well as portal hyperpressure (mimicked by PVS) independently activated SON and AVP secretion. BA accumulated in the brain after PH or BDL, and TGR5 was expressed in SON. SON activation was mimicked by the TGR5 agonist OA and inhibited in TGR5 KO mice after BDL. An afferent nerve pathway also contributed to post-PH AVP secretion, as capsaicin treatment or SCL resulted in a weaker SON activation after PH. CONCLUSIONS After PH in rodents, acute cholestasis and portal hypertension, via the nervous and endocrine routes, stimulate the secretion of AVP that may protect the liver against shear stress and bile acids overload. Data in living donors suggest that this pathway may also operate in humans.

Rapid Procoagulant Phosphatidylserine Exposure Relies on High Cytosolic Calcium Rather Than on Mitochondrial Depolarization

Objective—Relationships between intracellular Ca2+ concentration ([Ca2+]cyt) and apoptotic events, such as mitochondrial depolarization (&Dgr;&PSgr;m loss) and Bcl-2 and Bad phosphorylation, were analyzed in platelets and Jurkat cells in relation to rapid procoagulant phosphatidylserine (PS) exposure. Methods and Results—Platelets were stimulated with A23187, thapsigargin (TG) and thrombin plus convulxin (Thr/Cvx), and Jurkat cells with ionomycin, in the presence or absence of cyclosporin A (CsA), a mitochondrial permeability transition pore inhibitor. &Dgr;&PSgr;m loss occurred when platelets were stimulated in Ca2+ medium in conditions exposing PS, but also in EGTA medium. CsA inhibited PS exposure, [Ca2+]cyt increase, and &Dgr;&PSgr;m loss in platelets stimulated with TG and Thr/Cvx, but had no inhibitory effect on A23187 stimulation. CsA reduced TG-induced Ca2+ release from the endoplasmic reticulum and, consequently, external Ca2+ influx. In ionomycin-stimulated Jurkat cells, rapid PS exposure was evidenced but not &Dgr;&PSgr;m loss, and CsA did not inhibit the process. The status of phosphorylated Bad and Bcl-2 in both cell types remained unchanged on stimulation. Conclusions—Whether &Dgr;&PSgr;m loss occurs or not, PS exposure is triggered by a high [Ca2+]cyt increase. Data further demonstrate that CsA prevents membrane scrambling by inhibiting the high [Ca2+]cyt increase, independently of its effect on mitochondrial permeability transition pore.

Calcium Signalling and Liver Regeneration

After partial hepatectomy (PH) the initial mass of the organ is restored through a complex network of cellular interactions that orchestrate both proliferative and hepatoprotective signalling cascades. Among agonists involved in this network many of them drive Ca2+ movements. During liver regeneration in the rat, hepatocyte cytosolic Ca2+ signalling has been shown on the one hand to be deeply remodelled and on the other hand to enhance progression of hepatocytes through the cell cycle. Mechanisms through which cytosolic Ca2+ signals impact on hepatocyte cell cycle early after PH are not completely understood, but at least they include regulation of immediate early gene transcription and ERK and CREB phosphorylation. In addition to cytosolic Ca2+, there is also evidence that mitochondrial Ca2+ and also nuclear Ca2+ may be critical for the regulation of liver regeneration. Finally, Ca2+ movements in hepatocytes, and possibly in other liver cells, not only impact hepatocyte progression in the cell cycle but more generally may regulate cellular homeostasis after PH.

ERK activation by Ca2+ ionophores depends on Ca2+ entry in lymphocytes but not in platelets, and does not conduct membrane scrambling.

Abstract.Rapid Ca2+-dependent phospholipid (PL) reorganization (scrambling) at the plasma membrane is a mechanism common to hematopoietic cells exposing procoagulant phosphatidylserine (PS). The aim of this research was to determine whether activation of the extracellular signal-regulated kinase (ERK) pathway was required for PL scrambling, based on a single report analyzing both responses induced by Ca2+ ionophores in megakaryoblastic HEL cells. Ca2+ ionophore-stimulated ERK phosphorylation was induced in platelets without external Ca2+, whereas exogenous Ca2+ entry was crucial for ERK activation in Jurkat T cells. In both cells, membrane scrambling only occurred following Ca2+ entry and was not blocked by inhibiting ERK phosphorylation. Furthermore, ERK proteins are strongly phosphorylated in transformed B lymphoblastic cell lines, which do not expose PS in their resting state. Overall, the data demonstrated that ERK activation and membrane scrambling are independent mechanisms.

170 VASOPRESSIN-BASED REFLEX REGULATING PORTAL PRESSURE AND BILE HOMEOSTASIS AFTER PARTIAL HEPATECTOMY IN RATS AND HUMANS

Conclusions: Our data show that the stimulation of non-parenchymal liver cells by TLR agonists leads to induction or activation of cytokines or transcription factors that are known to have proand anti-regenerative properties. Therefore, the administration of TLR agonists with favorable ratios of proand anti-regenerative mediators before liver resection may be used for optimizing liver regeneration postoperatively.