Deconstructing the Role of PKC Epsilon in Glucose Homeostasis

doi:10.1016/j.tem.2020.01.016

Trends in Endocrinology & Metabolism

Volume 31, Issue 5, May 2020, Pages 344-356

https://doi.org/10.1016/j.tem.2020.01.016 Get rights and content

Highlights

Lipid-sensitive protein kinase C (PKC) isoforms are activated upon accumulation of the lipid metabolite diacylglycerol and are strong candidates for mediating insulin resistance.
PKCε has been linked to liver insulin resistance and global ablation has demonstrated a key role for this kinase in glucose homeostasis. However, recent genetic and phospho-proteomic studies have generated conflicting data on the tissue-specific function of PKCε and its role in the inhibition of insulin signal transduction.
The role of PKCε impacts on the broader debate concerning the relative importance of direct and indirect actions of insulin on the liver.
The measurement of PKCε activation by subcellular translocation assay is open to postextraction artefacts. Determinations in live cells and intact tissue are needed to confirm the response of the kinase to chronic lipid excess and to determine its true cellular location.

The failure of insulin to suppress glucose production by the liver is a key aspect of the insulin resistance seen in type 2 diabetes. Lipid-activated protein kinase C epsilon has long been identified as an important mediator of diet-induced glucose intolerance and hepatic insulin resistance and the current view emphasizes a mechanism involving phosphorylation of the insulin receptor by the kinase to inhibit downstream insulin action. However, the significance of this direct effect in the liver has now been challenged by tissue-specific deletion of PKCε, which demonstrated a more prominent role for the kinase in adipose tissue to promote glucose intolerance. New insights regarding the role of PKCε therefore contribute to the understanding of indirect effects on hepatic glucose metabolism.

Section snippets

Protein Kinase C as a Mediator of Insulin Resistance

Obesity and a chronic oversupply of fatty acids (FAs) are strongly associated with the development of insulin resistance (see Glossary), a key feature of type 2 diabetes (T2D). The reduced ability of insulin to promote glucose uptake into skeletal muscle and to suppress glucose production by the liver, together with a failure of insulin-secreting pancreatic β-cells to compensate for this insulin resistance, lead to dysregulated glucose homeostasis. This in turn causes complications, such as

PKCε and Insulin Resistance in the Liver

PKCε translocation and DAG accumulation have been reported in association with hepatic insulin resistance in animal studies using several dietary and genetic approaches. This led to the development of a widely accepted model for the induction of defective insulin action through direct disruption of proximal insulin signaling by the activated kinase [4]. However, alterations in hepatic DAG levels do not always correlate with the expected changes in PKCε translocation or insulin sensitivity [21.,

PKCε-Mediated Interference with Insulin Receptor Signaling

Several protein kinases reduce insulin signal transduction through Ser/Thr phosphorylation of IRS-1, which impairs subsequent insulin receptor-mediated Tyr phosphorylation [29]. Thus PKCε translocation in the liver was initially associated with diminished tyrosine phosphorylation of IRS-1 in fat-fed rats [30]. More recently, however, attention has been focused on direct phosphorylation of the insulin receptor by the kinase. Analysis by mass spectrometry of phosphopeptides, generated by in vitro

Tissue-Specific PKCε Deletion Highlights Extrahepatic Roles

Although the conventional whole body PKCε KO mouse provided evidence that PKCε played a causal role in the impaired glucose tolerance observed in fat-fed mice [25], because the kinase was deleted in every cell type, a direct effect in liver was not definitively established. Similarly, knockdown of PKCε with ASOs led to the abolition of PKCε expression not only in liver, but also in adipose tissue and possibly in several other additional tissues [27]. In order to determine the contribution that

Measuring PKCε Activation

PKC translocation from soluble to particulate fractions of cells and tissues has long been used as a convenient surrogate measure for enzyme activation (Box 1). Activation of PKC is mediated by an accumulation of intracellular DAG molecules that have access to the kinase and recruit it to membranes. It should be noted, however, that this procedure may be open to postextraction artefacts, because pools of DAG that are not in contact with PKC molecules in intact cells could promote membrane

Concluding Remarks

There is broad agreement that PKCε plays a significant role in the generation of lipid-induced insulin resistance and glucose intolerance. Until recently, the major focus has been on a direct effect of the kinase in the liver and a mechanism involving inhibitory phosphorylation of the insulin receptor and impaired downstream signaling. However, PKCε is ubiquitously expressed and earlier work had already elucidated a role for this isoform in the β-cell dysfunction that develops after insulin

Acknowledgments

Studies in the author’s laboratory have been supported by grants from the National Health and Medical Research Council of Australia (APP535917, APP1081869) and Diabetes Australia Research Program (Y15G-SCHC, Y18G-SCHC, Y19G-SCHC). The author is grateful to Prof. Trevor J. Biden for critical reading of the manuscript.

Glossary

Akt: a serine kinase, also known as protein kinase B, that is central to insulin signaling pathways that regulate glucose and lipid metabolism. It is transiently recruited to the plasma membrane in insulin-stimulated cells, where it is phosphorylated and activated.
Coatomer protein: one of several proteins that form a complex around vesicles involved in protein transport, either from the endoplasmic reticulum to the Golgi complex (COPII vesicles) or retrograde transport from the Golgi complex to the

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Thus, we consider that the effects of HH-extract on AMPK expression are due to the presence of hypericin, hyperforin, quercetin and chlorogenic acid in the HH extract. With respect to the PKCε concentration, there was significant decrease in the liver of HH-treated diabetic rats, which leads to inhibition of glycogenolysis and stimulation of net glycogen synthesis (Schmitz-Peiffer, 2020), resulting in the increased liver glycogen content. Moreover, this effect on PKCε could also attribute to activation of the AMPK signaling pathway in the liver (Jeon, 2016).
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Plant material of H. perforatum was harvested from a natural population in the Republic of North Macedonia during full flowering season. Methanol (80% v/v) was used to extract bioactive components from HH powder. The dissolved HH dry extract (in 0.3% CMC) was given daily as a single treatment (200 mg/kg bw) during 14 days both in healthy and streptozotocin-induced diabetic rats. As a positive control, we applied glibenclamide. The activity of key enzymes involved in carbohydrate methabolisam in the liver were assessed, along with substrate concentration, as well as AMPK mRNA levels, PKCε concentration, plasma insulin level and pancreatic PARP activity.
Compared to diabetic rats, treatment of diabetic rats with HH extract resulted with decreased activity of hepatic enzymes glucose-6-phospatase and fructose-1,6-bisphosphatase, increased liver glycogen and glucose-6-phosphate content, which resulted with reduced blood glucose concentration up to normoglycaemia. Non-significant changes were observed in the activity of hexokinase, glycogen phosphorylase and glucose-6-phospahte dehydrogenase. HH-treatment also caused an increase in plasma insulin concentration and increase in pancreatic PARP activity. Finally, HH treatment of diabetic rats showed significant increase in AMPK expression and decrease of PKCε concentration.
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Trends in Endocrinology & Metabolism

ReviewDeconstructing the Role of PKC Epsilon in Glucose Homeostasis

Highlights

Section snippets

Protein Kinase C as a Mediator of Insulin Resistance

PKCε and Insulin Resistance in the Liver

PKCε-Mediated Interference with Insulin Receptor Signaling

Tissue-Specific PKCε Deletion Highlights Extrahepatic Roles

Measuring PKCε Activation

Concluding Remarks

Acknowledgments

Glossary

Pharmacol. Res.

Cell Metab.

Trends Endocrinol. Metab.

Trends Cell Biol.

Cell Rep.

Trends Endocrinol. Metab.

Cell Metab.

J. Lipid Res.

Cell Metab.

J. Biol. Chem.

Mol. Cell. Proteomics

Cell Metab.

Mol Metab.

Biochim. Biophys. Acta Mol. Cell Biol. Lipids

Biochim. Biophys. Acta

Cell Metab.

J. Biol. Chem.

Cell. Signal.

J. Biol. Chem.

J. Biol. Chem.

Methods

Trends Endocrinol. Metab.

Trends Endocrinol. Metab.

Cell Metab.

Cell

Cell Metab.

Protein kinase C: perfectly balanced

Crit. Rev. Biochem. Mol. Biol.

Protein kinase C function in muscle, liver, and beta-cells and its therapeutic implications for type 2 diabetes

Diabetes

Mechanisms of insulin action and insulin resistance

Physiol. Rev.

Mammalian triacylglycerol metabolism: synthesis, lipolysis, and signaling

Chem. Rev.

DAG tales: the multiple faces of diacylglycerol--stereochemistry, metabolism, and signaling

Cell. Mol. Life Sci.

The substrates and binding partners of protein kinase Cε

Biochem. J.

Spatiotemporal dynamics of lipid signaling: protein kinase C as a paradigm

IUBMB Life

Protein kinase Cη is targeted to lipid droplets

Histochem. Cell Biol.

CGI-58 knockdown sequesters diacylglycerols in lipid droplets/ER-preventing diacylglycerol-mediated hepatic insulin resistance

Proc. Natl. Acad. Sci. U. S. A.

Diverse roles for protein kinase C delta and protein kinase C epsilon in the generation of high-fat-diet-induced glucose intolerance in mice: regulation of lipogenesis by protein kinase C delta

Diabetologia

PKCδ regulates hepatic insulin sensitivity and hepatosteatosis in mice and humans

J. Clin. Invest.

Protein kinase C is increased in the liver of humans and rats with noninsulin-dependent diabetes mellitus: an alteration not due to hyperglycemia

J. Clin. Invest.

Review
Deconstructing the Role of PKC Epsilon in Glucose Homeostasis