Functional Variant in the GCKR Gene Affects Lactate Levels Differentially in the Fasting State and During Hyperglycemia

Maykel López Rodríguez, Lilian Fernandes Silva, Jagadish Vangipurapu, Shalem Modi, Johanna Kuusisto, Minna U. Kaikkonen, Markku Laakso
2018 Scientific Reports  
The rs780094 single nucleotide polymorphism (SNP; C/T) of glucokinase regulatory protein gene (GCKR) is a regulatory genetic variant that has been associated with lactate levels in the fasting state. However, the association of this locus with lactate during hyperglycemia, and the mechanisms underlying these associations remain unknown. We investigated the association of rs780094 with lactate levels in a frequently sampled oral glucose tolerance test in humans and evaluated the effect of
more » ... he effect of increasing GCKR expression on lactate production in liver cells. The C allele of rs780094 was associated with lower lactate levels in fasting but increased lactate level during hyperglycemia independently of insulin levels. Increased expression of GKRP induced higher lactate level in HepG2 cells and in human primary hepatocytes (HPH) upon glucose stimulation by increasing the amount of GCK. Glucagon induced the expression of GCKR in HepG2 and HPH cells. Our results suggest that the association of rs780094 with lactate levels may involve differential GCKR expression between the carriers of the C and T alleles. Genome-wide association studies (GWAS) have identified over 100 gene variants associated with type 2 diabetes (T2D), including the Glucokinase Regulatory Protein gene (GCKR; MIM 613463) 1 . Glucokinase Regulatory Protein (GKRP; NP_001477), encoded by GCKR, is almost exclusively expressed in the liver, where it inhibits glucokinase (GCK) in the nucleus of the hepatocytes at low glucose concentrations 2 . An intronic variant rs780094 and a coding variant rs1260326 of GCKR are in high linkage disequilibrium (LD), and these variants have been found to be associated with glucose and lactate levels, and a wide range of other metabolic traits 3-9 , de novo lipogenesis in obese young individuals 10 , and the risk of non-alcoholic fatty liver disease, nonalcoholic steatohepatitis and liver fibrosis [11] [12] [13] [14] . Functional studies of rs1260326 have shown that the minor T allele of a non-synonymous P446L substitution of GCKR leads to a weak GCK binding at low glucose concentrations, and impaired response to fructose-6 phosphate 15,16 . We recently demonstrated that the intronic locus including rs780094 is a transcriptional enhancer that regulates GCKR expression in a haplotype specific manner 17 . The important role of GKRP and GCK in the regulation of glucose metabolism in the liver suggests that any functional variant affecting GCKR/GKRP could have an effect on glucose metabolism. Lactate is produced by the reduction of pyruvate coupled to the oxidation of reduced nicotinamide adenine dinucleotide (NADH) to NAD + . Glucose, contributing to about 65% of circulating lactate 18 and alanine are the main sources of lactate formation in humans. Plasma lactate levels represent a balance between its formation and clearance. In the fasting state, circulating lactate released mainly from skeletal muscle and adipose tissue is taken up by the liver and used for gluconeogenesis, whereas glucose is metabolized to lactate at high glucose availability 19 . Therefore, blood lactate levels reflect the rate of hepatic glucose metabolism. Previous studies have shown that P446L of GCKR is associated with lactate levels in the fasting state 6,10,20 , and after an intake of glucose and fructose 10 . The aims of our study were, i) to investigate the association of rs780094 of GCKR with lactate levels in the fasting state and during hyperglycemia, and ii) to examine the effects of increasing GKRP levels on lactate production to understand the mechanisms underlying the association of rs780094 of GCKR with lactate levels. Our Published: xx xx xxxx OPEN www.nature.com/scientificreports/
doi:10.1038/s41598-018-34501-9 fatcat:343fl3tafnc6lbdy4ouuvzvgnm