Highlights from the Top 100 Most Influential Articles Regarding the Nuclear Receptor PPAR-γ: A Bibliometric Analysis

BACKGROUND

PPAR-γ is one of three members of the PPAR group of the nuclear receptor superfamily and plays an important regulatory role as a ligand-dependent transcription factor.

OBJECTIVE

This study aimed to identify the top 100 most influential articles in the field of PPAR-γ. We hypothesized that a bibliometric and scientometric analysis of the PPAR-γ research field could render trends that provide researchers and funding agencies valuable insight into the history of the field, and potential future directions.

METHODS

A literature search of publications was carried out using the Web of Science (WOS) and Scopus database based on specific subject words on September 11, 2023. Articles were listed in descending order of the number of citations. Statistical analysis was performed on the data of the top 100 cited articles in terms of year of publication, journal, research direction, institution, author, and country. Meanwhile, co-authorship networks and co-citation networks were constructed by using VOSviewer software, and keywords were analyzed for co-occurrence.

RESULTS

A total of 9,456 articles regarding PPAR-γ were identified and analyzed based on the WOS database, and the top 100 cited articles in the field of PPAR-γ were ranked by citation. The most cited article was published in 1998, with 2,571 citations and a density of 102.80 citations/ year. Of the 100 articles, Harvard University was the institution with the highest number of articles published. Spiegelman, B. M. was the author with the highest number of articles published. Using the VOSviewer software, we found that the most used keywords were geneexpression, activated receptor-gamma, and adipocyte differentiation. PPAR-γ, one of the most widely studied transcription factors, is an important drug target for many diseases. Therefore, screening for small molecule compounds targeting PPAR-γ remains of great value.

CONCLUSION

The present study identified the top 100 most influential articles in the field of PPAR-γ, which help global researchers to better understand research perspectives and develop future research directions of PPAR-γ.

Potential of Sorbus berry extracts for management of type 2 diabetes: Metabolomics investigation of 1H NMR spectra, α-amylase and α-glucosidase inhibitory activities, and in vivo anti-hyperglycaemic activity of S. norvegica

ETHNOPHARMACOLOGICAL RELEVANCE

Berries of Sorbus species have been used to treat type 2 diabetes in many regions in Europe.

AIMS OF THE STUDY

To investigate the inhibitory activity of berry extract of Sorbus on the digestive enzymes α-amylase and α-glucosidase, two important targets for management of blood glucose for type 2 diabetics. Furthermore, to test the anti-hyperglycaemic potential of S. norvegica berry extract in vivo.

MATERIALS AND METHODS

70% acetone berry extracts of 16 Sorbus species were tested in vitro for inhibition of α-amylase and α-glucosidase. Single berry extracts were analysed by ¹H-NMR spectroscopy and principal component analysis to evaluate the chemical profiles of the extracts. The anti-hyperglycaemic effect was evaluated in an oral starch tolerance test in STZ-treated C57BL/6 mice.

RESULTS

The lowest IC₅₀ values against α-amylase and α-glucosidase were obtained with the Sorbus species belonging to the subspecies Aria, which have simple leaves compared to pinnately compound leaves of the other Sorbus species. Species belonging to subspecies Aria grouped together and away from the other Sorbus species in the score plot, indicating a difference in chemistry. Both the carbohydrate- and polyphenol-fraction contributed to the enzyme inhibition. Extract of the most active species, S. norvegica, had anti-hyperglycaemic activity, at a level 36 times lower than clinically used acarbose, corresponding to a needed daily dose of 900 mg extract.

CONCLUSIONS

Sorbus species of subspecies Aria have the potential to be used for management of type 2 diabetes.

Insulin Sensitizing Pharmacology of Thiazolidinediones Correlates with Mitochondrial Gene Expression rather than Activation of PPARγ

Insulin sensitizing thiazolidinediones (TZDs) are generally considered to work as agonists for the nuclear receptor peroxisome proliferative activated receptor-gamma (PPARγ). However, TZDs also have acute, non-genomic metabolic effects and it is unclear which actions are responsible for the beneficial pharmacology of these compounds. We have taken advantage of an analog, based on the metabolism of pioglitazone, which has much reduced ability to activate PPARγ. This analog (PNU-91325) was compared to rosiglitazone, the most potent PPARγ activator approved for human use, in a variety of studies both in vitro and in vivo. The data demonstrate that PNU-91325 is indeed much less effective than rosiglitazone at activating PPARγ both in vitro and in vivo. In contrast, both compounds bound similarly to a mitochondrial binding site and acutely activated PI-3 kinase-directed phosphorylation of AKT, an action that was not affected by elimination of PPARγ activation. The two compounds were then compared in vivo in both normal C57 mice and diabetic KKAy mice to determine whether their pharmacology correlated with biomarkers of PPARγ activation or with the expression of other gene transcripts. As expected from previous studies, both compounds improved insulin sensitivity in the diabetic mice, and this occurred in spite of the fact that there was little increase in expression of the classic PPARγ target biomarker adipocyte binding protein-2 (aP2) with PNU-91325 under these conditions. An examination of transcriptional profiling of key target tissues from mice treated for one week with both compounds demonstrated that the relative pharmacology of the two thiazolidinediones correlated best with an increased expression of an array of mitochondrial proteins and with expression of PPARγ coactivator 1-alpha (PGC1α), the master regulator of mitochondrial biogenesis. Thus, important pharmacology of the insulin sensitizing TZDs may involve acute actions, perhaps on the mitochondria, that are independent of direct activation of the nuclear receptor PPARγ. These findings suggest a potential alternative route to the discovery of novel insulin sensitizing drugs.

Cytotoxicity of thiazolidinedione-, oxazolidinedione- and pyrrolidinedione-ring containing compounds in HepG2 cells

Liver damage occurred in some patients who took troglitazone (TGZ) for type II diabetes. The 2,4-thiazolidinedione (TZD) ring in TGZ's structure has been implicated in its hepatotoxicity. To further examine the potential role of a TZD ring in toxicity we used HepG2 cells to evaluate two series of compounds containing different cyclic imides. N-phenyl analogues comprised 3-(3,5-dichlorophenyl)-2,4-thiazolidinedione (DCPT); 3-(3,5-dichlorophenyl)-2,4-oxazolidinedione (DCPO) and N-(3,5-dichlorophenyl)succinimide (NDPS). Benzylic compounds, which closely resemble TGZ, included 5-(3,5-dichlorophenylmethyl)-2,4-thiazolidinedione (DCPMT); 5-(4-methoxyphenylmethyl)-2,4-thiazolidinedione (MPMT); 5-(4-methoxyphenylmethylene)-2,4-thiazolidinedione (MPMT-I); 5-(4-methoxyphenylmethyl)-2,4-oxazolidinedione (MPMO); 3-(4-methoxyphenylmethyl)succinimide (MPMS) and 3-(4-methoxyphenylmethylene)succinimide (MPMS-I). Cytotoxicity was assessed using the MTS assay after incubating the compounds (0-250μM) with HepG2 cells for 24h. Only certain TZD derivatives (TGZ, DCPT, DCPMT and MPMT-I) markedly decreased cell viability, whereas MPMT had low toxicity. In contrast, analogues without a TZD ring (DCPO, NDPS, MPMO, MPMS and MPMS-I) were not cytotoxic. These findings suggest that a TZD ring may be an important determinant of toxicity, although different structural features, chemical stability, cellular uptake or metabolism, etc., may also be involved. A simple clustering approach, using chemical fingerprints, assigned each compound to one of three classes (each containing one active compound and close homologues), and provided a framework for rationalizing the activity in terms of structure.

Mitochondrial target of thiazolidinediones

Insulin-sensitizing thiazolidinediones exert a pleiotropic pharmacology with therapeutic potential in a number of disease states ranging from metabolic syndrome and diabetes to neurodegeneration and cancer. A growing understanding of their mechanism of action, working from the site of their binding in the mitochondrion, provides insight into the mechanism of action of the insulin sensitizers and the reasons for their pleiotropic pharmacology. This review helps to frame the direction of future work that should be helpful in setting a new direction for the discovery and development of new, more useful therapeutic agents for metabolic disease.

Pharmacokinetic-pharmacodynamic assessment of the interrelationships between tesaglitazar exposure and renal function in patients with type 2 diabetes mellitus

The effects of tesaglitazar on renal function (assessed as urinary clearance of 125I-sodium iothalamate or estimated by the modification of diet in renal disease formula) were studied in a 24-week open-label trial in type 2 diabetes mellitus patients randomized to daily doses of either tesaglitazar 2 mg or pioglitazone 45 mg. The aim of the analysis was to develop a population pharmacokinetic-pharmacodynamic model that could simultaneously describe the interrelationship between tesaglitazar exposure and reduction in renal function over time in patients with type 2 diabetes mellitus. The pharmacokinetic-pharmacodynamic model could adequately describe the interplay between tesaglitazar and glomerular filtration rate. A one-compartment model in which the apparent clearance was influenced by glomerular filtration rate characterized the pharmacokinetics of tesaglitazar. An indirect-response model was used for the slow time course of change in glomerular filtration rate, which decreased from 100 to 78 mL/min/1.73m(2) after 12 weeks of treatment. All tesaglitazar-treated patients had a reduction in glomerular filtration rate, and available demographic variables could not explain differences in response. Patients treated with an angiotensin converting enzyme inhibitor were more sensitive to tesaglitazar and had larger glomerular filtration rate decrease compared to nontreated patients. Approximately 8 weeks after discontinuing treatment, mean glomerular filtration rate had returned towards baseline. The model and data give valuable insights into the dynamic changes in glomerular filtration rate over time.

A model for glucose, insulin, and beta-cell dynamics in subjects with insulin resistance and patients with type 2 diabetes

Type 2 diabetes mellitus (T2DM) is a progressive, metabolic disorder characterized by reduced insulin sensitivity and loss of beta-cell mass (BCM), resulting in hyperglycemia. Population pharmacokinetic-pharmacodynamic (PKPD) modeling is a valuable method to gain insight into disease and drug action. A semi-mechanistic PKPD model incorporating fasting plasma glucose (FPG), fasting insulin, insulin sensitivity, and BCM in patients at various disease stages was developed. Data from 3 clinical trials (phase II/III) with a peroxisome proliferator-activated receptor agonist, tesaglitazar, were used to develop the model. In this, a modeling framework proposed by Topp et al was expanded to incorporate the effects of treatment and impact of disease, as well as variability between subjects. The model accurately described FPG and fasting insulin data over time. The model included a strong relation between insulin clearance and insulin sensitivity, predicted 40% to 60% lower BCM in T2DM patients, and realistic improvements of BCM and insulin sensitivity with treatment. The treatment response on insulin sensitivity occurs within the first weeks, whereas the positive effects on BCM arise over several months. The semi-mechanistic PKPD model well described the heterogeneous populations, ranging from nondiabetic, insulin-resistant subjects to long-term treated T2DM patients. This model also allows incorporation of clinical-experimental studies and actual observations of BCM.

Identification of a novel agonist of peroxisome proliferator-activated receptors alpha and gamma that may contribute to the anti-diabetic activity of guggulipid in Lep(ob)/Lep(ob) mice

The ethyl acetate extract of the gum of the guggul tree, Commiphora mukul (guggulipid), is marketed for the treatment of dyslipidaemia and obesity. We have found that it protects Lep(ob)/Lep(ob) mice from diabetes and have investigated possible molecular mechanisms for its metabolic effects, in particular those due to a newly identified component, commipheric acid. Both guggulipid (EC(50)=0.82 microg/ml) and commipheric acid (EC(50)=0.26 microg/ml) activated human peroxisome proliferator-activated receptor alpha (PPARalpha) in COS-7 cells transiently transfected with the receptor and a reporter gene construct. Similarly, both guggulipid (EC(50)=2.3 microg/ml) and commipheric acid (EC(50)=0.3 microg/ml) activated PPARgamma and both promoted the differentiation of 3T3 L1 preadipocytes to adipocytes. Guggulipid (EC(50)=0.66 microg/ml), but not commipheric acid, activated liver X receptor alpha (LXRalpha). E- and Z-guggulsterones, which are largely responsible for guggulipid's hypocholesterolaemic effect, had no effects in these assays. Guggulipid (20 g/kg diet) improved glucose tolerance in female Lep(ob)/Lep(ob) mice. Pure commipheric acid, given orally (960 mg/kg body weight, once daily), increased liver weight but did not affect body weight or glucose tolerance. However, the ethyl ester of commipheric acid (150 mg/kg, twice daily) lowered fasting blood glucose and plasma insulin, and plasma triglycerides without affecting food intake or body weight. These results raise the possibility that guggulipid has anti-diabetic activity due partly to commipheric acid's PPARalpha/gamma agonism, but the systemic bioavailability of orally dosed, pure commipheric acid appears poor. Another component may contribute to guggulipid's anti-diabetic and hypocholesterolaemic activity by stimulating LXRalpha.

Thiazolidinediones inhibit the progression of established hypertension in the Dahl salt-sensitive rat

We evaluated the effects of two thiazolidinediones (TZDs), the potent PPARgamma agonist rosiglitazone currently being used to treat diabetes, and a structurally similar experimental compound that is a poor PPARgamma agonist, in a non-diabetic, established hypertension model with continuous measurement of blood pressure by telemetry. Hypertension was induced in male Dahl salt-sensitive rats by a three-week pre-treatment with 4% salt before initiation of treatment. Fasting blood samples were taken for analysis of a biomarker panel to assess metabolic, anti-inflammatory and antioxidant activity of the treatments. Both TZDs significantly reduced both systolic and diastolic blood pressure. When used at the maximally effective doses established for metabolic improvement, both compounds produced equivalent reduction in lipids and elevation of adiponectin, yet the poorer PPARgamma agonist produced significantly greater reductions in blood pressure. Neither compound had a significant effect on circulating glucose or insulin in this animal model. The data demonstrate that these TZDs lower blood pressure significantly in Dahl rats and that this cardiovascular pharmacology is not directly correlated with the metabolic actions or with the magnitude of PPARgamma activation. These data suggest that it may be possible to find insulin-sensitising agents that have beneficial cardiovascular pharmacology with broad applications for disease prevention.

Discontinued drugs in 2005: endocrine and metabolic

This perspective describes the compounds from the endocrine and metabolic area that were discontinued during the calendar year 2005. This is a continuation of a series of perspectives of each of the editorial areas summarized by Expert Opinion on Investigational Drugs. The candidates for this summary were being developed in the areas of treatment of diabetes and diabetes complications, as well as reproductive and urogenital health issues.

Insulin sensitizers may prevent metabolic inflammation

The relative decreased response of peripheral tissues to insulin (insulin resistance) is a key metabolic disturbance that predisposes a large percentage of individuals to the development of type 2 diabetes and to cardiovascular disease. As detailed in an extensive literature over the last two decades, insulin resistance co-exists in varying degrees with a variety of other key risk factors, including dyslipidemia, hypertension, and vascular inflammation, that contribute to poor cardiovascular outcomes of individuals with type 2 diabetes and metabolic syndrome. Whereas insulin resistance is generally thought of as pathology unto itself, this commentary suggests that insulin resistance is a physiological compensation to inappropriate oxidative metabolism that induces a metabolic inflammatory response. Via signaling of this inflammatory response, the protective compensation to excessive oxidative metabolism dampens metabolism by reducing insulin action, fatty acid oxidation, and eventually mitochondrial function and numbers. Such a scenario could explain the coexistence of these phenomena with obesity and reduced mitochondrial function. Recent evidence suggests that thiazolidinediones exert pharmacology through modifications of mitochondrial metabolism, preventing the metabolic inflammation and allowing the up regulation of mitochondrial biogenesis. A further understanding of these mechanisms, which are likely to involve key redox signaling events emanating from mitochondrial biochemistry, is needed to fuel new therapeutic advances for the treatment of metabolic syndrome.

Outcomes associated with introduction of thiazolidinedione therapy in Medicaid enrolled patients with type 2 diabetes: an updated and expanded retrospective analysis

OBJECTIVES

In an earlier analysis, differences in health-care costs, medication adherence, and persistence were examined between patients with type 2 diabetes, enrolled in the North Carolina Medicaid, who had newly started thiazolidinedione (TZD) therapy and those starting other oral antidiabetic agents. In this analysis, the size of the cohort was increased by including 18 months of additional Medicaid data (until December 2004) and sought to: (1) replicate the results of the original study in a larger cohort; and (2) extend the original analysis by providing an additional 18 months of observational follow-up.

METHODS

A total of 2660 patients newly starting TZD therapy between July 2001 and December 2003 were compared to 2050 patients starting other oral antidiabetic medication for health-care costs and outcomes in the post-medication start year. In addition, the initial cohort was followed for an additional 18 months to examine if there were any differences in outcomes, such as hospitalization and total health-care costs, that could be associated with the type of therapy. Multivariate regression techniques, incorporating health-care utilization in the year prior to start of new therapy, were used to determine the net cost impact of one therapy versus the other.

RESULTS

Multiple regression analyses found that patients starting TZD have better treatment persistence in the post-medication start year compared to patients starting other oral antidiabetic agents (4% increase in therapy persistence index, p < 0.001). In addition, patients starting TZDs had 18.9% lower total annual health-care costs (p < 0.01) compared to patients starting other oral antidiabetic agents. Examination of the original cohort of 3191 patients, for up to an additional 18 months, showed TZD's association with improved adherence rates but not with persistence. Importantly, treatment adherence remained the strongest independent predictor of decreased hospitalization risk and health-care cost reduction in this population.

CONCLUSIONS

Introduction of thiazolidinedione therapy in a Medicaid-enrolled type 2 diabetic population was associated with significantly greater treatment adherence, in the post-start year, compared to patients starting other oral antidiabetic agents.

What has prevented the expansion of insulin sensitisers?

The ability to improve insulin sensitivity with synthetic compounds was uncovered by empirical discoveries by Takeda in the late 1970s. The potential of a class of thiazolidinediones for the treatment of Type 2 diabetes, by decreasing glucose and triglycerides alongside lowering circulating insulin, was made public during the 1980s. As the first of the chemicals (pioglitazone, troglitazone and rosliglitazone) proceeded to clinical trials, these observations were soon extended to demonstrate a rich and complex pharmacology. The promise of this mode of action included prevention of diabetes as well as making a significant impact on the incidence and severity of the life-shortening consequences of the established disease. There are now two of these drugs on the market: pioglitazone and rosiglitazone, and they are being used to treat significant numbers of diabetic patients. However, the use of these drugs and development of future generations of successful candidates has not met the expectations that were held out in the early 1980s. This can be attributed to two major prevailing conditions. Troglitazone became the first thiazolidinedione to be approved as a result of delays in the development of pioglitazone. Unfortunately, troglitazone produced a unique idiosyncratic and sometimes fatal, hepatoxicity that necessitated its removal from the marketplace; second, there has been an incomplete understanding of the biochemical mechanism of action of these drugs that has slowed (and perhaps derailed) attempts to produce second-generation compounds. The latter issue is the subject of this editorial, which suggests that it is time to take a fresh look at the pharmacology of insulin sensitisers.

Healthcare costs and prescription adherence with introduction of thiazolidinedione therapy in Medicaid type 2 diabetic patients: a retrospective data analysis

OBJECTIVES

Outcomes in patients with type 2 diabetes may vary depending on the antidiabetic medication used. Observational studies of outcomes of diabetes pharmacotherapy are needed to understand the implications of choice of controller in different populations. This study compared differences in total health care costs, medication adherence, and persistence in patients with type 2 diabetes enrolled in the North Carolina Medicaid Program that were newly started on thiazolidinedione (TZD) therapy with patients starting other oral antidiabetics during the same period. In addition differences among the TZDs with respect to these outcomes were examined.

METHODS

A total of 1774 patients newly starting TZD therapy between July 2001 and June 2002 were compared to 1709 patients starting other oral antidiabetic medication (metformin or sulfonylureas) for health care costs and outcomes in the post-medication start year. In addition, a sub-group analysis of health care costs in patients starting either TZD (pioglitazone [n = 1086] versus rosiglitazone [N = 688]) was compared. All included patients had complete enrollment for the 24 months of follow-up. Multivariate techniques incorporating health care utilization in the year prior to start of new therapy were utilized to determine the cost impact of one therapy versus another.

RESULTS

Results of multiple regression analyses suggest that patients starting TZD have better treatment adherence and persistence in the post-medication start year compared to patients starting other oral antidiabetics (13% increase in Medication Possession Ratios, and 10% increase in therapy persistence index, both p < 0.001). In addition, patients starting TZDs had 16.1% lower total annual health care costs (p < 0.01) compared to patients starting other oral antidiabetics. There were no differences in adherence and cost outcomes between the 2 TZDs.

CONCLUSIONS

Introduction of thiazolidinedione therapy in a Medicaid-enrolled type 2 diabetic population was associated with significantly improved treatment adherence, persistence, and lower annual health care costs in the post-start year compared to patients starting other oral antidiabetics.

Identification of a novel mitochondrial protein ("mitoNEET") cross-linked specifically by a thiazolidinedione photoprobe

Thiazolidinediones address underlying causes of type 2 diabetes, although their mechanism of action is not clearly understood. The compounds are thought to function as direct activators of the nuclear receptor PPARgamma (peroxisome proliferator-activated receptor-gamma), although pioglitazone, the weaker agonist of the two thiazolidinediones now in clinical use, seems to have more useful effects on circulating lipids. We have used tritiated pioglitazone and a photoaffinity cross-linker to identify a novel binding site in mitochondria. A saturable binding site for [3H]pioglitazone was solubilized from the membranes with CHAPS and migrated as a large complex by size exclusion chromatography. The binding correlated with a <17-kDa protein (m17), marked by a photoaffinity cross-linker, in both subcellular location and selectivity of competition by analogs. The protein was isolated and identified by mass spectrometry analysis and NH2-terminal sequencing. Three synthetic peptides with potential antigenic properties were synthesized from the predicted nontransmembrane sequence to generate antibodies in rabbits. Western blots show that this protein, which we have termed "mitoNEET," is located in the mitochondrial fraction of rodent brain, liver, and skeletal muscle, showing the identical subcellular location and migration on SDS-PAGE as the protein cross-linked specifically by the thiazolidinedione photoprobe. The protein exists in low levels in preadipocytes, and expression increases exponentially in differentiated adipocytes. The synthetic protein bound to solid phase associated with a complex of solubilized mitochondrial proteins, including the trifunctional beta-oxidation protein. It is possible that thiazolidinedione modification of the function of the mitochondrial target may contribute to lipid lowering and/or antidiabetic actions.