Alteration of De Novo Glucose Production Contributes to Fasting Hypoglycaemia in Fyn Deficient Mice

Abstract

Previous studies have demonstrated that glucose disposal is increased in the Fyn knockout (FynKO) mice due to increased insulin sensitivity. FynKO mice also display fasting hypoglycaemia despite decreased insulin levels, which suggested that hepatic glucose production was unable to compensate for the increased basal glucose utilization. The present study investigates the basis for the reduction in plasma glucose levels and the reduced ability for the liver to produce glucose in response to gluconeogenic substrates. FynKO mice had a 5-fold reduction in phosphoenolpyruvate carboxykinase (PEPCK) gene and protein expression and a marked reduction in pyruvate, pyruvate/lactate-stimulated glucose output. Remarkably, de novo glucose production was also blunted using gluconeogenic substrates that bypass the PEPCK step. Impaired conversion of glycerol to glucose was observed in both glycerol tolerance test and determination of the conversion of 13C-glycerol to glucose in the fasted state. α-glycerol phosphate levels were reduced but glycerol kinase protein expression levels were not changed. Fructose-driven glucose production was also diminished without alteration of fructokinase expression levels. The normal levels of dihydroxyacetone phosphate and glyceraldehyde-3-phosphate observed in the FynKO liver extracts suggested normal triose kinase function. Fructose-bisphosphate aldolase (aldolase) mRNA or protein levels were normal in the Fyn-deficient livers, however, there was a large reduction in liver fructose-6-phosphate (30-fold) and fructose-1,6-bisphosphate (7-fold) levels as well as a reduction in glucose-6-phosphate (2-fold) levels. These data suggest a mechanistic defect in the allosteric regulation of aldolase activity.

<p>Guidelines and Considerations for Metabolic Tolerance Tests in Mice</p>

“…injection), control of glycemia at 0, 15, 30, 45, 60, and 120 min after injection. [47,48] Alanine and glutamine tolerance tests Glutamine and alanine are two major amino acids transported from the muscles during periods of active gluconeogenesis. Therefore.…”

“…Several modifications of procedure have been Levels of blood glucose should be analyzed at 0, 15, 30, 45, 60, and 120 min after injection. [47,49] Metformintolerance test (MTT)…”

Thy1 (CD90) controls adipogenesis by regulating activity of the Src family kinase, Fyn

“…Fyn and Cdk5 are involved in a signal transduction pathway that mediates neuronal guidance (51), and it is interesting to speculate whether a similar complex is involved in insulin resistance and obesity in part through modification of PPARγ. Interestingly, Fyn‐nullmice have a 5‐fold lower level of expression of PEPCK , aPPARγ‐dependent gene, compared to wild‐type mice (18). These data further suggest a link between Fyn signaling and PPARγ activity.…”

“…Specifically, Fyn‐null mice display reduced free triglycerides, reduced lipid accumulation, and a reduced percentage of adipose tissue mass (17). Fyn‐null mice also express 5‐fold lower levels of phosphoenolpyruvate carboxykinase ( PEPCK ), a key adipogenic gene that is induced by PPARγ (18), suggesting that Fyn‐null mice have lower PPARγ activity than wild‐type animals. Although these studies demonstrate a clear link between Fyn, PPARγ, and adipose tissue, to our knowledge, there is no literature describing a role for Thy1 in adipose tissue accumulation and obesity.…”

Fyn kinase regulates translation in mammalian mitochondria

“…4). Validation of endogenous Fyn kinase expression and stimulation of mitochondrial translation in cells with higher oxidative phosphorylation capacity is also in agreement with its role in various converging signaling pathways, such as Wnt and insulin signaling to glucose and fatty acid oxidation [45–48].…”