Development of Biomarkers for Inhibition of SLC6A19 (B0AT1)—A Potential Target to Treat Metabolic Disorders

Abstract

Recent studies have established that dietary protein restriction improves metabolic health and glucose homeostasis. SLC6A19 (B0AT1) is the major neutral amino acid transporter in the intestine and carries out the bulk of amino acid absorption from the diet. Mice lacking SLC6A19 show signs of protein restriction, have improved glucose tolerance, and are protected from diet-induced obesity. Pharmacological blockage of this transporter could be used to induce protein restriction and to treat metabolic diseases such as type 2 diabetes. A few novel inhibitors of SLC6A19 have recently been identified using in vitro compound screening, but it remains unclear whether these compounds block the transporter in vivo. To evaluate the efficacy of SLC6A19 inhibitors biomarkers are required that can reliably detect successful inhibition of the transporter in mice. A gas chromatography mass spectrometry (GC-MS)-based untargeted metabolomics approach was used to discriminate global metabolite profiles in plasma, urine and faecal samples from SLC6A19ko and wt mice. Due to inefficient absorption in the intestine and lack of reabsorption in the kidney, significantly elevated amino acids levels were observed in urine and faecal samples. By contrast, a few neutral amino acids were reduced in the plasma of male SLC6A19ko mice as compared to other biological samples. Metabolites of bacterial protein fermentation such as p-cresol glucuronide and 3-indole-propionic acid were more abundant in SLC6A19ko mice, indicating protein malabsorption of dietary amino acids. Consistently, plasma appearance rates of [14C]-labelled neutral amino acids were delayed in SLC6A19ko mice as compared to wt after intra-gastric administration of a mixture of amino acids. Receiver operating characteristic (ROC) curve analysis was used to validate the potential use of these metabolites as biomarkers. These findings provide putative metabolite biomarkers that can be used to detect protein malabsorption and the inhibition of this transporter in intestine and kidney.

Restriction of essential amino acids dictates the systemic metabolic response to dietary protein dilution

“…On another note, even though dietary protein as a nutrient can be digested and absorbed as AA or short peptides, our data reinforce that dietary AA supply is paramount under conditions of dietary protein restriction. Importantly, metabolomic studies of postprandial plasma amino acid levels in Slc6a19 null mice, which mimic the effects of DPD, also revealed a prominent effect on Thr and Trp when fed standard laboratory chow 58 . By contrast, the ablation of intestinal peptide transport does not mimic the effects of DPD 59 .…”

Amino acid transporters in the regulation of insulin secretion and signalling

“…Both methionine and leucine are major substrates of B 0 AT1 and the requirement of multiple neutral amino acids for effective and cumulative FGF21 induction is consistent with the transporter's role as the primary uptake pathway for multiple neutral amino acids. The reduced plasma glucose recorded in these B 0 AT1 KO mice seemed to be a result of a 50% reduction in intestinal uptake (61) but without any downregulation of known sugar transporters (70). However, the plasma glucose lowering effect of FGF21 has also been attributed to activation of brown adipose tissue (BAT), the browning of white adipose tissue (WAT), and increased hepatic energy consumption, particularly in obese and male rodents (35,(71)(72)(73)(74).…”

Novel Chemical Scaffolds to Inhibit the Neutral Amino Acid Transporter B0AT1 (SLC6A19), a Potential Target to Treat Metabolic Diseases

“…Renal aminoaciduria is restricted to neutral amino acids and defines the disorder in the absence of other clinical symptoms. Typically, plasma amino levels are normal or at the lower end of the normal range in individuals with Hartnup disorder (Levy, 2001) and this is also observed in fasting B 0 AT1 knock-out mice (Javed et al, 2018). However, upon nutrient intake, amino acid absorption is delayed and differences in plasma neutral amino acid levels are observed (Singer et al, 2012;Jiang et al, 2015;Javed and Broer, 2019).…”