Deletion of protein kinase Cε in mice has limited effects on liver metabolite levels but alters fasting ketogenesis and gluconeogenesis

Abstract

Aims/hypothesis Protein kinase Cε (PKCε) is emerging as a key mediator of lipid-induced insulin resistance in liver and hepatic lipid metabolism itself. We investigated whether PKCε plays a role in other metabolic processes, to further examine its suitability as a therapeutic target. Methods We measured amino acid, organic acid and sugar levels by liquid and gas chromatography-mass spectrometry of liver extracts from chow and fat-fed wild-type (WT) and PKCε-deficient (Prkce −/− ) mice. Fed and fasting glucose, ketone and fatty acid levels were measured in blood. Triacylglycerol levels and gluconeogenic and ketogenic enzyme expression were measured in liver.The effect of fasting on epididymal fat pad mass was also determined.Results Metabolomic analysis indicated that the short-term high-fat diet affected over 20 compounds, including a 50% reduction in the glucogenic amino acid alanine. Prkce deletion resulted only in a reduction of 4-hydroxyproline and aspartate and an increase in glutamate. However, upon fasting, Prkce −/− mice were better able to maintain blood glucose levels and also exhibited lower levels of the ketone β-hydroxybutyrate compared with WT mice. Upon fasting, Prkce deletion also resulted in lower liver and plasma lipids and a smaller reduction in fat pad mass. Conclusions/interpretation Metabolomic analysis provided new insights into the effects of a high-fat diet on liver metabolite levels. Glucose homeostasis under fasting conditions is improved in Prkce −/− mice, which, in turn, may reduce the mobilisation of lipid from adipose tissue, reducing the availability of ketogenic substrate in the liver. Together with the protection against fat-diet-induced glucose intolerance previously observed in the fed state, these findings indicate PKCε as a unique therapeutic target for the improvement of glucose homeostasis.Keywords Alanine . Gluconeogenesis .

The tail wagging the dog – regulation of lipid metabolism by protein kinase C

“…In PKCε‐deficient mice, fat pad mass is not reduced to the same extent as in wild‐type mice after a 48 h fast, plasma free fatty acids are not as elevated, and liver lipid stores are not supplemented to the same extent. This reduced supply of ketogenic precursors to the liver results in plasma β‐hydroxybutyrate levels that are almost 50% lower than in wild‐type mice . This prevention of fatty acid release by adipose tissue under fasting conditions, together with the alterations in lipid flux exhibited by the liver and pancreatic islets in the fed state, contributes to the emerging theme of lipid partitioning towards TAG storage in the absence of PKCε.…”

“…Various roles for individual PKC isoforms have been identified by studying the deletion of specific kinases in vivo, as well as manipulating their expression in cultured cells. PKCb and PKCe have been most widely implicated in adipogenesis [55][56][57][58]60,61,64,73] and fat cell metabolism [13][14][15][39][40][41][42]. In the liver, PKCb, PKCd and PKCι/k have been associated with lipogenesis [19,20,[48][49][50][51]74], while PKCe appears to play a role in the acute response to fat over-supply [39], promoting b-oxidation in the initial phase of a high-fat diet.…”

“…While these findings support a direct role of PKCε in the inhibition of glucose disposal in response to insulin, additional studies indicated that this enzyme also contributes to the early response of the liver to dietary lipid excess. First, the sustained insulin action observed in the absence of PKCε during the early phase of a high‐fat diet was unexpectedly associated with an increase in hepatic TAG content and a defect in lipid oxidation . Thus, although liver lipid content was still much lower than in long term fat‐fed mice, it was up to 50% greater in the PKCε‐deficient mice compared to wild‐type mice, and this was associated with reduced energy consumption despite normal switching to fat as the major oxidative fuel .…”

“…In the liver, PKCβ, PKCδ and PKCι/λ have been associated with lipogenesis , while PKCε appears to play a role in the acute response to fat over‐supply , promoting β‐oxidation in the initial phase of a high‐fat diet. This enzyme also affects ketogenesis in the liver under fasting conditions, most likely by regulating fatty acid (FA) release from adipose tissue . In skeletal muscle, increased PKCβ activity is associated with enhanced lipogenesis , but the immediate effects of PKCθ in this tissue on lipid metabolism are unclear, but influence whole‐body energy expenditure .…”

Linking physiology to toxicity using DILIsym®, a mechanistic mathematical model of drug‐induced liver injury

“…Thus, hepatic fatty acid uptake, mitochondrial utilization and trafficking to triglycerides were added to the in vivo mitochondrial toxicity sub-model. Meal-provided nutrient input was also added to DILIsym ® , with feeding patterns and nutrient amounts consistent with those observed for humans, dogs, rats and mice [43][44][45][46]. Glucose uptake, entry to the glycolysis pathway and glycogen synthesis/utilization were carried over from MITOsym ™ .…”