Protective effects of prostaglandin E1 on hepatocytes

INTRODUCTION

Numerous studies have demonstrated the protective action of prostaglandin E1 (PGE1) on experimental animal models of liver injury and on patients with fulminant viral hepatitis. It could act on PGE1 receptor of the diseased vessels to dilate them and increase portal venous flow, improve the microcirculation of the liver, clear the metabolites of the liver cells, and increase oxygen supply to the liver tissues. PGE1 could also accumulate at the inflammentory por tion, inhibit the release of lethal factors, stablize the membrane of liver cell s and lysosome, inhibit the active oxygen, and promote the proliferation of the liver cells. It is now used to treat fulminant hepatic failure.

PROTECTIVE ACTION OF PGE1 ON EXPERIMENTAL ANIMAL MODELS OF LIVER INJURY

The protective action of PGE1 has been shown on both experimental animal models of liver injury and patients with fulminant viral hepatitis. Beck et al[1] examined the effects of long-term PGE treatment on liver and stomach in cirrhotic rats. Cirrhosis was induced by bile duct ligation. Sham-operation was performed an controls. Half of the rats received a PGE1 analogue, misoprostol (PGE1) (10 μg orally, daily) on d1-d29 postoperation, and the others received placebo only. Liver chemistry, portal pressures, and levels of prostaglandin E2, leukotriene B4, myeloperoxidase, and collagen in hepatic and gastric tissue of all rats on d31 were determined. PGE1-treated cirrhotic rats had less hepatosplenomegaly, lower serum alanine aminotransferase levels, and portal pressures and higher arterial pressure than placebo-treated cirrhotic rats. Hepatic and gastric le ukotriene B4, myeloperoxidase and collagen levels were significantly lower in th e PGE1-treated compared with placebo-treated cirrhotic rats. Placebo-treated cirrhotic rats had greater spontaneous and ethanol-induced gastric damage and failed to show gastric hyperemic response to ethanol, whereas PGE1-pretreated rats did. PGE1 did not significantly affect sham-operated rats. Beck suggested that long-term PGE1 administration was cytoprotective for both the liver and gastric mucosa in cirrhotic rats.

An animal model of hepatocytic necrosis was established by Gu[2] with injection of D-galactosamine into peritoneal cavity. Examination at regular intervals after injection of PGE showed that the level of increased serum TB, ALT and GST and the degree of histological changes in the liver were less marked in PGE-treated animals (n = 34) than those in PGE-untreated animals (n = 29), suggesting that PGE has definite protective effect on experimental hepatocytic necrosis.

The effects of PGE1.CD on dimethylnitrosamine (DMN)-induced acute liver damage with intravascular coagulation in rats were biochemically and histopathologicall y investigated by Suzuki[3]. PGE1.CD was administered i.v. 30 min before and 24 h after DMN-intoxication (pretreatment) and 30 min or 4 h to 24 h after DMN-intoxication (post-treatment). Pretreatment with P GE1.CD (0.2-2 μg·kg^-1·min^-1) dose-dependently suppressed the decrease of platelet counts and the elevation of blood biochemical parameters (PT, HPT, GOT, GPT, LDH, LAP, T-Bil) caused by DMN-intoxication. PGE1. CD (0.5 μg·kg^-1·min^-1 or over) significantly suppressed the DMN-induced histopathological changes (occurrence of hemorrhage and necrosis). Post-treatment with PGE1.CD (2 μg·kg^-1·min^-1) also suppressed the liver damage. Furthermore, pretreatment with PGE1.CD (2 μg·kg^-1·min^-1) not only suppressed the disruption of hepatocytes, but also prevented the damages of sinusoidal endothelial cells and lysosomal membrane, and it reduced the increase of lipid peroxidation. PGE1.CD (1 μg·kg^-1·min^-1 or over) significantly suppressed the decrease of hepatic tissue blood flow caused by DMN-intoxication. These results demonstrate that PGE1.CD has therapeutical efficacy against DMN-induced acute liver damage in rats, therefore, it will be clinically useful for the treatment of severe hepatitis such as fulminant hepatitis with intravascular coagulation in the sinusoid.

PGE1 AND SEVERE HEPATITIS

The effect of PG on patients with fulminant and subfulminant viral hepatitis (FH F) was studied by Sinclair[4]. Seventeen patients presented with FHF secondary to hepatitis A (n = 3), hepatitis B (n = 6), and non-A, non-B (NANB) hepatitis (n = 8). Fourteen of the 17 patients had stage III or IV hepatic encephalopathy (HE). The mean aspartate transaminase (AST) was 1844 U/L ± 1246 U/L, bilirubin 232 μmol/L ± 135 μmol/L, prothrombin time (PT) 34 s ± 18 s, partial thromboplastin time (PTT) 73 s ± 26 s, and coagulation Factors V and VII 8% ± 4% and 9% ± 5%, respectively. Intravenous PGE1 was initiated 24-48 h later after a rise in AST (2195 U/L ± 1810 U/L), bilirubin (341 μmol/L ± 148 μmol/L), PT (36 s ± 15 s), and PTT (75 s ± 18 s). Twelve of 17 responded rapidly with a decrease in AST from 1540 U/L ± 833 U/L to 188 U/L ± 324 U/L. Improvement in hepatic synthetic function was indicated by a decrease in PT from 27 s ± 7 s to 12 s ± 1 s and PTT from 61 s ± 10 s to 31 s ± 2 s, and an increase in Factor V from 9% ± 4% to 69% ± 18% and Factor VII from 11% ± 5% to 71% ± 20%. Five responders with NANB hepatitis relapsed upon discontinuation of therapy, with recurrence of HE and increases in AST and PT, and improvement was observed upon retreatment. After 4 wk of intravenous therapy oral PGE2 was substituted. Two patients with NANB hepatitis recovered completely and remained in remission 6 mo and 12 mo after cessation of therapy. Two additional patients maintained in remission after 2 mo and 6 mo of PGE2. No relapses were seen in the patients with hepatitis A virus and hepatitis B virus infection. Liver biopsies in all 12 surviving patients restored to normal. In the five no n-responders an improvement in hepatic function was indicated by a fall in AST (3767 U/L ± 2611 U/L to 2142 U/L ± 2040 U/L), PT (52 s ± 25 s to 33 s ± 18 s), and PTT (103 s ± 29 s to 77 s ± 44 s), but all deteriorated and died of cerebral edema (n = 3) or liver transplantation (n = 2). These results suggest that PGE has beneficial effect on FHF.

According to the severity, hepatic failure was divided into early stage, typical symptom stage and late stage. A treatment group of 55 cases received PGE1 therapy and a control group received basic support therapy only. T he results showed that difference of the total effective rate was not significant between these two groups, but in the early stage of hepatic failure, the effective rate in the treatment group was markedly higher than that in the control group. In addition, incidence of hepato-renal syndrome was lower in the treatment group[2]. This study indicates that division of severe viral hepatitis into three stages for evaluation of therapeutic effect is rational and useful and early use of PGE1 may certainly show some efficacy.

While orthotopic liver transplantation (OLT) has become the treatment of choice for most irreversible end-stage liver diseases, its role in patients with hepatitis B (HBV) infection is controversial. A high risk of reinfection of the trans planted graft, associated with significant morbidity and mortality, has been reported. Although passive and active immunization can delay the reappearance of virus in the allograft, there is not yet an effective therapy for recurrent HBV infection in liver transplant recipients. Twenty-eight OLT in 25 patients with acute and chronic HBV infections were performed[5]. Twelve of the patients were HBV DNA-negative, six were HBV DNA-positive, and seven were not tested prior to transplantation. Only 19 patients surviving more than 100 d after transplantation were considered to have sufficient duration of follow-up (mean 734 d) to include in the analysis of recurrence. Five (26%) were free of recurrent disease at the time of last follow-up (mean 1031 d). Recurrent HBV in the allograft, as defined by positive immunoperoxidase stains of biopsy sections for viral antigens, was detected in 74% at a mean of 134 d posttransplantation. Histological changes of viral hepatitis were evident in 13 of 14 with positive immunostaining. Twelve o f the 14 patients were treated, on an open trial basis, with intravenous and ora l prostaglandin E (PGE) because of deteriorating clinical condition. Eleven of t he twelve responded to PGE with an initial drop in serum transaminases, improvement in coagulopathy and resolution of encephalopathy. One patient failed to respond and died of myocardial infarction within 9 d of institution of therapy. Three of the eleven patients with an initial response relapsed and died of liver failure as a direct result of recurrent HBV after 13 d, 16 d and 37 d of treatment in association with generalized sepsis. Eight of the 12 patients (67%) had a sustained favorable response to PGE therapy (mean follow-up 737 d). All patients with a sustained response had accompanying improvement in histology and reduction in viral antigen staining in hepatocytes. Treatment with PGE appeared to be of benefit in recurrent HBV infection of the transplanted liver with an initial response rate of 92% and a sustained response rate of 67%.

The efficacy of PGE1 was demonstrated in the treatment of another 4 patients with subfulminant hepatitis of viral hepatitis B[6]. Three patients suffered from hepatic enchepalopathy of the first degree, and the remaining one of t he second degree. In three patients the clinical and biochemical improvement cam e relatively quickly, followed by recovery. In one patient, due to drug intolerance, the treatment was discontinued on the third day. The recurrence of illness was noted with the moderate increase of serum aminotransferase activities without clinical deterioration, necessitating no further use of prostaglandin E1. Boj ic suggested that prostaglandin E1, applied in the treatment of patients with subfulminant form of hepatitis, has favorable effect on the course of illness.

In a rare case of severe acute hepatitis A complicated by pure red cell aplasia (PRCA), plasma exchange transfusion and glucagon-insulin (GI) therapy improved the consciousness, but bilirubin, transaminase levels, and IgM anti-HAV titer remained high. Intravenous administration of lipophilized PGE1 (lipo-PGE1) was added to the GI therapy. Bilirubin and transaminase levels were normalized in the wk8 after the initiation of this combination therapy (17 wk after admission). The combined use of lipo-PGE1 with plasma exchange and GI therapy appeared to be useful for the severe hepatitis in this patient [7].

Two patients with HIV infection developed acute hepatitis B with liver insufficiency and hepatic encephalopathy. After alprostadil infusion was begun, they improved quickly and got a full recovery[8].

MECHANISMS OF HEPATIC CYTOPOTECTION OF PGE1

Indocyanine green disappearance enhanced by PGE1

Indocyanine green (ICG) is a reliable indicator reflecting hepatocyte function and hepatic blood flow. PGE1 has been indicated to increase hepatic blood flow and protect the hepatocyte. Tsukada[9] found that PGE1 administration increased ICG K in the liver cirrhosis (LC) and chronic hepatitis (CH) groups with normal liver function, and the ICG-K response was dose dependent when the dosage of PGE1 ranged from 0.01 μg·kg^-1·min^-1 to 0.05 μg·kg^-1·min^-1.

Inhibitory effects of PGE1 on T-cell mediated cytotoxicity

The effects of PG E1 on cell-mediated cytotoxicity against hepatocytes were investigated using in vitro cytotoxic assay system by Ogawa[10]. Isolated liver cells from normal C57BL/6 mice were used as the target cells, and effector cells were obtained from spleens of C57BL/6 mice in which experimental hepatitis had been induced by immunization with syngeneic liver antigens. In this assay system, spleen T cells adhering to nylon wool demonstrate d a high cytotoxic activity against target liver cells. The cytotoxicity was markedly reduced by PGE1 at concentrations greater than 10^-7 mol/L. Maximum suppressive activity was obtained when PGE1 was continuously present during the assay period. By contrast, indomethacin, a specific inhibitor of prostaglandin synthesis, enhanced the cytotoxic activity of effector cells. These data seem to indicate that exogenously added PGE1 has an inhibitory effect on cell-medi ated cytotoxicity of effector spleen cells against target hepatocytes.

PGE1 enhance DNA synthesis of injured liver after partial hepatectomy by stimulating cyclic AMP production and increasing ATP level in hepatic tissue

D-galactosamine (D-gal) damaged rats were infused with PGE1 through peripheral vein for 40 min. before and after partial hepatectomy. DNA synthesis following 68% partial hepatectomy was severely inhibited by the pretreatment of D-galactosamine. PGE1 infusion (0.5 μg·kg^-1·min^-1, 1.0 μg·kg^-1·min^-1) enhanced the DNA synthesis inhibited by D-gal 600 mg/kg significantly (P < 0.01). After 20 min of PGE1 infusion cyclic AMP levels of liver tissue was increased as compared with saline infusion in D gal (600 mg/kg) damaged rat (P < 0.05). Twenty min and 3 h after partial hepatectomy, ATP levels of liver tissue was enhanced in PGE1 treated group (P < 0.05). These results suggest that PGE1 enhances DNA synthesis of injured liver after partial hepatectomy by the mechanism of stimulating cyclic AMP production and increasing ATP level in hepatic tissue[11].

PGE1 could accelerate the recovery of mitochondrial respiratory function after reperfusion, stabilization of membrane microviscosity

PGE1 has been indicated to increase hepatic blood flow and protect the hepatocyte. Kurokawa et al[12] found that PGE1 could accelerate the recovery of mitochondrial respiratory function after reperfusion. When PGE1 was continuously administered to rats from 24 h before giving a dose of carbon tetrachloride, deranged serum glutamic pyruvic transaminase levels and prothrombin time were significantly reduced 12 h after intoxication compared with controls. A similar effect of PGE1 was seen at 24 h in D-galactosamine-intoxicated rats. Liver histology showed a comparable attenuation of injury in these rats. These results were consistent with reported effects of PGE2, suggesting that both prostaglandins may share a common pathway in protection against liver injury. When PGE1 or 16,16'-dimethyl PGE2 was added to the medium of primary cultured rat hepatocytes, lipid peroxidation- dependent killing of the cells by tert-butyl hydroperoxide was significantly attenuated without affecting the extent of malondialdehyde accumulation compared with controls. Both prostaglandins significantly reduced the extent of increased plasma membrane microviscosity of these cells. Masaki et al[13] concluded that PGE1 and PGE2 may possess cytoprotective effects on liver parenchymal cells through stabilization of membrane microviscosity, which may contribute t o protection against liver injury.

While the mechanisms of prostaglandin on protecting liver injury are not well understood, it has been demonstrated that dimethyl PGE2 abrogates the induction of tumour necrosis factor, leukotriene B4 (LTB4) and procoagulant activity by ma crophages as well as attenuating the expression of major histocompatibility class antigens on the surface of hepatocytes, and may inhibit viral replication. Fro m the present research, we came to a conclusion that increasing hepatic blood flow, accelerating the recovery of mitochondrial respiratory function after reperfusion, stabilizing the membrane microviscosity, decreasing the cell-mediated cytotoxicity against hepatocytes, enhancing DNA synthesis by stimulating cyclic AMP production and increasing ATP level in hepatic tissue made PGE1 as a hepatic cytoprotective agent.

TOXIC EFFECTS OF INTRAVENOUS AND ORAL PGE THERAPY ON PATIENTS WITH LIVER DISEASE

Prostaglandins are cytoprotective agents that have been shown to benefit patient s with a variety of acute and chronic liver diseases. Few data exist on the frequency of adverse effects of prostaglandins in these patients. Cattral et al[14] retrospectively studied 105 patients with liver disease who were treated with either i.v. or oral PGE. Forty-four patients with primary nonfunction after liver transplantation and 36 patients with fulminant hepatic failure received i.v. PGE1 for 4.5 d ± 2.6 d and 12.6 d ± 10.9 d, respectively. Twenty-five patients with recurrent hepatitis B viral infection after liver transplantation received oral PGE1 for 105 d ± 94 d or PGE2 for 464 d ± 399 d. Twenty-six of 80 patients (33%) receiving i.v. PGE1 developed gastrointestinal and/or cardiovascular side effects and 8% developed arthritis. Twenty-three (92%) of 25 patients who received high-dose oral PGE1 or PGE2 incurred arthritis and/or gastrointestinal adverse effects. Twenty-five patients receive d prolonged PGE therapy (oral > 60 d; i.v. > 28 d). Of this group, 23 (92%) developed clubbing and cortical hyperostosis resembling hypertrophic osteoarthropathy. All adverse effects were dose related and resolved with reduction or cessation of therapy. PGE therapy resulted in a wide spectrum of multisys tem adverse effects which were reversible with reduction or cessation of therapy. Although the administration of PGE was safe and generally well tolerated, close medical supervision is necessary to avoid serious side effects.

RECENT ADVANCES IN LIPID MICROSPHERE TECHNOLOGY FOR TARGETING PROSTAGLANDIN DELIVERY

Although PGE1 exhibits pharmacological activities in free for m, it has been hypothesized and experimentally verified that carrier can target them more effectively at lower doses, thus causing fewer side effects. Lipid microspheres (LM) with a diameter of 0.2 μm are drug carriers prepared from soybean oil and lecithin, and the drug is incorporated within the LM. Lipo-PGE1 is LM preparations of PGE1 that are designed to accumulate at the vascular lesions. The newly developed lipo-PGE1 (lipo-AS013) could overcome the disadvantages of the preparation currently available[15]. Lipo-AS013, a precursor of PGE1, is considered superior to free PGE1 in terms of its chemical stability and the retention ratio in LM in the body.