In vivo activities of cytokine oncostatin M in the regulation of plasma lipid levels

Oncostatin M: Risks and Benefits of a Novel Therapeutic Target for Atherosclerosis

BACKGROUND

Cardiovascular disease (CVD) is a leading cause of death worldwide. It is predicted that approximately 23.6 million people will die from CVDs annually by 2030. Therefore, there is a great need for an effective therapeutic approach to combat this disease. The European Cardiovascular Target Discovery (CarTarDis) consortium identified Oncostatin M (OSM) as a potential therapeutic target for atherosclerosis. The benefits of modulating OSM - an interleukin (IL)-6 family cytokine - have since been studied for multiple indications. However, as decades of high attrition rates have stressed, the success of a drug target is determined by the fine balance between benefits and the risk of adverse events. Safety issues should therefore not be overlooked.

OBJECTIVE

In this review, a risk/benefit analysis is performed on OSM inhibition in the context of atherosclerosis treatment. First, OSM signaling characteristics and its role in atherosclerosis are described. Next, an overview of in vitro, in vivo, and clinical findings relating to both the benefits and risks of modulating OSM in major organ systems is provided. Based on OSM's biological function and expression profile as well as drug intervention studies, safety concerns of inhibiting this target have been identified, assessed, and ranked for the target population.

CONCLUSION

While OSM may be of therapeutic value in atherosclerosis, drug development should also focus on de-risking the herein identified major safety concerns: tissue remodeling, angiogenesis, bleeding, anemia, and NMDA- and glutamate-induced neurotoxicity. Close monitoring and/or exclusion of patients with various comorbidities may be required for optimal therapeutic benefit.

Hepatic cholesterol transport and its role in non-alcoholic fatty liver disease and atherosclerosis

Non-alcoholic fatty liver disease (NAFLD) is a quickly emerging global health problem representing the most common chronic liver disease in the world. Atherosclerotic cardiovascular disease represents the leading cause of mortality in NAFLD patients. Cholesterol metabolism has a crucial role in the pathogenesis of both NAFLD and atherosclerosis. The liver is the major organ for cholesterol metabolism. Abnormal hepatic cholesterol metabolism not only leads to NAFLD but also drives the development of atherosclerotic dyslipidemia. The cholesterol level in hepatocytes reflects the dynamic balance between endogenous synthesis, uptake, esterification, and export, a process in which cholesterol is converted to neutral cholesteryl esters either for storage in cytosolic lipid droplets or for secretion as a major constituent of plasma lipoproteins, including very-low-density lipoproteins, chylomicrons, high-density lipoproteins, and low-density lipoproteins. In this review, we describe decades of research aimed at identifying key molecules and cellular players involved in each main aspect of hepatic cholesterol metabolism. Furthermore, we summarize the recent advances regarding the biological processes of hepatic cholesterol transport and its role in NAFLD and atherosclerosis.

Modulators of hepatic lipoprotein metabolism identified in a search for small-molecule inducers of tribbles pseudokinase 1 expression

Recent genome wide association studies have linked tribbles pseudokinase 1 (TRIB1) to the risk of coronary artery disease (CAD). Based on the observations that increased expression of TRIB1 reduces secretion of VLDL and is associated with lower plasma levels of LDL cholesterol and triglycerides, higher plasma levels of HDL cholesterol and reduced risk for myocardial infarction, we carried out a high throughput phenotypic screen based on quantitative RT-PCR assay to identify compounds that induce TRIB1 expression in human HepG2 hepatoma cells. In a screen of a collection of diversity-oriented synthesis (DOS)-derived compounds, we identified a series of benzofuran-based compounds that upregulate TRIB1 expression and phenocopy the effects of TRIB1 cDNA overexpression, as they inhibit triglyceride synthesis and apoB secretion in cells. In addition, the compounds downregulate expression of MTTP and APOC3, key components of the lipoprotein assembly pathway. However, CRISPR-Cas9 induced chromosomal disruption of the TRIB1 locus in HepG2 cells, while confirming its regulatory role in lipoprotein metabolism, demonstrated that the effects of benzofurans persist in TRIB1-null cells indicating that TRIB1 is sufficient but not necessary to transmit the effects of the drug. Remarkably, active benzofurans, as well as natural products capable of TRIB1 upregulation, also modulate hepatic cell cholesterol metabolism by elevating the expression of LDLR transcript and LDL receptor protein, while reducing the levels of PCSK9 transcript and secreted PCSK9 protein and stimulating LDL uptake. The effects of benzofurans are not masked by cholesterol depletion and are independent of the SREBP-2 regulatory circuit, indicating that these compounds represent a novel class of chemically tractable small-molecule modulators that shift cellular lipoprotein metabolism in HepG2 cells from lipogenesis to scavenging.

Advances in understanding the relationship between oncostatin M and liver regeneration and liver diseases

Oncostatin M (OSM) is a pleiotropic cytokine belonging to the interleukin (IL)-6 family of cytokines. It is closely related structurally and functionally to leukemia inhibitory factor (LIF). There are two types of functional OSM receptors (OSMR): I and II. The binding of OSM to its receptors activates the JAK-STAT and MAPK signal pathways. OSM not only inhibits the proliferation of tumor cells but also participates in several physiological and pathological processes in a variety of cell types and plays key roles in the pathogenesis of multiple diseases, including regulation of inflammatory responses, stimulation of hematopoiesis, regulation of cholesterol metabolism, and induction of neurotrophic peptides. Recent studies suggest that OSM participates in liver regeneration and is closely related to the occurrence and progression of viral hepatitis, non-alcoholic fatty liver disease, liver fibrosis, and liver cancer. This article reviews recent advances in understanding the relationship between OSM and liver generation and liver diseases.

Janus kinase activation by cytokine oncostatin M decreases PCSK9 expression in liver cells

PCSK9 degrades LDL receptor (LDLR) in liver and thereby influences the circulating level of LDL cholesterol. Hence, mechanisms inhibiting PCSK9 expression have potential for cholesterol-lowering intervention. Previously, we demonstrated that oncostatin M (OM) activates LDLR gene transcription, resulting in an increased LDL uptake in HepG2 cells and a reduction of plasma LDL in hypercholesterolemic hamsters. Here we identify the suppression of PCSK9 expression by OM as one new mechanism that increases LDLR protein in HepG2 cells. Treating HepG2 cells with OM decreases PCSK9 mRNA and protein levels. Inhibition studies and small interfering RNA -targeted depletion revealed a critical role for JAK1 and JAK2 in mediating this OM inhibitory effect. Furthermore, we showed that OM induces transient phosphorylation of STAT1, STAT3, and STAT5 and sustained activation of ERK signaling molecules. While depletion of STAT members in HepG2 cells did not affect OM inhibitory activity on PCSK9 expression, blocking activation of the MEK1/ERK signaling pathway resulted in attenuation of the OM inhibitory effect. Finally, by using an anti-hamster PCSK9 antibody, we demonstrated the in vivo suppression of liver PCSK9 mRNA and protein expression by OM in hypercholesterolemic hamsters. Our study uncovered a cytokine-triggered regulatory network for PCSK9 expression that is linked to JAKs and the ERK signaling pathway.

Elucidation of an SRE-1/SREBP-independent cellular pathway for LDL-receptor regulation: from the cell surface to the nucleus

Reduction in blood levels of low-density lipoprotein (LDL) cholesterol lowers the risk of coronary heart disease. The elucidation of cellular pathways that control LDL-receptor expression through a cholesterol-mediated negative feedback mechanism has provided a crucial molecular basis for the development and clinical applications of statins in the treatment of hypercholesterolemia. The characterization of signaling transduction pathways elicited by cytokine oncostatin M (OM) in liver cells has revealed a novel cellular pathway that activates LDL-receptor transcription independent of intracellular levels of cholesterol and sterol-regulatory element binding proteins. This transcriptional activation is achieved through interactions of the sterol-independent regulatory element of LDL-receptor promoter and transcription factors Egr1 and c/EBPbeta, and is dependent upon the activation of the extracellular signal-regulated kinase signaling cascade by OM. In vivo OM administration in hyperlipidemic animals reduces circulating cholesterol and prevents lipid accumulation in the liver. Exploring this sterol-independent cellular pathway may lead to new therapeutic advances.

Acyl-CoA synthesis, lipid metabolism and lipotoxicity

Although the underlying causes of insulin resistance have not been completely delineated, in most analyses, a recurring theme is dysfunctional metabolism of fatty acids. Because the conversion of fatty acids to activated acyl-CoAs is the first and essential step in the metabolism of long-chain fatty acid metabolism, interest has grown in the synthesis of acyl-CoAs, their contribution to the formation of signaling molecules like ceramide and diacylglycerol, and their direct effects on cell function. In this review, we cover the evidence for the involvement of acyl-CoAs in what has been termed lipotoxicity, the regulation of the acyl-CoA synthetases, and the emerging functional roles of acyl-CoAs in the major tissues that contribute to insulin resistance and lipotoxicity, adipose, liver, heart and pancreas.

Mechanisms of oncostatin M-induced pulmonary inflammation and fibrosis

Oncostatin M (OSM), an IL-6 family cytokine, has been implicated in a number of biological processes including the induction of inflammation and the modulation of extracellular matrix. In this study, we demonstrate that OSM is up-regulated in the bronchoalveolar lavage fluid of patients with idiopathic pulmonary fibrosis and scleroderma, and investigate the pathological consequences of excess OSM in the lungs. Delivery of OSM to the lungs of mice results in a significant recruitment of inflammatory cells, as well as a dose-dependent increase in collagen deposition in the lungs, with pathological correlates to characteristic human interstitial lung disease. To better understand the relationship between OSM-induced inflammation and OSM-induced fibrosis, we used genetically modified mice and show that the fibrotic response is largely independent of B and T lymphocytes, eosinophils, and mast cells. We further explored the mechanisms of OSM-induced inflammation and fibrosis using both protein and genomic array approaches, generating a "fibrotic footprint" for OSM that shows modulation of various matrix metalloproteinases, extracellular matrix components, and cytokines previously implicated in fibrosis. In particular, although the IL-4/IL-13 and TGF-beta pathways have been shown to be important and intertwined of fibrosis, we show that OSM is capable of inducing lung fibrosis independently of these pathways. The demonstration that OSM is a potent mediator of lung inflammation and extracellular matrix accumulation, combined with the up-regulation observed in patients with pulmonary fibrosis, may provide a rationale for therapeutically targeting OSM in human disease.

Combination of simvastatin with berberine improves the lipid-lowering efficacy

We have identified berberine (BBR) as a novel cholesterol-lowering drug acting through stabilization of the low-density lipoprotein receptor (LDLR) messenger RNA. Because the mechanism differs from that of statins, it is of great interest to examine the lipid-lowering activity of BBR in combination with statins. Our results showed that combination of BBR with simvastatin (SIMVA) increased the LDLR gene expression to a level significantly higher than that in monotherapies. In the treatment of food-induced hyperlipidemic rats, combination of BBR (90 mg/[kg d], oral) with SIMVA (6 mg/[kg d], oral) reduced serum LDL cholesterol by 46.2%, which was more effective than that of the SIMVA (28.3%) or BBR (26.8%) monotherapy (P < .01 for both) and similar to that of SIMVA at 12 mg/(kg d) (43.4%). More effective reduction of serum triglyceride was also achieved with the combination as compared with either monotherapy. Combination of BBR with SIMVA up-regulated the LDLR messenger RNA in rat livers to a level about 1.6-fold higher than the monotherapies did. Significant reduction of liver fat storage and improved liver histology were found after the combination therapy. The therapeutic efficacy of the combination was then evaluated in 63 hypercholesterolemic patients. As compared with monotherapies, the combination showed an improved lipid-lowering effect with 31.8% reduction of serum LDL cholesterol (P < .05 vs BBR alone, P < .01 vs SIMVA alone). Similar efficacies were observed in the reduction of total cholesterol as well as triglyceride in the patients. Our results display the rationale, effectiveness, and safety of the combination therapy for hyperlipidemia using BBR and SIMVA. It could be a new regimen for hypercholesterolemia.

Transcriptional activation of hepatic ACSL3 and ACSL5 by oncostatin m reduces hypertriglyceridemia through enhanced beta-oxidation

OBJECTIVE

In our previous studies that examined in vivo activities of oncostatin M (OM) in upregulation of hepatic LDL receptor (LDLR) expression, we observed reductions of LDL-cholesterol and triglyceride (TG) levels in OM-treated hyperlipidemic hamsters. Interestingly, the OM effect of lowering plasma TG was more pronounced than LDL-cholesterol reduction, suggesting additional LDLR-independent actions. Here, we investigated mechanisms underlying the direct TG-lowering effect of OM.

METHODS AND RESULTS

We demonstrate that OM activates transcription of long-chain acyl-coenzymeA (CoA) synthetase isoforms 3 and 5 (ACSL3, ACSL5) in HepG2 cells through the extracellular signal-regulated kinase (ERK) signaling pathway. Increased acyl-CoA synthetase activities in OM-stimulated HepG2 cells and in livers of OM-treated hamsters are associated with decreased TG accumulation and increased fatty acid beta-oxidation. We further show that overexpression of ACSL3 or ACSL5 alone in the absence of OM led to fatty acid partitioning into beta-oxidation. Importantly, we demonstrate that transfection of siRNAs targeted to ACSL3 and ACSL5 abrogated the enhancing effect of OM on fatty acid oxidation in HepG2 cells.

CONCLUSIONS

These new findings identify ACSL3 and ACSL5 as OM-regulated genes that function in fatty acid metabolism and suggest a novel cellular mechanism by which OM directly lowers the plasma TG in hyperlipidemic animals through stimulating the transcription of ACSL specific isoforms in the liver.

Pharmacological regulation of low density lipoprotein receptor expression: Current status and future developments

Plasma levels of low-density lipoprotein (LDL) cholesterol are considered to be a major risk factor for the development of cardiovascular diseases. The LDL receptor is the key component in the maintenance of cholesterol homeostasis in the body, playing a pivotal role by regulating the hepatic catabolism of LDL cholesterol. Many clinical studies using statins, which up-regulate the LDL receptor expression via a feedback mechanism, have demonstrated that the reduction of LDL cholesterol levels lowers the incidence of cardiovascular events in both primary and secondary prevention. In this context, new strategies designed to increase hepatic LDL receptor activity can be considered as attractive opportunities for future therapy. Several potential new drugs have been described in the last decade to up-regulate LDL receptor expression in vitro and in vivo, thus allowing the identification of new transcriptional and post-transcriptional mechanisms.

Blockage of oncostatin M-induced LDL receptor gene transcription by a dominant-negative mutant of C/EBPbeta

OM (oncostatin M) activates the human LDLR [LDL (low-density lipoprotein) receptor] gene transcription in HepG2 cells through the SIRE (sterol-independent regulatory element) of LDLR promoter. The SIRE sequence consists of a C/EBP (CCAAT/enhancer-binding protein)-binding site and a CRE (cAMP-response element). Our previous studies [Zhang, Ahlborn, Li, Kraemer and Liu (2002) J. Lipid Res. 43, 1477-1485; Zhang, Lin, Abidi, Thiel and Liu (2003) J. Biol. Chem. 278, 44246-44254] have demonstrated that OM transiently induces EGR-1 (early growth response gene product 1) expression and EGR-1 activates LDLR transcription primarily through a protein-protein interaction with C/EBPbeta, which serves as a co-activator of EGR-1. In the present study, we examined the direct role of C/EBPbeta as a transactivator in OM-regulated LDLR gene transcription independent of EGR-1. We show that OM induces C/EBPbeta expression with kinetics slower than EGR-1 induction. A significant increase in C/EBPbeta protein level is detected by 2 h of OM treatment and remains elevated for 24 h. Chromatin immunoprecipitation assays demonstrate that the amount of C/EBPbeta bound to the LDLR SIRE sequence is increased 2.8-fold of control by 2 h of OM treatment, reached the highest level of 8-fold by 4 h, and slowly declined thereafter. To further examine the requirement of C/EBPbeta in OM-stimulated LDLR expression, we developed a His-tagged dominant-negative mutant of C/EBPbeta (His-C/EBPbeta-P4; where P4 is plasmid 4 in our mutation series), consisting of the DNA-binding and leucine zipper domains of C/EBPbeta (amino acids 246-345). Expression of His-C/EBPbeta-P4 in HepG2 cells significantly diminishes the OM-induced increase in LDLR promoter activity and the elevation of endogenous LDLR mRNA expression. Taken together, these new findings identify C/EBPbeta as an OM-induced transactivator in LDLR gene transcription and provide a better understanding of the molecular mechanism underlying the sterol-independent regulation of LDLR expression.

Human low-density lipoprotein receptor gene and its regulation

The low-density lipoprotein (LDL) receptor is a transmembrane glycoprotein that mediates the binding and endocytosis of lipoproteins containing apolipoprotein B and E, especially the cholesterol-rich LDL. Mutations in the LDL receptor gene can produce dysfunctional LDL receptors and cause familial hypercholesterolemia. The expression of the LDL receptor gene is under an intriguing regulation by sterol and nonsterol mediators either at the transcriptional level or at the posttranscriptional level, both of which are linked to cell signaling pathways. Upregulation of liver LDL receptor expression is effective in treating hypercholesterolemia. In this review, we focus on the latest progress on the mechanisms and regulation of the LDL receptor gene expression.