Differential regulation of branched-chain alpha-ketoacid dehydrogenase kinase expression by glucocorticoids and acidification in LLC-PK1-GR101 cells

Association between branched-chain amino acids and renal function in the ELSA-Brasil study

BACKGROUND & AIMS

Epidemiologic studies show high circulating Branched-chain amino acids (BCAA) are associated with excess body weight, impaired fasting glucose, insulin resistance, high blood pressure, and dyslipidemia. There is scarce data on the association between renal function and circulating levels of BCAA. Therefore, we aim to study this association in a sample of the Brazilian Longitudinal Study of Adults (ELSA-Brasil) METHODS: We analyzed participants who had at the baseline BCAA: valine, isoleucine, and leucine measured through nuclear magnetic resonance. The outcomes evaluated were estimated glomerular function (eGFR - CKD-EPI without race) and 12h-albumin-creatinine ratio (ACR). In addition, we built unadjusted and adjusted multivariable linear regression models to investigate the association between the BCAA (total and individual) and eGFR and ACR.

RESULTS

We studied 4912 participants (age 51.7(±9.0) years, 53.4% women, 59.5% White (59.5%), 32.7% hypertension, and 18.2% diabetes). The mean BCAA level was 429.15 ± 87.15. The mean eGFR was 84.95 ± 15 ml/min/1.73 m2, and the median ACR was 6.5 (1.8-4920) mg/g. Descriptive analyses comparing eGFR stratified <60 ml/min/1.73 m2 and ACR≥30 mg/g demonstrate that BCAA levels are higher in patients with eGFR<60 and ACR ≥30. Regarding eGFR, an inverse association was detected with BCAA levels when adjusted for demographic variables, and it is not maintained after adjustments for other confounders. Also, a positive association was found for ACR≥30 mg/g, and BCAA levels, and this association is not confirmed after adjustments.

CONCLUSIONS

BCAA levels were inversely associated with eGFR and positively associated with ACR. Further studies are necessary to allow the comprehension of those associations.

External Validation of a Novel Multimarker GFR Estimating Equation

Key Points

Using multiple markers may improve GFR estimation especially in settings where creatinine and cystatin C are known to be limited. Panel eGFR is a novel multimarker eGFR equation consisting of age, sex, cystatin C, and nuclear magnetic resonance–measured creatinine, valine, and myo-inositol. eGFR-Cr and eGFR-Cr-CysC may underestimate measured GFR, while panel eGFR was unbiased among younger Black male individuals.

Background

Using multiple markers may improve accuracy in GFR estimation. We sought to externally validate and compare the performance of a novel multimarker eGFR (panel eGFR) equation among Black and White persons using the Genetic Epidemiology Network of Arteriopathy cohort.

Methods

We included 224 sex, race/ethnicity, and measured GFR (mGFR) category–matched persons, with GFR measured using urinary clearance of iothalamate. We calculated panel eGFR using serum creatinine, valine, myo-inositol, cystatin C, age, and sex. We compared its reliability with current eGFR equations (2021 CKD Epidemiology Collaboration creatinine [eGFR-Cr] and creatinine with cystatin C [eGFR-Cr-CysC]) using median bias, precision, and accuracy metrics. We evaluated each equation's performance in age, sex, and race subgroups.

Results

In the overall cohort, 49% were Black individuals, and mean mGFR was 79 ml/min per 1.73 m2. Panel eGFR overestimated mGFR (bias: −2.4 ml/min per 1.73 m2; 95% confidence interval [CI], −4.4 to −0.7), eGFR-Cr-CysC underestimated mGFR (bias: 4.8 ml/min per 1.73 m2; 95% CI, 2.1 to 6.7), while eGFR-Cr was unbiased (bias: 2.0 ml/min per 1.73 m2; 95% CI, −1.1 to 4.6). All equations had comparable accuracy. Among Black male individuals younger than 65 years, both eGFR-Cr (bias: 17.0 ml/min per 1.73 m2; 95% CI, 8.6 to 23.5) and eGFR-Cr-CysC (bias: 14.5 ml/min per 1.73 m2; 95% CI, 6.0 to 19.7) underestimated mGFR, whereas panel eGFR was unbiased (bias: 1.7 ml/min per 1.73 m2; 95% CI, −3.4 to 10.0). Metrics of accuracy for all eGFRs were acceptable in all subgroups except for panel eGFR in Black female individuals younger than 65 years (P30: 73.3%).

Conclusions

Panel eGFR can be used to estimate mGFR and may have utility among Black male individuals younger than 65 years where current CKD Epidemiology Collaboration equations are biased.

Emerging role for branched-chain amino acids metabolism in fibrosis

Fibrosis is a common pathological feature of organ diseases resulting from excessive production of extracellular matrix, which accounts for significant morbidity and mortality. However, there is currently no effective treatment targeting fibrogenesis. Recently, metabolic alterations are increasingly considered as essential factors underlying fibrogenesis, and especially research on metabolic regulation of amino acids is flourishing. Among them, branched-chain amino acids (BCAAs) are the most abundant essential amino acids, including leucine, isoleucine and valine, which play significant roles in the substance and energy metabolism and their regulation. Dysregulation of BCAAs metabolism has been proven to contribute to numerous diseases. In this review, we summarize the metabolic regulation of fibrosis and the changes in BCAAs metabolism secondary to fibrosis. We also review the effects and mechanisms of the BCAAs intervention, and its therapeutic targeting in hepatic, renal and cardiac fibrosis, with a focus on the fibrosis in liver and associated hepatocellular carcinoma.

Effect of renal tubule-specific knockdown of the Na+/H+ exchanger NHE3 in Akita diabetic mice

Na+/H+ exchanger isoform 3 (NHE3) contributes to Na+/bicarbonate reabsorption and ammonium secretion in early proximal tubules. To determine its role in the diabetic kidney, type 1 diabetic Akita mice with tubular NHE3 knockdown [Pax8-Cre; NHE3-knockout (KO) mice] were generated. NHE3-KO mice had higher urine pH, more bicarbonaturia, and compensating increases in renal mRNA expression for genes associated with generation of ammonium, bicarbonate, and glucose (phosphoenolpyruvate carboxykinase) in proximal tubules and H+ and ammonia secretion and glycolysis in distal tubules. This left blood pH and bicarbonate unaffected in nondiabetic and diabetic NHE3-KO versus wild-type mice but was associated with renal upregulation of proinflammatory markers. Higher renal phosphoenolpyruvate carboxykinase expression in NHE3-KO mice was associated with lower Na+-glucose cotransporter (SGLT)2 and higher SGLT1 expression, indicating a downward tubular shift in Na+ and glucose reabsorption. NHE3-KO was associated with lesser kidney weight and glomerular filtration rate (GFR) independent of diabetes and prevented diabetes-associated albuminuria. NHE3-KO, however, did not attenuate hyperglycemia or prevent diabetes from increasing kidney weight and GFR. Higher renal gluconeogenesis may explain similar hyperglycemia despite lower SGLT2 expression and higher glucosuria in diabetic NHE3-KO versus wild-type mice; stronger SGLT1 engagement could have affected kidney weight and GFR responses. Chronic kidney disease in humans is associated with reduced urinary excretion of metabolites of branched-chain amino acids and the tricarboxylic acid cycle, a pattern mimicked in diabetic wild-type mice. This pattern was reversed in nondiabetic NHE3-KO mice, possibly reflecting branched-chain amino acids use for ammoniagenesis and tricarboxylic acid cycle upregulation to support formation of ammonia, bicarbonate, and glucose in proximal tubule. NHE3-KO, however, did not prevent the diabetes-induced urinary downregulation in these metabolites.

Divergent Induction of Branched-Chain Aminotransferases and Phosphorylation of Branched Chain Keto-Acid Dehydrogenase Is a Potential Mechanism Coupling Branched-Chain Keto-Acid-Mediated-Astrocyte Activation to Branched-Chain Amino Acid Depletion-Mediated Cognitive Deficit after Traumatic Brain Injury

Deficient branched-chain amino acids (BCAAs) are implicated in cognitive dysfunction after traumatic brain injury (TBI). The mechanism remains unknown. BCAAs are catabolized by neuron-specific cytosolic and astrocyte-specific mitochondrial branched-chain aminotransferases (BCATc, BCATm) to generate glutamate and branched-chain keto-acids (BCKAs) that are metabolized by the mitochondrial branched-chain keto-acid dehydrogenase (BCKD) whose activity is regulated by its phosphorylation state. BCKD phosphorylation by BCKD kinase (BCKDK) inactivates BCKD and cause neurocognitive dysfunction, whereas dephosphorylation by specific phosphatase restores BCKD activity. Real-time polymerase chain reaction showed rapidly and significantly decreased BCATc messenger RNA (mRNA) levels, but significantly increased BCATm mRNA level post-CCI (controlled cortical impact). BCKD and BCKDK mRNA decreased significantly immediately after CCI-induced TBI (CCI) in the rat. Phosphorylated BCKD proteins (pBCKD) increased significantly in the ipsilateral-CCI hemisphere. Immunohistochemistry revealed significantly increased pBCKD proteins in ipsilateral astrocytes post-CCI. BCKD protein expression is higher in primarily cultured cortical neurons than in astrocytes, whereas pBCKD protein level is higher in astrocytes than in cortical neurons. Transforming growth factor beta treatment (10 μg/mL for 48 h) significantly increased pBCKD protein expression in astrocytes, whereas glutamate treatment (25 μM for 24 h) significantly decreased pBCKD protein in neurons. Because increased pBCKD would lead to increased BCKA accumulation, BCKA-mediated astrocyte activation, cell death, and cognitive dysfunction as found in maple syrup urine disease; thus, TBI may potentially induce cognitive deficit through diverting BCAA from glutamate production in neurons to BCKA production in astrocytes through the pBCKD-dependent mechanism.

Branched-chain amino acids in metabolic signalling and insulin resistance

Branched-chain amino acids (BCAAs) are important nutrient signals that have direct and indirect effects. Frequently, BCAAs have been reported to mediate antiobesity effects, especially in rodent models. However, circulating levels of BCAAs tend to be increased in individuals with obesity and are associated with worse metabolic health and future insulin resistance or type 2 diabetes mellitus (T2DM). A hypothesized mechanism linking increased levels of BCAAs and T2DM involves leucine-mediated activation of the mammalian target of rapamycin complex 1 (mTORC1), which results in uncoupling of insulin signalling at an early stage. A BCAA dysmetabolism model proposes that the accumulation of mitotoxic metabolites (and not BCAAs per se) promotes β-cell mitochondrial dysfunction, stress signalling and apoptosis associated with T2DM. Alternatively, insulin resistance might promote aminoacidaemia by increasing the protein degradation that insulin normally suppresses, and/or by eliciting an impairment of efficient BCAA oxidative metabolism in some tissues. Whether and how impaired BCAA metabolism might occur in obesity is discussed in this Review. Research on the role of individual and model-dependent differences in BCAA metabolism is needed, as several genes (BCKDHA, PPM1K, IVD and KLF15) have been designated as candidate genes for obesity and/or T2DM in humans, and distinct phenotypes of tissue-specific branched chain ketoacid dehydrogenase complex activity have been detected in animal models of obesity and T2DM.

Inhibition of PI3-kinase signaling by glucocorticoids results in increased branched-chain amino acid degradation in renal epithelial cells

Phosphatidylinositol 3-kinase(PI3K) is a pivotal enzyme involved in the control of a variety of diverse metabolic functions. Glucocorticoids have been shown to attenuate PI3K signaling in some nonrenal cell types, raising the possibility that some physiological effects of glucocorticoids in renal cells may be achieved by a similar mechanism. Therefore, we tested whether glucocorticoids affect signaling through the insulin receptor substrate (IRS)-1/PI3K/Akt signaling cascade in LLC-PK1-GR101 renal epithelial cells. Treatment of cells with dexamethasone for 24 h: 1) suppressed IRS-1-associated PI3K activity and Akt phosphorylation, 2) increased the level of the PI3K p85 regulatory subunit but not the p110 catalytic subunit, and 3) induced the phosphorylation of IRS-1 on inhibitory Ser307. We have previously reported that glucocorticoids increase branched-chain ketoacid dehydrogenase (BCKD) activity in LLC-PK1-GR101 cells. This response was achieved, in part, by alterations in the transcription of BCKD subunits and BCKD kinase, which inactivates the enzyme complex by phosphorylation. Therefore, we tested whether inhibition of PI3K signaling would mimick glucocorticoids by increasing branched-chain amino acid degradation. Expression of a dominant negative PI3K p85 regulatory subunit (Adp85ΔiSH2) increased BCKD activity, and dexamethasone did not further stimulate enzyme activity. Inhibition of PI3K using LY-294002 increased the transcription of the BCKD E2 subunit but not the E1α subunit or BCKD kinase. Thus, glucocorticoids inhibit signaling through the IRS-1/PI3K/Akt pathway with a consequence of increased branched-chain amino acid catabolism.

Application of branched-chain amino acids in human pathological states: renal failure

During renal failure, abnormalities of BCAA and branched-chain keto acid (BCKA) metabolism are due to both the lack of renal contribution to amino acid metabolism and the impact of renal failure and acidosis on whole-body nitrogen metabolism. Abnormal BCAA and BCKA metabolism result in BCAA depletion as reflected by low plasma BCAAs and cellular valine. BCAA metabolic disturbances can alter tissue activities, particularly brain function, and nutritional status. In dialysis patients, BCAA oral supplementation can induce an improvement of appetite and nutritional status. During chronic renal failure, the aims of nutritional interventions are to minimize uremic toxicity, avoid malnutrition and delay progression of kidney disease. BCAA and BCKA supplements have been proposed to decrease further protein intake while maintaining satisfactory nutritional status. In this setting, BCAAs or BCKAs have not been administrated solely but in association with other essential AA or keto analogs. Therefore, the proper effects of BCAAs and/or BCKAs have not been studied separately. Protein restriction together with keto acids and/or essential AAs has been reported to improve insulin sensitivity and hyperparathyroidism and to be compatible with a preservation of nutritional status. Nonetheless, a careful monitoring of protein-calorie intake and nutritional status is needed. A recent meta-analysis concluded that reducing protein intake in patients with chronic renal failure reduces the occurrence of renal death by approximately 40% as compared with larger or unrestricted protein intake. The additional effect of essential amino acids and keto acids on retardation of progression of renal failure has not been demonstrated.