Bilirubin Exerts Renoprotective Effects in Angiotensin II-Hypertension

The predictive effect of direct-indirect bilirubin ratio on clinical events in acute coronary syndrome: results from an observational cohort study in north China

Background:

Patients with extremely high-risk ASCVD usually suffered poor prognosis, bilirubin is considered closely related to cardiovascular outcomes. However, there is controversy over the relationship between bilirubin and coronary artery disease. This study aimed to evaluate the predictive value of the DIBIL ratio in patients with extremely high-risk ASCVD.

Methods:

10,260 consecutive patients with extremely high-risk ASCVD were enrolled in this study. All patients were divided into three groups according to their DIBIL ratio. The incidence of MACCEs was recorded, and in a competing risk regression, the incidence of MACCEs and their subgroups were recorded. The direct-indirect bilirubin ratio (DIBIL ratio) was calculated by the direct bilirubin (umol/L)/indirect bilirubin (umol/L) ratio, all laboratory values were obtained from the first fasting blood samples during hospitalization.

Results:

The area under the ROC curve of the DIBIL ratio to predict the occurrence of all-cause death was 0.668, the cut-off value of which is 0.275. Competing risk regression indicated that DIBIL ratio was positively correlated with all-cause death [1.829 (1.405–2.381), p < 0.001], CV death [1.600 (1.103, 2.321), p = 0.013]. The addition of DIBIL ratio to a baseline risk model had an incremental effect on the predictive value for all-cause death [IDI 0.004(0, 0.010), p < 0.001; C-index 0.805(0.783–0.827), p < 0.001].

Conclusion:

The DIBIL ratio was an excellent tool to predict poor prognosis, suggesting that this index may be developed as a biomarker for risk stratification and prognosis in extremely ASCVD patients.

Bilirubin as a metabolic hormone: the physiological relevance of low levels

Recent research on bilirubin, a historically well-known waste product of heme catabolism, suggests an entirely new function as a metabolic hormone that drives gene transcription by nuclear receptors. Studies are now revealing that low plasma bilirubin levels, defined as "hypobilirubinemia," are a possible new pathology analogous to the other end of the spectrum of extreme hyperbilirubinemia seen in patients with jaundice and liver dysfunction. Hypobilirubinemia is most commonly seen in patients with metabolic dysfunction, which may lead to cardiovascular complications and possibly stroke. We address the clinical significance of low bilirubin levels. A better understanding of bilirubin's hormonal function may explain why hypobilirubinemia might be deleterious. We present mechanisms by which bilirubin may be protective at mildly elevated levels and research directions that could generate treatment possibilities for patients with hypobilirubinemia, such as targeting of pathways that regulate its production or turnover or the newly designed bilirubin nanoparticles. Our review here calls for a shift in the perspective of an old molecule that could benefit millions of patients with hypobilirubinemia.

Non-urate transporter 1, non-glucose transporter member 9-related renal hypouricemia and acute renal failure accompanied by hyperbilirubinemia after anaerobic exercise: a case report

Background

Renal hypouricemia (RHUC) is an inherited heterogenous disorder caused by faulty urate reabsorption transporters in the renal proximal tubular cells. Anaerobic exercise may induce acute kidney injury in individuals with RHUC that is not caused by exertional rhabdomyolysis; it is called acute renal failure with severe loin pain and patchy renal ischemia after anaerobic exercise (ALPE).

RHUC is the most important risk factor for ALPE. However, the mechanism of onset of ALPE in patients with RHUC has not been elucidated. The currently known genes responsible for RHUC are SLC22A12 and SLC2A9.

Case presentation

A 37-year-old man presented with loin pain after exercising. Despite having a healthy constitution from birth, biochemical examination revealed hypouricemia, with a uric acid (UA) level of < 1 mg/dL consistently at every health check. We detected acute kidney injury, with a creatinine (Cr) level of 4.1 mg/dL, and elevated bilirubin; hence, the patient was hospitalized. Computed tomography revealed no renal calculi, but bilateral renal swelling was noted. Magnetic resonance imaging detected cuneiform lesions, indicating bilateral renal ischemia. Fractional excretion values of sodium and UA were 0.61 and 50.5%, respectively. Urinary microscopy showed lack of tubular injury. The patient’s older sister had hypouricemia. The patient was diagnosed with ALPE. Treatment with bed rest, fluid replacement, and nutrition therapy improved renal function and bilirubin levels, and the patient was discharged on day 5. Approximately 1 month after onset of ALPE, his Cr, UA, and TB levels were 0.98, 0.8, and 0.9 mg/dL, respectively. We suspected familial RHUC due to the hypouricemia and family history and performed genetic testing but did not find the typical genes responsible for RHUC. A full genetic analysis was opposed by the family.

Conclusions

To the best of our knowledge, this is the first report of ALPE with hyperbilirubinemia. Bilirubin levels may become elevated as a result of heme oxygenase-1 activation, occurring in exercise-induced acute kidney injury in patients with RHUC; this phenomenon suggests renal ischemia-reperfusion injury. A new causative gene coding for a urate transporter may exist, and its identification would be useful to clarify the urate transport mechanism.

Elevated bilirubin levels are associated with a better renal prognosis and ameliorate kidney fibrosis

Background

Bilirubin has been reported to protect against kidney injury. However, further studies highlighting the beneficial effects of bilirubin on renal fibrosis and chronic renal function decline are necessary.

Methods

We assessed a prospective cohort with a reference range of total bilirubin levels. The primary outcome was a 30% reduction in the estimated glomerular filtration rate (eGFR) from baseline, and the secondary outcome was a doubling of the serum creatinine levels, halving of the eGFR and the initiation of dialysis. In addition, experiments with tubular epithelial cells and C57BL/6 mice were performed to investigate the protective effects of bilirubin on kidney fibrosis.

Results

As a result, 1,080 patients were included in the study cohort. The study group with relative hyperbilirubinemia (total bilirubin 0.8–1.2 mg/dL) showed a better prognosis in terms of the primary outcome (adjusted hazard ratio (HR) 0.33, 95% confidence interval (CI) 0.19–0.59, P < 0.001) and the secondary outcome (adjusted HR 0.20, 95% CI 0.05 to 0.71, P = 0.01) than that of the control group. Moreover, the bilirubin-treated mice showed less fibrosis in the unilateral ureteral obstruction (UUO) model (P < 0.05). In addition, bilirubin treatment decreased fibronectin expression in tubular epithelial cells in a dose-dependent manner (P < 0.05).

Conclusions

Mildly elevated serum bilirubin levels were associated with better renal prognosis, and bilirubin treatment induced a beneficial effect on renal fibrosis. Therefore, bilirubin could be a potential therapeutic target to delay fibrosis-related kidney disease progression.

Bile Cast Nephropathy Caused by Obstructive Cholestasis

Acute kidney injury (AKI) is a major complication in patients with liver disease. Although hepatorenal syndrome is frequently involved, bile cast nephropathy, characterized by tubular bile cast formation, has been scarcely described in the setting of severe liver failure. Few renal histology studies are available in these patients. We describe a case of bile cast nephropathy in a patient with obstructive cholestasis caused by stones in the common bile duct. The kidney biopsy confirmed this diagnosis, with several green casts in tubular lumens, tubular injury, and bilirubin composition of the tubular casts with Hall stain. The patient had no confounding cause of kidney failure, and complete kidney recovery followed removal of the bile duct obstruction. This case shows that severe cholestasis is sufficient to cause AKI, and that AKI can be reversible after treatment of the biliary obstruction.

Length Polymorphisms in Heme Oxygenase-1 and AKI after Cardiac Surgery

Heme oxygenase-1 (HO-1) catalyzes the degradation of heme, which may be involved in the pathogenesis of AKI. Length polymorphisms in the number of GT dinucleotide repeats in the HO-1 gene (HMOX1) promoter inversely associate with HMOX1 mRNA expression. We analyzed the association between allelic frequencies of GT repeats in the HMOX1 gene promoter and postoperative AKI in 2377 white patients who underwent cardiac surgery with cardiopulmonary bypass. We categorized patients as having the short allele (S; <27 GT repeats) or long allele (L; ≥27 GT repeats), and defined AKI as an increase in serum creatinine ≥0.3 mg/dl within 48 hours or ≥50% within 5 days, or the need for RRT. Compared with patients with the SS genotype, patients with the LL genotype had 1.58-fold (95% confidence interval, 1.06 to 2.34; P=0.02) higher odds of AKI. After adjusting for baseline and operative characteristics, the odds ratio for AKI per L allele was 1.26 (95% confidence interval, 1.05 to 1.50; P=0.01). In conclusion, longer GT repeats in the HMOX1 gene promoter associate with increased risk of AKI after cardiac surgery, consistent with heme toxicity as a pathogenic feature of cardiac surgery-associated AKI, and with HO-1 as a potential therapeutic target.

Circulating bilirubin and defense against kidney disease and cardiovascular mortality: mechanisms contributing to protection in clinical investigations

Unconjugated bilirubin is an endogenous circulating antioxidant, bound to albumin, and therefore is retained in the vascular compartment. Bilirubin has well-documented neurotoxic effects in infants; however, current evidence indicates mildly elevated bilirubin is associated with protection from cardiovascular disease and all-cause mortality in adults. Recent clinical studies show mildly elevated bilirubin is associated with protection from kidney damage and dysfunction, in addition to cardiovascular events and all-cause mortality in patients undergoing hemodialysis. This is the first review to examine the clinical evidence and summarize the potential mechanisms of action that link bilirubin to protection from kidney damage, subsequent kidney failure, and dialysis-related mortality. With this understanding, it is hoped that new therapies will be developed to prevent renal dysfunction and mortality from cardiovascular disease in at-risk individuals.

The level of serum bilirubin associated with coronary lesion types in patients with coronary artery disease

BACKGROUND

Serum bilirubin has been proven to be associated with coronary artery disease (CAD). However, how serum bilirubin is related to the complexity of coronary artery lesions is still unknown.

METHODS AND RESULTS

One thousand two hundred and sixty patients (men 775, 61.5%, mean age, 59.3  ±  8.2 years) diagnosed with unstable angina were enrolled in the study. Patients were categorized into three major groups and group III was further divided into four subgroups according to the guidelines of AHA/ACC 1993 described in the Methods section. The total serum bilirubin levels showed significant differences among the three major groups (group I vs. group II, 14.8 ± 5.8 vs. 13.7 ± 4.7 μmol/l, P=0.017; group I vs. group III, 14.8 ± 5.8 vs. 12.6 ±  4.4 μmol/l, P<0.001; group II vs. group III, 13.7 ± 4.7 vs. 12.6 ± 4.4 μmol/l, P=0.009). The difference was further seen among the subgroups. Logistic regression analysis demonstrated that age, male sex, histories of hypertension and diabetes, and total serum bilirubin were independent risk factors for CAD. However, in the subgroups, only age, male sex, history of hypertension and total serum bilirubin were associated with CAD. Total serum bilirubin showed the strongest relationship (odds ratio=0.95, 95% confidence interval 0.91-0.98, P=0.001).

CONCLUSION

Total serum bilirubin level is an independent risk factor for CAD. It has a strong relationship with coronary artery lesion types.

Bilirubin, renal hemodynamics, and blood pressure

Bilirubin is generated from the breakdown of heme by heme oxygenase and the reduction of biliverdin by the enzyme biliverdin reductase. Several large population studies have reported a significant inverse correlation between plasma bilirubin levels and the incidence of cardiovascular disease. Protection from cardiovascular disease is also observed in patients with Gilbert’s syndrome which is a disease characterized by mutations in hepatic UGT1A1, the enzyme responsible for the conjugation of bilirubin into the bile. Despite the strong correlation between plasma bilirubin levels and the protection from cardiovascular disease, the mechanism by which increases in plasma bilirubin acts to protect against cardiovascular disease is unknown. Since the chronic antihypertensive actions of bilirubin are likely due to its renal actions, the effects of moderate increases in plasma bilirubin on renal hemodynamics as well as bilirubin’s potential effects on renal tubule function will be discussed in this review. Mechanisms of action as well as the potential for antihypertensive therapies targeting moderate increases in plasma bilirubin levels will also be highlighted.

Renal heme oxygenase-1 induction with hemin augments renal hemodynamics, renal autoregulation, and excretory function

Heme oxygenases (HO-1; HO-2) catalyze conversion of heme to free iron, carbon monoxide, and biliverdin/bilirubin. To determine the effects of renal HO-1 induction on blood pressure and renal function, normal control rats (n = 7) and hemin-treated rats (n = 6) were studied. Renal clearance studies were performed on anesthetized rats to assess renal function; renal blood flow (RBF) was measured using a transonic flow probe placed around the left renal artery. Hemin treatment significantly induced renal HO-1. Mean arterial pressure and heart rate were not different (115 ± 5 mmHg versus 112 ± 4 mmHg and 331 ± 16 versus 346 ± 10 bpm). However, RBF was significantly higher (9.1 ± 0.8 versus 7.0 ± 0.5 mL/min/g, P < 0.05), and renal vascular resistance was significantly lower (13.0 ± 0.9 versus 16.6 ± 1.4 [mmHg/(mL/min/g)], P < 0.05). Likewise, glomerular filtration rate was significantly elevated (1.4 ± 0.2 versus 1.0 ± 0.1 mL/min/g, P < 0.05), and urine flow and sodium excretion were also higher (18.9 ± 3.9 versus 8.2 ± 1.0 μL/min/g, P < 0.05 and 1.9 ± 0.6 versus 0.2 ± 0.1 μmol/min/g, P < 0.05, resp.). The plateau of the autoregulation relationship was elevated, and renal vascular responses to acute angiotensin II infusion were attenuated in hemin-treated rats reflecting the vasodilatory effect of HO-1 induction. We conclude that renal HO-1 induction augments renal function which may contribute to the antihypertensive effects of HO-1 induction observed in hypertension models.

Heme induction with delta-aminolevulinic Acid stimulates an increase in water and electrolyte excretion

Purpose. Studies were performed to examine hemodynamic and renal function before and after acute induction of the endogenous CO system with delta-aminolevulinic acid (DALA), which drives HO activity. Methods. In vivo studies were conducted on Inactin-anesthetized male Sprague Dawley rats (250–300 g) either with or without chronic pretreatment with L-NAME (50 mg/Kg, q12 hours x4d). Results. DALA (80 μmol/Kg, IV bolus) administration acutely increased endogenous CO production and HO-1 protein. In untreated and L-NAME-pretreated rats, DALA did not alter BP, GFR, or RBF but increased UF, U_NaV, and U_KV (untreated: Δ108.8 ± 0.28%, 172.1 ± 18.4%, and 165.2 ± 45.9%; pretreated: Δ109.4 ± 0.29%, 187.3 ± 26.9%, and 197.2 ± 45.7%). Acute administration of biliverdin (20 mg/kg, IV) and bilirubin (30 mg/kg, IV) to similarly treated animals did not alter UF, U_NaV, and U_KV. Conclusion. These results demonstrate that heme oxygenase induction increases urine and electrolyte excretion and suggest a direct tubular action of endogenous carbon monoxide.

Heme oxygenase metabolites inhibit tubuloglomerular feedback in vivo

Tubuloglomerular feedback (TGF) is a renal autoregulatory mechanism that constricts the afferent arteriole in response to increases in distal NaCl. Heme oxygenases (HO-1 and HO-2) release carbon monoxide (CO) and biliverdin, which may help control renal function. We showed in vitro that HO products inhibit TGF; however, we do not know whether this also occurs in vivo or the mechanism(s) involved. We hypothesized that in vivo HO-1 and HO-2 in the nephron inhibit TGF via release of CO and biliverdin. We first performed laser capture microdissection followed by real-time PCR and found that both HO-1 and HO-2 are expressed in the macula densa. We next performed micropuncture experiments in vivo on individual rat nephrons, adding different compounds to the perfusate, and found that an HO inhibitor, stannous mesoporphyrin (SnMP), potentiated TGF (P < 0.05, SnMP vs. control). The CO-releasing molecule (CORM)-3 partially inhibited TGF at 50 μmol/l (P < 0.01, CORM-3 vs. control) and blocked it completely at higher doses. A soluble guanylyl cyclase (sGC) inhibitor, LY83583, blocked the inhibitory effect of CORM-3 on TGF. Biliverdin also partially inhibited TGF (P < 0.01, biliverdin vs. control), most likely attributable to decreased superoxide (O(2)(-)) because biliverdin was rendered ineffective by tempol, a O(2)(-) dismutase mimetic. We concluded that HO-1 and HO-2 in the nephron inhibit TGF by releasing CO and biliverdin. The inhibitory effect of CO on TGF is mediated by the sGC/cGMP signaling pathway, whereas biliverdin probably acts by reducing O(2)(-).

The role of oxidative stress in the pathophysiology of hypertension

Hypertension is considered to be the most important risk factor in the development of cardiovascular disease. An increasing body of evidence suggests that oxidative stress, which results in an excessive generation of reactive oxygen species (ROS), has a key role in the pathogenesis of hypertension. The modulation of the vasomotor system involves ROS as mediators of vasoconstriction induced by angiotensin II, endothelin-1 and urotensin-II, among others. The bioavailability of nitric oxide (NO), which is a major vasodilator, is highly dependent on the redox status. Under physiological conditions, low concentrations of intracellular ROS have an important role in the normal redox signaling maintaining vascular function and integrity. However, under pathophysiological conditions, increased levels of ROS contribute to vascular dysfunction and remodeling through oxidative damage. In human hypertension, an increase in the production of superoxide anions and hydrogen peroxide, a decrease in NO synthesis and a reduction in antioxidant bioavailability have been observed. In turn, antioxidants are reducing agents that can neutralize these oxidative and otherwise damaging biomolecules. The use of antioxidant vitamins, such as vitamins C and E, has gained considerable interest as protecting agents against vascular endothelial damage. Available data support the role of these vitamins as effective antioxidants that can counteract ROS effects. This review discusses the mechanisms involved in ROS generation, the role of oxidative stress in the pathogenesis of vascular damage in hypertension, and the possible therapeutic strategies that could prevent or treat this disorder.