Effects of rapamycin on cultured hepatocyte proliferation and gene expression

Liver Regeneration after Hepatectomy and Partial Liver Transplantation

The liver is a unique organ with an abundant regenerative capacity. Therefore, partial hepatectomy (PHx) or partial liver transplantation (PLTx) can be safely performed. Liver regeneration involves a complex network of numerous hepatotropic factors, cytokines, pathways, and transcriptional factors. Compared with liver regeneration after a viral- or drug-induced liver injury, that of post-PHx or -PLTx has several distinct features, such as hemodynamic changes in portal venous flow or pressure, tissue ischemia/hypoxia, and hemostasis/platelet activation. Although some of these changes also occur during liver regeneration after a viral- or drug-induced liver injury, they are more abrupt and drastic following PHx or PLTx, and can thus be the main trigger and driving force of liver regeneration. In this review, we first provide an overview of the molecular biology of liver regeneration post-PHx and -PLTx. Subsequently, we summarize some clinical conditions that negatively, or sometimes positively, interfere with liver regeneration after PHx or PLTx, such as marginal livers including aged or fatty liver and the influence of immunosuppression.

Alloresponses of Mixed Lymphocyte Hepatocyte Culture to Immunosuppressive Drugs as an In-Vitro Model of Hepatocyte Transplantation

Background

Hepatocyte transplantation (HCTx) has the potential for the treatment of end-stage liver disease. However, failure of engraftment and the long-term acceptance of cellular allografts remain significant challenges for its clinical application. The aim of this study was to investigate the efficacy of the immunosuppressive agents, Cyclosporine, Everolimus, and Belatacept to suppress the alloresponse of primary human hepatocytes in a mixed lymphocyte-hepatocyte culture (MLHC) and their potential hepatotoxicity in vitro.

Material/Methods

Primary human hepatocytes were co-cultured with allogeneic peripheral blood mononuclear cells (PBMCs) in an MLHC. Proliferative alloresponses were determined by flow cytometry, and cytokine secretion was measured using Luminex-based multiplex technology. Using an MLHC, the alloresponses of primary human hepatocytes were compared in the presence and absence of Cyclosporine, Everolimus, and Belatacept. Cultured primary human hepatocytes were assessed for the production of albumin, urea, aspartate transaminase (AST) and DNA content. Metabolic activity was determined with the MTT assay.

Results

Immune responses induced by primary human hepatocytes were effectively suppressed by Cyclosporine, Everolimus, and Belatacept. Everolimus significantly reduced the metabolic activity of primary human hepatocytes in vitro, suggesting impairment of cell viability. However, further functional analysis showed no significant differences between treated and untreated controls.

Conclusions

Cyclosporine, Everolimus, and Belatacept suppressed the alloresponse of primary human hepatocytes in an MLHC without significant cytotoxicity or functional cell impairment.

Comparison of Liver Graft Regeneration Between ABO-Compatible and ABO-Incompatible Living Donor Liver Transplantation: A Propensity Score Matching Analysis

BACKGROUND ABO-incompatible (ABOi) living donor liver transplantation (LDLT) was accepted as a feasible therapy for end-stage liver disease after the introduction of rituximab. The present study investigated the association between ABO incompatibility and graft regeneration in patients who underwent LDLT. MATERIAL AND METHODS A total of 335 adult patients who underwent elective LDLT were divided into ABO-compatible (ABOc) and ABOi LDLT groups using propensity score (PS) matching of graft regeneration-related factors. Postoperative serial changes in graft volumes were compared between the groups. The factors associated with graft volume on postoperative day (POD) 21 were investigated in patients who underwent ABOi LDLT. RESULTS In total, 300 (89.6%) patients underwent ABOc LDLT and 35 (10.4%) patients underwent ABOi LDLT. After PS matching, the ABOc and ABOi groups each included 32 paired patients. The absolute liver graft volumes on POD 21 were significantly lower in the ABOi group than those in the ABOc group in the PS-matched patients (1098.4 [964.0-1,162.0] vs. 1202.0 [1107.8-1455.2] mL; p=0.007). Major complications, including overall patient mortality during the follow-up period, did not differ between the groups. In patients who underwent ABOi LDLT, the preoperative graft volume/standard liver volume ratio and CD4+ cell level on POD 14 were independent factors related to liver graft volume on POD 21. CONCLUSIONS These results suggest that ABO incompatibility could affect postoperative liver graft regeneration. Therefore, graft regeneration must be investigated using a volumetric assessment in patients who have undergone ABOi LDLT.

eIF4E Phosphorylation in Prostate Cancer

Prostate cancer (PCa) progression involves a shift from endocrine to paracrine and eventually autocrine control resulting from alterations in molecular mechanisms in the cells. Deregulation of RNA translation is crucial for tumor cells to grow and proliferate; therefore, overactivation of the translation machinery is often observed in cancer. The two most important signal transduction pathways regulating PCa progression are PI3K/Akt/mTOR and Ras/MAPK. These two pathways converge on the eukaryotic translation initiation factor 4E (eIF4E) which binds to the protein scaffold eIF4G upon mechanistic target of rapamycin (mTOR) activation and is phosphorylated by the mitogen-activated protein kinase (MAPK) interacting protein kinases (Mnk1/2). This review describes the role of eIF4E in mRNA translation initiation mediated by its binding to the methylated 5′ terminal structure (m7G-cap) of many mRNAs, and the ability of many tumor cells to bypass this mechanism. Hormonal therapy and chemotherapy are two of the most prevalent therapies used in patients with advanced PCa, and studies have implicated a role for eIF4E phosphorylation in promoting resistance to both these therapies. It appears that eIF4E phosphorylation enhances the rate of translation of oncogene mRNAs to increase tumorigenicity.

Current methods of the analysis of immunosuppressive agents in clinical materials: A review

More than 100000 solid organ transplantations are performed every year worldwide. Calcineurin (cyclosporine A, tacrolimus), serine/threonine kinase (sirolimus, everolimus) and inosine monophosphate dehydrogenase inhibitor (mycophenolate mofetil), are the most common drugs used as immunosuppressive agents after solid organ transplantation. Immunosuppressive therapy, although necessary after transplantation, is associated with many adverse consequences, including the formation of secondary metabolites of drugs and the induction of their side effects. Calcineurin inhibitors are associated with nephrotoxicity, cardiotoxicity and neurotoxicity; moreover, they increase the risk of many diseases after transplantation. The review presents a study of the movement of drugs in the body, including the processes of absorption, distribution, localisation in tissues, biotransformation and excretion, and also their accompanying side effects. Therefore, there is a necessity to monitor immunosuppressants, especially because these drugs are characterised by narrow therapeutic ranges. Their incorrect concentrations in a patient's blood could result in transplant rejection or in the accumulation of toxic effects. Immunosuppressive pharmaceuticals are macrolide lactones, peptides, and high molecular weight molecules that can be metabolised to several metabolites. Therefore the two main analytical methods used for their determination are high performance liquid chromatography with various detection methods and immunoassay methods. Despite the rapid development of new analytical methods of analysing immunosuppressive agents, the application of the latest generation of detectors and increasing sensitivity of such methods, there is still a great demand for the development of highly selective, sensitive, specific, rapid and relatively simple methods of immunosuppressive drugs analysis.

mTOR signaling in liver regeneration: Rapamycin combined with growth factor treatment

AIM: To investigate the effects of mammalian target of rapamycin (mTOR) inhibition on liver regeneration and autophagy in a surgical resection model.

METHODS: C57BL/6 mice were subjected to a 70% partial hepatectomy (PH) and treated intraperitoneally every 24 h with a combination of the mTOR inhibitor rapamycin (2.5 mg/kg per day) and the steroid dexamethasone (2.0 mg/kg per day) in phosphate buffered saline (PBS) or with PBS alone as vehicle control. In the immunosuppressant group, part of the group was treated subcutaneously 4 h prior to and 24 h after PH with a combination of human recombinant interleukin 6 (IL-6; 500 μg/kg per day) and hepatocyte growth factor (HGF; 100 μg/kg per day) in PBS. Animals were sacrificed 2, 3 or 5 d after PH and liver tissue and blood were collected for further analysis. Immunohistochemical staining for 5-Bromo-2’-deoxyuridine (BrdU) was used to quantify hepatocyte proliferation. Western blotting was used to detect hepatic microtubule-associated protein 1 light chain 3 (LC3)-II protein expression as a marker for autophagy. Hepatic gene expression levels of proliferation-, inflammation- and angiogenesis-related genes were examined by real-time reverse transcription-polymerase chain reaction and serum bilirubin and transaminase levels were analyzed at the clinical chemical core facility of the Erasmus MC-University Medical Center.

RESULTS: mTOR inhibition significantly suppressed regeneration, shown by decreased hepatocyte proliferation (2% vs 12% BrdU positive hepatocyte nuclei at day 2, P < 0.01; 0.8% vs 1.4% at day 5, P = 0.02) and liver weight reconstitution (63% vs 76% of initial total liver weight at day 3, P = 0.04), and furthermore increased serum transaminase levels (aspartate aminotransferase 641 U/L vs 185 U/L at day 2, P = 0.02). Expression of the autophagy marker LC3-II, which was reduced during normal liver regeneration, increased after mTOR inhibition (46% increase at day 2, P = 0.04). Hepatic gene expression showed an increased inflammation-related response [tumor necrosis factor (TNF)-α 3.2-fold upregulation at day 2, P = 0.03; IL-1Ra 6.0-fold upregulation at day 2 and 42.3-fold upregulation at day 5, P < 0.01] and a reduced expression of cell cycle progression and angiogenesis-related factors (HGF 40% reduction at day 2; vascular endothelial growth factor receptor 2 50% reduction at days 2 and 5; angiopoietin 1 60% reduction at day 2, all P≤ 0.01). Treatment with the regeneration stimulating cytokine IL-6 and growth factor HGF could overcome the inhibitory effect on liver weight (75% of initial total liver weight at day 3, P = 0.02 vs immunosuppression alone and P = 0.90 vs controls) and partially reversed gene expression changes caused by rapamycin (TNF-α and IL-1Ra levels at day 2 were restored to control levels). However, no significant changes in hepatocyte proliferation, serum injury markers or autophagy were found.

CONCLUSION: mTOR inhibition severely impairs liver regeneration and increases autophagy after PH. These effects are partly reversed by stimulation of the IL-6 and HGF pathways.

Liver regeneration after liver transplantation

BACKGROUND/PURPOSE

The liver has a remarkable capacity to regenerate after injury or resection. The aim of this review is to outline the mechanisms and factors affecting liver regeneration after liver transplantation.

METHODS

Relevant studies were reviewed using Medline, PubMed and Springer databases.

RESULTS

A variety of cytokines (such as interleukin-6 and tumor necrosis factor-α), growth factors (like hepatocyte growth factor and transforming growth factor-α) and cells are involved in liver regeneration. Several factors affect liver regeneration after transplantation such as ischemic injury, graft size, immunosuppression, steatosis, donor age and viral hepatitis.

CONCLUSION

Liver regeneration has been studied for many years. However, further research is essential to reveal the complex processes affecting liver regeneration, which may provide novel strategies in the management of liver transplantation recipients and donors.

Current status of immunosuppressive agents for solid organ transplantation in children

Immunosuppression after organ transplantation is complex and ever evolving. Over the past two decades, newer immunosuppressive agents have been introduced with an aim to provide better patient and graft survival. Improved therapeutic strategies have been developed offering the option to use combinations of drugs with non-overlapping toxicities. There are, however, only a few clinical studies with robust data to rationalize the use of these agents in children. This review will discuss the newer immunosuppressive agents used for solid organ transplant, their current status in post-transplant management and prevention of allograft rejection.

The impact of sirolimus on hepatocyte proliferation after living donor liver transplantation

BACKGROUND

There is a lack of data on the use of sirolimus after partial liver transplantation, especially regarding its impact on post-transplant regeneration.

METHODS

We reviewed adult living donor transplantations, with de novo sirolimus (n = 7) and without sirolimus (n = 21). Liver biopsies were stained for KI-67, a proliferation marker. Controls included specimens with normal liver parenchyma (n = 13).

RESULTS

Both groups had similar demographics, graft and patient survival and complication rates. During the first six wk and over the whole first year post-transplant, the use of sirolimus was associated with lower levels of hepatocyte proliferation compared to sirolimus-free patients, (overall, 0.3 [0-7.2] vs. 3 [0-49] KI-67 positive hepatocytes per high power field, p ≤ 0.05). The levels observed in the sirolimus group were similar to those seen in non-transplanted control patients with normal parenchyma (0.2 [0-1.3], p = NS). Post-transplant hepatocyte proliferation correlated with the serum levels of sirolimus (p ≤ 0.05), but not with those of tacrolimus or with the dose of mycophenolate mofetil (p = 0.9 and 0.3, respectively).

CONCLUSIONS

These data suggest that sirolimus is associated with decreased post-transplant hepatocyte proliferation. The clinical significance of this observation remains to be fully determined.

Rapamycin prevents concanavalin A-induced liver injury by inhibiting lymphocyte activation

BACKGROUND AND AIMS

Liver injury induced by concanavalin A (Con A) is often used as a model to study the pathophysiology of immune mediated liver injury. Rapamycin (Rapa) is an effective immunosuppressant widely used for preventing immune activation and transplant rejection. However, the effect of Rapa on liver injury caused by Con A has not been carefully examined. In the present study, we examined the effect of Rapa on liver injury caused by Con A.

METHODS

Mice received intraperitoneal Rapa injection before Con A intravenous administration. The liver injury was examined by measuring serum transaminase and pathology, and the level of cytokines was detected by enzyme linked immunosorbent assay (ELISA).

RESULTS

In the present study, we examined the effect of Rapa on liver injury after Con A injection in mice. We found that the treatment of mice with Rapa protected the liver from Con A-induced injury. Pretreatment with Rapa dramatically ameliorated Con A-induced mortality. This protection was associated with reduced transaminase levels in the blood and further confirmed by liver histology. ELISA showed that Rapa suppressed pro-inflammatory cytokines IFN-gamma and TNF-alpha production as compared with the untreated controls. Furthermore, intrahepatic lymphocyte proliferation was significantly inhibited.

CONCLUSION

These findings suggested that Rapa has the therapeutic potential for treatment of immune-mediated liver injury in the clinic.

Akt-mediated liver growth promotes induction of cyclin E through a novel translational mechanism and a p21-mediated cell cycle arrest

The control of hepatocyte growth is relevant to the processes of liver regeneration, development, metabolic homeostasis, and cancer. A key component of growth control is the protein kinase Akt, which acts downstream of mitogens and nutrients to affect protein translation and cell cycle progression. In this study, we found that transient transfection of activated Akt triggered a 3-4-fold increase in liver size within days but only minimal hepatocyte proliferation. Akt-induced liver growth was associated with marked up-regulation of cyclin E but not cyclin D1. Analysis of liver polyribosomes demonstrated that the post-transcriptional induction of cyclin E was associated with increased translational efficiency of this mRNA, suggesting that cell growth promotes expression of this protein through a translational mechanism that is distinct from the cyclin D-E2F pathway. Treatment of Akt-transfected mice with rapamycin only partially inhibited liver growth and did not prevent the induction of cyclin E protein, indicating that target of rapamycin activity is not necessary for this response. In the enlarged livers, cyclin E-Cdk2 complexes were present in high abundance but were inactive due to increased binding of p21 to these complexes. Akt transfection of p21(-/-) mice promoted liver growth, activation of Cdk2, and enhanced hepatocyte proliferation. In conclusion, growth promotes cyclin E expression through a novel translational mechanism in the liver, suggesting a new link between cell growth and the cell cycle machinery. Furthermore, p21 suppresses proliferation in the overgrown livers and may play a role in preventing cell cycle progression in response to organ size homeostatic mechanisms.

A role for Akt in epidermal growth factor-stimulated cell cycle progression in cultured hepatocytes: generation of a hyperproliferative window after adenoviral expression of constitutively active Akt

Epidermal growth factor (EGF) stimulation of cell cycle progression in cultured primary hepatocytes has previously been reported to be dependent on the mammalian target of rapamycin (mTOR) elements of the phosphoinositide 3-kinase (PI3K) signaling cascade and not the Akt pathway. Here we have established conditions of combined treatment of rat hepatocytes with insulin and EGF that favor cell cycle progression. The resulting cell population expresses albumin and retains receptor regulation of the signaling pathways leading to glycogen phosphorylase activation. We then investigated the hypothesis that the Akt limb of the PI3K pathway plays a central role in this insulin/EGF enhancement of cell cycle progression. The phosphorylation of Akt, central to the PI3K pathway, was increased by both insulin (sustained) and EGF (transient). The stimulation of Akt phosphorylation was inhibited in a concentration-dependent manner by the PI3K inhibitor, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one (LY294002). Cell cycle progression in these cultures was reduced, but not abolished, by this inhibitor. The mTOR inhibitor, rapamycin, also inhibited entry into S phase. The novel Akt inhibitor A-443654 [(S)-1-(1H-indol-3-ylmethyl)-2-[5-(3-methyl-1H-indazol-5-yl)-pyridin-3-yloxy]-ethylamine] blocked both EGF-stimulated cell cycle progression and phosphorylation of the Akt substrate glycogen synthase kinase-3. Infection of cells with an adenoviral vector expressing a constitutively active form of Akt but not a kinase-dead form increased hepatocyte proliferation probably through enhanced cell cycle progression and reduced apoptosis. These results show that the Akt element of the PI3K cascade is necessary for EGF-stimulated cell cycle progression and provide evidence that the sustained elevation of Akt alone generates a hyperproliferative window in hepatocyte cultures.

Sirolimus improves early microcirculation, but impairs regeneration after pancreatic ischemia-reperfusion injury

Ischemia and reperfusion injury remains a relevant problem in clinical pancreas transplantation. We investigated the effect of sirolimus (SRL) in a rodent model of 90-min warm pancreatic ischemia. Four groups were studied: (1) sham surgery and vehicle; (2) sham surgery and SRL; (3) warm ischemia and vehicle; (4) warm ischemia and SRL. SRL (1.5 mg/kg/day) and vehicle were administered intraperitoneally for 3 days prior to surgery until the animals were killed. Microcirculation was assessed immediately after reperfusion by means of intravital fluorescence microscopy. Histopathological injury, apoptosis, proliferation and biochemical parameters were analyzed at 2 h, 1 day and 5 days after surgery. Ninety minutes after ischemia, intravital microscopy revealed an improved functional capillary density (p < 0.05) and reduction of adherent leucocytes (p < 0.01) and platelets (p < 0.05) in the SRL-treated group compared to the vehicle-treated controls. In contrast, on day 5 after ischemia, the pancreatic tissue of SRL-treated animals showed a higher grade of histological injury (p < 0.05) and higher rate of apoptotic cells (p < 0.05) than the vehicle controls. In summary, our data indicate that administration of SRL improves microcirculation at a very early stage, but results in an impairment of the recovery phase after pancreatic ischemia-reperfusion injury.

Remnant liver volume to body weight ratio > or =0.5%: A new cut-off to estimate postoperative risks after extended resection in noncirrhotic liver

BACKGROUND

Before extended hepatectomy of five or more segments, the remnant liver volume (RLV) is usually calculated as a ratio of RLV to total liver volume (RLV-TLV) and must be >20% to 25%. This method can lead to compare parts of normal liver parenchyma to others compromised by biliary or vascular obstruction or by portal vein embolization. Extrapolating from living-donor liver transplantation, we hypothesized that RLV to body weight ratio (RLV-BWR) could accurately assess the functional limit of hepatectomy.

STUDY DESIGN

From September 2000 to December 2004, volumetric measurements of RLV using computed tomography were obtained before right-extended hepatectomy in 31 patients. RLV-BWR of 0.5% as a critical point for patient course was compared with stratification by RLV-TLV (< or =25% or >25% and < or =20% or >20%).

RESULTS

Three-month morbidity and mortality were not significantly different between groups RLV-TLV < or = and >25% and between groups RLV-TLV < or = and >20%, but increased significantly in group RLV-BWR < or = 0.5% compared with group RLV-BWR > 0.5% (p = 0.038 and p = 0.019, respectively) with an non-significant increase in death from liver failure (p = 0.077).

CONCLUSIONS

RLV-BWR was more specific than RLV-TLV in predicting postoperative course after extended hepatectomy. Patients with an anticipated RLV < or = 0.5% of body weight are at considerable risk for hepatic dysfunction and postoperative mortality.

Immunosuppression in liver transplantation: beyond calcineurin inhibitors

Although calcineurin inhibitors (CNIs) remain the mainstay of immunosuppression in liver transplantation (LTX), their long-term toxicity significantly contributes to morbidity and mortality. The elucidation of mechanisms of alloimmunity and leukocyte migration have provided novel targets for immunosuppression development. The toxicities of these agents differ from that of the CNI and act additively or synergistically. CNI avoidance protocols in LTX have not been achieved routinely; however, pilot trials have begun to delineate the limitations and promises of such approaches. CNI-sparing protocols appear to be much more promising in balancing the early need for minimizing rejection while tapering doses and minimizing long-term toxicity.

The hepatoprotective effect of hepatic stimulator substance (HSS) against liver regeneration arrest induced by acute ethanol intoxication

Male Wistar rats were randomized to receive ethanol (2.5 ml/kg by gastric intubation every 8 hr; group I), equal volumes of isocaloric to ethanol sucrose solution (group II), or ethanol and HSS (100 mg/kg intraperitoneally 10 and 16 hr after partial hepatectomy; groups III and IV, respectively) for up to 96 hr after partial hepatectomy, with ethanol administration starting 1 hr prior to partial hepatectomy. Animals were killed at 8, 12, 16, 20, 24, 28, 32, 36, 40, 44, 48, 60, and 96 hr after partial hepatectomy. The rate of liver regeneration was evaluated by the mitotic index in H&E-stained sections, immunochemical detection of Ki67 nuclear antigen, rate of [3H]thymidine incorporation into hepatic DNA, and liver thymidine kinase enzymatic activity. The biological activity of HSS in groups I and II rats was evaluated using a bioassay. Ethanol administration arrested liver regeneration during the first 32 hr after partial hepatectomy and suppressed HSS activity throughout the period examined. Liver regeneration progressed after 32 hr despite the low levels of HSS activity. HSS administration at 10 and 16 hr reversed liver regeneration arrest induced by ethanol. Acute ethanol administration induces cell cycle arrest during the first 32 hr after partial hepatectomy and suppression of HSS biological activity seems to contribute to this effect. HSS administration reversed the inhibitory effect of ethanol on liver regeneration and caused synchronized entrance of hepatocytes in the S phase of the cell cycle. HSS seems to participate in the network of growth factors controlling the G1/S cell cycle checkpoint.

Benefit-Risk Assessment of Sirolimus in Renal Transplantation

Sirolimus (rapamycin) is a macrocyclic lactone isolated from a strain of Streptomyces hygroscopicus that inhibits the mammalian target of rapamycin (mTOR)-mediated signal-transduction pathways, resulting in the arrest of cell cycle of various cell types, including T- and B-lymphocytes. Sirolimus has been demonstrated to prolong graft survival in various animal models of transplantation, ranging from rodents to primates for both heterotopic, as well as orthotopic organ grafting, bone marrow transplantation and islet cell grafting. In human clinical renal transplantation, sirolimus in combination with ciclosporin (cyclosporine) efficiently reduces the incidence of acute allograft rejection. Because of the synergistic effect of sirolimus on ciclosporin-induced nephrotoxicity, a prolonged combination of the two drugs inevitably leads to progressive irreversible renal allograft damage. Early elimination of calcineurin inhibitor therapy or complete avoidance of the latter by using sirolimus therapy is the optimal strategy for this drug. Prospective randomised phase II and III clinical studies have confirmed this approach, at least for recipients with a low to moderate immunological risk. For patients with a high immunological risk or recipients exposed to delayed graft function, sirolimus might not constitute the best therapeutic choice--despite its ability to enable calcineurin inhibitor sparing in the latter situation--because of its anti-proliferative effects on recovering renal tubular cells. Whether lower doses of sirolimus or a combination with a reduced dose of tacrolimus would be advantageous in these high risk situations remains to be determined. Clinically relevant adverse effects of sirolimus that require a specific therapeutic response or can potentially influence short- and long-term patient morbidity and mortality as well as graft survival include hypercholesterolaemia, hypertriglyceridaemia, infectious and non-infectious pneumonia, anaemia, lymphocele formation and impaired wound healing. These drug-related adverse effects are important determinants in the choice of a tailor-made immunosuppressive drug regimen that complies with the individual patient risk profile. Equally important in the latter decision is the lack of severe intrinsic nephrotoxicity associated with sirolimus and its advantageous effects on arterial hypertension, post-transplantation diabetes mellitus and esthetic changes induced by calcineurin inhibitors. Mild and transient thrombocytopenia, leukopenia, gastrointestinal adverse effects and mucosal ulcerations are all minor complications of sirolimus therapy that have less impact on the decision for choosing this drug as the basis for tailor-made immunosuppressive therapy. It is clear that sirolimus has gained a proper place in the present-day immunosuppressive armament used in renal transplantation and will contribute to the development of a tailor-made immunosuppressive therapy aimed at fulfilling the requirements outlined by the individual patient profile.

Evidence that cyclin D1 mediates both growth and proliferation downstream of TOR in hepatocytes

Signaling through the target of rapamycin is required for increased protein synthesis, cell growth, and proliferation in response to growth factors. However, the downstream mediators of these responses, and the elements linking growth and proliferation, have not been fully elucidated. Rapamycin inhibits hepatocyte proliferation in culture and liver regeneration in vivo. In cultured rat hepatocytes, rapamycin prevented the up-regulation of cyclin D1 as well as proteins acting downstream in the cell cycle. Transfection with cyclin D1 or E2F2, but not cyclin E or activated Akt, overcame the rapamycin-mediated cell cycle arrest. Rapamycin also inhibited the induction of global protein synthesis after growth factor stimulation, and cyclin D1 overcame this inhibition. Rapamycin inhibited hepatocyte proliferation and cyclin D1 expression in the mouse liver after 70% partial hepatectomy. In rapamycin-treated mice, transfection with cyclin D1 induced hepatocyte proliferation, increased hepatocyte cell size, and promoted growth of the liver. These results suggest that cyclin D1 is a key mediator of increased protein synthesis, cell growth, and proliferation downstream of target of rapamycin in mitogen-stimulated hepatocytes.

PI3K-FRAP/mTOR pathway is critical for hepatocyte proliferation whereas MEK/ERK supports both proliferation and survival

Growth factors are known to favor both proliferation and survival of hepatocytes. In this work, we investigated the role of 2 main signaling pathways, phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase (MEK)/extracellular signal-regulated kinase (ERK), in these processes. First, evidence was provided that the PI3K cascade as well as the MEK/ERK cascade is a key transduction pathway controlling hepatocyte proliferation, as ascertained by arrest of DNA synthesis in the presence of LY294002, a specific PI3K inhibitor. Inhibition of FRAP/mTOR by rapamycin also abrogated DNA replication and protein synthesis induced by growth factor. We showed that expression of cyclin D1 at messenger RNA (mRNA) and protein levels was regulated by this pathway. We highlighted that 4E-BP1 phosphorylation was not activated by epidermal growth factor (EGF) but was under an insulin-regulation mechanism through a PI3K-FRAP/mTOR activation that could account for the permissive role of insulin on hepatocyte proliferation. No interference between the MEK/ERK pathway and 4E-BP1 phosphorylation was detected, whereas p70S6K phosphorylation induced by EGF was under a U0126-sensitive regulation. Last, we established that the antiapoptotic function of EGF was dependent on MEK, whereas LY294002 and rapamycin had no direct effect on cell survival. Taken together, these data highlight the regulation and the role of 2 pathways that mediate growth-related response by acting onto distinct steps. In conclusion, hepatocyte progression in late G1 phase induced by EGF generates survival signals depending on MEK activation, whereas PI3K and MEK/ERK cascades are both necessary for hepatocyte replication.

Pretreatment with FK506 up-regulates insulin receptors in regenerating rat liver

This report examines the effect of FK506 pretreatment on liver insulin receptor expression in partially (70%) hepatectomized rats. FK506 pretreatment led to an increased insulin receptor number 24 hours after hepatectomy, detected by means of insulin binding and cross-linking procedures. This increase was related to enhanced insulin receptor expression determined by in vitro mRNA translation and Western blot techniques. We also tested the functionality of the expressed insulin receptors by [(3)H] thymidine incorporation into DNA in insulin-stimulated hepatocytes. The results show that FK506 pretreatment elicits an increase in the amount of insulin receptor alpha-subunits as measured by Western blot. Maximum alpha-subunit expression recorded 24 hours after surgery was preceded by increased insulin receptor mRNA levels, which were detected 6 hours after hepatectomy. Moreover, in FK506-pretreated rat hepatocytes, obtained from remnant livers 24 hours after partial hepatectomy (PH), the increase in insulin receptor number was associated with improved sensitivity to the hormone. However, in both experimental groups (FK506-pretreated and nonpretreated rats), the sensitivity of hepatocytes toward epidermal growth factor (EGF) showed no significant change, which suggests a specific effect of FK506 on insulin receptor expression. In conclusion, our findings suggest that FK506 pretreatment induces insulin receptor expression in regenerating rat liver and promotes liver regeneration in hepatectomized rats.

Newer immunosuppressive drugs: a review

In recent years, many new immunosuppressive drugs have been discovered and developed for clinical use in transplantation. This review focuses on those drugs (leflunomide, mycophenolate mofetil, sirolimus, tacrolimus) that have been shown to have immunosuppressive activity in patients. Different anti-interleukin-2 receptor antibodies are also reviewed as an example of a resurgence of development in the area of monoclonal antibodies. The price for reducing the incidence of allograft rejection by improved immunosuppression was thought to be a proportional increase in the incidence of infection and malignancy. Data from Phase III clinical trials of new immunosuppressants, however, show a statistically significant reduction in the incidence of acute rejection produced by these new drugs, which has not been accompanied by increases in infection and malignancy rates. The wide array of new drugs offers the opportunity to use combinations that block different pathways of immune activation while at the same time selecting drug combinations with nonoverlapping toxicity profiles so that doses of each single drug can be reduced below toxicity levels. The immunosuppressive therapy for patients can be tailored according to their individual needs.

The therapeutic prescription for the organ transplant recipient: the linkage of immunosuppression and antimicrobial strategies

Infection and rejection, the two major barriers to successful organ transplantation, are closely linked, with immunosuppressive therapy being central to the pathogenesis of both. After almost two decades when azathioprine and prednisone, supplemented by antilymphocyte antibody therapy, were the cornerstones of post-transplant immunosuppressive programs, there has been a major increase in the therapeutic armamentarium available to treat rejection: cyclosporine, tacrolimus, mycophenolate mofetil, rapamycin, and antibodies directed against the interleukin-2 receptor. These agents are potent inhibitors of microbial specific T cell function, thus potentiating opportunistic infection with herpes group viruses, fungal and mycobacterial species, Strongyloides stercoralis, and a variety of intracellular pathogens. The mechanisms by which each of these drugs exerts its effects are an important determinant of the antimicrobial strategies that will be necessary to combat infection. Indeed, strategies to limit these infections are being linked to the nature of the immunosuppressive therapy required in a particular patient. Thus, the therapeutic prescription for the transplant patient is said to have two components: an immunosuppressive component to prevent and treat rejection, and an antimicrobial one to make it safe. In addition to using antimicrobial agents therapeutically, in the transplant patient prevention is stressed in which antibiotics are deployed prophylactically or preemptively.

Rapamune (RAPA, rapamycin, sirolimus): mechanism of action immunosuppressive effect results from blockade of signal transduction and inhibition of cell cycle progression

OBJECTIVE

Rapamune is a novel immunosuppressive agent in Phase III clinical trial in renal transplantation. Its unique mechanism of action has created great interest in its use as a biochemical probe of signal transduction pathways that has provided insight into its molecular mechanism of action. This article reviews the current state of our understanding of the mechanism of action of rapamune.

Inhibition by rapamycin of ornithine decarboxylase and epithelial cell proliferation in intestinal IEC-6 cells in culture

1. Induction of the enzyme ornithine decarboxylase (ODC) appears to be controlled primarily at the level of ODC mRNA translation. The immunosuppressant drug, rapamycin, blocked the induction of ODC in response to serum by roughly 50% but was without effect on transport of putrescine into the intracellular space. The effect on ODC was specific for the intracellular signalling pathway leading to activation of p70S6k, as the immunosuppressant FK 506 was without effect on ODC activity. 2. Exposure of IEC-6 duodenal epithelial cells to rapamycin inhibited cellular proliferation. The effect of rapamycin was cytostatic in that removal of the immunosuppressant from the medium resulted in renewed cell division. Conversely, addition of exogenous putrescine, the product of the ODC catalysed reaction, was unable to reverse the cytostatic effects of rapamycin. 3. At a concentration of 10 nM, rapamycin inhibited the induction of ODC by 50%, a level of inhibition which could not be enhanced by exposure cells to 1000 nM rapamycin. This observation suggests that other intracellular signalling pathways, in addition to the p70S6k cascade, might be involved in regulation of translation of ODC mRNA or that rapamycin does not completely inhibit p70S6k.

Rapamycin dissociates p70(S6K) activation from DNA synthesis stimulated by bombesin and insulin in Swiss 3T3 cells

Phosphorylation and subsequent activation of p70 S6 kinase (S6K) are events that are highly conserved in the cellular response to mitogens. The neuropeptide bombesin, which is a potent mitogen for Swiss 3T3 cells, stimulated a time- and dose-dependent activation of p70(S6K) as determined by gel mobility shift and immune complex kinase assays. This effect was inhibited by the immunosuppressant rapamycin at 10 nM, which also completely abolished bombesin-stimulated DNA synthesis at identical concentrations. In striking contrast, the combination of bombesin and insulin in synergy stimulated maximum DNA synthesis in these cells despite persistent inhibition of p70(S6K) by rapamycin throughout G1. These results indicate that activation of p70(S6K) is not required for the transition of quiescent cells to the S phase of the cell cycle. The inhibitory effects of rapamycin on bombesin-stimulated cell cycle progression did not involve accumulation of the cyclin-dependent kinase inhibitor p27(kip1) but a striking inhibition of the expression of cyclin D1. This effect was circumvented by bombesin with insulin, suggesting that a rapamycin-insensitive pathway stimulated by this combination leads to cyclin D1 expression. Thus, these findings, dissociating the mitogenic effects of bombesin in synergy with insulin from activation of p70(S6K), support the hypothesis that this kinase is a component of one of the parallel pathways that can lead to DNA synthesis rather than an obligatory point of convergence in mitogenic signaling.

The 70 kDa S6 kinase: regulation of a kinase with multiple roles in mitogenic signalling

The activation of the 70kDa S6 kinase, pp70S6k, is a well documented mitogenic response, yet until recently little was known of how pp70S6k is activated, or of the identities of its crucial targets. The past year has revealed the complexity of pp70S6k regulation, with the overriding theme being that enzymes which have proven or putative roles in phospholipid metabolism mediate its activation. Studies also indicate that pp70S6k may regulate many more pathways than previously recognized.

Mechanism of action of the immunosuppressant rapamycin

Rapamycin has potent immunosuppressive properties reflecting its ability to disrupt cytokine signaling that promotes lymphocyte growth and differentiation. In IL-2-stimulated T cells, rapamycin impedes progression through the G1/S transition of the proliferation cycle, resulting in a mid-to-late G1 arrest. Two major biochemical alterations underlie this mode of action. The first one affects the phosphorylation/activation of the p70 S6 kinase (p70s6k), an early event of cytokine-induced mitogenic response. By inhibiting this enzyme, whose major substrate is the 40S ribosomal subunit S6 protein, rapamycin reduces the translation of certain mRNA encoding for ribosomal proteins and elongation factors, thereby decreasing protein synthesis. A second, later effect of rapamycin in IL-2-stimulated T cells is an inhibition of the enzymatic activity of the cyclin-dependent kinase cdk2-cyclin E complex, which functions as a crucial regulator of G1/S transition. This inhibition results from a prevention of the decline of the p27 cdk inhibitor, that normally follows IL-2 stimulation. To mediate these biochemical alterations, rapamycin needs to bind to intracellular proteins, termed FKBP, thereby forming a unique effector molecular complex. However, neither(p70s6k) inhibition, nor p27-induced cdk2-cyclin E inhibition are directly caused by the FKBP-rapamycin complex. Instead, this complex physically interacts with a novel protein, designated "mammalian target of rapamycin" (mTOR), which has sequence homology with the catalytic domain of phosphatidylinositol kinases and may therefore be itself a kinase. mTOR may act upstream of (p70s6K) and cdk2-cyclin E in a linear or bifurcated pathway of growth regulation. Molecular dissection of this pathway should further unravel cytokine-mediated signaling processes and help devise new immunosuppressants.

Identification of an 11-kDa FKBP12-rapamycin-binding domain within the 289-kDa FKBP12-rapamycin-associated protein and characterization of a critical serine residue

Complexed with its intracellular receptor, FKBP12, the natural product rapamycin inhibits G1 progression of the cell cycle in a variety of mammalian cell lines and in the yeast Saccharomyces cerevisae. Previously, a mammalian protein that directly associates with FKBP12-rapamycin has been identified and its encoding gene has been cloned from both human (designated FRAP) [Brown, E.J., Albers, M.W., Shin, T.B., Ichikawa, K., Keith, C.T., Lane, W.S. & Schreiber, S.L. (1994) Nature (London) 369, 756-758] and rat (designated RAFT) [Sabatini, D.M., Erdjument-Bromage, H., Lui, M., Tempst, P. & Snyder, S.H. (1994) Cell 78, 35-43]. The full-length FRAP is a 289-kDa protein containing a putative phosphatidylinositol kinase domain. Using an in vitro transcription/translation assay method coupled with proteolysis studies, we have identified an 11-kDa FKBP12-rapamycin-binding domain within FRAP. This minimal binding domain lies N-terminal to the kinase domain and spans residues 2025-2114. In addition, we have carried out mutagenesis studies to investigate the role of Ser2035, a potential phosphorylation site for protein kinase C within this domain. We now show that the FRAP Ser2035-->Ala mutant displays similar binding affinity when compared with the wild-type protein, whereas all other mutations at this site, including mimics of phosphoserine, abolish binding, presumably due to either unfavorable steric interactions or induced conformational changes.

RAPT1, a mammalian homolog of yeast Tor, interacts with the FKBP12/rapamycin complex

Rapamycin is a potent immunosuppressant that blocks the G1/S transition in antigen-activated T cells and in yeast. The similar effects of rapamycin in animal cells and yeast suggest that the biochemical steps affected by rapamycin are conserved. Using a two-hybrid system we isolated mammalian clones that interact with the human FK506/rapamycin-binding protein (FKBP12) in the presence of rapamycin. Specific interactors, designated RAPT1, encode overlapping sequences homologous to yeast Tor, a putative novel phosphatidylinositol 3-kinase. A region of 133 amino acids of RAPT1 is sufficient for binding to the FKBP12/rapamycin complex. The corresponding region in yeast Tor contains the serine residue that when mutated to arginine confers resistance to rapamycin. Introduction of this mutation into RAPT1 abolishes its interaction with the FKBP12/rapamycin complex.

A mammalian protein targeted by G1-arresting rapamycin-receptor complex

The structurally related natural products rapamycin and FK506 bind to the same intracellular receptor, FKBP12, yet the resulting complexes interfere with distinct signalling pathways. FKBP12-rapamycin inhibits progression through the G1 phase of the cell cycle in osteosarcoma, liver and T cells as well as in yeast, and interferes with mitogenic signalling pathways that are involved in G1 progression, namely with activation of the protein p70S6k (refs 5, 11-13) and cyclin-dependent kinases. Here we isolate a mammalian FKBP-rapamycin-associated protein (FRAP) whose binding to structural variants of rapamycin complexed to FKBP12 correlates with the ability of these ligands to inhibit cell-cycle progression. Peptide sequences from purified bovine FRAP were used to isolate a human cDNA clone that is highly related to the DRR1/TOR1 and DRR2/TOR2 gene products from Saccharomyces cerevisiae. Although it has not been previously demonstrated that either of the DRR/TOR gene products can bind the FKBP-rapamycin complex directly, these yeast genes have been genetically linked to a rapamycin-sensitive pathway and are thought to encode lipid kinases.

Differential inhibition of cutaneous T-cell-mediated reactions and epidermal cell proliferation by cyclosporin A, FK-506, and rapamycin

Although cyclosporin A is a highly effective treatment for several skin disorders, particularly psoriasis, its use in dermatology appears limited due to drug-induced hypertension and nephrotoxicity. Newer, similar-acting anti-T-cell agents such as FK-506 and rapamycin may be more effective; therefore a comparison was made with cyclosporin A to assess their inhibitory action on T-cell responses and keratinocyte proliferation. Using a guinea-pig model of delayed-type hypersensitivity to dinitrofluorobenzene (DNFB), drugs were given systemically (25 mg/kg cyclosporin A, rapamycin; 2.5 mg/kg FK-506) and topically (0.02% and 2%) at the time of DNFB challenge or several hours after and were assessed with respect to erythema and the numbers of infiltrating T lymphocytes entering skin-challenge sites. FK-506, at all concentrations, significantly inhibited both T-cell infiltration and skin reddening when used by both routes. Rapamycin displayed no inhibitory effect, whereas cyclosporin A only suppressed the erythema response when given systemically. The inhibition of normal human keratinocyte growth by the drugs was assessed using a protein dye-binding assay. After 2 weeks, FK-506 had no effect, whereas cyclosporin A and rapamycin both inhibited keratinocyte growth in a dose-dependent fashion and almost equivalently in serum-containing and serum-free keratinocyte growth medium. The findings showed that in vivo only FK-506 suppressed T-cell involvement in sensitized animals. In contrast, it failed to have any effect on keratinocyte growth, whereas rapamycin was more potent than cyclosporin A in inhibiting their proliferation. The future benefit of these drugs in dermatology may ultimately lie in their combined use.