How do SGLT2 inhibitors protect the kidney? A mediation analysis of the EMPA-REG OUTCOME trial.

INTRODUCTION: Mechanisms underlying kidney benefits with sodium-glucose cotransporter-2 (SGLT2) inhibition in heart failure and/or type 2 diabetes (T2D) with established cardiovascular disease are currently unclear. METHODS: We evaluated post hoc the factors mediating the effect of empagliflozin on a composite kidney outcome (first sustained estimate glomerular filtration rate ≥40% reduction from baseline, initiation of renal replacement therapy, or death due to kidney disease) in EMPA-REG OUTCOME. Variables, calculated as change from baseline or updated mean, were evaluated as time-dependent covariates and using a landmark approach (at Week 12) in Cox regression analyses. In multivariable analyses, variables with the greatest mediating effect were added using a step-up procedure. RESULTS: In univariable time-dependent updated mean covariate analyses, the strongest mediator was hematocrit (99.5% mediation). Hemoglobin, uric acid, and urine albumin-to-creatinine ratio mediated 79.4%, 33.2%, and 31.0%, respectively. Multivariable analyses were not performed due to the very strong mediation effect of hematocrit. In univariable Week 12 landmark change from baseline analyses, the strongest mediators included hematocrit (40.7%), glycated hemoglobin (28.3%), systolic blood pressure (16.8%), and free fatty acids (16.5%), which yielded a combined mediation of 78.9% in multivariable analysis. CONCLUSIONS: Changes in hematocrit and hemoglobin were the strongest mediators of empagliflozin's kidney benefits in EMPA-REG OUTCOME participants with T2D and cardiovascular disease.

Empagliflozin and serum potassium in heart failure: an analysis from EMPEROR-Pooled.

AIMS: Hyperkalaemia frequently leads to interruption and discontinuation of neurohormonal antagonists, which may worsen heart failure prognosis. Some studies suggested that sodium-glucose cotransporter 2 inhibitors reduce hyperkalaemia, an effect that may have important clinical implications. This analysis evaluates the effect of empagliflozin on the occurrence of hyper- and hypokalaemia in HF. METHODS AND RESULTS: EMPEROR-Pooled (i.e. EMPEROR-Reduced and EMPEROR-Preserved combined) included 9583 patients with available serum potassium levels at baseline (98.6% of the total EMPEROR-Pooled population, n = 9718). Hyperkalaemia was identified by investigators' reports of adverse events, and by a laboratory serum potassium value above 5.5 mmol/L and 6.0 mmol/L. The main outcome was a composite of investigator-reported hyperkalaemia or initiation of potassium binders. Patients with high potassium at baseline were more frequently diagnosed with diabetes and ischaemic HF aetiology and had lower left ventricular ejection fraction and estimated glomerular filtration rate but were more frequently treated with sacubitril/valsartan or mineralocorticoid receptor antagonists. Empagliflozin (compared with placebo) reduced the composite of investigator-reported hyperkalaemia or initiation of potassium binders [6.5% vs. 7.7%, hazard ratio (HR) 0.82, 95% confidence interval (CI) 0.71-0.95, P = 0.01]. Empagliflozin reduced hyperkalaemia rates regardless of the definition used (serum potassium >5.5 mmol/l: 8.6% vs. 9.9%, HR 0.85, 95% CI 0.74-0.97, P = 0.017; serum potassium >6.0 mmol/l: 1.9% vs. 2.9%, HR 0.62, 95% CI 0.48-0.81, P < 0.001). The incidence of hypokalaemia (investigator-reported or serum potassium <3.0 mmol/l) was not significantly increased with empagliflozin. CONCLUSIONS: Empagliflozin reduced the incidence of hyperkalaemia without significant increase in hypokalaemia.

Characterization and implications of the initial estimated glomerular filtration rate 'dip' upon sodium-glucose cotransporter-2 inhibition with empagliflozin in the EMPA-REG OUTCOME trial.

Treatment with sodium-glucose co-transporter-2 inhibitors induces an initial 3-5 ml/min/1.73 m2 decline in estimated glomerular filtration rate (eGFR). Although considered to be of hemodynamic origin and largely reversible, this 'eGFR dip' may cause concern in clinical practice, which highlights the need to better understand its incidence and clinical implications. In this post hoc analysis of the EMPA-REG OUTCOME trial, 6,668 participants randomized to empagliflozin 10 mg, 25 mg or placebo with eGFR available at baseline and week four were categorized by initial eGFR change into three groups; over 10% decline ('eGFR dipper'), over 0 and up to 10% decline ('eGFR intermediate'), no eGFR decline ('eGFR non-dipper'). Baseline characteristics of 'eGFR intermediate' and 'eGFR non-dipper' were generally comparable. An initial 'eGFR dip' was observed in 28.3% of empagliflozin versus 13.4% of placebo-treated participants; odds ratio 2.7 [95% Confidence Interval 2.3-3.0]. In multivariate logistic regression, diuretic use and higher KDIGO risk category at baseline were independently predictive of an 'eGFR dip' in empagliflozin versus placebo. Safety and beneficial treatment effects with empagliflozin on cardiovascular and kidney outcomes were consistent across subgroups based on these predictive factors. The initial 'eGFR dip' did not have a major impact on the treatment effect of empagliflozin on subsequent cardiovascular death, hospitalization for heart failure, and incident or worsening kidney disease. Thus, patients with type 2 diabetes with more advanced kidney disease and/or on diuretic therapy were more likely to experience an 'eGFR dip' of over 10% with empagliflozin, but reduction in cardiovascular and kidney outcomes was not relevantly modified by such 'eGFR dip.'

Body weight predicts Nicotinamide N-Methyltransferase activity in mouse fat.

AIM: Nicotinamide N-methyltransferase (NNMT) is a novel regulator of energy homeostasis in adipose tissue. NNMT expression is higher in obese mice than in lean mice, and NNMT knockdown prevents diet-induced obesity. Little is known about the regulation of enzyme activity during the development of obesity. The aim of this study was to analyze NNMT activity in tissues of mice with incipient and established obesity. METHODS: A fluorescence-based, sensitive, low-volume, high-throughput method was developed to assay NNMT activity. C57BL/6 mice were fed a high-fat diet for 4 weeks (incipient obesity) and for 12 weeks (established obesity). Tissues and serum were harvested and analyzed. RESULTS: NNMT activity was highest in subcutaneous white fat (55.0 µU/mg), followed by epididymal white fat (35.6 µU/mg), brown adipose tissue (7.8 µU/mg), liver (7.6 µU/mg), and lung (7.3 µU/mg). Little activity was detected in heart, skeletal muscle, and kidney. No activity was found in serum samples. Body weight predicted NNMT activity in white fat, but not in brown fat or any other tissue, and only in incipient obesity. With established obesity, this association was lost. CONCLUSIONS: As obesity develops, body weight predicts NNMT activity in white adipose tissue, but not in any other tissue, consistent with a specific role of adipose-tissue NNMT in the regulation of body weight.

Novel role for retinol-binding protein 4 in the regulation of blood pressure.

Elevated levels of serum retinol-binding protein 4 (RBP4) contribute to insulin resistance and correlate with increased prevalence of hypertension and myocardial infarction. We sought to determine whether lowering RBP4 would improve blood pressure (BP) and protect against obesity- or angiotensin (Ang)-II-induced hypertension. Systolic and diastolic BP were lower in the RBP4-knockout (RBP4-KO) mice and higher in the RBP4-overexpressing (RBP4-Tg) mice compared with BP in the wild-type (WT) littermates. Carbachol-induced vasodilatation was increased in arteries from the RBP4-KO compared with the WT mice and was impaired in the RBP4-Tg mice. Aortic eNOS(Ser1177) phosphorylation was enhanced ∼50% in the RBP4-KO mice, with no change in total eNOS protein. Feeding a high-fat diet increased BP in the RBP4-KO mice only to the level in the WT mice fed chow and had no effect on aortic eNOS(Ser1177) phosphorylation. Ang-II infusion resulted in 22 mmHg lower systolic BP in the RBP4-KO than in the WT mice, although the relative BP increase over saline infusion was ∼30% in both. Ang-II treatment decreased aortic eNOS(Ser1177) phosphorylation in the WT and RBP4-KO mice, but phosphorylation remained higher in the RBP4-KO mice. Cardiac hypertrophy with Ang-II treatment was diminished by 56% in the RBP4-KO mice. Thus, elevated serum RBP4 raises BP and lack of RBP4 reduces it, with commensurate changes in aortic eNOS(Ser1177) phosphorylation. Lowering RBP4 may reduce BP through enhanced eNOS-mediated vasodilatation and may be a novel therapeutic approach for hypertension.

Kidney and heart failure events are bidirectionally associated in patients with type 2 diabetes and cardiovascular disease.

AIMS: This study aimed to evaluate the bidirectional relationship between kidney and cardiovascular (CV) events in trial participants with type 2 diabetes and CV disease. METHODS AND RESULTS: Post hoc analyses of EMPA-REG OUTCOME using Cox regression models were performed to assess the association of baseline factors with risk of a kidney event and bidirectional associations of incident kidney events and CV events. Among placebo-treated participants, baseline factors significantly associated with greater kidney event risk included lower baseline estimated glomerular filtration rate, albuminuria, higher uric acid, low-density lipoprotein cholesterol levels, and prior heart failure (HF). Coronary artery disease was not associated with increased risk. In placebo-treated participants, occurrence of an incident non-fatal kidney event increased the subsequent risk of hospitalization for HF (HHF) but not 3-point major adverse CV events (non-fatal stroke, non-fatal myocardial infarction, and CV death). Vice versa, HHF (but not myocardial infarction/stroke) increased the risk of subsequent kidney events. These associations were generally also seen in empagliflozin-treated participants and in the overall population. Interestingly, the risk of kidney events following HHF was not significantly increased in the relatively small number of placebo-treated participants already diagnosed with HF at baseline. CONCLUSIONS: These findings demonstrate a bidirectional inter-relationship between HHF and kidney events. Further exploration of this relationship and strategies to optimize the use of therapies to reduce both kidney and HF outcomes is warranted.

The clinical burden of newly diagnosed Heart failure among patients with Reduced, mildly Reduced, and preserved ejection fraction.

Background: Contemporary analyses of the distribution of heart failure (HF) patients by groups of ejection fraction are not available or are limited to hospitalized patients. Our objective was to quantify the per-person and system level clinical burden of a broad population of HF patients. Methods: We studied 16,516 patients with a new HF diagnosis recorded in the electronic medical record of a U.S. integrated delivery system between 2005 and 2017. We used the diagnosis date as the index date and the nearest echocardiogram result to classify patients as HFrEF (n = 2,430), HFmrEF (n = 1,646), HFpEF (n = 12,440) and followed them through 2019 for major clinical outcomes (all-cause mortality, HF hospitalizations [HHF], all-cause hospitalizations, incident chronic kidney disease [CKD], progression of eGFR category, progression of CKD, incident type 2 diabetes [T2D], and progression to insulin use). We compared age and sex adjusted incidence rates and rate ratios of the outcomes between the HF types. Results: Incidence rates for most outcomes were significantly higher among patients with HFrEF compared with HFpEF. HHF was 59 % greater, mortality 31 % greater, and CKD incidence 55 % greater, (p < 0.001 for all comparisons). However, the larger size of the HFpEF group generated 4.7-6.7 times as many total outcomes. Conclusions: Regardless of subtype, the presence of HF was associated with poor clinical outcomes. Incidence rates were higher for HFrEF than HFpEF, but as the latter represented 75% of the study population, HFpEF caused a greater overall burden on the health care system, reflecting the high unmet need of target therapies for HFpEF.

Medical Costs of Chronic Kidney Disease and Type 2 Diabetes Among Newly Diagnosed Heart Failure Patients With Reduced, Mildly Reduced, and Preserved Ejection Fraction.

The economic burden of heart failure (HF) is enormous, but studies of HF costs typically consider the disease to be a single entity. We sought to distinguish the medical costs for patients with HF with reduced ejection fraction (HFrEF), mildly reduced ejection fraction (HFmrEF), and HF with preserved ejection fraction (HFpEF). We identified 16,516 adult patients with an incident HF diagnosis and an echocardiogram from 2005 to 2017 in the electronic medical record of Kaiser Permanente Northwest. Using the echocardiogram nearest to the first diagnosis date, we classified patients with HFrEF (ejection fraction [EF] ≤40%), HFmrEF (EF 41% to 49%), or HFpEF (EF ≥50%). We calculated annualized inpatient, outpatient, emergency, pharmaceutical medical utilization and costs and total costs in $2,020, adjusted for age and gender using generalized linear models, with further analysis of the effects of co-morbid chronic kidney disease (CKD) and type 2 diabetes (T2D). For all HF types, 1 in 5 patients were affected by both CKD and T2D, and costs were significantly higher when both co-morbidities were present. Total per-person costs were significantly higher for HFpEF ($33,740, 95% confidence interval $32,944 to $34,536) than HFrEF ($27,669, $25,649 to $29,689) or HFmrEF ($29,484, $27,166 to $31,800), driven by in- and outpatient visits. Across HF types, visits approximately doubled with the presence of both co-morbidities. Due to greater prevalence, HFpEF accounted for the majority of total and resource-specific treatment costs of HF, regardless of the presence of CKD and/or T2D. In summary, the economic burden was greater per HFpEF patient and was further amplified by co-morbid CKD and T2D. HFpEF accounted for the large majority of total HF costs, underscoring the need to implement effective treatments.

Differential Modulation of Markers of Oxidative Stress and DNA Damage in Arterial Hypertension.

Patients with arterial hypertension have an increased risk of developing tumors, particularly renal cell carcinoma. Arterial hypertension is linked to DNA damage via the generation of oxidative stress, in which an upregulated renin-angiotensin-aldosterone system plays a crucial role. The current study investigated surrogates of oxidative stress and DNA damage in a group of hypertensive patients (HypAll, n = 64) and subgroups of well (HypWell, n = 36) and poorly (HypPoor, n = 28) controlled hypertensive patients compared to healthy controls (n = 8). In addition, a longitudinal analysis was performed with some of the hypertensive patients. Markers for oxidative stress in plasma (SHp, D-ROM, and 3-nitrotyrosine) and urine (8-oxodG, 15-F2t-isoprostane, and malondialdehyde) and markers for DNA damage in lymphocytes (γ-H2AX and micronuclei) were measured. In HypAll, all markers of oxidative stress except malondialdehyde were increased compared to the controls. After adjustment for age, this association was maintained for the protein stress markers SHp and 3-nitrotyrosine. With regard to the markers for DNA damage, there was no difference between HypAll and the controls. Further, no significant differences became apparent in the levels of both oxidative stress and DNA damage between HypWell and HypPoor. Finally, a positive correlation between the development of blood pressure and oxidative stress was observed in the longitudinal study based on the changes in D-ROM and systolic blood pressure. In conclusion, we found increased oxidative stress in extensively treated hypertensive patients correlating with the level of blood-pressure control but no association with DNA damage.

Cardiac and kidney benefits of empagliflozin in heart failure across the spectrum of kidney function: Insights from the EMPEROR-Preserved trial.

AIM: In the EMPEROR-Preserved trial, empagliflozin improved clinical outcomes of patients with heart failure (HF) with preserved ejection fraction. In this pre-specified analysis, we aim to study the effect of empagliflozin on cardiovascular and kidney outcomes across the spectrum of kidney function. METHODS AND RESULTS: Patients were categorized by the presence or absence of chronic kidney disease (CKD) at baseline (CKD defined by an estimated glomerular filtration rate [eGFR] <60 ml/min/1.73 m2 or urine albumin to creatinine ratio >300 mg/g). The primary and key secondary outcomes were (i) a composite of cardiovascular death or first HF hospitalization (primary outcome); (ii) total number of HF hospitalization, (iii) eGFR slope; and a pre-specified exploratory composite kidney outcome including a sustained ≥40% decline in eGFR, chronic dialysis or renal transplant. The median follow-up was 26.2 months. A total of 5988 patients were randomized to empagliflozin or placebo, of whom 3198 (53.5%) had CKD. Irrespective of CKD status, empagliflozin reduced the primary outcome (with CKD: hazard ratio [HR] 0.80, 95% confidence interval [CI] 0.69-0.94; without CKD: HR 0.75, 95% CI 0.60-0.95; interaction p = 0.67) and total (first and recurrent) hospitalizations for HF (with CKD: HR 0.68, 95% CI 0.54-0.86; without CKD: HR 0.89, 95% CI 0.66-1.21; interaction p = 0.17). Empagliflozin slowed the slope of eGFR decline by 1.43 (1.01-1.85) ml/min/1.73 m2 /year in patients with CKD and 1.31 (0.88-1.74) ml/min/1.73 m2 /year in patients without CKD (interaction p = 0.70). Empagliflozin did not reduce the pre-specified kidney outcome in patients with or without CKD (with CKD: HR 0.97, 95% CI 0.71-1.34; without CKD: HR 0.92, 95% CI 0.58-1.48; interaction p = 0.86) but slowed progression to macroalbuminuria and reduced the risk of acute kidney injury. The effect of empagliflozin on the primary composite outcome and the key secondary outcomes was consistent across five baseline eGFR categories (all interaction p >0.05). Empagliflozin was well tolerated independent of CKD status. CONCLUSIONS: In EMPEROR-Preserved, empagliflozin had a beneficial effect on the key efficacy outcomes in patients with and without CKD. Overall, the benefit and safety of empagliflozin was consistent across a wide range of kidney function spectrum, down to a baseline eGFR of 20 ml/min/1.73 m2 .

Nephrotic-range proteinuria in type 2 diabetes: Effects of empagliflozin on kidney disease progression and clinical outcomes.

BACKGROUND: Diabetic kidney disease with nephrotic-range proteinuria (NRP) is commonly associated with rapid kidney function loss, increased cardiovascular risk, and premature mortality. We explored the effect of empagliflozin in patients with type 2 diabetes and cardiovascular disease, complicated by presence of this major risk factor for progressive kidney disease, in a post-hoc analysis of data from the EMPA-REG OUTCOME trial (NCT01131676). METHODS: Cox proportional hazards models were used to investigate the risk of cardiovascular and kidney outcomes in participants with and without NRP, defined by urine albumin-to-creatinine ratio (UACR) ≥2200 mg/g at baseline. Annual loss of eGFR during chronic treatment (eGFR slopes) and hypothetical time to projected end-stage kidney disease (ESKD), conditioning upon linearity of eGFR change over time if a patient did not decease before projected ESKD, were calculated using a random-intercept random-coefficient model. Safety was described based on investigator-reported adverse events. FINDINGS: 112 participants (pooled empagliflozin, n = 70; placebo, n = 42; median on-treatment follow-up of 1·9 years on placebo compared with 2·3 years on empagliflozin) presented with NRP at baseline; eGFR and UACR were balanced between treatments. Empagliflozin benefits on cardiovascular death, hospitalisation for heart failure, or kidney outcomes, were consistent in participants with and without NRP (pinteraction >0·1). Treatment effects of empagliflozin on adjusted annual mean eGFR slope were more pronounced in participants with NRP versus those without (pinteraction 0·005). Empagliflozin was estimated to double the median hypothetical time to projected ESKD in participants with NRP. The overall safety profile of empagliflozin was comparable between participants with and without NRP at baseline. INTERPRETATION: Our data suggests that empagliflozin might slow kidney function loss and delay the estimated onset of projected ESKD in patients with type 2 diabetes and cardiovascular disease complicated by NRP.

Increased myocardial sodium signal intensity in Conn's syndrome detected by 23Na magnetic resonance imaging.

AIMS: Sodium intake has been linked to left ventricular hypertrophy independently of blood pressure, but the underlying mechanisms remain unclear. Primary hyperaldosteronism (PHA), a condition characterized by tissue sodium overload due to aldosterone excess, causes accelerated left ventricular hypertrophy compared to blood pressure matched patients with essential hypertension. We therefore hypothesized that the myocardium constitutes a novel site capable of sodium storage explaining the missing link between sodium and left ventricular hypertrophy. METHODS AND RESULTS: Using 23Na magnetic resonance imaging, we investigated relative sodium signal intensities (rSSI) in the heart, calf muscle, and skin in 8 PHA patients (6 male, median age 55 years) and 12 normotensive healthy controls (HC) (8 male, median age 61 years). PHA patients had a higher mean systolic 24 h ambulatory blood pressure [152 (140; 163) vs. 125 (122; 130) mmHg, P < 0.001] and higher left ventricular mass index [71.0 (63.5; 106.8) vs. 55.0 (50.3; 66.8) g/m2, P = 0.037] than HC. Compared to HC, PHA patients exhibited significantly higher rSSI in the myocardium [0.31 (0.26; 0.34) vs. 0.24 (0.20; 0.27); P = 0.007], calf muscle [0.19 (0.16; 0.22) vs. 0.14 (0.13; 0.15); P = 0.001] and skin [0.28 (0.25; 0.33) vs. 0.19 (0.17; 0.26); P = 0.014], reflecting a difference of +27%, +38%, and +39%, respectively. Treatment of PHA resulted in significant reductions of the rSSI in the myocardium, calf muscle and skin by -13%, -27%, and -29%, respectively. CONCLUSION: Myocardial tissue rSSI is increased in PHA patients and treatment of aldosterone excess effectively reduces rSSI, thus establishing the myocardium as a novel site of sodium storage in addition to skeletal muscle and skin.

Quantitative assessment of adipocyte differentiation in cell culture.

Adipocyte cell culture is an important tool for mechanistic studies of energy metabolism. Many factors affect the differentiation of adipocytes in culture. Oil red O staining can be used to assess the degree of differentiation. However, the validity of this method for quantitative analysis has not yet been established. Here we show that a protocol with arbitrarily chosen parameters does not measure in the linear range and is not suitable for quantitative analysis (R2 = 0.077, p = 0.382), and develop and validate an optimized protocol for quantitative oil red O staining of cultured adipocytes. 3T3-L1 preadipocytes and adipocytes are fixed with 4% formaldehyde and stained with 0.2% oil red O solution in 40% 2-propanol for 30 minutes. Dye is eluted with 2-propanol, and absorption of the eluate is measured photometrically at 510 nm. This optimized protocol achieves excellent correlation between defined amounts of differentiated adipocytes on constant-size culture plates and photometric absorption (R2 = 0.972, p = 6.585E-14). The performance of the method is independent of the culture plates used. Thus, the optimized oil red O staining protocol can be universally employed to quantitatively assess adipocyte differentiation.

Downregulation of STRA6 in adipocytes and adipose stromovascular fraction in obesity and effects of adipocyte-specific STRA6 knockdown in vivo.

To investigate the mechanisms by which elevated retinol-binding protein 4 (RBP4) causes insulin resistance, we studied the role of the high-affinity receptor for RBP4, STRA6 (stimulated by retinoic acid), in insulin resistance and obesity. In high-fat-diet-fed and ob/ob mice, STRA6 expression was decreased 70 to 95% in perigonadal adipocytes and both perigonadal and subcutaneous adipose stromovascular cells. To determine whether downregulation of STRA6 in adipocytes contributes to insulin resistance, we generated adipose-Stra6(-/-) mice. Adipose-Stra6(-/-) mice fed chow had decreased body weight, fat mass, leptin levels, insulin levels, and adipocyte number and increased expression of brown fat-selective markers in white adipose tissue. When fed a high-fat diet, these mice had a mild improvement in insulin sensitivity at an age when adiposity was unchanged. STRA6 has been implicated in retinol uptake, but retinol uptake and the expression of retinoid homeostatic genes (encoding retinoic acid receptor ß [RARß], CYP26A1, and lecithin retinol acyltransferase) were not altered in adipocytes from adipose-Stra6(-/-) mice, indicating that retinoid homeostasis was maintained with STRA6 knockdown. Thus, STRA6 reduction in adipocytes in adipose-Stra6(-/-) mice fed chow resulted in leanness, which may contribute to their increased insulin sensitivity. However, in wild-type mice with high-fat-diet-induced obesity and in ob/ob mice, the marked downregulation of STRA6 in adipocytes and adipose stromovascular cells does not compensate for obesity-associated insulin resistance.

Retinol-binding protein 4 inhibits insulin signaling in adipocytes by inducing proinflammatory cytokines in macrophages through a c-Jun N-terminal kinase- and toll-like receptor 4-dependent and retinol-independent mechanism.

Retinol-binding protein 4 (RBP4), the sole retinol transporter in blood, is secreted from adipocytes and liver. Serum RBP4 levels correlate highly with insulin resistance, other metabolic syndrome factors, and cardiovascular disease. Elevated serum RBP4 causes insulin resistance, but the molecular mechanisms are unknown. Here we show that RBP4 induces expression of proinflammatory cytokines in mouse and human macrophages and thereby indirectly inhibits insulin signaling in cocultured adipocytes. This occurs through activation of c-Jun N-terminal protein kinase (JNK) and Toll-like receptor 4 (TLR4) pathways independent of the RBP4 receptor, STRA6. RBP4 effects are markedly attenuated in JNK1-/- JNK2-/- macrophages and TLR4-/- macrophages. Because RBP4 is a retinol-binding protein, we investigated whether these effects are retinol dependent. Unexpectedly, retinol-free RBP4 (apo-RBP4) is as potent as retinol-bound RBP4 (holo-RBP4) in inducing proinflammatory cytokines in macrophages. Apo-RBP4 is likely to be physiologically significant since RBP4/retinol ratios are increased in serum of lean and obese insulin-resistant humans compared to ratios in insulin-sensitive humans, indicating that higher apo-RBP4 is associated with insulin resistance independent of obesity. Thus, RBP4 may cause insulin resistance by contributing to the development of an inflammatory state in adipose tissue through activation of proinflammatory cytokines in macrophages. This process reveals a novel JNK- and TLR4-dependent and retinol- and STRA6-independent mechanism of action for RBP4.