Rapamycin-FKBP12 blocks proliferation, induces differentiation, and inhibits cdc2 kinase activity in a myogenic cell line

Advances in the mTOR signaling pathway and its inhibitor rapamycin in epilepsy

INTRODUCTION

Epilepsy is one of the most common and serious brain syndromes and has adverse consequences on a patient's neurobiological, cognitive, psychological, and social wellbeing, thereby threatening their quality of life. Some patients with epilepsy experience poor treatment effects due to the unclear pathophysiological mechanisms of the syndrome. Dysregulation of the mammalian target of the rapamycin (mTOR) pathway is thought to play an important role in the onset and progression of some epilepsies.

METHODS

This review summarizes the role of the mTOR signaling pathway in the pathogenesis of epilepsy and the prospects for the use of mTOR inhibitors.

RESULTS

The mTOR pathway functions as a vital mediator in epilepsy development through diverse mechanisms, indicating that the it has great potential as an effective target for epilepsy therapy. The excessive activation of mTOR signaling pathway leads to structural changes in neurons, inhibits autophagy, exacerbates neuron damage, affects mossy fiber sprouting, enhances neuronal excitability, increases neuroinflammation, and is closely associated with tau upregulation in epilepsy. A growing number of studies have demonstrated that mTOR inhibitors exhibit significant antiepileptic effects in both clinical applications and animal models. Specifically, rapamycin, a specific inhibitor of TOR, reduces the intensity and frequency of seizures. Clinical studies in patients with tuberous sclerosis complex have shown that rapamycin has the function of reducing seizures and improving this disease. Everolimus, a chemically modified derivative of rapamycin, has been approved as an added treatment to other antiepileptic medicines. Further explorations are needed to evaluate the therapeutic efficacy and application value of mTOR inhibitors in epilepsy.

CONCLUSIONS

Targeting the mTOR signaling pathway provides a promising prospect for the treatment of epilepsy.

Cell synchronization by Rapamycin improves the developmental competence of buffalos (Bubalus bubalis) somatic cell nuclear transfer embryos

This study mainly explored the effects of Rapamycin on the growth of the Buffalo ear fibroblast (BEF) and embryonic developmental competence of somatic cell nuclear transfer (SCNT). The results show that the appropriate concentration (1 μM) of Rapamycin could significantly improve the proportion of the G0/G1 phase in BEF cells treated at a certain time (72 hr). Simultaneously, the percentage of the G0/G1 phase also was significantly higher than the serum starvation and control group. This may be related to Rapamycin inhibiting the phosphorylation of mTOR and affecting the expression of cell cycle-related genes (CDK2, CDK4, P27, CycleD1, and CycleD3). Besides, compared with the control group and serum-starved group, Rapamycin significantly decreased BEF cell apoptosis by reducing ROS generation. Moreover, these results also indicated that the proportion of BEF cells with normal chromosome multiples treated by Rapamycin is significantly higher than that of the serum-starved group (p < .05). Finally, this study explored the effects of Rapamycin and serum starvation on the embryonic developmental competence of SCNT. The results show that Rapamycin significantly increased the rate of 8-cell and blastocyst, compared with the control group and serum starvation group (p < .05). To summarize, these results indicate that Rapamycin improved the embryonic development competence of SCNT, which may be related to Rapamycin increasing the percentage of G0/G1 phase and maintaining BEF cell quality.

Promotion of Myoblast Differentiation by Fkbp5 via Cdk4 Isomerization

Fkbp5 is a widely expressed peptidyl prolyl isomerase that serves as a molecular chaperone through conformational changes of binding partners. Although it regulates diverse protein functions, little is known about its roles in myogenesis. We found here that Fkbp5 plays critical roles in myoblast differentiation through two mechanisms. First, it sequesters Cdk4 within the Hsp90 storage complex and prevents the formation of the cyclin D1-Cdk4 complex, which is a major inhibitor of differentiation. Second, Fkbp5 promotes cis-trans isomerization of the Thr172-Pro173 peptide bond in Cdk4 and inhibits phosphorylation of Thr172, an essential step for Cdk4 activation. Consistent with these in vitro findings, muscle regeneration is delayed in Fkbp5^−/− mice. The related protein Fkbp4 also sequesters Cdk4 within the Hsp90 complex but does not isomerize Cdk4 or induce Thr173 phosphorylation despite its highly similar sequence. This study demonstrates protein isomerization as a critical regulatory mechanism of myogenesis by targeting Cdk4.

The Role of AMPK/mTOR Modulators in the Therapy of Acute Myeloid Leukemia

Differentiation therapy of acute promyelocytic leukemia with all-trans retinoic acid represents the most successful pharmacological therapy of acute myeloid leukemia (AML). Numerous studies demonstrate that drugs that inhibit mechanistic target of rapamycin (mTOR) and activate AMP-kinase (AMPK) have beneficial effects in promoting differentiation and blocking proliferation of AML. Most of these drugs are already in use for other purposes; rapalogs as immunosuppressants, biguanides as oral antidiabetics, and 5-amino-4-imidazolecarboxamide ribonucleoside (AICAr, acadesine) as an exercise mimetic. Although most of these pharmacological modulators have been widely used for decades, their mechanism of action is only partially understood. In this review, we summarize the role of AMPK and mTOR in hematological malignancies and discuss the possible role of pharmacological modulators in proliferation and differentiation of leukemia cells.

Evolution of Cancer Pharmacological Treatments at the Turn of the Third Millennium

The medical history of cancer began millennia ago. Historical findings of patients with cancer date back to ancient Egyptian and Greek civilizations, where this disease was predominantly treated with radical surgery and cautery that were often ineffective, leading to the death of patients. Over the centuries, important discoveries allowed to identify the biological and pathological features of tumors, without however contributing to the development of effective therapeutic approaches until the end of the 1800s, when the discovery of X-rays and their use for the treatment of tumors provided the first modern therapeutic approach in medical oncology. However, a real breakthrough took place after the Second World War, with the discovery of cytotoxic antitumor drugs and the birth of chemotherapy for the treatment of various hematological and solid tumors. Starting from this epochal turning point, there has been an exponential growth of studies concerning the use of new drugs for cancer treatment. The second fundamental breakthrough in the field of oncology and pharmacology took place at the beginning of the '80s, thanks to molecular and cellular biology studies that allowed the development of specific drugs for some molecular targets involved in neoplastic processes, giving rise to targeted therapy. Both chemotherapy and target therapy have significantly improved the survival and quality of life of cancer patients inducing sometimes complete tumor remission. Subsequently, at the turn of the third millennium, thanks to genetic engineering studies, there was a further advancement of clinical oncology and pharmacology with the introduction of monoclonal antibodies and immune checkpoint inhibitors for the treatment of advanced or metastatic tumors, for which no effective treatment was available before. Today, cancer research is always aimed at the study and development of new therapeutic approaches for cancer treatment. Currently, several researchers are focused on the development of cell therapies, anti-tumor vaccines, and new biotechnological drugs that have already shown promising results in preclinical studies, therefore, in the near future, we will certainly assist to a new revolution in the field of medical oncology.

Preparation and experimental research into retrievable rapamycin- and heparin-coated vena cava filters: a pilot study

The use of retrievable vena cava filters (RVCFs) was once commonplace, but filter retrieval was often very difficult. Most unsuccessful retrieval was due to intimal hyperplasia of the inferior vena cava and in-filter thrombosis. This pilot study aimed to design a drug-eluting RVCF. The hypothesis was that coated drugs could be released continuously to inhibit vena intimal hyperplasia and thrombosis, and thus improve the retrieval rates of RVCFs. Various concentrations of polycaprolactone (PCL)/chloroform solution were made from a mixture of Rapamycin and Heparin according to the quality of PCL. The drug was coated onto the surface of the filters by a process of dipping. In vitro tests were performed to check stability and in vitro drug release. Animals receiving filter implantation were divided into 4 groups, the experimental intervention group (EI), experimental laparotomy group (EL), control intervention group (CI), and control laparotomy group (CL). Filters were retrieved by laparotomy in the EL and CL groups, and by interventional operation in the EI and CI groups at 10, 20 and 30 days after implantation. Pathological endothelia biopsies were performed with wood grain-eosin (HE) staining and immunohistochemical examination, with the proliferating cell nuclear antigen (PCNA) index, and the results were compared between the experimental and control groups. The weight of thrombus within the filters was also measured by scale and compared. The coating concentration that succeeded in completely covering the surface was 0.2 g/ml. There was better coverage by SEM at this concentration, and the coated drugs had no obvious loss after filter release. The drug release curves showed that the amount of Heparin released was more than 50 % at day 1; Rapamycin released little in the first few days, beginning in earnest at 20 to 30 days. The filters were easy to retrieve at 10 days for both groups, while neither could be retrieved at 30 days. However, at 20 days the filter in the EI group could be retrieved with some difficulty, but the filter in the CI group couldn't be removed at all. The pathological examination and immunohistochemical PCNA examination results showed that the use of drug-eluting filters could effectively inhibit endothelial hyperplasia at 10 and 20 days, but was less effective at 30 days. There was no apparent difference in the total weight of blood clots between the experimental and control groups. We successfully conducted a pilot study into preparing Rapamycin- and Heparin-coated RVCFs. In vitro and in vivo tests further proved the possibility of improving the retrieval rates of RVCFs by effectively inhibiting vein endothelial proliferation, but the anticoagulation and antithrombosis effects of Heparin were unsatisfactory.

Combination of LC3 shRNA plasmid transfection and genistein treatment inhibited autophagy and increased apoptosis in malignant neuroblastoma in cell culture and animal models

Malignant neuroblastoma is an extracranial solid tumor that usually occurs in children. Autophagy, which is a survival mechanism in many solid tumors including malignant neuroblastoma, deters the efficacy of conventional chemotherapeutic agents. To mimic starvation, we used 200 nM rapamycin that induced autophagy in human malignant neuroblastoma SK-N-BE2 and IMR-32 cells in cell culture and animal models. Combination of microtubule associated protein light chain 3 short hairpin RNA (LC3 shRNA) plasmid transfection and genistein (GST) treatment was tested for inhibiting rapamycin-induced autophagy and promoting apoptosis. The best synergistic efficacy caused the highest decrease in cell viability due to combination of 50 nM LC3 shRNA plasmid transfection and 25 µM GST treatment in rapamycin-treated SK-N-BE2 cells while combination of 100 nM LC3 shRNA plasmid transfection and 25 µM GST treatment in rapamycin-treated IMR-32 cells. Quantitation of acidic vesicular organelles confirmed that combination of LC3 shRNA plasmid transfection and GST treatment prevented rapamycin-induced autophagy due to down regulation of autophagy promoting marker molecules (LC3 II, Beclin 1, TLR-4, and Myd88) and upregulation of autophagy inhibiting marker molecules (p62 and mTOR) in both cell lines. Apoptosis assays showed that combination therapy most effectively activated mitochondrial pathway of apoptosis in human malignant neuroblastoma in cell culture and animal models. Collectively, our current combination of LC3 shRNA plasmid transfection and GST treatment could serve as a promising therapeutic strategy for inhibiting autophagy and increasing apoptosis in human malignant neuroblastoma in cell culture and animal models.

Synergistic anti-tumor effects of RAD001 with MEK inhibitors in neuroendocrine tumors: a potential mechanism of therapeutic limitation of mTOR inhibitor

Mammalian target of rapamycin (mTOR) inhibitors have been clinically used as anticancer agents in several types of human malignancies including neuroendocrine tumor (NET) but the development of clinical resistances or their therapeutic limitations have been also reported. This clinical resistance has been proposed to be partly due to a compensatory activation of an mTOR upstream factor Akt and MEK/ERK pathway in NET cells but its details have not necessarily been reported. Therefore, in this study, we examined the effects of mTOR inhibitors on these activations and of the concomitant treatment of mTOR and MEK inhibitors in two NET cell lines, NCI-H727 and COLO320. We evaluated the effects of RAD001, mTOR inhibitor, and U0126, MEK inhibitor, on cell proliferation and migration of these cells. In addition, an alteration of the factors involved in Akt/mTOR and MEK/ERK pathways was also examined under administration of these agents. RAD001 and U0126 treatment significantly inhibited cell proliferation and their combined treatment synergistically decreased it in both cell lines. Additionally, these treatments above decreased the expression of cell cycle-related factors, suggestive of an involvement of cell cycle arrest in therapeutic effects. The combined treatment also inhibited the cell migration in NCI-H727 via the decrement of MMP2 and 9 in an additive manner. We demonstrated the potential synergistic/combined effects of inhibitors of mTOR and MEK on cell proliferation and migration. These results suggest the potential therapeutic efficacy of the combined therapy of mTOR and MEK inhibitors or a dual inhibitor for the treatment of NET patients.

Novel classification based on immunohistochemistry combined with hierarchical clustering analysis in non-functioning neuroendocrine tumor patients

Somatostatin analogues ameliorated many symptoms caused by neuroendocrine tumors (NET), but their antitumor activities are limited especially in non-functioning cases. An overactivation of signaling pathways under receptor tyrosine-kinase (RTK) has been recently demonstrated in some NET patients, but its details have remained largely unknown. Therefore, in this study, we immunolocalized therapeutic factors and evaluated the data to study the clinical significance of the molecules in non-functioning Japanese gastrointestinal NET. Fifty-two NET cases were available for examination in this study and expression of somatostatin receptor (sstr) 1, 2A, 2B, 3 and 5, activated form of mammalian target of rapamycin (mTOR), eukaryotic initiation factor 4-binding protein 1 (4EBP1), ribosomal protein s6 (S6), extracellular signal-regulated kinase (ERK) and insulin-like growth factor 1 receptor (IGF-1R) was evaluated using immunohistochemistry. We then studied the correlation among the immunohistochemical results of the individual cases using hierarchical clustering analysis. Results of clustering analysis demonstrated that NET cases were basically classified into Cluster I and II. Cluster I was associated with higher expression of sstr1, 2B and 3 and Cluster II was characterized by an activation of the PI3K/Akt pathway and IGF-1R and higher proliferative status. Cluster II was further classified into Cluster IIa and IIb. Cluster IIa was associated with higher expression of sstr1 and 5 and higher proliferative status and Cluster IIb was characterized by ERK activation. Hierarchical clustering analysis of immunoreactivity of the therapeutic factors can classify NET cases into three distinctive groups and the medical treatment may be determined according to this novel classification method for non-functioning NET patients.

Kinetics of FKBP12.6 binding to ryanodine receptors in permeabilized cardiac myocytes and effects on Ca sparks

RATIONALE

FK506-binding proteins FKBP12.6 and FKBP12 are associated with cardiac ryanodine receptors (RyR2), and cAMP-dependent protein kinase A (PKA)-dependent phosphorylation of RyR2 was proposed to interrupt FKBP12.6-RyR2 association and activate RyR2. However, the function of FKBP12.6/12 and role of PKA phosphorylation in cardiac myocytes are controversial.

OBJECTIVE

To directly measure in situ binding of FKBP12.6/12 to RyR2 in ventricular myocytes, with simultaneous Ca sparks measurements as a RyR2 functional index.

METHODS AND RESULTS

We used permeabilized rat and mouse ventricular myocytes, and fluorescently-labeled FKBP12.6/12. Both FKBP12.6 and FKBP12 concentrate at Z-lines, consistent with RyR2 and Ca spark initiation sites. However, only FKBP12.6 inhibits resting RyR2 activity. Assessment of fluorescent FKBP binding in myocyte revealed a high FKBP12.6-RyR2 affinity (K(d)=0.7+/-0.1 nmol/L) and much lower FKBP12-RyR2 affinity (K(d)=206+/-70 nmol/L). Fluorescence recovery after photobleach confirmed this K(d) difference and showed that it is mediated by k(off). RyR2 phosphorylation by PKA did not alter binding kinetics or affinity of FKBP12.6/12 for RyR2. Using quantitative immunoblots, we determined endogenous [FKBP12] in intact myocytes is approximately 1 micromol/L (similar to [RyR]), whereas [FKBP12.6] is <or=150 nmol/L.

CONCLUSIONS

Only 10% to 20% of endogenous myocyte RyR2s have FKBP12.6 associated, but virtually all myocyte FKBP12.6 is RyR2-bound (because of very high affinity). FKBP12.6 but not FKBP12 inhibits basal RyR2 activity. PKA-dependent RyR2 phosphorylation has no significant effect on binding of either FKBP12 or 12.6 to RyR2 in myocytes.

Intracellular parasitism with Toxoplasma gondii stimulates mammalian-target-of-rapamycin-dependent host cell growth despite impaired signalling to S6K1 and 4E-BP1

The Ser/Thr kinase mammalian-target-of-rapamycin (mTOR) is a central regulator of anabolism, growth and proliferation. We investigated the effects of Toxoplasma gondii on host mTOR signalling. Toxoplasma invasion of multiple cell types rapidly induced sustained mTOR activation that was restricted to infected cells, as determined by rapamycin-sensitive phosphorylation of ribosomal protein S6; however, phosphorylation of the growth-associated mTOR substrates 4E-BP1 and S6K1 was not detected. Infected cells still phosphorylated S6K1 and 4E-BP1 in response to insulin, although the S6K1 response was blunted. Parasite-induced S6 phosphorylation was independent of S6K1 and did not require activation of canonical mTOR-inducing pathways mediated by phosphatidylinositol 3-kinase-Akt and ERK. Host mTOR was localized in a vesicular pattern surrounding the parasitophorous vacuole, suggesting potential activation by phosphatidic acid in the vacuolar membrane. In spite of a failure to phosphorylate 4E-BP1 and S6K1, intracellular T. gondii triggered host cell cycle progression in an mTOR-dependent manner and progression of infected cells displayed increased sensitivity to rapamycin. Moreover, normal cell growth was maintained during parasite-induced cell cycle progression, as indicated by total cellular S6 levels. The Toxoplasma-infected cell provides a unique example of non-canonical mTOR activation supporting growth that is independent of signalling through either S6K1 or 4E-BP1.

Approaches for prevention of restenosis

Coronary artery disease is characterized by a narrowing (stenosis) of the arteries that supply blood to the tissue of the heart. Continued restriction of blood flow manifests itself as angina and ultimately myocardial infarction (heart attack) for the patient. Heart bypass was once the only treatment for this condition, but over the years percutaneous coronary intervention (PCI) has become an increasingly attractive alternative to medical therapy and surgical revascularization for the treatment of coronary artery disease. A vascular stent is a medical device designed to serve as a temporary or permanent internal scaffold, to maintain or increase the lumen of a blood vessel. Metallic coronary stents were first introduced to prevent arterial dissections and to eliminate vessel recoil and intimal hyperplasia associated with PCI. Further advancement in the treatment of coronary artery disease is the development of drug-eluting stents that dramatically reduce the incidence of in-stent restenosis to less than 5%. Local drug delivery offers the advantages of allowing a relatively high local concentration of drug at the treatment site while minimizing systemic toxic effect. This review describes approaches for prevention of restenosis. It focuses on drugs for prevention of restenosis, bare metal stents, and drug-eluting stents. It also describes recent advances in bioresorbable stents. One of the chapters is dedicated to our novel composite bioresorbable drug-eluting fibers, designed to be used as basic elements in drug-eluting stents.

Role of calcineurin in the regulation of human lung mast cell and basophil function by cyclosporine and FK506

BACKGROUND AND PURPOSE

Cyclosporine and FK506 are thought to act by targeting the Ca2+-dependent protein phosphatase, calcineurin. The aim of the present study was to determine whether cyclosporine and FK506 stabilize mast cells and basophils by interacting with calcineurin.

EXPERIMENTAL APPROACH

The effects of cyclosporine and FK506 on the IgE-mediated release of histamine from mast cells and basophils were evaluated. The presence of calcineurin in cells was determined by Western blotting. Ca2+-dependent protein phosphatase activities were assessed in cell extracts using a synthetic phosphorylated peptide that is known to serve as a substrate for calcineurin.

KEY RESULTS

FK506 was about 100-fold more potent than cyclosporine as an inhibitor of IgE-dependent histamine release from mast cells and basophils. Immunoblotting of solubilized preparations of purified cells demonstrated the presence of calcineurin in mast cells and basophils. In enzyme assays, mast cells expressed approximately 7-fold higher Ca2+-dependent protein phosphatase activity than basophils. Whereas cyclosporine effectively inhibited Ca2+-dependent protein phosphatase activity in cell extracts, FK506 was considerably less effective.

CONCLUSIONS AND IMPLICATIONS

FK506 and cyclosporine inhibit the stimulated release of histamine from mast cells and basophils. However, the ability of cyclosporine, but not FK506, to inhibit Ca2+-dependent protein phosphatase activity questions whether FK506 stabilizes mast cells and basophils by interacting with calcineurin.

In vivo antitumor effect of the mTOR inhibitor CCI-779 and gemcitabine in xenograft models of human pancreatic cancer

Mammalian target of rapamycin (mTOR) is considered to be a major effector of cell growth and proliferation that controls protein synthesis through a large number of downstream targets. We investigated the expression of the phosphatidylinositol 3'-kinase (PI3K)/mTOR signaling pathway in human pancreatic cancer cells and tissues, and the in vivo antitumor effects of the mTOR inhibitor CCI-779 with/without gemcitabine in xenograft models of human pancreatic cancer. We found that the Akt, mTOR and p70 S6 kinase (S6K1) from the PI3K/mTOR signaling pathway were activated in all of the pancreatic cancer cell lines examined. When surgically resected tissue specimens of pancreatic ductal adenocarcinoma were examined, phosphorylation of Akt, mTOR and S6K1 was detected in 50, 55 and 65% of the specimens, respectively. Although CCI-779 had no additive or synergistic antiproliferative effect when combined with gemcitabine in vitro, it showed significant antitumor activity in the AsPC-1 subcutaneous xenograft model as both a single agent and in combination with gemictabine. Furthermore, in the Suit-2 peritoneal dissemination xenograft model, the combination of these 2 drugs achieved significantly better survival when compared with CCI-779 or gemcitabine alone. These results demonstrate promising activity of the mTOR inhibitor CCI-779 against human pancreatic cancer, and suggest that the inhibition of mTOR signaling can be exploited as a potentially tumor-selective therapeutic strategy.

Rapamycin antagonizes NF-kappaB nuclear translocation activated by TNF-alpha in primary vascular smooth muscle cells and enhances apoptosis

Several lines of evidence support the view that rapamycin inhibits NF-kappaB. TNF-alpha, a potent inducer of NF-kappaB, is released after artery injury (e.g., balloon angioplasty) and plays an important role in inflammation and restenosis. We investigated the effect of rapamycin on NF-kappaB activation and apoptosis in vascular smooth muscle cells (VSMCs) stimulated with TNF-alpha. Using EMSA, we found that TNF-alpha caused NF-kappaB nuclear translocation in VSMCs after 1 h of incubation. Rapamycin inhibited IkappaBalpha degradation, thereby preventing nuclear translocation. Activation of NF-kappaB was accompanied by an increase of Bcl-xL and Bfl-1/A1 proteins, detected by Western blot assay, whereas rapamycin prevented the TNF-alpha-induced enhancement of these antiapoptotic proteins. The extent of apoptosis of VSMCs exposed to TNF-alpha was significantly enhanced by rapamycin. The effect of rapamycin appeared to be independent of the phosphatidylinositol 3-kinase/Akt-protein kinase B survival pathway, because the phosphatidylinositol 3-kinase inhibitor wortmannin neither prevented IkappaBalpha degradation nor increased apoptosis of cells incubated with TNF-alpha. Finally, we demonstrate that the large immunophilin FK-506 binding protein FKBP51 is essential for TNF-alpha-induced NF-kappaB activation in VSMCs. Our findings show that rapamycin inhibits NF-kappaB activation and acts in concert with TNF-alpha in induction of VSMC apoptosis.

Rapamycin: an anti-cancer immunosuppressant?

Rapamycin and its derivatives are promising therapeutic agents with both immunosuppressant and anti-tumor properties. These rapamycin actions are mediated through the specific inhibition of the mTOR protein kinase. mTOR serves as part of an evolutionarily conserved signaling pathway that controls the cell cycle in response to changing nutrient levels. The mTOR signaling network contains a number of tumor suppressor genes including PTEN, LKB1, TSC1, and TSC2, and a number of proto-oncogenes including PI3K, Akt, and eIF4E, and mTOR signaling is constitutively activated in many tumor types. These observations point to mTOR as an ideal target for anti-cancer agents and suggest that rapamycin is such an agent. In fact, early preclinical and clinical studies indicate that rapamycin derivatives have efficacy as anti-tumor agents both alone, and when combined with other modes of therapy. Rapamycin appears to inhibit tumor growth by halting tumor cell proliferation, inducing tumor cell apoptosis, and suppressing tumor angiogenesis. Rapamycin immunosuppressant actions result from the inhibition of T and B cell proliferation through the same mechanisms that rapamycin blocks cancer cell proliferation. Therefore, one might think that rapamycin-induced immunosuppression would be detrimental to the use of rapamycin as an anti-cancer agent. To the contrary, rapamycin decreases the frequency of tumor formation that occurs in organ transplant experiments when combined with the widely used immunosuppressant cyclosporine compared with the tumor incidence observed when cyclosporine is used alone. The available evidence indicates that with respect to tumor growth, rapamycin anti-cancer activities are dominant over rapamycin immunosuppressant effects.

Drug-eluting stents: Sirolimus and paclitaxel differentially affect cultured cells and injured arteries

Sirolimus and paclitaxel eluted from stents inhibit cell proliferation and other cellular processes by dramatically different mechanisms. In this study, the effects of sirolimus and paclitaxel on cultured human coronary artery smooth muscle and endothelial cell function or cell cycle changes in balloon-injured arteries were directly compared. Both sirolimus and paclitaxel inhibited smooth muscle and endothelial cell proliferation. However, only paclitaxel inhibited smooth muscle and endothelial cell migration at low (nM) concentrations. Sirolimus arrested smooth muscle and endothelial cells in the G0/G1 phase of the cell cycle without inducing apoptosis while paclitaxel produced apoptosis in both cell types at low nanomolar concentrations. Although both agents blocked neointimal formation, sirolimus applied locally to injured rat carotid arteries increased the percentage of cycling vascular cells in G0/G1 without inducing apoptosis while paclitaxel increased the percentage of cycling cells in S and G2/M phases while inducing apoptosis. These results suggest that sirolimus reduces neointimal hyperplasia through a cytostatic mechanism while paclitaxel produces apoptotic cell death.

Therapeutic potential of target of rapamycin inhibitors

Target of rapamycin (TOR) functions within the cell as a transducer of information from various sources, including growth factors, energy sensors, and hypoxia sensors, as well as components of the cell regulating growth and division. Blocking TOR function mimics amino acid, and to some extent, growth factor deprivation and has a cytostatic effect on proliferating cells in vivo. Inhibition of TOR in vivo, utilising its namesake rapamycin, leads to immunosuppression. This property has been exploited successfully with the use of rapamycin and its derivatives as a therapeutic agent in the prevention of organ rejection after transplantation with relatively mild side effects when compared to other immunosuppressive agents. The cytostatic effect of TOR on vascular smooth muscle cell proliferation has also recently been exploited in the therapeutic application of rapamycin to drug eluting stents for angioplasty. These stents significantly reduce the amount of arterial reblockage that results from proliferating vascular smooth muscle cells. In cancer, the effect of blocking TOR function on tumour growth and disease progression is currently of major interest and is the basis for a number of ongoing clinical trials. However, different cell types and tumours respond differently to TOR inhibition, and TOR is clearly not cytostatic for all types of cancer cells in vitro or in vivo. As the molecular details of how TOR functions and the targets of TOR activity are further elucidated, tumour and tissue specific functions are being identified that implicate TOR in angiogenesis, apoptosis, and the reversal of some forms of cellular transformation. This review will describe our current understanding of TOR function, describe the current strategies for employing TOR inhibitors in clinical and preclinical development, and outline future strategies for appropriate targets of TOR inhibitors in the treatment of disease.

Compensatory PI3-kinase/Akt/mTor activation regulates imatinib resistance development

BCR/ABL-kinase mutations frequently mediate clinical resistance to the selective tyrosine kinase inhibitor Imatinib mesylate (IM, Gleevec). However, mechanisms that promote survival of BCR/ABL-positive cells before clinically overt IM resistance occurs have poorly been defined so far. Here, we demonstrate that IM-treatment activated the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTor)-pathway in BCR/ABL-positive LAMA-cells and primary leukemia cells in vitro, as well as in a chronic phase CML patient in vivo. In fact, PI3K/Akt-activation critically mediated survival during the early phase of IM resistance development before manifestation of BCR/ABL-dependent strong IM resistance such as through a kinase mutation. Accordingly, inhibition of IM-induced Akt activation using mTor inhibitors and Akt-specific siRNA effectively antagonized development of incipient IM-resistance in vitro. In contrast, IM-resistant chronic myeloid leukemia (CML) patients with BCR/ABL kinase mutations (n=15), and IM-refractory BCR/ABL-positive acute lymphatic leukemia patients (n=2) displayed inconsistent and kinase mutation-independent autonomous patterns of Akt-pathway activation, and mTor-inhibition overcame IM resistance only if Akt was strongly activated. Together, an IM-induced compensatory Akt/mTor activation may represent a novel mechanism for the persistence of BCR/ABL-positive cells in IM-treated patients. Treatment with mTor inhibitors may thus be particularly effective in IM-sensitive patients, whereas Akt-pathway activation variably contributes to clinically overt IM resistance.

PABPN1 overexpression leads to upregulation of genes encoding nuclear proteins that are sequestered in oculopharyngeal muscular dystrophy nuclear inclusions

Oculopharyngeal muscular dystrophy (OPMD) is an adult-onset disease caused by expanded (GCN)12-17 stretches encoding the N-terminal polyalanine domain of the poly(A) binding protein nuclear 1 (PABPN1). OPMD is characterized by intranuclear inclusions (INIs) in skeletal muscle fibers, which contain PABPN1, molecular chaperones, ubiquitin, proteasome subunits, and poly(A)-mRNA. We describe an adenoviral model of PABPN1 expression that produces INIs in most cells. Microarray analysis revealed that PABPN1 overexpression reproducibly changed the expression of 202 genes. Sixty percent of upregulated genes encode nuclear proteins, including many RNA and DNA binding proteins. Immunofluorescence microscopy revealed that all tested nuclear proteins encoded by eight upregulated genes colocalize with PABPN1 within the INIs: CUGBP1, SFRS3, FKBP1A, HMG2, HNRPA1, PRC1, S100P, and HSP70. In addition, CUGBP1, SFRS3, and FKBP1A were also found in OPMD muscle INIs. This study demonstrates that a large number of nuclear proteins are sequestered in OPMD INIs, which may compromise cellular function.

Rapamycin inhibits fibronectin-induced migration of the human arterial smooth muscle line (E47) through the mammalian target of rapamycin

The matrix protein fibronectin (FN) is a potent agoinst of vascular smooth muscle cell (SMC) migration. The role of rapamycin and the mammalian target of rapamycin (mTOR) in matrix protein-induced migration has not yet been defined. In these studies, we found that rapamycin (10 nM) markedly diminished chemotaxis of E47 cells (a cell line derived from human atherosclerotic plaques) and rat aortic SMCs toward FN as well as type I collagen and laminin; however, a period of preincubation >20 h was required. Subsequently, we showed that treatment with FN induced a rapid activation of mTOR as well as its downstream effector, S6 kinase (S6K). Moreover, FN-induced activation of both proteins was inhibited by preincubation with rapamycin for only 30 min. We then explored the upstream signaling pathway through which FN might mediate mTOR activation. A blocking antibody to alpha(v)beta(3) inhibited FN-induced mTOR/S6K activation as well as E47 cell chemotaxis, implicating alpha(v)beta(3) as the integrin receptor responsible for initiating FN-induced migration. Moreover, preincubation of E47 cells with wortmannin or LY-294002 blocked FN-induced mTOR/S6K activation, demonstrating that phosphatidylinositol 3-kinase (PI3K) plays a critical role in this rapamycin-sensitive signaling pathway. It has been previously suggested that rapamycin's effect on migration maybe related to enhancement of p27(kip1). However, treatment of E47 cells with rapamycin did not alter the level of p27(kip1) in the presence or absence of FN. Taken together, our data demonstrate that rapamycin inhibits FN-induced SMC migration through a pathway that involves at least alpha(v)beta(3)-integrin, PI3K, mTOR, and S6K.

Rapamycin attenuates vascular wall inflammation and progenitor cell promoters after angioplasty

Rapamycin combines antiproliferative and antiinflammatory properties and reduces neointima formation after angioplasty in patients. Its effect on transcriptional programs governing neointima formation has not yet been investigated. Here, we systematically analyzed the effect of rapamycin on gene expression during neointima formation in a human organ culture model. After angioplasty, renal artery segments were cultured for 21 or 56 days in absence or presence of 100 ng/ml rapamycin. Gene expression analysis of 2312 genes revealed 264 regulated genes with a peak alteration after 21 days. Many of those were associated with recruitment of blood cells and inflammatory reactions of the vessel wall. Likewise, chemokines and cytokines such as M-CSF, IL-1beta, IL-8, beta-thromboglobulin, and EMAP-II were found up-regulated in response to vessel injury. Markers indicative for a facilitated recruitment and stimulation of hematopoetic progenitor cells (HPC), including BST-1 and SDF-1, were also induced. In this setting, rapamycin suppressed the coordinated proadhesive and proinflammatory gene expression pattern next to down-regulation of genes related to metabolism, proliferation, and apoptosis. Our study shows that mechanical injury leads to induction of a proinflammatory, proadhesive gene expression pattern in the vessel wall even in absence of leukocytes. These molecular events could provide a basis for the recruitment of leukocytes and HPC. By inhibiting the expression of such genes, rapamycin may lead to a reduced recruitment of leukocytes and HPC after vascular injury, an effect that may play a decisive role for its effectiveness in reducing restenosis.

Identification of c-Kit receptor as a regulator of adult neural stem cells in the mammalian eye: interactions with Notch signaling

Neural stem cells are present in specific regions of the adult central nervous system (CNS). Recent evidence suggests that the ciliary epithelium (CE), a CNS derivative, in the adult mammalian eye, harbors a quiescent population of neural stem cells. Here, we report the identification of c-Kit signaling as one of the regulators of adult CE neural stem cells in vitro. c-Kit receptors are expressed in proliferating adult CE neural stem cells and colocalized with neural progenitor markers. Perturbation of c-Kit signaling influences the self-renewal and differentiation of CE neural stem cells, thus demonstrating the role of c-Kit signaling in the maintenance of these cells. In addition, we observed an influence of c-Kit-mediated signaling on the expression of Notch1, another critical regulator of neural stem cells. Our observations suggest that, given the importance of preservation of a stem cell pool for generating different cell types at different times, multiple signaling pathways act in concert for the maintenance of neural stem cells.

Evidence for Cyclin D3 as a Novel Target of Rapamycin in Human T Lymphocytes

The immunosuppressant rapamycin has been shown to inhibit G(1)/S transition of the cell cycle. This inhibition is thought to be mediated by maintenance of the threshold levels of cyclin-dependent kinase (CDK) inhibitor p27(Kip1) (p27) and inhibition of p70 s6 kinase (p70(s6k)). However, recent evidence suggests that cells still remain sensitive to rapamycin in the absence of functional p27 or p70(s6k). Here, we show that rapamycin represses cyclin D3 levels in activated human T lymphocytes with no inhibitory effects on cyclin D2. Furthermore, rapamycin elicits similar cyclin D3 modulatory effects in B lymphocytes. The overall effect of rapamycin on cyclin D3 leads to impaired formation of active complexes with Cdk4 or Cdk6 and subsequent inhibition of cyclin D3/CDK kinase activity. Decrease in cyclin D3 protein levels is due to translational repression and not due to attenuated transcription of the cyclin D3 gene. Importantly, stable overexpression of cyclin D3 (2-2.5 fold) in Jurkat T cell transfectants renders them resistant to lower doses (1-10 ng/ml) of rapamycin. These results point to a critical role of cyclin D3 in rapamycin-mediated immunosuppressive effects in T cells and cell cycle regulation in lymphocytes in general.

Drug-Eluting Stents

Advances in catheter and stent design have made stent implantation the standard coronary angioplasty procedure. Unfortunately, in-stent restenosis continues to plague this procedure, with the optimum binary restenosis rates reaching ~10% to 20%. In the past few years, it has become clear that in-stent restenosis is largely due to the migration and proliferation of vascular smooth muscle cells to form a neointima. To address this issue, stents coated with drug-delivery vehicles have been developed to deliver antiproliferative therapeutics. Two drugs, rapamycin and taxol, have been the lead compounds for testing the idea of a drug-eluting stent. These drugs have been successful largely because of the solid mechanistic understanding of their effects and extensive preclinical examination. The result of these years of work is that the rapamycin-coated stent entered the US market in April of 2003, and the taxol-coated stent appears poised to follow soon.

Inositol 1,4,5-trisphosphate receptor (type 1) phosphorylation and modulation by Cdc2

Calcium (Ca2+) release from the endoplasmic reticulum (ER) controls numerous cellular functions including proliferation, and is regulated in part by inositol 1,4,5-trisphosphate receptors (IP3Rs). IP3Rs are ubiquitously expressed intracellular Ca2+-release channels found in many cell types. Although IP3R-mediated Ca2+ release has been implicated in cellular proliferation, the biochemical pathways that modulate intracellular Ca2+ release during cell cycle progression are not known. Sequence analysis of IP3R1 reveals the presence of two putative phosphorylation sites for cyclin-dependent kinases (cdks). In the present study, we show that cdc2/CyB, a critical regulator of eukaryotic cell cycle progression, phosphorylates IP3R1 in vitro and in vivo at both Ser(421) and Thr(799) and that this phosphorylation increases IP3 binding. Taken together, these results indicate that IP3R1 may be a specific target for cdc2/CyB during cell cycle progression.

Rapamycin: signaling in vascular smooth muscle

Rapamycin (sirolimus) was initially developed as an antibiotic, then as an immunosuppressant, and recently has been identified as one of the most promising novel agents for prevention of coronary artery stent restenosis. The story of how rapamycin was developed for the prevention of stent restenosis involves the discovery of its antiproliferative and antimigratory actions in vascular smooth muscle and ultimately the demonstration that it inhibits neointimal hyperplasia in a large animal model of restenosis. Rapamycin upregulates the cyclin-dependent kinase inhibitor p27(kip1), resulting in cell-cycle arrest at the G1 to S transition. Rapamycin also inhibits other important cellular functions, including protein translation. The precise mechanisms underlying rapamycin's actions have not been fully elucidated. However, its ability to potently inhibit vascular smooth muscle cell migration and proliferation has been the basis for developing rapamycin-eluting coronary artery stents that have reduced in-stent restenosis from about 30% to less than 5% in large clinical trials.

Myogenic differentiation is dependent on both the kinase function and the N-terminal sequence of mammalian target of rapamycin

The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase known to control initiation of translation through two downstream pathways: eukaryotic initiation factor 4E-binding protein 1 (4E-BP1)/eukaryotic initiation factor 4E and ribosomal p70 S6 kinase (S6K1). We previously showed in C2C12 murine myoblasts that rapamycin arrests cells in G(1) phase and completely inhibits terminal myogenesis. To elucidate the pathways that regulate myogenesis, we established stable C2C12 cell lines that express rapamycin-resistant mTOR mutants (mTORrr; S2035I) that have N-terminal deletions (Delta10 or Delta91) or are full-length kinase-dead mTORrr proteins. Additional clones expressing a constitutively active S6K1 were also studied. Our results show that Delta10mTORrr signals 4E-BP1 and permits rapamycin-treated myoblasts to differentiate, confirming the mTOR dependence of the inhibition of myogenesis by rapamycin. C2C12 cells expressing either Delta91mTORrr or kinase-dead mTORrr(D2338A) could not phosphorylate 4E-BP1 in the presence of rapamycin and could not abrogate the inhibition of myogenesis. Taken together, our results indicate that both the kinase function of mTOR and the N terminus (residues 11-91, containing part of the first HEAT domain) are essential for myogenic differentiation. In contrast, constitutive activation of S6K1 does not abrogate rapamycin inhibition of either proliferation or myogenic differentiation.

The mammalian target of rapamycin regulates C2C12 myogenesis via a kinase-independent mechanism

Rapamycin inhibits differentiation of mouse C2C12 myoblasts, a tissue culture model for skeletal muscle differentiation. The mechanism by which a rapamycin-sensitive signaling pathway regulates myogenesis is largely unknown. The mammalian target of rapamycin (mTOR) is a central regulator of cell growth and proliferation, but its role in myogenesis has not been examined directly. Here we report the investigation of the function of mTOR and its downstream effectors in muscle differentiation. Rapamycin exerts an inhibitory effect on C2C12 myogenesis at different stages, implying that a rapamycin-sensitive pathway may be required for multiple processes during muscle differentiation. The mTOR protein level increases 10-fold during differentiation, via a post-transcriptional mechanism. As the first direct demonstration of the essential role of mTOR in muscle differentiation, we show that a rapamycin-resistant mTOR, but not S6 kinase 1, can rescue rapamycin-inhibited myogenesis. Remarkably, the myogenic function of mTOR does not require its kinase activity. Two downstream effectors of the rapamycin-sensitive pathway, S6 kinase 1 and eIF4E-binding protein 1, undergo differential regulation during myogenesis, but neither protein is the relevant effector for the myogenic signaling of mTOR. Taken together, our observations suggest a novel mTOR signaling mechanism essential for skeletal muscle differentiation.

Cell cycle protein expression in vascular smooth muscle cells in vitro and in vivo is regulated through phosphatidylinositol 3-kinase and mammalian target of rapamycin

Cell cycle progression represents a key event in vascular proliferative diseases, one that depends on an increased rate of protein synthesis. An increase in phosphatidylinositol 3-kinase (PI 3-kinase) activity is associated with vascular smooth muscle cell proliferation, and rapamycin, which blocks the activity of the mammalian target of rapamycin, inhibits this proliferation in vitro and in vivo. We hypothesized that these 2 molecules converge on a critical pathway of translational regulation that is essential for successful upregulation of cell cycle-regulatory proteins in activated smooth muscle cells. p70(S6) kinase, a target of PI 3-kinase and the mammalian target of rapamycin, was rapidly activated on growth factor stimulation of quiescent coronary artery smooth muscle cells and after balloon injury of rat carotid arteries. The translational repressor protein 4E-binding protein 1 was similarly hyperphosphorylated under these conditions. These events were associated with increases in the protein levels of cyclin B1, cyclin D1, cyclin E, cyclin-dependent kinase 1, cyclin-dependent kinase 2, proliferating cell nuclear antigen, and p21(Cip1) in vivo and in vitro, whereas inhibition of the PI 3-kinase signaling pathway with either rapamycin or wortmannin blocked the upregulation of these cell cycle proteins, but not mRNA, and arrested the cells in vitro before S phase. In contrast to findings in other cell types, growth factor- or balloon injury-induced downregulation of the cell cycle inhibitor p27(Kip1) was not affected by rapamycin treatment. These data suggest that cell cycle progression in vascular cells in vitro and in vivo depends on the integrity of the PI 3-kinase signaling pathway in allowing posttranscriptional accumulation of cell cycle proteins.

Induction of differentiation of human myeloid leukemia cells by immunosuppressant macrolides (rapamycin and FK506) and calcium/calmodulin-dependent kinase inhibitors

OBJECTIVE

Potent immunosuppressants, such as rapamycin, FK506, and ascomycin, are known to regulate the phosphorylation of proteins. The purpose of this study was to investigate the effects of these immunosuppressants on differentiation of several human myeloid leukemic cell lines.

MATERIALS AND METHODS

Human myeloid leukemic cell lines were cultured with each immunosuppressant, and several differentiation markers were assayed.

RESULTS

Rapamycin effectively induced granulocytic differentiation of human myeloid leukemic HL-60 and ML-1 cells. In addition to morphologic differentiation, it also induced nitroblue tetrazolium reduction, lysozyme activity, and expression of CD11b in HL-60 cells. The commitment to differentiation was observed after treatment with rapamycin for 1 day, indicating that the effect of rapamycin was irreversible. FK506 and ascomycin induced differentiation of HL-60 cells, but at higher concentrations than rapamycin. A calcium/calmodulin-dependent kinase (CaMK) was copurified with FKBP52 immunophilin, a binding protein of immunosuppressants. We also found that the CaMK inhibitors KN62 and KN93 induced differentiation of HL-60 cells. Rapamycin and CaMK inhibitors induced differentiation of human myeloid leukemia ML-1 and K562, but not of other cell lines such as NB4, U937, or HEL.

CONCLUSION

Immunosuppressants and CaMK inhibitors induced differentiation of HL-60, ML-1, and K562 cells.

Salicylate-induced growth arrest is associated with inhibition of p70s6k and down-regulation of c-myc, cyclin D1, cyclin A, and proliferating cell nuclear antigen

Salicylate and its pro-drug form aspirin are widely used medicinally for their analgesic and anti-inflammatory properties, and more recently for their ability to protect against colon cancer and cardiovascular disease. Despite the wide use of salicylate, the mechanisms underlying its biological activities are largely unknown. Recent reports suggest that salicylate may produce some of its effects by modulating the activities of protein kinases. Since we have previously shown that the farnesyltransferase inhibitor l-744, 832 inhibits cell proliferation and p70(s6k) activity, and salicylate inhibits cell proliferation, we examined whether salicylate affects p70(s6k) activity. We find that salicylate potently inhibits p70(s6k) activation and phosphorylation in a p38 MAPK-independent manner. Interestingly, low salicylate concentrations (</=250 microm) inhibit p70(s6k) activation by phorbol myristate acetate, while higher salicylate concentrations (>/=5 mm) are required to block p70(s6k) activation by epidermal growth factor + insulin-like growth factor-1. These data suggest that salicylate may selectively inhibit p70(s6k) activation in response to specific stimuli. Inhibition of p70(s6k) by salicylate occurs within 5 min, is independent of the phosphatidylinositol 3-kinase pathway, and is associated with dephosphorylation of p70(s6k) on its major rapamycin-sensitive site, Thr(389). A rapamycin-resistant mutant of p70(s6k) is resistant to salicylate-induced Thr(389) dephosphorylation.

Interaction of RAFT1 with gephyrin required for rapamycin-sensitive signaling

RAFT1 (rapamycin and FKBP12 target 1; also called FRAP or mTOR) is a member of the ATM (ataxia telangiectasia mutated)-related family of proteins and functions as the in vivo mediator of the effects of the immunosuppressant rapamycin and as an important regulator of messenger RNA translation. In mammalian cells RAFT1 interacted with gephyrin, a widely expressed protein necessary for the clustering of glycine receptors at the cell membrane of neurons. RAFT1 mutants that could not associate with gephyrin failed to signal to downstream molecules, including the p70 ribosomal S6 kinase and the eIF-4E binding protein, 4E-BP1. The interaction with gephyrin ascribes a function to the large amino-terminal region of an ATM-related protein and reveals a role in signal transduction for the clustering protein gephyrin.

Inhibition of intimal thickening after balloon angioplasty in porcine coronary arteries by targeting regulators of the cell cycle

BACKGROUND

Although percutaneous transluminal coronary angioplasty (PTCA) is a highly effective procedure to reduce the severity of stenotic coronary atherosclerotic disease, its long-term success is significantly limited by the high rate of restenosis. Several cellular and molecular mechanisms have been implicated in the development of restenosis post-PTCA, including vascular smooth muscle cell (VSMC) activation, migration, and proliferation. Recently, our group demonstrated that rapamycin, an immunosuppressant agent with antiproliferative properties, inhibits both rat and human VSMC proliferation and migration in vitro. In the present study, we investigated (1) whether rapamycin administration could reduce neointimal thickening in a porcine model of restenosis post-PTCA and (2) the mechanism by which rapamycin inhibits VSMCs in vivo.

METHODS AND RESULTS

PTCA was performed on a porcine model at a balloon/vessel ratio of 1.7+/-0.2. Coronary arteries were analyzed for neointimal formation 4 weeks after PTCA. Intramuscular administration of rapamycin started 3 days before PTCA at a dose of 0.5 mg/kg and continued for 14 days at a dose of 0.25 mg/kg. Cyclin-dependent kinase inhibitor (CDKI) p27(kip1) protein levels and pRb phosphorylation within the vessel wall were determined by immunoblot analysis. PTCA in the control group was associated with the development of significant luminal stenosis 4 weeks after the coronary intervention. Luminal narrowing was a consequence of significant neointimal formation in the injured areas. Rapamycin administration was associated with a significant inhibition in coronary stenosis (63+/-3.4% versus 36+/-4.5%; P<0.001), resulting in a concomitant increase in luminal area (1.74+/-0.1 mm2 versus 3. 3+/-0.4 mm2; P<0.001) after PTCA. Inhibition of proliferation was associated with markedly increased concentrations of the p27(kip1) levels and inhibition of pRb phosphorylation within the vessel wall.

CONCLUSIONS

Rapamycin administration significantly reduced the arterial proliferative response after PTCA in the pig by increasing the level of the CDKI p27(kip1) and inhibition of the pRb phosphorylation within the vessel wall. Therefore, pharmacological interventions that elevate CDKI in the vessel wall and target cyclin-dependent kinase activity may have a therapeutic role in the treatment of restenosis after angioplasty in humans.

p70 S6 kinase activation is not required for insulin-like growth factor-induced differentiation of rat, mouse, or human skeletal muscle cells

Insulin-like growth factors (IGFs) are potent stimulators of muscle differentiation, and phosphatidylinositol 3-kinase (PI 3-kinase) is an essential second messenger in this process. Little is known about the downstream effectors of the IGF/PI 3-kinase myogenic cascade, and contradictory observations have been reported concerning the involvement of p70 S6 kinase. In an attempt to clarify the role of p70 S6 kinase in myogenesis, here we have studied the effect of rapamycin on rat, mouse, and human skeletal muscle cell differentiation. Both insulin and IGF-II activated p70 S6 kinase in rat L6E9 and mouse Sol8 myoblasts, which was markedly inhibited at 1 ng/ml rapamycin concentrations. Consistent with previous observations in a variety of cell lines, rapamycin exerted a potent inhibitory effect on L6E9 and Sol8 serum-induced myoblast proliferation. In contrast, even at high concentrations (20 ng/ml), rapamycin had no effect on IGF-II-induced proliferation or differentiation. Indeed, neither the morphological differentiation, as assessed by myotube formation, nor the expression of muscle-specific markers such as myogenin, myosin heavy chain, or GLUT4 (glucose transporter-4) glucose carriers was altered by rapamycin. Moreover, here we extended our studies on IGF-II-induced myogenesis to human myoblasts derived from skeletal muscle biopsies. We show that, as observed for rat and mouse muscle cells, human myoblasts can be induced to form multinucleated myotubes in the presence of exogenous IGF-II. Moreover, IGF-II-induced human myotube formation was totally blocked by LY294002, a specific PI 3-kinase inhibitor, but remained unaffected in the presence of rapamycin.

Late signals from the PDGF receptors leading to the activation of the p70S6-kinase are necessary for the transition from G1 to S phase in AKR-2B cells

Platelet-derived growth factor AB (PDGF-AB) has to be permanently present in the culture medium to achieve full proliferation (>90%) of AKR-2B fibroblasts. Upon removal after 1 h incubation time, only a small number of cells (<20%) entered the cell cycle. Concomitantly there was no increase in RNA- and protein-synthesis. The PDGF-receptor autophosphorylation reached a maximum after 30 min incubation with PDGF-AB. Tyrosine phosphorylation was no longer detectable after 2-4 h. The clustering of receptors into coated pits, analyzed by indirect immunofluorescence using a specific antibody against PDGF-beta-receptor, showed in contrast to autophosphorylation a biphasic kinetic. A first maximum was reached after 30 min, followed by a complete disappearance of coated pits, which regenerated in a second phase after 3 h and were long lasting. If PDGF-AB was removed after 1 h, the second phase was obliterated. The involvement of two different signalling pathways in these two phases was investigated in detail: (1) The ras-raf-MAP-kinase pathway and (2) the PI-3-kinase/p70(S6)-kinase pathway. PDGF-AB addition caused a fast (10 min) activation of MAP-kinase, which returned to background level after 1 h without any further activation later on. In contrast PDGF-AB led to a rapid (15-30 min) activation of the p70(S6)-kinase that persisted for 8-12 h just prior to the entry of the cells into S-phase. If PDGF-AB was removed after 1 h, the activation of this kinase ceased 3 h later. PDGF-AA, which is unable to promote division of AKR-2B cells, induced only a shortlasting p70(S6)-kinase activation. These observations add further evidence for the involvement of the p70(S6)-kinase pathway in the proliferation control of AKR-2B fibroblasts in the late G1 phase (4-8 h after growth factor addition). On the other hand, if the p70(S6)-kinase activation was prevented by the addition of 10 nM rapamycin, the cell division was not inhibited but only delayed by 4 h. Similar kinetics were observed when the PI-3-kinase was inhibited by 400 nM wortmannin. It is suggested that a regulatory element exists upstream of the p70(S6)-kinase and the PI-3-kinase. This regulatory element should be responsible for the transmission of late signals required for the progression through the cell cycle. This element is not involved in the immediate responses after PDGF-AB addition but must be stimulated within a second later phase of PDGF activation.

Osteoblastic differentiation is enhanced by rapamycin in rat osteoblast-like osteosarcoma (ROS 17/2.8) cells

The effects of three immunosuppressants (rapamycin, FK506 and cyclosporin A) on the proliferation and differentiation of rat osteoblasts-like osteosarcoma cell line, ROS 17/2.8 (ROS) cells were examined in vitro. All immunosuppressants showed a direct inhibition on the proliferation of ROS cells with different potencies. Growth inhibition by rapamycin was stronger than that by FK506 or cyclosporin A. Rapamycin caused a significant increase in alkaline phosphatase (ALP) activity and in the expression of osteopontin and osteocalcin mRNAs. FK506 caused a moderate increase in ALP activity and a decreased expression of osteopontin mRNA. Cyclosporin A caused a decrease in ALP activity and in the expression of type 1 alpha 1 collagen mRNA. Our study indicates that rapamycin directly acts on ROS cells and induces osteoblastic differentiation, however, the effect of FK506 and cyclosporin A is weak. Rapamycin significantly enhances the differentiation induced by 1,25(OH)2-vitaminD3.

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.