Abstract Body: Objective: Insulin resistance (IR) is the key environmental factor associated with hyperuricemia; however, the underlying mechanisms are not fully understood. Although several studies suggested that excessive salt intake results in hyperuricemia, the association still remains controversial. Focusing on urate transporter 1 (URAT1), we here investigated the mechanisms by which insulin and salt intake contribute to hyperuricemia.

Methods: In a large UK Biobank (UKB) dataset involving 377,358 participants, we evaluated the association of TyG index (a surrogate of IR) and habitual salt intake with serum uric acid levels using multivariable regression models. We addressed whether single nucleotide polymorphisms (SNPs) that influence URAT1 expression (eQTL) modify the association of TyG index with serum uric acid levels. We also analyzed underlying mechanisms by using HEK cells expressing hURAT1.

Results: In UKB dataset, TyG index and habitual salt intake were significantly and independently associated with elevated serum uric acid levels (P < 0.05). From previous studies and imputation accuracy, we extracted an eQTL SNP for SLC22A12 gene and found that there is a positive interaction between TyG Index and the eQTL SNP. As post-translational modification is the key mechanism that enables adaptation to environmental changes, we evaluated the contribution of phosphorylation by AGC kinases, such as Akt, as possible downstream effectors of insulin signaling and salt-sensing pathway. Western blotting with phospho-Akt substrate motif antibody confirmed URAT1 phosphorylation and identified T350 and T408 as candidate sites. Among them, URAT1 carrying non-phosphorylatable A408 substitution (T408A-URAT1) suppressed glycosylation and plasma membrane translocation, while T350A had no effect. By using kinase screen assay involving 53 AGC and related Ser/Thr kinases, we found that SGK1, PKA, and PKG also phosphorylated URAT1-T408. Insulin and NaCl loading induced Akt and SGK1 expression respectively, which resulted in URAT1 phosphorylation and increased cell-surface membrane expression; this effect was abolished in T408A-URAT1. Finally, we found that URAT1-T408 phosphorylation was also detected in human kidney sections.

Conclusion: IR and excessive salt intake contribute to hyperuricemia, and the effect is in part mediated by hURAT1. Mechanistically, insulin and NaCl promote membrane trafficking of URAT1 by inducing T408 phosphorylation through Akt and SGK1, respectively.